small molecule activation and transformation using aluminum-based frustrated … · 2013-08-09 ·...

Small Molecule Activation and Transformation Using Aluminum-Based Frustrated Lewis Pairs

by

Gabriel Ménard

A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy

Department of Chemistry University of Toronto

© Copyright by Gabriel Ménard 2013

ii

Small Molecule Activation and Transformation Using Aluminum-

Based Frustrated Lewis Pairs

Gabriel Ménard

Doctor of Philosophy

Department of Chemistry

University of Toronto

2013

Abstract

While hundreds of papers have been published on frustrated Lewis pairs (FLPs) – the

combination of bulky Lewis acids and bases which cannot form adducts – few of these use

aluminum-based Lewis acids. The research outlined in this thesis expands the small molecule

activation chemistry of FLPs to include Al.

Combinations of bulky phosphines and AlX3 (X = halide or C6F5) with CO2 leads to the rapid

activation to form the complexes R3P(CO2)(AlX3)2 (R = otol, Mes). Subsequent treatment with

ammonia-borane (AB) results in the rapid reduction of the CO2 fragment to methanol after water

quench. Subsequent reactivity studies have established that AB adducts of AlX3, which react

with CO2, are key intermediates in this chemistry.

Further studies with Mes3P(CO2)(AlX3)2 revealed that these can reduce exogenous CO2 to CO,

along with the generation of Mes3P(C(OAlX2)2O)(AlX3) and [Mes3PX][AlX4]. Detailed

experimental and theoretical mechanistic investigations outline a possible mechanism involving

direct CO2 insertion into free AlX3, followed by nucleophilic attack by PMes3 resulting in the

expulsion of CO.

iii

Reactions with olefins were also investigated. While addition products of the type

R3P(CH2CH2)AlX3 could be obtained with ethylene, C–H bond activation occurred with bulkier

olefins. The resulting allyl species underwent subsequent C–C bond forming reactions with other

olefins or CO2.

Hydrogen was also activated using PR3/AlX3 FLPs to form species of the general formula,

[R3PH][(H)(AlX3)2] (X = I, C6F5). These were found to reduce unactivated olefins, generating

the redistributed products [R3PH][AlX4] and RAlX2 (R = alkyl). Attempts to circumvent this

redistribution and favour alkyl protonation, thus generating a catalytic hydrogenation catalyst,

are also discussed.

Finally, the activation of N2O was also examined. While addition products could be formed,

unexpected aromatic or benzylic C–H bond activation chemistry occurred in the presence of

excess Al. A radical reaction pathway is proposed.

iv

Acknowledgments

First and foremost, I would like to acknowledge my supervisor, Prof. Doug Stephan. Thank you

for always being there with your office door open, never turning anyone back. Thank you for the

countless fruitful discussions mostly about chemistry, but also about life experiences, politics,

and wine! I am deeply grateful for all the opportunities you have given me, whether in allowing

me to explore the chemistry I want or by sending me to countless conferences, allowing me to

network with others and showcase our work.

Next, I am deeply indebted to the Stephan group for countless useful discussions and help

throughout this experience. There have been far too many of you, past and present to name you

all. I am also very grateful to all the crystallographers of the group, past and present, including

Meghan Dureen, Stephen Geier, Michael Sgro, Michael Boone, Chris Caputo, Fatme Dahcheh,

Conor Pranckevicius, Adam McKinty, Chris Brown, Stephanie Granville, and Xiaoxi Zhao. I

would also like to thank Jillian Hatnean for her several trips to Windsor to run EPR experiments.

Thanks also to my undergraduate student, Lina Tran, for a job well done.

I would also like to acknowledge my committee members Professors Bob Morris and Datong

Song. Thank you both for the advice throughout the years and for reading this thesis.

This work would not have been possible without the excellent support staff in the department.

Thanks to Shanna Pritchard for keeping our lab organized. I am deeply indebted to the terrific

NMR staff for all their help, including Timothy Burrow, Darcy Burns, Adina Golombek, and

Dmitry Pichugin. Thanks also to Kenton Greaves, John Ford, Jack O’Donnell, Jack Jackiewicz,

Frank Bures, Anna Liza Villavelez, Alan Lough, Rose Balazs, and Dan Mathers.

I would like to thank my family for their support throughout this very long endeavour of both

undergraduate and graduate studies. Thanks Mom for your constant love and for always being

there. Thanks also to Josée, Travis, Anik, Wayne, Zach, Marie-France, and of course the little

ones who always make me smile, Liam, Zoë, and Addyson. Thanks so much to all in the Nowak

and Fallenbuchl families for their constant love and support. Thanks especially to Papa and

Yolande for being such wonderful people. Finally, none of this would have been possible

without the constant support, understanding, encouragement, and love of my lifelong partner,

best friend, and wife, Joey. Merci très sincèrement ma chère, je t’aime bien gros!

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Table of Contents

Abstract ........................................................................................................................................... ii

Acknowledgments .......................................................................................................................... iv

Table of Contents ............................................................................................................................ v

List of Figures ................................................................................................................................. x

List of Schemes ............................................................................................................................ xiv

List of Tables ............................................................................................................................. xviii

List of Symbols and Abbreviations .............................................................................................. xix

Chapter 1 Introduction ............................................................................................................. 1

1.1 Chemistry, Development, and the Environment ................................................................. 1

1.1.1 Aluminum: Natural abundance, extraction, and prevalence ................................... 2

1.1.2 Aluminum chemistry .............................................................................................. 3

1.2 A Short History of Frustrated Lewis Pair Chemistry .......................................................... 4

1.2.1 H2 activation and mechanism .................................................................................. 4

1.2.2 Hydrogenation chemistry ........................................................................................ 6

1.2.3 Other small molecules ............................................................................................. 7

1.2.3.1 Olefins and alkynes .................................................................................. 7

1.2.3.2 CO2 and SO2 ............................................................................................. 8

1.2.3.3 N2O and NO ............................................................................................. 9

1.2.4 Al-based FLPs ....................................................................................................... 10

1.3 Scope of Thesis ................................................................................................................. 12

Chapter 2 Activation and Reduction of CO2 to Methanol using Al-Based Frustrated

Lewis Pairs and Ammonia-Borane .......................................................................................... 14

2.1 Introduction ....................................................................................................................... 14

2.2 Results and Discussion ..................................................................................................... 16

2.2.1 Activation of CO2 using AlX3 and a phosphine .................................................... 16

vi

2.2.2 Reduction of CO2 to methanol using ammonia-borane ........................................ 19

2.2.3 Reactions of AB with EX3 (E = B, Al; X = halide or C6F5): redistribution vs.

adduct formation ................................................................................................... 20

2.2.4 Reactions of Al–AB species with CO2 ................................................................. 23

2.2.5 Possible reaction pathway for the reduction of CO2 from Mes3P(CO2)(AlX3)2 ... 28

2.3 Conclusions ....................................................................................................................... 32

2.4 Experimental Section ........................................................................................................ 32

2.4.1 General considerations .......................................................................................... 32

2.4.2 Synthesis of compounds ....................................................................................... 33

2.4.3 X-Ray crystallography .......................................................................................... 40

2.4.3.1 X-Ray data collection and reduction ...................................................... 40

2.4.3.2 X-Ray data solution and refinement ....................................................... 41

2.4.3.3 Selected crystallographic data ................................................................ 42

Chapter 3 Stoichiometric Reduction of CO2 to CO Using AlX3-Based Frustrated Lewis

Pairs........... ............................................................................................................................... 46

3.1 Introduction ....................................................................................................................... 46


3.2.1 In situ reduction of CO2 to CO using PMes3/AlX3 solutions ................................ 48

3.2.2 Reaction of R3P(CO2)(AlX3)2 (X = halide or C6F5) with CO2.............................. 52

3.2.3 Mechanistic study of the reduction of CO2 to CO using AlX3 ............................. 56

3.2.3.1 Kinetic study ........................................................................................... 56

3.2.3.2 Rate inhibition using cyclohexene .......................................................... 58

3.2.3.3 Labelling experiments ............................................................................ 59

3.2.3.4 (otol)3P(CO2)(AlI3)2: Rate inhibition due to strong P–Al bonding ........ 61

3.2.3.5 Proposed mechanism .............................................................................. 64

3.2.3.6 The in situ reduction conundrum ............................................................ 67

3.2.4 Theoretical investigation ....................................................................................... 69

vii

3.2.4.1 CO2 insertion into Al–X bonds and CO formation ................................ 70

3.2.4.2 Exchange of [X2AlOAlX3]- with the AlX3 of Mes3P(CO2)(AlX3)2 ....... 71

3.2.4.3 Thermodynamic product formation ........................................................ 71

3.2.4.4 Competitive cyclohexene binding .......................................................... 71

3.2.5 Alternative mechanism: Open-shell pathway? ..................................................... 72

3.3 Conclusions ....................................................................................................................... 75

3.4 Experimental Section ........................................................................................................ 76

3.4.1 General considerations .......................................................................................... 76

3.4.2 Synthesis of compounds ....................................................................................... 77

3.4.3 Quantification of CO produced from the reduction of CO2 .................................. 82

3.4.4 IR analysis of CO produced .................................................................................. 84

3.4.5 Kinetic study details .............................................................................................. 84

3.4.6 Computational detail ............................................................................................. 86

3.4.7 EPR measurements ............................................................................................... 87

3.4.8 X-Ray crystallography .......................................................................................... 88

3.4.8.1 X-Ray data collection and reduction ...................................................... 88

3.4.8.2 X-Ray data solution and refinement ....................................................... 88

3.4.8.3 Selected crystallographic data ................................................................ 90

Chapter 4 Divergent Reactivity of P/Al Frustrated Lewis Pairs with Olefins: Addition vs.

C–H Bond Activation Pathways .............................................................................................. 93

4.1 Introduction ....................................................................................................................... 93


4.2.1 Reversible vs. irreversible activation of ethylene ................................................. 95

4.2.2 Reactions of PR3/AlX3 with propylene: unexpected deprotonation and

dimerization reactions ........................................................................................... 98

4.2.3 C–H bond activation to generate bis-aluminum σ-allyl complexes .................... 100

4.2.4 C–C bond forming reactions ............................................................................... 105

viii

4.2.4.1 Reaction with ethylene ......................................................................... 105

4.2.4.2 Reaction with CO2 ................................................................................ 108

4.3 Conclusions ..................................................................................................................... 110

4.4 Experimental Section ...................................................................................................... 110

4.4.1 General considerations ........................................................................................ 110

4.4.2 Synthesis of compounds ..................................................................................... 111

4.4.3 Computational detail ........................................................................................... 119

4.4.4 X-Ray crystallography ........................................................................................ 119

4.4.4.1 X-Ray data collection and reduction .................................................... 119

4.4.4.2 X-Ray data solution and refinement ..................................................... 119

4.4.4.3 Selected crystallographic data .............................................................. 121

Chapter 5 H2 Activation and Hydride Transfer to Unactivated Olefins: Towards

Transition Metal-Free Catalytic Hydrogenation .................................................................... 124

5.1 Introduction ..................................................................................................................... 124

5.2 Results and Discussion ................................................................................................... 125

5.2.1 H2 Activation ...................................................................................................... 125

5.2.2 Hydride delivery to olefin ................................................................................... 127

5.2.3 Possible mechanism ............................................................................................ 129

5.2.4 Attempts to generate a catalytic cycle ................................................................ 133

5.2.4.1 Using more acidic phosphonium cations .............................................. 134

5.2.4.2 Synthesizing a less Lewis acidic alane ................................................. 136

5.2.4.3 Steric inhibition using a bulky alane .................................................... 137

5.3 Conclusions ..................................................................................................................... 146




ix





Chapter 6 Frustrated Lewis Pair-Mediated C–H Bond Activation Using N2O ................... 160

6.1 Introduction ..................................................................................................................... 160

6.2 Results and Discussion ................................................................................................... 163

6.2.1 Synthesis of a P/Al N2O complex and subsequent phosphine C–H bond

activation ............................................................................................................. 163

6.2.2 Isolation and crystal structure of a phosphoniumyl cation ................................. 167

6.2.3 C–H bond activation of exogenous substrates .................................................... 173

6.2.3.1 C–H bond activation of toluene ............................................................ 173

6.2.3.2 C–H bond activation of bromobenzene ................................................ 175

6.3 Conclusions ..................................................................................................................... 177




6.4.3 EPR measurements ............................................................................................. 182





Chapter 7 Conclusion .......................................................................................................... 187

7.1 Thesis Summary .............................................................................................................. 187

7.2 Future Work .................................................................................................................... 188

References ................................................................................................................................... 190

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List of Figures

Figure 1.1 – Depiction of the proposed “encounter” complex transition state where H2 is split. .. 6

Figure 2.1 – POV-Ray depiction of the molecular structure of 2.1.. ............................................ 17

Figure 2.2 – POV-Ray depiction of the molecular structure of 2.4.. ............................................ 18

Figure 2.3 – POV-Ray depictions of a) 2.7; b) 2.8; and c) 2.10.. ................................................. 21

Figure 2.4 – POV-Ray depiction of 2.11. ..................................................................................... 23

Figure 2.5 – POV-Ray depiction of 2.13.. .................................................................................... 24

Figure 2.6 – Space filled POV-Ray depictions of a) 2.7; b) 2.8; and c) 2.10. .............................. 26

Figure 2.7 – POV-Ray depiction of the anion of 2.15. ................................................................. 27

Figure 2.8 – POV-Ray depiction of 2.17. ..................................................................................... 29

Figure 3.1 – Important industrial processes involving carbon monoxide. .................................... 46

Figure 3.2 – POV-Ray depiction of the molecular structure of 3.1. ............................................. 49

Figure 3.3 – POV-Ray depiction of the molecular structure of 3.2. ............................................. 50

Figure 3.4 – Representative spectra of the analyzed headspace gas (black) from the reaction of

PMes3 + 1.9 AlI3 + CO2 after 16 h compared to an authentic spectrum (red) of CO. .................. 51

Figure 3.5 – 31

P{1H} NMR spectra of a r.t. J-Young solution of PMes3 + 1.9 AlBr3 under N2

(top), after initial CO2 addition (middle), and after 48 h under an atmosphere of CO2 (bottom)

showing the disappearance of 2.5 and the appearance of 3.3 and 3.4 .......................................... 52

Figure 3.6 – POV-Ray depiction of the molecular structure of 3.5 .............................................. 54



xi

Figure 3.9 – Stacked time-dependent 31

P{1H} NMR spectra at 40 °C of the reaction of 2.6 (7.2

mM) with 5 atm CO2 (~ 0.38 M) with peaks at 22 ppm (2.6) and 19.5 ppm (3.1) (top left).

Representative plot of the first order decay of 2.6 at 60 °C (top right) and corresponding Eyring

plot (bottom) analyzed using the equation ln(k/T) = (-≠R)(1/T) + (ln(kB/h) + (S

≠R)) ........ 57

Figure 3.10 – 31

P{1H} NMR spectra of the reaction of [

13C]-2.6 (0.027 M) (with added 3.8

(0.080 M)) in C6D5Br with approximately: a) 0.5; b) 1.0; c) 2.0; d) 3.0 atm 12

CO2 to produce 3.1

after 8 h total reaction time in a J-Young NMR tube. .................................................................. 60

Figure 3.11 – POV-Ray depiction of the molecular structure of 3.9. ........................................... 62

Figure 3.12 – Plot of G vs. T allowing for the determination of H and S using

G = H - TS. G values were obtained using the relation G = -RT•ln(K). .......................... 63

Figure 3.13 – X-band EPR spectrum of a bromobenzene solution of PMes3 + 2 AlBr3 (spectrum

in blue, simulation in red). Analogous signals are observed when using AlI3. ............................ 72

Figure 3.14 – Intensity of 384 nm (blue) and 572 nm (red) absorption bands over time (s) with

stirring upon mixing PMes3 (0.95 mM) with AlI3 (1.9 mM) in bromobenzene. Inset is a sample

spectrum of the reaction mixture. ................................................................................................. 74

Figure 3.15 – Picture taken of the CO trapping experiment ......................................................... 83

Figure 3.16 – Representative 31

P NMR (C7D8) spectrum of a mixture of Cp*RuCl(PCy3) (41.1

ppm) and Cp*RuCl(PCy3)(CO) (52.1 ppm) resulting from the reduction of CO2 to CO by PMes3

and AlI3. ........................................................................................................................................ 83

Figure 3.17 – Linear plot of kobs vs. PCO2 indicating a first order dependence on CO2 pressure

using solutions of 2.6 (0.027 M) with 3.8 (0.080 M) monitored by 31

P{1H} NMR spectroscopy at

r.t. under different pressures. ........................................................................................................ 85

Figure 3.18 – Linear plot of [cyc] vs. [CO2]/kobs indicating an inverse-first order dependence on

cyclohexene based on the derived equation below. ...................................................................... 86


xii

Figure 4.2 – POV-Ray depiction of the molecular structures of 4.4 (left) and 4.5 (right). .......... 98

Figure 4.3 – 1H NMR (600 MHz) spectrum of 4.7 in C6D5Br. ..................................................... 99

Figure 4.4 – 1H NMR (400 MHz) spectrum of a PMes3 + 2 Al(C6F5)3 solution in C6D5Br after

exposure to propylene for 12 h. The assigned resonances are shown in red and the appearance of

the pendant olefin by-product is also highlighted. ...................................................................... 101

Figure 4.5 – POV-ray depiction of the molecular structure of 4.8 ............................................. 102

Figure 4.6 – POV-ray depiction of the molecular structure of 4.9. ............................................ 104

Figure 4.7 – 13

C{1H} (top) and

13C{

1H,

19F} (bottom) NMR spectra of the terminal olefinic

resonance of 4.11 in C6D5Br. ...................................................................................................... 106

Figure 4.8 – Computed geometry of 4.11 showing Al–C distances and Mulliken charges on

olefinic carbon atoms. ................................................................................................................. 107

Figure 4.9 – Partial variable temperature 1H NMR spectra of 4.8 in C6D5Br. ........................... 116

Figure 5.1 – POV-Ray depictions of the molecular structures of a) 5.1 and b) 5.2 .................... 126

Figure 5.2 – Partial 1H NMR spectra of the reactions of 5.3 (top) or [D2]-5.3 (bottom) with

ethylene (1 atm) in C6D5Br after ca. 15 min at r.t. ...................................................................... 128

Figure 5.3 – POV-Ray depiction of the molecular structure of 5.6 ............................................ 131

Figure 5.4 – POV-Ray depiction of the molecular structure of 5.8 ............................................ 132

Figure 5.5 – POV-Ray depiction of the molecular structure of 5.10 .......................................... 133




Figure 5.9 – POV-Ray space filling models of 5.15 ................................................................... 145

xiii

Figure 6.1 – Various binding modes of tBu3P(N2O)(Zn(C6F5)2)n, where: a) n = 1; b) n = 1.5; c) n

= 2.0. ........................................................................................................................................... 162

Figure 6.2 – POV-Ray depiction of 6.1 ...................................................................................... 164

Figure 6.3 – 1H NMR spectrum of 6.2 in C6D5Br. ..................................................................... 165



Figure 6.6 – X-band EPR spectrum of a bromobenzene solution of 6.3 .................................... 170

Figure 6.7 – UV-Vis spectrum of a bromobenzene sample of 6.3 ............................................. 171


Figure 6.9 ‒ R.t. X-band solid state EPR spectrum of 6.3 .......................................................... 183

xiv

List of Schemes

Scheme 1.1 – The aufbau reaction with ethylene ........................................................................... 3

Scheme 1.2 – Hydroalumination of an alkyne followed by cross-coupling reaction with an

unsaturated organic halide to yield newly formed coupled products. ............................................ 4

Scheme 1.3 – Synthesis of phosphonium borohydride species and reversible H2 activation. ........ 5

Scheme 1.4 – Proposed mechanism for the hydrogenation of imines. ........................................... 7

Scheme 1.5 – Reactions of olefins (left) or alkynes (right) with FLPs. .......................................... 8

Scheme 1.6 – Activation of CO2 or SO2 using bimolecular (top) or linked (bottom) P/B FLP

systems. The relative stereochemistry using SO2 has been omitted for simplicity. The reversible

activation using CO2 is also highlighted in the boxes. .................................................................... 9

Scheme 1.7 – FLP-mediated activation of N2O and subsequent N2 release (top) or exchange

reactions with other Lewis acids (bottom). ................................................................................... 10

Scheme 1.8 – Activation of NO using linked P/B FLPs. Subsequent H-atom transfer reaction

with 1,4-cyclohexadiene is shown as one of many118

examples. .................................................. 10

Scheme 1.9 – Example synthesis of a geminal P/Al FLP and its reactivity with alkyne or CO2. 11

Scheme 2.1 – Initial examples of the reversible activation of CO2 using FLPs. .......................... 15

Scheme 2.2 – Reduction of CO2 to MeOH using the TMP/B(C6F5)3 system and H2. .................. 15

Scheme 2.3 – Reduction of CO2 to MeOH after water quench using 2.4-2.6 and excess AB. .... 20

Scheme 2.4 – Synthesis of 2.11 and 2.12 via redistribution of B(C6F5) with 2 equiv. of Me3

AB. 23

Scheme 2.5 – Proposed divergent reaction pathways of: (top) the bulky adduct (2.8) with the

product of CO2 insertion (2.13) leading mostly to deactivation to 2.14 and formic acid after water

quench, and; (bottom) attack of the less bulky adducts 2.7 and 2.10 with the products of initial

xv

CO2 insertion (A and B, respectively) leading to further reduction to MeOH after water quench.

Circles represent relative steric bulk of substituents. .................................................................... 28

Scheme 2.6 – Proposed reaction pathway for the reaction of Me3

AB with excess 2.17. ............... 31

Scheme 3.1 – Reduction of CO2 to CO using a digermyne as reported by Jones et al.189

........... 48

Scheme 3.2 – Reduction of CO2 to CO using PMes3/AlX3 FLPs (X = Br, I)............................... 51

Scheme 3.3 – Proposed rate-determining step involving CO2 insertion in Al–X bonds (X = Br, I)

to generate A following its dissociation from Mes3P(CO2)(AlX3)2. ............................................. 58

Scheme 3.4 – Proposed competitive coordination of cyclohexene vs. CO2 to AlX3, as suggested

by the inverse first-order dependence on cyclohexene. The subsequent proposed rate-determining

insertion step following coordination of CO2 is shown in the box. .............................................. 59

Scheme 3.5 – Proposed equilibrium involving the 1:1 species R3P(CO2)(AlX3) and the FLP

PR3/AlX3. The resulting free PR3 is available to attack the generated intermediate A resulting in

CO expulsion and generation of the compound C. The boxed segment represents proposed

pathways. ...................................................................................................................................... 61

Scheme 3.6 – Proposed mechanism for the reduction of 12

CO2 (blue) to 12

CO using a generic 13

C-

labelled (red) R3P(CO2)(AlX3)2 species to produce [13

C]-3.1 and 3.2 (when R = Mes; X = I).

Cyclohexene inhibition is shown in the top right box. Scrambling of [13

C]-2.6 to [12

C]-2.6 when

12CO2 (blue) is limited, resulting in a hindered rate-determining step (green) to produce A, is

shown in the left box. Note also that under experimental conditions [12

CO2] ~ 50 x [13

CO2]. .... 65

Scheme 3.7 – Proposed thermodynamic vs. kinetic reaction pathways of PMes3/AlX3 FLPs with

CO2. The box describes the proposed intermediate equilibrium mixture en route to the

thermodynamic products. .............................................................................................................. 69

Scheme 3.8 – Possible reversible electron transfer and disproportionation reactions involving

PMes3/AlX3 FLPs. The formation of the CO2 complexes is also shown in the box. .................... 73

xvi

Scheme 3.9 – Insertion of CE2 into the dialane (R = CH(SiMe3)2; E = S) to yield the products D

and E as reported by Uhl.58

Possible reaction pathway for the reduction of CO2 to CO if R = Br

or I and E = O for this chapter. ..................................................................................................... 73

Scheme 3.10 – EPR measurements of 2.6 or 3.9 with and without CO2. ..................................... 87

Scheme 3.11 – EPR measurements of 2.6 with excess (3 equiv.) of 3.8 with and without CO2. . 87

Scheme 3.12 – EPR measurements of 3.9 with excess (3 equiv.) of 3.8 with and without CO2. . 88

Scheme 4.1 – Divergent addition vs. deprotonation reactivity of PR3/E(C6F5)3 FLPs with alkynes

(R = tBu, otol, Ph; E = B, Al). ...................................................................................................... 94

Scheme 4.2 – Activation of olefins using PtBu3/B(C6F5)3 FLPs (top), and proposed mechanism

of addition involving the intermediate formation of a “van der Waals” complex (bottom). ........ 94

Scheme 4.3 – Reversible vs. irreversible ethylene activation using PMes3/AlX3 FLPs. .............. 97

Scheme 4.4 – Synthesis of 4.6 and 4.7 by proposed C–H deprotonation to generate an

intermediate allyl fragment, followed by insertion of a second propylene to generate the pendant

locked olefin product. ................................................................................................................... 99

Scheme 4.5 – Reaction pathways of 4.8 with ethylene affording 4.4 (minor) and 4.10/4.11

(major). ........................................................................................................................................ 108

Scheme 4.6 – Synthesis of 4.12 from 4.8 and CO2. .................................................................... 109

Scheme 4.7 – Proposed divergent reaction pathways of 4.8 (right) vs. 4.9 (left) toward attack at

ethylene to generate either the known B compound,101

tBu3P(CH2CH2)B(C6F5)3, or the new Al

compounds 4.10 and 4.11. Analogous pathways can be applied to the chemistry with CO2. .... 110

Scheme 5.1 – Reactions of 5.1 or 5.2 with olefin yielding the salts 4.10 or 5.5, respectively, and

the alanes 5.4. .............................................................................................................................. 129

Scheme 5.2 – Possible mechanisms for the reaction of 5.1 with olefin. ..................................... 130

xvii

Scheme 5.3 – Proposed competitive redistribution (deactivation) vs. protonation (catalytic)

reaction pathways for olefin hydrogenation. .............................................................................. 134

Scheme 5.4 – Product distribution following the reaction of 5.11 with 5.10. ............................ 135

Scheme 5.5 – Proposed catalytic olefin hydrogenation pathway using extremely bulky X groups

at Al in order to prevent the deactivating redistribution pathway............................................... 138

Scheme 5.6 – Proposed reaction pathway for the generation of B(C12F9)3 or the salt

[Li][FAl(C12F9)3]. ........................................................................................................................ 139

Scheme 6.1 – Reaction of an Fe(II) pyrrole anionic species with N2O leading to N2 evolution and

a HAT reaction (from solvent or H-atom donor) as reported by Chang and co-workers.286,287

. 161

Scheme 6.2 – N2O capture by the PtBu3/ B(C6F5)3 FLP solution and subsequent N2 release. ... 162

Scheme 6.3 – Proposed homolytic mechanism for the reaction of 6.1 with Al(C6F5)3 to generate

6.2. ............................................................................................................................................... 167

Scheme 6.4 – Divergent HAT reaction pathways following the reaction of PMes3 + 2

Al(C6F5)3•tol + N2O in toluene leading to the formation of 6.3 and the by-product 6.4.

Subsequent controlled HAT of 6.3 with DHA is shown for quantification purposes (m = ~75%,

n = ~25%). .................................................................................................................................. 173

Scheme 6.5 – Divergent reaction pathways depending on solvent of P(naph)3 + 2 Al(C6F5)3•tol +

N2O producing 6.5 or 6.7 as major products. ............................................................................. 174

Scheme 7.1 – Proposed generation of frustrated radical pairs and possible new reactivity with

small molecules. .......................................................................................................................... 189

xviii

List of Tables

Table 2.1 – Selected crystallographic data for 2.1, 2.2 and 2.4. ................................................... 42


Table 2.3 – Selected crystallographic data for 2.8, 2.10 and 2.11. ............................................... 44

Table 2.4 – Selected crystallographic data for 2.13, 2.15 and 2.17. ............................................. 45



Table 3.3 – Selected crystallographic data for 3.7 and 3.9. .......................................................... 92

Table 4.1 – Selected crystallographic data for 4.1, 4.2, and 4.3. ................................................ 121


Table 4.3 – Selected crystallographic data for 4.9. ..................................................................... 123


Table 5.2 – Selected crystallographic data for 5.8, 5.10, and 5.13. ............................................ 158

Table 5.3 – Selected crystallographic data for 5.15 and 5.16. .................................................... 159

Table 6.1 – Selected crystallographic data for 6.1 and 6.2. ........................................................ 185

Table 6.2 – Selected crystallographic data for 6.3 and 6.8. ........................................................ 186

xix

List of Symbols and Abbreviations

°C degrees Celcius

µ bridging

Å angstrom, 10-10

m

δ chemical shift

heat

G Gibbs free energy

H enthalpy

H≠

enthalpy of activation

S entropy

S≠ entropy of activation

η eta (bonding mode)

λ wavelength

π pi orbital

σ sigma orbital

υ frequency

υ1/2 frequency difference at half-height

atm atmosphere

Bpin pinacolborane

C6D5Br deuterated bromobenzene

C6F5 pentafluorophenyl

C6H5Br bromobenzene

CD2Cl2 deuterated dichloromethane

CFC chlorofluorocarbon

Cy cyclohexyl

xx

d day

DFT density functional theory

DHA 9,10-dihydroanthracene

DMF dimethylformamide

equiv. equivalent

Et ethyl

FLP frustrated Lewis pair

FRP frustrated radical pair

FT-IR Fourier transform infrared

g gram

GHG greenhouse gas

h hour

h Planck’s constant, 6.626 x 10-34

m2•kg/s

HAT H atom transfer

HMBC heteronuclear multiple bond correlation

HSQC heteronuclear single quantum correlation

Hz Hertz

I nuclear spin

K Kelvin

kJ kilojoule

M molar (mol/L)

M06-2X a functional for DFT

MAO methylaluminoxane

Me methyl

Mes mesityl, 2,4,6-trimethylphenyl

min minute

xxi

mmol millimole

naph 1-naphthyl

nBu n-butyl

NHC N-heterocyclic carbene

nJx-y n-bond scalar coupling constant between X and Y atoms

NMR nuclear magnetic resonance

ODS ozone depleting substance

OG2R3 ONIOM G2R3

Ph phenyl

POV-Ray Persistence of Vision Raytracer

ppb parts per billion, 10-9

ppm parts per million, 10-6

r.t. room temperature

RAB N-alkyl amine-borane

s seconds

SCRF polarized continuum solvent model

T temperature

tBu tert-butyl

TMP 2,2,6,6-tetramethylpiperidine

tol toluene

1

Chapter 1 Introduction

1.1 Chemistry, Development, and the Environment

The past two centuries have seen tremendous advances in science and technology. These in turn

have led to improved life expectancies, better standards of living for many, and an increasingly

interconnected world. Scientific discoveries often pave the way for revolutionary technologies.

For example, the Haber-Bosch process for ammonia production is central to the synthesis of

nitrogen-based fertilizers, which in turn were key to the Green Revolution, saving millions from

famine.1 Intense research in the late twentieth century also led to the development of anti-

retroviral drugs used to treat and reduce the transmission of HIV/AIDS.2,3

Ziegler and Natta’s

chemistry in the mid-twentieth century led to the development and worldwide use of plastics,

ubiquitous in today’s world.4-6

In these and many other cases, chemistry was integral to these discoveries. However, chemistry

can also lead to the development of dangerous substances such as mustard gases used in World

War I, and nerve agents such as Sarin which threaten future chemical warfare. The development

of several refrigerants such as chlorofluorocarbons, while needed, also led to the near destruction

of the ozone layer in the 1970s and 1980s. Furthermore, expanding chemical industries have had

significant environmental impacts, such as causing acid rain.

As a result of these technological and scientific advances, the global population has increased

exponentially, putting increased pressure on the world’s natural resources. Today, over 80% of

energy used comes from fossil fuels, energy which was stored in the ground millions of years

ago.7 The ever-increasing need for energy

8 and resulting burning of fossil fuels have led to rapid

increases in greenhouse gas (GHG) emissions since the industrial revolution,9-11

paving the way

for near irreversible changes to the climate system.12

Such strains on the world’s resources require chemists to develop new, less intensive processes

to fulfill our needs. In this aspect, green chemistry has emerged as a new philosophy to reduce

the impact of chemical processes.13

In addition, significant research is focusing on using more

benign, readily available transition metals, such as Fe,14-17

Co,18-21

and Ni22-26

for effecting

2

chemical transformations normally reserved to their heavier, precious metal analogs.27

Using

abundant elements reduces the environmental impacts involved in extensive mining and ore

processing required in particular for the rare precious metals, such as Ir, Rh, and Pt.28

Main

group elements, particularly the lighter elements, are relatively abundant on the planet and have

also been shown, in some cases, to behave similarly to transition metals.29

This thesis will examine such main group chemistry using abundant aluminum (Al) as part of

frustrated Lewis pair (FLP) systems. FLPs are defined as combinations of Lewis acids and bases

which are sterically prohibited from forming stable adducts. This resulting “frustration” has been

shown to lead to enhanced reactivity with small molecules.30

Notably, the majority of FLPs have

incorporated abundant elements, such as B and P, as their Lewis acid and base centres,

respectively. The research in this thesis extends this to Al and P pairs, as this chemistry remains

far less explored than with B.30-32

The following will outline the basics of Al and pertinent Al

chemistry, as well as a short history of FLP chemistry, and concludes by giving the scope of this

thesis.

1.1.1 Aluminum: Natural abundance, extraction, and prevalence

Aluminum is the most abundant metal in the Earth’s crust and is always combined with other

elements, such as O, F, Si, alkali and alkaline earth metals, or is combined with hydroxides,

sulfates, and phosphates. It is almost always in the 3+ oxidation state and forms remarkably

stable minerals. Pure Al is first obtained from the ore bauxite (30-54% Al oxides with some Si,

Ti, and Fe contaminants) and is processed into pure alumina (Al2O3) in the Bayer process.33

In

this process, the bauxite is treated to a hot NaOH solution, dissolving the Al oxides and forming

the soluble NaAl(OH)4 which is then filtered from the insoluble impurities. Upon cooling,

Al(OH)3 precipitates and is further calcinated at 1000 °C to generate pure alumina.34

The

extremely basic filtered bauxite residue (red mud) presents significant disposal concerns and can

lead to environmental disasters if containment is breached.35

The following step involves the electrolytic conversion of the alumina into pure Al as part of the

Hall-Héroult process. Here, the alumina is dissolved in the cryolite (Na3AlF6) electrolyte

between 940-970 °C and is electrolyzed using carbon anodes resulting in the production of CO

and CO2 (C from anode and O from alumina) and molten Al metal.33

Because of these extreme

conditions, the production of Al is highly energy intensive. Furthermore, significant

3

environmental concerns exist in the production of Al, in particular with regards to air emissions

and solid waste. Significant emissions of fluorine containing compounds, such as HF, CF4, C2F6,

and SiF4 – many of which are potent GHGs – arising from side reaction of the cryolite with

moisture or the anode, are also of significant concern. Solid waste generated from the cell lining

also poses important disposal concerns.33

Despite significant environmental and energy concerns involved in the production of Al, the

sheer abundance of Al in the Earth’s crust makes this the most widely produced non-ferrous

metal (~ 30 Mt/year).33,36

The global per capita stock of Al in use (cars, electronics, other

objects) is 80 kg with developed nations like Canada using closer to 350-500 kg per capita.37

Furthermore, because of its abundance and mass production, Al remains one of the cheapest

metals to produce costing on average 2.50 $/kg over the past decade.33

Also beneficial is the rate

of Al recycling with over half of produced Al being recycled, and requiring as little as 5% of the

energy required to produce Al from bauxite.36

1.1.2 Aluminum chemistry

While not all Al chemistry can feasibly be summarized, the objective here is to provide a brief

overview of some of the most relevant chemistry of the past half century, as well as the

chemistry most pertinent to this thesis. Aluminum compounds are widely used in industrial and

academic settings. Perhaps the most well known uses are as co-catalysts, in particular in the form

of methylaluminoxane (MAO),38

in Ziegler-Natta olefin polymerization.39,40

The role of these

co-catalysts in polymerization and oligomerization catalysis has been investigated by several

groups.41-44

In addition to its role as a co-catalyst, Ziegler observed early on that olefins could

directly insert into Al-C bonds in what has become known as the aufbau (growth) reaction

(Scheme 1.1).45,46

While this reaction is sluggish at best, several other groups have more recently

reported olefin polymerization using Al species.47-49

However, the exact mechanism remains

unclear.50

Scheme 1.1 – The aufbau reaction with ethylene

4

In addition to polymerization catalysis, Al compounds have been widely used by Negishi and

others in hydroalumination reactions with alkynes. The resulting alkenyl-alanes are then used in

Ni or Pd-catalyzed cross-coupling reactions with unsaturated organic halides to yield newly

formed C–C bonds (Scheme 1.2).51,52

Scheme 1.2 – Hydroalumination of an alkyne followed by cross-coupling reaction with an

unsaturated organic halide to yield newly formed coupled products.

In the past few decades, Al compounds have been used as catalysts for other C–C bond forming

reactions. For example, Olah reported the chemoselective carboxylation of aromatics using CO2

and a mixed AlCl3/Al system to yield arylcarboxylic acids.53

Okuda recently reported the

synthesis of allyl-Al species which can undergo C–C bond forming reactions with allyl halides or

pyridines.54,55

Finally, C–H bond activation using Al compounds has recently been reported by

Berben. Here, an Al complex containing redox-active ligands was oxidized with pyridine N-

oxide to yield a proposed reactive Al-oxo intermediate which then cleaves C–H bonds to yield an

Al-OH final product.56

The examples of this paragraph were selected as they either resemble

and/or are relevant to the chemistry presented in this thesis. While not the focus of this thesis, it

should also be noted that several other groups, including those of Uhl,57-64

Schnöckel,65-70

Cowley,71,72

and Roesky73-75

have investigated the chemistry of lower-valent Al(I) and Al(II)

species.

1.2 A Short History of Frustrated Lewis Pair Chemistry

1.2.1 H2 activation and mechanism

In studying the chemistry of phosphonium borate zwitterions as activators and additives to olefin

polymerization chemistry,76,77

linked zwitterions of the type R2P(H)(C6F4)B(F)(C6F5)2 (R = alkyl

or aryl groups) were synthesized in the Stephan group. This was accomplished by the

nucleophilic attack of a sterically bulky secondary phosphine at the para position of the bulky

Lewis acid B(C6F5)3.78

Treatment of these species with Me2SiHCl led to the formation of the

phosphonium borohydride species R2P(H)(C6F4)B(H)(C6F5)2.79

In a remarkable case (R = Mes),

5

this phosphonium borohydride was shown to reversibly activate H2 (Scheme 1.3),80

representing

the first example of reversible H2 activation using main group compounds.

Scheme 1.3 – Synthesis of phosphonium borohydride species and reversible H2 activation.

Subsequent to this discovery, H2 activation was extended to bimolecular systems with bulky

Lewis bases and boranes.81-84

Except with the use of the weaker Lewis acid B(p-C6F4H)3,85

most

of these bimolecular systems did not activate H2 reversibly.

While different mechanisms were initially proposed for the activation of H2 using these newly

discovered FLPs,80,81

subsequent theoretical investigations shed more light on this. Both the

groups of Pápai86,87

and Grimme88

launched theoretical investigations into the heterolytic

splitting of H2. The initial proposed mechanism involving coordination of H2 to either the

phosphine or the borane – both of which were in part based on experimental evidence89-91

–

followed by approach of the respective FLP partner for final H–H cleavage was not found to be

supported computationally. Rather, using the PtBu3/B(C6F5)3 FLP as a test system, it was found

that considerable C–H---F dispersion interactions existed between the phosphine methyl groups

and the borane C6F5 rings (Figure 1.1). The resulting close proximity of the phosphine and

borane centres creates an electric field within its cavity. Once H2 enters, it becomes polarized

and is subsequently heterolytically split.

6

Figure 1.1 – Depiction of the proposed “encounter” complex transition state where H2 is split.

The proposed “encounter complex” mechanism for H2 activation, and perhaps for other small

molecules (vide infra), is generally the accepted mechanism for small molecule activation using

FLPs. However, it should be noted, as will be presented in Chapter 2, that certain classical Lewis

pairs also exhibit FLP behaviour. In a separate study by Rhee,92

it was found that in certain

cases, the solvent can also play an important role in dissociating a Lewis acid from a Lewis base,

effectively creating “solvent induced frustration” and conferring certain classical Lewis pairs

FLP-type reactivity.

1.2.2 Hydrogenation chemistry

Following the discovery of facile H2 activation using FLPs, the Stephan group and other groups

undertook concerted efforts to deliver H2 to unsaturated substrates. It was found that the linked

systems, R2P(H)(C6F4)B(H)(C6F5)2 (R = tBu, Mes), of Scheme 1.3 were capable of catalytically

reducing a variety of bulky imines and N-protected nitriles to the corresponding primary amines

using 5% catalyst, 5 atm of H2, and temperatures varying from 80 – 120 °C.93

In a subsequent

report,94

it was found that the imine itself could act as the Lewis base, thus removing the need for

a phosphine. Therefore, using B(C6F5)3 as the Lewis acid, various bulky imines were reduced

catalytically under H2 to yield the corresponding amines in various yields. A general mechanism

was proposed for these results and is shown in Scheme 1.4.

7

Scheme 1.4 – Proposed mechanism for the hydrogenation of imines.

Various other functional groups have been reduced since these initial reports, including

aziridines,93,94

enamines,95

and silyl enol ethers,96

as well as the partial hydrogenation of

polyaromatic hydrocarbons97

and N-containing heterocycles.98

In remarkable recent examples,

FLPs have also been shown to catalytically hydrogenate activated olefins, such as 1,1-

diphenylethylene,99

as well as stoichiometrically hydrogenate the aromatic rings of anilines to

cyclohexylammonium derivatives.100

1.2.3 Other small molecules

In addition to the breadth of H2 activation and reduction chemistry mediated by FLPs, a variety

of other small molecules can be activated by FLPs and, in some cases, also undergo further

transformation. As many of these transformations will be described separately in the following

chapters of this thesis, only a brief overview will be provided here.

1.2.3.1 Olefins and alkynes

The first reported paper of small molecule activation other than H2 using FLPs was with olefins.

It was found that the reaction of the bulky phosphine, PtBu3, and the bulky borane, B(C6F5)3, in

the presence of simple olefins resulted in the addition across the double bond (Scheme 1.5,

left).101

In contrast, in a subsequent paper with terminal alkynes, and employing a similar

strategy, divergent deprotonation vs. addition chemistry was found to occur depending on the

phosphine used (Scheme 1.5, right).102

8

Scheme 1.5 – Reactions of olefins (left) or alkynes (right) with FLPs.

Much of this chemistry has been further expanded,103-109

including by using Al-based FLPs as

will be described in Chapter 4.

1.2.3.2 CO2 and SO2

The activation of GHG molecules, such as CO2 and N2O (vide infra), as well as acid rain-causing

gases, such as SO2, were investigated using FLPs. In two separate joint reports by the Stephan

and Erker groups,110,111

the activation of both CO2 and SO2 were presented using either the

bimolecular FLP system, PtBu3/B(C6F5)3, or the ethyl linked P/B systems as shown in Scheme

1.6 (top and bottom, respectively). Due to the nature of the S atom, the products of SO2 addition

contained stereogenic S centres (omitted in scheme for simplicity), as opposed to the products

with CO2. Also different is the reversible nature of the CO2 activation compared to that of SO2.

9

Scheme 1.6 – Activation of CO2 or SO2 using bimolecular (top) or linked (bottom) P/B FLP

systems. The relative stereochemistry using SO2 has been omitted for simplicity. The reversible

activation using CO2 is also highlighted in the boxes.

Similar FLP-mediated activation and subsequent reduction reactions with CO2 have also been

reported by O’Hare112

using H2, and by Piers113

using silanes. Reduction of CO2 using Al-based

FLPs is the subject of Chapters 2 and 3.

1.2.3.3 N2O and NO

In related chemistry, the activation of nitrogen oxides, such as N2O and NO, has also been

investigated by the Stephan and Erker groups, respectively. In similar addition-type chemistry to

the CO2 and SO2 reactions, the bimolecular FLP PtBu3/B(C6F5)3 was found to add N2O to yield

the product tBu3P(NNO)B(C6F5)3, the first crystallographically characterized main-group N2O

complex (Scheme 1.7).114

This complex was found to release N2 under forcing conditions to

form the phosphine oxide adduct of B(C6F5)3. In addition, several related R3P(NNO)BR'3 species

were found to undergo exchange reactions with other Lewis acids, including group 4 Lewis acids

(Scheme 1.7).115,116

10

Scheme 1.7 – FLP-mediated activation of N2O and subsequent N2 release (top) or exchange

reactions with other Lewis acids (bottom).

Using the ethyl linked P/B FLPs, the Erker group has recently reported the capture of NO to

yield a new type of persistent N-oxyl radical (Scheme 1.8).117

These, and related species,118

have

been shown to undergo subsequent H-atom transfer reactions with various substrates. The use of

these radical species as radical polymerization initiators is also reported.118

Scheme 1.8 – Activation of NO using linked P/B FLPs. Subsequent H-atom transfer reaction

with 1,4-cyclohexadiene is shown as one of many118

examples.

This section has attempted to provide a partial overview of B-based FLP chemistry for the past 6

years. Many of the reactions presented here serve as good background knowledge to the

chemistry that will be presented throughout this thesis using Al-based FLPs. Before this,

however, an overview of this Al FLP chemistry will be presented.

1.2.4 Al-based FLPs

The majority of FLP chemistry reported to date uses B Lewis acids, most often in the form of

B(C6F5)3 (vide supra). When the candidate joined the Stephan group in 2009, only one paper102

described the use of an Al Lewis acid, Al(C6F5)3, in FLP chemistry (see Scheme 1.5). Since then,

Al-based FLP chemistry has expanded modestly due to contributions from the Stephan group,

the Uhl group, and others.

11

In 2010, Chen reported the rapid r.t. polymerization of methyl methacrylate and naturally

renewable methylene butyrolactones using Al(C6F5)3-based classical and frustrated Lewis pairs

with phosphines or N-heterocyclic carbenes (NHC).119

A subsequent study examined the full

scope of this reactivity.120

The research group of Fontaine later reported the surprising activation

of CO2 using linked classical Lewis pairs of the type R2PCH2AlMe2 (R = Me, Ph) which are

known to form dimers.121

No crystal structures were obtained here, precluding the definitive

assignment of the molecular geometry.

The research group of Uhl, often in collaboration with others, has provided several reports of

small molecule activation using linked P/Al systems.122-125

The general strategy involves the

synthesis of geminal P/Al FLPs by the hydroalumination reaction of readily available bulky

alkynylphosphines. In the initial report, the facile activation of terminal alkynes and CO2 using

these systems was demonstrated (Scheme 1.9).122

Scheme 1.9 – Example synthesis of a geminal P/Al FLP and its reactivity with alkyne or CO2.

In a subsequent example of the cooperative reactivity of the geminal P/Al FLPs, the phase-

transfer catalysis of alkali metal hydrides was reported, a first in FLP chemistry.123

While these

hydrides are typically very insoluble in standard organic solvents, the use of the P/Al system

shown in Scheme 1.9 significantly increased the solubility of these hydrides, further favouring

their reaction with substrates. In the report, a slurry of NaH was mixed with 10% of the P/Al

catalyst and shown to quantitatively convert Ph3SiCl into Ph3SiH and NaCl under reflux

conditions after 48 h. No reaction is reported without the use of this P/Al phase-transfer catalyst

under similar conditions.123

12

Subsequent reports by Uhl et al.124,125

have further expanded this chemistry to include various

other geminal P/Al systems, as well as alkenyl linked N/Al systems. The reactivity of these

systems with various small molecules, such as CO2, alkynes, carbodiimides, and isocyanates is

also reported.

1.3 Scope of Thesis

As will be described throughout this thesis, the objective of this graduate research was to survey

the reactivity of Al-based FLPs. A recurring focus throughout is on using benign and

inexpensive aluminum halides (AlX3) as Lewis acids, although the use of perfluorinated alanes

becomes more common in the later sections of the thesis. Approximately half of this research

focuses on the activation of GHG molecules, CO2 and N2O. The further reduction of CO2 to

either methanol (Chapter 2) or CO (Chapter 3) is described. Aromatic and benzylic C–H bond

activation using N2O is also presented (Chapter 6). Chapter 4 expands on the previously reported

olefin chemistry by introducing the unexpected C–H bond activation of substituted olefins to

produce novel Al (and B) allyl species. The former species are shown to undergo subsequent C–

C bond forming reactions. Finally, Chapter 5 builds on the known H2 chemistry with FLPs by

describing H2 activation and hydride delivery to unactivated olefins using Al, a feat that had not

yet been accomplished using FLPs.

With the exception of EPR spectroscopy, elemental analyses, and X-ray experiments, all

synthetic work and characterizations were performed by the author. DFT calculations for

Chapter 3 were performed by Prof. Thomas M. Gilbert at Northern Illinois University.

Compound 3.6 was initially synthesized by an exchange student, Anne Kraft, as part of a

collaboration with Prof. Ingo Krossing. Part of the syntheses for compounds 4.1-4.3 were done

by an undergraduate student under the author’s supervision, Lina Tran. Dr. Alan J. Lough solved

the structures of compounds 6.2, 6.3, and 6.8. All other structures were solved by Prof. Stephan;

however, at the time of submission of this thesis, several structures required further refinement

and will be completed prior to publication.

Portions of each chapter are either published or have been drafted at the time of writing:

Chapter 2: 1) Ménard, G.; Stephan, D. W. “Room Temperature Reduction of CO2 to Methanol

by Al-Based Frustrated Lewis Pairs and Ammonia Borane.” J. Am. Chem. Soc. 2010, 132, 1796-

13

1797. 2) Ménard, G.; Stephan, D. W. “CO2 Reduction via Aluminum Complexes of Ammonia

Boranes.” Dalton Trans. 2013, DOI: 10.1039/c3dt00098b.

Chapter 3: 3) Ménard, G.; Stephan, D. W. “Stoichiometric Reduction of CO2 to CO by

Aluminum-Based Frustrated Lewis Pairs.” Angew. Chem. Int. Ed. 2011, 50, 8396-8399. 4)

Ménard, G.; Gilbert, T. M.; Hatnean, J. A.; Kraft, A.; Krossing, I.; Stephan, D. W. “Reduction of

CO2 to CO: An Experimental and Theoretical Mechanistic Investigation.” Drafted.

Chapter 4: 5) Ménard, G.; Stephan, D. W. “C–H Activation of Isobutylene Using Frustrated

Lewis Pairs: Aluminum and Boron σ-Allyl Complexes.” Angew. Chem. Int. Ed. 2012, 51, 4409-

4412. 6) Ménard, G.; Tran, L.; Stephan, D. W. “Reversible Activation vs. Dimerization of

Olefins Using AlX3-Based Frustrated Lewis Pairs.” Drafted.

