pi-bonding and reactivity in transition metal nitrosyl ... · pi-bonding and reactivity in...
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Pi-Bonding and Reactivity in Transition Metal Nitrosyl Complexes
by
KEVIN MICHAEL SMITH
B.Sc, The University of Toronto, 1992
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF
T H E REQUIREMENTS FOR T H E D E G R E E OF
DOCTOR OF PfflLOSOPHY
in
T H E F A C U L T Y OF G R A D U A T E STUDIES
Department of Chemistry
We accept this thesis as conforming
to the requjr^d standard.
T H E UNIVERSITY OF BRITISH COLUMBIA
January 1998
© Kevin M . Smith, 1998
In presenting this thesis in partial fulfilment of the requirements for an advanced
degree at the University of British Columbia, 1 agree that the Library shall make it
freely available for reference and study. 1 further agree that permission for extensive
copying of this thesis for scholarly purposes may be granted by the head of my
department or by his or her representatives. It is understood that copying or
publication of this thesis for financial gain shall not be allowed without my written
permission.
Department of
The University of British Columbia Vancouver, Canada
Date
DE-6 (2/88)
11
Abstract
This Thesis investigates the effect of 7t-bonding interactions on the electronic
configuration and reactivity of Cr and M o organometallic complexes that contain C p '
( C 5 R 5 ; R = H (Cp), C H 3 (Cp*)) and N O ligands.
From metrical parameters obtained from X-ray crystallographic studies and
geometry optimization computations, the orbitals of CpCr-containing compounds in a
variety o f spin states are examined using Extended Hiickel molecular-orbital calculations.
The ligand effects are evaluated in the context o f Spectrochemical and Nephelauxetic
effects.
Reaction o f C p ' C r ( N O ) 2 C l with A g O T f yields Cp 'Cr (NO) 2 OTf . The covalently-
bound trifiate ligand in CpCf(NO) 2 (OTf) can be displaced by amines to afford
[CpCr(NO) 2 (NHR 2 ) ] [OTfj salts ( N H R 2 = N H 3 , N H M e 2 , N H 2
r B u , N H 2 P h ) . Treatment o f
basic, aqueous solutions o f C p C r ( N O ) 2 O T f with FLXL reagents (FLXL = acetylacetone,
salicylaldehyde, picolinic acid) results in the precipitation o f C p C r ( N O ) ( L X ) complexes.
These mononitrosyl complexes may be independently synthesized from [CpCr(NO)(p-
I)] 2 and the appropriate chelating ligand in either anhydrous organic solvents or aqueous
solution.
C p M o ( N O ) ( C H 2 P h ) C l reacts with R 2 M g (R = M e , Ph) or L i R (R = C=CPh)
reagents to form C p M o ( N O ) ( C H 2 P h ) R complexes. These products are stabilized by the
7] -benzyl ligand; the corresponding 16e C p M o ( N O ) R 2 species are too thermally unstable
to be isolated. The r | 2 - C H 2 P h interaction is disrupted by introducing other ligands capable
of multihapto-bonding, as evidenced by the synthesis o f C p 2 M o ( N O ) ( C H 2 P h ) . Reaction
of C p M o ( N O ) ( C H 2 P h ) C l with A g O T f yields CpMo(NO)(CH 2 Ph)OTf , which reacts with
P P h 3 or pyridine to afford [CpMo(NO)(CH 2 Ph)L][OTfJ salts.
Cp*Cr (NO)(CO) 2 reacts with PC1 5 or I 2 in N C M e to form [Cp*Cr(NO)(M--X)] 2
dimers via the C p * C r ( N O ) ( N C M e ) X adducts ( X = CI, I). The coordinating solvent is
required in order to avoid overoxidation and subsequent decomposition via N O loss.
Cr (NO)(N 'Pr 2 ) (0 2 CPh) 2 reacts with Mg(CH 2SiMe3) 2 to afford
Cr(NO)(N'Pr 2)(CH 2SiMe3) 2. The unusual nature o f this 14e, pseudo-tetrahedral,
monomeric, diamagnetic bis(alkyl) compound is in part accounted for by the 7t-bonding
properties of the ancillary amide and nitrosyl ligands.
iv
Table of Contents
Abstract • 1 1
Table of Contents i v
List of Tables viii
List of Figures x *
List of Schemes x ' v
List of Abbreviations x v
Acknowledgments x * x
Quotations x x
Chapter 1: Introduction 1
Reactivity in Organometallic Compounds 2
Importance of n Bonding 2
Synthesis and Theory 5
TI Bonding in Cp'M(NO)-Containing Complexes 7
Outline of This Thesis 9
References and Notes 11
Chapter 2: Theoretical Investigations of Cp Chromium Complexes 14
Introduction 15
Results and Discussion 20
[CpCrL 3]" 2 0
CpM(NO)Cl 2 (M = Cr or Mo). 24
CpCr(NO)(NH 2)X (X = CI or CH 3 ) 26
CpCr(NO)(CH 3)X (X = CI or CH 3 ) 31
Spectrochemical and Nephelauxetic Effects 34
Summary 40
Computational Details 41
References and Notes 42
Chapter 3: Pi-Bonding and NO Loss from CpCr(NO)2Y Species 48
Introduction 49
Results and Discussion 52
Synthesis o f Cp 'Cr(NO) 2 (OTf) 52
Reaction o f 3.1 with a-donor ligands 55
Pi-Bonding and N O loss 58
Hydrogen Bonding in CpCr(NO) 2 (OH) (aq) 62
Synthesis o f C p C r ( N O ) ( L X ) 64
Aqueous Chemistry o f [ C p C r ( N O ) ( H 2 0 > 2 ] + 67
Summary 69
Future Work :70
Chelate assisted N O displacement 70
Generation of C p C r ( N O ) Y species 70
Experimental 73
Methods 73
Synthesis o f C p C r ( N O ) 2 O T f (3.1) 73
Synthesis o f C p * C r ( N O ) 2 O T f (3.2) 73
Synthesis o f [CpCr(NO) 2 L][OTf] (3.3-3.6) 74
Synthesis o f C p C r ( N O ) 2 ( N 2 C 5 H 7 ) (3.7) 74
Synthesis o f C p C r ( N O ) ( L X ) (3.8-3.10) 75
Synthesis o f [CpCr(NO)(H20)2][BPh4] (3.11) 75
Synthesis o f [CpCr(NO)(p.-OH)] 2 (3.12) 76
Characterization Data 77
vi
References and Notes 79
Chapter 4: Synthetic Utility of the [CpMo(NO)(CH2Ph)]+ Fragment 85
Introduction 86
Results and Discussion 90
Direct Synthesis of CpMo(NO)(CH2Ph)R 90
Stabilizing Effects of ri 2-CH 2Ph 95
Synthesis of [CpMo(NO)(CH2Ph)L][OTfJ 96
Deprotonation of [CpMo(NO)(CH2Ph)(PPh3)][OTfJ 97
Summary 99
Future Work 100
Carbene transfer from CpMo(NO)(=CHPh)(L2) intermediates 100
le Reduction of [CpMo(NO)(CH2Ph)L][OTfJ 100
Ligand-based reactivity of CpMo(NO)(CH2Ph)R 101
Cp'M(NO)(CH 2Ph)Cl as Cp 2Zr(CH 3)Cl analog 102
Synthesis and reactivity of Cp 2M(NO)H 103
Experimental 105
Methods :.. 105
Synthesis of CpMo(NO)(CH2Ph)R (4.1-4.4) 105
Synthesis of CpMo(NO)(CH2Ph)(OTf) (4.5) .106
Synthesis of [CpMo(NO)(CH2Ph)L][OTf] (4.6-4.7). 106
Characterization Data 107
References and Notes 109
Chapter 5: Towards CpCr(NO)R 2 116
Introduction 117
Results and Discussion 120
Oxidation of Cp'M(NO)(CO) 2 with Halogen Sources 120
Amine Elimination Reactions of Cr(NO)(N'Pr2)3 125
Synthesis of Cr(NO)(N'Pr2)(CH2SiMe3)2 (5.3) 128
Alternative routes to CpCr(NO)R 2 134
Summary.. 136
Future Work 137
le Oxidation of 17e Cp'Cr(NO)R 2 anionic complexes 137
Diamagnetic 12e tris(alkyl) species 137
Paramagnetic 16e Cr(0) compounds 138
Diamagnetic 16e M(NO)(NR 2)(L)R 2 bis(alkyls) of Cr, Mo, and W. 139
Alternative route to Cp*Cr(NO) 2Cl and [Cp*Cr(NO)(u-Cl)]2 140
15e Cp*Cr(NO)R species 141
Experimental 142
Methods ; 142
Synthesis of [Cp*Cr(NO)(p.-I)]2 (5.1) 142
Synthesis of [Cp*Cr(NO)(^Cl)]2 (5.2) 143
Synthesis of Cr(NO)(N'Pr2)(CH2SiMe3) (5.3) 143
Reaction of 5.3 with CsHe 144
Reaction of Cr(NO)(N'Pr2)(02CPh)2 with PhC0 2 H 144
Reaction of Cr(NO)(0'Bu)3 with Na(DME)Cp, CpSiMe 3, LiCp 145
Reaction of Cr(NO)(O fBu)3 with H X 145
Generation of Cp*Cr(NO)(CH2Ph)2 146
Reaction of Cr(NO)(0'Bu)3 with LiCH(SiMe 3) 2 146
Reaction of Cr(NO)(N'Pr2)(02CPh)2 with dppe and M 147
References and Notes 148
Appendix...... 157
vm
List of Tables
Table 1.1. Summary o f Thesis by Chapters 9
Table 2.1 Optimized geometries and energies for CpM(NO)Cl2 18
Table 2.2 Optimized geometries and energies for CpCr(NO)(NR"2)X. 18
Table 2.3 Optimized geometries and energies for C p C r ( N O ) ( C H 3 ) X 19
Table 2.4 E H M O frontier orbital splittings for singlet C p C r ( N O ) X 2 38
Table 2.5 Interelectron repulsion energy parameters for CpM(NO)X2 39
Table 3.1 Selected Bond Lengths and Angles for Cp*Cr(NO) 2 (OTf) (3.2) 54
Table 3.2 Selected Bond Lengths and Angles for CpCr(NO>2(3,5-Me2pyrazolyl)
(3.7) 57
Table 3.3 Selected Bond Lengths and Angles for CpCr(NO)(acetylacetonate)
(3.8) 66
Table 3.4 Numbering Scheme, Color, Yie ld and Elemental Analysis Data 77
Table 3.5 Infrared and Mass Spectral Data 77
Table 3.6 T i and 1 3 C N M R Data.. 78