Chapter 5: 7) Ménard, G.; Stephan, D. W. “H2 Activation and Hydride Transfer to Olefins by

Al(C6F5)3-Based Frustrated Lewis Pairs.” Angew. Chem. Int. Ed. 2012, 51, 8272-8275.

Chapter 6: 8) Ménard, G.; Hatnean, J. A.; Lough, A. J.; Stephan, D. W. “Frustrated Lewis Pair-

Mediated C–H Bond Activation Using N2O.” Drafted.

14

Chapter 2 Activation and Reduction of CO2 to Methanol using Al-Based Frustrated Lewis Pairs and Ammonia-Borane

2.1 Introduction

Carbon dioxide is ubiquitous in our environment and maintained in gentle balance through the

combined actions of photosynthesis, animal respiration, decomposing organic material, etc. as

part of the carbon cycle. Humans have begun to alter this balance since the dawn of the industrial

revolution through the rapid increase of CO2 emissions to the atmosphere, resulting in global

climate change.11

With CO2 levels reaching a critical point towards irreversible changes to the

climate,12

solutions are needed to mitigate further emissions.126

While mitigation through

reduced consumption, the use of renewable,7 and a shift away from C-based fuels

8,127,128 are

likely the best remedies, strategies to reduce the impacts of the current C-based system are

needed. One such example would be through Olah’s “Methanol Economy” wherein CO2 is

captured and used as a C1 feedstock to produce a fuel.129,130

However, the further development of

CO2 activation and reduction strategies are needed to develop such an economy.

Several transition metal and main-group based reductions of CO2 have recently been reported. A

variety of metals, such as Zr,131

Nb,132

Re,133

Fe,134

Ru,135

Ir,136

Ni,26,137

and Cu,138,139

in

conjunction with H2 or an external reductant, have been used to reduce CO2 to CO, formic acid,

MeOH, and even methane, most often following acid/water quenching. Non-metal mediated

routes have also emerged, such as through the use of carbenes to activate CO2 for further

reduction.140,141

A recent report by Bertrand also demonstrates the activity of an apparently

benign dithiocarbamate in the activation of CO2; however, reduction of this fragment was not

reported.142

In conjunction with the Erker group, the Stephan group published the first example of CO2

activation using frustrated Lewis pairs.110

These phosphine/borane systems were found to

reversibly activate CO2 at different temperatures depending on the system used (Scheme 2.1).

While these systems could offer a new avenue for carbon capture technologies, they were found

to be inadequate to undergo further reduction at the CO2 moiety. For example, treatment of these

systems with H2 resulted in the release of CO2 and the generation of the known products

[tBu3PH][HB(C6F5)3]81

or Mes2P(H)CH2CH2B(H)(C6F5)2.84

15

Scheme 2.1 – Initial examples of the reversible activation of CO2 using FLPs.

Shortly after this report, Ashley and O’Hare reported the direct insertion of CO2 at 100 °C into

the borohydride fragment [HB(C6F5)3]- generated by the heterolytic cleavage of H2 using 2,2,6,6-

tetramethylpiperidine (TMP) and B(C6F5)3.112

This formate species was found to be reversibly

generated as it slowly releases CO2 and regenerates the salt [TMPH][HB(C6F5)3] (Scheme 2.2)

below 100 °C. Nonetheless, under forcing conditions, heating 1 equiv. of CO2 with 4 equiv. of

TMP/B(C6F5)3 under an atmosphere of H2 to 160 °C for 6 days in a sealed system resulted in the

isolation of 17-25% MeOH after solvent distillation, as well as several decomposition products

(Scheme 2.2). Although the yield is low and the conditions forceful, this was the first example of

the reduction of CO2 to a fuel using FLPs.

Scheme 2.2 – Reduction of CO2 to MeOH using the TMP/B(C6F5)3 system and H2.

Following these reports, we were interested in exploring inexpensive and commercially available

Lewis acids for CO2 activation. We also wanted to explore milder routes to the reduction of CO2.

This chapter will describe our results in the irreversible capture of CO2 using aluminum halides

(AlX3; X = Cl, Br, I), as well as its subsequent rapid, stoichiometric reduction to MeOH at r.t.

16

following water quench and using ammonia-borane as reductant. A possible reaction mechanism

involving the ammonia-borane adduct of AlX3 is presented.

2.2 Results and Discussion

2.2.1 Activation of CO2 using AlX3 and a phosphine

Our initial explorations using commercially available Lewis acids focused on boron halide

species, BX3 (X = F (SMe2 adduct), Cl, Br). Treatment of these Lewis acids with PtBu3 (Tolman

cone angle of 182°)143

resulted in the formation of strong adducts as evidenced by the P–B 1J

couplings in both the 31

P and 11

B NMR spectra. No reaction with CO2 was observed with these

strong adducts. Using the sterically encumbered phosphines P(otol)3 and PMes3 (Tolman cone

angles of 194° and 212°, respectively)143

again resulted in no activation of CO2. However, this

was surprising as these systems displayed possible FLP behaviour (except with P(otol)3/BCl3), as

no Lewis acid/base interactions were observed by NMR spectroscopy.

Next we focused our attention on AlX3 (X = Cl, Br, I) combinations with bulky phosphines. As

the Al centres are more accessible than B due to longer Al–X bonds, we used the bulkiest

phosphine, PMes3, in order to avoid or weaken adduct formation. In contrast to the BX3 systems,

1:1 solutions of PMes3/AlX3 in bromobenzene react to form weak Lewis adducts. This is

evidenced by the very broad multiplet (Al: I = 5/2) in the 31

P NMR spectrum in the -10 to -30

ppm region, which is significantly downfield of the resonance derived from free PMes3 (-35

ppm). The broadness in these resonances is attributable to an equilibrium involving free

phosphine and AlX3, likely aided by solvent coordination to Al (vide infra).92

Indeed, this

equilibrium is supported by the presence of a small, broad signal centred at -35 ppm in the 31

P

NMR spectrum indicative of free PMes3. These adducts give rise to broad doublets in the 27

Al

NMR spectra (X (δ, 1JAl-P): Cl (110 ppm, 258 Hz); Br (100 ppm, 222 Hz); I (38 ppm, 217 Hz).

The large chemical shift difference in the 27

Al spectra is consistent with previous reports using

different halides of Al.144

These classical Lewis adducts could be isolated in 81% (X = Cl, 2.1),

84% (X = Br, 2.2), and 59% (X = I, 2.3) yields. They were fully characterized and in all cases

the 1H NMR spectra displayed resonances indicating that the ortho-methyl groups and meta-H

atoms of the mesityl groups are inequivalent, consistent with a barrier to rotation of the mesityl

rings. This notion is further supported by the solid state structures of 2.1 and 2.2 in which the

mesityl rings are shown to adopt propeller-like orientations. Both structures are analogous and

17

that of 2.1 is shown in Figure 2.1. The P–Al bond distances in 2.1 and 2.2 were found to be

2.5239(11) and 2.5360(10) Å, respectively. This is comparable to the literature values of

2.58(2) Å for Me3P-AlCl3 adduct predicted on the basis of a solid state NMR study.145

Figure 2.1 – POV-Ray depiction of the molecular structure of 2.1. H atoms are omitted for

clarity. C: black; P: orange; Al; teal; Cl: green.

To probe whether the weak adducts 2.1-2.3 could exhibit FLP-type reactivity, these solutions

were exposed to 1 atm of CO2 in deuterated bromobenzene (C6D5Br) in sealed J-Young NMR

tubes. In all cases, monitoring of the reactions by 31

P NMR spectroscopy showed the liberation

of PMes3 and the formation of new products in an approximate 1:1 ratio. The new species gave

rise to 31

P NMR signals at 20, 22, and 22 ppm for the reactions of 2.1, 2.2, and 2.3, respectively.

1H NMR spectra also showed broad peaks in the methyl and aromatic regions suggesting

possible exchange of free PMes3 and the newly formed compounds 2.4, 2.5, and 2.6,

respectively. The generation of free phosphine further suggests that the stoichiometry of reaction

of CO2 with the adducts 2.1-2.3 is not 1:1. Thus, 1:2 mixtures of PMes3/AlX3 were prepared and

reacted with CO2. These reactions yielded the immediate formation of the previously observed

species 2.4-2.6 with no evidence of free PMes3. These compounds were subsequently isolated by

precipitation in 82% (2.4), 83% (2.5), and 87% (2.6) yield. While the spectroscopic data were

consistent with the inclusion of the constituents, the structures of these products were ultimately

identified by X-ray crystallographic studies. All structures are analogous and, thus, only that of

2.4 is shown in Figure 2.2.

18


clarity. C: black; P: orange; Al; teal; Cl: green; O: red.

These products, formulated as Mes3P(CO2)(AlX3)2, were comprised of phosphine bound to the

C-atom of CO2 while AlX3 units are bonded to each of the O-atoms. The resulting P–C bond

lengths in 2.4, 2.5, and 2.6 were found to be 1.927(8), 1.918(5), and 1.903(8) Å, respectively,

while the O–Al distances were 1.807(5) and 1.808(6) Å in 2.4, 1.829(4) and 1.803(3) Å in 2.5,

and 1.809(6) and 1.841(6) Å in 2.6. The C–O bond lengths were determined to be 1.233(8) and

1.251(8) Å in 2.4, equivalent at 1.248(6) Å in 2.5, and 1.226(10) and 1.268(10) Å in 2.6. The C–

O bond lengths are significantly longer than terminal C=O bonds in tBu3PCO2B(C6F5)3

(1.2081(15) Å) and Mes2PCH2CH2B(C6F5)2(CO2) (1.209(4) Å), but shorter than C–O–B linkages

at 1.2988(15) and 1.284(4) Å, respectively.110

The similarity of the two C–O bond distances in

2.4-2.6 is consistent with delocalization of the formal negative charge over the C(OAlX3)2

fragments and approximate bond orders of 1.5 for each of the C–O bonds. The O–C–O angles

are 126.6(7)°125.8(4)°, and 125.0(8)° in 2.4, 2.5, and 2.6, respectively, while the C–O–Al

angles differed substantially from each other being 141.3(5)°and 165.2(6)° in 2.4, 140.0(3)°and

178.7(4)°in 2.5, and 146.1(6)° and 175.7(6)° in 2.6. The binding of both O-atoms to Al in 2.4-

2.6 clearly has an impact on the C–O bond strength. IR spectra do not reveal a clear or typical

C=O stretch and the vibration could not be assigned. This is consistent with the remarkable

stability of these species to CO2 loss even on heating to 80 °C under a vacuum. This stands in

contrast to the phosphine/borane adducts tBu3PCO2B(C6F5)3 and Mes2PCH2CH2B(C6F5)2(CO2)

19

which rapidly lose CO2 at 80 and -20 °C, respectively.110

The formation of two Al–O in these

systems, as opposed to one B–O bond in the latter cases, could explain this enhanced stability.

These systems are rare examples of double activation of CO2, although similar systems

exist146,147

or have been proposed.53

The activation of CO2 from the isolated adducts 2.1-2.3 seems to contradict a fundamental

premise of FLP chemistry, wherein steric bulk precludes adduct formation and leads to the

observed reactivity with small molecules.30

Indeed, with the exception of a few examples,119,148

classical adducts such as 2.1-2.3 are not typically reactive with small molecules, thus

distinguishing them from FLPs. In order to probe this apparent contradiction, Rhee and co-

workers92

studied the 2.1/2.4 system computationally. It was found that, unlike traditional FLPs

where steric bulk hinders adduct formation and results in small molecule activation, the solvent

(bromobenzene) plays a key role in dissociating AlCl3 from PMes3 in 2.1. This allows for

subsequent capture of CO2 by AlCl3 and attack by PMes3 forming the 1:1 species

Mes3P(CO2)AlCl3 prior to coordination of a second AlCl3 to form 2.4. Thus, the authors describe

this as “solvent-assisted frustration” giving this adduct FLP-like reactivity.92

2.2.2 Reduction of CO2 to methanol using ammonia-borane

With compounds 2.4-2.6 in hand, we wanted to investigate the reduction of the CO2 moiety to a

more valuable substance, such as formic acid, formaldehyde, or methanol. The reductions were

initially attempted under an atmosphere of H2, but the outcome remained unclear after prolonged

heating and was complicated by side reactions (see Chapter 3). Other reducing reagents such as

silanes or alanes were also used, but again the results remained unclear.

Ammonia-borane (NH3BH3 = AB) is known as an excellent H2 source149

and an excess amount

(3 equiv.) was added to compounds 2.4-2.6. The reactions were monitored by NMR

spectroscopy using the 13

CO2 isotopologues, [13

C]-2.4-2.6. The 13

C resonances for these species

are observed at 172, 173, and 168 ppm, each with a 1JC-P of 123, 119, and 119 Hz, respectively,

for 2.4, 2.5, and 2.6. In less than 15 min after reaction with AB, the 11

B NMR spectral data

indicate dehydrogenation of AB to borazine and other dehydrogenated products.150

The 13

C

NMR spectra showed the loss of the CO2 signal derived from [13

C]-2.4-2.6 and the appearance of

approximately 3-4 quartet resonances between 50-65 ppm with a 1JC-H of 146-149 Hz which

correlate to doublets (due to H–13

C coupling) in the 3-4.5 ppm region of the 1H NMR, as

20

ascertained by 2D 1H–

13C HSQC NMR experiments. These data infer the formation of B or Al

methoxy species. Interestingly, no evidence exists for the formation of other reduction products,

such as formate or formaldehyde derivatives. Following quenching of the reaction with D2O, the

organic phase was shown to contain free PMes3 by 31

P NMR spectroscopy while the aqueous

phase exhibited a 13

C resonance as a quartet at 49 ppm (1JC-H = 142 Hz), identical to an authentic

sample of CH3OH in D2O (Scheme 2.3). The yield of CH3OH was quantified by integration of

the 1H NMR with an internal standard of 1,4-dioxane, and showed average extracted yields of

37-51%.

Scheme 2.3 – Reduction of CO2 to MeOH after water quench using 2.4-2.6 and excess AB.

While the second O-atom is thought to be incorporated into Al or B by-products, its precise fate

remains unknown. The following sections will describe a probable partial reaction pathway for

this complex transformation.

2.2.3 Reactions of AB with EX3 (E = B, Al; X = halide or C6F5): redistribution vs. adduct formation

In order to examine the reduction chemistry of the species Mes3P(CO2)(AlX3)2 with excess AB,

a series of control experiments were performed and it was found that AB, Me2NHBH3 (Me2

AB),

or Me3NBH3 (Me3

AB) do not react on their own with CO2. Addition of PMes3 to these samples

also does not result in reaction with CO2. However, to our surprise, solutions of AB, Me2

AB, or

Me3AB with AlX3 do result in reaction with CO2 to produce reduction product signals, such as

methoxy and/or formate, as observed in the 13

C and 1H NMR spectra.

In order to probe the interaction of ammonia-boranes with AlX3, the bromobenzene soluble

AlBr3 was combined with an equimolar amount of the soluble Me3

AB in C6D5Br in a J-Young

NMR tube. While either coordination of the NMe3 group to AlBr3 and/or scrambling of the BH3

with AlBr3 were expected, to our surprise, the 27

Al NMR spectrum showed the appearance of a

single broad peak centred at 79 ppm. The 11

B and 1H NMR spectra showed only slightly shifted

21

and broadened signals in comparison to an authentic sample of Me3

AB. This data suggests

possible adduct formation between AlBr3 and Me3

AB. This synthesis was scaled up in

bromobenzene and the compound isolated in pure form by precipitation with hexanes in 70%

yield. Elemental analysis of the product indicated a composition of AlBr3•H3BNMe3 (2.7) and

single crystals confirmed the geometry as a H-bridged 3-centre-2-electron adduct between AlBr3

and Me3

AB (Figure 2.3a). The synthesis of the AB adduct of AlBr3 was also attempted; however,

a highly insoluble product was obtained and its composition was difficult to confirm.

Decomposition or the formation of a H-bonded species are suspected.

The analogous synthesis was also attempted with the bulkier alane, Al(C6F5)3, and Me3

AB. After

mixing the reagents for ca. 15 min in toluene, pentane was added to precipitate a pure product in

77% yield. While the 27

Al NMR spectrum contained no discernible peaks, the 11

B and 1H NMR

signals for the Me3

AB fragment were again slightly shifted and broadened. Furthermore, the 19

F

signals for Al(C6F5)3 were also slightly shifted. The analogous Al(C6F5)3•H3BNMe3 (2.8)

compound formed was isolated in good yield and its solid state structure is shown in Figure 2.3b.

The Me2

AB (2.9) and AB (2.10) adducts of Al(C6F5)3 were also successfully synthesized in an

analogous fashion in 87% and 55% yield, respectively, and the solid state structure of 2.10 is

shown in Figure 2.3c.

Figure 2.3 – POV-Ray depictions of a) 2.7; b) 2.8; and c) 2.10. C: black, N: blue, B: yellow-

green, Al: teal, Br: scarlet, F: pink, H: white. Methyl H’s are omitted for clarity.

Of interest in the structures of Figure 2.3 are the N–B and B–Al distances. In the case of 2.7 and

2.8, the N–B bond distances of 1.574(5) Å and 1.589(2) Å, respectively, are shorter than those

reported for free Me3

AB (1.617(4) Å in the solid phase151

and 1.66 Å in the gas phase152

), perhaps

22

due to an increased positive dipole at B due to polarization of the B–H bond by Al. This effect

seems less pronounced with 2.10 as the N–B distance (1.582(4) Å) is similar to the one reported

for free AB (1.58(2) Å in the solid phase153

and 1.66 Å in the gas phase154

). The B–Al distances

are also noteworthy as they differ significantly for 2.7 (2.312(5) Å) and 2.8 (2.4978(19) Å). This

large difference is attributed to the increased steric congestion between the C6F5 and Me groups

of 2.8 vs. the smaller Br and Me groups of 2.7. Consistent with this argument, the B–Al distance

in 2.10 is found to be much shorter than in 2.8 and similar to 2.7 at 2.357(3) Å due to its smaller

NH3 group.

Interestingly, while Al compounds have been used in the dehydrogenation chemistry of amine-

boranes,155,156

the present compounds represent rare examples of intermolecular B–H–Al bonds.

This B–H–M bonding motif is reminiscent of several known early157,158

and late metal

complexes,22,159-163

as well as s-block main group164-166

compounds bound to various substituted

and unsubstituted amine-boranes. However, amine-borane complexes of d0 elements typically

contain intramolecular B–H–M (M = Mg, Ca, Sc)157,164-166

bonds derived from chelating anions

{H3BNMe2BH2Me2N}- or {H3BR2N

-} with both terminal B–H and amide (R2N

-) groups bound

to the same metal centre. In contrast, compounds 2.7-2.10 represent the first intermolecular

group 13-AB adducts known, where no chelate is present.

Compounds 2.7-2.10 are stable in solution for at least 24 h, in particular for 2.8; however, all

compounds readily decompose when heated to 70 °C for several hours. The stability of these

compounds stands in marked contrast to the analogous reaction using B(C6F5)3 as the Lewis acid,

where the dehydrogenation of AB with B(C6F5)3 has been reported.167

Indeed, we observed the

same phenomenon except when using Me3

AB. Mixing a 1:1 solution of B(C6F5)3 and Me3

AB

resulted in broad signals in the 11

B, 19

F, and 1H NMR spectra. Some of the initial identifiable

species included the [HB(C6F5)3]- anion

81 and the [(µ-H)(H2BNMe3)2]

+ cation

168; however, after

standing overnight in a J-Young NMR tube, several products were observed. The reaction was

much cleaner when using a 1:2 ratio of B(C6F5)3:Me3

AB and the progress could be monitored by

NMR spectroscopy. After mixing in a J-Young NMR tube for 20 min, the intermediate salt

[(µ-H)(H2BNMe3)2][HB(C6F5)3] was cleanly generated; however, attempts to isolate this salt

were unsuccessful. Instead, the reaction proceeds to form 2 new compounds 2.11 and 2.12.

Compound 2.11 was isolated by crystallization of a toluene/hexanes solution at -38 °C. The 11

B

NMR spectrum of the product contained a broadened triplet (1JB-H = 95 Hz) at -10 ppm with

23

corresponding peaks in the 19

F NMR spectrum at -129, -157, and -164 ppm, indicative of a four-

coordinate boron centre. The 1H NMR spectrum contained 2 peaks which integrated in a 9:2 ratio

representing the three methyl and two B–H peaks, respectively. Furthermore, the B–H peaks

formed a triplet (4JH-F = 6.8 Hz) in the

1H{

11B} spectrum indicative of H–F coupling to the

ortho-fluorines of the C6F5 ring. Single crystal X-ray crystallography confirmed the structure as

(C6F5)BH2•NMe3 and is shown in Figure 2.4.

Figure 2.4 – POV-Ray depiction of 2.11. C: black, N: blue, B: yellow-green, F: pink, H: white.

Methyl H’s are omitted for clarity.

The balanced equation for this redistribution reaction is shown in Scheme 2.4 and stands in

contrast to the adduct chemistry with Al(C6F5)3. This is likely driven by the higher of B–H vs.

Al–H bond strengths.169

A similar reaction using a combination of B(C6F5)3 and BH3•SMe2 has

been reported, generating the SMe2 adduct analog of 2.11.170

Interestingly, while the Al adduct

of Me3

AB (2.8) is stable in solution, addition of Me3

AB to 2.8 leads to decomposition with

concurrent formation of some 2.11 as observed by NMR spectroscopy. The second product in

Scheme 2.4 (2.12) is presumed to be the adduct-free (C6F5)BH2 based on NMR spectroscopy;

however, attempts to isolate this species were unsuccessful.

Scheme 2.4 – Synthesis of 2.11 and 2.12 via redistribution of B(C6F5) with 2 equiv. of Me3

AB.

2.2.4 Reactions of Al–AB species with CO2

In order to establish whether the new Al–AB species 2.7-2.10 play a role in the mechanism of

CO2 reduction from 2.4-2.6, these species were treated to 1 atm of CO2. Compounds 2.7-2.10

24

were all found to be reactive to CO2. The reaction of 2.7 with 1 atm 13

CO2 leads to the rapid

formation (< 20 min) of several formate peaks in the 170-175 ppm region of the 13

C NMR

spectrum, as well as several peaks in the methoxy region at 50-70 ppm, all of which correlate to

doublets (due to H–13

C coupling) in the 1H NMR spectrum. Several unidentified peaks were also

observed in the 11

B and 27

Al NMR spectra. No distinct, single species could be isolated.

However, the analogous reaction of 2.8 with 13

CO2 led to the clean formation of a single formate

species (2.13) with a peak at 173 ppm in the 13

C NMR spectrum correlating to a peak at 8.14

ppm in the 1H NMR spectrum. While the

27Al NMR spectrum again showed no peaks, the

11B

NMR resonance shifted from -9.1 ppm (2.8) to 3.8 ppm. The 19

F NMR signals also shifted

slightly. After mixing under CO2 for ca. 15 min, the product was isolated by precipitation with

pentane in 72% yield. X-Ray diffraction provided the solid state structure of 2.13 and is shown in

Figure 2.5.

Figure 2.5 – POV-Ray depiction of 2.13. C: black, N: blue, B: yellow-green, O: red, Al: teal, F:

pink, H: white. Methyl H’s are omitted for clarity.

The bonding in 2.13 is quite unique and is the first example, to our knowledge, of a formate

moiety bound between two different group 13 elements, B and Al. Similar formate species bound

between two Lewis acidic B centres have been reported by Piers113

and by our group.171

The C–

O bond lengths in 2.13 are virtually identical at 1.251(2) and 1.255(2) Å and the Al–O

(1.8236(14) Å) is similar to those reported earlier in this chapter. The B–O length (1.533(3) Å) is

slightly shorter than in similar formate species bound between two B centres, whereas the O–C–

O angle of 121.42 (19)° is similar.113,171

25

The in situ reaction of 2.8 with 13

CO2 appears to only generate 2.13 as observed by NMR

spectroscopy over time; however, further reduction occurs with the analogous reactions using 2.9

and 2.10. Whereas the reaction of CO2 with 2.9 displays similar patterns in the NMR spectra,

some methoxy peaks are present, suggesting that 2.9 is more susceptible to further reducing CO2

than 2.8 and similar to 2.7. Attempted isolation of a formate species analogous to 2.13 using 2.9

as the starting material was unsuccessful. Compound 2.10 was found to be even more reactive

with CO2, leading to very broad signals in the 13

C NMR spectrum at 172 and 52 ppm (typical

regions for formate and methoxy peaks, respectively) after only 15 min of mixing. At least 2

broad signals were also present in the 11

B NMR spectrum at 34 and -15 ppm which could

represent dehydrogenated species (HNBH)n and (H2NBH2)n,150

respectively; however, the exact

identity of these species remains unknown. Quenching this sample with D2O and subsequent 1H

NMR analysis of this water phase reveals a major doublet (due to H–13

C coupling) at 3.34 ppm

and a minor doublet at 8.43 ppm consistent with methanol and formic acid, respectively.

The increase in reactivity using different amine-boranes or aluminum centres is likely related to

sterics around the N and/or Al centres (see space filling models, Figure 2.6). For example,

compound 2.8 with bulky C6F5 rings combined with 3 methyl groups at N is the most bulky with

the most sterically protected B–H–Al bond, and thus least reactive of the Al–AB species yielding

only reduction to formate upon exposure to CO2. Reducing the bulk at Al to Br groups (2.7) –

making the B–H–Al bond more accessible – leads to rapid reduction to formate (major) and

methoxy (minor) peaks after 20 min under CO2 as observed by 1H and

13C NMR spectroscopy.

After mixing overnight, the methoxy peaks become major and the formate peaks minor.

Similarly, reducing the bulk at N from 2.8 to 2.9 and 2.10 results in less sterically shielded B–H–

Al bonds and progressively more reactive species generating more reduced products upon

exposure to CO2 as described above.

26

Figure 2.6 – Space filled POV-Ray depictions of a) 2.7; b) 2.8; and c) 2.10. C: black, N: blue, B:

yellow-green, Al: teal, Br: scarlet, F: pink, H: white.

In order to determine if further reduction could occur at the formate moiety of 2.13, this

compound was treated with 1 equiv. of 2.8 and monitored over time by NMR spectroscopy.

While it was found that some further reduction to methoxy occurs over time (minor), an initial

competing reaction involves the formation of a major intermediate species

[(µ-H)(H2BNMe3)2][(µ-η1:η

1-HCO2)(Al(C6F5)3)2] (2.14) as observed by NMR spectroscopy.

Attempts to isolate this species were unsuccessful; however, the cation [(µ-H)(H2BNMe3)2]+ is

known168

and the anion was independently synthesized by reacting the salt

[tBu3PH][(µ-H)(Al(C6F5)3)2] (5.1, Chapter 5) with CO2. The solid state structure of

[(tBu)3PH][(µ-η1:η

1-HCO2)(Al(C6F5)3)2] (2.15) is shown in Figure 2.7. With these spectroscopic

signatures, the identity of the intermediate (2.14) was confirmed.

27

Figure 2.7 – POV-Ray depiction of the anion of 2.15. C: black, O: red, Al: teal, F: pink, H:

white. The cation, [tBu3PH]+, is omitted for clarity.

The anion of 2.15 was found to be unreactive to further reduction upon exposure to the bulky

compound 2.8; however further reduction to methanol is observed using the less bulky

compounds, 2.7 and 2.10, following water quench. This further supports the notion that sterics

play a significant role in the reduction chemistry as the bulky groups around the electrophilic

carbonyl centre (–BH2NMe3 and Al(C6F5)3 (2.13) or Al(C6F5)3 (2.15)) preclude further attack by

the bulky reducing agent 2.8 (Scheme 2.5, top), but does remain susceptible to attack using the

less bulky compounds, 2.7 and 2.10. Scheme 2.5 details the proposed divergent pathways for the

reduction of CO2 to formic acid (top) or to formic acid and methanol (bottom). In each of the

cases, it is assumed that compounds 2.7 and 2.10, like 2.8, undergo initial CO2 insertion to form

formate intermediates (A and B, respectively) analogous to 2.13. Due to the increased bulk of

2.8/2.13, a major deactivation pathway involves the formation of 2.14. However, with 2.7 and

2.10, further reduction at the formate moiety (A or B) is possible and generates some methanol

after water quench. The intermediate formation of a deactivated analog of 2.14 does not preclude

further reduction in these cases. Finally, as stated above, 2.9 was found to be slightly more

reactive than 2.8 producing some methanol after water quench (minor compared to the reactions

using 2.7 and 2.10, but more than the reaction with 2.8). It should be noted that no formaldehyde

intermediate was observed in this chemistry.

28

Scheme 2.5 – Proposed divergent reaction pathways of: (top) the bulky adduct (2.8) with the

product of CO2 insertion (2.13) leading mostly to deactivation to 2.14 and formic acid after water

quench, and; (bottom) attack of the less bulky adducts 2.7 and 2.10 with the products of initial

CO2 insertion (A and B, respectively) leading to further reduction to MeOH after water quench.

Circles represent relative steric bulk of substituents.

2.2.5 Possible reaction pathway for the reduction of CO2 from Mes3P(CO2)(AlX3)2

As described in Section 2.2.2 above, treatment of the complexes Mes3P(CO2)(AlX3)2 (2.4-2.6)

with excess AB readily leads to the reduction of the CO2 moiety generating MeOH within

minutes following water quench (Scheme 2.3). A mechanistic investigation of this complex

reaction was complicated by the extremely poor solubility of AB in bromobenzene, the very fast

reaction rate, the lack of useful Al or halide NMR handles, and the multiple by-products formed.

In order to gain more information into this transformation, the analogous reactions were

undertaken using the more soluble Me2

AB and Me3

AB. We used 2.4 (X = Cl) as this is the most

soluble of the Mes3P(CO2)(AlX3)2 compounds. Treatment of 2.4 with 0.33, 0.66, or 1 equiv.

Me2AB or

Me3AB results in decreasing formation of a formate species and increasing formation of

29

methoxy species. Using Me3

AB instead of Me2

AB or AB also eliminates the formation of the

[Mes3PH]+ cation and replaces it with the free phosphine PMes3 in the

31P NMR spectrum after

the reaction is complete. However, even with these more soluble amine-boranes, it was difficult

to propose a mechanism for this reaction due to a lack of useful NMR handles from AlX3. We

therefore synthesized the Al(C6F5)3 analog of 2.4, Mes3P(CO2)(Al(C6F5)3)2 (2.16) in the hopes of

obtaining more spectroscopic information. Unfortunately, while combustion analysis results

demonstrated that 2.16 had been formed, the compound was found to be extremely insoluble in

bromobenzene precluding useful acquisition of NMR data. In contrast, the analog

(otol)3P(CO2)(Al(C6F5)3)2 (2.17), synthesized in a similar fashion to 2.4-2.6, was found to be

readily soluble in bromobenzene. Its solid state structure is shown in Figure 2.8.

Figure 2.8 – POV-Ray depiction of 2.17. C: black, O: red, Al: teal, F: pink, P: orange. H atoms

are omitted for clarity.

We were surprised to find that 2 equiv. of the bulky Al(C6F5)3 coordinated the CO2 moiety,

similar to the AlX3 analogs. This stands in contrast to the reported172

tBu3P(CO2)Al(C6F5)3

complex where only one Al centre is present even though PtBu3 is reported to have a smaller

Tolman cone angle (182°) than P(otol)3 (194°) or PMes3 (212°).143

Indeed, while a 1:1 CO2

complex of P(otol)3 and Al(C6F5)3 could be observed by NMR spectroscopy, attempts to isolate

it resulted in the isolation of 2.17 in lower yield. While the O–C–O angle in 2.17 (126.8(4)°) is

similar to those reported for 2.4-2.6, the O–Al bond lengths are slightly elongated at 1.856(3) Å

30

each compared to an average 1.816 Å for 2.4-2.6 and likely the result of the larger Al(C6F5)3

groups. Also, while the solid state structure of 2.17 displays different environments for the

Al(C6F5)3 centres, the 19

F NMR spectrum displays only 3 peaks indicating a fluxional system

with rapidly rotating C6F5 rings.

Mechanistic insight into the reduction of CO2 was greatly facilitated using the 13

C-labelled

compound, [13

C]-2.17, along with the soluble amine-boranes, Me2

AB and Me3

AB. Treatment of

[13

C]-2.17 with 0.33 equiv. of Me3

AB monitored frequently over 12 h by 31

P{1H},

11B{

1H},

13C{

1H},

19F{

1H}, and

1H NMR spectroscopy allowed for the elucidation of a possible reaction

pathway (Scheme 2.6). The initial spectra showed broadening of the 31

P NMR peak attributable

to 2.17, as well as some free P(otol)3. This is indicative of dissociation of Al(C6F5)3 from 2.17

creating an equilibrium mixture. This is further confirmed by the immediate formation of the

formate species 2.13 as evidenced by 11

B{1H},

13C{

1H},

19F{

1H}, and

1H NMR spectroscopy. As

with the Al–AB chemistry described above, this formate species (2.13) reacts with some in situ

generated 2.8 to form the salt 2.14 as shown in Scheme 2.5 (top). What remains after 12 h is

some unreacted [13

C]-2.17, the formate anion of 2.14, as well as the cation [(otol)3P-BH2NMe3]+

which was independently confirmed by the reaction of the known salt168

[(µ-H)(H2BNMe3)2][B(C6F5)4] with excess P(otol)3.

31

Scheme 2.6 – Proposed reaction pathway for the reaction of Me3

AB with excess 2.17.

Increasing the equivalents of Me3

AB to 1 and 3 equiv. consumes all remaining 2.17 converting it

mostly to 2.13, 2.14, and some excess unreacted 2.8. Only upon heating this reaction mixture to

60 °C for several hours did further reduction from formate to methoxy species begin to occur.

Alternatively, treating 2.17 to the less bulky Me2

AB or AB leads to further reduction to methoxy,

similar to the results presented in the previous section and following the steric arguments

described in Scheme 2.5.

The results obtained using 2.17 as a model compound allow us to propose a general reaction

pathway for the initially reported reduction of CO2 from Mes3P(CO2)(AlX3)2 to MeOH using

excess AB (Scheme 2.3). In an analogous fashion to the model experiments of these past

sections, it is proposed that AB coordinates to the AlX3 of Mes3P(CO2)(AlX3)2, forcing the

dissociation of the Lewis acid and catalyzing the decomposition of Mes3P(CO2)(AlX3)2 to its

components, PMes3, AlX3 and CO2. The non-bulky AlX3-AB adduct formed would be highly

reactive toward CO2 reduction (Scheme 2.5 bottom), readily reducing it completely to methoxy

32

species. Consistent with the reactions in Scheme 2.6, reducing the equivalents of AB from 3

equiv. to 1 equiv. or less leads to the formation of more formate and fewer methoxy species as

observed by 13

C and 1H NMR spectroscopy as noted earlier. Also consistent with the steric

arguments of the previous section, using the bulkier Me2

AB or Me3

AB with Mes3P(CO2)(AlX3)2

(2.4-2.6) leads to similar reduction products; however, the rate of reaction is diminished

compared to using AB. Combining the results of this section with the previous, it can be

concluded that the complexes Mes3P(CO2)(AlX3)2 serve only as convenient stoichiometric

sources of CO2 for reduction by Al-AB complexes. Finally, while a detailed computational paper

on the possible mechanism of reduction by Paul and co-workers173

proposed direct attack of AB

to the CO2 moiety of Mes3P(CO2)(AlX3)2, the experimental evidence outlined here clearly

suggests an alternative pathway.

2.3 Conclusions

This chapter has presented the results of the rapid and irreversible activation of CO2 using

PMes3/AlX3 FLPs to generate the Mes3P(CO2)(AlX3)2 species. The facile reduction of this

activated CO2 moiety using ammonia-borane to methanol after water quench was also presented.

A possible reaction pathway is proposed based on the observed chemistry of novel isolated RAB

adducts of AlX3 (X = halide or C6F5) with CO2. The increasing steric congestion around the

hydride moieties of these adducts is believed to lead to diminished reactivity with CO2. In

summary, it is believed that AB acts as a simple Lewis base to catalyze the decomposition of the

Mes3P(CO2)(AlX3)2 species into its constituents PMes3, AlX3, and CO2, in turn generating AB-

AlX3 adducts which can then react rapidly with the released CO2. These observations stand in

marked contrast to a proposed theoretical study on the mechanism.173

2.4 Experimental Section

2.4.1 General considerations

All manipulations were performed under an atmosphere of dry, oxygen-free N2 by means of

standard Schlenk or glovebox techniques (Innovative Technology glovebox equipped with a

-38 ºC freezer). Hexanes, pentane, and toluene (Aldrich) were dried using an Innovative

Technologies solvent system and degassed prior to use. Fluorobenzene and bromobenzene (-H5

and -D5) were purchased from Aldrich and dried on P2O5 for several days and vacuum distilled

33

onto 4 Å molecular sieves prior to use. Dichloromethane-d2 and toluene-d8 were purchased from

Aldrich, dried on CaH2 and vacuum distilled onto 4 Å molecular sieves prior to use. D2O was

purchased from Cambridge Isotopes and used as received. Trimethylaluminum (TMA), P(otol)3,

PtBu3, and PMes3 were purchased from Strem and used without further purification. 13

CO2,

NH3BH3, 1,4-dioxane, and Me2Si(H)Cl were purchased from Aldrich and used without further

purification. Me2NHBH3 and Me3NBH3 were purchased from Strem and sublimed prior to use.

AlX3 were purchased from Strem and sublimed three times prior to use under vacuum using a

-78 °C cold finger and a 100 °C (X = Cl), 80 °C (X = Br), or 150 °C (X = I) bath. B(C6F5)3 was

purchased from Boulder Scientific, sublimed under vacuum, then treated with excess

Me2Si(H)Cl for 4 h and re-sublimed after removal of volatiles. CO2 (grade 4.0) was purchased

from Linde and passed through a Drierite column prior to use. [tBu3PH][(µ-H)(Al(C6F5)3)2]174

was prepared according to literature procedure. Al(C6F5)3•tol was prepared from B(C6F5)3 and

TMA in toluene by a known procedure.175

NMR spectra were obtained on a Bruker Avance 400 MHz or a Varian 400 MHz and spectra

were referenced to residual solvent of C6D5Br (1H = 7.28 ppm for meta proton;

13C = 122.4 ppm

for ipso carbon), C7H8 (1H = 2.08 ppm for methyl;

13C = 20.43 ppm for methyl), D2O (

1H = 4.79

ppm for residual HDO peak), and CD2Cl2 (1H = 5.32 ppm;

13C = 53.84 ppm), or externally (

27Al:

Al(NO3)3,11

B: (Et2O)BF3, 31

P: 85% H3PO4, 19

F: CFCl3). Chemical shifts (δ) listed are in ppm and

absolute values of the coupling constants are in Hz. NMR assignments are supported by

additional 2D experiments. IR spectra were collected on a Perkin Elmer Spectrum One FT-IR

instrument. Elemental analyses (C, N, H) and X-ray crystallography were performed in house.

2.4.2 Synthesis of compounds

Synthesis of Mes3P-AlCl3 (2.1): A 50 mL round bottom flask equipped with a magnetic stir bar

in the glovebox was charged with PMes3 (200 mg, 0.51 mmol) and AlCl3 (69 mg, 0.51 mmol).

Bromobenzene (10 mL) was added to this all at once. The solution was allowed to stir for 30 min

at which point ca. 15 mL hexanes was added dropwise with rapid stirring. The white precipitate

that forms was collected on a glass frit and washed with hexanes (ca. 5 mL) followed by pentane

(ca. 5 mL) and was dried (215 mg, 0.41 mmol, 81%). Vapour diffusion of a bromobenzene

solution of the compound with pentane yielded single crystals suitable for X-ray crystallography.

34

1H NMR (400 MHz, C6D5Br): δ 6.79 (bs, 3H, m-Mes), 6.66 (bs, 3H, m-Mes), 2.56 (s, 9H, o-

CH3Mes

), 2.02 (s, 9H, p-CH3Mes

), 1.78 (s, 9H, o-CH3Mes

). 31

P{1H} NMR (161 MHz, C6D5Br): δ -

14 – -21 (bm). 27

Al NMR (104 MHz, C6D5Br): δ 110.3 (bd, 1JAl-P = 258 Hz).

13C{

1H} NMR

(100 MHz, C6D5Br): δ 144.8 (d, 2JC-P = 17.2 Hz, o-C6H2), 143.4 (d,

2JC-P = 2.5 Hz, o-C6H2),

141.9 (d, 4JC-P = 2.1 Hz, p-C6H2), 132.4 (d,

3JC-P = 8.1 Hz, m-C6H2), 131.4-130.9 (m-C6H2, signal

lost in solvent peak), 121.0 (d, 1JC-P = 45.8 Hz, i-C6H2), 25.1 (d,

3JC-P = 10.9 Hz, o-CH3

Mes), 24.1

(d, 3JC-P = 3.6 Hz, o-CH3

Mes), 21.0 (s, p-CH3

Mes). Anal. Calc. Satisfactory EA for this compound

could not be obtained.

Synthesis of Mes3P-AlBr3 (2.2): Prepared in an analogous fashion to 2.1 using 200 mg of

PMes3 (0.51 mmol) and 137 mg of AlBr3 (0.51 mmol). Yield: (280 mg, 0.43 mmol, 84%).

Vapour diffusion of a fluorobenzene solution of the compound with pentane yielded single

crystals suitable for X-ray crystallography.


CH3Mes

), 2.02 (s, 9H, p-CH3Mes

), 1.79 (s, 9H, o-CH3Mes

). 31


11 – -21 (bm). 27

Al NMR (104 MHz, C6D5Br): δ 99.9 (bd, 1JAl-P = 222 Hz).

13C{

1H} NMR (100

MHz, C6D5Br): δ 145.1 (d, 2JC-P = 17.2 Hz, o-C6H2), 143.4 (d,

2JC-P = 3.6 Hz, o-C6H2), 142.0 (d,

4JC-P = 1.9 Hz, p-C6H2), 132.5 (d,

3JC-P = 8.2 Hz, m-C6H2), 131.4-130.9 (m-C6H2, signal lost in

solvent peak), 120.8 (d, 1JC-P = 45.8 Hz, i-C6H2), 26.4 (d,

3JC-P = 10.3 Hz, o-CH3

Mes), 24.2 (d,

3JC-P = 4.0 Hz, o-CH3

Mes), 21.0 (s, p-CH3

Mes). Anal. Calc. for C27H33PAlBr3: C, 49.49; H, 5.08.

Found: C, 49.51; H, 5.34.

Synthesis of Mes3P-AlI3 (2.3): A 50 mL round bottom flask equipped with a magnetic stir bar in

the glovebox was charged with PMes3 (250 mg, 0.64 mmol), AlI3 (262 mg, 0.64 mmol) and

bromobenzene (6-7 mL). The solution was allowed to stir for 5-10 min while a precipitate

formed. The white precipitate was collected on a glass frit and washed with bromobenzene

(several drops) and hexanes (ca. 5 mL) and was dried (300 mg, 0.38 mmol, 59%).


CH3Mes

), 2.02 (s, 9H, p-CH3Mes

), 1.79 (s, 9H, o-CH3Mes

). 31


19 – -29 (bm). 27

Al NMR (104 MHz, C6D5Br): 37 (bd, 1JAl-P = 193 Hz).

13C{

1H} NMR (100

MHz, C6D5Br): δ 145.6 (bs), 143.3 (bs), 142.0 (bs), 132.5 (bs, m-C6H2), 131.5 (m-C6H2, signal

35

lost in solvent peak), 120.5 (d, 1JC-P = 40.0 Hz, i-C6H2), 28.6 (bs, o-CH3

Mes), 24.4 (bs, o-CH3

Mes),

21.0 (s, p-CH3Mes

). Anal. Calc. for C27H33AlI3P: C, 40.73; H, 4.18. Found: C, 40.65; H, 3.81.

Synthesis of Mes3P(CO2)(AlX3)2 (2.4-2.6): These compounds were all synthesized in a similar

fashion including the 13

CO2 isotopologues; therefore, only one synthesis is described.

A 50 mL Schlenk bomb equipped with a Teflon cap was charged with PMes3 (0.500 g, 1.29

mmol) and AlI3 (1.05 g, 2.58 mmol) in 20 mL bromobenzene. The bomb was transferred to the

Schlenk line equipped with a CO2 outlet. The bomb was degassed at r.t., filled with CO2, and

sealed. The solution was stirred rapidly for ca. 10 min in the glovebox. The CO2 atmosphere was

removed. Precipitation using hexanes (ca. 20 mL) afforded a white solid which was filtered and

dried on a frit.

2.4: Isolated yield: 82%. Vapour diffusion of a bromobenzene solution of the compound with

pentane yielded single crystals suitable for X-ray crystallography.

1H NMR (400 MHz, C6D5Br): δ 6.80 (bs, 3H, m-Mes), 6.68 (bs, 3H, m-Mes), 2.32 (bs, 9H, o-

CH3Mes

), 2.03 (s, 9H, p-CH3Mes

), 1.85 (bs, 9H, o-CH3Mes

). 31

P{1H} NMR (161 MHz, C6D5Br): δ

20.1. 27

Al NMR (104 MHz, C6D5Br): δ 105.0 (bs, υ1/2 = 750 Hz). 13

C{1H} NMR (100 MHz,

C6D5Br): δ 171.5 (d, 1JC-P = 123 Hz, CO2), 146.8 (d,

4JC-P = 3.2 Hz, p-C6H2), 145.0 (bs, o-C6H2),

144.4 (bs, o-C6H2), 133.6 (bs, 2 x m-C6H2 (coalesced)), 114.6 (d, 1JC-P = 76 Hz, i-C6H2), 24.5

(bs, o-CH3Mes

), 23.6 (bs, o-CH3Mes

), 21.2 (s, p-CH3Mes

). Anal. Calc. for C28H33PO2Al2Cl6: C,

48.10; H, 4.76. Found: C, 48.24; H, 4.71.

[13

C]-2.4: 31

P{1H} NMR (161 MHz, C6D5Br): δ 19.8 (d,

1JP-C = 123 Hz).

2.5: Isolated yield: 83%. Vapour diffusion of a fluorobenzene solution of the compound with


1H NMR (400 MHz, C6D5Br): δ 6.84 (bs, 3H, m-Mes), 6.76 (bs, 3H, m-Mes), 2.32 (bs, 9H, o-

CH3Mes

), 2.08 (s, 9H, p-CH3Mes

), 1.90 (bs, 9H, o-CH3Mes

). 31


22.3. 27

Al NMR (104 MHz, C6D5Br): δ Blank – signal likely lost in probe signal. 13

C{1H} NMR

(100 MHz, C6D5Br): δ 172.5 (d, 1JC-P = 119 Hz, CO2), 147.2 (d,

4JC-P = 3.0 Hz, p-C6H2), 145.3

(bs, o-C6H2), 144.1 (bs, o-C6H2), 133.9 (s, m-C6H2), 133.8 (s, m-C6H2), 114.1 (d, 1JC-P = 76 Hz, i-

36

C6H2), 24.9 (bs, o-CH3Mes

), 23.8 (bs, o-CH3Mes

), 21.3 (s, p-CH3Mes

). Anal. Calc. for

C28H33PO2Al2Br6: C, 34.82; H, 3.44. Found: C, 34.78; H, 3.64.

[13

C]-2.5: 31

P{1H} NMR (161 MHz, C6D5Br): δ 21.0 (d,

1JP-C = 121 Hz).

2.6: Isolated yield: 87 %. Vapour diffusion of a bromobenzene solution of the compound with


1H NMR (400 MHz, C6D5Br): δ 6.83 (d,

4JH-H = 4.4 Hz, 3H, m-Mes), 6.70 (d,

4JH-H = 4.4 Hz,

3H, m-Mes), 2.48 (s, 9H, o-CH3Mes

), 2.06 (s, 9H, p-CH3Mes

), 1.90 (s, 9H, o-CH3Mes

). 31

P{1H}

NMR (161 MHz, C6D5Br): δ 22.0. 27

Al NMR (104 MHz, C6D5Br): δ 20 (bs, υ1/2 = ca. 1500 Hz).

13C{

1H} NMR (100 MHz, C6D5Br): δ 167.8 (d,

1JC-P = 119 Hz, CO2), 146.5 (d,

4JC-P = 3.0 Hz, p-

C6H2), 144.9 (d, 2

JC-P = 11.6 Hz, o-C6H2), 144.4 (d, 2

JC-P = 10.3 Hz, o-C6H2), 134.5 (d, 3

JC-P =

12.2 Hz, m-C6H2), 133.7 (d, 3

JC-P = 12.5 Hz, m-C6H2), 115.0 (d, 1JC-P = 74.5 Hz, i-C6H2), 25.5 (d,

3JC-P = 5.7 Hz, o-CH3

Mes), 23.9 (d,

3JC-P = 5.2 Hz, o-CH3

Mes), 21.2 (d,

5JC-P = 1.5 Hz, p-CH3

Mes).

Anal. Calc. for C28H33PO2Al2I6: C, 26.95; H, 2.67. Found: C, 26.91; H, 2.71.

[13

C]-2.6: 31

P{1H} NMR (161 MHz, C6D5Br): δ 22.0 (d,

1JP-C = 119 Hz).

Synthesis of Me3NBH3•AlBr3 (2.7): AlBr3 (300 mg, 1.1 mmol) and Me3NBH3 (82 mg, 1.1

mmol) were combined in bromobenzene (5 mL) in a screw cap vial equipped with a magnetic

stirbar. After stirring for 5 min, hexanes (ca. 10 mL) were added to precipitate a product. The

product was filtered on a glass frit, washed with pentane and dried (265 mg, 0.78 mmol, 70%).

Vapour diffusion of a bromobenzene solution of the compound with pentane yielded single

crystals suitable for X-ray crystallography.

1H NMR (400 MHz, C6D5Br): δ 2.62 (bq,

1JH-B = 89 Hz, 3H, BH3), 2.07 (s, 9H, 3 x CH3).

11B

NMR (128 MHz, C6D5Br): δ -10.2 (bq, 1JB-H = 89 Hz).

27Al NMR (104 MHz, C6D5Br): 78.5

(bs). 13

C{1H} NMR (100 MHz, C6D5Br): δ 53.5. Anal. Calc. for C3H12BAlBr3N: C, 10.61; H,

3.56; N, 4.12. Found: C, 10.83; H, 3.22; N, 4.11.

Synthesis of Me3NBH3•Al(C6F5)3 (2.8): Al(C6F5)3•tol (750 mg, 1.2 mmol) and Me3NBH3 (88

mg, 1.2 mmol) were combined in toluene (10 mL) in a 50 mL Schlenk flask equipped with a

magnetic stirbar. After stirring for 5 min, pentane (ca. 30 mL) was added to precipitate a product.

The product was filtered on a glass frit, washed with pentane and dried (560 mg, 0.93 mmol,

37

77%). Slow cooling a saturated toluene solution to -38 °C yielded single crystals suitable for X-

ray crystallography.