Table 4.1 Selected Bond Lengths, Bond Angles, and Torsion Angles for
C p 2 M o ( N O ) ( C H 2 P h ) (4.4). 93
Table 4.2 Numbering Scheme, Color, Yie ld and Elemental Analysis Data 107
Table 4.3 Infrared and Mass Spectral Data 107
Table 4.4 ! H and 1 3 C N M R Data 108
Table 5.1 Selected Bond Lengths and Angles for [Cp*Cr(NO)(p.-Cl)] 2 (5.2).. 123
Table 5.2 M - M Lengths in Group 6 Dimers 124
Table 5.3 Selected Bond Lengths, Bond Angles, and Torsion Angles for
Cr(NO)(N'Pr2)(CH2SiMe3)2 (5.3) 130
Table A l Crystallographic Data for Complexes 3.2, 3.7, and 3.8 158
ix
Table A 2 Crystallographic Data for Complexes 5.2 and 5.3 159
Table A3 Fractional Coordinates of DFT-optimized geometry o f
C p M o ( N O ) C l 2 160
Table A 4 Fractional Coordinates of DFT-optimized geometry o f
C p C r ( N O ) C l 2 161
Table A 5 Fractional Coordinates of DFT-optimized geometry o f
C p C r ( N O ) ( N H 2 ) C l 162
Table A 6 Fractional Coordinates of DFT-optimized geometry o f
C p C r ( N O ) ( N H 2 ) ( C H 3 ) 163
Table A 7 Fractional Coordinates of DFT-optimized geometry o f
C p C r ( N O ) ( C H 3 ) 2 164
Table A 8 Fractional Coordinates of DFT-optimized geometry o f
CpCr (NO)(CH 3 ) 165
Table A 9 Fractional Coordinates and B e q for 3.2 166
Table A10 Fractional Coordinates and U e q for 3.7 167
Table A l 1 Fractional Coordinates and B e q for 3.8 168
Table A12 Fractional Coordinates and B e q for 4.4 169
Table A l 3 Fractional Coordinates and U e q for 5.2 170
Table A l 4 Fractional Coordinates and B e q for 5.3 171
Table A l 5 Bond Lengths for 3.2 173
Table A l 6 Bond Lengths for 3.7 174
Table A l 7 Bond Lengths for 3.8 175
Table A l 8 Bond Lengths for 4.4 176
Table A l 9 Bond Lengths for 5.2 177
Table A20 Bond Lengths for 5.3 178
Table A21 Bond Angles for 3.2 179
X
Table A22 Bond Angles for 3.7 180
Table A23 Bond Angles for 3.8 181
Table A24 Bond Angles for 4.4 182
Table A25 Bond Angles for 5.2 183
Table A26 Bond Angles for 5.3 184
xi
List of Figures
Figure 1.1 a - and 7t-Bonding in organic and organometallic complexes 3
Figure 1.2 Frontier orbital effects of 7t-donor and 7i-acceptor ligands 4
Figure 1.3 Synthesis, structure and bonding in organometallic chemistry 6
Figure 1.4 Frontier orbitals of pseudo-octahedral Cp'M(NO)(ligand)2
complexes 7
Figure 1.5 Solid-state molecular structure o f CpCr(NO)(NPh 2 )1 8
Figure 2.1 Relative energies and D F T - B 3 L Y P optimized geometries of singlet and
triplet C p M ( N O ) X 2 17
Figure 2.2 E H M O energy-level diagram for [CpCr(CO) 3 ]" , [CpCrCl 3T~, and
[CpCr(NO)Cl 2 ] " 20
Figure 2.3 Pictorial representation of the 7t-bonding interactions o f the five Cr 3d
orbitals in [CpCrL 3 ]~ (L - C O , CI) 21
Figure 2.4 Pictorial representation of the Cp and N O 7i-bonding interactions with the
five C r 3d orbitals in [CpCr(NO)Cl 2 ] " 22
Figure 2.5 E H M O diagram for singlet and triplet C p M ( N O ) C l 2
( M = M o , Cr) 24
Figure 2.6 E H M O diagram for singlet and triplet C p C r ( N O ) ( N H 2 ) X
( X = C1, C H 3 ) 26
Figure 2.7 Pictorial representation o f the N H 2 7t-bonding interactions in singlet
C p C r ( N O ) ( N H 2 ) C l 27
Figure 2.8 Pictorial representation of the N O and N H 2 7t-bonding interactions in
triplet CpCr (NO) (NH 2 )C l 29
Figure 2.9 E H M O diagram for singlet and triplet C p C r ( N O ) ( C H 3 ) X
( X = C1, C H 3 ) 31
Figure 2.10 Pictorial representation o f the Cp and N O 7t-bonding and CH3 a -
bonding interactions in H O M O -5 o f triplet C p C r ( N O ) ( N H 2 ) C l 32
Figure 2.11 Qualitative energy-level diagram illustrating 7t-donor effects on orbital-
splitting energy and spin state in d 4 M L . 6 36
Figure 2.12 Qualitative energy-level diagram illustrating N H 2 7t-donor effects on
orbital-splitting energy and spin state in C p C r ( N O ) ( N H 2 ) X 37
Figure 3.1 O R T E P plot of Cp*Cr(NO) 2 (OTf) (3.2). Thermal ellipsoids o f 50%
probability are shown 54
Figure 3.2 O R T E P plot o f CpCr(NO) 2 (3,5-Me 2 pyrazolyl) (3.7). Thermal ellipsoids of
50% probability are shown for the non-hydrogen atoms 57
Figure 3.3 Pictorial representation of N O 7t-bonding interactions in
C p C r ( N O ) 2 C l 58
Figure 3.4 Pictorial representation o f N O and N R 2 7t-bonding interactions in
C p C r ( N O ) 2 ( N R 2 ) 59
Figure 3.5 Qualitative enthalpy diagram of 7t-donor-assisted ligand