1H NMR (400 MHz, C6D5Br): δ 2.14 (overlapping s (3 x CH3) and bs (BH3), 12H).

11B NMR

(128 MHz, C6D5Br): δ -9.1 (bs). 27

Al NMR (104 MHz, C6D5Br): blank. 19

F{1H} NMR (376

MHz, C6D5Br): δ -122.4 (dd, 3JF-F = 26.3 Hz,

4JF-F = 11.3 Hz, 6F, o-C6F5), -152.4 (t,

3JF-F = 18.8

Hz, 3F, p-C6F5), -161.3 (m, 6F, m-C6F5). 13

C{1H} NMR (100 MHz, C6D5Br): δ 150.0 (dm,

1JC-F

= 235 Hz), 141.9 (dm, 1

JC-F = 253 Hz), 136.9 (dm, 1

JC-F = 253 Hz), 113.2 (m, i-C6F5), 53.3 (s,

N(CH3)3). Anal. Calc. for C21H12BAlF15N: C, 41.96; H, 2.01; N, 2.33. Found: C, 41.72; H, 1.99;

N, 2.36.

Synthesis of Me2NHBH3•Al(C6F5)3 (2.9): This compound was synthesized in an analogous

fashion to 2.8 using Al(C6F5)3•tol (500 mg, 0.8 mmol), Me2NHBH3 (47 mg, 0.8 mmol), and

toluene (5 mL). Following filtration of the solid, the filtrate was stored at -38 °C to obtain a

second crop (410 mg (total), 0.70 mmol, 87%).

1H NMR (400 MHz, C6D5Br): δ 3.73 (bs, 1H, NH), 2.15 (overlapping d (

3JH-H = 5.6 Hz,

N(CH3)2) and bs (BH3), 9H total). 11

B NMR (128 MHz, C6D5Br): δ -16.5 (bs). 27

Al NMR (104

MHz, C6D5Br): blank. 19

F{1H} NMR (376 MHz, C6D5Br): δ -123.2 (dd,

3JF-F = 26.3 Hz,

4JF-F =

7.5 Hz, 6F, o-C6F5), -151.5 (t, 3JF-F = 20.7 Hz, 3F, p-C6F5), -160.0 (m, 6F, m-C6F5).

13C{

1H}

NMR (100 MHz, C6D5Br): δ 149.7 (dm, 1JC-F = 234 Hz), 141.7 (dm,

1JC-F = 252 Hz), 136.9 (dm,

1JC-F = 255 Hz), 113.5 (m, i-C6F5), 43.8 (s, N(CH3)2). Anal. Calc. for C20H10BAlF15N: C, 40.92;

H, 1.72; N, 2.39. Found: C, 40.58; H, 1.70; N, 2.39.

Synthesis of NH3BH3•Al(C6F5)3 (2.10): Al(C6F5)3•tol (300 mg, 0.48 mmol) and NH3BH3 (15

mg, 0.48 mmol) were combined in fluorobenzene (10 mL) in a vial equipped with a magnetic

stirbar. After stirring for 20 min, the mixture was filtered through Celite. The filtrate volume was

reduced to ca. 2-3 mL and pentane (ca. 10 mL) was added to precipitate a product. The product

was filtered on a glass frit, washed with pentane and dried (150 mg, 0.27 mmol, 55%). Slow

cooling a fluorobenzene/pentane solution to -38 °C yielded single crystals suitable for X-ray

crystallography.

1H NMR (400 MHz, C6D5Br): δ 2.99 (bs, 3H, NH3), 2.10 (bs, 3H, BH3).

11B NMR (128 MHz,

C6D5Br): δ -24.0 (bs). 27


F{1H} NMR (376 MHz,

38

C6D5Br): δ -123.1 (dd, 3JF-F = 28.2 Hz,

4JF-F = 18.8 Hz, 6F, o-C6F5), -151.6 (t,

3JF-F = 18.8 Hz,

3F, p-C6F5), -159.9 (m, 6F, m-C6F5). 13


1JC-F =

235 Hz), 141.7 (dm, 1

JC-F = 252 Hz), 136.9 (dm, 1

JC-F = 255 Hz), 113.5 (m, i-C6F5). Anal. Calc.

for C18H6AlBF15N: C, 38.67; H, 1.08; N, 2.51. Found: C, 38.30; H, 0.95; N, 3.26.

Synthesis of (Me3N)•H2B(C6F5) (2.11): In a vial in the glovebox were combined B(C6F5)3 (200

mg, 0.39 mmol) and Me3NBH3 (57 mg, 0.78 mmol) in a 1:2 solution of toluene (5 mL) and

hexanes (10 mL). Precipitation initially occurs but the mixture became a solution after stirring

overnight. The next morning, the solution was put in the -38 °C freezer. The crystals that formed

were filtered on a glass frit and washed with minimal cold hexanes and dried in vacuo (100 mg,

0.42 mmol, 54%). Slow cooling a concentrated toluene/hexanes solution of the compound to

-38 °C yielded single crystals suitable for X-ray crystallography.

1H NMR (400 MHz, C6D5Br): δ 2.49 (bq, 2H, (C6F5)BH2), 2.07 (s, 9H, N(CH3)3).

11B{

1H}

NMR (128 MHz, C6D5Br): -9.9 (s). 19


3JF-F =

26.3 Hz, 4JF-F = 11.3 Hz, 2F, o-C6F5), -157.4 (t,

3JF-F = 20.7 Hz, 1F, p-C6F5), -163.6 (m, 2F, m-

C6F5). 13


1JC-F = 236 Hz), 139.9 (dm,

1JC-F = 249

Hz), 137.0 (dm, 1JC-F = 249 Hz), 117.4 (bs, i-C6F5), 51.7 (s, N(CH3)3). Anal. Calc. for

C9H11BF5N: C, 45.23; H, 4.64; N, 5.86. Found: C, 45.27; H, 4.92; N, 5.99.

Synthesis of Me3NBH2O(CH)OAl(C6F5)3 (2.13): Me3NBH3•Al(C6F5)3 (155 mg, 0.26 mmol)

(2.8) was dissolved in toluene (5 mL) in a 50 mL Schlenk flask equipped with a Teflon screw

cap and a magnetic stirbar. The bomb was transferred to the Schlenk line equipped with a CO2

outlet. The bomb was degassed, filled with CO2 (1 atm), and sealed. The solution was stirred for

30 min after which time the CO2 atmosphere was removed and pentane (ca. 20 mL) was added.

After stirring rapidly for several minutes, a product precipitated and was filtered, washed with

pentane, and dried (120 mg, 0.19 mmol, 72%). Slow cooling a toluene/pentane solution to -38 °C

yielded single crystals suitable for X-ray crystallography.

1H NMR (400 MHz, C7D8): δ 7.55 (s, 1H, H-CO2), 2.17 (bs, 2H, BH2), 1.33 (s, 9H, 3 x CH3).

11B NMR (128 MHz, C7D8): δ 3.2 (bs).

27Al NMR (104 MHz, C7D8): 121 (bs, υ1/2 = ca. 2000

Hz). 19

F{1H} NMR (376 MHz, C7D8): δ -124.1 (dd,

3JF-F = 26.3 Hz,

4JF-F = 11.3 Hz, 6F, o-C6F5),

-154.0 (t, 3JF-F = 20.7 Hz, 3F, p-C6F5), -162.7 (m, 6F, m-C6F5).

13C{

1H} NMR (100 MHz, C7D8):

39

δ 173.3 (s, HCO2), 150.5 (dm, 1JC-F = 235 Hz), 142.0 (dm,

1JC-F = 251 Hz), 137.3 (dm,

1JC-F =

253 Hz), 48.1 (s, 3 x CH3). Anal. Calc. for C22H12BAlF15NO2: C, 40.96; H, 1.87; N, 2.16.

Found: C, 40.37; H, 1.94; N, 2.17.

Synthesis of [tBu3PH][(µ-η1:η

1-HCO2)(Al(C6F5)3)2] (2.15): A 50 mL Schlenk bomb equipped

with a Teflon screw cap and a magnetic stirbar was charged with [tBu3PH][(µ-H)(Al(C6F5)3)2]

(400 mg, 0.32 mmol) dissolved in fluorobenzene (ca. 5 mL). The bomb was transferred to the

Schlenk line equipped with a CO2 outlet, degassed and filled with CO2 (1 atm). The solution was

stirred for 12 h after which the solvent was removed in vacuo. Hexanes (ca. 10 mL) were added

to the residue and the precipitate that forms was stirred for 30 min before being filtered on a

glass frit (260 mg, 0.20 mmol, 63%). Vapour diffusion of a bromobenzene solution of the

compound with hexanes yielded single crystals suitable for X-ray crystallography.

1H NMR (400 MHz, C6D5Br): δ 8.65 (s, 1H, HCO2-), 4.12 (d,

1JH-P = 426 Hz, 1H, P-H), 0.96 (d,

3JH-P = 15.6 Hz, 27H, tBu).

31P{

1H} NMR (161 MHz, C6D5Br): δ 60.0.

27Al NMR (104 MHz,

C6D5Br): blank. 19


3JF-F = 26.3 Hz,

4JF-F = 11.3

Hz, 12F, o-C6F5), -153.6 (t, 3

JF-F = 20.7 Hz, 6F, p-C6F5), -161.6 (m, 12F, m-C6F5). 13

C{1H} NMR

(100 MHz, C6D5Br): δ 172.8 (s, HCO2-), 150.1 (dm, 1JC-F = 235 Hz), 141.4 (dm,

1JC-F = 250 Hz),

136.7 (dm, 1JC-F = 252 Hz), 114.5 (m, i-C6F5), 36.9 (d,

1JC-P = 26.4 Hz, PCMe3), 29.3 (s, PCMe3).

Anal. Calc. for C49H29Al2F30O2P: C, 45.11; H, 2.24. Found: C, 44.58; H, 2.40.

Synthesis of Mes3P(CO2)(Al(C6F5)3)2 (2.16): A 50 mL Schlenk bomb equipped with a Teflon

screw cap and a magnetic stirbar was charged with PMes3 (125 mg, 0.32 mmol), Al(C6F5)3•tol

(400 mg, 0.64 mmol) and bromobenzene (5 mL). The bomb was transferred to the Schlenk line

equipped with a CO2 outlet. The bomb was degassed, filled with CO2 (1 atm), and sealed. The

solution became a mixture and was stirred for 30 min after which time the CO2 atmosphere was

removed and hexanes (ca. 5-10 mL) were added dropwise to the stirring mixture in the glovebox.

The precipitate was filtered on a glass frit, washed with hexanes and dried in vacuo (410 mg,

0.27 mmol, 85%).

1H NMR (400 MHz, CD2Cl2): δ 7.21 (d,

4JH-H = 4.0 Hz, 3H, m-Mes), 7.05 (d,

4JH-H = 4.0 Hz,

3H, m-Mes), 2.38 (s, 9H, CH3), 2.28 (s, 9H, CH3), 1.97 (s, 9H, CH3). 31

P{1H} NMR (161 MHz,

CD2Cl2): δ 17.0. 27

Al NMR (104 MHz, CD2Cl2): blank. 19

F{1H} NMR (376 MHz, CD2Cl2): δ -

122.2 (dd, 3JF-F = 25.6 Hz,

4JF-F = 9.8 Hz, 12F, o-C6F5), -154.7 (bs, 6F, p-C6F5), -163.3 (m, 12F,

40

m-C6F5). 13

C{1H} NMR (100 MHz, CD2Cl2), partial: δ 150.2 (dm,

1JC-F = 232 Hz), 146.8 (d,

4JC-

P = 3.0 Hz, p-C6H2), 145.2 (d, 2JC-P = 11.4 Hz, o-C6H2), 144.2 (d,

2JC-P = 9.5 Hz, o-C6H2), 141.8

(dm, 1JC-F = 250 Hz), 136.9 (dm,

1JC-F = 252 Hz), 133.9 (d,

3JC-P = 12.1 Hz, m-C6H2), 133.3 (d,

3JC-P = 11.6 Hz, m-C6H2), 131.2 (d,

1JC-P = 135 Hz, CO2), 117.6 (d,

1JC-P = 77.8 Hz, i-C6H2), 24.7

(d, 3JC-P = 4.2 Hz, o-CH3

Mes), 24.2 (d,

3JC-P = 5.2 Hz, o-CH3

Mes), 21.3 (s, p-CH3

Mes). Anal. Calc.

for C64H33Al2F30O2P: C, 51.63; H, 2.23. Found: C, 51.24; H, 2.56.

Synthesis of (otol)3P(CO2)(Al(C6F5)3)2 (2.17): Synthesized in an analogous fashion to 2.16

using 76 mg (0.25 mmol) P(otol)3 and 311 mg (0.50 mmol) Al(C6F5)3•tol. Isolated yield is 325

mg (0.22 mmol, 90%). Vapour diffusion of a bromobenzene solution of the compound with


1H NMR (400 MHz, C6D5Br): δ 7.60-6.85 (m, 12H), 1.81 (bs, 9H, o-CH3), 1.27-1.13 (m, 4H,

0.5•(CH3(CH2)CH3)), 0.84 (t, 3JH-H = 6.8 Hz, 3H, 0.5•(CH3(CH2)CH3)).

31P{

1H} NMR (161

MHz, C6D5Br): δ 30.0. 27


F{1H} NMR (376 MHz,

C6D5Br): δ -121.8 (bd, 3JF-F = 19.2 Hz, 12F, o-C6F5), -152.5 (t,

3JF-F = 19.9 Hz, 6F, p-C6F5), -

161.4 (m, 12F, m-C6F5). 13

C{1H} NMR (100 MHz, C6D5Br): δ 167.3 (d,

1JC-P = 128 Hz, CO2),

149.9 (dm, 1JC-F = 234 Hz), 144.2 (d,

2JC-P = 8.4 Hz, C-Me), 141.7 (dm,

1JC-F = 252 Hz), 136.7

(dm, 1

JC-F = 253 Hz), 136.4 (d, JC-P = 1.8 Hz), 135.2 (d, JC-P = 11.8 Hz), 133.7 (d, JC-P = 11.4

Hz), 127.7 (d, JC-P = 13.5 Hz), 112.9 (m, i-C6F5), 112.1 (d, 1JC-P = 78 Hz, i-C6H4), 31.8 (s,

C6H14), 22.9 (s, C6H14), 22.4 (bs, o-CH3), 14.4 (s, C6H14). Anal. Calc. for C61H28Al2F30O2P (2.17

+ 0.5•C6H14): C, 50.61; H, 1.95. Found: C, 50.36; H, 2.06.

2.4.3 X-Ray crystallography

2.4.3.1 X-Ray data collection and reduction

Crystals were coated in paratone oil in the glovebox, mounted on a MiTegen Micromount and

placed under an N2 stream, thus maintaining a dry, O2-free environment for each crystal. The

data were collected on a Bruker Apex II diffractometer employing Mo Kα radiation (λ = 0.71073

Å). Data collection strategies were determined using Bruker Apex software and optimized to

provide >99.5% complete data to a 2θ value of at least 55°. The frames were integrated with the

Bruker SAINT software package using a narrow-frame algorithm. Data were corrected for

absorption effects using the empirical multi-scan method (SADABS).

41

2.4.3.2 X-Ray data solution and refinement

Non-hydrogen atomic scattering factors were taken from the literature tabulations.176

The heavy

atom positions were determined using direct methods employing the SHELXTL direct methods

routine. The remaining non-hydrogen atoms were located from successive difference Fourier

map calculations. The refinements were carried out by using full-matrix least squares techniques

on F, minimizing the function (Fo-Fc)2 where the weight is defined as 4Fo

2/2 (Fo

2) and Fo

and Fc are the observed and calculated structure factor amplitudes, respectively. In the final

cycles of each refinement, all non-hydrogen atoms were assigned anisotropic temperature factors

in the absence of disorder or insufficient data. In the latter cases atoms were treated isotropically.

C-H atom positions were calculated and allowed to ride on the carbon to which they are bonded

assuming a C-H bond length of 0.95 Å. H-atom temperature factors were fixed at 1.20 times the

isotropic temperature factor of the C-atom to which they are bonded. The H-atom contributions

were calculated, but not refined. The locations of the largest peaks in the final difference Fourier

map calculation as well as the magnitude of the residual electron densities in each case were of

no chemical significance.

42

2.4.3.3 Selected crystallographic data

Table 2.1 – Selected crystallographic data for 2.1, 2.2 and 2.4.

2.1 (+0.5 C6H5Br) 2.2 (+0.5 C6H14) 2.4

Formula C30H36Al1Br0.50Cl3P1 C30H40Al1Br3P1 C28H33Al2Cl6O2P1

Formula wt. 600.84 698.3 699.17

Crystal system triclinic triclinic orthorhombic

Space group P-1 P-1 Pbca

a(Å) 10.3628(5) 10.5729(4) 10.5382(10)

b(Å) 11.1182(5) 11.2503(4) 20.0664(15)

c(Å) 13.0439(6) 13.1735(5) 31.987(3)

α(deg) 89.470(3) 84.716(2) 90

β(deg) 88.254(3) 85.767(2) 90

γ(deg) 80.848(2) 76.403(2) 90

V(Å3) 1483.0(1) 1514.4(1) 6764.2(1)

Z 2 2 8

T (K) 150(2) 150(2) 150(2)

d(calc) g/cm3 1.435 1.531 1.373

Abs coeff, μ, mm-1

1.084 4.095 0.632

Data collected 26346 42871 32202

Rint 0.0304 0.0334 0.1449

Data used 7205 11306 7807

Variables 334 316 352

R (>2σ) 0.0563 0.0467 0.1023

wR2 0.1671 0.1333 0.2715

GOF 1.049 1.025 1.018

43


2.5 2.6 2.7

Formula C28H33Al2Br6O2P1 C28H33Al2I6O2P1 C3H12Al1B1Br3N1

Formula wt. 965.93 1247.92 339.66

Crystal system triclinic triclinic monoclinic

Space group P-1 P-1 P21/c

a(Å) 10.7976(6) 11.1673(3) 13.7292(11)

b(Å) 10.9304(6) 11.3849(3) 10.3051(9)

c(Å) 16.1191(10) 16.6312(5) 16.6848(14)

α(deg) 100.641(3) 78.9400(10) 90

β(deg) 90.487(3) 89.955(2) 108.256(4)

γ(deg) 107.466(3) 70.2720(10) 90

V(Å3) 1779.3(2) 1948.7(1) 2241.8(3)

Z 2 2 8

T (K) 150(2) 150(2) 150(2)

d(calc) g/cm3 1.803 2.127 2.013

Abs coeff, μ, mm-1

6.887 4.891 10.812


Rint 0.0373 0.0287 0.0361

Data used 7476 8880 5598


R (>2σ) 0.0457 0.0809 0.0376

wR2 0.1329 0.2758 0.0882

GOF 1.062 1.723 1.008

44


2.8 2.10 2.11

Formula C21H12Al1B1F15N1 C18H6Al1B1F15N1 C9H11B1F5N1

Formula wt. 601.1 559.02 239.00

Crystal system triclinic monoclinic monoclinic

Space group P-1 P21/n P21/c

a(Å) 10.4136(6) 9.9492(14) 9.9549(5)

b(Å) 10.9685(6) 12.0774(18) 9.8224(5)

c(Å) 12.1639(7) 17.098(3) 11.8944(5)

α(deg) 110.843(3) 90 90

β(deg) 111.842(3) 98.863(6) 113.294(3)

γ(deg) 94.792(3) 90 90

V(Å3) 1167.3(1) 2029.9(5) 1068.2(1)

Z 2 4 4

T (K) 150(2) 150(2) 150(2)

d(calc) g/cm3 1.702 1.819 1.486

Abs coeff, μ, mm-1

0.218 0.244 0.148


Rint 0.0207 0.0551 0.0416

Data used 5642 4691 3374


R (>2σ) 0.0383 0.0428 0.0452

wR2 0.1417 0.1445 0.1214

GOF 1.025 0.854 1.028

45


2.13 2.15 2.17 (+ C6H5Br)

Formula C22H12Al1B1F15N1O2 C49H29Al2F30O2P1 C64H26Al2Br1F30O2P1

Formula wt. 645.10 1304.65 1561.68

Crystal system orthorhombic triclinic triclinic

Space group Pbca P-1 P-1

a(Å) 19.5240(7) 12.6630(8) 12.1156(7)

b(Å) 12.5631(5) 13.8558(8) 12.9796(8)

c(Å) 19.7628(7) 15.7986(10) 20.8875(13)

α(deg) 90 92.518(3) 84.386(3)

β(deg) 90 100.938(3) 88.395(3)

γ(deg) 90 108.147(3) 77.233(3)

V(Å3) 4847.5(3) 2570.4(3) 3188.0(3)

Z 8 2 2

T (K) 150(2) 150(2) 150(2)

d(calc) g/cm3 1.738 1.686 1.671

Abs coeff, μ, mm-1

0.223 0.238 0.832


Rint 0.0456 0.0386 0.0359

Data used 5584 11687 16301


R (>2σ) 0.0354 0.0582 0.0712

wR2 0.0850 0.1560 0.2136

GOF 1.002 1.025 1.087

46

Chapter 3 Stoichiometric Reduction of CO2 to CO Using AlX3-Based

Frustrated Lewis Pairs

3.1 Introduction

Rapidly increasing levels of greenhouse gases in the atmosphere, especially levels of CO2,

requires urgent action to mitigate emissions.11

While the direct use of CO2 as a C1 feedstock for

chemical transformation is attractive and being used in some cases,177,178

a far more useful C1

feedstock is carbon monoxide (CO).179

This gas is used on a large scale in the Cativa process

(formerly the Monsanto process) to produce acetic acid from CO and methanol (Figure 3.1).180

Perhaps more importantly is the role of CO in Fischer-Tropsch chemistry where “syn gas”

(CO/H2) is transformed into liquid hydrocarbons.130

This syn gas is presently formed by steam

reforming of hydrocarbons, often using methane. Hydrocarbons are useful serving as both the C

and H sources, both needed in Fischer-Tropsch chemistry; however, on an industrial scale, steam

reforming is combined with the water-gas shift reaction (WGSR) to produce even more H2, at the

expense of CO2 release. Lastly, hydroformylation is also used on a massive scale to produce

aldehydes, which serve as important precursors to plasticizers and detergents (Figure 3.1).21

Figure 3.1 – Important industrial processes involving carbon monoxide.

47

In each of these examples, the CO used is derived mainly from the fossil fuel-dependent steam

reforming reaction, as is the H2 (in addition to the WGSR). As shown by this last reaction, CO

combined with water can serve as a H2 source, but at the expense of CO2 release. Therefore, a

process where CO2 is captured and recycled to regenerate CO is desirable as this could

ultimately provide the necessary syn gas central to these reactions without adverse CO2

emissions.130

However, this transformation remains difficult due to the strong O=C(O) bond

strength (estimated to be 532 kJ/mol)138

presenting significant kinetic barriers to

functionalization.177

As a significant part of the carbon cycle, the enzyme carbon monoxide dehydrogenase (CODH)

is nature’s way of effecting the 2e-/2H

+ multistep reversible reduction of CO2 to CO and H2O.

181

In academic settings, the photochemical reduction of CO2 to CO is being intensely

investigated.182-185

For example, a recent contribution has demonstrated the efficient, catalytic

reduction of CO2 to CO using H2 as the 2e- reductant to produce CO and H2O under visible light

conditions.133

Electrochemical reduction has also been explored.186

In a recent example, CO2 was

reduced electrochemically in an ionic liquid medium at low overpotentials and high Faradaic

efficiencies to produce CO with good turnover.187

Alternatively, chemical reduction of CO2

remains under intense investigation using both metal and non-metal mediated routes. In all cases,

the formation of strong element-oxygen bonds is often used as the thermodynamic driving force

to effect these reactions. For example, Sadighi reported the catalytic reduction of CO2 mediated

by a homogeneous Cu(I) catalyst using the sacrificial reductant, bis(pinacolato)diboron (pinB–

Bpin), to form CO and the strongly bound pinB–O–Bpin by-product.138

In a similar manner,

Meyer reported the production of CO from CO2 using a U(III) starting material where a strong

U–O–U product drives the reaction to completion.188

Cummins has recently reported a reduction

sequence involving CO2 activation by a terminal Nb nitride anion to generate a nucleophilic

carbamate species. This species then reacts with acyl halides and, following reduction of the

resulting Nb centre, regenerates the Nb nitride starting material, CO, and a carboxylate by-

product.132

Main group mediated routes for CO2 reduction to CO have also been reported.112,113,140,189,190

In a

similar report to Cummins’s, CO2 activated by an N-heterocyclic carbene (NHC) was found to

generate a nucleophilic carboxylate which unexpectedly reacted with aldehydes generating the

carboxylic acid and CO catalytically.140

And finally, in a very recent report by Jones, an amido-

48

digermyne (Ge(I)) species has been found to spontaneously react with CO2 to generate a Ge–O–

Ge (Ge(II)) bond through CO expulsion.189

The mechanism of this chemistry, shown in Scheme

3.1, is similar to Meyer’s U chemistry188

and may be relevant to the chemistry of this chapter.

Scheme 3.1 – Reduction of CO2 to CO using a digermyne as reported by Jones et al.189

In a similar process involving strong element-oxygen bond formation, this chapter will describe

the FLP-mediated stoichiometric reduction of CO2 to CO using aluminum halides. The

generation of strong Al–O–Al is believed to be the central driving force in this chemistry. The

results of in-depth mechanistic and theoretical investigations into the possible mechanism will

also be presented.


3.2.1 In situ reduction of CO2 to CO using PMes3/AlX3 solutions

In the previous chapter, we described the FLP chemistry of aluminum halides (AlX3) and PMes3

with CO2 to give the species Mes3P(CO2)(AlX3)2 (X = Cl (2.4), Br (2.5), I (2.6)). Subsequent

treatment of these products with ammonia-borane, followed by hydrolysis resulted in the

stoichiometric reduction of CO2 to methanol. During the exploration of this chemistry, we

discovered new chemistry involving 2.4-2.6 and CO2 reduction.

In synthesizing 2.6, it was found that additional products were sometimes present depending on

the mixing time with CO2 as evidenced by 31

P NMR spectroscopy. The reaction was optimized

by mixing PMes3 (1 equiv.) with a sub-stoichiometric amount of AlI3 (1.9 equiv.) under an

atmosphere of CO2 for 16 h resulting in the disappearance of the 31

P NMR signal attributable to

2.6 (22 ppm) and the appearance of two new products at 20 ppm (3.1) and -15 ppm (3.2) in a 1:1

ratio. When using labeled 13

CO2, the 31

P NMR resonance for 3.1 exhibited a 1JP-C coupling of

118 Hz. The 27

Al NMR spectrum showed a broad peak at 31 ppm (υ1/2 = ca. 170 Hz) which is

slightly downfield from 2.6 (20 ppm). Single crystals suitable for X-ray diffraction were

obtained by slow cooling a saturated fluorobenzene solution to -38 °C. The solid state structure

49

of 3.1 is shown in Figure 3.2. The product contains a six-membered ring comprised of a CO2

fragment linked to a I2AlOAlI2 fragment. PMes3 is carbon bound in an exocyclic position, while

an additional equivalent of AlI3 is bound to the transannular oxygen atom. The P–C distance of

1.906(5) Å is similar to those in compounds 2.4-2.6. The C–O bond distances were found to be

1.241(5) and 1.261(4) Å and the related Al–O distances were 1.847(3) and 1.860(3) Å. The Al–O

distances in the I2AlOAlI2 fragment were 1.806(3) and 1.811(3) Å while the O–AlI3 distance is

longer (1.831(3) Å) consistent with the nature of this dative bond. It is noteworthy that while the

X-ray data confirms the presence of two distinct Al environments in 3.1, the 27

Al NMR data

shows only a single broad resonance. As dissymmetric aluminum centres often give broad

resonances, this could arise from overlapping signals. Alternatively, a fluxional process could

account for this observation, but the poor solubility of 3.1 precluded the acquisition of 27

Al NMR

data at low temperature.


clarity. C: black; P: orange; Al; teal; I: pink; O: red.

The additional product 3.2 gives rise to a 31

P NMR resonance at -15 ppm and a sharp 27

Al NMR

singlet at -25 ppm. These data are consistent with the formulation of the product 3.2 as the salt

[Mes3PI][AlI4]. This was confirmed by a crystallographic study of single crystals obtained from

independently synthesized 3.2 by combining PMes3, AlI3, and I2. The structure is shown in

Figure 3.3. The metric parameters of this salt are unexceptional.

50


clarity. C: black; P: orange; Al; teal; I: pink.

The capture of oxide and consequent formation of the Al–O–Al fragment in 3.1, infer the

reduction of CO2 to CO. Infrared spectroscopy of the headspace gas revealed an absorption

centred at 2143 cm-1

, confirming CO is produced in this reaction (Figure 3.4).191

When

employing 13

CO2, the reaction was shown to generate a new peak at 184.5 ppm in the 13

C NMR

spectrum while the corresponding headspace gas gave rise to an absorption centred at 2096 cm-1

.

These data are the spectroscopic signatures of 13

CO and this was confirmed by comparison to an

authentic sample. The liberated CO was captured by exposure of the head gas to a solution of

[Cp*RuCl(PCy3)],

132 prompting the formation of [Cp

*Ru(CO)Cl(PCy3)] in 80% yield based on

the stoichiometry in which one equivalent of CO arises from reaction of two equivalents of

phosphine.191

Collectively, these data imply that the reaction of two equivalents of phosphine

and CO2 react with approximately four equivalents of AlI3 (~3.8 equiv.) to produce one

equivalent each of 3.1, 3.2, and CO (Scheme 3.2). The reduction of CO2 to CO and the oxide that

is incorporated in 3.1 is concurrent with formal oxidation of phosphine to iodophosphonium,

generating the salt 3.2.

51

05001000150020002500300035000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Wavenumbers [1/cm]

Tra

nsm

itta

nce

Wavenumbers (cm-1)

Tra

nsm

itta

nce

2172

2115

05001000150020002500300035000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Wavenumbers [1/cm]

Tra

nsm

itta

nce

Wavenumbers (cm-1)

Tra

nsm

itta

nce

2172

2115

Figure 3.4 – Representative spectra of the analyzed headspace gas (black) from the reaction of

PMes3 + 1.9 AlI3 + CO2 after 16 h compared to an authentic spectrum (red) of CO.

Scheme 3.2 – Reduction of CO2 to CO using PMes3/AlX3 FLPs (X = Br, I).

The analogous reaction of PMes3 and AlBr3 was also performed. In a similar fashion exposure of

the solution to CO2 for 2 days at r.t. afforded a mixture of two products, 3.3 and 3.4, in a 1:1

ratio. Vapour diffusion of cyclohexane into the bromobenzene solution afforded crystals of

Mes3P(C(OAlBr2)2O)(AlBr3) (3.3). This product exhibited a 31

P NMR signal at 20 ppm while a

broad 27

Al NMR resonance was observed at 88 ppm. Employing 13

CO2, the species [13

C]-3.3 was

prepared and a resonance at 176.8 ppm with a 1JC-P coupling constant of 123 Hz was seen in the

13C NMR spectrum. The second product 3.4 was confirmed to be the salt [Mes3PBr][AlBr4] by

independent synthesis from PMes3, AlBr3, and Br2. This latter species gave rise to a 31

P NMR

shift at 39 ppm and an 27

Al NMR signal at 81 ppm. Compounds 3.3 and 3.4 were also

characterized crystallographically and were found to be analogous to 3.1 and 3.2.191

In the case

of 3.3, the geometry was almost identical to that described above for 3.1, with the Al–O bond

lengths to the PCO2 fragment being 1.837(3) and 1.840(3) Å, while the Al–O bond lengths in the

52

Br2AlOAlBr2 fragment are 1.792(3) and 1.795(3) Å. While spectroscopic data suggested the

formation of the chloride analogues may occur, the prolonged heating of PMes3 and AlCl3 under

CO2 led to a mixture of products that were not separable. Possible reasons for this reduced

reactivity will be discussed later.

3.2.2 Reaction of R3P(CO2)(AlX3)2 (X = halide or C6F5) with CO2

Efforts to garner some insight into the mechanism of CO2 reduction were undertaken.

Monitoring solutions of PMes3 and AlX3 (X = Br or I) by 31

P{1H} NMR spectroscopy showed

only a broad resonance attributable to the formation of weak donor-acceptor adducts in rapid

exchange with excess AlX3. Upon exposure to CO2, there is rapid and near quantitative

generation of the initial species Mes3P(CO2)(AlX3)2 as the only major product. Subsequent

monitoring of these reactions over time showed the decline of the resonances from

Mes3P(CO2)(AlX3)2 and the appearance of the peaks resulting from the corresponding products

3.1/3.2 and 3.3/3.4 (Figure 3.5).

Figure 3.5 – 31

P{1H} NMR spectra of a r.t. J-Young solution of PMes3 + 1.9 AlBr3 under N2

(top), after initial CO2 addition (middle), and after 48 h under an atmosphere of CO2 (bottom)

showing the disappearance of 2.5 and the appearance of 3.3 and 3.4.

53

These spectral data seem to imply that the species 2.6 and 2.5 are intermediates en route to

3.1/3.2 and 3.3/3.4, respectively. This suggests that using the isolated compounds 2.5 and 2.6

should lead to the same products. To our surprise, it was found that these isolated compounds

were virtually unreactive at r.t. These were only found to undergo reduction at elevated

temperatures under an atmosphere of CO2. Higher CO2 pressure also accelerated this reaction

(vide infra). As also noted in Figure 3.5 and in the previous section, a sub-stoichiometric amount

of AlX3 was needed to facilitate the in situ reaction. Indeed, altering the equivalents from 1.9 to

2.0 to 2.1 equiv. of freshly sublimed AlX3 with 1 equiv. PMes3 under CO2 (1 atm) resulted in

decreased product formation. While the 1.9 equiv. reaction showed complete conversion after

13 h in bromobenzene for AlI3, the 2.0 and 2.1 equiv. reactions were approximately 60% and

33% complete, respectively, as observed by 31

P NMR spectroscopy. Similar relative conversions

occurred with AlBr3, but with longer reaction times.

The reaction rate also appears to be solvent dependant. Whereas the in situ AlI3 reduction is

complete in 13 h in bromobenzene, the reaction takes ca. 4 days in fluorobenzene. In addition,

whereas isolated 2.6 is unreactive under CO2 in fluorobenzene at r.t., the reaction does proceed

slowly in iodobenzene over 5 days to produce 3.1, 3.2, and CO. The solvent impact on the rate

follows as fluorobenzene < bromobenzene < iodobenzene, perhaps signaling the importance of

the increasing Lewis basicity at the halide centre (I > Br > F).

The role of a Lewis basic solvent in the initial activation of CO2 by PMes3/AlCl3 FLPs was

recently probed computationally92

and discussed in Chapter 2. Here, it was found that

bromobenzene plays an important role in favouring the dissociation of AlCl3 from PMes3 (in 2.1)

by forming a weak bromobenzene adduct with AlCl3, allowing it to then capture CO2 and

generate 2.4. A similar solvent-induced dissociation of AlX3 (X = Br, I) may be at play in

complexes 2.5 and 2.6. While attempts to isolate a bromobenzene adduct of these acids failed, a

bromobenzene adduct of Al(C6F5)3 to generate Al(C6F5)3(C6H5Br)2 (3.5) was obtained (Figure

3.6). The coordination of 2 bromobenzene molecules to the bulky Al(C6F5)3 centre suggests that

a similar coordination mode to the less bulky AlX3 centres is likely and may play an important

role in dissociating these fragments from 2.5 or 2.6. However, it should be noted that possible π

interactions, which may exist in 3.5 between the phenyl ring of bromobenzene and the C6F5 rings

of Al(C6F5)3, would not be present with AlX3.

54


clarity. C: black; Al; teal; F: pink; Br: scarlet.

The results obtained thus far suggest that dissociation of AlX3 from 2.5 or 2.6 – aided by the

presence of a weakly Lewis basic solvent and/or through the application of heat – may be the

initial step in the reduction of CO2 to CO. Dissociation of the more Lewis acidic192

AlBr3 and

AlCl3 from 2.5 and 2.4, respectively, is expected to be more difficult, and may explain the more

sluggish nature of the reduction using these compounds.

In order to garner more information on this reaction, the analogous CO2 complex,

(otol)3P(CO2)(Al(C6F5)3)2 (2.17), reported in Chapter 2, was used in order to gain an NMR

handle at the Lewis acidic end. Considering the Lewis acidity of Al(C6F5)3 is calculated to be

comparable to AlCl3,193

a slower reaction rate may be expected with CO2. Exposure of a

J-Young NMR solution of the 13

CO2-labelled compound [13

C]-2.17 in C6D5Br to 1 atm of 13

CO2

at r.t. did not lead to any discernible reaction; however, heating the reaction mixture to 90 ⁰C for

2 days led to the gradual disappearance of the doublet (1JP-C = 128 Hz) at 30 ppm in the

31P{

1H}

NMR attributable to [13

C]-2.17, and the appearance of a new doublet (1JP-C = 121 Hz) at 12 ppm.

This change was concomitant with the disappearance of the P–CO2 peak at 167.3 ppm for 2.17 in

the 13

C NMR and the appearance of 2 new peaks: a singlet at 168 ppm and a doublet at 160 ppm

(1JC-P = 121 Hz), indicative of a new P–CO2 species. The doublets in the

31P and

13C NMR

spectra have very similar shifts and coupling constants to a 1:1 solution of P(otol)3:Al(C6F5)3

exposed to an atmosphere of 13

CO2 in a J-Young NMR tube. The 19

F resonances are also the

same and this suggests the formation of the 1:1 species (otol)3P(CO2)Al(C6F5)3, although

55

attempts to isolate this were unsuccessful. However, a 1:1 species could be isolated using the

very bulky super Lewis acid, Al(OC(CF3)3)3, initially reported by Krossing.194

The compound

(otol)3P(CO2)Al(OC(CF3)3)3 (3.6) was prepared by exposing a 1:1 solution of

P(otol)3:Al(OC(CF3)3)3 to CO2 followed by recrystallization.191

Single crystals suitable for X-ray

diffraction were obtained and the structure is shown in Figure 3.7. While the metric parameters

are unexceptional, importantly, the 31

P and 13

C (for CO2) NMR resonances of this compound are

analogous to those for the assumed 1:1 species (otol)3P(CO2)Al(C6F5)3.


clarity. C: black; P: orange; Al; teal; F: pink; O: red.

In addition to the (otol)3P(CO2)Al(C6F5)3 product obtained after heating 2.17 under CO2, the

second product contained 6 peaks in the 19

F NMR spectrum – 2 sets of 3 peaks integrating in a

2:1 ratio. This single species was independently prepared by the insertion reaction of CO2 into

Al(C6F5)3 yielding the symmetric dimeric species [(C6F5)C(O)OAl(C6F5)2]2 (3.7) which was

shown to have a singlet in the 13

C NMR attributable to the CO2 moiety at 168 ppm, consistent

with the above results. The solid state structure of 3.7 is shown in Figure 3.8. The CO2 moieties

in 3.7 are bound inequivalently to the Al centres. While the C1–O1 and C2–O2 bond lengths are

nearly equivalent at 1.2573(17) Å and 1.2617(18) Å, the resulting O–Al bond lengths are slightly

different at 1.7730(12) Å and 1.8141(12) Å, respectively. The bond angles are also different with

a more bent angle of 135.14(10)⁰ for C1–O1–Al1 and a more linear angle of 160.41(11)⁰ for C2–

O2–Al2. The bridging of 2 Al centres by a carboxylate is a common motif in Al

56

chemistry.146,195,196

The insertion of CO2 into Al–C bonds has also been frequently reported,

although mainly with alkyl- and not aryl-aluminum compounds.196-198

Figure 3.8 – POV-Ray depiction of the molecular structure of 3.7. C: black; Al; teal; F: pink; O:

red.

The data presented in this section using the isolated complexes Mes3P(CO2)(AlX3)2 (2.5 or 2.6)

in concert with the insertion chemistry shown using the variant (otol)3P(CO2)(Al(C6F5)3)2 (2.17)

suggests a reaction mechanism involving dissociation of an AlX3 fragment followed by insertion

of CO2. The following section will further probe the importance of this step, along with the

possible subsequent steps of the mechanism.

3.2.3 Mechanistic study of the reduction of CO2 to CO using AlX3

3.2.3.1 Kinetic study

A kinetic study using Mes3P(CO2)(AlI3)2 (2.6) was initiated in order to gain more information on

the mechanism of the reduction of CO2 to CO. The reaction was found to be first order in both

CO2 pressure and 2.6 (second order overall).191

After an exhaustive search, Henry’s Law

constant for CO2 solubility in bromobenzene was not found; however, its solubility could be

estimated using Raoult’s Law based on a known mole fraction of 0.0079 for CO2 in

bromobenzene at 1.0 atm.199

Using a known volume (0.50 mL) and, thus, moles of

bromobenzene, we were able to estimate the moles of dissolved CO2, in turn establishing its

57

concentration as approximately 0.38 M when the pressure is 5.0 atm. This provides an

approximate 50-fold excess of CO2 compared to 2.6, allowing for study of the system under

pseudo-first order conditions. Solutions of 2.6 were prepared and allowed to stand under CO2 in

a J-Young NMR tube. The samples were monitored by timed 31

P{1H} NMR experiments over a

30 K range (303-333 K) allowing for the determination of the activation parameters (Figure 3.9):

H≠ = 85(5) kJ/mol and S

≠ = -45(15) J/(mol•K).

Figure 3.9 – Stacked time-dependent 31

P{1H} NMR spectra at 40 °C of the reaction of 2.6 (7.2

mM) with 5 atm CO2 (~ 0.38 M) with peaks at 22 ppm (2.6) and 19.5 ppm (3.1) (top left).

Representative plot of the first order decay of 2.6 at 60 °C (top right) and corresponding Eyring

plot (bottom) analyzed using the equation ln(k/T) = (-≠R)(1/T) + (ln(kB/h) + (S

≠R)) with k

= rate constant, T = temperature, R = gas constant, kB = Boltzmann’s constant, h = Planck’s

constant.191

58

The results obtained from the Eyring analysis suggest an associative mechanism based on the

negative activation entropy value. Furthermore, the dependence on CO2 pressure is indicative of

a rate-determining step involving CO2. Combined with the CO2 insertion chemistry with

Al(C6F5)3 above, we propose that the reduction of CO2 to CO using AlX3 follows an initial rate-

determining CO2 insertion step into the Al–X bond generating the intermediate A (Scheme 3.3).

Scheme 3.3 – Proposed rate-determining step involving CO2 insertion in Al–X bonds (X = Br, I)

to generate A following its dissociation from Mes3P(CO2)(AlX3)2.

3.2.3.2 Rate inhibition using cyclohexene

In order to support the proposed rate-determining CO2 insertion step, we set out to investigate the

role of weakly coordinating donors in inhibiting the insertion reaction. Cyclohexene was used as

it was found to form a weak dative bond with Al(C6F5)3 as evidenced by the crystal structure of

Al(C6F5)3•(η2-C6H10) (5.6) shown in Chapter 5. Unsuccessful attempts were made to isolate a

similar structure with AlX3; however, we were able to gain spectroscopic evidence for the

interaction of cyclohexene with AlX3. For example, treatment of a cyclohexene solution in

bromobenzene to 20% AlBr3 led to the downfield shift of the olefinic protons from 5.63 ppm to

5.80 ppm, indicative of its coordination to Al. This also suggests stronger coordination of the

dilute cyclohexene to Al than bromobenzene (Figure 3.6).

With this in mind, we conducted a series of kinetic experiments to determine if cyclohexene

would play an inhibiting role by coordinating to the presumed free AlX3 in the reduction

chemistry of CO2 to CO (Scheme 3.3). An inverse first-order dependence on cyclohexene was

determined191

supporting competitive binding of cyclohexene over CO2 to AlX3 and further

supporting the rate-determining CO2 insertion step (Scheme 3.4).

59

Scheme 3.4 – Proposed competitive coordination of cyclohexene vs. CO2 to AlX3, as suggested

by the inverse first-order dependence on cyclohexene. The subsequent proposed rate-determining

insertion step following coordination of CO2 is shown in the box.

3.2.3.3 Labelling experiments

To further probe this reaction, we undertook labelling experiments using natural abundance CO2

(denoted 12

CO2), as well as 13

C-enriched CO2 (denoted 13

CO2). The reaction of AlX3 (X = Cl, Br,

I) with 13

CO2 in bromobenzene leads to little reaction at r.t. as observed by 13

C NMR

spectroscopy; however, increasing the temperature leads to the formation of several by-products

likely the result of Friedel Crafts-type carboxylation chemistry with the solvent.53

Increasing the

temperature accelerates this decomposition reaction; however, little useful information was

gained here.

In studying the reduction chemistry with 2.6, it was found that mixing it with a source of the

[AlI4]- anion (typically in the form of [Mes3PMe][AlI4] (3.8)) greatly enhanced its solubility in

bromobenzene. This was attributed to possible I- exchange from [AlI4]

- to AlI3 and/or

coordination of [AlI4]- to AlI3 generating the weakly bridged species [I3Al(µ-I)AlI3]

-. In either

case, significant broadening of the sharp 27

Al NMR resonance for 3.8 was observed upon mixing

with 2.6, and a subsequent accelerated rate of reaction was also observed. Enhanced solubility

and increased rates were also observed in the in situ protocol above and a possible explanation

will be given later. In either case, using the isolated compound 2.6, combinations of 2.6/3.8, or

the in situ protocol, resulted in analogous results in the following labelling experiments. Thus,

treating [12

C]-2.6 (with 3.8) to different pressures of 13

CO2 resulted in a slower reaction rate due

to the rate dependence on CO2 at lower pressures, as described above. However, in addition to a

decreased rate, increased scrambling of the CO2 moiety from 12

CO2 to 13

CO2 in the reactant

(2.6), and consequently in the product (3.1), was observed at lower pressures of CO2 (Figure

60

3.10). This unusual increase in scrambling at lower CO2 pressures is unexpected and an

explanation for this observation is presented in the proposed mechanism section (vide infra).

Figure 3.10 – 31

P{1H} NMR spectra of the reaction of [

13C]-2.6 (0.027 M) (with added 3.8

(0.080 M)) in C6D5Br with approximately: a) 0.5; b) 1.0; c) 2.0; d) 3.0 atm 12

CO2 to produce 3.1

after 8 h total reaction time in a J-Young NMR tube. The results indicate reduced scrambling of

13CO2 (doublets) for

12CO2 (singlets) with increasing pressures. It is noteworthy that c and d

show similar rates indicating the point at which there is no longer a dependence on CO2 pressure

(pseudo first-order conditions).

In addition to this NMR data, it was found that at high pressures only exogenous CO2 (from the

added atmosphere) is reduced to CO as confirmed by FT-IR analysis of the headspace gas for

12CO vs.

13CO (2143 cm

-1 vs. 2096 cm

-1, respectively), as well as

13C NMR analysis when

13CO2

is reduced to 13

CO. Thus, if [12

C]-2.6 is treated to 13

CO2 (5 atm), the products of reduction are

[12

C]-3.1, 13

CO, and 3.2. The reverse reaction using [13

C]-2.6 treated to 12

CO2 also leads to the

reversely labelled products, [13

C]-3.1, 12

CO, and 3.2.

61

3.2.3.4 (otol)3P(CO2)(AlI3)2: Rate inhibition due to strong P–Al bonding

While the experiments thus far indicate a rate-determining CO2 insertion step, the subsequent

steps in the mechanism also need to be determined or proposed. Thus, we propose that the

intermediate A, generated by the insertion of CO2 into the Al–X bond, undergoes nucleophilic

attack resulting in the expulsion of CO and the generation of the oxo-bridged compound C

(Scheme 3.5 box). Two types of nucleophilic attack are possible: the first involves nucleophilic

attack of the phosphine at the halogen atom (blue path), while the second involves nucleophilic

attack at the carbonyl centre en route to the phosphonium halide product (red path). The red

pathway is more typical of classical organic chemistry involving nucleophilic attack at an acyl

centre generating an acylphosphonium species. Alkylacylphosphoniums, [R3PC(O)R']+, are

known and can be prepared by the reaction of a phosphine with an alkylacyl halide, XC(O)R'.200

However haloacylphosphoniums, [R3PC(O)X]+, have not been isolated, but are proposed as

intermediates en route to CO loss and generation of the halophosphonium cation, [R3PX]+, when

reacting some phosphines with phosgene.201,202

Alternatively, computational studies indicate that

the blue pathway involving direct attack of the phosphine at the polarized halogen atom, due to

the very bulky nature of PMes3, is possible and perhaps even more likely (vide infra).

Unfortunately, either nucleophilic pathway resulting in CO evolution proved impossible to

demonstrate experimentally due to the inability to synthesize the intermediate A; however,

experimental support could be obtained for the first portion of this step (Scheme 3.5 left).

Scheme 3.5 – Proposed equilibrium involving the 1:1 species R3P(CO2)(AlX3) and the FLP

PR3/AlX3. The resulting free PR3 is available to attack the generated intermediate A resulting in

CO expulsion and generation of the compound C. The boxed segment represents proposed

pathways.

62

In our initial explorations of CO2 activation using P/Al FLPs, we also found that combinations of

P(otol)3/AlI3 were capable of activating CO2. In stark contrast to the rapid r.t. activation to

produce 2.6, the generation of the analogous species (otol)3P(CO2)(AlI3)2 (3.9) required heating

the P(otol)3/AlI3 solution to 80 °C for 1 h under an atmosphere of CO2. Single crystals suitable

for X-ray diffraction were obtained and the structure is shown in Figure 3.11. While the structure

is largely analogous to 2.6, the P–CO2 bond length in 3.9 is shorter (1.877(4) Å) than 2.6

(1.903(8) Å). The O–C–O angle in 3.9 is also slightly larger (127.4(4)°) than 2.6 (125.0(8)°).

Both these metric parameters reflect the reduced steric bulk around P(otol)3 vs. PMes3 (Tolman

cone angles of 196° and 212°, respectively).143

Figure 3.11 – POV-Ray depiction of the molecular structure of 3.9. C: black; P: orange; Al: teal;

I: pink; O: red. H atoms are omitted for clarity.

The impact of reduced bulk around the phosphine can also be seen in the isolated adducts of

Mes3P–AlI3 (2.3) and (otol)3P–AlI3 (3.10). This latter adduct was synthesized in an analogous

manner to 2.3. Interestingly, while both displayed broad 31

P NMR resonances due to coupling to

the quadrupolar (I = 5/2) 27

Al nucleus, the spectrum for 2.3 displayed a second broad resonance

centred at -35 ppm indicative of free PMes3. This suggests an equilibrium mixture is present

between the adduct (2.3) and free phosphine and AlI3. This equilibrium has a G value of 8.2

kJ/mol with an equilibrium constant of 3.72•10-2

M at 25 °C. Monitoring the solution over a 30

K range (298-328 K) allowed for the determination of the thermodynamic parameters (Figure

63

3.12): H = 42(1) kJ/mol and S 113(3) J/(K•mol). These data are consistent with an

equilibrium mixture favouring the adduct 2.3. Indeed at 25 °C, only 16% dissociation of the

adduct is observed and this increases to 30% at 55 °C. This dissociation is significantly reduced

using the stronger Lewis acids AlCl3 and AlBr3 in compounds 2.1 and 2.2, respectively. In

contrast to the trend for 2.3, the adduct 3.10 does not display a 31

P NMR resonance attributable

to free P(otol)3 over the same temperature range. This is indicative of a stronger adduct present

and is attributed to the reduced steric congestion around the P centre. While PMes3 is expected to

be a stronger Lewis base than P(otol)3 due to its electron rich mesityl rings, its steric bulk likely

prevents it from forming as strong an adduct with AlI3.

Figure 3.12 – Plot of G vs. T allowing for the determination of H and S using

G = H - TS. G values were obtained using the relation G = -RT•ln(K). The equilibrium

constant (K) was obtained by relative integration of the 31

P{1H} NMR resonances for 2.3 and

PMes3 (= [AlI3]) in bromobenzene between 298-328 K.

Unlike 2.6 which reduces exogenous CO2 (5 atm) to CO in approximately 1 h at 60 °C, a similar

reaction only occurs with 3.9 upon heating to 90 °C for several days. Furthermore, as with 2.6,

addition of excess [Mes3PMe][AlI4] (3.8) accelerates the reaction of 3.9 with CO2. While CO2

reduction using a 1:3 solution of 2.6:3.8 is complete in 6 h at r.t., the analogous reduction of a

1:3 solution of [12

C]-3.9:3.8 under 13

CO2 takes 5 days to complete at r.t. The generation of 13

CO

is observed here by 13

C NMR spectroscopy. Two new peaks in the 31

P NMR spectrum at 25 and

-10 ppm in an approximate 1:1 ratio are also present and follow the gradual disappearance of 3.9.

The peak at 25 ppm is slightly upfield of the starting material 3.9 (27 ppm) and splits into a 1JP-C

64

doublet when the [13

C]-3.9 starting material is used. The peak at -10 ppm is indicative of the

[(otol)3PI]+ cation.

203 Two

27Al resonances are also observed in the

27Al NMR spectrum and have

the same shifts as those in 3.1 and 3.2. While attempts to separate these products failed, these

spectroscopic patterns are similar to the 2 products (3.1 and 3.2) obtained from the reduction

using 2.6. Thus, we believe that the P(otol)3 analogs of 3.1 and 3.2 are formed here, along with

the observed CO.

The significantly higher barrier for the reduction of CO2 to CO using P(otol)3/AlI3 compared to

PMes3/AlI3 FLPs is consistent with the stronger P–Al adducts formed using P(otol)3 vs. PMes3 as

described in this section. The stronger adduct (3.10) limits the amount of free phosphine

available in solution for further nucleophilic reaction with the proposed intermediate A (Scheme

3.5). In contrast, using PMes3 provides a weaker adduct, allowing for free PMes3 to readily react

with A as it is formed.

3.2.3.5 Proposed mechanism

Based on all the experimental data presented in the last several sections, we propose a general

reaction mechanism shown in Scheme 3.6.

65

Scheme 3.6 – Proposed mechanism for the reduction of 12

CO2 (blue) to 12

CO using a generic

13C-labelled (red) R3P(CO2)(AlX3)2 species to produce [

13C]-3.1 and 3.2 (when R = Mes; X = I).

Cyclohexene inhibition is shown in the top right box. Scrambling of [13

C]-2.6 to [12

C]-2.6 when

12CO2 (blue) is limited, resulting in a hindered rate-determining step (green) to produce A, is

shown in the left box. Note also that under experimental conditions [12

CO2] ~ 50 x [13

CO2].

The proposed mechanism is consistent with all the experimental detail outlined in the past

several sections of this chapter. First, the proposed equilibrium a is consistent with the

observation that excess AlX3 (from 1.9 to 2.1 equiv.) significantly hampers the reduction

chemistry by pushing the equilibrium more to the left. The solvent also plays an important role in

dissociating AlX3 from the R3P(CO2)(AlX3)2 as evidenced by the coordinating ability of

bromobenzene to Al centres, such as in the structure of Al(C6F5)3(C6H5Br)2 (3.5; Figure 3.6).

The increased reactivity from fluoro- to bromo- to iodo-benzene also suggests that the increasing

coordinating ability of these solvents is important. In a similar fashion, it is believed that addition

of the [AlI4]- source (3.8) plays a similar role by facilitating the dissociation of AlX3 from the

starting material. The equilibrium a is also supported by the observation that the analogous

reduction chemistry using AlBr3 (to produce 3.3 and 3.4) and AlCl3 (products not isolated) is

66

significantly slower, likely due to the increased Lewis acidity of these species192

preventing

dissociation; although, additional reasons for this reduced reactivity may also exist (vide infra).

Finally, the proposed equilibrium a and subsequent insertion step c are consistent with the

experiments done with (otol)3P(CO2)(Al(C6F5)3)2 (2.17) to generate the inserted dimer product

[(C6F5)C(O)OAl(C6F5)2]2 (3.7) and the proposed 1:1 species (otol)3P(CO2)Al(C6F5)3, which is

spectroscopically similar to the isolated 1:1 compound (otol)3P(CO2)Al(OC(CF3)3)3 (3.6).

Second, the kinetics experiments further support the mechanism outlined here. A first order

dependence was found on both 2.6 and CO2 pressure (second order overall). The dependence on

the former is consistent with the generation of AlX3 by equilibrium a. The dependence on CO2 is

consistent with a rate-determining CO2 insertion step into the Al–X bond to generate the

intermediate A, similar to the isolated compound 3.7 when using Al(C6F5)3. The negative

activation entropy value of -45(15) J/(mol•K) derived from the Eyring plot further supports this

associative rate-determining step involving CO2 and AlX3. Furthermore, the inverse first-order

dependence on cyclohexene suggests a competitive coordination with CO2 to the Al centre,

significantly hampering the proposed rate-determining step (Scheme 3.4).

Third, the proposed nucleophilic attack of the phosphine at A demands the availability of this

phosphine in solution. As evidenced by 31

P NMR spectroscopy, it was found that the adduct

Mes3P–AlI3 (2.3) is in equilibrium with its free components, PMes3 and AlI3 in bromobenzene.

This is in contrast to the adduct (otol)3P–AlI3 (3.10) which does not form an appreciable

equilibrium. It was found that whereas the parent CO2 compound Mes3P(CO2)(AlI3)2 (2.6)

readily reduces exogenous CO2 to CO, the related compound, (otol)3P(CO2)(AlI3)2 (3.9), requires

much more forceful conditions. This supports the stronger adduct formation, governed by

equilibrium b2, precluding the availability of this P(otol)3 compared to PMes3 to undergo

nucleophilic attack at A.

Fourth, labelling experiments also support the mechanism outlined here where exogenous CO2 is

reduced to CO. The unusual decrease in scrambling at elevated pressures, combined with the

observed first-order dependence on CO2 supports the presence of competitive reactions governed

by the equilibria b/b2, as well as the rate-determining step c. It is proposed that at lower

pressures, the intermediate A is limiting ([PMes3] > [A]) allowing the free PMes3 to recombine

with AlX3 and exogenous 12

CO2 (blue), generating the scrambled product [12

C]-2.6 (left box of

67

Scheme 3.6). However, at elevated 12

CO2 pressure, the free PMes3 becomes the limiting reagent

relative to A ([PMes3] < [A]) allowing for an irreversible nucleophilic attack to generate

[Mes3PX]+ (path d) and reduce the exogenous

12CO2 to

12CO generating the oxo-bridged anion

[X2AlOAlX3]- (path e). The final step involves a proposed exchange of this chelating anion

[X2AlOAlX3]- from C with the two AlX3 from [

13C]-2.6 generating the unscrambled product

[13

C]-3.1 and 3.2.

Finally, while attempts were made to prove the nucleophilic attack at A experimentally, the

synthesis of such a compound proved impossible. Attempts were also made to synthesize an oxo

anion of the type [X2AlOAlX3]-, but were also unsuccessful. We therefore turned to theoretical

calculations to determine the likelihood of these last steps (vide infra). In summary, however, we

believe that the substantial amount of experimental evidence presented thus far supports the

proposed mechanism of Scheme 3.6.

3.2.3.6 The in situ reduction conundrum

As described earlier in this chapter, in our initial discovery of the reduction of CO2 to CO using

PMes3/AlX3 FLPs (X = Br, I), we performed the reaction in situ using a sub-stoichiometric

amount of AlX3 (1.9 equiv.). While the r.t. reaction appeared to go through the intermediate

formation of the Mes3P(CO2)(AlX3)2 species (2.5 or 2.6; Figure 3.5) en route to 3.3/3.4 and

3.1/3.2, respectively, treating the isolated compounds 2.5 or 2.6 to the same reaction conditions

at r.t. led to insignificant product formation in the same amount of time. It was only upon heating

2.5 or 2.6 that the reactions proceeded to completion. Furthermore, while the in situ reaction

involved a formal 1:1.9 PMes3:AlX3 stoichiometry, pre-treating the isolated complexes (2.5 or

2.6) (0.95 equiv.) to 0.05 equiv. PMes3, forming the same 1:1.9 ratio of PMes3:AlX3 did not lead

to a similar rate as the in situ generated system. In fact, the rate here was further reduced as

compared to using the isolated compound. This is consistent with the equilibria –b2/-b/-a of

Scheme 3.6 wherein addition of PMes3 pushes the equilibrium back to the starting material

Mes3P(CO2)(AlX3)2. While a kinetic study was not performed on the in situ system due to the

difficulty in precisely measuring exact stoichiometry for every run, a qualitative picture can be

gained by considering that the r.t. in situ reduction of CO2 to CO using a 1:1.9 mixture of

PMes3:AlI3 forms the products 3.1 and 3.2 is ca. 12-14 h under only 1 atm of CO2. In contrast the

isolated reaction using 2.6 under excess CO2 (5 atm) takes at least 5 days to complete at r.t.

68

These results raise questions as to the differences involved in the in situ vs. the isolated

mechanisms. We postulate that the same mechanism is at play in both cases. Both cases were

found to reduce only exogenous CO2 at high pressures. Therefore, we believe that the differences

in rate are due to competing thermodynamic vs. kinetic reaction pathways (Scheme 3.7). Both

experimental and computational (vide infra) methods indicate that 3.1/3.2 from 2.6 or 3.3/3.4

from 2.5 are the thermodynamic products. However, as observed by monitoring the in situ

reaction, the CO2 compounds 2.5 or 2.6 are clearly the major kinetic products being initially

almost quantitatively formed as observed by 31

P NMR spectroscopy (see Figure 3.5). Thus, we

propose that a small part of the initial FLP solution reacts directly to form the thermodynamic

products 3.1/3.2 or 3.3/3.4 through the intermediate formation of A (Scheme 3.6). This would

then deplete the free FLP solution (PMes3/AlX3, centre of Scheme 3.7) pushing the dissociation

equilibrium from Mes3P(CO2)(AlX3)2 to the left to regenerate some of this FLP solution

(Scheme 3.7 box). Using a sub-stoichiometric amount of AlX3 also pushes this equilibrium to the

left. In contrast, with the isolated system we begin completely on the right side of the

equilibrium, requiring more time to generate the FLP constituents needed to form the

thermodynamic products. This results in a reduced reaction rate which can be accelerated with a

more Lewis basic solvent, the addition of [AlI4]-, and/or heating to overcome the kinetic barriers

proposed in Scheme 3.7 (from right to left).

69

Scheme 3.7 – Proposed thermodynamic vs. kinetic reaction pathways of PMes3/AlX3 FLPs with

CO2. The box describes the proposed intermediate equilibrium mixture en route to the

thermodynamic products.

These proposed divergent reaction pathways are, at least in part, consistent with NMR

observations of the appearance of the final thermodynamic products very early on in the in situ

reduction chemistry as shown in Figure 3.5 (middle spectrum). Furthermore, as noted earlier, a

complex equilibrium mixture involving these kinetic, thermodynamic, and FLP components

likely contributes to the significantly enhanced solubility (an order of magnitude higher)

observed for the in situ reduction as compared to the isolated starting materials, 2.5 or 2.6.

3.2.4 Theoretical investigation

As described, many of the proposed steps could not be investigated experimentally. Therefore, in

order to establish the likelihood of these (Scheme 3.6), we collaborated with Prof. Thomas M.

Gilbert from Northern Illinois University to undertake a theoretical study of some of the key

steps of the mechanism. Some of these results are presented here.

70

3.2.4.1 CO2 insertion into Al–X bonds and CO formation

A key premise of the proposed mechanism involves the insertion of CO2 into the Al–X bond of

AlX3. This proposal is based on the observed chemistry with Al(C6F5)3 to form 3.7, as well as the

observed rate inhibition caused by the addition of cyclohexene. The barrier to insertion of CO2

into AlI3 (path c of Scheme 3.6) was estimated using scans of the potential energy surface at the

M06-2X(SCRF) level and found to be 75 kJ/mol, consistent with the experimental barrier of

85(5) kJ/mol based on the Eyring plot. The reaction energies were calculated at the

OG2R3(SCRF) level and the reaction was found to be essentially thermoneutral (4 kJ/mol);

however, coordination of the second AlI3 to form A rendered this reaction highly exothermic

(-145 kJ/mol). In contrast to this, the barrier for CO2 insertion into AlCl3 was also probed and

estimated to be smaller (40 kJ/mol); however, the overall reaction energy to form the species

Cl2AlOC(O)Cl was higher (36 kJ/mol) compared to 4 kJ/mol for AlI3. This data supports the

insertion chemistry observed with AlX3 and is also consistent with the observed favourable

reaction with AlI3 compared to AlCl3.

As proposed in Scheme 3.5, two separate reaction pathways were postulated involving free

PMes3 and the intermediate compound A. In either case, the mechanism of attack of the

phosphine at either the carbonyl or the halide centres would be challenging to model well

computationally as this involves the formation of ion pairs from neutral compounds in a low

polarity solvent. Nonetheless, both were probed computationally. For the red pathway (Scheme

3.5), the decomposition of the acylphosphonium cation, [Mes3PC(O)I]+, into [Mes3PI]

+ and CO

was calculated at the OG2R3(SCRF) level ([AlI4]- was used as the counteranion for simplicity).

The reaction is estimated to be endothermic (46 kJ/mol) with bromobenzene as implicit solvent

(54 kJ/mol in the gas phase), with a reaction barrier in excess of 150 kJ/mol. Similar values are

obtained at the M06-2X(SCRF) level. In stark contrast, the anion of B (Scheme 3.5, blue

pathway), generated from attack at the halide, was found to eject CO in a barrierless and

exothermic (50 kJ/mol) process at the M06-2X(SCRF) level to generate the oxo anion of C.

These results support the blue pathway as the more likely CO-evolving step and support steps d

and e of the proposed mechanism (Scheme 3.6).

71

3.2.4.2 Exchange of [X2AlOAlX3]- with the AlX3 of Mes3P(CO2)(AlX3)2

As stated earlier, attempts were made to independently synthesize the anion [X2AlOAlX3]-;

however, mixtures of products were obtained. Therefore, the last step of the mechanism,

involving the replacement of the two AlI3 groups of 2.6 by the chelating [I2AlOAlI3]- of C

(Scheme 3.6) was investigated theoretically at the M06-2X(SCRF) level. To simplify

calculations, the neutral fragment [I2AlOAlI2] was used which, in the presence of AlI3, is likely

in equilibrium with the anion. The oxo species was found to easily displace the two AlI3 in

sequential coordination steps of the µ-oxo to the AlI3 centres, followed by subsequent cyclization

to generate the thermodynamic product 3.1 with a remaining AlI3. This reaction was found to be

very favourable and exothermic with an energy of -99 kJ/mol.

3.2.4.3 Thermodynamic product formation

The overall thermodynamics for the conversion of 2.6 to 3.1, 3.2, and CO was investigated at the

OG2R3(SCRF)//ONI(M06-2x)(SCRF) level and found to have an overall exothermic value of

37 kJ/mol. This is consistent with the argument that the formation of 2.6 represents the kinetic

product en route to the thermodynamically favourable products 3.1, 3.2, and CO.

3.2.4.4 Competitive cyclohexene binding

As described earlier, an inverse first-order dependence on cyclohexene was found. We proposed

that this was due to competitive binding of cyclohexene vs. CO2 to the free AlX3 moiety, thereby

hindering the rate-determining insertion step to generate A (Scheme 3.4). Since we could not

observe the proposed η1 coordination of CO2 to AlX3 spectroscopically, we turned to

calculations. The results of calculations performed using the OG2R3(SCRF) approach indicate

that the (η2-C6H10)AlI3 complex is more stable by 34.0 kJ/mol than the (η

1-CO2)AlI3 complex in

bromobenzene solvent (43 kJ/mol in the gas phase). This supports the proposed competitive

coordination of cyclohexene to AlX3 proposed in our mechanism.

The theoretical calculations performed by Prof. Thomas M. Gilbert described here, combined

with the experimental detail of this chapter are both consistent with the proposed mechanism

outlined in Scheme 3.6. Nonetheless, the following section outlines a possible alternative open-

shell radical mechanism.

72

3.2.5 Alternative mechanism: Open-shell pathway?

Throughout our studies with the PMes3/AlX3 systems, we noticed that these mixtures formed

deep purple solutions in bromobenzene, similar to what has been reported for PMes3/B(C6F5)3

FLPs.81

This colour has been reported to arise from the trimesitylphosphoniumyl radical

cation,204-206

although no EPR signal had been reported with the B(C6F5)3 pairs. However,

monitoring solutions of PMes3/AlX3 (X = Br, I) by X-band EPR spectroscopy revealed the

presence of a doublet (g = 2.0056, aP = 239 G) consistent with literature reports of the Mes3P•+

radical (Figure 3.13).206

This doublet increased in intensity when 0.5 to 1.0 to 2.0 equiv. of AlX3

were used, but did not change with an excess of 2.0 equiv.

Figure 3.13 – X-band EPR spectrum of a bromobenzene solution of PMes3 + 2 AlBr3 (spectrum

in blue, simulation in red). Analogous signals are observed when using AlI3. Simulation was

done using PIP4Win v. 1.2.207

Interestingly, while these solutions are EPR active, they are also NMR active displaying broad

signals in the 31

P NMR spectrum, as well as the 1H and

27Al NMR indicative of the formation of

the adducts 2.2 or 2.3. Also surprisingly, the EPR spectrum did not show any other signals

indicative of a radical anion. This could either be due to a loss of the signal in the baseline due to

the quadrupolar 27

Al nucleus, a rapid reversible electron transfer from the phosphoniumyl cation,

73

and/or a disproportionation reaction generating bound diamagnetic Al(II) centres. Some of these

possibilities are shown in Scheme 3.8.

Scheme 3.8 – Possible reversible electron transfer and disproportionation reactions involving

PMes3/AlX3 FLPs. The formation of the CO2 complexes is also shown in the box.

An Al(II) halide species, (L)Br2Al–AlBr2(L) (L = anisole), has been reported by Schnöckel;

however, its reactivity has not been studied further.66

In contrast, an Al(II) species, R2Al–AlR2

(R = CH(SiMe3)2), has been reported by Uhl57

and its reactivity studied extensively.60,61,63,208

While no reaction with CO2 has been reported, the reaction with CS2 leads to the isolation of 2

species (D and E) as shown in Scheme 3.9.58

These species are analogous to the computed

intermediate (similar to D) and final products observed in the amido-digermyne chemistry

reported by Jones189

and described in the introduction (Scheme 3.1) for the reduction of CO2 to

CO. As outlined in Scheme 3.9, a possible reaction could be envisaged for the reduction

chemistry reported in this chapter (R = Br, I) to generate the X2AlOAlX2 key intermediate (vide

supra). The synthesis of adduct-free X2AlAlX2 (X = Cl, Br) starting materials has been reported

by high temperature or matrix conditions209

; however, these results have been discussed

critically.66

While we considered synthesizing these, further experiments suggested that a radical

reaction pathway may not be at play here.

Scheme 3.9 – Insertion of CE2 into the dialane (R = CH(SiMe3)2; E = S) to yield the products D

and E as reported by Uhl.58

Possible reaction pathway for the reduction of CO2 to CO if R = Br

or I and E = O for this chapter.

As determined by UV-Vis spectroscopy, diagnostic absorption bands for the Mes3P•+

radical are

found at 384 and 572 nm (see Chapter 6), and the amount of this radical was found to be stable

74

over time (Figure 3.14); unfortunately, this radical could not be isolated on its own and, thus, the

quantity of radical present in solution could not be determined since we could not obtain an

extinction coefficient. Nonetheless, addition of CO2 to this solution leads to the rapid

disappearance of the peaks attributable to the radical. In a similar fashion, monitoring solutions

of 2.6 under 5 atm of CO2 by X-Band EPR spectroscopy did not reveal the presence of any

radical intermediate during observations over 48 h. Interestingly, addition of 3 equiv. of the salt

[Mes3PMe][AlI4] (3.8) to 2.6 initially led to the generation of the doublet by EPR spectroscopy;

however, this doublet disappeared upon addition of the CO2 atmosphere. This result is consistent

with the proposed role of 3.8 in dissociating AlI3 from 2.6, favouring the formation of the free

constituents, PMes3 and AlI3, and thus catalyzing the reduction of CO2 to CO with free AlI3 (vide

supra). These FLP solutions are thought to give rise to the radical signal.

Figure 3.14 – Intensity of 384 nm (blue) and 572 nm (red) absorption bands over time (s) with

stirring upon mixing PMes3 (0.95 mM) with AlI3 (1.9 mM) in bromobenzene. Inset is a sample

spectrum of the reaction mixture.

Next, it was found that solutions of AlX3 (X = Br, I) with P(otol)3 are EPR inactive, consistent

with the higher oxidation potential of this phosphine.206

The related CO2 complex, 3.9, with

added 3.8 (3 equiv.) was also monitored by EPR spectroscopy before and after addition of CO2

and was found to exhibit no signals indicating the formation of radical species at any time.

75

While solutions of PMes3/AlX3 indicate the formation of some Mes3P•+ radical cation, the

disappearance of any radical signal as observed by EPR and UV-Vis spectroscopy upon addition

of CO2 to 2.6 or 2.6/3.8 is consistent with the equilibrium scenario of Scheme 3.8 and suggests

that an open-shell mechanism is perhaps unlikely. While a radical mechanism occurring faster

than the EPR timescale is possible, the lack of any radical signals using P(otol)3 at any time,

either as an FLP with AlX3 or a 3.9/3.8 mixture, also suggests that a radical pathway is unlikely.

Furthermore, while PMes3 is reported to form a stable phosphoniumyl radical cation,206

P(otol)3

is not; therefore, if a radical pathway were at play, we would expect to see phosphine

decomposition of P(otol)3 upon reduction of CO2 to CO, something that is not observed. We

therefore believe that the diamagnetic mechanism of Scheme 3.6, proposed and supported

experimentally and theoretically, is the more likely pathway.

3.3 Conclusions

This chapter has presented the results of the spontaneous stoichiometric reduction of CO2 to CO

using PR3 (R = otol, Mes) and AlX3 (X = Br, I). This reaction could be performed either from

isolated starting materials, 2.5, 2.6, or 3.9, or in situ from FLP solutions of PMes3 with a sub-

stoichiometric amount and AlX3 (1.9 equiv.). Kinetic vs. thermodynamic reaction pathways are

proposed for the different rates observed in these two cases. Detailed mechanistic studies into

this reduction chemistry were performed and it was found that solvent, temperature, and the

addition of the [AlI4]- anion all play important roles. Labelling studies combined with FT-IR

spectroscopy confirmed that exogenous 13

CO2 is reduced when a 13

CO2 atmosphere is used,

generating 13

CO and the unscrambled [12

C]-3.1 and 3.2 from [12

C]-2.6. Detailed kinetic

experiments with 2.6 were also performed and a first-order dependence on both 2.6 and CO2

pressure (second order overall) were found, as well as an inverse-first order dependence on

cyclohexene. A negative activation entropy suggests an associative mechanism. Using Al(C6F5)3

as a model compound, it is proposed that the reduction chemistry proceeds through a rate-

determining associative insertion of exogenous CO2 into the Al–X bond of AlX3, forming the

intermediate X2AlO(X)C=O–AlX3 (A) after coordination of a second AlX3. Subsequent

nucleophilic attack by phosphine and exchange chemistry to produce the final labelled

compounds is proposed and supported by detailed theoretical calculations performed by Prof.

Thomas M. Gilbert. Finally, while a radical mechanism is possible, it was ultimately deemed

unlikely as indicated by EPR and UV-Vis spectroscopic studies.

76





-38 ºC freezer). Hexanes, pentane, and toluene (Aldrich) were dried using an Innovative

Technologies solvent system. Fluorobenzene and bromobenzene (-H5 and -D5) were purchased

from Aldrich and dried on P2O5 for several days and vacuum distilled onto 4 Å molecular sieves

prior to use. Cyclohexene, dichloromethane-d2 and toluene-d8 were purchased from Aldrich,

dried on CaH2 and vacuum distilled onto 4 Å molecular sieves prior to use. Cyclohexane was

dried over Na and distilled prior to use. Tris(2,4,6-trimethylphenyl)phosphine (PMes3), tris(o-

tolyl)phosphine (P(otol)3), tricyclohexylphosphine (PCy3), trimethylaluminum (TMA), and

triethylaluminum (TEA) were purchased from Strem, and Br2, I2 (resublimed), methyl iodide,

RuCl3•3H2O, 1,2,3,4,5-pentamethylcyclopentadiene (Cp*H), and Et3BHLi (super-hydride) were

purchased from Aldrich, and (CF3)3COH was purchased from Apollo Scientific, and B(C6F5)3

was purchased from Boulder Scientific and all these reagents were used without further

purification. The compound Cp*RuCl(PCy3) was synthesized from Cp

*RuCl and PCy3 by a

known literature procedure.210

The precursor Cp*RuCl was synthesized from RuCl3•3H2O,

Cp*H, and super-hydride through a known procedure.211

Al(C6F5)3•tol was prepared from

B(C6F5)3 and TMA in toluene by a known procedure.175

Al(OC(CF3)3)3•C6H5F was prepared

from TEA and (CF3)3COH in fluorobenzene according to a literature procedure.194

AlX3 (X = Br,

I) were purchased from Strem and sublimed three times prior to use under vacuum using a

-78 °C cold finger and an 80 °C (X = Br) or 150 °C (X = I) bath. Compounds 2.3, 2.5, 2.6, and

2.17 were synthesized according to Chapter 2. 13

CO2 was purchased from Aldrich and used

without further purification. CO2 (grade 4.0) was purchased from Linde and passed through a

Drierite column prior to use.



13C = 122.4 ppm

for ipso carbon), C7H8 (1H = 2.08 ppm for methyl;

13C = 20.43 ppm for methyl), and CD2Cl2 (

1H

= 5.32 ppm; 13

C = 53.84 ppm), or externally (27

Al: Al(NO3)3, 31

P: 85% H3PO4, 19

F: CFCl3).

Chemical shifts (δ) listed are in ppm and absolute values of the coupling constants are in Hz.

77

NMR assignments are supported by additional 2D experiments. IR spectra were collected on a

Perkin Elmer Spectrum One FT-IR instrument using the G-2 gas cell (10 cm long) obtained from

International Crystal Laboratories. UV-Vis spectra were obtained on an Agilent 8453 UV-Vis

spectrophotometer using Quartz cells modified with a J-Young NMR cap to assure a near-perfect

seal. Elemental analyses (C, H) and X-ray crystallography were performed in house. EPR spectra

were recorded at the University of Windsor by a Stephan Group post-doctoral fellow, Dr. Jillian

A. Hatnean, as part of a collaboration. The spectra were recorded at r.t. or 77 K on a Bruker

EMXplus X-band spectrometer controlled using Xenon software on a PC operating under Linux.

The spectra were modeled using PIP4Win v. 1.2.207


Reduction of CO2 using AlI3 to produce 3.1 and 3.2: A 100 mL Schlenk bomb equipped with

a Teflon screw cap and a magnetic stirbar in the glovebox was charged with PMes3 (0.5 g, 1.29

mmol) and AlI3 (1.0 g, 2.45 mmol). Bromobenzene (20 mL) or fluorobenzene (20 mL) was

added to this all at once. (NOTE: Product isolation was greatly facilitated by using

fluorobenzene due to its lower b.p.; however, the reaction was faster in bromobenzene). The

bomb was transferred to the Schlenk line equipped with a CO2 outlet. The bomb was degassed at

r.t., filled with CO2 (ca. 2 atm) and sealed. The solution was stirred rapidly overnight (ca. 16

hours in bromobenzene) or for 4 days (in fluorobenzene) in the glovebox and changed from a

purple solution to a dark yellow solution. The solvent was then removed in vacuo and hexanes

(ca. 10 mL) was added to the residue. The precipitate was rapidly stirred for ca. 10 min then

filtered on a glass frit. Obtained 1.35 g of a 1:1 mixture of 3.1 and 3.2.

1H,

31P{

1H},

27Al,

13C{

1H} NMR: Analogous to a 1:1 mixture of isolated compounds 3.1 and 3.2

(see below).

Isolation of 3.1: Small portions of 3.1 could be separated from a 1:1 mixture of 3.1:3.2 by slow

cooling a fluorobenzene solution to -38 °C. Crystals obtained were suitable for X-ray

crystallography. The crystals were partly soluble in deuterated bromobenzene.

1H NMR (400 MHz, C6D5Br): δ 7.09-6.83 (m, 7-8H, 1.5•C6H5F), 6.82 (bs, 3H, m-Mes), 6.70

(bs, 3H, m-Mes), 2.33 (bs, 9H, o-CH3Mes

), 2.06 (s, 9H, p-CH3Mes

), 1.83 (bs, 9H, o-CH3Mes

).

31P{

1H} NMR (161 MHz, C6D5Br): δ 19.5.

27Al NMR (104 MHz, C6D5Br): δ 31 (s, υ1/2 = ca.

78

170 Hz). 13

C{1H} NMR (100 MHz, C6D5Br): δ 173.9 (d,

1JC-P = 118 Hz, CO2), 162.8 (d,

1JC-F =

244 Hz, i-C6H5F), 147.0 (d, 4JC-P = 3.0 Hz, p-C6H2), 144.7 (bs, 2C, o-C6H2), 133.5 (bs, 2C, m-

C6H2), 130.0 (d, 3JC-F = 7.7 Hz, m-C6H5F), 124.1 (d,

4JC-F = 3.1 Hz, p-C6H5F), 115.3 (d,

2JC-F =

20.6 Hz, o-C6H5F), 113.6 (d, 1JC-P = 76.2 Hz, i-C6H2), 26.0 (bs, o-CH3

Mes), 23.8 (bs, o-CH3

Mes),

21.3 (s, p-CH3Mes

).. 19

F NMR (376 MHz, C6D5Br): δ -112.4. Anal. Calc. for C74H81Al6F3I14O6P2

(2•3.1 + 3•C6H5F)): C, 28.45; H, 2.61. Found: C, 28.36; H, 2.60.

Synthesis of [Mes3PI][AlI4] (3.2): A 50 mL round bottom Schlenk flask equipped with a

magnetic stir bar was charged with PMes3 (0.300 g, 0.77 mmol) and AlI3 (0.315 g, 0.77 mmol).

Toluene (20 mL) was added to this all at once. To this mixture was then added dropwise a

solution of I2 (0.196 g, 0.77 mmol) in ca. 5 mL toluene. The mixture turned to a pale yellow oily

solution and was allowed to stir for 30 min. The solvent was removed in vacuo to obtain a pale

orange solid. The solid was stirred in hexanes (ca. 10 mL) for 10 min and the mixture was

filtered on a glass frit and washed with hexanes (ca. 5 mL) and dried (0.72 g, 89%). Vapour

diffusion of a bromobenzene solution of the compound with cyclohexane yielded single crystals

suitable for X-ray crystallography.

1H NMR (400 MHz, C6D5Br): δ 6.82 (d,

4JH-P = 4.4 Hz, 3H, m-Mes), 6.63 (d,

4JH-P = 6.0 Hz, 3H,

m-Mes), 2.17 (s, 9H, o-CH3Mes

), 2.12 (s, 9H, p-CH3Mes

), 1.73 (s, 9H, o-CH3Mes

). 31

P{1H} NMR

(161 MHz, C6D5Br): δ -14.5. 27

Al NMR (104 MHz, C6D5Br): δ -25 (s). 13

C{1H} NMR (100

MHz, C6D5Br): δ 146.4 (d, 4JC-P = 3.4 Hz, p-C6H2), 145.4 (d,

2JC-P = 11.8 Hz, o-C6H2), 143.4 (d,

2JC-P = 12.1 Hz, o-C6H2), 133.4 (d,

3JC-P = 12.3 Hz, m-C6H2), 132.9 (d,

3JC-P = 12.3 Hz, m-C6H2),

119.3 (d, 1JC-P = 65.5 Hz, i-C6H2), 26.3 (d,

3JC-P = 6.4 Hz, o-CH3

Mes), 24.4 (d,

3JC-P = 4.3 Hz, o-

CH3Mes

), 21.4 (d, 5JC-P = 1.8 Hz, p-CH3

Mes). Anal. Calc. for C27H33PAlI5: C, 30.88; H, 3.17.

Found: C, 31.22; H, 3.05.

Reduction of CO2 using AlBr3 to produce 3.3 and 3.4: Performed in an analogous fashion to

the reduction producing 3.1/3.2 above using PMes3 (0.500 g, 1.29 mmol), AlBr3 (0.652 g, 2.45

mmol) and fluorobenzene (20 mL). (NOTE: Product isolation was again greatly facilitated by

using fluorobenzene due to its lower b.p.; however, the reaction was again faster in

bromobenzene). The solution was stirred rapidly for 2 days (in bromobenzene) or for 6 days (in

fluorobenzene) in the glovebox and changed from a purple solution to a dark yellow solution.

Obtained 0.97 g of a 1:1 mixture of 3.3 and 3.4 after analogous workup.

79

1H,

31P{

1H},

27Al,

13C{

1H} NMR: Analogous to a 1:1 mixture of isolated compounds 3.3 and 3.4

(see below).

Isolation of 3.3: Small portions of 3.3 could be separated from a 1:1 mixture of 3.3:3.4 by

vapour diffusion of a bromobenzene solution with cyclohexane. The crystals obtained were

suitable for X-ray crystallography.

1H NMR (400 MHz, CD2Cl2): δ 7.52-7.24 (m, 5H, C6H5Br), 7.17 (bs, 6H, m-Mes), 2.41 (s, 9H,

p-CH3Mes

), 2.32 (bs, 9H, o-CH3Mes

), 2.11 (bs, 9H, o-CH3Mes

). 31

P{1H} NMR (161 MHz, CD2Cl2):

δ 19.5. 27

Al NMR (104 MHz, CD2Cl2): δ 88 (bs). 13

C{1H} NMR (100 MHz, CD2Cl2): δ 176.8

(d, 1JC-P = 123 Hz, CO2), 147.9 (d,

4JC-P = 3.0 Hz, p-C6H2), 145.5 (bd,

2JC-P = 11 Hz, o-C6H2),

134.0 (bd, 3

JC-P = 12.4 Hz, m-C6H2), 131.9 (s, C6H5Br), 130.5 (s, C6H5Br), 127.4 (s, C6H5Br),

122.7 (s, C6H5Br), 113.7 (d, 1JC-P = 76.6 Hz, i-C6H2), 25.1 (bs, o-CH3

Mes), 24.3 (bs, o-CH3

Mes),

21.5 (d, 5JC-P = 1.6 Hz, p-CH3

Mes). Anal. Calc. for C34H38Al3Br8O3P (3.3 + C6H5Br): C, 32.78;

H, 3.07. Found: C, 32.52; H, 3.11.

Synthesis of [Mes3PBr][AlBr4] (3.4): A 50 mL round bottom Schlenk flask equipped with a

magnetic stir bar was charged with PMes3 (0.300 g, 0.77 mmol) and AlBr3 (0.206 g, 0.77 mmol).

Fluorobenzene (10 mL) was added to this all at once. To this mixture was added dropwise Br2

(40 µL, 0.78 mmol). The mixture turned to a pale yellow solution and was allowed to stir for 30

min. Hexanes (ca. 10 mL) was added dropwise with rapid stirring and the off-white precipitate

that forms was collected on a glass frit and washed with hexanes (ca. 5 mL) and dried (0.56 g,

89%). Vapour diffusion of a bromobenzene solution of the compound with pentane yielded

single crystals suitable for X-ray crystallography.

1H NMR (400 MHz, C6D5Br): δ 6.87 (d,

4JH-P = 4.4 Hz, 3H, m-Mes), 6.71 (d,

4JH-P = 6.4 Hz, 3H,

m-Mes), 2.14 (s, 9H, p-CH3Mes

), 2.13 (s, 9H, o-CH3Mes

), 1.76 (s, 9H, o-CH3Mes

). 31

P{1H} NMR

(161 MHz, C6D5Br): δ 38.5. 27

Al NMR (104 MHz, C6D5Br): δ 81 (s). 13

C{1H} NMR (100 MHz,

C6D5Br): δ 147.2 (d, 4JC-P = 3.0 Hz, p-C6H2), 145.4 (d,

2JC-P = 10.3 Hz, o-C6H2), 143.7 (d,

2JC-P =

15.4 Hz, o-C6H2), 133.7 (d, 3

JC-P = 12.1 Hz, m-C6H2), 133.2 (d, 3

JC-P = 13.5 Hz, m-C6H2), 119.0

(d, 1JC-P = 74.4 Hz, i-C6H2), 24.9 (d,

3JC-P = 6.2 Hz, o-CH3

Mes), 24.4 (d,

3JC-P = 5.5 Hz, o-

CH3Mes

), 21.5 (d, 5JC-P = 1.5 Hz, p-CH3

Mes). Anal. Calc. for C27H33PAlBr5: C, 39.79; H, 4.08.

Found: C, 39.87; H, 4.05.

80

Synthesis of Al(C6F5)3(C6H5Br)2 (3.5): Single crystals suitable for X-ray diffraction of this

compound were obtained by slow cooling a bromobenzene solution of Al(C6F5)3•tol to -38 °C

over several days.

1H,

27Al,

19F,

13C{

1H} NMR: Analogous to those reported for the isolated complex

Al(C6F5)3•tol.175

Synthesis of (otol)3P(CO2)Al(OC(CF3)3)3 (3.6): A 50 mL Schlenk bomb equipped with a

Teflon screw cap and a magnetic stirbar was charged with P(otol)3 (20 mg, 66 µmol), PhF-

Al(OC(CF3)3)3 (54 mg, 66 µmol) and fluorobenzene (0.7 mL). The bomb was transferred to the

Schlenk line equipped with a CO2 outlet. The bomb was degassed, filled with CO2 (1 atm), and

sealed. The solution was stirred for 10 min after which time the CO2 atmosphere was removed.

Slow cooling of the solution to -38⁰C yielded single crystals suitable for X-ray crystallography

(66 mg, 61 µmol, 93%).

1H NMR (400 MHz, C6H5F): δ 7.84 (m, 3H, o-CH), 7.59 (m, 3H, p-CH), 7.39 (m, 3H, m-CH),

7.36 (m, 3H, m-CH), 2.29 (s, 9H, CH3). 31

P{1H} NMR (161 MHz, C6H5F): δ 11.2.

27Al NMR

(104 MHz, C6H5F): 37 (bs). 19

F{1H} NMR (376 MHz, C6H5F): δ -75.2 (s, 27F, CF3).

13C{

1H}

NMR (100 MHz, C6H5F): δ 159.1 (d, 1JP-C = 124 Hz, CO2), 144.4 (d,

2JP-C = 8.4 Hz, o-C6H4),

135.0 (d, 4JP-C = 2.9 Hz, p-C6H4), 134.8 (d,

2JP-C = 12 Hz, o-C6H4), 133.2 (d,

3JP-C = 11 Hz, m-

C6H4), 127.4 (d, 3JP-C = 13 Hz, m-C6H4), 121.5 (q,

1JC-F = 291 Hz, OC(CF3)3), 115.5 (d,

1JP-C =

82 Hz, i-C6H4), 78.9 (m, OC(CF3)3), 21.5 (d, 3JP-C = 5 Hz, CH3). Anal. Calc. for

C34H21AlF27O5P: C, 37.80; H, 1.96. Found: C, 37.72; H, 2.04.

Synthesis of [(C6F5)CO2Al(C6F5)2]2 (3.7): A 50 mL Schlenk bomb equipped with a Teflon

screw cap and a magnetic stirbar was charged with Al(C6F5)3•tol (300 mg, 0.48 mmol) and

fluorobenzene (5 mL). The bomb was transferred to the Schlenk line equipped with a CO2 outlet.

The bomb was degassed, filled with CO2 (1 atm), and sealed. The solution was heated to 90⁰C

and stirred rapidly for 24 h after which the CO2 atmosphere was removed. The product was

isolated by crystallization at -38⁰C and filtered (220 mg, 0.38 mmol, 79%). Slow cooling of a

fluorobenzene solution to -38⁰C of the compound yielded single crystals suitable for X-ray

crystallography.

81

27Al NMR (104 MHz, C6D5Br): blank.

19F{

1H} NMR (376 MHz, C6D5Br): δ. -122.8 (bd,

2JF-F =

18.0 Hz, 4F, o-C6F5-Al), -131.5 (bs, 2F, o-C6F5-CO2), -135.0 (tt, 3JF-F = 22.6 Hz,

4JF-F = 11.3 Hz,

1F, p-C6F5-CO2), -148.3 (t, 3JF-F = 19.9 Hz, 2F, p-C6F5-Al), -156.5 (m, 2F, m-C6F5-CO2), -159.1

(m, 4F, m-C6F5-Al). 13

C{1H} NMR (100 MHz, C6D5Br), partial: δ 168.0 (s, CO2). Anal. Calc.

for C19AlF15O2: C, 39.88. Found: C, 39.79.

Synthesis of [Mes3PMe][AlI4] (3.8): A 50 mL round bottom Schlenk flask equipped with a

magnetic stir bar was charged with PMes3 (1.0 g, 2.57 mmol) and AlI3 (1.1 g, 2.7 mmol).

Fluorobenzene (15 mL) was added to this all at once. To this mixture was added dropwise a

solution of MeI (0.384 g, 2.7 mmol) in fluorobenzene (5 mL). The mixture turned to a pale

yellow solution and was allowed to stir for 1 h. The solvent was removed in vacuo to obtain a

beige oily residue. The residue was stirred in hexanes (ca. 20 mL) for 10 min. An off-white

precipitate formed which was filtered on a glass frit and washed with hexanes (ca. 2 x 5 mL) and

dried (2.1 g, 87%).

1H NMR (400 MHz, C6D5Br): δ 6.87 (s, 3H, m-Mes), 6.69 (s, 3H, m-Mes), 2.39 (d,

2JH-P = 11.6

Hz, 3H, Me), 2.14 (s, 9H, p-CH3Mes

), 1.99 (s, 9H, o-CH3Mes

), 1.72 (s, 9H, o-CH3Mes

). 31

P{1H}

NMR (161 MHz, C6D5Br): δ 7.0. 27

Al NMR (104 MHz, C6D5Br): δ -25 (s). 13

C{1H} NMR (100

MHz, C6D5Br): δ 145.1 (d, 4JC-P = 2.9 Hz, p-C6H2), 143.6 (d,

2JC-P = 10.6 Hz, o-C6H2), 142.8 (d,

2JC-P = 10.4 Hz, o-C6H2), 133.2 (d,

3JC-P = 11.4 Hz, m-C6H2), 133.1 (d,

3JC-P = 11.6 Hz, m-C6H2),

119.6 (d, 1JC-P = 79.0 Hz, i-C6H2), 25.7 (d,

1JC-P = 62.4 Hz, PMe), 24.1 (d,

3JC-P = 4.7 Hz, o-

CH3Mes

), 24.0 (d, 3JC-P = 5.2 Hz, o-CH3

Mes), 21.4 (s, p-CH3

Mes). Anal. Calc. for C28H36PAlI4: C,

35.85; H, 3.87. Found: C, 35.88; H, 3.94.

Synthesis of (otol)3P(CO2)(AlI3)2 (3.9): A solution of P(otol)3 (0.200 g, 0.66 mmol) and AlI3

(0.535 g, 1.31 mmol) in bromobenzene (10 mL) was prepared in a 100 mL Schlenk bomb

equipped with a Teflon cap and a magnetic stir bar in the glovebox. The bomb was transferred to

the Schlenk line equipped with a CO2 outlet. The bomb was degassed at r.t., filled with CO2 (ca.

2 atm) and sealed. The solution was stirred rapidly and heated to 80 °C for 1 hr. Precipitation

using hexanes (ca. 20 mL) afforded a white solid which was filtered and dried on a frit (0.55 g,

72%). Vapour diffusion of a bromobenzene solution of the compound with pentane yielded


82

1H NMR (400 MHz, C6D5Br): δ 7.57-7.51 (m, 3H), 7.36-7.32 (m, 3H), 7.16-7.09 (m, 6H), 2.08

(bs, 9H, o-CH3). 31

P{1H} NMR (161 MHz, C6D5Br): δ 27.5.

27Al NMR (104 MHz, C6D5Br): δ

23 (bs, υ1/2 = ca. 1500 Hz). 13

C{1H} NMR (100 MHz, C6D5Br): δ 164.3 (d,

1JC-P = 123 Hz, CO2),

144.5 (d, JC-P = 9.3 Hz), 136.9 (d, JC-P = 3.0 Hz), 135.7 (d,

JC-P = 12.7 Hz), 134.5 (d,

JC-P = 11.6

Hz), 128.5 (d, JC-P = 13.6 Hz), 111.2 (d, JC-P = 81.2 Hz, i-C6H4), 23.4 (d,

3JC-P = 4.9 Hz, o-CH3).

Anal. Calc. for C22H21PO2Al2I6: C, 22.71; H, 1.82. Found: C, 23.22; H, 2.00.

Synthesis of (otol)3P-AlI3 (3.10): A 50 mL round bottom flask equipped with a magnetic stir bar

in the glovebox was charged with P(otol)3 (0.400 g, 1.31 mmol) and AlI3 (0.535 g, 1.31 mmol).

Bromobenzene (20 mL) was added to this all at once. The mixture was allowed to stir for 30 min

at which point ca. 20 mL hexanes was added dropwise with rapid stirring. The white precipitate

was collected on a glass frit and washed with hexanes (ca. 5 mL) and dried (0.7 g, 75%).

1H NMR (400 MHz, C6D5Br): δ 7.23-6.90 (m, 12H), 2.63 (bs, 9H, o-CH3).

31P{

1H} NMR (161

MHz, C6D5Br): δ -8 to -15 (bm). 27

Al NMR (104 MHz, C6D5Br): δ 45 (d, 1

JAl-P = 138 Hz).

13C{

1H} NMR (100 MHz, C6D5Br): δ 144.3 (d, JC-P = 11.9 Hz), 134.4 (d, JC-P = 7.3 Hz), 132.5

(d, JC-P = 2.5 Hz), 132.4 (d, JC-P = 9.3 Hz), 127.0 (d, JC-P = 9.3 Hz), 121.2 (d, JC-P = 46.8 Hz, i-

C6H4), 26.9 (d, JC-P = 5.1 Hz, o-CH3). Anal. Calc. for C21H21PAlI3: C, 35.42; H, 2.97. Found: C,

35.53; H, 3.00.

3.4.3 Quantification of CO produced from the reduction of CO2

Quantification of CO from the reaction of PMes3/AlI3/CO2: A 50 mL Schlenk bomb

equipped with a teflon cap and a magnetic stirbar in the glovebox was charged with PMes3 (50

mg, 0.13 mmol) and AlI3 (100 mg, 0.24 mmol). Bromobenzene (2 mL) was added to this all at

once. The bomb was transferred to the Schlenk line equipped with a CO2 outlet. The bomb was

degassed at r.t., filled with CO2 (ca. 2 atm) and sealed. The solution was stirred rapidly for ca. 16

hours in the glovebox and changed from a purple solution to a dark yellow solution. A second 50

mL Schenk bomb equipped with a teflon cap and a magnetic stirbar was charged with

Cp*RuCl(PCy3) (71 mg, 0.13 mmol) in toluene-d8 (1 mL) and was attached to the outlet of the

first Young bomb by a short piece of Tygon tubing (see Figure 3.15). Both flasks were open to

each other and allowed to stir for ca. 3 hours. The Ru containing solution was then analyzed by

31P NMR spectroscopy and the ratios integrated as shown below (Figure 3.16) to yield 61:39

mixture of Cp*RuCl(PCy3):Cp

*RuCl(PCy3)(CO).

83

Figure 3.15 – Picture taken of the CO trapping experiment. Each bomb is open to one another

and connected together by Tygon tubing.

Figure 3.16 – Representative 31

P NMR (C7D8) spectrum of a mixture of Cp*RuCl(PCy3) (41.1

ppm) and Cp*RuCl(PCy3)(CO) (52.1 ppm) resulting from the reduction of CO2 to CO by PMes3

and AlI3.

Quantification of CO from the reaction of PMes3/AlBr3/CO2: Performed in an analogous

fashion to the reaction above with AlI3; however, the reaction was stirred for 2 days at r.t. The

Ru containing solution was then analyzed by 31

P NMR spectroscopy and the ratios integrated to

yield 70:30 mixture of Cp*RuCl(PCy3):Cp

*RuCl(PCy3)(CO).

84

3.4.4 IR analysis of CO produced

The produced CO was analyzed by IR spectroscopy by attaching the gas cell to the reduction

flask of PMes3/AlI3 by a Tygon tubing and purging the cell with N2 prior to use. The reduction

flask was cooled to ca. -110 °C using an EtOH/N2(l) bath to solidify the CO2 and bromobenzene

while keeping the CO in gaseous form (b.p. -192 °C). Once frozen, the system was exposed to

the IR cell for approximately 30 min to allow the CO to diffuse to the cell. The cell was then

sealed and its contents analyzed by IR spectroscopy. A representative spectrum is shown in

Figure 3.4 of the Chapter.

3.4.5 Kinetic study details

Sample preparation: All kinetic experiments were performed in identical thick-walled J-Young

NMR tubes (total internal volume of 2 mL) under an atmosphere of CO2 at elevated pressures.

Saturated solutions of 2.6 (7.2 mM) were prepared by dissolving 36 mg (29 µmol) in 4.0 mL of

C6D5Br. An exact volume of 0.50 mL was used for each run. The CO2 pressure was estimated to

be 5 atm by filling a 7.5 mL vessel with 1 atm of CO2, then condensing this using N2(l) directly in

the 1.5 mL headspace of the J-Young NMR tube and sealing. The concentration of CO2 in

solution was assumed to be 0.38 M as derived from the experimental molar fraction value of

Raoult’s Law for CO2 in bromobenzene at 1 atm.199

First order plots and Eyring equation: Under these high pressure conditions, first order plots

were obtained when monitoring the 31

P{1H} NMR spectra over time as shown in detail in Figure

3.9. The 31

P{1H} NMR experiments were acquired with an exact delay between acquisitions so

as to know the exact starting point of each experiment. Samples were injected directly in the pre-

warmed spectrometer and the initial spectra were acquired after waiting for 5 min to equilibrate.