dissociation 61
Figure 3.6 O R T E P plot o f CpCr(NO)(acetylacetonate) (3.8). Thermal ellipsoids of
50% probability are shown 66
Figure 4.1 r i 2 -Benzyl interaction in C p ' M ( N O ) ( C H 2 P h ) X species 89
Figure 4.2 O R T E P plot o f C p 2 M o ( N O ) ( C H 2 P h ) (4.4). Thermal ellipsoids o f 50%
probability are shown 93
Figure 4.3 18e CpM(NO)R-containing complexes 95
Figure 5.1 O R T E P plot of [Cp*Cr(NO)(p.-Cl) 2] (5.2). Thermal ellipsoids o f 50%
probability are shown 123
Figure 5.2 O R T E P plot of C r (NO) (N 'P r 2 ) (CH 2 SiMe 3 ) 2 (5.3). Thermal ellipsoids o f
50% probability are shown 130
x i i i
Figure 5.3 Qualitative energy-level diagram illustrating N O and NR2 Tt-bonding
interactions in tetrahedral Cr(II) complexes 131
Figure 5.4 * H N M R spectrum o f Cr(NO)(N'Pr2)(CH2SiMe3)2 (5.3) in C 6 D 6 133
Figure 5.5 Qualitative orbital splitting and amide rotation barriers in Cr (NO) and
Cr(N) species 134
xiv
List of Schemes
Scheme 3.1 NO loss reactivity postulated for CpCr(NO) 2Y species 50
Scheme 3.2 Hydrogen-bonding, protonolysis, and chelate-assisted NO loss 64
Scheme 3.3 Synthesis of [CpCr(NO)(H 20) 2] + 67
Scheme 3.4 Postulated importance of steric bulk in the generation of Cp'Cr(NO)(Y)X
species from CpCr(NO) 2Y precursors 71
Scheme 3.5 Possible synthetic routes to Cp'Cr(NO)Y complexes 72
Scheme 4.1 Application of CpMo-containing complexes to organic synthesis... 86
Scheme 4.2 Potential extension of known CpMo-based organic synthesis to C - H bond
activation of C - C bond formation reactions 87
Scheme 4.3 Planned reactions to investigate synthetic utility of [CpMo(NO)(CH2Ph)]+
fragment 88
Scheme 4.4 Possible reactivity modes of Cp 2M(NO)H 104
Scheme 5.1 Oxidative halogenation of CpM(NO)(CO) 2 (M = Cr, Mo, W) and
subsequent alkylation reactions 118
Scheme 5.2 Oxidation of Cp'Cr(NO)(CO) 2 with halogen sources 121
Scheme 5.3 Steric influences on the reactivity of Cr tris(amide) complexes 126
Scheme 5.4 Derivitization of Cr(NO)(N'Pr2)3 127
Scheme 5.5 Current synthetic routes to Cp*Cr(NO)-containing complexes...... 140
XV
List of Abbreviations
The following is a list o f abbreviations and symbols employed in this Thesis, most
of which are in common use in the chemical literature.
ne n valence electrons
A angstrom, 10~ 1 0 m
acac acetylacetonate
anal. analysis
atm atmosphere
B 3 L Y P three-parameter form of the Becke, Lee, Yang and Parr functional
br broad
"Bu w-butyl, - C H 2 C H 2 C H 2 C H 3
'Bu / -bu ty l , -CMe 3
1 3 C carbon-13
1 3 C { 'H} proton-decoupled carbon-13
°C degree Celsius
C3H5 al lyl , - C H 2 C H = C H 2
cal calorie
calcd calculated
C p ' Cp or Cp*
Cp cyclopentadienyl, C5H5
C P centroid of the C5H5 ring
Cp* pentamethylcyclopentadienyl, CsMes
8 chemical shift
d days, or doublet (in a N M R spectrum)
xvi
D deuterium, H
D B U l,8-diazabicyclo[5.4.0]undec-7-ene
DFT density functional theory
dppe diphenylphospinoethane, Pl^PCFfeCHsPPl^
D M E dimethoxyethane, CH3OCH2CH2OCH3
E H M O Extended Huckel molecular orbital
EI electron impact
eq equation
equiv equivalents
Et ethyl, - C H 2 C H 3
FAB fast atom bombardment
*H proton
h hours
HF Hartree-Fock
HOMO highest occupied molecular orbital
Hz hertz, s - 1
'Pr /so-propyl, -CH(CH 3 ) 2
DR. infrared
J joule, kgm2s~2
J coupling constant
K degrees Kelvin
k rate constant
L Lewis basic, 2-electron-donor ligand; liter, 10 - 3 m 3
L U M O lowest unoccupied molecular orbital
M metal (usually group 6); molar, mole/liter
m multiplet
xvi i
m/z mass-to-charge ratio
M e methyl, - C H 3
min minutes
mmol millimole, 10" 3 mole
M O molecular orbital
mol mole
M S mass spectrum
v stretching frequency
N M R nuclear magnetic resonance
no. number
O R T E P Oak Ridge Thermal Ell ipsoid Program
P + parent molecular ion
Ph p h e n y l , - C 6 H 5
ppm parts per million
py pyridine, C5H5N
R hydrocarbyl
s singlet (in a N M R spectrum); strong (in a IR spectrum)
S O M O singly-occupied molecular orbital
t triplet
O T f trifluromethanesulfonate, triflate, OSO2CF3
o-tol or/Ao-tolyl, 2-C6H4CH3
/7-tol para-to\y\, 4 -C6H4CH3
T H F tetrahydrofuran, C4H8O
U V ultraviolet
vis visible
vs versus
xviii
X halide or other anionic 1-electron-donor ligand
Y amide, alkoxide or sulfide ligand
xix
Acknowledgements
The efforts of many people were required to see this work through to completion.