The plots were obtained over a 30 K range (303 – 333 K) in order to obtain the thermodynamic

activation parameters by plotting the Eyring plot as described in the chapter.

Order determination: The order in CO2 was determined at r.t. by monitoring solutions of 2.6

(0.027 M) with 3.8 (0.080 M) (to increase the rate at r.t.) under different CO2 pressures. A linear

plot of kobs vs. PCO2 was obtained indicating a first order dependence on CO2 pressure (Figure

3.17).

85

Figure 3.17 – Linear plot of kobs vs. PCO2 indicating a first order dependence on CO2 pressure

using solutions of 2.6 (0.027 M) with 3.8 (0.080 M) monitored by 31

P{1H} NMR spectroscopy at

r.t. under different pressures.

As described in this chapter, an inverse first-order dependence on cyclohexene (cyc) was

observed. Concentrated solutions of 2.6 were prepared as described above and treated to

different equivalents of cyclohexene under 5 atm of CO2 and monitored at 50 °C by timed

31P{

1H} NMR experiments. The resulting linear plot of [cyc] vs. [CO2]/kobs is linear (see

derivation of this equation), consistent with the inverse first order dependence on cyclohexene

(Figure 3.18).

86

Figure 3.18 – Linear plot of [cyc] vs. [CO2]/kobs indicating an inverse-first order dependence on

cyclohexene based on the derived equation below. Concentrated solutions of 2.6 (7.2 mM) were

monitored under 5 atm of CO2 ([CO2] assumed to be 0.38 M) by timed 31

P{1H} NMR

spectroscopy at 50 °C.

Derivation of relation for Figure 3.18, assuming inverse order dependence:

obs

obsobsk

COkcyc

cyc

COkkk

cyc

COkrate

][][.:

][

][.:].[

][

]][.[ 222 6262

The equation was rearranged to include [cyc] = 0 into the plot above.

3.4.6 Computational detail

Computational studies were performed by Prof. Thomas M. Gilbert at Northern Illinois

University. The Gaussian suite (G09)212

was utilized for all calculations. Optimizations

employed the ONIOM213,214

composite approach, within which the DFT model M06-2X215

was

used. The appropriateness of this approach was affirmed by observation of good agreement

between computationally- and X-ray diffraction-determined structural parameters. Single point

energy calculations at the ONIOM-determined geometries were performed using the ONIOM

G2R3 (OG2R3)216,217

composite approach, which approximates a CCSD(T)/6-311+G(2df, 2p)

energy, providing a more accurate prediction at modest cost.218,219

Bromobenzene solvent effects

were examined using both explicit bromobenzene molecules and/or a polarized continuum

solvent model (denoted SCRF in the text).220,221

Inclusion of solvent in either form had only a

small effect on predicted structures, but showed a detectable effect on the reaction energetics,

87

presumably because all species were extremely polar. Energies were corrected using unscaled

zero point energies from the frequency analyses, since scaling factors for ONIOM-based

frequency determinations are undetermined.

3.4.7 EPR measurements

The following EPR measurements were taken as described below.

Scheme 3.10 – EPR measurements of 2.6 or 3.9 with and without CO2.

Details for Scheme 3.10: An EPR spectrum was acquired for a ~0.009 M bromobenzene solution

of R3P(CO2)(AlI3)2 (2.6 or 3.9) in a J-Young NMR tube and found to contain no EPR active

species. This mixture was then freeze-pump-thaw degassed and 5 atm of CO2 added. The

solution was monitored for 48 h at r.t. by EPR spectroscopy. At no time was an EPR signal

observed.

Scheme 3.11 – EPR measurements of 2.6 with excess (3 equiv.) of 3.8 with and without CO2.

Details for Scheme 3.11: A bromobenzene solution (0.6 mL) of 2.6 (20 mg, 0.016 mmol) and 3.8

(45 mg, 0.048 mmol) were thoroughly mixed and transferred to a J-Young NMR tube. An initial

r.t. X-band solution EPR spectrum revealed the presence of a doublet (g = 2.0056, aP = 239 G)

identified as the P-based [Mes3P•]+ radical cation (identical to Figure 3.13) with spectral

parameters in excellent agreement with previously reported trimesitylphosphonium radical

cation.206

This mixture was then freeze-pump-thaw degassed and 5 atm of CO2 added. The

solution was monitored for an additional 48 h at r.t. by EPR and NMR spectroscopy. At no time

88

was an EPR signal observed after addition of CO2 gas. NMR spectroscopy confirmed the

expected appearance of the products 3.1 and 3.2 along with the disappearance of 2.6.

Scheme 3.12 – EPR measurements of 3.9 with excess (3 equiv.) of 3.8 with and without CO2.

Details for Scheme 3.12: A bromobenzene solution (0.6 mL) of 3.9 (20 mg, 0.017 mmol) and 3.8

(48 mg, 0.051 mmol) were thoroughly mixed and transferred to a J-Young NMR tube. An initial

r.t. EPR spectrum acquired was found to contain no EPR active species. This mixture was then

freeze-pump-thaw degassed and 5 atm of CO2 added. The solution was monitored for 48 h at r.t.

by EPR and NMR spectroscopy. At no time was an EPR signal observed. The appearance of

reduction products, as described in this chapter, were apparent by NMR spectroscopy.












The heavy





2/2 (Fo

2) and Fo


89



C–H atom positions were calculated and allowed to ride on the carbon to which they are bonded

assuming a C–H bond length of 0.95 Å. H-atom temperature factors were fixed at 1.20 times the





90



3.1 3.2 3.3

Formula C37H41Al3F1.50I7O3P1 C27H33Al1I5P1 C34H38Al3Br8O3P1

Formula wt. 1562.41 1049.98 1245.83

Crystal system monoclinic orthorhombic monoclinic

Space group P21/n P212121 P21/c

a(Å) 12.5568(9) 14.1843(12) 11.6820(3)

b(Å) 20.6941(15) 15.1713(12) 18.7583(4)

c(Å) 19.2801(13) 15.7407(12) 21.1200(5)

α(deg) 90 90 90

β(deg) 97.810(2) 90 97.6750(10)

γ(deg) 90 90 90

V(Å3) 4963.5(6) 3387.3(5) 4586.7(2)

Z 4 4 4

T (K) 150(2) 150(2) 150(2)

d(calc) g/cm3 2.091 2.059 1.804

Abs coeff, μ, mm-1

4.499 4.679 7.117


Rint 0.0436 0.0508 0.0448

Data used 11295 7796 10527


R (>2σ) 0.0351 0.0427 0.0420

wR2 0.0751 0.1147 0.1022

GOF 1.001 0.988 1.015

91


3.4 3.5 3.6

Formula C27H33Al1Br5P1 C30H10Al1Br2F15 C34H21Al1F27O5P1

Formula wt. 815.03 842.18 1080.46

Crystal system orthorhombic monoclinic triclinic

Space group P212121 P21/c P-1

a(Å) 13.5071(18) 16.566(2) 13.5607(10)

b(Å) 14.8431(18) 7.4086(9) 17.0003(13)

c(Å) 15.3001(17) 24.971(3) 18.6463(14)

α(deg) 90 90 96.300(4)

β(deg) 90 107.620(5) 100.964(4)

γ(deg) 90 90 100.812(4)

V(Å3) 3067.5(6) 2920.8(6) 4097.9(5)

Z 4 4 4

T (K) 150(2) 150(2) 150(2)

d(calc) g/cm3 1.765 1.915 1.751

Abs coeff, μ, mm-1

6.647 2.924 0.253


Rint 0.0325 0.0356 0.0688

Data used 6910 8761 13673


R (>2σ) 0.0468 0.0307 0.0675

wR2 0.1211 0.1102 0.1454

GOF 1.122 0.803 1.023

92

Table 3.3 – Selected crystallographic data for 3.7 and 3.9.

3.7 3.9

Formula C38Al2F30O4 C22H21Al2I6O2P1

Formula wt. 574.19 1163.76

Crystal system monoclinic monoclinic

Space group P121/c1 P21/c

a(Å) 10.8523(13) 10.9541(7)

b(Å) 13.0808(16) 18.4541(13)

c(Å) 14.0173(18) 16.5285(11)

α(deg) 90 90

β(deg) 109.453(3) 93.151(2)

γ(deg) 90 90

V(Å3) 1876.3(4) 3336.2(4)

Z 4 24

T (K) 150(2) 150(2)

d(calc) g/cm3 2.033 13.661

Abs coeff, μ, mm-1

0.274 34.219

Data collected 21454 53121

Rint 0.0367 0.0354

Data used 5703 7633

Variables 334 301

R (>2σ) 0.0393 0.0314

wR2 0.1037 0.1277

GOF 1.022 1.034

93

Chapter 4 Divergent Reactivity of P/Al Frustrated Lewis Pairs with

Olefins: Addition vs. C–H Bond Activation Pathways

4.1 Introduction

The functionalization of olefins to form polymers for the petrochemical industry has been one of

the central driving forces in the development of modern organometallic chemistry in the 20th

century.128,222

Central to this functionalization is the need for the olefin to coordinate to a metal

centre prior to undergoing C–C bond formation and subsequent polymerization.223

Both early

and late transition metal catalysts can catalyze these reactions with varying degrees of success.224

Olefins are also central to hydroformylation chemistry wherein C–C bond formation with CO

leads to the production of aldehydes needed for the synthesis of plasticizers and detergents.21

An alternative to C–C bond formation with olefins involves allylic C–H bond activation to

generate allyl fragments desirable for synthetic chemistry. Allyl groups are important fragments

in organic synthesis;225-227

however, the installation of such substituents usually requires the use

of harsh bases, such as KOtBu/nBuLi.228

Alternatively, activated olefins,25,229

such as allyl

halides,54,230

ethylene ketals,231

allyl alkoxides,232

or trimethylsilyl-olefin derivatives233

have

been used to install allyl substituents.227,228,234

While such reagents can be employed to allylate

carbonyl functionalities,54,230,235-237

the allylation of olefins has only been achieved in a limited

number of cases.25,238

Despite their diverse applications and ubiquitous use in the chemical industry, olefins are wasted

every day through the flaring activities of the oil industry, increasing local air pollution and

global CO2 levels.239

New synthetic strategies for accessing olefin derivatives are needed in order

to render this type of waste economically unfeasible. To this end, frustrated Lewis pairs have

been used to activate unsaturated C–C bonds, such as alkynes and olefins. In the former case, it

was found that phosphine and E(C6F5)3 (E = B, Al) combinations led either to addition across the

triple bond, or deprotonation of the terminal C–H bond depending on the phosphine used

(Scheme 4.1).102

In either case, the proposed mechanism involves initial π coordination to the

Lewis acidic centre, polarizing the C centre and increasing the terminal C–H acidity, followed

either by nucleophilic attack at the C or deprotonation of the C–H bond depending on the

94

Brønsted basicity of the phosphine used. Many other examples of alkyne activation by FLPs

have since been reported.104,105,109,122

Scheme 4.1 – Divergent addition vs. deprotonation reactivity of PR3/E(C6F5)3 FLPs with alkynes

(R = tBu, otol, Ph; E = B, Al).

Similar chemistry with olefins and FLPs has also been explored (Scheme 4.2, top).101

In this

case, however, no C–H deprotonation of olefinic or allylic C–H bonds was reported. Addition

products were obtained and a similar mechanism involving π coordination to the Lewis acidic

centre, followed by nucleophilic attack of a phosphine is believed to occur. To corroborate this

mechanism, more recent work has spectroscopically probed the interactions of olefins with

Lewis acidic B centres. Through detailed NMR studies, it was established that pendant olefins

attached to B form weak van der Waals complexes (Scheme 4.2, bottom).107

These interactions

were found to polarize the olefin making it susceptible to nucleophilic attack,106,107

consistent

with the mechanisms proposed in the initial olefin and alkyne results (Schemes 4.2 (top) and 4.1,

respectively).

Scheme 4.2 – Activation of olefins using PtBu3/B(C6F5)3 FLPs (top), and proposed mechanism

of addition involving the intermediate formation of a “van der Waals” complex (bottom).

95

In this chapter, we explore the olefin chemistry using Al-based Lewis acids and phosphines.

While some of the addition chemistry is similar, the chemistry with substituted olefins is

markedly different than outlined in Scheme 4.2 with B, and features unexpected C–H

deprotonation to generate σ-allyl fragments. This deprotonation chemistry is similar to the alkyne

chemistry of Scheme 4.1. In a specific case, a σ-allyl borate species could also be synthesized.

While this species is relatively unreactive to further chemistry, the Al σ-allyls were found to

undergo olefin, as well as CO2, insertion chemistry to generate new C–C bonds.


4.2.1 Reversible vs. irreversible activation of ethylene

As in Chapters 2 and 3, our initial investigation into olefin chemistry focused on the use of

PMes3/AlX3 FLPs (X = Cl, Br, I). We therefore exposed 1:1 solutions of PMes3/AlX3 in C6D5Br

in J-Young NMR tubes to ethylene. All reactions indicated complete conversion within 2 h to a

single product as observed by the formation of new singlets in the 31

P{1H} NMR spectrum

centred at 20 ppm, as well as 2 new broad resonances in the aliphatic region of the 1H NMR

spectrum, each integrating to 2 protons. While the NMR data suggests the formation of the

species Mes3P(CH2CH2)AlX3, analogous to the B chemistry above, attempts to isolate these

products were unsuccessful. While powders could be obtained upon precipitation with added

hexanes, single crystals or satisfactory elemental analyses for these products could not be

obtained. While we were initially puzzled, closer inspection of the 1H NMR spectrum of these

isolated powders indicated the presence of a small amount of ethylene in solution, indicative of a

reversible reaction.

In a recent study by our group using boron halide Lewis acids,190

it was found that the bis-

borane, 1,2-C6H4(BCl2)2, had significantly higher Lewis acidity than many other boranes110,147

for CO2 activation. This enhanced Lewis acidity was attributed to the formation of a bridging Cl

between the two B centres upon complexation with CO2; therefore, it is thought that one B centre

enhances the Lewis acidity of the other by “pulling” on one of its chlorides.190

While it was not

initially clear if addition of excess AlX3 to the ethylene chemistry would change the course of the

reaction, we decided to alter the system by exposing 1:2 mixtures of PMes3/AlX3 to ethylene.

While NMR analysis did not immediately indicate a significant change in the chemistry, scaled

up preparations resulted in the isolation of powders in good yields which did not appear to lose

96

ethylene once re-dissolved. Furthermore, combustion analysis was consistent with a formulation

of Mes3P(CH2CH2)(AlX3)2 (X = Cl (4.1), Br (4.2), I (4.3)). Single crystals suitable for X-Ray

diffraction were obtained for each of these compounds. Since the geometries are analogous, only

the structure for 4.2 is shown in Figure 4.1.

Figure 4.1 – POV-Ray depiction of the molecular structure of 4.2. Analogous structures are

obtained for 4.1 and 4.3. H atoms are omitted for clarity. C: black; P: orange; Al; teal; Br:

scarlet.

The structures of 4.1, 4.2, and 4.3 display remarkably similar features with analogous P–C(ethyl)

bond lengths of 1.874(3), 1.862(3), and 1.885(10) Å, respectively, consistent with literature P–C

bond values.101,240

The ethyl C–C bonds are also similar at 1.544(4) and 1.535(5) Å for 4.1 and

4.2, respectively, and slightly elongated for 4.3 at 1.572(15) Å, all of which are indicative of a

C–C single bond. Finally, the C–Al bonds are also analogous at 1.950(3), 1.946(3), and

1.951(10) Å, respectively, similar to reported C–Al bond lengths.102,174

The chemistry described with PMes3/AlX3 FLPs with ethylene displays reversible vs. irreversible

character (Scheme 4.3). The irreversible nature of this is attributed to the enhanced Lewis acidity

at the ethyl-bound AlX3 caused by the second equivalent of AlX3 in a similar manner to the

reported bis-borane chemistry.190

The reversible chemistry using 1:1 ratios of PMes3/AlX3 is in

contrast to the initially reported ethylene activation using PtBu3/B(C6F5)3 FLPs which were not

reported to be reversible.101

97

Scheme 4.3 – Reversible vs. irreversible ethylene activation using PMes3/AlX3 FLPs.

Next, we were interested in exploring ethylene activation with Al(C6F5)3. A 1:1 solution of

PMes3 and Al(C6F5)3•tol was exposed to 1 atm of ethylene. The 31

P and 1H NMR resonances of

the resulting solution were indicative of the formation of Mes3P(CH2CH2)Al(C6F5)3. The 19

F

NMR spectrum also indicated the presence of a 4-coordinate aluminate anion. Addition of

hexanes to a scaled-up preparation in bromobenzene resulted in precipitation of a solid; however

this solid gradually became soluble again. Analysis of this resulting solution by NMR

spectroscopy revealed the reversion to the initial components, PMes3 and Al(C6F5)3. Therefore, it

is believed that the 1:1 compound Mes3P(CH2CH2)Al(C6F5)3 forms under an atmosphere of

ethylene, but rapidly loses ethylene once this atmosphere is replaced with N2. Unlike in the AlX3

case, addition of excess Al(C6F5)3 does not appear to significantly stabilize this product.

In contrast to this reactivity, using the less sterically hindered phosphines PtBu3 or P(otol)3 with

Al(C6F5)3 leads to the clean isolation of single isolated products after exposure to ethylene. The

combustion analyses indicate a formulation of R3P(CH2CH2)Al(C6F5)3 (R = tBu (4.4), otol (4.5))

and single crystals suitable for X-ray diffraction were obtained for each of these products (Figure

4.2). The stable formation of these products, as opposed to with PMes3, suggests that sterics play

an important role.

98

Figure 4.2 – POV-Ray depiction of the molecular structures of 4.4 (left) and 4.5 (right). H atoms

are omitted for clarity. C: black; P: orange; Al; teal; F: pink.

The structure of 4.4 is analogous to the reported B analog tBu3P(CH2CH2)B(C6F5)3.101

Except for

the expected longer Al–Cethyl bond length (1.997(2) Å) vs B–Cethyl length (1.653(4) Å), all other

metric parameters are very similar. The P–Cethyl bond lengths of 4.4 and 4.5 (1.833(2) and

1.830(7) Å, respectively) are shorter than the average P–Cethyl bond lengths in 4.1-4.3 of 1.87 Å,

consistent with the reduced bulk around PtBu3 and P(otol)3. It should be noted that the B analog

of 4.5 has not been reported, perhaps due to the increased steric protection around B.

4.2.2 Reactions of PR3/AlX3 with propylene: unexpected deprotonation and dimerization reactions

Subsequent to the ethylene chemistry, we set out to explore the chemistry with propylene to see

if addition products, analogous to 4.1-4.3, would be obtained. We exposed 1:2 solutions of

PMes3/AlX3 to propylene in J-Young NMR tubes. To our surprise, no addition product was

observed by 31

P{1H} NMR spectroscopy; instead, a peak at -26 ppm which split into a doublet

(1JP-H = 482 Hz) in the

31P NMR appeared, indicative of the formation of the [Mes3PH]

+ cation.

The formation of this product was significantly accelerated using AlI3, likely due to the weaker

adduct with PMes3 (see Chapter 3). The reaction was scaled up in fluorobenzene and allowed to

react for 24 h under propylene. Following solvent removal, the 27

Al NMR spectrum indicated the

presence of 2 separate signals at 130 ppm (broad) and -25 ppm (sharp). This latter signal is

indicative of the [AlI4]- anion (Chapter 3). Addition of hexanes to this residue resulted in the

99

precipitation and isolation of the salt [Mes3PH][AlI4] (4.6) after filtration. Removal of the

solvent from the filtrate resulted in the isolation of an oil with the 27

Al NMR resonance at 130

ppm. The 1H NMR spectrum displayed a complex set of nine separate peaks, each of which was

found to be coupled to at least one other resonance. After detailed 1H multiplet analysis, and 2D

1H–

1H COSY and

1H–

13C HSQC experiments, this product was established to be the pendant

olefin (4.7) shown in Figure 4.3. This product is believed to form by initial C–H deprotonation to

generate an intermediate allyl fragment which subsequently attacks a second propylene.

Subsequent redistribution generates the salt 4.6 and the pendant olefin 4.7 (Scheme 4.4).

Figure 4.3 – 1H NMR (600 MHz) spectrum of 4.7 in C6D5Br. Inset is shown a dddd, one of

many complex signals from this spectrum. He features a ddddd and, although Hd appears as a q,

it is actually a ddd.191

Scheme 4.4 – Synthesis of 4.6 and 4.7 by proposed C–H deprotonation to generate an

intermediate allyl fragment, followed by insertion of a second propylene to generate the pendant

locked olefin product.

100

In contrast to the pendant olefin chemistry described for B-based Lewis acids (Scheme 4.2),107

the pendant olefin with Al (4.7) was found to be locked into position as observed by the complex

coupling pattern of the 1H NMR spectrum (Figure 4.3), as well as the unusually downfield

shifted 13

C NMR resonance for the olefinic (C2) atom at 163.5 ppm, indicative of a polarized

bond. This locked conformation is likely the result of the increased Lewis acidity at Al,193

as well

as its more diffuse empty p orbital.

4.2.3 C–H bond activation to generate bis-aluminum σ-allyl complexes

In continuing our exploration of C–H bond activation with propylene, we exposed a 1:2 solution

of PMes3/Al(C6F5)3 in C6D5Br to propylene and monitored it by NMR spectroscopy. While we

initially expected to see redistribution products similar to those observed with AlI3, the NMR

spectra for this reaction were markedly different. Monitoring this reaction by 19

F NMR

spectroscopy revealed the initial formation of a single product concomitant with the appearance

of the [Mes3PH]+ cation in the

31P NMR spectrum. A distinct triplet of triplets and broad singlet

was also observed in the 1H NMR integrating to 1 and 4 protons, respectively, relative to the 1

P–H proton of [Mes3PH]+ (Figure 4.4). These signatures are attributed to the generation of a

symmetric allyl species between two Al(C6F5)3 moieties. Attempts to isolate this product were

unsuccessful due to the presence of the presumed redistributed products similar to 4.6 and 4.7,

making the separation of the 3 compounds impossible (Figure 4.4). The data presented here is

consistent with the proposed intermediate generation of an allyl species as proposed in Scheme

4.4 with AlI3.

101

Figure 4.4 – 1H NMR (400 MHz) spectrum of a PMes3 + 2 Al(C6F5)3 solution in C6D5Br after

exposure to propylene for 12 h. The assigned resonances are shown in red and the appearance of

the pendant olefin by-product is also highlighted.

While this data supports the intermediate formation of an allyl species, we were nonetheless

interested in isolating such a species. Considering the results of the AlI3 chemistry suggest a

Markovnikov-type addition of the allyl to the secondary C of propylene (Scheme 4.4), we

reasoned that using an olefin with a tertiary C centre would eliminate this substitution pathway.

Thus, treatment of a 1:2 solution of PMes3:Al(C6F5)3 to isobutylene was monitored by NMR

spectroscopy and led to the gradual appearance of resonances attributable to an allyl fragment as

above. This reaction was significantly accelerated and product isolation facilitated using the

more basic phosphine, PtBu3. Therefore, treatment of a 1:2 ratio solution of PtBu3:Al(C6F5)3 in

fluorobenzene to isobutylene led to the formation of a single product after 1 h. This species (4.8)

could be isolated by trituration with hexanes. The 31

P{1H} and

31P NMR spectra revealed a

resonance at 60 ppm with a strong P–H coupling of 426 Hz, consistent with one-bond coupling

and thus the formation of the [tBu3PH]+ cation. The

19F{

1H} NMR spectrum revealed only three

peaks, similar to above, suggesting the Al(C6F5)3 fragments exist in equivalent environments.

The 1H NMR spectrum revealed the presence of a broad singlet at 3.91 ppm and a sharp singlet

102

at 2.10 ppm integrating for four and three protons, respectively. Low temperature (-30 °C) 1H

NMR experiments show the splitting of the peak at 3.91 ppm into two separate but broad

singlets.191

Collectively, this data implies the formation of an anion derived from a bridging allyl

moiety between two equivalent Al(C6F5)3 fragments with the corresponding phosphonium cation

similar to the proposed species in Scheme 4.4 and Figure 4.4 using PMes3. The NMR data

support an allyl geometry that is likely µ2-1

:1 although a µ

2-3

:3 geometry could not be

dismissed. Single crystal X-ray crystallography ultimately confirmed the former binding mode

(Figure 4.5). While the metric parameters of the cation are unexceptional, the anion is rather

interesting. The planar allyl fragment is linked to the two Al centres via σ interactions with the

methylene units, giving rise to Al–C distances of 2.080(4) and 2.094(4) Å. The two Al(C6F5)3

units are oriented in a transoid disposition which minimizes the steric congestion. The C–C

distances between these terminal carbon atoms and the central C of the allyl fragment are found

to be 1.415(6) Å and 1.411(6) Å with a corresponding CAl–C–CAl’ angle of 121.8(4)°.

Figure 4.5 – POV-ray depiction of the molecular structure of 4.8. H atoms, except P–H, are

omitted for clarity. C: black; P: orange; Al: teal; F: pink; H: white.

103

While a number of recent reports have described early metal,228,241,242

as well as Al-based allyl

compounds,54,55

the present example is, to the best of our knowledge, the first example in which

an isolable allyl species is derived from facile C–H activation. Furthermore, compound 4.8 is the

first example of a bridging allyl between two Al centres. It is noteworthy that the formation of

4.8 appears to be reversible with prolonged heating at 60 °C under vacuum, slowly regenerating

the initial FLP mixture with concurrent release of isobutylene.

Since the B olefin chemistry outlined in the introduction never involved the use of tertiary

olefins (Scheme 4.2),101,106,107

the analogous reaction employing B(C6F5)3 was performed in

fluorobenzene solution. A 1:2 mixture of PtBu3:B(C6F5)3 was exposed to an atmosphere of

isobutylene for 1 h. Addition of pentane to the solution resulted in the isolation of a product (4.9)

after filtration in modest yield (< 50%). 31

P{1H} NMR analysis in CD2Cl2 revealed the formation

of the expected [tBu3PH]+ cation. Furthermore, the

11B{

1H} NMR spectrum revealed a sharp

singlet at -13 ppm and the 19

F{1H} NMR spectrum showed signals at -131.6, -164.3, and

-167.2 ppm all of which are consistent with C6F5 rings on a four-coordinate boron centre.

However, the 1H NMR revealed four separate signals in addition to the peaks for the

phosphonium cation at 5.02 and 1.59 ppm. Two doublets each integrating for one proton and

coupling to each other were found in the olefinic region at 4.01 and 3.83 ppm. Two singlets in

the aliphatic region at 2.20 and 1.45 ppm integrated to two and three protons, respectively. This

data was consistent with a dissymmetric allyl fragment and suggested the possible presence of an

allyl-borate anion. Indeed, X-ray crystallography confirmed the formulation of 4.9 as

[tBu3PH][(C6F5)3B(CH2)2CMe] (Figure 4.6). The geometry about B is tetrahedral with the B–C

bond distance for the allyl fragment being 1.665(2) Å. The B–C–C angle was found to be

116.01(8)°. The central C atom of the allyl fragment is bound to the methylene bound to B, a

terminal methylene, and a terminal methyl group with C–C bond distances of 1.507(2), 1.337(2),

and 1.501(2) Å, respectively.

104

Figure 4.6 – POV-ray depiction of the molecular structure of 4.9. tBu H atoms are omitted for

clarity. C: black; P: orange; B: yellow-green; F: pink; H: white.

The synthesis of 4.9 was achieved in far higher yield (83%) using the identical protocol but

adjusting the stoichiometry of PtBu3 and B(C6F5)3 to 1:1. The facile synthesis of 4.9 at ambient

temperature using commercially available reagents stands in marked contrast to the often multi-

step syntheses used for known allyl-borates.235,237

Interestingly, monitoring a solution of 4.9 in either CD2Cl2 or C6D5Br by NMR spectroscopy in a

J-Young NMR tube reveals the formation of ca. 1 equiv. of free PtBu3, free isobutylene and

significant broadening of the 19

F, 11

B, and allylic 1H NMR signals. This suggests the loss of

isobutylene is facile. In the presence of excess borane, an allyl-bis-borate analog of 4.8 was not

observed spectroscopically. Indeed efforts to generate and isolate an allyl-bis-borate species via

alterations of the stoichiometry were unsuccessful. The apparent preference of B to form an

allyl-mono-borate anion incorporating a terminal olefinic unit is consistent with the weak

interactions of B(C6F5)3 with olefins107

and the weaker Lewis acidity of B(C6F5)3 compared to

Al(C6F5)3.193

105

4.2.4 C–C bond forming reactions

4.2.4.1 Reaction with ethylene

The isolation of the allyl species 4.8 is consistent with the observed allyl species generated using

propylene and observed by NMR spectroscopy. However, with propylene, this intermediate

could not be isolated due to subsequent C–C bond formation with a second equivalent of

propylene at the secondary C. The isobutylene compounds 4.8 and 4.9 do not undergo this

second addition due to the presence of a tertiary C precluding subsequent attack. We were thus

interested to see if C–C bond formation was still possible using 4.8 or 4.9 with other olefins.

The reactions of 4.8 and 4.9 with ethylene were probed. In the case of 4.9, treatment with

ethylene resulted only in the release of isobutylene and generation of the known ethylene

addition product tBu3P(CH2CH2)B(C6F5)3.101

Additional borane had no effect on product

formation. In marked contrast, reaction of 4.8 with ethylene resulted in a reaction that was slow

at ambient temperature and accelerated at 60 °C and was complete in 6 h. NMR analysis

revealed the formation of three products, 4.10, 4.11, and 4.4 in addition to the generation of a

small amount of Al(C6F5)3. Based on the integration of the 19

F{1H} NMR spectrum of the

reaction, compound 4.4 and Al(C6F5)3 accounted for about 20% of the Al while the major

products were compounds 4.10 and 4.11. This mixture could be separated by careful workup

(vide infra). NMR data for 4.10 revealed the presence of the [tBu3PH]+ cation along with three

multiplets in the 19

F{1H} NMR spectrum at -122.1, -157.4 and -163.5 ppm and a broad singlet in

the 27

Al NMR spectrum at 116 ppm, indicative of a symmetric aluminate species. Together with

elemental analysis, compound 4.10 was identified as the salt [tBu3PH][Al(C6F5)4].

Detailed 1D and 2D NMR analysis of 4.11 revealed the incorporation of an ethylene fragment

into the Al-allyl moiety of 4.8. This species also exhibited 19

F resonances at -122.0, -152.1, and

-160.8 ppm while the 27

Al signal was too broad to be observed. Together with elemental

analysis, this data suggests the formulation of 4.11 as CH2=C(Me)(CH2)3Al(C6F5)2, derived from

ethylene insertion into an Al-allyl in a similar mechanism to the observed chemistry with

propylene above (Scheme 4.4). This insertion of olefin into Al–C bonds is also reminiscent of

chemistry first published by Ziegler (see Chapter 1).4,45

106

Further inspection of the 13

C{1H} NMR spectrum of 4.11 reveals the substituted carbon of the

olefinic residue exhibits a resonance at 188.5 ppm, significantly downfield shifted compared to

the corresponding resonance of the free alkene analog 2-methyl-1-pentene (147 ppm). This is

similar to the deshielded resonance noted for the pendant olefin 4.7, as well as previously

observed Al-vinyl species.243

In addition to this, the terminal olefinic carbon is observed as a

pentet at 105.5 ppm in the 13

C{1H} NMR spectrum.

13C NMR with both

1H and

19F decoupling

reduced this signal to a singlet implying 13

C–19

F coupling of this carbon to the four equivalent

ortho-fluorine atoms of the two C6F5 rings (Figure 4.7). Collectively, these data suggest that the

terminal C is weakly bound to the Al centre resulting in a polarized double bond and a locked

conformation analogous to that observed in 4.7.

Figure 4.7 – 13

C{1H} (top) and

13C{

1H,

19F} (bottom) NMR spectra of the terminal olefinic

resonance of 4.11 in C6D5Br.

Attempts to obtain crystals of 4.11 suitable for X-ray diffraction failed; therefore, its structure

was probed via DFT computations performed at the M06-2X/6-311++G(d,p) level215

(Figure

4.8). The computed geometry shows a slight pyramidalization of Al (sum of angles is 352.9⁰) as

the olefinic fragment is oriented near Al with Al–C distances of 2.39 Å and 2.67 Å for the

terminal and substituted carbons, respectively. Mulliken charges at the terminal and substituted

carbons were found to be -1.149 and 0.689, consistent with a highly polarized olefin. This is also

107

consistent with the 13

C NMR data and the analysis of the molecular orbitals which reveals an

interaction of the terminal C with the Al centre. This is similar to the closely related B species

shown in Scheme 4.2,107

as well as Al species where interactions with olefin fragments have

been observed.65

Figure 4.8 – Computed geometry of 4.11 showing Al–C distances and Mulliken charges on

olefinic carbon atoms.

The formation of 4.10, 4.11, and a minor amount of 4.4 is consistent with competing reaction

pathways resulting from the equilibrium governing the formation of 4.8 (Scheme 4.5). Thus,

thermal loss of isobutylene generates free phosphine and alane which can react with ethylene to

give 4.4 with residual Al(C6F5)3. Alternatively, direct reaction of ethylene with 4.8 results in

insertion into the Al–C bond of the Al-allyl fragment and subsequent aryl group redistribution

affording the salt 4.10 and the alane 4.11. While separation of the salts 4.4 and 4.10 from the

alanes 4.11 and Al(C6F5)3 is readily achieved based on solubility, the isolation of 4.11 from

Al(C6F5)3 required an additional strategy. Based on the integration of the 19

F NMR spectrum, an

amount of PtBu3 equivalent to that of Al(C6F5)3 present was added. Further exposure to ethylene,

converted the residual Al(C6F5)3 and PtBu3 to 4.4, allowing the extraction of 4.11 into hexanes.

In a separate procedure, 4.10 was separated from 4.4 by careful precipitation following the

removal of the Al(C6F5)3 and 4.11 residues.191

108

Scheme 4.5 – Reaction pathways of 4.8 with ethylene affording 4.4 (minor) and 4.10/4.11

(major).

4.2.4.2 Reaction with CO2

In a similar strategy to the ethylene chemistry, C–C bond forming reactions were probed using

other small molecules. Compounds 4.8 and 4.9 were separately exposed to an atmosphere of

13CO2 in a J-Young NMR tube. In a similar fashion to the ethylene chemistry, compound 4.9

rapidly released isobutylene and generated the known compound tBu3P(CO2)B(C6F5)3.110

In

contrast, 4.8 generated a new species with a resonance at 183.1 ppm from the 13

CO2 moiety in

the 13

C NMR spectrum. Two new broad singlets each integrating to one proton were seen at 4.82

and 4.75 ppm in the 1H NMR spectrum, indicative of two olefinic protons. A methyl and a

methylene resonance adjacent to a carboxylate, each integrating to three and two protons,

respectively, were seen at 1.73 and 3.40 ppm. The 19

F NMR spectrum displayed only slightly

shifted resonances as compared to the starting material 4.8, and is indicative of equivalent C6F5

rings around both Al centres. While crystals suitable for X-ray diffraction could not be obtained

for this product, this data, together with combustion analysis, is indicative of the product derived

from attack of the allyl at a CO2 molecule to generate a bis-aluminum carboxylate product (4.12)

in good yield as shown in Scheme 4.6.

109

Scheme 4.6 – Synthesis of 4.12 from 4.8 and CO2.

It is interesting to note that the product 4.12 does not redistribute to form the salt 4.10 and an

alane as seen with the ethylene insertion reaction into 4.8 to generate 4.10 and 4.11. While a

[(µ-Me)(Al(C6F5)3)2]- anion is known

244 and is not reported to redistribute, it is likely that the

larger alkyl group in 4.11 sterically disfavours the formation of a stable bridging alkyl species. In

contrast, the bidentate nature of the carboxylate group in 4.12 likely prevents the redistribution

from occurring due to both reduced sterics and strong Al–O bonds.

While the reason for the difference in reactivity of 4.8 and 4.9 toward C–C bond formation

remains uncertain, it is possible that the second equivalent of Lewis acid in 4.8 temporarily

dissociates from the allyl moiety to activate a second molecule, whether olefin or CO2, for

subsequent nucleophilic attack by the resulting mono allyl-aluminate (Scheme 4.7). Attempts to

demonstrate this premise were unsuccessful; however, as described in Chapter 3 and as

demonstrated by 4.11, Lewis acidic Al centres can polarize olefinic bonds thereby making them

susceptible to nucleophilic attack. This activation by the smaller, less Lewis acidic B centre may

be significantly reduced (even with excess B(C6F5)3), providing enough time for sequential

equilibrium release of isobutylene and generation of the known addition products as described

here.

110

Scheme 4.7 – Proposed divergent reaction pathways of 4.8 (right) vs. 4.9 (left) toward attack at

ethylene to generate either the known B compound,101

tBu3P(CH2CH2)B(C6F5)3, or the new Al

compounds 4.10 and 4.11. Analogous pathways can be applied to the chemistry with CO2.

4.3 Conclusions

This chapter has described the reversible vs. irreversible addition chemistry of PR3/AlX3 FLPs

with ethylene (X = C6F5 (R = tBu, otol, Mes); X = halide (R = Mes)). The reactivity of these

FLPs with a bulkier olefin, propylene, proceeded in an unexpected fashion through C–H bond

activation followed by attack at a second propylene and redistribution to yield a phosphonium

aluminate, [R3PH][AlX4], and an alane, R'AlX2 (see products 4.6/4.7 and 4.10/4.11). These

reactions imply the intermediate formation of bis-aluminum allyl fragments. Such a species was

successfully obtained using the basic phosphine, PtBu3, and the substituted olefin, isobutylene. A

B σ-allyl complex was also obtained. These species are the first examples of FLP-based C–H

activation of unactivated olefins. While the mono-B-allyl anion forms reversibly, the bis-Al-allyl

anion reacts with either ethylene or CO2 to effect insertion into the Al–C bond affording a new

approach to C–C bond formation and olefin elaboration.





-38 °C freezer). Diethyl ether, hexanes, and pentane (Aldrich) were dried using an Innovative


111


prior to use. Dichloromethane-d2 was purchased from Aldrich, dried on CaH2 and was vacuum

distilled onto 4 Å molecular sieves prior to use. Trimethylaluminum (TMA), PtBu3, and PMes3

were purchased from Strem and used without further purification. AlX3 were purchased from

Strem and sublimed three times prior to use under vacuum using a -78 °C cold finger and a

100 °C (X = Cl), 80 °C (X = Br), or 150 °C (X = I) bath. Me2SiHCl was purchased from Aldrich

and used without further purification. B(C6F5)3 was purchased from Boulder Scientific, sublimed

under vacuum, then treated with excess Me2SiHCl for 4 h and re-sublimed after removal of

volatiles. Al(C6F5)3•tol was prepared from B(C6F5)3 and TMA in toluene by a known

procedure.175

Isobutylene was purchased from Aldrich and used without further purification.

Ethylene (grade 3.0) and propylene (grade 2.5) were purchased from Linde and passed through a

Restek oxygen scrubber and a Restek moisture trap prior to use. CO2 (grade 4.0) was purchased

from Linde and passed through a Drierite column prior to use.

NMR spectra were obtained on a Bruker Avance 400 MHz, a Varian 400 MHz, an Agilent DD2

500 MHz, or an Agilent 600 MHz NMR spectrometer and spectra were referenced to residual

solvent of C6D5Br (1H = 7.28 ppm for meta proton;

13C = 122.4 ppm for ipso carbon) and

CD2Cl2 (1H = 5.32 ppm;

13C = 53.84 ppm), or externally (

27Al: Al(NO3)3,

11B: (Et2O)BF3,

31P:

85% H3PO4, 19

F: CFCl3). Chemical shifts (δ) listed are in ppm and absolute values of the

coupling constants are in Hz. NMR assignments are supported by additional 2D experiments.

Elemental analyses (C, H) and X-ray crystallography were performed in house.


Synthesis of Mes3P(CH2CH2)(AlX3)2 (4.1-4.3): These compounds were all synthesized in a

similar fashion; therefore, only one synthesis is described.

A 50 mL Schlenk bomb equipped with a Teflon cap was charged with PMes3 (150 mg, 0.39

mmol) and AlBr3 (206 mg, 0.77 mmol) in 10 mL of fluorobenzene. The bomb was transferred to

the Schlenk line equipped with an ethylene outlet. The bomb was degassed at r.t., filled with

ethylene, and sealed. The solution was stirred rapidly overnight (ca. 12 h). The ethylene

atmosphere was removed. Precipitation using hexanes (ca. 5-10 mL) afforded a white solid

which was filtered and dried on a frit.

112


hexanes yielded single crystals suitable for X-ray crystallography.

1H NMR (400 MHz, C6D5Br): δ 6.68 (bs, 6H), 3.28 (dt,

2JH-P = 8.6 Hz,

3JH-H = 8.6 Hz, 2H, P-

CH2), 2.08 (bs, 9H, o-CH3Mes

), 2.05 (s, 9H, p-CH3Mes

), 1.73 (bs, 9H, o-CH3Mes

), 0.68 (bs, 2H,

CH2-Al). 31

P{1H} NMR (161 MHz, C6D5Br): δ 20.0.

27Al NMR (104 MHz, C6D5Br): δ 106.0

(bs, υ1/2 = 1200 Hz). 13

C{1H} NMR (100 MHz, C6D5Br): δ 144.3 (d,

4JC-P = 3.0 Hz, p-C6H2),

143.6 (d, 2JC-P = 10.1 Hz, o-C6H2), 132.8 (d,

3JC-P = 11.5 Hz, m-C6H2), 120.2 (d,

1JC-P = 73 Hz, i-

C6H2), 34.8 (d, 1JC-P = 39 Hz, P-CH2), 24.9 (bs, o-CH3

Mes), 24.1 (bs, o-CH3

Mes), 21.1 (s, p-

CH3Mes

), 10.0 (bs, CH2-Al). Anal. Calc. for C29H37Al2Cl6P: C, 50.98; H, 5.46. Found: C, 50.76;

H, 5.48.

4.2: Isolated yield: 79%. Slow cooling a saturated bromobenzene solution to -38 °C yielded


1H NMR (400 MHz, C6D5Br): δ 6.69 (bs, 6H), 3.31 (dt,

2JH-P = 8.7 Hz,

3JH-H = 8.7 Hz, 2H, P-

CH2), 2.11 (bs, 9H, o-CH3Mes

), 2.05 (s, 9H, p-CH3Mes

), 1.74 (bs, 9H, o-CH3Mes

), 0.89 (bs, 2H,

CH2-Al). 31

P{1H} NMR (161 MHz, C6D5Br): δ 21.0.

27Al NMR (104 MHz, C6D5Br): δ blank

(signal likely lost in the probe signal). 13

C{1H} NMR (100 MHz, C6D5Br): δ 144.3 (d,

4JC-P = 3.0

Hz, p-C6H2), 143.6 (d, 2JC-P = 9.7 Hz, o-C6H2), 132.8 (d,

3JC-P = 11.1 Hz, m-C6H2), 120.1 (d,

1JC-

P = 74 Hz, i-C6H2), 34.9 (d, 1JC-P = 39 Hz, P-CH2), 25.0 (bs, o-CH3

Mes), 23.9 (bs, o-CH3

Mes), 21.0

(s, p-CH3Mes

), 13.0 (bs, CH2-Al). Anal. Calc. for C29H37Al2Br6P: C, 36.67; H, 3.93. Found: C,

37.24; H, 4.01.



1H NMR (400 MHz, C6D5Br): δ 6.76 (bs, 3H), 6.63 (bs, 3H), 3.34 (dt,

2JH-P = 9.5 Hz,

3JH-H = 8.3

Hz, 2H, P-CH2), 2.18 (bs, 9H, o-CH3Mes

), 2.05 (s, 9H, p-CH3Mes

), 1.74 (bs, 9H, o-CH3Mes

), 1.19

(dt, 3

JH-P = 10.0 Hz, 3

JH-H = 10.0 Hz, 2H, CH2-Al). 31

P{1H} NMR (161 MHz, C6D5Br): δ 18.8.

27Al NMR (104 MHz, C6D5Br): δ -15.0 (bs, υ1/2 = 1500 Hz).

13C{

1H} NMR (100 MHz,

C6D5Br): δ 144.4 (d, 4JC-P = 2.9 Hz, p-C6H2), 143.6 (d,

2JC-P = 9.3 Hz, o-C6H2), 132.9 (d,

3JC-P =

11.5 Hz, m-C6H2), 120.2 (d, 1JC-P = 73 Hz, i-C6H2), 35.6 (d,

1JC-P = 39 Hz, P-CH2), 25.6 (bs, o-

113

CH3Mes

), 24.0 (bs, o-CH3Mes

), 21.1 (s, p-CH3Mes

), 14.3 (bs, CH2-Al). Anal. Calc. for

C29H37Al2I6P: C, 28.27; H, 3.03. Found: C, 28.41; H, 3.27.

Synthesis of R3P(CH2CH2)Al(C6F5)3 (4.4 and 4.5): These compound were synthesized in an

analogous manner to 4.1-4.3 using their respective phosphines, PtBu3 (4.4) and P(otol)3 (4.5),

and Al(C6F5)3•tol.



1H NMR (400 MHz, C6D5Br): δ 2.26-2.19 (m, 2H, PCH2), 0.99 (d,

3JH-P = 13.2 Hz, 27H, tBu),

0.88-0.81 (m, 2H, CH2Al). 31

P{1H} NMR (161 MHz, C6D5Br): δ 45.1.

27Al NMR (104 MHz,

C6D5Br): δ 132 (bs, υ1/2 = ca. 1100 Hz). 19


3JF-F =

29.3 Hz, 4JF-F = 12.0 Hz, 6F, o-C6F5), -156.0 (t,

3JF-F = 19.9 Hz, 3F, p-C6F5), -162.2 (m, 6F, m-

C6F5). 13


1JC-F = 228 Hz), 140.4 (dm,

1JC-F = 242

Hz), 136.7 (dm, 1

JC-F = 250 Hz), 118.9 (m, i-C6F5), 38.5 (d, 1

JC-P = 27.7 Hz, PCMe3), 29.2 (s,

PCMe3), 16.7 (d, 1

JC-P = 21.7 Hz, PCH2), 6.0 (bs, CH2Al). Anal. Calc. for C32H31AlF15P: C,

50.67; H, 4.12. Found: C, 50.37; H, 4.08.



1H NMR (400 MHz, C6D5Br): δ 7.37-6.91 (m, 12H), 3.34 (dt,

2JH-P = 7.7 Hz,

3JH-H = 7.7 Hz, 2H,

P-CH2), 1.85 (bs, 9H, o-CH3), 0.63-0.55 (m, 2H, CH2Al). 31


29. 27

Al NMR (104 MHz, C6D5Br): δ 142 (bs, υ1/2 = ca. 2300 Hz). 19

F{1H} NMR (376 MHz,

C6D5Br): δ -121.4 (dd, 3JF-F = 29.6 Hz,

4JF-F = 12.2 Hz, 6F, o-C6F5), -155.9 (t,

3JF-F = 20.0 Hz,

3F, p-C6F5), -162.2 (m, 6F, m-C6F5). 13


1JC-F =

236 Hz), 142.9 (d, JC-P = 7.5 Hz), 140.4 (dm, 1JC-F = 248 Hz), 136.6 (dm,

1JC-F = 249 Hz), 134.6

(d, JC-P = 2.3 Hz), 134.4 (d,

JC-P = 9.9 Hz), 133.5 (d,

JC-P = 10.1 Hz), 127.2 (d,

JC-P = 11.5 Hz),

118.9 (m, i-C6F5), 117.2 (d, JC-P = 78.7 Hz, i-C6H4), 23.4 (d, 1

JC-P = 33.5 Hz, PCH2), 22.2 (d,

3JC-P = 3.8 Hz, o-CH3), 6.0 (bs, CH2Al). Anal. Calc. for C41H25AlF15P + 0.5•C6H5F: C, 58.16; H,

3.05. Found: C, 58.65; H, 2.93.

114

Reaction of PMes3/AlI3 with propylene to generate 4.6 and 4.7: A 50 mL Schlenk bomb

equipped with a Teflon cap was charged with PMes3 (286 mg, 0.74 mmol) and AlI3 (600 mg,

1.47 mmol) in 10 mL of fluorobenzene. The bomb was transferred to the Schlenk line equipped

with a propylene outlet. The bomb was degassed at r.t., filled with propylene, and sealed. The

mixture was stirred rapidly for 36 h. The propylene atmosphere and solvent were removed in

vacuo. Hexanes (ca. 20 mL) were added to the remaining oily residue and the mixture was stirred

vigorously for 1 h upon which time a white precipitate formed. The solid was filtered on a glass

frit and washed with copious amounts of hexanes. Isolated yield of 4.6 was 480 mg (0.52 mmol,

71%).

The solvent was thoroughly removed from the filtrate to obtain an oil. Pentane (10 mL) was

added to this and a residual precipitate was filtered on Celite. The solvent from this filtrate was

thoroughly removed to obtain 120 mg of 4.7 as a viscous oil (0.33 mmol, 45%).

4.6: 1H NMR (400 MHz, C6D5Br): δ 8.19 (d,

1JH-P = 482 Hz, 1H, P-H), 6.78 (bs, 3H), 6.73 (bs,

3H), 2.16 (bs, 9H, o-CH3Mes

), 2.11 (s, 9H, p-CH3Mes

), 1.80 (bs, 9H, o-CH3Mes

). 31

P{1H} NMR

(161 MHz, C6D5Br): δ -26.0. 27

Al NMR (104 MHz, C6D5Br): δ -25. 13

C{1H} NMR (100 MHz,

C6D5Br): δ 146.7 (d, 4JC-P = 2.7 Hz, p-C6H2), 143.7 (bs, o-C6H2), 142.9 (bs, o-C6H2), 133.1 (bs,

m-C6H2), 131.9 (bs, m-C6H2), 111.4 (d, 1JC-P = 83 Hz, i-C6H2), 22.3 (bs, o-CH3

Mes), 22.1 (bs, o-

CH3Mes

), 21.6 (s, p-CH3Mes

). Anal. Calc. for C27H34AlI4P: C, 35.09; H, 3.71. Found: C, 35.01; H,

3.75.

4.7: 1H NMR (600 MHz, C6D5Br): δ 6.73 (dddd,

3JHc-Hb = 18.2,

3JHc-Hd = 11.0,

3JHc-Ha = 9.3,

3JHc-

He = 4.9, 1H, Hc), 5.62 (dd, 3JHa-Hc = 9.4,

2JHa-Hb = 2.4, 1H, Ha), 5.48 (ddd,

3JHb-Hc = 18.3,

2JHb-Ha =

2.9, 4JHb-He = 1.5, 1H, Hb), 1.93 (ddddd,

2JHe-Hd = 11.9,

3JHe-Hc = 5.1,

3JHe-Hf = 3.4,

4JHe-Hb = 1.6,

4JHe-Hh = 1.6, 1H, He), 1.83 – 1.71 (m, 1H, Hf), 1.37 (ddd,

2JHd-He = 11.3,

3JHd-Hc = 11.3,

3JHd-Hf =

11.3, 1H, Hd), 0.87 (d, 3JH-Hf = 6.5, CH3, 3H), 0.57 (ddd,

2JHh-Hg = 14.3,

3JHh-Hf = 4.3,

4JHh-He =

1.7, 1H, Hh), 0.03 (dd, 2JHg-Hh = 14.3,

3JHg-Hf = 12.3, 1H, Hg).

27Al NMR (104 MHz, C6D5Br): δ

130.0 (bs, υ1/2 = 2000 Hz). 13

C{1H} NMR (150 MHz, C6D5Br): δ 163.5 (s, C2), 123.0 (s, C1),

45.4 (s, C3), 33.4 (s, C4), 25.7 (s, CH3), 23.8 (bs, C5). Anal. Calc. Satisfactory EA for this

compound could not be obtained.

115

Synthesis of 4.8: A 50 mL Schlenk bomb equipped with a Teflon screw cap and a magnetic

stirbar was charged with PtBu3 (122 mg, 0.60 mmol) and Al(C6F5)3•tol (750 mg, 1.21 mmol) in

the glovebox. Fluorobenzene (10 mL) was added to this all at once. The bomb was transferred to

the Schlenk line equipped with an isobutylene outlet. The bomb was degassed, filled with

isobutylene (1 atm), and sealed. The solution was stirred rapidly for 1 h after which the

isobutylene atmosphere and solvent were removed in vacuo. In the glovebox, hexanes (ca. 10

mL) were added to the residue. A white precipitate formed and the mixture was stirred rapidly

for an additional 30-60 min.. The precipitate was filtered on a glass frit, washed with hexanes

and dried in vacuo (680 mg, 0.52 mmol, 86%). Careful slow cooling of a concentrated

bromobenzene solution at -38 °C of the compound yielded single crystals suitable for X-ray

crystallography.

1H NMR (400 MHz, C6D5Br): δ 4.13 (d,

1JH-P = 426 Hz, 1H, P-H), 3.91 (bs, 4H, 2 x allyl-CH2),

2.10 (s, 3H, allyl-CH3), 0.98 (d, 3JH-P = 16.0 Hz, 27H, tBu).

31P{

1H} NMR (161 MHz, C6D5Br):

δ 60.2. 27

Al NMR (104 MHz, C6D5Br): δ 139 (bs, υ1/2 = ca. 2400 Hz). 19

F{1H} NMR (376 MHz,

C6D5Br): δ -120.9 (dd, 3JF-F = 28.2 Hz,

4JF-F = 11.3 Hz, 12F, o-C6F5), -154.2 (t,

3JF-F = 19.9 Hz,

6F, p-C6F5), -161.5 (m, 12F, m-C6F5). 13


1JC-F =

227 Hz), 141.0 (dm, 1JC-F = 249 Hz), 136.8 (dm,

1JC-F = 250 Hz), 115.9 (m, i-C6F5), 59.6 (s, allyl-

CH2), 36.9 (d, 1

JC-P = 26.2 Hz, PCMe3), 29.3 (s, PCMe3), 29.1 (s, allyl-CH3), 6.3 (allyl-C) *this

peak is blank (very broad) in the 13

C NMR but was located by 1H-

13C HMBC experiment. Anal.

Calc. for C52H35Al2F30P: C, 47.50; H, 2.68. Found: C, 47.04; H, 2.81.

116

Figure 4.9 – Partial variable temperature 1H NMR spectra of 4.8 in C6D5Br.


stirbar was charged with PtBu3 (119 mg, 0.59 mmol) and B(C6F5)3 (300 mg, 0.59 mmol) in the

glovebox. Fluorobenzene (5 mL) was added to this all at once. The bomb was transferred to the

Schlenk line equipped with an isobutylene outlet. The bomb was degassed, filled with

isobutylene (1 atm), and sealed. The solution was stirred rapidly for 1 h after which the

isobutylene atmosphere was removed. In the glovebox, pentane (ca. 10 mL) was added dropwise.

A white precipitate formed which was filtered on a glass frit, washed with pentane and dried in

vacuo (375 mg, 0.49 mmol, 83%). Slow cooling of a concentrated bromobenzene solution at

-38 °C of the compound yielded single crystals suitable for X-ray crystallography. Due to the

instability of this compound in solution, the NMR spectra were collected at -40 °C.

1H NMR (400 MHz, CD2Cl2, -40 °C): δ 5.02 (d,

1JH-P = 429 Hz, 1H, P-H), 4.01 (d,

2JH-H = 2.8

Hz, 1H, =CH2), 3.83 (d, 2JH-H = 2.8 Hz, 1H, =CH2), 2.20 (s, 2H, B-CH2), 1.59 (d,

3JH-P = 16.0

Hz, 27H, tBu), 1.45 (s, 3H, CH3). 11

B{1H} NMR (128 MHz, CD2Cl2, -40⁰C): -13.2 (s).

19F{

1H}

NMR (376 MHz, CD2Cl2, -40 °C): δ -131.6 (bd, 3JF-F = 22.6 Hz, 6F, o-C6F5), -164.3 (t,

3JF-F =

21.0 Hz, 3F, p-C6F5), -167.2 (bt, 3JF-F = 20.3 Hz, 6F, m-C6F5).

13C{

1H} NMR (100 MHz,

CD2Cl2, -40 °C): δ 152.5 (s, CH2=C), 147.9 (dm, 1JC-F = 237 Hz), 137.1 (dm,

1JC-F = 240 Hz),

136.0 (dm, 1

JC-F = 244 Hz), 126.5 (m, i-C6F5), 105.8 (s, CH2=C), 37.4 (d, 1

JC-P = 26.9 Hz,

PCMe3), 33.5 (bs, CH2-B), 29.7 (s, PCMe3), 23.4 (s, Me). Anal. Calc. for C34H35BF15P: C,

53.01; H, 4.58. Found: C, 53.21; H, 4.60.

117

Reaction of 4.8 with ethylene to produce 4.10 and 4.11: A 50 mL Schlenk bomb equipped

with a Teflon screw cap and a magnetic stirbar was charged with a fluorobenzene solution of 4.8

in the glovebox. The bomb was transferred to the Schlenk line equipped with an ethylene outlet.

The bomb was degassed, filled with ethylene (1 atm), and sealed. The solution was stirred

rapidly for 6 h at 60⁰C. The following steps for the isolation of 4.10 and 4.11 are different and

are described separately below.

Isolation of 4.10: In this preparation, 250 mg (0.19 mmol) of 4.8 was used in 5 mL

fluorobenzene. After heating with ethylene, the bomb was brought in the glovebox and the

ethylene atmosphere removed. The solution was filtered on Celite and the solvent removed in

vacuo in a vial with a stirbar. Hexanes (ca. 5 mL) were added to the residue and the solution was

stirred rapidly for 10 min after which time a white precipitate had formed. The precipitate was

filtered on a glass frit, washed with hexanes and dried in vacuo to afford 135 mg of a mixture of

4.10 and 4.4. This mixture was dissolved in fluorobenzene (3 mL) and diethyl ether (1 mL).

Hexanes were added very slowly with rapid stirring. A total of 5 mL was added. Compound 4.10

crashed out of solution after the addition of hexanes. The solution was decanted and the white

solid washed with hexanes and dried in vacuo (50 mg, 56 µmol, 30 % based on the amount of

4.8 used).

1H NMR (400 MHz, C6D5Br): δ 4.16 (d,

1JH-P = 428 Hz, 1H, P-H), 0.94 (d,

3JH-P = 16.0 Hz,

27H, tBu). 31

P{1H} NMR (161 MHz, C6D5Br): δ 60.2.

27Al NMR (104 MHz, C6D5Br): δ 116

(bs, υ1/2 = ca. 500 Hz). 19


3JF-F = 26.7 Hz,

4JF-F =


13C{

1H}

NMR (100 MHz, C6D5Br), partial: δ 150.2 (dm, 1JC-F = 231 Hz), 140.4 (dm,

1JC-F = 246 Hz),

136.5 (dm, 1

JC-F = 252 Hz), 36.8 (d, 1

JC-P = 26.4 Hz, PCMe3), 29.2 (s, PCMe3). Anal. Calc. for

C36H28AlF20P: C, 48.12; H, 3.14. Found: C, 48.38; H, 3.45.

Isolation of 4.11: In this preparation, 500 mg (0.38 mmol) of 4.8 was used in 10 mL

fluorobenzene. After heating with ethylene, the bomb was brought in the glovebox and the

ethylene atmosphere removed. A sample of the solution was taken and the 1H NMR spectrum of

it was recorded in C6D5Br in order to determine the ratio of 4.4 present, taken with reference to

4.11. In a typical experiment, 20-25 % of 4.8 produced 4.4 and Al(C6F5)3 through release of

isobutylene and capture of ethylene. A molar equivalent to 4.4 of PtBu3 (ca. 18 mg, 89 µmol (for

118

~24%)) dissolved in fluorobenzene (1 mL) was added to the bulk solution and the flask was

refilled with ethylene (1 atm) and the solution stirred for 1 h at r.t. The ethylene atmosphere was

removed, the solution was filtered on Celite, and the solvent removed in vacuo in a vial with a

stirbar. Hexanes (ca. 10 mL) were added to the residue and the solution was stirred rapidly for

ca. 10 min after which time a white precipitate had formed. The precipitate was filtered on a

glass frit, washed with hexanes (2 x 5 mL) and dried in vacuo to afford 320 mg of a mixture of

4.10 and 4.4. The filtrate was brought to dryness in vacuo. Fresh hexanes (ca. 10 mL) were

added to the residue and the solution was filtered on Celite. The solvent was removed to obtain

an oil which was placed in the -38 °C freezer overnight. The off-white solid that formed was

isolated and put under vacuum for several minutes yielding pure 4.11 (65 mg, 0.15 mmol, 38 %

based on the amount of 4.8 used).

1H NMR (400 MHz, C6D5Br): δ 5.35 (d,

2JH-H = 4.0 Hz, 1H, Ha), 5.24 (d,

2JH-H = 4.0 Hz, 1H,

Hb), 2.02 (m, 2H, C(4)

H2), 1.90 (m, 2H, C(5)

H2), 1.43 (s, 3H, C(1)

H3), 0.40 (bt, 3

JH-H = 6.0 Hz, 2H,

C(6)

H2). 27



3JF-F = 28.2 Hz,

4JF-F = 11.7 Hz, 4F, o-C6F5), -152.1 (t,

3JF-F = 19.9 Hz, 2F, p-C6F5), -160.8 (m,

4F, m-C6F5). 13

C{1H} NMR (100 MHz, C6D5Br), partial: δ 188.5 (s, C

(2)), 149.6 (dm,

1JC-F = 233

Hz), 141.7 (dm, 1JC-F = 253 Hz), 136.8 (dm,

1JC-F = 253 Hz), 105.5 (p,

4JC-F = 3.0 Hz, C

(3)), 42.8

(s, C(4)

), 26.3 (s, C(5)

), 25.4 (s, C(1)

), 8.4 (bs, C(6)

). Anal. Calc. for C18H11AlF10: C, 48.67; H,

2.50. Found: C, 48.52; H, 2.62.


stirbar was charged with a fluorobenzene (5 mL) solution of 4.8 (275 mg, 0.21 mmol) in the

glovebox. The bomb was transferred to the Schlenk line equipped with a CO2 outlet. The bomb

was degassed, filled with CO2 (1 atm), and sealed. The solution was stirred rapidly for 4 h at r.t.

The gas and solvent were then removed in vacuo to obtain an oily residue. Addition of hexanes

(ca. 10-15 mL) to the residue followed by stirring for 30 min resulted in the isolation of a pure

white powder after filtration on a glass frit (210 mg, 0.15 mmol, 74%).

119

1H NMR (400 MHz, C6D5Br): δ 4.82 (s, 1H, =CH2), 4.75 (s, 1H, =CH2), 4.13 (d,

1JH-P = 426 Hz,

1H, P-H), 3.40 (s, 2H, CH2), 1.73 (s, 3H, CH3), 0.97 (d, 3JH-P = 15.6 Hz, 27H, tBu).

31P{

1H}

NMR (161 MHz, C6D5Br): δ 60.4. 27


F{1H} NMR (376

MHz, C6D5Br): δ -122.9 (dd, 3JF-F = 27.0 Hz,

4JF-F = 10.5 Hz, 12F, o-C6F5), -154.9 (t,

3JF-F =

19.9 Hz, 6F, p-C6F5), -162.7 (m, 12F, m-C6F5). 13

C{1H} NMR (100 MHz, C6D5Br): δ 183.1 (s, -

CO2), 150.2 (dm, 1JC-F = 232 Hz), 141.2 (dm,

1JC-F = 248 Hz), 136.6 (dm,

1JC-F = 251 Hz), 136.4

(s, CH2=C), 116.7 (s, CH2=C), 115.2 (m, i-C6F5), 46.3 (s, CH2-CO2), 36.9 (d, 1

JC-P = 26.3 Hz,

PCMe3), 29.3 (s, PCMe3), 22.1 (s, Me). Anal. Calc. for C53H35Al2F30O2P: C, 46.85; H, 2.60.

Found: C, 46.34; H, 2.84.

4.4.3 Computational detail

DFT calculations were performed using Gaussian 09.245

The geometry of 4.11 was optimized at

the M06-2X/6-311++G(d,p) and the structure was confirmed to be a minimum by frequency

analysis showing no imaginary frequencies.












The heavy





2/2 (Fo

2) and Fo


120









121


Table 4.1 – Selected crystallographic data for 4.1, 4.2, and 4.3.

4.1 4.2 4.3

Formula C29H37Al2Cl6P1 C29H37Al2Br6P1 C29H37Al2I6P1

Formula wt. 683.26 949.96 1231.97

Crystal system triclinic triclinic triclinic

Space group P-1 P-1 P-1

a(Å) 10.9674(10) 11.2147(15) 11.611(4)

b(Å) 11.7777(12) 11.8073(15) 12.081(4)

c(Å) 14.9368(15) 15.117(2) 15.584(5)

α(deg) 80.549(4) 80.640(7) 82.234(11)

β(deg) 87.525(4) 87.175(6) 86.866(11)

γ(deg) 63.969(4) 63.311(5) 62.200(10)

V(Å3) 1709.2(3) 1764.0(4) 1915.8(1)

Z 2 2 2

T (K) 150(2) 150(2) 150(2)

d(calc) g/cm3 1.328 1.789 2.073

Abs coeff, μ, mm-1

0.619 6.942 4.967


Rint 0.0404 0.0277 0.0318

Data used 7658 8145 8574


R (>2σ) 0.0494 0.0301 0.0618

wR2 0.1614 0.1066 0.2246

GOF 1.006 0.752 1.451

122


4.4 4.5 4.8 (+ 2•C6H5Br)

Formula C32H31Al1F15P1 C41H25Al1F15P1 C64H45Al2Br2F30P1

Formula wt. 758.52 860.57 1628.75

Crystal system monoclinic monoclinic triclinic

Space group P21/c P21/n P-1

a(Å) 11.1182(4) 11.4324(7) 12.6080(7)

b(Å) 18.3639(7) 19.0421(14) 13.8369(8)

c(Å) 17.0112(6) 20.4558(14) 19.2878(11)

α(deg) 90 90 96.396(2)

β(deg) 106.677(2) 104.824(2) 97.763(2)

γ(deg) 90 90 101.782(2)

V(Å3) 3327.1(2) 4304.9(5) 3230.1(3)

Z 4 4 2

T (K) 223(2) 150(2) 150(2)

d(calc) g/cm3 1.514 1.451 1.675

Abs coeff, μ, mm-1

0.216 1.106 1.435


Rint 0.0336 0.0467 0.0326

Data used 7967 9879 15134


R (>2σ) 0.0527 0.1445 0.0736

wR2 0.1564 0.4206 0.2289

GOF 1.032 2.415 1.018

123

Table 4.3 – Selected crystallographic data for 4.9.

4.9

Formula C34H35B1F15P1

Formula wt. 770.4

Crystal system triclinic

Space group P-1

a(Å) 8.6571(3)

b(Å) 13.0426(5)

c(Å) 15.3468(6)

α(deg) 84.625(2)

β(deg) 85.379(2)

γ(deg) 78.860(2)

V(Å3) 1689.2(1)

Z 2

T (K) 150(2)

d(calc) g/cm3 1.515

Abs coeff, μ, mm-1

0.190

Data collected 46438

Rint 0.0300

Data used 12858

Variables 474

R (>2σ) 0.0395

wR2 0.1104

GOF 1.036

124

Chapter 5 H2 Activation and Hydride Transfer to Unactivated Olefins:

Towards Transition Metal-Free Catalytic Hydrogenation

5.1 Introduction

The homogeneous catalytic reduction of olefins has been deemed to be the purview of transition

metal catalysts since the seminal work of Wilkinson on Rh and Ru species, some 50 years.246,247

Since then, numerous advances have evolved such metal-based technologies to include Nobel

prize-winning work on asymmetric hydrogenations.248-252

More recent developments have given

Ru-based olefin-specific reduction catalysts,253

as well as remarkably active hydrogenation

catalysts based on Fe and Co.18,254

An alternative strategy garnering attention is based on non-

transition metal systems. As far back as the 1970s, it was reported that strong Brønsted acids,

such as HBr•AlBr3, could mediate the reduction of some aromatics to alkanes; however, changes

to the C–C framework of the arenes were often observed.255

More recently, main group systems

such as cyclic (alkyl)(amino)carbenes256

or heavier group 14 alkene analogs257,258

have been

shown by the groups of Bertrand or Power, respectively, to effect H2 activation. In very recent

work, the research groups of Harder259

and Okuda260

have developed Ca-based species capable

of reducing the activated olefin, 1,1-diphenylethene.

Frustrated Lewis pairs have been shown to be versatile reagents for the activation of H2, most

often using B-based Lewis acids.30,81,85

The B-based catalytic hydrogenation of several types of

unsaturated bonds, such as imines,93-95

nitriles,93,94

aziridines,93,94

enamines,95

and silyl enol

ethers96

have been reported. Most recently, FLPs have been shown to effect the stoichiometric

reductions of anilines to cyclohexylammonium derivatives,100

as well as effect the catalytic

hydrogenation of activated olefins, such as 1,1-diphenylethylene.99

This latter result is of

particular interest as it is formally the first example of catalytic olefin hydrogenation using FLPs;

however, the olefin substrate scope was limited to those capable of stabilizing carbocation

intermediates as the mechanism was found to proceed through initial protonation of the double

bond, followed by hydride delivery.99

Even with these reported results, the catalytic H2 delivery to simple, unactivated olefins by any

main group system has not been reported. As noted throughout this thesis, Al-based FLP

chemistry remains much less explored than B-based systems. The Uhl group has explored the

125

chemistry of linked P/Al and N/Al systems for the activation of small molecules, such as

alkynes,122,124,125,261

CO2,122,125

and other functional groups.124,125

The group of Fontaine has also

explored CO2 activation using intermolecular P/Al Lewis pairs.121

However, with the exception

of the results presented in this chapter, H2 activation mediated by Al-based FLPs has not been

reported, and thus neither has the subsequent hydrogenation of olefins. This chapter will outline

the results of Al-based FLPs capable of activating H2 and reacting stoichiometrically with the

unactivated olefins ethylene and cyclohexene, to give alkyl-alanes. A plausible mechanism is

considered and is thought to involve aluminum-activation of olefin. This view is supported by

the characterization of the first simple intermolecular aluminum-olefin adduct. A significant

deactivation pathway preventing the catalytic hydrogenation of olefins involves a redistribution

reaction similar to the one observed in the previous chapter. Strategies to develop a system where

this redistribution is suppressed are detailed here in an attempt to promote the catalytic

hydrogenation of unactivated olefins.


5.2.1 H2 Activation

The initial work with FLPs focused heavily on H2 activation. In the first report of intermolecular

H2 activation, PR3 (R = tBu, Mes) was combined with B(C6F5)3 and exposed to H2 to yield the

heterolytically cleaved salts [R3PH][HB(C6F5)3].81

In our own explorations of Al-based FLPs, we

decided to probe H2 activation using the analogous strategy; however, as with most Al FLP

chemistry, it was soon realized that a 1:2 ratio of PR3 to Al(C6F5)3 was required. Thus,

combining PtBu3 and Al(C6F5)3•tol in a 1:2 ratio in fluorobenzene under 4 atm of H2 at 25 °C

resulted in the subsequent isolation of a product (5.1) in 88% yield. The 1H NMR spectrum of

5.1 showed a broad signal at 4.30 ppm (H–Al) and doublets at 4.09 and 0.94 ppm attributable to

the P–H and tBu resonances, respectively. The corresponding 31

P{1H} NMR resonance was

observed at 60.0 ppm, while the 27

Al NMR spectrum showed no discernible resonance. The

19F{

1H} NMR spectrum gave rise to signals at -120.5, -153.1 and -161.8 ppm. These data,

together with elemental analysis, are consistent with the formulation of 5.1 as

[tBu3PH][(-H)(Al(C6F5)3)2] (Figure 5.1a). In an analogous fashion, the species

[Mes3PH][(-H)(Al(C6F5)3)2] (5.2) was prepared in 60% yield.191

The structures of these

compounds were both confirmed by X-ray crystallography (Figure 5.1).

126

Figure 5.1 – POV-Ray depictions of the molecular structures of a) 5.1 and b) 5.2. H atoms,

except µ-H and P–H, are omitted for clarity. C: black; P: orange; Al: teal; F: pink; H: white.

While the metric parameters of the cations are unexceptional, a single H-atom bridges the two

pseudo-tetrahedral Al-centres with Al–H distances of 1.715 and 1.618 Å for 5.1 and identical at

1.818 Å for 5.2. The resulting Al–H–Al angles are 173° and 138.0°, respectively. While these

values appear very different from one another, they are likely insignificant and due to the

inaccuracy of measuring the precise location of H atoms using X-Ray diffraction since both

anions have very similar Al–Al bond distances of 3.327 and 3.395 Å for 5.1 and 5.2,

respectively.

The formation of bridging hydride anions stands in contrast to the analogous borane chemistry,81

although the anionic fragment is reminiscent of the anion [(-Me)(Al(C6F5)3)2]-,244

as well as

other bridging hydrides between two Al centres.262-264

Similar to the salts [R3PH][HB(C6F5)3] (R

= tBu, Mes),81

the salts 5.1 and 5.2 show no evidence of H2 loss when heating under vacuum.

Nonetheless, all efforts to prepare the related 1:1 species [R3PH][HAl(C6F5)3] resulted only in

the isolation of reduced yields of the 1:2 species 5.1 and 5.2. This observation is consistent with

both the presumed nucleophilicity of the anion [HAl(C6F5)3]- and the Lewis acidity of Al(C6F5)3.

Interestingly, the reaction with the electron poor phosphine, P(otol)3, proceeded much more

slowly and attempts to isolate a pure product failed as the 1H NMR spectrum of the isolated

powder indicated the presence of H2 upon dissolution, indicative of a reversible reaction. The

presumed increased Brønsted acidity of this phosphonium cation, [(otol)3PH]+, will be discussed

later.

127

Next, we were interested in investigating H2 activation using PMes3/AlX3 (X = Cl, Br, I) FLPs.

Exposing 1:2 solutions of PMes3:AlX3 in bromobenzene to H2 at r.t. resulted in no reaction over

24 h as observed by NMR spectroscopy. The 31

P NMR spectrum displayed only signals

attributable to the adducts 2.1-2.3. In order to favour this dissociation, we heated the reaction

mixtures under H2. While the reactions with AlCl3 and AlBr3 were sluggish, heating to 75 °C a

1:2 mixture of PMes3:AlI3 in fluorobenzene under 4 atm of H2 for 4 h resulted in the isolation of

a pure white powder (5.3) after solvent removal and trituration with hexanes. The 27

Al NMR

spectrum displayed a single broad resonance at 36 ppm, while the 1H and

31P NMR spectra

displayed signals indicative of the [Mes3PH]+ cation. In addition to these

1H resonances, an

additional broad peak integrating to 1 proton was observed at 6.26 ppm, likely indicating the

presence of an Al–H bond. The isotopologue was synthesized using D2 and the product

([D2]-5.3) displayed the expected 1:1:1 triplet at -26 ppm in the 31

P NMR spectrum, as well as

the disappearance of the P–H and presumed Al–H resonances in the 1H NMR spectrum. While

repeated attempts to obtain crystals of this compound failed, this data, combined with

combustion analysis, is consistent with a formulation for 5.3 of [Mes3PH][H(AlI3)2].

Unfortunately, the exact structure of the anion – whether with a µ-H or a µ-I – could not be

unambiguously established.

5.2.2 Hydride delivery to olefin

With compounds 5.1-5.3 in hand, we set out to explore H2 delivery to unactivated olefins. As

described in the introduction, while FLPs are capable of hydrogenating polar unsaturated bonds,

as well as activated olefins, H2 delivery to unactivated simple olefins such as ethylene remains a

challenge. Therefore, compounds 5.1-5.3 were each exposed to 1 atm of ethylene in a J-Young

NMR tube at r.t. Whereas, a very slow reaction occurs with 5.1 and 5.2, an immediate reaction

occurs with 5.3. The 27

Al NMR spectrum displays a broad singlet at -17 ppm, downfield shifted

from the starting material at 36 ppm. While the 31

P and 1H NMR spectra display an unchanged

[Mes3PH]+ cation, the signal attributable to the Al–H bond is replaced by a triplet at 1.09 ppm

and a quartet at 0.92 ppm, attributable to the presence of an ethyl fragment. Using the

isotopologue [D2]-5.3 reduces the quartet to a broad triplet (Figure 5.2). Attempts to obtain a

crystal structure of this product were unsuccessful, precluding the exact assignment of the

structure of the anion; however, it should be noted that this 27

Al NMR shift is very similar to the

one observed for the ethylene activated product Mes3P(CH2CH2)(AlI3)2 (4.3) perhaps suggesting

128

that an iodide bridged ethyl anion of the type [(I3Al)(µ-I)(AlI2Et)]- is present here. Regardless of

the exact structure, attempts to protonate this ethyl fragment with the [Mes3PH]+ cation were

unsuccessful even upon heating this solution up to 120 °C for 24 h under H2. However,

protonation with water resulted in the evolution of ethane. An analogous rapid hydride transfer

reaction to cyclohexene using 5.3, followed by protonation with water to generate cyclohexane is

also readily achieved.

Figure 5.2 – Partial 1H NMR spectra of the reactions of 5.3 (top) or [D2]-5.3 (bottom) with

ethylene (1 atm) in C6D5Br after ca. 15 min at r.t.

The reaction of 5.1 and 5.2 with ethylene proceeded in a similar fashion. While the reaction at r.t.

is very slow, exposure of 5.1 to an atmosphere of ethylene at 60 °C for 2 h results in the

consumption of 5.1 and the formation of two products 5.4a and the known salt seen in the

previous chapter, [tBu3PH][Al(C6F5)4] (4.10). Using 5.2 yielded the product 5.4a along with the

salt, [Mes3PH][Al(C6F5)4] (5.5), which was confirmed by independent synthesis.191

Compound

5.4a gave a triplet and a broad quartet at 1.15 and 0.57 ppm in the 1H NMR spectrum, inferring

the presence of an ethyl fragment. The 19

F NMR signals were at -120.5, -150.9, and -160.2 ppm,

similar to those previously reported for alkyl alanes of the general formula RAl(C6F5)2.265

Compound 5.4a was independently generated by the redistribution reaction of AlEt3 and

B(C6F5)3 in a 3:2 ratio, resulting in the release of the volatile by-product, BEt3.265

The

corresponding reaction of 5.1 with cyclohexene again generates 4.10 and the cyclohexyl-alane

derivative 5.4b. By analogy to the previously reported alane [MeAl(C6F5)2]2,265

the compounds

129

5.4 are thought to be dimers of the form [RAl(C6F5)2]2 (Scheme 5.1), although this was not

unambiguously confirmed.

Scheme 5.1 – Reactions of 5.1 or 5.2 with olefin yielding the salts 4.10 or 5.5, respectively, and

the alanes 5.4.

Both salts 4.10 and 5.5 can be isolated by washing away the alane 5.4 with hexanes. This

suggests that the redistribution reaction is irreversible once the [Al(C6F5)4]- anion is generated.

This separation of products is also achieved when 5.3 is used. Furthermore, in a similar attempt

to the chemistry with 5.3, the products 4.10 or 5.5 and the alane 5.4 were heated up to 120 °C

under H2 in an attempt to protonate the alkyl groups and generate a catalytic system. These

reactions were unsuccessful and, in part, complicated by the formation of by-products at high

temperatures (vide infra). In an attempt to better understand this hydride delivery and

redistribution chemistry, we attempted to elucidate a possible mechanism.

5.2.3 Possible mechanism

Several possibilities arise in considering the mechanism of the reaction of 5.1-5.3 with olefin.

Scheme 5.2 outlines two of these possibilities using compound 5.1 as an example. The first

possible pathway (Scheme 5.2a) involves the redistribution of 5.1 to generate 4.10 and

HAl(C6F5)2. Subsequent reaction of the latter alane with olefin would afford 5.4. However,

variable temperature NMR studies of a salt of the anion in 5.1 (compound 5.9: vide infra) from

25 °C to 120 °C in C6D5Br did not show any appreciable changes in the 19

F NMR spectra

inferring that redistribution to generate HAl(C6F5)2 is unlikely.

130

Scheme 5.2 – Possible mechanisms for the reaction of 5.1 with olefin.

A possible alternative mechanism (Scheme 5.2b) involves olefin interception by Al(C6F5)3 from

5.1. The proposed olefin-Al(C6F5)3 would then be significantly activated and susceptible to

attack by the anion [HAl(C6F5)3]-, leading to the formation of the alkyl-aluminate anion,

[RAl(C6F5)3]-, and free Al(C6F5)3. A subsequent redistribution reaction would afford 4.10 and

5.4. This possibility is supported by the reaction of 5.1 or 5.2 with Lewis bases, such as Et2O or

PMe3, which results in the rapid loss of H2 and the formation of the Lewis base adducts

L•Al(C6F5)3 (L = Et2O, PMe3). In this case, the donor acts to sequester Al(C6F5)3 from the anion

[(µ-H)(Al(C6F5)3)2]- generating the salt [R3PH][HAl(C6F5)3] which is unstable with respect to H2

loss in the absence of other electrophiles.

Support for the notion of an olefin-Al(C6F5)3 interaction was derived from the following

reaction: cooling a solution of Al(C6F5)3•tol in neat cyclohexene afforded the subsequent

isolation of crystals of the complex 5.6 in 85% yield. While elemental analysis confirmed the

formulation of the crystals as Al(C6F5)3•(C6H10), the strength of the aluminum-olefin interaction

appears to be rather weak as dissolution in bromobenzene gave the 1H NMR signature of free

cyclohexene. Nonetheless, crystallographic analysis of 5.6 showed the aluminum centre is

131

pseudo tetrahedral with coordination of the olefinic unit to the aluminum centre in an 2-fashion

(Figure 5.3). The Al–Colefin distances were found to be 2.471(2) and 2.540(2) Å, while the C=C

bond is 1.340(3) Å.

Figure 5.3 – POV-Ray depiction of the molecular structure of 5.6. C: black; Al: teal; F: pink; H:

white.

The interaction of olefins with Lewis acidic aluminum centres has been demonstrated in

Chapters 3 and 4 of this thesis. Previous spectroscopic evidence for olefin interaction at Lewis

acidic Al266

or B107

centres has also been reported, and Schnöckel and co-workers have reported

the tetrametallic dimer derived from 1,4-dialumina-2,5-cyclohexadiene which incorporates

aluminum-olefin interactions with Al–C distances of 2.355 Å.65

Nonetheless, compound 5.6 is to

the best of our knowledge, the first crystallographically characterized species derived from the

interaction of a simple free olefin with Al.

With 5.6 representing a portion of the reaction sequence in Scheme 5.2b, we next wanted to

probe the transient role of a mono-Al hydride species [HAl(C6F5)3]-. Therefore, the known salt

267

K[HAl(C6F5)3] was prepared; however, its solubility in bromobenzene proved extremely poor

and consequently attempted cation exchange was very sluggish. An alternative synthetic

approach involved the initial preparation of [Et4N][ClAl(C6F5)3] (5.7) in 92% yield via the

combination of [Et4N]Cl and Al(C6F5)3•tol.191

Subsequent treatment of 5.7 with LiAlH4 at r.t.,

followed by filtration resulted in the isolation of the highly pyrophoric salt 5.8. The 1H NMR

spectrum of 5.8 showed a hydride signal at 4.79 ppm, downfield shifted compared to 5.1 (4.30

132

ppm). The molecular structure of 5.8 (Figure 5.4) confirmed the formulation of the salt as

[Et4N][HAl(C6F5)3]. The metric parameters were generally unexceptional, although the Al–H

bond lengths in the two independent molecules of the asymmetric unit averaged 1.51(1) Å

comparable to that seen in K[HAl(C6F5)3] (1.59(5) Å)267

and significantly shorter than those seen

in 5.1 and 5.2.

Figure 5.4 – POV-Ray depiction of the molecular structure of 5.8. C: black; Al: teal; F: pink; N:

blue; H: white. H atoms, except Al–H, are omitted for clarity.

Compound 5.8 alone does not react with added olefin, even on heating to 60 °C for 12 h. This

observation is consistent with the known inability of LiAlH4 to react with isolated olefins on its

own.268-270

However, addition of one equivalent of Al(C6F5)3 to 5.8 in C6D5Br produced the

rather poorly soluble salt [Et4N][(µ-H)(Al(C6F5)3)2] (5.9) as evidenced by 19

F and 1H NMR

spectroscopy. Heating 5.8/Al(C6F5)3, or alternatively the isolated salt 5.9, under an atmosphere

of ethylene at 60 °C for 12 h resulted in the complete conversion to 5.4a and the salt

[Et4N][Al(C6F5)4] as evidenced by NMR spectroscopy. These observations further support the

notion that thermal/substrate induced dissociation of Al(C6F5)3 from the anion of 5.1/5.2

facilitates both the activation of olefin and hydride transfer. This proposed mechanism is also

closely related to the reported hydride delivery from [HB(C6F5)3]- to the borane-activated olefin

fragment of (C6F5)2B(CH2)4CH=CH2 and (C6F5)2BOC(CF3)2(CH2)CH=CH2.106

133

In the final step of the pathway (Scheme 5.2b), a redistribution reaction is proposed to give the

salt [R3PH][Al(C6F5)4] (4.10 or 5.5) and 5.4. To model this, the salt [Et4N][EtAl(C6F5)3] (5.10)

was prepared in 79% yield by treating the precursor 5.7 with EtMgBr.191

The solid state structure

of 5.10 was confirmed crystallographically (Figure 5.5). The 1H NMR signals arising from the

Al–Et fragment are observed at 1.40 and 0.75 ppm, considerably downfield shifted to those in

5.4a (1.15, 0.57 ppm). Similarly, the 19

F resonances attributable to the para and meta-F atoms

are shifted to -159 and -164 ppm, respectively, from -151 and -160 ppm for the corresponding

fragments in 5.4a.

Figure 5.5 – POV-Ray depiction of the molecular structure of 5.10. C: black; Al: teal; F: pink;

N: blue. H atoms are omitted for clarity.

Reaction of 5.10 with one equivalent of Al(C6F5)3•tol in C6D5Br leads to the rapid redistribution

affording 5.4a and the salt [Et4N][Al(C6F5)4]. This observation stands in contrast to the

analogous reaction of the anion [MeAl(C6F5)3]- where the bridging methyl analog

[(-Me)(Al(C6F5)3)2]- is isolated.

244 This redistribution reaction is also very similar to the one

observed in Chapter 4 following ethylene insertion into 4.8 to generate the salt 4.10 and the

pendant alane 4.11. It appears that such redistribution reactions are driven by steric congestion as

smaller substituents such as hydride, methyl or fluoride do not redistribute.244,271

5.2.4 Attempts to generate a catalytic cycle

The data of the previous section suggest that a reaction pathway consistent with Scheme 5.2b is

at play in the hydride delivery to unactivated olefins. Compounds 5.1-5.3 all seem to undergo a

134

deactivating redistribution reaction following hydride delivery to olefin yielding the anion

[AlX4]- (X = C6F5, I) and an alane RAlX2 (R = ethyl, cyclohexyl). The only observable

difference in this chemistry is with 5.3 where a weak interaction appears to exist between the

product anion [AlI4]- and the alane RAlI2, presumably through halogen bridging. Assuming a

pathway as in Scheme 5.2b, we devised several strategies to help prevent the deactivating

redistribution reaction in order to favour a catalytic cycle. These strategies are outlined in this

section.

5.2.4.1 Using more acidic phosphonium cations

In order to generate a catalytic cycle for olefin hydrogenation, we modified the systems above to

include more acidic phosphonium cation intermediates. The intent here was to generate a

competitive reaction pathway to the redistribution chemistry and facilitate the regeneration of the

FLP mixture for catalytic olefin hydrogenation (Scheme 5.3).

Scheme 5.3 – Proposed competitive redistribution (deactivation) vs. protonation (catalytic)

reaction pathways for olefin hydrogenation.

In order to qualitatively test the relative acidities of the different phosphines used, the salts

[R3PH][Al(C6F5)4] (R = otol (5.11), Mes (5.5), tBu (4.10)) were independently synthesized.191

These salts were treated in a 1:1 ratio with the ethyl aluminate salt 5.10 and monitored by 1H

NMR spectroscopy in a J-Young NMR tube to observe the evolution of ethane. The reaction of

5.11 immediately generated ethane at r.t. after 2 h and was complete upon standing overnight.

The reaction products were ethane, the alane 5.4a, P(otol)3, the salt [Et4N][Al(C6F5)4], as well as

unreacted 5.11 (Scheme 5.4), as observed by 31

P, 19

F, and 1H NMR spectroscopy. The product

135

5.4a arises from the competitive redistribution reaction of 5.10 with Al(C6F5)3, which itself is

produced following protonation of the ethyl group from 5.10.

Scheme 5.4 – Product distribution following the reaction of 5.11 with 5.10.

In contrast to this reactivity, the analogous reaction with the electron rich phosphine in 5.5

resulted in no reaction at r.t. after 6 h. Heating the sample to 70 °C for 48 h also did not result in

the production of ethane or the alane 5.4a. Similar results were obtained with 1:1 combinations

of 4.10:5.10. The increased basicity of PtBu3 over PMes3 was also confirmed by combining 1

equiv. of 5.5 with 1 equiv. PtBu3 resulting in the quantitative generation of 4.10 and free PMes3.

In addition to these phosphines, the Brønsted basicity of the electron deficient bulky phosphine,

P(2,6-Cl2C6H3)3,272

was also examined. As the analogous salt to 5.11 and 5.5 of this phosphine

could not be synthesized, the phosphonium cation was generated by treating a 1:1 solution of

P(2,6-Cl2C6H3)3:AlCl3 to HCl to generate the salt [(2,6-Cl2C6H3)3PH][AlCl4] (5.12) in good

yield.191

Treatment of this salt to 1 equiv. of P(otol)3 or PMes3 resulted in the quantitative

generation of the cations [(otol)3PH]+ and [Mes3PH]

+, respectively, confirming the more acidic

nature of the [(2,6-Cl2C6H3)3PH]+ cation. Furthermore, treating 5.12 to the ethyl salt 5.10, again

resulted in the facile generation of ethane, similar to the reaction with 5.11. Together this

qualitative data is consistent with an increasing phosphonium pKa trend of [(2,6-Cl2C6H3)3PH]+

< [(otol)3PH]+ < [Mes3PH]

+ < [tBu3PH]

+.

With this knowledge, we performed an extensive study by repeating the hydrogenation reactions

in situ and altering all variables, including the ratio of PR3:Al(C6F5)3 (R = 2,6-Cl2C6H3, otol,

Mes, tBu), the ratio of cyclohexene to FLP, the solvent (fluorobenzene or bromobenzene), the

temperature, and the time of reaction. Cyclohexene was not found to generate any allyl

intermediates and was generally used instead of ethylene in order to avoid the formation of

ethylene addition products of the type R3P(CH2CH2)Al(C6F5)3 (see Chapter 4). While trace

amounts of cyclohexane were sometimes observed using P(otol)3, these results were not

reproducible and cyclohexane was not generated in any other case under all reaction conditions

136

attempted. Similar results were obtained when ethylene was used; however, higher temperatures

were generally needed to dissociate the addition products R3P(CH2CH2)Al(C6F5)3 that may form.

It should be noted that the phosphine P(2,6-Cl2C6H3)3 was incapable of activating H2 at r.t. with

Al(C6F5)3. While the activation at lower temperatures may be possible based on previous results

obtained with B(C6F5)3,99

lower temperatures were found to significantly inhibit the hydride

delivery reaction, consistent with the required dissociation of Al(C6F5)3 from the

[(µ-H)(Al(C6F5)3)2]- anion prior to olefin activation (Scheme 5.2b). In all cases, with the other

phosphines, the redistributed products [R3PH][Al(C6F5)4] and 5.4a or 5.4b were ultimately

obtained with negligible alkane production.

Finally, a similar extensive study was undertaken with AlX3 (X = Cl, Br, I) and olefin. No

evidence existed for the activation of H2 using the less bulky phosphines P(otol)3 and PtBu3 due

to strong adduct formation. In contrast, the bulky phosphine P(2,6-Cl2C6H3)3 does show evidence

of H2 activation and can deliver hydride to ethylene; however, even with the generation of this

acidic phosphonium cation, protonation of the generated alkyl fragments did not occur in a

reproducible manner under all reaction conditions attempted.

5.2.4.2 Synthesizing a less Lewis acidic alane

In a complementary approach to using acidic phosphonium cations to protonate alkyl fragments,

we thought that using a poorly Lewis acidic alane could generate a weaker Al–Calkyl bond which

could be more easily protonated. In addition to this rationale, we also found that the chemistry

with Al(C6F5)3 was plagued by decomposition reactions at high temperatures (> 100 °C)

characterized by C–Fpara bond activation generating the known [(µ-F)(Al(C6F5)3)2]- anion

271 and

a resulting C–Hpara substituted alane through an unknown mechanism. For these reasons, we

sought to make the para-H substituted alane, Al(C6F4H)3, analogous to the weaker Lewis acid,

B(C6F4H)3, previously reported by our group.85

Following the known literature procedure to synthesize Al(C6F5)3 from B(C6F5)3 and AlMe3,175

we combined 1 equiv. AlMe3 with 1 equiv. of B(C6F4H)3 in toluene overnight. Recrystallization

of the solution at -38 °C yielded crystals of the product in 70% yield. The 19

F NMR spectrum

displayed 2 multiplets at -121.6 and -137.5 ppm and the 1H NMR spectrum displayed a triplet of

triplets (due to H–F coupling) at 6.62 ppm, as well as one equivalent of toluene. The structure

137

was confirmed by X-ray crystallography as the η1 toluene adduct, Al(C6F4H)3•tol (5.13) (Figure

5.6) similar to the reported Al(C6F5)3•tol adduct.175


H: white. Toluene H atoms are omitted for clarity.

Compound 5.13 (2 equiv.) was combined with either P(otol)3 or PMes3 (1 equiv.) in a J-Young

NMR tube and exposed to H2. The reduced Lewis acidity of 5.13 was confirmed by the slower

rate of H2 activation with these phosphines, as well as the lack of hydride delivery to olefin at

70 °C when using P(otol)3. This is in contrast to the analogous reaction using Al(C6F5)3 and

suggests that H2 activation does not occur at this temperature. While hydride delivery to ethylene

or cyclohexene was observed at lower temperatures with P(otol)3, the resulting protonation of the

cyclohexyl fragment did not occur. Furthermore, following hydride delivery, the appearance of a

broad signal in the 27

Al NMR spectrum at 116 ppm, as well as corresponding signals in the 19

F

and 1H NMR spectra were indicative of a similar redistribution reaction to generate the anion

[Al(C6F4H)4]- and the alane RAl(C6F4H)2 analogous to the products observed in the Al(C6F5)3

chemistry above.

5.2.4.3 Steric inhibition using a bulky alane

The results described thus far suggest that the deactivating redistribution reaction of Scheme 5.2b

to generate the aluminate [Al(C6F4E)4]- anion and alane RAl(C6F4E)2 (E = H, F) is faster than

protonation of the proposed intermediate alkyl fragment [RAl(C6F4E)3]- by the phosphonium

cation. As a final approach to promote the catalytic hydrogenation of olefins, we sought to

138

synthesize an alane which is sterically prohibited from forming a symmetric aluminate product,

[AlX4]- (X = sterically bulky group). It is hoped that such a species would be forced to undergo

protonation at an alkyl aluminate centre [RAlX3]- and, thus, generate a catalytic cycle (Scheme

5.5). In addition to this steric requirement, this alane would also have to be Lewis acidic enough

to bind olefin for subsequent hydride delivery.

Scheme 5.5 – Proposed catalytic olefin hydrogenation pathway using extremely bulky X groups

at Al in order to prevent the deactivating redistribution pathway.

An extensive literature search of extremely bulky Lewis acidic alanes revealed that very few are

sterically bulkier than Al(C6F5)3; however, in a previous paper by Marks,273

the extremely bulky

and Lewis acidic borane, tris(1-perfluorobiphenyl)borane, B(C12F9)3, was reported. This borane

and its derivatives have been used extensively as co-catalysts in transition metal olefin

polymerization.271,273,274

A very recent report by O’Hare has also demonstrated the activity of

this borane as a Lewis acid partner for heterolytic cleavage of H2 with a phosphine.275

Its Lewis

acidity has also been determined to be higher than B(C6F5)3.

In Marks’s original paper, the attempted synthesis of the alane analog was also outlined;

however, the fluoride product [Li(OEt2)][FAl(C12F9)3] was consistently obtained. A proposed

reaction pathway leading to this product was proposed (Scheme 5.6) and involves the

139

intermediate formation of the very Lewis acidic alane, Al(C12F9)3, followed by fluoride

abstraction from a lithiated perfluorobiphenyl fragment to yield the observed product.

Scheme 5.6 – Proposed reaction pathway for the generation of B(C12F9)3 or the salt

[Li][FAl(C12F9)3].

The reaction to generate the salt [Li(OEt2)][FAl(C12F9)3] proceeds through elimination of a

perfluorinated benzyne by-product; thus, 4 equiv. of the starting material, C12F9Br, was used

relative to AlCl3 in the report to optimize the yield of the reaction.273

The generation of this

fluoride product, although an inconvenience, demonstrates that a fourth perfluorinated biphenyl

is sterically prohibited from coordinating to the trivalent Al centre to generate the anion

[Al(C12F9)4]-. Combined with the high Lewis acidity of perfluorinated alanes,

193 this proposed

Lewis acid intermediate, Al(C12F9)3, offers both the steric and electronic requirements required

for the proposed chemistry of Scheme 5.5. We therefore attempted to synthesize this Lewis acid

independently.

Our first attempt focused on transmetallation chemistry, similar to that used for the synthesis of

Al(C6F5)3 or the alane 5.13, using the appropriate borane precursor and AlMe3. Therefore, the

borane B(C12F9)3 was combined with 1 equiv. AlMe3 in toluene. No interaction of the 2 Lewis

acids was observed by 27

Al, 11

B, and 19

F NMR spectroscopy even upon heating. The lack of

140

reactivity with this borane as compared to B(C6F5)3 is attributed to the increased steric bulk

around the B centre. With this in mind, our second attempt focused on using the related known

species, Zn(C12F9)2•tol.276

Therefore, we mixed a 3:2 solution of Zn(C12F9)2•tol:AlMe3 in toluene

and observed the progress by 19

F NMR spectroscopy. In contrast to the borane chemistry, broad

signals indicative of some exchange were observed in the 19

F spectrum; however, no clean

product could be isolated from this reaction mixture. This reaction was also attempted in

bromobenzene using the soluble AlBr3 in the hopes that precipitation of the insoluble ZnBr2

would favour this reaction; while precipitation and exchange chemistry was observed by 19

F

NMR, again no clean product could be isolated. Several other attempts to modify the original

synthesis reported by Marks were attempted; however, none were successful.

Having exhausted most conventional approaches, we turned our attention to the fluoride product,

[Li(OEt2)][FAl(C12F9)3].273

While Al–F bonds are extremely strong, we rationalized that

replacing this bond with a much weaker Al–H bond could be beneficial as this could serve as a

viable precursor to the free alane, Al(C12F9)3, through treatment with the trityl salt,

[CPh3][B(C6F5)4]. Before embarking on a potentially long synthesis, we tested this hypothesis

using the mono-Al hydride species [Et4N][HAl(C6F5)3] (5.8). Treatment of this salt with 1 equiv.

of the trityl cation resulted in hydride transfer generating Ph3CH, the free alane Al(C6F5)3, and

the salt [Et4N][B(C6F5)4], confirming the viability of this approach.

In a similar strategy to the synthesis of 5.8 from the chloride 5.7 and LiAlH4, we found that

treating the compound [Li(OEt2)][FAl(C12F9)3] with 1 equiv. of LiAlH4 leads to the gradual

disappearance of the Al–F resonance as observed by 19

F NMR spectroscopy. This strategy

yielded a cleaner product by first removing the Li cation through cation exchange chemistry with

Et4NCl. Therefore, 1 equiv. of [Li(OEt2)][FAl(C12F9)3] was combined with 1 equiv. Et4NCl in

dichloromethane. Removal of the solvent, followed by filtration through Celite using

fluorobenzene and subsequent precipitation with pentane yielded a new product in 70% yield.

The 19

F NMR spectrum displayed 7 aromatic F signals for the biphenyl ring (where the C6F5

rings are freely rotating and result in 3 separate signals), as well as a broad singlet at -175 ppm,

indicative of the Al–F bond. The 1H NMR contained the resonances attributable to the [Et4N]

+

cation. Together with elemental analysis, this data is consistent with the formulation of the salt as

[Et4N][FAl(C12F9)3] (5.14).

141

Next, we treated 5.14 to a slight excess of LiAlH4 in a mixed fluorobenzene:Et2O solvent system

at r.t. for 16 h. Filtration followed by precipitation with pentane resulted in the isolation of a

powder in 87% yield. The 19

F NMR spectrum again displayed 7 separate slightly shifted

resonances with the notable absence of the Al–F resonance from 5.14. The 27

Al NMR spectrum

contained a shifted broad singlet at 122 ppm, compared to the signal at 112 ppm for 5.14. The 1H

NMR displayed the signals attributable to the [Et4N]+ cation with a very broad Al–H resonance

located by a 1H{

27Al} NMR experiment at 2.50 ppm. Elemental analysis confirmed the

formulation of the product as [Et4N][HAl(C12F9)3] (5.15) and single crystals suitable for X-Ray

diffraction were obtained. The structure of 5.15 is shown in Figure 5.7. All bond lengths are

similar to those reported for 5.8.