Thanks to Peter, for his good humor, unflagging support, attention to detail, and
steadfast insistence on producing nothing but the finest.
Thanks to all the members of the Legzdins' Group, past and present, for showing
what is possible. I 'm especially thankful for the camaraderie of a pair o f Steves (English
(Sayers) and Vancouver (McNeil)) , and for the opportunity to work with two very
talented fourth-year undergraduate students (Vick i Tong and Jane Kuzelka).
Thanks to the staff at U B C , particularly M . Austria, P. Borda, and L . Darge; to
Drs. S. Rettig and V . Young for solving the crystal structures, and to Prof. R. Pol i for his
computational work and helpful discussions.
Thanks to my family, for unceasing encouragement and support.
Finally, a limoful o f cash and prizes to Teresa, insufficient thanks for your songs
and understanding and silliness and insight and love.
Quotations
"This skein o f unusual reactions must surely be thought fanciful, were it not
the fact that it renders comprehensible an otherwise inexplicable transformation."
Seth N . Brown and James M . Mayer
Inorg. Chem. 1995, 34, 3560-3562
Chapter 1: Introduction
Reactivity in Organometallic Compounds 2
Importance of 7t Bonding 2
Synthesis and Theory 5
7i Bonding in Cp'M(NO)-Containing Complexes 7
Outline of This Thesis 9
References and Notes 11
2
Reactivity in Organometallic Compounds
Organometallic chemistry involves compounds that contain a bond between a
carbon atom and a metal a tom. 1 - 4 The work described in this Thesis belongs to the specific
subcategory of monomelic, homogeneous, transition-metal organometallic chemistry and
is particularly concerned with the synthesis, characterization and reactivity of these kinds
of complexes. For a given transition metal, two important factors can influence the
reactivity of organometallic compounds. The first involves the properties of the metal-
bound ligands whose effects can be divided into steric and electronic components. The
second is the electronic configuration of the metal, which reflects both the oxidation state
and the spin state. The studies presented in this Thesis were initiated to investigate how
one type of ligand property, namely n bonding, influences the electronic configuration and
reactivity of one class of organometallic compound, namely complexes o f Cr and M o
containing N O and C p ' ( C 5 R 5 ; R = H (Cp), R = M e (Cp*)) ligands.5
Importance of n Bonding
In order to understand bonding in organometallic chemistry, instructive parallels
may be drawn to simple organic chemistry (Figure 1.1). The a bond that exists between a
metal and a methyl group is fundamentally similar to the C - C single bond in ethane.
Because a bonds are cylindrically symmetric along the internuclear axis, there is no
electronic barrier to rotation about this type of bond.
However, n bonds possess a single nodal plane that contains the internuclear axis.
Rotation about this axis disrupts the orbital overlap essential for the existence of the 7t
bond, leading to a barrier to rotation. There are two types of organometallic 7t-bonding
interactions, classified by the formal direction of electron donation in the metal-ligand
bond. The N R 2 group shown in Figure 1.1 is an example o f a 7t-donor ligand since
electron density is donated from the filled N p orbital of the amide ligand into the empty
3
metal d orbital. Since the empty TC* orbital of the H2C=CH2 group is accepting electron
density from the filled metal d orbital, the ethylene ligand is a TC acceptor! Note the
orientation of the amide and ethylene ligands required to overlap with the metal d orbital.
F i g u r e 1.1. a- and TC-Bonding in organic and organometallic complexes
While organometallic TC bonds are generally weaker than a bonds, re-bonding
interactions can exert a powerful influence over the reactivity of organometallic
compounds by modifying the relative energies of the frontier orbitals. The degenerate set
of dxy, 6x2, and dyz orbitals shown in the center of Figure 1.2 constitutes the frontier orbitals
for a hypothetical, octahedral d 1 M L 6 complex. The left side of Figure 1.2 illustrates how
the empty, high energy TC* H2C=CH2 ligand orbital can interact with the d^ orbital if one
of the L ligands is replaced by ethylene. While the antibonding combination of this
interaction is too high in energy to be considered one of the frontier orbitals (indicated by
the box in Figure 1.2), the bonding combination is slightly lower in energy than the
corresponding d^ orbital in ML6. The net overall effect is that %-acceptor ligands typically
lower the energy of d orbitals. The situation is reversed for the interaction with the d^
orbital with the low energy, filled N p orbital of the amide ligand, shown at the right of
4
Figure 1.2. In this case, the occupied, bonding combination is low in energy, and the
antibonding combination lies slightly above the remaining d^ and d_̂ orbitals, and so 7t-
donor ligands are considered to raise the energy o f d orbitals.
y **- . ^
\
H2CCH2sv
— M ;
Figure 1.2. Frontier-orbital effects o f 7t-donor and 7t-acceptor ligands.