N: blue; H: white. Ethyl H atoms are omitted for clarity.

With 5.15 in hand, we next attempted to isolate the free alane, Al(C12F9)3, by treatment with the

compound [CPh3][B(C6F5)4]. It should be noted that the order of addition here is very important.

Adding [CPh3][B(C6F5)4] to a solution of 5.15 led to the continuous disappearance of the

characteristic yellow colour of trityl after every drop added; however, the colour ceased to

disappear once half the solution of trityl was added. It is believed that the generated Al(C12F9)3

combines with an equivalent of 5.15 to form the bridging salt [Et4N][(µ-H)(Al(C12F9)3)2] similar

to compound 5.9 above. The steric protection around this hydride could make hydride

142

abstraction by trityl more difficult. In contrast, slow addition of a solution of 5.15 to a solution of

[CPh3][B(C6F5)4] resulted in the disappearance of the yellow trityl colour once all the compound

was added. This addition sequence also resulted in a higher isolated yield. After the addition, the

solvent was removed and the salt by-product, [Et4N][B(C6F5)4], was separated from the alane by

dissolving the latter in hexanes and filtering through Celite. While a new set of 7 downfield

shifted F resonances was observed by 19

F NMR spectroscopy, and while the broad 27

Al NMR

resonance of 5.15 was now gone, indicating the presence of a free alane, this product could not

be isolated in pure form due to the presence of the by-product, Ph3CH. Attempts to crystallize a

toluene or cyclohexene adduct of this alane, similar to Al(C6F5)•tol, 5.13, or 5.6, were also

unsuccessful presumably due to the sterically congested Al centre. Nonetheless, these results do

suggest that access to the free alane is possible and can be used for further reactivity.

While it was not immediately clear whether H2 activation with this alane would yield a 1:1 or a

1:2 P:Al product with a bridging hydride, the reported coordination chemistry of the anion

[FAl(C12F9)3]- to bulky metal centres through M–F–Al bonds suggested that a 1:2 product could

be obtained.273

In order to test this, we first pretreated the hydride 5.15 with 1 equiv. of

[CPh3][B(C6F5)4] in fluorobenzene, followed by workup in hexanes. Following removal of the

solvent, the Al(C12F9)3/Ph3CH residue was combined with approximately 0.5 equiv. PMes3 in

bromobenzene and exposed to 4 atm of H2 for 12 h. A resulting poorly soluble white powder was

obtained in low yield (38%). The 31

P and 1H NMR spectra indicated the presence of the

[Mes3PH]+ cation and the possible presence of an Al–H–Al bond, similar in shift to those

observed for 5.1 and 5.2, although the poor solubility made this difficult to confirm. The 19

F

NMR provided little information as it contained at least 13 separate overlapping resonances,

likely indicative of restricted fluoroaryl ring rotation, similar in trend to the M–F–Al(C12F9)3

compounds previously reported.273

Combustion analysis was consistent with the formulation as

[Mes3PH][H(Al(C12F9)3)2] (5.16). Fortunately, X-ray crystallography confirmed the geometry as

a bridging bis-Al hydride species as shown in Figure 5.8.

143

Figure 5.8 – POV-Ray depiction of the molecular structure of 5.16. C: black; Al: teal; F: pink; P:

orange; H: white. Mesityl H atoms are omitted for clarity.

The structure of 5.16 contains a slightly elongated Al–Al distance of 3.448 Å in comparison to

3.327 and 3.395 Å for 5.1 and 5.2, respectively, and indicative of a more sterically congested

environment. Most surprising in the structure of 5.16 is the incorporation of the 2 extremely

bulky Al(C12F9)3 centres bridging the hydride.

Based on the Brønsted acidity tests described above, the phosphonium cation [Mes3PH]+ would

presumably not be acidic enough to protonate an alkyl-Al fragment generated after hydride

transfer to an olefin. However, we synthesized 5.16 to first establish if hydride transfer would

even be possible with this bulkier alane moiety. Thus, a mixture of 5.16 was treated to ethylene

and monitored by NMR spectroscopy in an attempt to observe the formation of an ethyl-Al

fragment, similar to the chemistry observed with 5.1 and ethylene. Unfortunately, even after

prolonged heating, no ethyl fragment was generated. Instead, the slow decomposition of the

anion of 5.16 was observed as new unknown signals gradually appeared in the 19

F NMR

spectrum.

144

Regardless of these decomposition results, the hydrogenation experiments were next attempted

using the poorly Brønsted basic phosphines, P(otol)3 and P(2,6-Cl2C6H3)3, as well as the

sterically less encumbered phosphines P(naph)3 (naph = 1-naphthyl) and PhP(otol)2. The

experiments were conducted under H2 using ethylene, propylene or cyclohexene, at different

temperatures. While P(otol)3 displayed evidence for slow H2 activation, all other phosphines did

not. For the P(naph)3 and PhP(otol)2 phosphines, this is attributed to the presence of a weak

adduct with Al(C12F9)3, whereas for P(2,6-Cl2C6H3)3, this is again attributed to its very weak

Brønsted basicity as observed with Al(C6F5)3. Unfortunately, under a range of different

conditions, no catalytic hydrogenation of olefins was evident; furthermore, no evidence of

hydride transfer was present. Under forcing conditions (> 70 °C), the Lewis acid again displayed

several new peaks by 19

F NMR spectroscopy, some of which are likely Al–F bonds.273

We attempted to rationalize the lack of reactivity of 5.16 or PR3/Al(C12F9)3 combinations with

olefins. Several reasons may exist. It is possible that the increased Lewis acidity of this alane

creates a stronger Al–H–Al bond rendering its dissociation more difficult. Alternatively, as noted

earlier, the toluene or cyclohexene adducts of the isolated alane could not be obtained, in contrast

to with Al(C6F5)3. Therefore, it is possible that the extremely congested Al centre here is

incapable of activating olefin for subsequent attack by the intermediate mono-Al hydride,

[HAl(C12F9)3]-. To demonstrate this point, space filling models of the anion of 5.15 are shown in

Figure 5.9. Thus, the extremely bulky nature of the biphenyl groups, which prevent the formation

of the symmetric aluminate [Al(C12F9)4]- deactivation product, may also be responsible for the

lack of reactivity.

145

Figure 5.9 – POV-Ray space filling models of 5.15. Left structure line of sight is perpendicular

to the Al–H plane, whereas the right structure has a line of sight down (parallel) the Al–H bond.

The [Et4N]+ cation is omitted for clarity. C: black; Al: teal; F: pink; H: white.

The lack of reactivity could also be attributed to an alternative mechanism, namely the one

shown in Scheme 5.2a involving rearrangement of an [(µ-H)(AlX3)2]- anion into the anion

[AlX4]- and the alane X2AlH. This latter fragment could easily undergo a hydroalumination

reaction with an olefin. Such a rearrangement would likely be facilitated with the halide species

[H(AlI3)2]- of 5.3 as halides easily bridge (see Chapter 4), and may explain its enhanced

reactivity with olefins as compared to the thermally induced hydride delivery from the

[(µ-H)(Al(C6F5)3)2]- anion of 5.1/5.2. Thus, the lack of reactivity with the bulkier Al(C12F9)3

could be attributed to its inability to form the aluminate anion [Al(C12F9)4]- and corresponding

alane (C12F9)2AlH. However, as noted earlier, variable temperature NMR experiments with the

anion [(µ-H)(Al(C6F5)3)2]- of 5.9 does not suggest that such a rearrangement occurs. In summary,

while considerable evidence to support the mechanism in Scheme 5.2b has been provided in this

chapter, it is not yet possible to fully discount the mechanism in Scheme 5.2a, or any alternative

mechanism. Gaining a better understanding of the hydride delivery mechanism may provide the

necessary clues needed to establish a viable catalytic olefin hydrogenation process using FLPs.

146

5.3 Conclusions

In conclusion, we have presented the facile activation of H2 using Al-based FLPs and

demonstrated subsequent hydride transfer to unactivated olefins. These reaction are thought to

involve Al-olefin activation, nucleophilic attack by an anion [HAlX3]- and subsequent

redistribution to aluminate and alane. This proposed route is supported by the isolation of the

first Al-olefin complex as well as parallel reactivity studies. Strategies were also outlined to

promote the subsequent protonation of the proposed intermediate alkyl-aluminate species in

order to generate a catalytic cycle. Such strategies included the use of more acidic phosphonium

cation intermediates, the use of a poorer Lewis acidic alane, or the use of a newly synthesized,

extremely bulky Lewis acidic alane which cannot undergo redistribution. While catalytic

hydrogenation was not achieved in these cases, important information was gained into the

reduction of unactivated olefins which may help in the development of future non-transition

metal based catalysts.





-38 ºC freezer). Hexanes, pentane, dichloromethane, and diethyl ether (Aldrich) were dried using

an Innovative Technologies solvent system. Fluorobenzene and bromobenzene (-H5 and -D5)

were purchased from Aldrich and dried on P2O5 for several days and vacuum distilled onto 4Å

molecular sieves prior to use. Cyclohexene (inhibitor free) was purchased from Alfa, dried over

CaH2 and distilled. PtBu3, P(otol)3, PMes3, AlMe3, and AlEt3 (93%) were purchased from Strem

and used without further purification. LiAlH4 was purchased from Strem and purified according

to literature procedure.277

AlX3 (X = Cl, I) were purchased from Strem and sublimed three times

prior to use under vacuum using a -78 °C cold finger and a 100 °C (X = Cl) or 150 °C (X = I)

bath. PCl3 was purchased from Aldrich and distilled prior to use. Me2SiHCl, EtMgBr (3.0 M in

Et2O), 1,3-dichlorobenzene, BF3(OEt2), BuLi (1.6 M in hexanes), and HCl (2.0 M in Et2O) were

purchased from Aldrich and used without further purification. Et4NCl was purchased from

Aldrich and dried under vacuum for several days. B(C6F5)3 was purchased from Boulder

Scientific, sublimed under vacuum, then treated with excess Me2SiHCl for 4 h and re-sublimed

147

after removal of volatiles. [CPh3][B(C6F5)4] was purchased from Boulder Scientific and purified

by precipitation from a saturated dichloromethane solution with pentane. C6BrF5 and C6BrF4H

were purchased from Apollo Scientific and used without further purification. Al(C6F5)3•tol was

prepared from B(C6F5)3 and AlMe3 according to a literature procedure.175

The phosphine

P(2,6-Cl2C6H3)3 was prepared from PCl3 and 1,3-dichlorobenzene according to a literature

procedure.272

The borane B(C6F4H)3 was prepared from C6BrF4H and BF3 according to a

literature procedure.85

The starting material C12BrF9 was prepared from C6F5H and BuLi, and

[Li(OEt2)][FAl(C12F9)3] was prepared from C12BrF9 and AlCl3 according to a literature

procedure.273

Propylene (grade 3.0) was purchased from Linde and passed through a Restek

oxygen scrubber and a Restek moisture trap prior to use. H2 (grade 5.0) was purchased from

Linde and dried through a Nanochem WeldAssure purifier column prior to use.



13C = 122.4 ppm

for ipso carbon), or externally (27

Al: Al(NO3)3, 31

P: 85% H3PO4, 19

F: CFCl3). Chemical shifts (δ)

listed are in ppm and absolute values of the coupling constants are in Hz. NMR assignments are

supported by additional 2D experiments. Elemental analyses (C, H, N) and X-ray

crystallography were performed in house.


Synthesis of [tBu3PH][H(Al(C6F5)3)2] (5.1): A 50 mL Schlenk bomb equipped with a Teflon

screw cap and a magnetic stirbar was charged with PtBu3 (82 mg, 0.41 mmol), Al(C6F5)3•tol

(500 mg, 0.81 mmol) and fluorobenzene (ca. 5 mL). The bomb was transferred to the Schlenk

line equipped with a H2 outlet. The bomb was immersed in a liquid N2 bath, degassed, filled with

H2, and slowly warmed to r.t. The solution was stirred for 12 h in the glovebox. Hexanes (ca. 5-

10 mL) were added dropwise to the stirring solution and the precipitate that formed was filtered

on a glass frit, washed with hexanes and dried in vacuo (450 mg, 0.36 mmol, 88%). Slow cooling

a concentrated fluorobenzene solution of the compound to -38 °C yielded single crystals suitable

for X-ray crystallography.

1H NMR (400 MHz, C6D5Br): δ 4.30 (bs, 1H, Al-H-Al), 4.09 (d,

1JH-P = 426 Hz, 1H, PH), 0.94

(d, 3JH-P = 15.6 Hz, 27H, tBu).

31P{

1H} NMR (161 MHz, C6D5Br): δ 60.0 (s).

27Al NMR (104

MHz, C6D5Br): blank. 19


3JF-F = 26.3 Hz,

4JF-F =

148

11.3 Hz, 12F, o-C6F5), -153.1 (t, 3

JF-F = 22.6 Hz, 6F, p-C6F5), -161.8 (m, 12F, m-C6F5). 13

C{1H}

NMR (100 MHz, C6D5Br), partial: δ 150.2 (dm, 1JC-F = 217 Hz), 136.9 (dm,

1JC-F = 251 Hz),

36.8 (d, 1

JC-P = 26.4 Hz, PCMe3), 29.2 (s, PCMe3). Anal. Calc. for C48H29Al2F30P: C, 45.73; H,

2.32. Found: C, 45.39; H, 2.40.

Synthesis of [Mes3PH][H(Al(C6F5)3)2] (5.2): Synthesized in an analogous fashion to 5.1 using

157 mg (0.40 mmol) PMes3 and 500 mg (0.81 mmol) Al(C6F5)3•tol and bromobenzene (ca. 5

mL). Isolated yield is 350 mg (0.24 mmol, 60%).

1H NMR (400 MHz, C6D5Br): δ 7.99 (d,

1JH-P = 478 Hz, 1H, PH), 6.75 (bs, 3H, m-Mes), 6.70

(bs, 3H, m-Mes), 4.33 (bs, 1H, Al-H-Al), 2.08 (s, 9H, p-CH3Mes

), 1.98 (s, 9H, o-CH3Mes

), 1.75 (s,

9H, o-CH3Mes

). 31

P{1H} NMR (161 MHz, C6D5Br): δ -26 (s).

27Al NMR (104 MHz, C6D5Br):

blank. 19

F{1H} NMR (376 MHz, C6D5Br): δ -121.5 (bd,

3JF-F = 18.8 Hz, 12F, o-C6F5), -154.3 (t,

3JF-F = 18.8 Hz, 6F, p-C6F5), -162.9 (m, 12F, m-C6F5).

13C{

1H} NMR (100 MHz, C6D5Br): δ

150.2 (dm, 1JC-F = 236 Hz), 147.2 (d,

4JC-P = 2.8 Hz, p-C6H2), 143.8 (bs, o-C6H2), 142.3 (bs, o-

C6H2), 141.5 (dm, 1JC-F = 251 Hz), 136.6 (dm,

1JC-F = 251 Hz), 133.1 (bs, m-C6H2), 131.7 (bs, m-

C6H2), 113.7 (m, i-C6F5), 111.1 (d, 2JC-P = 82.6 Hz, i-C6H2), 21.8 (bs, o-CH3

Mes), 21.3 (d,

5JC-P =

1.1 Hz, p-CH3Mes

), 20.9 (bs, o-CH3Mes

). Anal. Calc. for C63H35Al2F30P: C, 52.30; H, 2.44. Found:

C, 52.12; H, 2.55.

Synthesis of [Mes3PH][H(AlI3)2] (5.3): A 50 mL Schlenk bomb equipped with a Teflon screw

cap and a magnetic stirbar was charged with PMes3 (300 mg, 0.77 mmol), AlI3 (630 mg, 1.54

mmol) and fluorobenzene (ca. 10 mL). The bomb was transferred to the Schlenk line equipped

with a H2 outlet. The bomb was immersed in a liquid N2 bath, degassed, filled with H2, and

warmed to r.t. The mixture was heated to 75 °C with rapid stirring on an oil bath behind a blast

shield for 4 h yielding a pale yellow solution. The solution was cooled to r.t. and filtered through

Celite. The solvent was removed in vacuo and hexanes (ca. 10 mL) were added to the residue

and the mixtures stirred rapidly for 2-3 h. The resulting white powder was filtered, washed with

hexanes, and dried (680 mg, 0.56 mmol, 73%).

1H NMR (400 MHz, C6D5Br): δ 8.10 (d,


(bs, 3H, m-Mes), 6.26 (bs, 1H, Al-H), 2.11 (s, 9H, p-CH3Mes

), 2.10 (bs, 9H, o-CH3Mes

), 1.79 (bs,

9H, o-CH3Mes

). 31

P{1H} NMR (161 MHz, C6D5Br): δ -26 (s).

27Al NMR (104 MHz, C6D5Br): 36

(bs, υ1/2 = ca. 800 Hz). 13

C{1H} NMR (100 MHz, C6D5Br): δ 146.9 (d,

4JC-P = 3.0 Hz, p-C6H2),

149

143.7 (bs, o-C6H2), 142.7 (bs, o-C6H2), 133.1 (bs, m-C6H2), 131.9 (bs, m-C6H2), 111.3 (d, 2JC-P =

83.8 Hz, i-C6H2), 22.3 (bs, o-CH3Mes

), 22.0 (bs, o-CH3Mes

), 21.6 (d, 5JC-P = 1.5 Hz, p-CH3

Mes).

Anal. Calc. for C27H35Al2I6P: C, 26.89; H, 2.93. Found: C, 26.48; H, 3.01.

Synthesis of EtAl(C6F5)2 (5.4a): This compound was synthesized in a similar fashion to the

previously reported [MeAl(C6F5)2]2 analog.265

In a vial equipped with a stirbar in the glovebox

was charged B(C6F5)3 (200 mg, 0.39 mmol) in hexanes (5 mL). In a separate vial, AlEt3 (76 mg,

0.67 mmol) was dissolved in hexanes (1 mL) and added dropwise to the stirring B(C6F5)3

solution (Note: 1.7 equiv. of AlEt3 was needed instead of the expected 1.5 equiv. due to the low

purity (93%) of AlEt3). The solution was stirred for 2 h after which the solvent was removed in

vacuo. The oily residue was dissolved in minimal pentane and crystallized at -38 °C. The

pentane was rapidly decanted and the solids dried briefly in vacuo. The solids that initially

formed at -38 °C again produced an oil at r.t. (ca. 160 mg, 0.41 mmol, 70%). Note: in solution

EtAl(C6F5)2 exists in equilibrium with Al(C6F5)3 and Et2Al(C6F5) due to the rapid Schlenk

equilibrium, but is the major product (> 90%).

1H NMR (400 MHz, C6D5Br): δ 1.15 (t,

3JH-H = 8.0 Hz, 3H, Al-CH2CH3), 0.57 (bq, 2H, Al-

CH2CH3). 27


F{1H} NMR (376 MHz, C6D5Br): δ -120.5

(bs, 4F, o-C6F5), -150.9 (bs, 2F, p-C6F5), -160.2 (bs, 4F, m-C6F5). 13

C{1H} NMR (100 MHz,

C6D5Br), partial: δ 149.7 (dm, 1JC-F = 236 Hz), 141.9 (dm,

1JC-F = 257 Hz), 136.7 (dm,

1JC-F =

257 Hz), 8.5 (bs, Al-CH2CH3), 3.3 (bs, Al-CH2CH3). Anal. Calc. for C14H5AlF10: C, 43.10; H,

1.29. Found: C, 43.45; H, 1.88.

Synthesis of [Mes3PH][Al(C6F5)4] (5.5): This compound could be isolated in a similar fashion

to the salt [tBu3PH][Al(C6F5)4] (4.10). A 50 mL Schlenk bomb equipped with a Teflon screw cap

and a magnetic stirbar was charged with PMes3 (100 mg, 0.26 mmol), Al(C6F5)3•tol (319 mg,

0.51 mmol), and fluorobenzene (ca. 5 mL). The bomb was transferred to the Schlenk line

equipped with a propylene outlet. The bomb was degassed and filled with propylene (1 atm). The

solution was stirred on a 50 °C oil bath overnight. The bomb was cooled and the solvent

removed in vacuo. Hexanes (ca. 10 mL) were added to the residue and the mixture was stirred

rapidly until a white precipitate appeared. The precipitate was filtered and dried (160 mg, 0.15

mmol, 57%).

150

1H NMR (400 MHz, C6D5Br): δ 7.99 (d,


(bs, 3H, m-Mes), 2.08 (s, 9H, p-CH3Mes

), 1.98 (s, 9H, o-CH3Mes

), 1.75 (s, 9H, o-CH3Mes

). 31

P{1H}

NMR (161 MHz, C6D5Br): δ -26 (s). 27

Al NMR (104 MHz, C6D5Br): 116 (s). 19

F{1H} NMR

(376 MHz, C6D5Br): δ -122.3 (bd, 3JF-F = 18.8 Hz, 8F, o-C6F5), -157.9 (t,

3JF-F = 18.8 Hz, 4F, p-

C6F5), -163.9 (m, 8F, m-C6F5). 13

C{1H} NMR (100 MHz, C6D5Br), partial: δ 150.2 (dm,

1JC-F =

236 Hz), 147.2 (d, 4JC-P = 2.8 Hz, p-C6H2), 143.8 (bs, o-C6H2), 142.3 (bs, o-C6H2), 140.3 (dm,

1JC-F = 246 Hz), 136.6 (dm,

1JC-F = 251 Hz), 133.1 (bs, m-C6H2), 131.7 (bs, m-C6H2), 111.1 (d,

2JC-P = 82.6 Hz, i-C6H2), 21.8 (bs, o-CH3

Mes), 21.3 (d,

5JC-P = 1.1 Hz, p-CH3

Mes), 20.9 (bs, o-

CH3Mes

). Anal. Calc. for C51H34AlF20P: C, 56.47; H, 3.16. Found: C, 56.17; H, 3.10.

Synthesis of Al(C6F5)3•(C6H10) (5.6): Al(C6F5)3•tol (300 mg, 0.48 mmol) was dissolved in

cyclohexene (3-4 mL) and stored in the -38 °C freezer overnight. The solvent was decanted and

the crystals washed with cold cyclohexene (ca. 1 mL). The crystals were briefly (10-15s) dried in

vacuo (250 mg, 0.41 mmol, 85%). The crystals obtained were of suitable quality for X-ray

diffraction.

1H NMR (400 MHz, C6D5Br): δ 5.64 (s, 2H, CH), 1.89 (m, 4H, CH2), 1.50 (m, 4H, CH2).

27Al

NMR (104 MHz, C6D5Br): blank. 19


3JF-F = 26.7

Hz, 4JF-F = 9.4 Hz, 6F, o-C6F5), -150.1 (t,

3JF-F = 19.9 Hz, 3F, p-C6F5), -160.4 (m, 6F, m-C6F5).

13C{

1H} NMR (100 MHz, C6D5Br): δ 150.0 (dm,

1JC-F = 237 Hz), 142.6 (dm,

1JC-F = 254 Hz),

136.9 (dm, 1JC-F = 258 Hz), 127.3 (s, CH), 110.5 (m, i-C6F5), 25.4 (s, CH2), 22.8 (s, CH2). Anal.

Calc. for C24H10AlF15: C, 47.23; H, 1.65. Found: C, 46.86; H, 1.93.

Synthesis of [Et4N][ClAl(C6F5)3] (5.7): Et4NCl (267 mg, 1.61 mmol) and Al(C6F5)3•tol (1.0 g,

1.61 mmol) were combined in a vial in fluorobenzene (10 mL). After stirring for 15 min, the

solution was filtered on Celite and pentane (15-20 mL) was added with the solution stirring

rapidly. The precipitate that forms was filtered, washed with pentane, and dried (1.025 g, 1.48

mmol, 92%).

1H NMR (400 MHz, C6D5Br): δ 2.34 (q,

3JH-H = 7.2 Hz, 8H, N(CH2CH3)4), 0.60 (tt,

3JH-H = 7.2

Hz, 3

JH-N = 1.8 Hz, 12H, N(CH2CH3)4). 27

Al NMR (104 MHz, C6D5Br): 123 (bs, υ1/2 = ca. 1500

Hz). 19


3JF-F = 28.2 Hz,

4JF-F = 11.7 Hz, 6F, o-

C6F5), -156.7 (t, 3JF-F = 19.6 Hz, 3F, p-C6F5), -163.6 (m, 6F, m-C6F5).

13C{

1H} NMR (100 MHz,

C6D5Br): δ 150.2 (dm, 1JC-F = 233 Hz), 140.7 (dm,

1JC-F = 249 Hz), 136.6 (dm,

1JC-F = 252 Hz),

151

118.6 (m, i-C6F5), 52.2 (t, 1

JC-N = 3.0 Hz, N(CH2CH3)4), 6.7 (s, N(CH2CH3)4). Anal. Calc. for

C26H20AlClF15N: C, 45.01; H, 2.91; N, 2.02. Found: C, 44.95; H, 2.97; N, 2.05.

Synthesis of [Et4N][HAl(C6F5)3] (5.8): In a vial equipped with a magnetic stirbar in the

glovebox was dissolved 5.7 (400 mg, 0.58 mmol) in fluorobenzene (8 mL). In a separate vial was

dissolved LiAlH4 (24 mg, 0.63 mmol) in Et2O (2 mL). The Et2O solution was added dropwise to

the fluorobenzene solution at r.t. with rapid stirring. A precipitate formed and the mixture was

allowed to stir for 2 h. The solvent was removed in vacuo and the residue was dissolved in

fluorobenzene and filtered on Celite. The solvent was removed from the filtrate and hexanes (ca.

10 mL) were added to the oily residue. A nice white precipitate forms after stirring rapidly for 15

min. The precipitate was filtered and dried (290 mg, 0.44 mmol, 76%). Vapour diffusion of a

bromobenzene solution with pentane yielded single crystals suitable for X-ray crystallography.

1H NMR (400 MHz, C6D5Br): δ 4.79 (bs, 1H, Al-H), 2.35 (q,

3JH-H = 7.2 Hz, 8H, N(CH2CH3)4),

0.61 (tm, 3JH-H = 7.2 Hz, 12H, N(CH2CH3)4).

27Al NMR (104 MHz, C6D5Br): 115 (bs, υ1/2 = ca.

1000 Hz). 19


3JF-F = 29.0 Hz,

4JF-F = 12.4 Hz, 6F,

o-C6F5), -156.8 (t, 3JF-F = 19.9 Hz, 3F, p-C6F5), -162.8 (m, 6F, m-C6F5).

13C{

1H} NMR (100

MHz, C6D5Br): δ 150.1 (dm, 1JC-F = 230 Hz), 140.2 (dm,

1JC-F = 247 Hz), 136.5 (dm,

1JC-F = 251

Hz), 119.8 (m, i-C6F5), 52.2 (t, 1

JC-N = 3.0 Hz, N(CH2CH3)4), 6.7 (s, N(CH2CH3)4). Anal. Calc.

for C26H21AlF15N: C, 47.36; H, 3.21; N, 2.12. Found: C, 46.97; H, 3.22; N, 2.18.

Synthesis of [Et4N][H(Al(C6F5)3)2] (5.9): In a vial equipped with a magnetic stirbar in the

glovebox were combined 5.8 (200 mg, 0.30 mmol) and Al(C6F5)3•tol (188 mg, 0.30 mmol) in

fluorobenzene (10 mL). A precipitate rapidly forms and the mixture was stirred for 30 min.

Pentane (5 mL) was added and the solids were filtered and dried (350 mg, 0.29 mmol, 97%).

1H NMR (400 MHz, C6D5Br, 80 °C): δ 4.28 (bs, 1H, Al-H), 2.35 (q,

3JH-H = 7.2 Hz, 8H,

N(CH2CH3)4), 0.65 (tm, 3JH-H = 7.2 Hz, 12H, N(CH2CH3)4).

27Al NMR (104 MHz, C6D5Br,

80 °C): blank. 19

F{1H} NMR (376 MHz, C6D5Br, 80 °C): δ -120.2 (dd,

3JF-F = 26.7 Hz,

4JF-F =


13C{

1H}

NMR (100 MHz, C6D5Br, 80 °C): δ 150.0 (dm, 1JC-F = 238 Hz), 141.6 (dm,

1JC-F = 252 Hz),

136.7 (dm, 1JC-F = 254 Hz), 113.6 (m, i-C6F5), 52.6 (t,

1JC-N = 3.1 Hz, N(CH2CH3)4), 6.7 (s,

N(CH2CH3)4). Anal. Calc. for C44H21Al2F30N: C, 44.50; H, 1.78; N, 1.18. Found: C, 44.0; H,

1.94; N, 1.21.

152

Synthesis of [Et4N][EtAl(C6F5)3] (5.10): In a vial equipped with a magnetic stirbar in the

glovebox was dissolved 5.7 (500 mg, 0.72 mmol) in fluorobenzene (10 mL). EtMgBr (3.0 M in

Et2O, 0.24 mL, 0.72 mmol) was added dropwise at r.t. to the rapidly stirring fluorobenzene

solution. A precipitate formed and the mixture was allowed to stir for 2 h. The solvent was

thoroughly removed in vacuo and the residue was dissolved in fluorobenzene and filtered on

Celite. The solvent was removed from the filtrate and hexanes (ca. 10 mL) were added to the oily

residue. A nice white precipitate forms after stirring rapidly for 15 min. The precipitate was

filtered and dried (390 mg, 0.57 mmol, 79%). Vapour diffusion of a bromobenzene solution with


1H NMR (400 MHz, C6D5Br): δ 2.26 (q,

3JH-H = 7.4 Hz, 8H, N(CH2CH3)4), 1.40 (t,

3JH-H = 8.0

Hz, 3H, Al-CH2CH3), 0.75 (q, 3JH-H = 8.0 Hz, 2H, Al-CH2CH3), 0.57 (tt,

3JH-H = 7.4 Hz,

3JH-N =

1.8 Hz, 12H, N(CH2CH3)4). 27

Al NMR (104 MHz, C6D5Br): 134 (bs, υ1/2 = ca. 2000 Hz).

19F{

1H} NMR (376 MHz, C6D5Br): δ -121.6 (dd,

3JF-F = 30.5 Hz,

4JF-F = 12.0 Hz, 6F, o-C6F5), -

158.9 (t, 3JF-F = 19.9 Hz, 3F, p-C6F5), -164.2 (m, 6F, m-C6F5).

13C{

1H} NMR (100 MHz,

C6D5Br), partial: δ 150.1 (dm, 1JC-F = 233 Hz), 139.7 (dm,

1JC-F = 247 Hz), 136.4 (dm,

1JC-F =

251 Hz), 52.1 (t, 1

JC-N = 3.0 Hz, N(CH2CH3)4), 10.3 (s, Al-CH2CH3), 6.6 (s, N(CH2CH3)4), 3.6

(bs, Al-CH2CH3). Anal. Calc. for C28H25AlF15N: C, 48.92; H, 3.67; N, 2.04. Found: C, 48.63; H,

3.67; N, 2.14.

Synthesis of [(otol)3PH][Al(C6F5)4] (5.11): A 50 mL Schlenk bomb equipped with a Teflon

screw cap and a magnetic stirbar was charged with P(otol)3 (98 mg, 0.32 mmol), Al(C6F5)3•tol

(400 mg, 0.64 mmol), ca. 50 equiv. cyclohexene (1.60 mL, 15.8 mmol), and fluorobenzene (ca. 5

mL). The bomb was transferred to the Schlenk line equipped with a H2 outlet. The bomb was

immersed in a liquid N2 bath, degassed, filled with H2, and slowly warmed to r.t. The solution

was stirred on a 60 °C oil bath for 72 h. The bomb was cooled, depressurized and ca. 1.5 mL

Et2O was added to the stirring solution. Hexanes (ca. 10 mL) were then added dropwise with

rapid stirring. The precipitate that forms was filtered and dried (190 mg, 0.19 mmol, 59%).

1H NMR (400 MHz, C6D5Br): δ 8.04 (d,

1JH-P = 482 Hz, 1H, PH), 7.33 (t, J = 8.0 Hz, 3H), 7.07-

7.01 (m, 6H), 6.84-6.78 (m, 3H), 1.99 (s, 9H, o-CH3). 31


13.2 (s). 27

Al NMR (104 MHz, C6D5Br): 116 (s). 19

F{1H} NMR (376 MHz, C6D5Br): δ -122.3

(bd, 3JF-F = 18.8 Hz, 8F, o-C6F5), -157.7 (t,

3JF-F = 18.8 Hz, 4F, p-C6F5), -163.9 (m, 8F, m-C6F5).

153

13C{

1H} NMR (100 MHz, C6D5Br), partial: δ 150.2 (dm,

1JC-F = 231 Hz), 143.0 (d, JC-P = 8.9

Hz), 140.3 (dm, 1

JC-F = 250 Hz), 136.5 (dm, 1

JC-F = 251 Hz), 136.4 (d, JC-P = 2.9 Hz), 134.2 (d,

JC-P = 12.8 Hz), 132.9 (d, JC-P = 10.6 Hz), 128.2 (d, JC-P = 13.9 Hz), 111.2 (d, 1JC-P = 87 Hz, i-

C6H4), 20.4 (d, 3JC-P = 8.5 Hz, o-CH3). Anal. Calc. for C45H22AlF20P: C, 54.02; H, 2.22. Found:

C, 53.64; H, 2.49.

Synthesis of [(2,6-Cl2C6H3)3PH][AlCl4] (5.12): A 50 mL Schlenk flask was charged with P(2,6-

Cl2C6H3)3 (200 mg, 0.43 mmol), AlCl3 (57 mg, 0.43 mmol), and fluorobenzene (ca. 5 mL). The

flask was transferred to the Schlenk line and HCl (2.0 M in Et2O; 0.23 mL, 0.46 mmol) was

added dropwise with rapid stirring. A precipitate formed which was filtered on a swivel frit and

washed with hexanes (160 mg, 0.25 mmol, 58%).

1H NMR (400 MHz, C6D5Br): δ 9.33 (d,

1JH-P = 561 Hz, 1H, PH), 7.21 (t,

3JH-H = 8.0 Hz, 3H, p-

Cl2C6H3), 7.09-7.06 (m, 6H). 31

P{1H} NMR (161 MHz, C6D5Br): δ -19 (s).

27Al NMR (104

MHz, C6D5Br): 104 (s). Anal. Calc. for C18H10AlCl10P: C, 33.85; H, 1.58. Found: C, 33.71; H,

1.63.

Synthesis of Al(C6F4H)3•tol (5.13): This compound was synthesized in an analogous fashion to

Al(C6F5)3•tol.175

A round bottom flask in the glovebox was charged with B(C6F4H)3 (350 mg,

0.76 mmol) and AlMe3 (55mg, 0.76 mmol) in toluene (ca. 10 mL). The solution was allowed to

stir rapidly overnight. The next morning, the solvent volume was reduced to 7-8 mL and the hot

solution put in the -38 °C freezer. The crystals obtained were filtered on a frit and washed with

minimal hexanes. The solid was lightly dried in vacuo (300 mg, 0.53 mmol, 70%).

1H NMR (400 MHz, C6D5Br): δ 7.18-6.98 (m, 5H, C6H5CH3), 6.62 (tt,

3JH-F = 9.3 Hz,

4JH-F = 7.1

Hz, 3H, C6F4H), 2.16 (s, C6H5CH3). 27


F{1H} NMR (376

MHz, C6D5Br): δ -121.6 (m, o-C6F4H, 6F), -137.5 (m, m-C6F4H, 6F). 13

C{1H} NMR (100 MHz,

C6D5Br): δ 149.7 (dm, 1JC-F = 236 Hz), 145.4 (dm,

1JC-F = 250 Hz), 137.5 (s, C6H5CH3), 129.1 (s,

C6H5CH3), 128.3 (s, C6H5CH3), 125.4 (s, C6H5CH3), 117.9 (m, i-C6F4H), 108.3 (t, 2JC-F = 22.8

Hz, p-C6F4H), 21.5 (s, C6H5CH3). Anal. Calc. for Al(C6F4H)3•0.8(C7H8): C, 51.74; H, 1.73.

Found: C, 52.14; H, 1.74.

Synthesis of [Et4N][FAl(C12F9)3] (5.14): [Li(OEt2)][FAl(C12F9)3] (1.5 g, 1.40 mmol) was

dissolved in dichloromethane (15 mL) in a 50 mL flask in the glovebox. A solution of Et4NCl

154

(232 mg, 1.40 mmol) in dichloromethane (5 mL) was added dropwise to the rapidly stirring

solution. Precipitation occurs immediately. The mixture was allowed to stir for an additional 2 h.

The solvent was thoroughly removed in vacuo and fluorobenzene (30 mL) was added to the

residue. The mixture was filtered on Celite. Pentane (10-15 mL) was added dropwise to the

rapidly stirring filtrate to yield a white precipitate which was filtered, washed with pentane, and

dried (1.1 g, 0.98 mmol, 70%).

1H NMR (400 MHz, C6D5Br): δ 2.28 (q,

3JH-H = 7.4 Hz, 8H, N(CH2CH3)4), 0.57 (tt,

3JH-H = 7.4

Hz, 3

JH-N = 1.8 Hz, 12H, N(CH2CH3)4). 27

Al NMR (104 MHz, C6D5Br): 112 (bs, υ1/2 = ca. 900

Hz). 19

F{1H} NMR (376 MHz, C6D5Br): δ -121.6 (m, 3F), -139.0 (m, 3F), -141.6 (m, 6F), -156.8

(m, 3F), -157.3 (m, 3F), -157.9 (m, 3F), -165.2 (m, 6F), -175.7 (bs, 1F, Al-F). 13

C{1H} NMR

(100 MHz, C6D5Br), partial: δ 151.7 (dm, 1JC-F = 234 Hz), 145.3 (dm,

1JC-F = 247 Hz), 144.8

(dm, 1JC-F = 242 Hz), 141.3 (dm,

1JC-F = 252 Hz), 139.6 (dm,

1JC-F = 245 Hz), 137.3 (dm,

1JC-F =

229 Hz), 116.3 (bs), 110.7 (bm), 52.1 (t, 1

JC-N = 3.0 Hz, N(CH2CH3)4), 6.6 (s, N(CH2CH3)4).

Anal. Calc. for C44H20AlF28N: C, 47.12; H, 1.80; N, 1.25. Found: C, 46.64; H, 2.06; N, 1.69.

Synthesis of [Et4N][HAl(C12F9)3] (5.15): In a 50 mL flask in the glovebox was dissolved 5.14

(900 mg, 0.80 mmol) in fluorobenzene (10 mL). A solution of LiAlH4 (37 mg, 0.97 mmol) in

diethyl ether (3 mL) was added dropwise to the rapidly stirring solution. The solution was

allowed to stir overnight (12 h) during which time precipitation occurred. The solvent was

thoroughly removed in vacuo and fluorobenzene (20 mL) was added to the residue. The mixture

was filtered on Celite and the filtrate concentrated to 10 mL. Pentane (10 mL) was added

dropwise to the rapidly stirring solution to yield a white precipitate which was filtered, washed

with pentane, and dried (770 mg, 0.70 mmol, 87%).

1H NMR (400 MHz, C6D5Br): δ 2.50 (bs, 1H, Al-H), 2.32 (q,

3JH-H = 7.4 Hz, 8H, N(CH2CH3)4),

0.62 (tt, 3JH-H = 7.4 Hz,

3JH-N = 1.8 Hz, 12H, N(CH2CH3)4).

27Al NMR (104 MHz, C6D5Br): 124

(bs, υ1/2 = ca. 1100 Hz). 19

F{1H} NMR (376 MHz, C6D5Br): δ -118.2 (m, 3F), -138.8 (m, 3F), -

140.5 (m, 6F), -155.7 (m, 3F), -156.6 (m, 3F), -157.8 (m, 3F), -163.8 (m, 6F). 13

C{1H} NMR

(100 MHz, C6D5Br), partial: δ 151.6 (dm, 1JC-F = 226 Hz), 145.2 (dm,

1JC-F = 253 Hz), 144.6

(dm, 1JC-F = 248 Hz), 141.3 (dm,

1JC-F = 253 Hz), 139.7 (dm,

1JC-F = 246 Hz), 139.2 (dm,

1JC-F =

250 Hz), 137.5 (dm, 1JC-F = 250 Hz), 116.4 (bm), 110.9 (bm), 52.1 (t,

1JC-N = 3.0 Hz,

155

N(CH2CH3)4), 6.6 (s, N(CH2CH3)4). Anal. Calc. for C44H21AlF27N: C, 47.89; H, 1.92; N, 1.27.

Found: C, 47.82; H, 2.16; N, 1.50.

Synthesis of [Mes3PH][(µ-H)(Al(C12F9)3)2] (5.16): In a vial in the glovebox was dissolved

[CPh3][B(C6F5)4] (251 mg, 0.27 mmol) in fluorobenzene (3-4 mL). To this stirring solution was

added a solution of 5.15 (300 mg, 0.27 mmol) dropwise. The initial orange solution turned pale

yellow and was allowed to stir rapidly for 10 min. The solvent was thoroughly removed and the

residue was dissolved in hexanes and filtered on Celite. The filter cake was washed with copious

amounts of hexanes. The filtrate solvent was removed and PMes3 (53 mg, 0.14 mmol) in

bromobenzene (10 mL) was added to this. The solution was transferred to a 50 mL Schlenk

bomb equipped with a Teflon screw cap and a magnetic stirbar. The bomb was transferred to the

Schlenk line equipped with a H2 outlet. The bomb was immersed in a liquid N2 bath, degassed,

filled with H2, and slowly warmed to r.t. The solution was stirred for 12 h in the glovebox after

which time a precipitate had formed. Hexanes (ca. 5-7 mL) were added dropwise to the stirring

mixture and the precipitate was filtered on a glass frit, washed with hexanes and dried in vacuo

(120 mg, 0.051 mmol, 38%). Vapour diffusion of a bromobenzene solution with hexanes yielded


1H NMR (400 MHz, C6D5Br), partial: δ 8.02 (d,

1JH-P = 478 Hz, 1H, PH), 6.78 (bs, 3H, m-Mes),

6.73 (bs, 3H, m-Mes), 2.10 (s, 9H, p-CH3Mes

), 2.01 (s, 9H, o-CH3Mes

), 1.78 (s, 9H, o-CH3Mes

).

31P{

1H} NMR (161 MHz, C6D5Br): δ -26 (s).

27Al NMR (104 MHz, C6D5Br): blank.

19F{

1H}

NMR (376 MHz, C6D5Br): δ -112 (bm, 4F), -134.7 (bm, 4F), -135.3 (bm, 2F), -136.8 (bm, 6F), -

137.7 (bm, 2F), -138.0 (bm, 4F), -154.6 (bm, 6F), -155.0 (bm, 6F), -156.0 (bm, 2F), -156.9 (bm,

4F), -162.1 (bm, 4F), -162.3 (bm, 4F), -162.7 (bm, 6F). 13

C{1H} NMR (100 MHz, C6D5Br): δ

due to the extremely poor solubility of this compound, a reliable 13

C NMR spectrum could not be

obtained. Anal. Calc. for C99H35Al2F54P: C, 50.92; H, 1.51. Found: C, 50.78; H, 1.85.






156







The heavy





2/2 (Fo

2) and Fo










157



5.1 5.2 5.6

Formula C48H29Al2F30P1 C69H40Al2Br1F30P1 C24H10Al1F15

Formula wt. 1260.64 1603.85 610.30

Crystal system monoclinic triclinic monoclinic

Space group P21/c P-1 P21/c

a(Å) 13.2334(6) 11.8376(8) 12.8841(11)

b(Å) 17.8939(11) 16.9501(11) 8.2440(7)

c(Å) 21.5964(9) 17.5071(11) 21.1402(17)

α(deg) 90 86.868(3) 90

β(deg) 96.924(2) 78.578(3) 92.776(4)

γ(deg) 90 74.546(4) 90

V(Å3) 5076.7(4) 3318.7(4) 2242.8(3)

Z 4 2 4

T (K) 150(2) 150(2) 150(2)

d(calc) g/cm3 1.649 1.494 1.807

Abs coeff, μ, mm-1

0.235 0.792 0.229


Rint 0.0587 0.0294 0.0459

Data used 11636 15007 5897


R (>2σ) 0.0566 0.0951 0.0410

wR2 0.1437 0.3114 0.0990

GOF 1.012 2.166 1.022

158


5.8 5.10 5.13

Formula C26H21Al1F15N1 C28H25Al1F15N1 C25H11Al1F12

Formula wt. 659.42 687.47 566.32

Crystal system monoclinic monoclinic triclinic

Space group P21 P21/n P-1

a(Å) 10.9199(6) 12.6210(5) 9.9703(5)

b(Å) 13.3504(7) 13.7433(5) 10.2507(5)

c(Å) 19.4497(11) 16.9159(7) 12.0527(6)

α(deg) 90 90 90.089(3)

β(deg) 104.310(3) 91.721(2) 113.851(2)

γ(deg) 90 90 96.326(3)

V(Å3) 2747.5(3) 2932.8(2) 1118.3(1)

Z 4 4 2

T (K) 150(2) 150(2) 150(2)

d(calc) g/cm3 1.594 1.557 1.682

Abs coeff, μ, mm-1

0.194 0.185 0.204


Rint 0.0247 0.0577 0.0226

Data used 12522 7035 4996


R (>2σ) 0.0356 0.0421 0.0344

wR2 0.0824 0.1005 0.1262

GOF 1.013 0.997 0.957

159


5.15 5.16

Formula C44H21Al1F27N1 C99H35Al2F54P1

Formula wt. 1103.60 2335.19

Crystal system monoclinic triclinic

Space group P21/n P-1

a(Å) 16.2527(9) 12.4265(18)

b(Å) 15.8456(9) 16.935(3)

c(Å) 17.1208(8) 21.347(3)

α(deg) 90 88.445(6)

β(deg) 106.582(2) 86.663(6)

γ(deg) 90 83.482(6)

V(Å3) 4225.8(4) 4454.6(1)

Z 4 2

T (K) 150(2) 150(2)

d(calc) g/cm3 1.735 1.458

Abs coeff, μ, mm-1

0.204 0.170


Rint 0.0458 0.0670

Data used 10516 22320

Variables 671 1423

R (>2σ) 0.0451 0.0881

wR2 0.1556 0.2628

GOF 0.901 1.319

160

Chapter 6 Frustrated Lewis Pair-Mediated C–H Bond Activation

Using N2O

6.1 Introduction

The rapid increase in the global human population since the industrial revolution is often

attributed to better health and sanitation, as well as the ability to produce more food on less land

using artificial fertilizers. The Haber-Bosch process to form ammonia (NH3) from N2 and H2

remains the largest industrial source of NH3, which is mostly used for the synthesis of N-based

fertilizers. The resulting nitrification of the ground stimulates bacteria which then denitrify the

ecosystem, releasing mostly N2, but also reactive N-containing products, such as nitrous oxide

(N2O).278

While most concern with global warming has been focused on CO2 emissions, N2O

emissions are equally rapidly rising. This is largely a result of increased agriculture, but also as a

by-product of fossil fuel combustion and other industrial processes.279

While its atmospheric

concentration is orders of magnitude lower than CO2 (323 ppb vs. 391 ppm),280

its global

warming potential compared to CO2 (= 1) is orders of magnitude higher (298).281

In addition to

this concern, while N2O is relatively stable in the troposphere, it decomposes in the stratosphere

into reactive nitrogen species, in particular NO and NO2, which are in turn potent stratospheric

ozone depleting substances (ODS). In fact, the Montreal Protocol, which bans or limits the

emissions of chlorofluorocarbons (CFCs) due to concerns over a growing ozone hole in the late

20th

century, does not cover N2O which has now become the dominant ODS of the 21st

century.282

In nature, denitrification of N2O to N2 and H2O is undertaken by a tetranuclear Cu site of the

nitrous oxide reductase enzyme within certain bacteria.283,284

Synthetic approaches to the

characterization of transition metal N2O complexes have remained challenging due to its poor

σ-donating and π-accepting abilities. Furthermore, while it is thermodynamically a good oxidant,

it remains kinetically inert. This kinetic barrier can be overcome through coordination and

subsequent reaction at transition metal centres.285

For example, in a recent example by the Chang

group,286,287

coordination of N2O to an Fe(II) pyrrole platform precedes its subsequent oxidation

to an Fe(IV) oxo centre intermediate through N2 release. This intermediate then undergoes a final

161

hydrogen atom transfer (HAT) reaction from a H-atom donor, such as 1,4-cyclohexadiene, to

generate the Fe(III) hydroxide species (Scheme 6.1).

Scheme 6.1 – Reaction of an Fe(II) pyrrole anionic species with N2O leading to N2 evolution and

a HAT reaction (from solvent or H-atom donor) as reported by Chang and co-workers (K+ cation

is omitted for clarity).286,287

In a similar report, the Berben group reported HAT reactions using Al species containing

iminopyridine ligands.56

When oxidized with suitable O-atom transfer reagents, such as pyridine-

N-oxide, these compounds were found to undergo subsequent HAT reactions with the solvent to

produce Al–OH products. The chemistry presented in this chapter shares some similarities with

both this Fe and Al chemistry.

In addition to oxidation chemistry, N2O is known to insert into metal carbon288

or metal

hydrogen289

bonds with concomitant release of N2. An alternative to this oxygen atom transfer

chemistry involves N–N bond scission of N2O at a metal centre. Recent examples by Sita290

involve the CO-induced N–N bond scission of N2O at a Mo centre to generate a nitrosyl

isocyanate species.

More recent developments in non-metal mediated routes to N2O activation and functionalization

have recently been reported. The Severin group has outlined the direct activation of N2O by an

N-heterocyclic carbene (NHC) resulting in the addition of the carbene at the terminal N atom.291

These same species were capable of undergoing sequential O-atom transfer to a V(III) complex,

followed by N–N bond scission of the product.292

Frustrated Lewis pairs have also been shown to

activate N2O, initially reported using PtBu3 and B(C6F5)3 to form the product

tBu3P(N2O)B(C6F5)3, the first crystallographically characterized N2O complex.114

This product

was found to undergo N–O bond cleavage and formation of the phosphine oxide, tBu3PO, adduct

of B(C6F5)3 through a proposed Staudinger-type mechanism293

under photolysis conditions or

prolonged heating (Scheme 6.2).

162

Scheme 6.2 – N2O capture by the PtBu3/ B(C6F5)3 FLP solution and subsequent N2 release.

Since then, N2O chemistry using FLPs has also been expanded to using Zn(C6F5)2 as the Lewis

acid component. In particular, it was found that the similar N2O complex tBu3P(N2O)B(C6H4F)3

undergoes exchange chemistry with the stronger Zn(C6F5)2 Lewis acid to form a variety of

products with different bonding modes depending on the applied stoichiometry (Figure 6.1).115

The R3PNNO moiety has also been found to coordinate to various other Lewis acidic centres,

such as bis-boranes, trityl cation, and group IV elements (Ti, Zr).116

Figure 6.1 – Various binding modes of tBu3P(N2O)(Zn(C6F5)2)n, where: a) n = 1; b) n = 1.5; c) n

= 2.0.