B y influencing the orbitals that are near the dividing energy between occupied and
unoccupied orbitals, n-bonding interactions can have a dramatic impact on several
important aspects of the chemistry of organometallic species. These include the oxidation
state of the metal and spin state of the complex as well as the orientation, binding modes
and rotation barriers of the ligands.6 When several K ligands are present in a single
complex, the resulting competition for the available rc-bonding orbitals can also have
important consequences for the reactivity of the compound. 7 Consideration o f 7i-bonding
interactions alone does not guarantee a complete understanding of the bonding o f an
organometallic complex which often also involves important a-bonding effects, subtle
geometric distortions, and critical steric influences.8 However, establishing a working
description of the n bonding present in a compound provides both a conceptual
5
framework with which to explain many important experimental observations and an
excellent starting point from which a more complete picture of the bonding may be
achieved.
Synthesis and Theory
While attaining a fuller understanding of the fundamental factors which determine
the behavior of organometallic compounds may be a sufficient goal in and of itself for
theoreticians, chemists concerned with more practical matters may rightfully ask how %-
bonding considerations may be applied to the actual synthesis of new complexes. After all,
most of the remarkable achievements of modern synthetic organometallic chemistry have
been attained without resorting to any theoretical concepts more advanced than the 18e
rule. At the risk of imposing artificial order on the naturally messy business of scientific
discovery, the use of theory to assist synthesis can be divided into three progressive
stages:
1. Explanation: using theory to account for trends that have already been observed for a
known class of related compounds, such as stability or reactivity patterns that may
result upon systematically altering the ligands or the metal.
2. Prediction: extrapolating from known trends to assist in the identification of new
target molecules or reactivity modes.
3. Design: using theoretical insights to design a complex that can accomplish a specific,
predetermined application.
Figure 1.3 illustrates how synthesis, structural determination and theoretical
modeling might be harnessed to further the development of organotransition-metal
chemistry. Once a new compound has been made, its solid-state molecular structure can
be determined using single crystal X-ray diffraction techniques. The crystallographically
determined parameters can then be used as the basis for molecular-orbital calculations.
F r o m the resulting theoretical description, critical bonding features o f the molecule can be
identified that will help guide subsequent synthetic studies.
N e w
Target
Molecules
Synthesis
Characterization
Techniques
Theoretical Description
Bondi ng Theory
Structural Determination
Figure 1.3. Synthesis, structure and bonding in organometallic chemistry.
Whi le interactions between synthetic and theoretical organometallic chemistry have
been possible for decades, 9 recent progress in computer technology has created
spectacular new opportunities to employ this interdisciplinary approach. Improvements in
data acquisition and processing techniques have made X - r a y diffraction studies so much
more accessible that crystallographic structural determinations are now comparatively
routine. Computational software is now readily available commercially, ranging from
simple molecular mechanics and Extended H u c k e l molecular-orbital programs for use on
standard home computers to advanced ab initio and hybrid density functional theory
packages for powerful workstations. T h e insights afforded by theoretical analysis may then
be directed towards the development o f new target molecules, thereby completing the
feedback cycle between synthesis, structure and theory.
7
7t Bonding in Cp'M(NO)-Containing Complexes
A simple model is sufficient to describe many of the 7t-bonding and reactivity
relationships observed for Cp'M(NO)-containing complexes. Figure 1.4 illustrates the
frontier orbitals of a pseudo-octahedral, d 4 to d 6 Cp 'ML 3 complex ( L - M - L = 90°,
C N T - M - L = 125.6°) where the Cp ligand occupies three facial coordination sites. If one
L ligand is replaced with NO, two of the "t2g-type" orbitals are lowered in energy due to
the two orthogonal M - N O 7t-acceptor interactions. The third orbital, designated d^ if the
z-axis lies along the M - N O bond, is of critical importance to the Cp'M(NO)(ligand)2
complex.10 For M(II), d 4 compounds, this orbital will be empty.11 Much of the known
reactivity of 16e Cp'M(NO)R 2 species consists of nucleophilic attack of small molecules
such as H 2 , CO, or 0 2 at this L U M O , and subsequent insertion into the metal-alkyl
bonds.5 If the M(II) complex contains a single-faced 7C-donor ligand such as an amide or
an alkylidene, the ligand will align co-planar with the M - N O bond in order to 7i-donate
into the vacant d^ orbital.12 Similarly, in M(0), d 6 compounds the dxy orbital is occupied,
and a 7i-acceptor ligand will also adopt an orientation which maximizes the 7t-bonding
interaction.13
Figure 1.4. Frontier orbitals of pseudo-octahedral Cp'M(NO)(ligand)2 complexes.