As part of our continued interest in exploring the chemistry of Al-based FLPs with small

molecules, we set out to probe the differences and similarities between Al and other Lewis acids

for N2O activation. What we found, as will be described in this chapter, were some similarities to

P/B N2O complexes, but also some striking differences, such as the divergent chemistry of these

systems upon addition of excess Al (as compared to the Zn chemistry). Addition of excess Al

leads to a proposed homolytic cleavage of the P–N and N–O bonds, ejecting N2 and generating

an intermediate radical species which undergoes a HAT reaction from a phosphine C–H group,

or from aromatic or benzylic C–H bonds. This chemistry resembles the Fe chemistry above, but

with the distinct difference of activating much more stable C–H bonds.

163


6.2.1 Synthesis of a P/Al N2O complex and subsequent phosphine C–H bond activation

In our initial explorations into N2O chemistry with Al, we set out to synthesize the analogous Al

compound of the previously reported species, tBu3P(N2O)B(C6F5)3 (Scheme 6.2).114

Exposing a

1:1 solution of PtBu3:Al(C6F5)3•tol in C6D5Br to 1 atm of N2O in a J-Young NMR tube resulted

in the formation of 3 major peaks in the 31

P{1H} NMR spectrum at 73 (broad), 66, and 43 ppm.

The 19

F{1H} NMR spectrum also displayed 3 separate sets of ortho, meta, and para peaks,

consistent with the formation of 3 separate products. While the peak at 66 ppm in the 31

P{1H}

NMR spectrum is similar to the one reported114

for the B product, tBu3P(N2O)B(C6F5)3, the

identity of the other 2 major products remained unknown. Efforts were made to isolate this

species and was successfully done on a large scale by slow addition of N2O to a cooled (-78 °C)

solution containing 2 equiv. of PtBu3 to Al(C6F5)3•tol in toluene. Warming the solution to r.t.,

followed by precipitation using hexanes resulted in the isolation of a clean product in 91% yield.

The excess unreacted PtBu3 was removed in the hexanes wash. The 31

P{1H} NMR spectrum of

the product contained only one signal at 66 ppm, while the 19

F{1H} NMR spectrum contained a

set of 3 peaks at -121.4, -154.0, and -161.7 ppm. Elemental analysis confirmed the composition

as tBu3P(N2O)Al(C6F5)3 (6.1) and crystals suitable for X-ray diffraction were obtained. The

structure of 6.1 is shown in Figure 6.2. The P–N, N–N, N–O, and O–Al bond distances in 6.1 are

1.7075(18), 1.254(2), 1.328(2), and 1.8203(15) Å, respectively. The P–N–N, N–N–O, and N–O–

Al angles are 115.79(15)°, 110.71(17)°, and 118.54(12)°, respectively. With the exception of the

longer O–Al bond (1.5430(18) Å for O–B), all metrical parameters for 6.1 are very similar to

those reported for the analog, tBu3P(N2O)B(C6F5)3.114

164

Figure 6.2 – POV-Ray depiction of 6.1. C: black, N: blue, P: orange, F: pink, Al: teal, O: red. H

atoms are omitted for clarity.

With 6.1 in hand, we next set out to determine the composition of the other 2 unknown by-

products outlined above. We found that reversing the stoichiometry of that used for 6.1 and

exposing a pre-cooled (-40 °C) 1:2 solution of tBu3P:Al(C6F5)3•tol to N2O resulted in the

formation of the single major product at 43 ppm in the 31

P{1H} NMR spectrum. The product

(6.2) was isolated by precipitation with pentane in 62% yield. The 19

F{1H} NMR spectrum

displayed 3 peaks at -122.5, -153.5, and -161.3 ppm, slightly shifted from the peaks for 6.1. In

contrast, the 1H NMR spectrum was very different than the usual single doublet (JH-P) obtained

for the tBu peaks of PtBu3 (Figure 6.3).

165

Figure 6.3 – 1H NMR spectrum of 6.2 in C6D5Br.

The spectrum consisted of 3 doublets (due to H–P coupling) in the aliphatic region integrating to

18:3:3 protons. Surprisingly, the 3 remaining protons of PtBu3 were found in the olefinic region

of the spectrum as 2 P-coupled doublets at 5.67 and 5.23 ppm, along with an overlapping broad

singlet at 5.64 ppm. Notably, these two doublets are found to weakly couple to each other, as

well as to the methyl doublet at 1.56 ppm as observed by 2D 1H–

1H COSY NMR experiments.

The olefinic doublets are also found on the same C atom as observed by 2D 1H–

13C HSQC NMR

experiments. Furthermore, both this C atom and the C of the methyl group at 1.56 ppm were

found to be adjacent to an ipso-type C atom with strong JC-P coupling of 58 Hz as determined by

2D 1H–

13C HMBC NMR. This data is consistent with the presence of an allyl group bound to P

at the secondary C atom. It should also be noted that the broad singlet at 5.64 ppm is not C

coupled as observed by 2D experiments. The structure of this compound (6.2) was

unambiguously confirmed by X-ray crystallography and is consistent with the NMR data

presented here. The solid state structure is shown in Figure 6.4.

166

Figure 6.4 – POV-Ray depiction of 6.2. C: black, P: orange, F: pink, Al: teal, O: red, H: white.

H atoms, except OH, are omitted for clarity.

The structure of 6.2 represents the product of a surprising chain of events involving N2 ejection

from N2O, followed by methyl group transfer from a tBu group to the P atom, and formation of a

µ-OH group, wherein this H atom emanates from another methyl group of the PtBu3 moiety.

Similar reactivity with Zn or B-based FLPs has not been reported.114-116

Furthermore, addition of

excess B(C6F5)3 to the reported compound114

tBu3P(N2O)B(C6F5)3 does not lead to any C–H

bond activation chemistry as observed by NMR spectroscopy. However, in a similar manner to

the Zn exchange chemistry with tBu3P(N2O)B(C6H4F)3,115

addition of 2 equiv. of Al(C6F5)3•tol

to a solution of tBu3P(N2O)B(C6F5)3 does lead to the displacement of B(C6F5)3 for the more

Lewis acidic Al(C6F5)3,193

and formation of 6.2. Alternatively, the product 6.2 can be synthesized

in higher yields (71%) by slow addition of 1 equiv. of Al(C6F5)3•tol to a cooled solution of 6.1 in

fluorobenzene. Both these reactions support the intermediate formation of 6.1 en route to 6.2.

With this data in hand, a proposed mechanism involving the homolytic cleavage of the P–N and

N–O bonds, initiated by excess Al(C6F5)3, is proposed (Scheme 6.3).

167

Scheme 6.3 – Proposed homolytic mechanism for the reaction of 6.1 with Al(C6F5)3 to generate

6.2.

The last unidentified broad peak at 73 ppm in the 31

P{1H} NMR spectrum of the 1:1 P:Al

reaction with N2O outlined at the beginning of this section remains unknown; however, its shift

is close to the 31

P{1H} NMR shift reported for the 1:1.5 and 1:2 P:Zn N2O complexes outlined in

Figure 6.1 (b and c, respectively).115

This could represent another intermediate between 6.1 and

6.2; however, the exact nature of this species remains unclear as attempts to isolate it failed. The

following section will outline further evidence for the proposed homolytic pathway outlined in

Scheme 6.3.

6.2.2 Isolation and crystal structure of a phosphoniumyl cation

In order to determine whether or not a homolytic radical pathway is at play in the transformation

of 6.1 to 6.2, a J-Young NMR tube was loaded with a 1:2 bromobenzene solution of

PtBu3:Al(C6F5)3•tol. An X-band EPR spectrum of the sample was acquired and no signals were

apparent. The sample was frozen and 1 atm of N2O was condensed into the tube. The frozen

sample was slowly thawed and monitored continuously by EPR spectroscopy. No signals were

observed during the course of the reaction; however, the proposed oxygen radical anion (O•-) and

phosphoniumyl radical cation (P•+

) are both known to be very reactive. For instance, many

studies have examined the role of the O•- in atmospheric and/or mass spectral studies and found

it to react readily with small molecules such as CO2, CO, aromatics, and simple alkanes, such as

168

methane.294

The reactions with hydrocarbons were often found to proceed in a facile manner

through HAT reactions. A similar HAT reaction between the proposed [(µ-O•)(Al(C6F5)3)2]

-

intermediate and a methyl group H atom of tBu3P•+

may occur here; however, the cited

references294

looked only at the O•- atom and not that of a bridged radical. Furthermore, EPR

spectra of the O•- radical are typically only obtained on surfaces and not in solution.

295 Combined

with this premise, tertiary phosphine radical cations (R3P•+

, R = alkyl or arene) are all known to

be unstable, being oxidized irreversibly.296

The by-products produced can include dimer

formation with one equivalent of PR3 to form [R3PPR3]•+

,296,297

P–R bond scission,298

or

reactions with adventitious O2 or H2O to form R3PO or R3PH+.296

The only exceptions to the

instability of the phosphoniumyl cation are with sterically protected phosphines, in particular

PMes3204,206,296,299

and its related bulkier analog, PMes*3 (Mes

* = 2,4,6-triisopropylphenyl),

205 as

well as other bulky triarylphosphines272

and phosphites.298

No reference could be found of an

EPR observable trialkylphosphoniumyl radical cation. For these reasons, the lack of an EPR

signal in this case, even a transient one, is not surprising due to the likely very short lifetimes of

these species.

With this in mind, we set out to explore the chemistry of PMes3 and Al(C6F5)3 with N2O. It

should be noted that, in contrast to with PtBu3, a weak EPR signal is present upon mixing PMes3

and Al(C6F5)3 in bromobenzene, similar to what was observed in Chapter 3 with aluminum

halides. Again, however, NMR resonances for a PMes3/Al(C6F5)3 FLP are also apparent, thus

indicating the presence of an equilibrium electron transfer as proposed in Scheme 3.8. It should

also be noted that no reactions have been reported to occur with B(C6F5)3 or Zn(C6F5)2 with N2O

and phosphines bulkier than PtBu3.114-116

This is attributed to the lower Lewis basicity of bulkier

phosphines such as P(otol)3 and PMes3, although a steric argument could also be made with

B(C6F5)3.114

Indeed a reaction does occur at the sterically less hindered Al centre when a 1:2

solution of PMes3:Al(C6F5)3•tol in C6D5Br is exposed to N2O. The immediate formation of a

deep purple solution occurs. Both 19

F{1H} and

1H NMR spectroscopy revealed the presence of

the [(µ-HO)(Al(C6F5)3)2]- anion, the same as in 6.2. Interestingly, with the exception of some

minor by-products, the signals attributable to the PMes3 moiety were absent in both the 1H and

31P NMR spectra. This data suggests the formation of the phosphoniumyl radical cation, Mes3P

•+,

as the major P-containing product. The intensely deep purple colour of the solution is consistent

with the colour reported for solutions of this radical, as well as that reported in Chapter 3.296

169

The synthesis of the proposed product, [Mes3P•][(µ-HO)(Al(C6F5)3)2] (6.3), could be scaled up

and proceeded more cleanly in a cooled (-40 °C) toluene solution. Slow addition of N2O to this

solution, followed by stirring at -40 °C for 1 h and warming to r.t. leads to the formation of a

deep purple oily solution. The appearance of the deep purple colour is immediate at -40 °C upon

addition of N2O. Careful crystallization with pentane of the supernatant at -40 °C yielded single

crystals suitable for X-ray diffraction (yield ca. 10%).191

The solid state structure of 6.3 is shown

in Figure 6.5.

Figure 6.5 – POV-Ray depiction of 6.3. C: black, P: orange, F: pink, Al: teal, O: red, H: white.

H atoms, except OH, are omitted for clarity.

While X-Ray diffraction is limited in definitively locating H atoms, the difference Fourier map

does not show the presence of a P–H bond at the PMes3 moiety. Furthermore, the sum of the 3

C–P–C angles is 349.5° slightly more planar than previously reported Mes3PH+ cations with

either [AlBr4]- or [(µ-H)(Al(C6F5)3)2]

- anions at 345.23° and 344.7°, respectively,

174,240 and

consistent with a planarized phosphoniumyl radical cation.206

X-band EPR spectroscopy of a

solution of 6.3 dissolved in bromobenzene (Figure 6.6) reveals the presence of an intense doublet

(g = 2.0056, aP = 239 G) consistent with literature reports of the Mes3P•+

radical.206

A solid state

spectrum is shown in the experimental section. NMR analysis of the sample in bromobenzene

displays an insignificant amount of Mes3PH+ compared to the OH signal as integrated by

1H

NMR spectroscopy. The little amount present is likely due to the reaction of adventitious water

170

with the phosphoniumyl radical, as previously reported for these species.296

A minor amount of

another impurity is also present (vide infra). The UV-Vis spectrum of a dilute sample of 6.3 in

bromobenzene was taken and is shown in Figure 6.7. The spectrum features strong absorption

bands at 286, 382 (max), and 573 nm. The determination of an extinction coefficient could not

be unambiguously determined due to the presence of the minor impurity. The cation of 6.3 is a

rare example of a crystallographically characterized phosphoniumyl radical cation.300

Figure 6.6 – X-band EPR spectrum of a bromobenzene solution of 6.3 (spectrum in blue,

simulation in red). Simulation was done using PIP4Win v. 1.2.207

171

Figure 6.7 – UV-Vis spectrum of a bromobenzene sample of 6.3. Major absorption bands are at

286, 382 (max), and 573 nm.

The formation of 6.3 and, in particular, the presence of the anion [(µ-HO)(Al(C6F5)3)2]- required

us to investigate the origin of the H atom in this anion. While this H atom clearly emanated from

phosphine C–H bond activation in the reaction of 6.1 to 6.2, it was not immediately clear where

the H atom originated from in 6.3. The reaction to produce 6.3 yields some unknown aromatic

and aliphatic by-product signals in the 1H NMR spectrum after addition of N2O. To determine if

these were the result of a HAT reaction from Mes3P•+

to the proposed oxygen radical anion

intermediate, [(µ-O•)(Al(C6F5)3)2]

-, the perdeuterated PMes3-d27 (where all Me groups are

deuterated) was prepared as reported.206

The synthesis afforded ~95% deuteration of the methyl

groups as observed by 1H NMR spectroscopy.

Monitoring the reaction of a 1:2 C6H5Br solution of PMes3-d27:Al(C6F5)3•tol with a pseudo-

internal C6D6 (dissolved) standard over 16 h by both 1H and

2H NMR spectroscopy revealed the

disappearance of ca. 70% of the initial PMes3-d27 2H resonances. Interestingly, the peak

attributable to µ-OD was negligible, accounting for less than 10% of the expected value if the

HAT reaction occurred solely at the PMes3-d27 moiety. In contrast, the 1H NMR spectrum clearly

displayed a µ-OH resonance, presumably emanating either from the bromobenzene solvent or the

toluene from Al(C6F5)3•tol. GC-MS analysis of this sample was inconclusive; however, analysis

of the solvent of a scaled up preparation in toluene (used to isolate single crystals of 6.3) clearly

indicated the presence of dimethylbiphenyl and/or phenyltolylmethane isomers. These may form

172

as a result of the combination of 2 benzyl radicals following HAT reactions with the anion,

[(µ-O•)(Al(C6F5)3)2]

-. A similar reaction and/or further isomerization likely occur when using

bromobenzene as solvent. The proposed reactivity of this radical anion with toluene is consistent

with literature reports of a similar benzyl C–H bond activation of toluene by the oxygen radical

anion (O•-).

301

As mentioned, the 2H NMR spectrum reveals a small amount of the [(µ-OD)(Al(C6F5)3)2]

- anion.

Also described above, the 31

P{1H} and

1H NMR analysis of single crystals of 6.3 reveal the

presence of a diamagnetic impurity, precluding the assignment of a definitive extinction

coefficient for 6.3. This impurity has a 31

P{1H} NMR signal at 29 ppm, as well as a complex

1H

NMR spectrum indicative of a C1 symmetric phosphine, with diagnostic resonances for the

(µ-OH) signal (1 proton) and a P-coupled doublet at 3.88 ppm (2 protons).191

Together, this data

indicates the presence of a PMes3 that has undergone a HAT reaction at one of the ortho methyl

groups to form a cyclic product (6.4) with a new P–Cortho bond. Approximate quantification of

both products was done by a combination of 1H NMR integration (6.4 vs. µ-OH signal), as well

as using 9,10-dihydroanthracene (DHA) to deliver H atoms to the Mes3P•+

of 6.3, in turn

generating anthracene and the known diamagnetic cation Mes3PH+. While the ratio of 6.3 to 6.4

varied depending on the batch, the isolated crystals generally revealed the presence of 20-30% of

6.4 to 6.3. A general reaction scheme outlining the details of this section is presented in Scheme

6.4. While a HAT reaction from the para methyl group of Mes3P•+

may occur, this could not be

unambiguously confirmed.

173

Scheme 6.4 – Divergent HAT reaction pathways following the reaction of PMes3 + 2

Al(C6F5)3•tol + N2O in toluene leading to the formation of 6.3 and the by-product 6.4.

Subsequent controlled HAT of 6.3 with DHA is shown for quantification purposes (m = ~75%,

n = ~25%).

As shown in this scheme, we propose the formation of an intermediate

[Mes3P•+

][(µ-O•-)(Al(C6F5)3)2] radical which cannot be self-quenched due to the steric protection

around both the P• and O

• centres. We term this species a “frustrated radical pair.” A similar

intermediate was proposed in the transformation of 6.1 to 6.2 (Scheme 6.3). Using the

oxidatively stable PMes3 instead of the unstable PtBu3 has allowed us to isolate the radical

product following C–H activation of toluene as the major product (6.3). However, even with this

stable cation, C–H activation of a methyl group of PMes3 by the oxygen radical,

[(µ-O•)(Al(C6F5)3)2]

-, or a generated benzyl radical, is still possible and occurs to form the by-

product 6.4. Using phosphines with no alkyl substituents may eliminate this side reaction

completely and favour C–H bond activation of exogenous substrates.

6.2.3 C–H bond activation of exogenous substrates

6.2.3.1 C–H bond activation of toluene

In order to probe the reactivity of the proposed generated frustrated radical pairs outlined in the

previous sections, we investigated the N2O chemistry using triarylphosphines containing no alkyl

substituents. Triphenylphosphine (PPh3) and the bulkier tri(1-naphthyl)phosphine (P(naph)3)

were investigated with a combination of 2 equiv. Al(C6F5)3•tol in C6D5Br under N2O in a

174

J-Young NMR tube. PPh3 was found to form a very strong, unreactive adduct with Al(C6F5)3.

P(naph)3 also appears to form an adduct with Al(C6F5)3 precipitating upon mixing; nonetheless,

this classical Lewis adduct displays FLP character as the mixture becomes a solution after ca.

20 min under an N2O atmosphere. Analysis by 19

F and 1H NMR spectroscopy reveals the

formation of the [(µ-OH)(Al(C6F5)3)2]- anion. Two major peaks are observed in the

31P{

1H}

NMR spectrum at 26 and 23 ppm (approximate 1:1 ratio). While the 1H NMR spectrum contains

mostly un-assignable aromatic peaks, a set of four diagnostic signals are present: a set of three

1H–

1H COSY coupled aromatic signals integrating for 5 protons in the 6.85-6.45 ppm range, and

a P-coupled doublet integrating to 2 protons at 4.55 ppm. These signals integrate to

approximately 50% of the (µ-OH) proton resonance and likely represent the product of benzyl

C–H bond activation of toluene to generate [(naph)3PCH2Ph][(µ-OH)(Al(C6F5)3)2] (6.5).

Attempts to isolate this product from the other major product were unsuccessful (Scheme 6.5,

top).

In order to unambiguously confirm the nature of the unreported cation in 6.5, we independently

synthesized it. Using a literature procedure for similar phosphonium salts,302

P(naph)3 was

combined with benzyl bromide in dimethyl formamide (DMF) for 20 h at 100 °C. Following

workup, the salt [(naph)3PCH2Ph][Br] (6.6) was obtained in 97% isolated yield.191

While this salt

is completely insoluble in C6D5Br (the solvent used to generate 6.5), it is soluble in CD2Cl2.

Thus, removing the C6D5Br from 6.5 and redissolving the residue in CD2Cl2 confirmed the

identity of the cation in 6.5 as being [(naph)3PCH2Ph]+.

Scheme 6.5 – Divergent reaction pathways depending on solvent of P(naph)3 + 2 Al(C6F5)3•tol +

N2O producing 6.5 or 6.7 as major products.

175

Attempts to promote the formation of 6.5 by conducting the reaction in toluene were

unsuccessful. This reaction instead resulted in the formation of a new major broad peak at

-16 ppm in the 31

P{1H} NMR spectrum, indicative of the (naph)3PH

+ cation,

303 as well as the

[(µ-OH)(Al(C6F5)3)2]- anion (6.7) as observed by

1H and

19F NMR spectroscopy (Scheme 6.5,

bottom). Similar to the reaction forming 6.3, GC-MS analysis revealed the formation of isomers

of mainly phenyltolylmethane, as confirmed by NMR analysis, although dimethylbiphenyl

isomers may also be present. Attempts to isolate 6.7 from the minor product, 6.5, were

unsuccessful.

The products 6.5 and 6.7 are indicative of a reaction involving initial methyl HAT from toluene

to [(µ-O•)(Al(C6F5)3)2]

- to generate the [(µ-OH)(Al(C6F5)3)2]

- anion consistent with literature

reports of the reactivity of the oxygen radical anion (O•-).

301 The following rapid combination of

this benzyl radical to solvent toluene and ejection of a second H atom captured by the

phosphoniumyl radical, [naph3P•+

], to generate the major product

[(naph)3PH][(µ-OH)(Al(C6F5)3)2] (6.7) is proposed to occur in toluene. In contrast, the

observation of 6.5 as a major product in bromobenzene (Scheme 6.5, top) suggests that

combination of the benzyl radical intermediate with bromobenzene is significantly slower,

resulting in its capture by the phosphoniumyl radical cation to generate 6.5.

6.2.3.2 C–H bond activation of bromobenzene

Next, we set out to investigate the nature of the other major product of this reaction in

bromobenzene (Scheme 6.5, top). We performed the reaction using the benzene adduct of

Al(C6F5)3 in order to eliminate the benzyl C–H activation of toluene. A J-Young NMR tube was

charged with a 1:2 mixture of P(naph)3:Al(C6F5)3•PhH in C6D5Br and exposed to 1 atm of N2O.

Again the initial adduct mixture becomes a solution after ~20 min of mixing. Analysis by NMR

spectroscopy reveals the peak at 23 ppm in the 31

P{1H} NMR spectrum as the major peak. The

peak at 26 ppm in the 31

P{1H} NMR spectrum, as well as the tolyl resonances in the

1H NMR

attributable to [(naph)3PCH2Ph]+ (6.5) are now gone. The

19F NMR spectrum indicated the

formation of the [(µ-OH)(Al(C6F5)3)2]- anion as the only major species. What remains in the

1H

NMR spectrum are the peak for the (µ-OH), as well as several un-assignable aromatic peaks. The

reaction was scaled up and a pure powder was obtained in 50% yield (6.8). Single crystals

176

suitable for X-ray diffraction were obtained and the solid state structure is shown in Figure 6.8.

The metric parameters of 6.8 are unexceptional.

Figure 6.8 – POV-Ray depiction of 6.8. C: black, P: orange, F: pink, Al: teal, O: red, Br: scarlet,

H: white. H atoms, except OH, are omitted for clarity.

The structure of 6.8 is the result of para C–H bond activation of bromobenzene and represents,

to the best of our knowledge, the first example of aromatic C–H bond cleavage mediated by the

activation of N2O using abundant main group elements. In contrast to the reactions generating

6.7, no dimerization of bromobenzene seems to occur as observed by GC-MS and NMR

spectroscopy. Due to the steric bulk around the proposed [(µ-O•)(Al(C6F5)3)2]

- anion which is

proposed to initiate the HAT reaction, combined with the steric protection offered by the

bromine atom at bromobenzene, it is of little surprise that para C–H bond activation is the main

product. Some minor products were observed in the 31

P NMR; however, establishing whether

these were the meta or ortho products proved impossible.

In conclusion, we propose that the aromatic C–H bond cleavage of bromobenzene proceeds

through a homolytic pathway catalyzed by the intermediate formation of the frustrated radical

pair, [naph3P•][(µ-O

•)(Al(C6F5)3)2] in a mechanism analogous to the one shown in Scheme 6.3.

The isolation of the phosphoniumyl radical, Mes3P•+

(6.3), subsequent to HAT from the toluene

solvent to [(µ-O•)(Al(C6F5)3)2]

-, suggests a radical pathway wherein this oxygen radical anion

177

initiates the homolytic C–H bond cleavage. This is consistent with the formation of 6.2 and 6.3;

however, direct attack of the phosphoniumyl cation, [naph3P•+

], at the bromobenzene solvent

also cannot be completely discounted. Previous anodic oxidation studies of PPh3 at a Pt electrode

in the presence of benzene revealed the formation of some phosphonium cation, Ph4P+, proposed

to have formed from the phosphorylation reaction of the Ph3P•+

radical with benzene.304

Regardless of the exact sequence, it is clear that alkylphosphine C–H bond cleavage is favoured

using trialkylphosphines, such as in the reaction of 6.1 to 6.2, and to some extent with PMes3 to

form 6.4. In contrast, the use of the triarylphosphine, P(naph)3, precludes this reaction pathway

allowing for the clean homolytic C–H bond cleavage of exogenous substrates, such as toluene

and bromobenzene. The further exploration of C–H bond cleavage mediated by N2O and FLPs is

currently underway in our laboratory.

6.3 Conclusions

This chapter has presented the activation chemistry of N2O using combinations of PR3 (R = tBu,

Mes, naph) and Al(C6F5)3. The formation of the N2O adduct tBu3P(N2O)(Al(C6F5)3)2 was found

to precede subsequent N2 ejection and phosphine C–H bond activation and rearrangement to 6.2.

Using the oxidatively stable PMes3 allowed for the isolation and crystallographic

characterization of the proposed phosphoniumyl radical intermediate, Mes3P•+

, following a HAT

reaction from the solvent to the proposed oxygen radical intermediate, [(µ-O•)(Al(C6F5)3)2]

-.

Eliminating phosphine alkyl groups using P(naph)3 allowed for the activation of exogenous C–H

bonds, such as toluene and bromobenzene, cleanly using N2O.





-38ºC freezer). Hexanes, pentane, and toluene (Aldrich) were dried using an Innovative



prior to use. Benzene (-H5 and -D5) was purchased from Aldrich, dried over Na/benzophenone

and distilled prior to use. Trimethylaluminum (TMA), potassium hydride, PtBu3, P(naph)3, and

178

PMes3 were purchased from Strem, and B(C6F5)3 was purchased from Boulder Scientific, and

DMSO-d6 was purchased from Aldrich and all were used without further purification. N2O was

purchased from Aldrich and dried through a Drierite column before use. Al(C6F5)3•(tol or PhH)

was prepared from B(C6F5)3 and TMA in toluene or benzene, respectively, by a known

procedure.175

PMes3-d27 was prepared from PMes3, DMSO-d6, and KH according to a literature

procedure.206

NMR spectra were obtained on a Bruker Avance 400 MHz, a Varian 400 MHz, or an Agilent

600 MHz (2H) spectrometer and spectra were referenced to residual solvent of C6D5Br (

1H =

7.28 ppm for meta proton; 13

C = 122.4 ppm for ipso carbon), C6D6 (2H = 7.15 ppm) or externally

(27

Al: Al(NO3)3, 31

P: 85% H3PO4, 19

F: CFCl3). Chemical shifts (δ) listed are in ppm and absolute

values of the coupling constants are in Hz. NMR assignments are supported by additional 2D

experiments. UV-Vis spectra were obtained on an Agilent 8453 UV-Vis spectrophotometer using

Quartz cells modified with a J-Young NMR cap to assure a near-perfect seal. Elemental analyses

(C, N, H) and X-ray crystallography were performed in house. EPR spectra were recorded at the

University of Windsor by a Stephan Group post-doctoral fellow, Dr. Jillian A. Hatnean, as part

of a collaboration. The spectra were recorded at r.t. or 77 K on a Bruker EMXplus X-band

spectrometer controlled using Xenon software on a PC operating under Linux. The spectra were

modeled using PIP4Win v. 1.2.207


Synthesis of tBu3P(N2O)Al(C6F5)3 (6.1): A 50 mL Schlenk bomb equipped with a Teflon screw

cap and a magnetic stirbar was charged with PtBu3 (196 mg, 0.97 mmol), Al(C6F5)3•tol (300 mg,

0.48 mmol) and toluene (10 mL). The bomb was transferred to the Schlenk line equipped with an

N2O outlet and cooled to -78 °C with a dry ice/acetone bath. The bomb was degassed, slowly

filled with N2O (1 atm), and sealed. The solution became a mixture and was allowed to warm to

r.t. over 1 h after which time the N2O atmosphere was removed and hexanes (ca. 5 mL) were

added dropwise to the stirring mixture in the glovebox. The precipitate was filtered on a glass

frit, washed with hexanes and dried in vacuo (340 mg, 0.44 mmol, 91%).

1H NMR (400 MHz, C6D5Br): δ 0.92 (d,

3JH-P = 14.4 Hz, 27H, tBu).

31P{

1H} NMR (161 MHz,

C6D5Br): δ 65.8. 27

Al NMR (104 MHz, C6D5Br): 117 (bs, υ1/2 = ca. 1600 Hz). 19

F{1H} NMR

(376 MHz, C6D5Br): δ -121.4 (dd, 3JF-F = 30.0 Hz,

4JF-F = 11.3 Hz, 6F, o-C6F5), -154.0 (t,

3JF-F =

179

18.8 Hz, 3F, p-C6F5), -161.7 (m, 6F, m-C6F5). 13

C{1H} NMR (100 MHz, C6D5Br), partial: δ

150.1 (dm, 1JC-F = 233 Hz), 141.1 (dm,

1JC-F = 251 Hz), 136.7 (dm,

1JC-F = 252 Hz), 40.6 (d,

1JC-P

= 28.9 Hz, P(CMe3)3), 28.6 (s, P(CMe3)3). Anal. Calc. for C30H27AlF15N2OP: C, 46.52; N, 3.62;

H, 3.51. Found: C, 46.19; N, 3.73; H, 3.90.

Synthesis of [tBu2P(Me)(2-allyl)][(µ-OH)(Al(C6F5)3)2] (6.2): Method 1: A 50 mL Schlenk

bomb equipped with a Teflon screw cap and a magnetic stirbar was charged with PtBu3 (49 mg,

0.24 mmol), Al(C6F5)3•tol (300 mg, 0.48 mmol) and fluorobenzene (5 mL). The bomb was

transferred to the Schlenk line equipped with an N2O outlet and cooled to -40 °C. The bomb was

degassed, slowly filled with N2O (1 atm), and sealed. The solution was allowed to warm to r.t.

over 1 h after which time the N2O atmosphere was removed and pentane (ca. 5-10 mL) was

added dropwise to the stirring solution in the glovebox. The precipitate was filtered on a glass

frit, washed with pentane and dried in vacuo (190 mg, 0.15 mmol, 62%).

Method 2: Separate solutions of 6.1 (150 mg, 0.19 mmol) and Al(C6F5)3•tol (120 mg, 0.19

mmol), each in fluorobenzene (3 mL) were cooled to -40 °C. The cold Al(C6F5)3•tol solution was

added dropwise to the cold solution of 1. The resulting solution was allowed to warm slowly to

r.t. and stirred for 5-6 h. Pentane (ca. 10 mL) was added dropwise to precipitate a product which

was filtered, washed with pentane and dried in vacuo (175 mg, 0.14 mmol, 71%).

1H NMR (400 MHz, C6D5Br): δ 5.67 (d,

3JH-P = 40.6 Hz, 1H, MeC=CH2), 5.64 (bs, 1H, OH),

5.23 (d, 3JH-P = 18.0 Hz, 1H, MeC=CH2), 1.56 (dm,

3JH-P = 11.5 Hz, 3H, MeC=CH2), 0.97 (d,

2JH-

P = 11.2 Hz, 3H, PMe), 0.78 (d, 3

JH-P = 15.8 Hz, 18H, 2 x tBu). 31

P{1H} NMR (161 MHz,

C6D5Br): δ 43.3. 27


F{1H} NMR (376 MHz, C6D5Br): δ -

122.5 (dd, 3JF-F = 26.0 Hz,

4JF-F = 8.8 Hz, 12F, o-C6F5), -153.5 (t,

3JF-F = 19.7 Hz, 6F, p-C6F5), -

161.3 (m, 12F, m-C6F5). 13


1JC-F = 234 Hz), 141.3

(dm, 1JC-F = 253 Hz), 137.5 (d,

2JC-P = 5 Hz, MeC=CH2), 136.7 (dm,

1JC-F = 252 Hz), 125.3 (d,

1JC-P = 58 Hz, MeC=CH2), 115.2 (m, i-C6F5), 34.1 (d,

1JC-P = 36.2 Hz, P(CMe3)2), 26.3 (s,

P(CMe3)2), 22.8 (d, 2JC-P = 9 Hz, MeC=CH2), -0.1 (d,

1JC-P = 52 Hz, PMe). Anal. Calc. for

C48H27Al2F30OP: C, 45.23; H, 2.14. Found: C, 45.03; H, 2.39.

Synthesis of [Mes3P•+

][(µ-OH)(Al(C6F5)3)2] (6.3): A 50 mL Schlenk bomb equipped with a

Teflon screw cap and a magnetic stirbar was charged with PMes3 (63 mg, 0.16 mmol),

Al(C6F5)3•tol (200 mg, 0.32 mmol) and toluene (7 mL). The bomb was transferred to the Schlenk

180

line equipped with an N2O outlet and cooled to -40 °C. The bomb was degassed, slowly filled

with N2O (1 atm), and sealed. The solution immediately turned dark purple upon addition of N2O

and was allowed to react at -40 °C for 1h before warming to r.t. The N2O atmosphere was

removed and pentane (ca. 5 mL) was added dropwise to the rapidly stirring oily solution in the

glovebox. The solution was allowed to settle without stirring for 15-30 min after which the top

layer was carefully decanted and transferred to a vial. The remaining deep purple oil was

discarded. The decant vial was wrapped in glass wool and put in the -38 °C for slow cooling.

Deep purple single crystals appeared after several days (20 mg, 14 µmol, 8%).

NMR spectra of [(µ-OH)(Al(C6F5)3)2]-:

1H NMR (400 MHz, C6D5Br): δ 5.64 (bs, 1H, OH).

27Al

NMR (104 MHz, C6D5Br): blank. 19


3JF-F = 26.0

Hz, 4JF-F = 8.8 Hz, 12F, o-C6F5), -153.5 (t,

3JF-F = 19.7 Hz, 6F, p-C6F5), -161.3 (m, 12F, m-C6F5).

13C{

1H} NMR (100 MHz, C6D5Br): δ 150.1 (dm,

1JC-F = 234 Hz), 141.3 (dm,

1JC-F = 253 Hz),

136.7 (dm, 1JC-F = 252 Hz), 115.2 (m, i-C6F5). Anal. Calc. for C70H42Al2F30OP (6.3•tol): C,

54.10; H, 2.72. Found: C, 54.10; H, 2.70.

NMR data for 6.4: Crystals of 6.3 were found to contain ~25% of 6.4 depending on the batch.

1H NMR (400 MHz, C6D5Br): δ 6.81 (d,

JH-P = 8.0 Hz, 1H), 6.67 (d,

JH-P = 4.8 Hz, 4H), 6.62 (s,

1H), 5.66 (bs, 1H, OH), 3.88 (d, 2

JH-P = 12.4 Hz, 2H, P-CH2), 2.12 (s, 3H), 2.07 (s, 3H), 2.05 (s,

6H), 1.97 (s, 3H), 1.93 (s, 9H). 31

P{1H} NMR (161 MHz, C6D5Br): δ 29.3.

27Al NMR (104 MHz,

C6D5Br): blank. 19

F{1H} NMR same as 6.3.

NMR data for 6.5: A J-Young NMR tube was charged with P(naph)3 (10 mg, 24 µmol),

Al(C6F5)3•tol (30 mg, 49 µmol) and C6D5Br (0.5 mL). N2O was added all at once to the mixture

and the sample was mixed several times over the next 20 min until a solution was formed. NMR

spectroscopy reveals an approximate 1:1 mixture of 6.5 and 6.8 (see below for spectral details of

6.8). The NMR data for 6.5 is provided here.

1H NMR (400 MHz, C6D5Br): δ 8.2-6.9 (overlapping naphthyl aromatic signals with 6.8, 21H),

6.78 (t, 3

JH-H = 7.7 Hz, 1H, p-C6H5), 6.67 (dd, 3

JH-H = 7.7 Hz, 2H, m-C6H5), 6.49 (dd, 3

JH-H = 7.7

Hz, 2H, o-C6H5), 5.66 (bs, 1H, OH), 4.55 (d, 2

JH-P = 12.7 Hz, 2H, P-CH2). 31

P{1H} NMR (161

MHz, C6D5Br): δ 26.1. 27


F{1H} NMR same as 6.3.

181

Synthesis of [(naph)3PCH2Ph][Br] (6.6): A 25 mL Schlenk round bottom equipped with a

reflux condenser and a magnetic stirbar was charged with P(naph)3 (482 mg, 1.2 mmol), benzyl

bromide (100 mg, 0.6 mmol) and DMF (3 mL). The cloudy mixture was stirred for 20 h at

100 °C after which the yellow solution became transparent. The solvent was removed in vacuo

and the resultant oil transferred to a vial. Toluene (10 mL) was added and a white precipitate

immediately formed. This solution was left undisturbed for 48 h in the –40 °C freezer. The

supernatant was decanted, and the precipitate was rinsed with toluene (5 mL). The resultant

white solid was filtered, and triturated with diethyl ether (10 mL). This process was repeated 3

times and then the solid was dried (425 mg, 0.57 mmol, 97%). The elemental analysis revealed

the presence of 1 equiv DMF and 0.5 equiv toluene that could not be removed in vacuo, as

confirmed by NMR spectroscopy.

1H NMR (400 MHz, CDCl3): δ 8.55 (bs, 3H, naphthyl-CH), 8.21 (d,

3JH-H = 8.5 Hz, 3H,

naphthyl-CH), 7.92 (d, 3JH-H = 8.5 Hz, 3H, naphthyl-CH), 7.82 (d,

3JH-H = 8 Hz, 3H, naphthyl-

CH), 7.68 (bs, 3H, naphthyl-CH), 7.47 (dd, 3JH-H = 7.4, 6.7 Hz, 3H, naphthyl-CH), 7.36 (dd,

3JH-H = 7.4, 6.7 Hz, 3H, naphthyl-CH), 6.87 (t,

3JH-H = 7.4 Hz, 1H, p-C6H5), 6.79 (m, 2H,

o-C6H5), 6.71 (m, 2H, m-C6H5), 5.64 (d, 2JH-P = 11.9 Hz, 2H, CH2).

31P{

1H} NMR (161 MHz,

CDCl3): δ 27.3. 13

C{1H} NMR (100 MHz, CDCl3): δ 138.4 (d, JC-P = 12 Hz, naphthyl-CH),

137.0 (d, JC-P = 3 Hz, naphthyl-CH), 134.2 (d, JC-P = 10 Hz, naphthyl-CH), 132.5 (d, JC-P = 9 Hz,

naphthyl-CH), 130.5 (d, JC-P = 7 Hz, naphthyl-CH), 130.3 (d, JC-P = 2 Hz, naphthyl-CH), 128.9

(d, JC-P = 4 Hz, naphthyl-CH), 128.9 (s, p-C6H5), 128.5 (d, JC-P = 3 Hz, naphthyl-CH), 127.9 (d,

JC-P = 4 Hz, naphthyl-CH), 127.4 (s, m-C6H5), 125.7 (d, 3JC-P = 3 Hz, o-C6H5), 125.6 (d,

2JC-P = 8

Hz, i-C6H5), 114.5 (d, 1JC-P = 81 Hz, naphthyl-C), 33.5 (d,

1JC-P = 48 Hz, CH2). Anal. Calc. for

C87H78Br2N2O2P2: C, 74.36; H, 5.59; N, 1.99. Found: C, 74.73; H, 5.95, N, 2.17.

Synthesis of [(naph)3P(C6H5Br)][(µ-OH)(Al(C6F5)3)2] (6.8): A 50 mL Schlenk bomb equipped

with a Teflon screw cap and a magnetic stirbar was charged with P(naph)3 (102 mg, 0.25 mmol),

Al(C6F5)3•PhH (300 mg, 0.49 mmol) and bromobenzene (10 mL). The bomb was transferred to

the Schlenk line equipped with an N2O outlet. The bomb was degassed, slowly filled with N2O

(1 atm), and sealed. The mixture was stirred for 16 h after which the mixture became a pale

yellow solution. The solvent was removed in vacuo and the residue dissolved in a saturated 1:1

fluorobenzene:pentane solution. The solution was slow cooled in the -38 °C freezer for several

182

days whereupon single crystals suitable for X-ray diffraction were obtained (200 mg, 0.12 mmol,

49%).

1H NMR (400 MHz, C6D5Br): δ 8.1-6.8 (m, 25H), 5.66 (bs, 1H, OH).

31P{

1H} NMR (161 MHz,

C6D5Br): δ 22.7. 27


F{1H} NMR (376 MHz, C6D5Br):

same as 6.3. 13

C{1H} NMR (100 MHz, C6D5Br), partial: δ 150.1 (dm,

1JC-F = 234 Hz), 141.3

(dm, 1JC-F = 253 Hz), 137.9 (s), 136.7 (dm,

1JC-F = 252 Hz), 135.5 (d, JC-P = 11 Hz), 134.4 (bs),

134.1 (d, JC-P = 13 Hz), 132.9 (bs), 130.6 (s), 128 (s), 127.9 (s), 125.5 (d, JC-P = 15 Hz), 117.8 (d,

JC-P = 90 Hz), 115.2 (m, i-C6F5). Anal. Calc. for C84H36Al2BrF32OP (6.8 + 2•C6H5F): C, 55.01;

H, 1.98. Found: C, 55.53; H, 1.79.

6.4.3 EPR measurements

Reaction of PtBu3/Al(C6F5)3 with N2O to produce 6.2: A bromobenzene solution (0.6 mL) of

PtBu3 (3 mg, 0.016 mmol) and Al(C6F5)3•tol (20 mg, 0.032 mmol) was prepared and transferred

to a J-Young NMR tube. A spectrum of the solution was acquired and no signals were observed.

The sample was then freeze-pump-thaw degassed and 1 atm of N2O added. The frozen solution

was placed into the EPR spectrometer and continuously monitored as the solution warmed. At no

time was an EPR signal observed.

Solid-state spectrum of 6.3: The solid state spectrum of 6.3 revealed an axial spectrum with a

large 31

P hyperfine coupling. The average of the anisotropic parameters (g = 2.0073, aP = 247 G)

are in good agreement with the isotropic solution values and are comparable with literature

values.205,206,305,306

Some additional features are present in the solid state sample located around g

= 2.02. The origin of these additional features is less evident, but the lack of a large hyperfine

interaction to 31

P or other nuclei may suggest Mes3POO•+

which has been proposed in the

reaction chemistry of R3P•+

ions but not spectroscopically characterized.307

This radical may

arise from adventitious oxygen.

183

Figure 6.9 ‒ R.t. X-band solid state EPR spectrum of 6.3 (spectrum in blue, simulation in red).

Simulation was done using PIP4Win v. 1.2.207












The heavy





2/2 (Fo

2) and Fo

184










185



6.1 6.2

Formula C30H27Al1F15N2O1P1 C48H27Al2F30O1P1

Formula wt. 774.48 1274.62

Crystal system monoclinic triclinic

Space group P21/c P-1

a(Å) 10.4886(2) 10.1990(15)

b(Å) 14.3224(3) 13.428(2)

c(Å) 21.3093(5) 18.404(3)

α(deg) 90 86.466(7)

β(deg) 94.7020(10) 87.338(7)

γ(deg) 90 88.940(7)

V(Å3) 3190.4(1) 2512.6(6)

Z 4 2

T (K) 150(2) 150(2)

d(calc) g/cm3 0.858 1.650

Abs coeff, μ, mm-1

0.189 0.239


Rint 0.0399 0.0734

Data used 7272 11824

Variables 460 741

R (>2σ) 0.0408 0.985

wR2 0.1374 0.2937

GOF 0.848 1.474

186


6.3 6.8 (+ 2•C6H5F)

Formula C63H34Al2F30O1P1 C84H36Al2Br1F32O1P1

Formula wt. 1461.83 1833.97

Crystal system triclinic monoclinic

Space group P-1 P21/n

a(Å) 11.6291(4) 13.8953(10)

b(Å) 16.9162(5) 14.9435(10)

c(Å) 17.4333(5) 35.121(3)

α(deg) 85.883(2) 90

β(deg) 81.665(2) 99.212(4)

γ(deg) 75.891(2) 90

V(Å3) 3288.5(2) 7198.7(9)

Z 2 4

T (K) 147(2) 296(2)

d(calc) g/cm3 1.476 1.642

Abs coeff, μ, mm-1

1.779 0.746


Rint 0.0670 0.1480

Data used 10997 12324

Variables 912 1031

R (>2σ) 0.0803 0.0874

wR2 0.2409 0.2995

GOF 1.059 1.015

187

Chapter 7 Conclusion

7.1 Thesis Summary

This thesis has presented the results of research studying Al-based frustrated Lewis pairs for

small molecule activation. This field remains relatively unexplored compared to B based FLPs.

The main conclusions are briefly outlined here:

First, the activation of CO2 using commercially available aluminum halide (AlX3) Lewis acids

was investigated. It was found that PMes3/AlX3 FLPs could rapidly and irreversibly generate the

Mes3P(CO2)(AlX3)2 species. The CO2 moiety of these compounds could then undergo facile

reduction to methanol by treatment with ammonia-borane (AB). A reaction pathway involving

initial dissociation of AlX3 by AB is supported by parallel reactivity studies involving the

isolation of Al–AB adducts and their reactivity with CO2.

Second, these same PMes3/AlX3 FLPs were found to react further when exposed to a CO2

atmosphere resulting in the reduction of CO2 to CO. Detailed experimental and theoretical

mechanistic studies were performed. The resulting proposed mechanism involves AlX3

dissociation from the kinetic product Mes3P(CO2)(AlX3)2 (X = Br, I), followed by a rate-

determining CO2 insertion step into the Al–X bond generating the X2AlO(X)C=O–AlX3

intermediate (after coordination of a second AlX3). Subsequent nucleophilic attack of PMes3 at

the halide centre is supported computationally and found to result in the ejection of CO. Ligand

redistribution is proposed to yield the observed isotopically labelled products.

Third, the reaction of Al-based FLPs was investigated with various olefins. While addition

products are formed with ethylene, the use of substituted olefins, such as propylene and

isobutylene, resulted in the unexpected C–H bond activation to yield -allyl species of Al. A bis-

Al allyl species is isolated using isobutylene. These species are found to undergo subsequent C–

C bond forming reactions with other olefins or CO2. A -allyl species of B was also isolated.

The facile generation of these allyl species stands in marked contrast to current synthetic

methods to yield reactive allyl species.

188

Fourth, H2 activation using Al-based FLPs was investigated and found to yield bridging hydride

species. Notably, these compounds are the first FLP products to successfully undergo subsequent

hydrogenation chemistry with unactivated olefins. A mechanism involving Al activation of the

olefin, followed by hydride delivery, and redistribution is proposed based on reactivity studies.

Several attempts were made to prevent the redistribution reaction and yield a catalytic cycle,

which included the synthesis of the new, extremely bulky alane, Al(C12F9)3. While catalytic

hydrogenation was not achieved, important information was gained which may facilitate the

development of a future non-transition metal based olefin hydrogenation catalyst.

Fifth, the activation of the greenhouse and ozone-depleting gas, N2O, was also investigated. In

contrast to the reported B and Zn chemistry, activation of N2O using Al-based Lewis acids

resulted in unexpected C–H bond activation chemistry. The intermediate formation of the

frustrated radical pairs, [R3P•][(µ-O

•)(Al(C6F5)3)2], is proposed which results in either phosphine

or solvent (aromatic or benzylic) C–H bond cleavage depending on the phosphine used. The

isolation of the first crystallographically characterized phosphoniumyl radical cation is also

described.

7.2 Future Work

As described in the introduction of this thesis, Al is the most abundant element in the Earth’s

crust and the second most used metal in the world, second to Fe. While the extraction process for

Al is by no means “green,” its sheer abundance and recyclability does make it environmentally

more sustainable than several precious metals, such as Rh, Ir, and Pt. Replacing a metal such as

Rh, involved in Wilkinson’s olefin hydrogenation, with Al would yield both enormous economic

and environmental benefits. A significant portion of this graduate work focused on this

proposition (Chapter 5) and while a catalytic olefin hydrogenation catalyst was not obtained,

significant progress towards this end goal was accomplished. The further development of either

bulky and/or linked ligands should be explored.

The final chapter of this thesis outlined the initial results of C–H bond activation using N2O and

mediated by Al-based FLPs. The P(naph)3/Al(C6F5)3 FLP offers the best framework to probe

further C–H activation of exogenous substrates. While not described in this thesis, initial results

suggest that C–H activation of benzene, fluorobenzene, and hexamethylbenzene is also possible.

189

While attempts to activate alkyl C–H bonds were inconclusive, the reaction of N2O with these

substrates should be investigated further.

Finally, a significant avenue of potential important future discovery involves the discovery of

possible “frustrated radical pairs” (FRPs). While these were proposed as intermediates in the

N2O chemistry, it was also found that PMes3/Al FLPs displayed some radical character as

evidenced by EPR spectroscopy (Chapters 3 and 6). Due to the presence of both EPR and NMR

signals, it is believed that this electron transfer between PMes3 and the Al centre is reversible.

Therefore, using an Al Lewis acid with a higher reduction potential should lead to the complete

transfer of an electron from the phosphine centre to the alane, as proposed in Scheme 7.1.

Scheme 7.1 – Proposed generation of frustrated radical pairs and possible new reactivity with

small molecules.

While the reduction potential of PMes3 is known,206

those of the alanes used are not. Further

studies probing these potentials should be undertaken in order to generate genuine FRPs. These

pairs may provide fundamental insights into the reactivity of FLPs as electron transfer steps are

proposed to occur synergistically with H2 activation in the encounter complex between the

phosphine and alane (see Chapter 1).86

Alternatively, these FRPs could offer new reaction

pathways with small molecules not seen before with traditional FLPs, such as in C–H bond

activation reactions (Chapter 6). Further studies into these FRPs are currently under intense

investigation in our laboratory.

190

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