8
The same pseudo-octahedral bonding description was employed by W . Stephen
M c N e i l to explain the ligand-loss reactions which accompanied the reduction or oxidation
o f Cr(I), d 5 complexes. 1 4 For the Cr(I)/Cr(0) redox couple in [ C p C r ( N O ) L 2 ] + / 0
compounds, loss or gain of an electron was observed to induce lability in the L ligands. 1 5
The N O ligand was shown to dissociate when [CpCr(NO)Cl 2 ]~ was oxidized from Cr(I) to
Cr(II) . 1 6 These studies laid the groundwork for much o f the content o f this Thesis.
Figure 1.5. Solid-state molecular structure of CpCr(NO)(NPh 2 )I .
Another source of inspiration came from the solid-state molecular structure of
CpCr(NO)(NPh 2 ) I (Figure 1.5). The 1979 report o f this X-ray crystallographic study
provided only cursory synthetic details: "rj-Cyclopentadienyl(diphenylamido)iodonitrosyl-
chromium was obtained from chloro(r|-cyclopentadienyl)dinitrosylchromium by reaction
with lithium diphenylamide ( M e l + L i + N P h 2 H ) " . 1 7 The structure itself, however,
provides eloquent testimony to the importance o f amide Tt-donation to the Cr(II), d 4 metal
center. The amide ligand is planar, and the N(ni t rosy l ) -M-N-C( ipso) dihedral angles are
less than 175.1° and - 1 0 . 1 ° , which places one of the phenyl groups in a sterically
unfavorable position with respect to the Cp ring. Consequently, this orientation must be
9
ascribed to an electronic interaction, namely that of the amide group functioning as a TC-
donor ligand to the empty orbital perpendicular to the Cr-NO bond.
Outline of This Thesis
On initial inspection, the focus of this Thesis may seem narrow and limited. In fact,
the investigation of 7C-bonding and reactivity in transition-metal nitrosyl complexes has
enabled me to conduct research that is of potential relevance to some of the most exciting
areas of current organometallic chemistry. Table 1.1 lists the appropriate area for each
chapter of this Thesis, as well as the desired goal and specific results from each study.
Table 1 . 1 . Summary of Thesis by Chapters.
Title Relevant Area Goal Results
2 Theoretical Investigation of Cp Chromium Complexes
Spin State and Reactivity in Open-Shell Organometallics
Application of E H M O to interpret CT- and TC-Bonding Effects
Paradigms of Coordination Chemistry are applicable to Organometallics
3 Pi-Bonding and NO Loss from CpCr(NO) 2Y Species
(a) Aqueous Organometallics (b) NO release in Aqueous Solution
pH-Dependant NO loss from CpCr(NO)2(OTf)
TC-Donor Assisted Ligand Loss via Stabilization of Dissociative Intermediate
4 Synthetic Utility of CpMo(NO)(CH2Ph)+
C-HBond Activation and C - C Bond Formation
Generate CpMo(NO)(CHPh) via Intermolecular Deprotonation
rjz-Benzyl Interaction Stabilizes reactive hydrocarbyl Ligands
5 Towards CpCr(NO)R2
Chromium-Based Olefin Polymerization Catalysts
Develop a synthetic route to CpCr(NO)R2
Species
Highly Unsaturated, Diamagnetic Complexes accessible from Cr(NO)(N'Pr2)3
Each chapter is divided into seven sections: Introduction, Results and Discussion,
Summary, Future Work, Experimental, Characterization Data, and References and Notes.
Tables, figures, schemes, equations and complexes are all numerically sequenced within
each chapter. An Appendix to the Thesis contains crystallographic details and tables of
atomic fractional coordinates for all compounds structurally characterized during this
work.
11
References and Notes
(1) For an introductory treatment of organometallic chemistry and 71-bonding, see:
Shriver, D. F.; Atkins, P.; Langford, C. H. Inorganic Chemistry, 2nd ed.; W. H. Freeman
and Co.: New York, NY, 1994.
(2) For advanced textbooks on organotransition-metal chemistry, see: (a) Collman,
J. P.; Hegedus, L . S.; Norton, J. R.; Finke, R. G. Principles and Applications of
Organotransition Metal Chemistry, University Science Books: Mill Valley, CA, 1987. (b)
Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 2nd ed.; Wiley-
Interscience: Toronto, 1994.
(3) For recent, multi-volume reference works on organometallic complexes, see:
(a) Comprehensive Organometallic Chemistry II, Abel, E. W., Stone, F. G. A.,
Wilkinson, G., Eds.; Elsevier: Oxford, 1995. (b) Dictionary of Organometallic
Compounds, 2nd ed.; Chapman & Hall: New York, 1995.
(4) For current research in organometallic chemistry, see: (a) Organometallics
(published biweekly by the American Chemical Society) (b) Journal of Organometallic
Chemistry (published biweekly by Elsevier).
(5) Legzdins, P.; Veltheer, J. E. Acc. Chem. Res. 1993, 26, 41.
\ (6) (a) Caulton, K. G. New J. Chem. 1994,18, 25. (b) Gibson, V. C. J. Chem.
Soc, Dalton Trans. 1994, 1607. (c) Poli, R. Chem. Rev. 1996, 96, 2135.
(7) (a) Kubacek, P.; Hoffmann, R. J. Am. Chem. Soc. 1981,103, 4320. (b)
Templeton, J. L . ; Winston, P. B.; Ward, B. C. J. Am. Chem. Soc. 1981,103,11X3. (c)
Brower, D. C ; Templeton, J. L.; Mingos, D. M . P. J. Am. Chem. Soc. 1987,109, 5203.
(d) Su, F. -M.; Bryan, J. C ; Jang, S.; Mayer, J. M . Polyhedron 1989, 8, 1261. (e) Huber,
S. R.; Baldwin, T. C ; Wigley, D. E. Organometallics 1993,12, 91. (f) Atagi, L. M . ;
Mayer, J. M . Organometallics 1994,13, 4794.
12
(8) (a) Wolczanski, P. T. Polyhedron 1995,14, 3335. (b) Heyn, R. H.; Macgregor,
S. A.; Nadasdi, T. T.; Ogasawara, M . ; Eisenstein, O.; Caulton, K. G. Inorg. Chim. Acta
1997, 259, 5.
(9) Hoffmann, R. Science 1981, 211, 995.
(10) (a) Schilling, B. E. R.; Hoffmann, R.; Faller, J. W. J. Am. Chem. Soc. 1979,
101, 592. (b) Legzdins, P.; Rettig, S. J.; Sanchez, L.; Bursten, B E . ; Gatter, M . G. J. Am.
Chem. Soc. 1985,107, 1411. (c) Bursten, B. E. ; Cayton, R. H. Organometallics 1987, 6,
2004.
(11) Throughout this thesis, the formal oxidation states and d-electron counts of
nitrosyl compounds are arrived at by considering the nitrosyl ligand as NO + . The
electronic configurations of transition-metal nitrosyl compounds are more accurately
described using Enemark-Feltham notation, see. Enemark, J. H ; Feltham, R. D. Coord.
Chem. Rev. 1974, 13, 339.
(12) (a) Hermann, W. A.; Hubbard, J. L. ; Bernal, I.; Korp, J. D.; Haymore, B. L.;
Hillhouse, G. L. Inorg. Chem. 1984, 23, 2978. (b) Ashby, M . T.; Enemark, J. H. J. Am.
Chem. Soc. 1986,108, 730. (c) Hubbard, J. L.; McVicar, W. K. Inorg. Chem. 1992, 31,
910. (d) Legzdins, P.; Ross, K. J.; Sayers, S. F.; Rettig, S. J. Organometallics 1997,16,
190. (e) Kuzelka, J.; Legzdins, P.; Rettig, S. J.; Smith, K. M . Organometallics 1997,16,
3569. (f) Tran, E. ; Legzdins, P. J. Am. Chem. Soc. 1997, 119, 5071. (g) Legzdins, P.;
Sayers, S. F. Chem. Eur. J. 1997, 3, 1579.
(13) (a) Boone, B. J.; Klein, D. P.; Seyler, J. W.; Mendez, N. Q.; Arif, A. M . ;
Gladysz, J. A. J. Am. Chem. Soc. 1996, 118, 2411. (b) Gladysz, J. A.; Boone, B. R.
Angew. Chem., Int. Ed. Eng. 1997, 36, 550. (c) Burkey, D. J.; Debad, J. D.; Legzdins, P.
J. Am. Chem. Soc. 1997,119, 1139.
(14) McNeil, W. S. Ph.D. Thesis, University of British Columbia, Dec. 1995.
13
(15) Legzdins, P.; McNeil, W. S.; Batchelor, R. J.; Einstein, F. W. B. J. Am.
Chem. Soc. 1995,117, 10521.
(16) Legzdins, P.; McNeil, W. S.; Rettig, S. J.; Smith, K. M . J. Am. Chem. Soc.
1997, 119, 3513.
(17) Sim, G. A.; Woodhouse, D. I.; Knox, G. R. J. Chem. Soc, Dalton Trans.
1979, 83.
Chapter 2: Theoretical Investigations of Cp Chromium Complexes
I n t r o d u c t i o n 15
Resu l t s a n d D i s c u s s i o n 20
S u m m a r y 40
C o m p u t a t i o n a l D e t a i l s 41
References a n d Notes 42
15
Introduction
Unlike the subsequent chapters of this thesis, this chapter contains no synthetic
chemistry. Instead, it describes the Extended Huckel Molecular Orbital (EHMO)
calculations conducted as part of a larger, ongoing synthetic and theoretical exploration
of CpCr(NO)X2 (X = halide, amide, alkyl) species.
Some background information is necessary to put the E H M O study in context.
The impetus for the investigation of CpCr(NO)X2 complexes came in part from the rich
chemistry exhibited by related Mo and W compounds.1 For example, CpMo(NO)X2
compounds are derived from treatment of CpMo(NO)(CO)2 with I22 or P C I 5 3 and
subsequent metathesis reactions of the bis(halide) species.4 The 16e, diamagnetic
CpMo(NO)(alkyl)2 species are of particular interest due to their unusual bonding,5*
synthesis,5b thermolytic decomposition50 and subsequent derivatization.5d In contrast, no
CpCr(NO)X2 compounds are isolable from analogous halogenation reactions of
CpCr(NO)(CO) 2
6 Chemical or electrochemical oxidation of [NEt4][CpCr(NO)Cl2]
results instead in the isolation of products consistent with the loss of NO from the
initially formed neutral CpCr(NO)Cl2 intermediate.7 Complexes such as
CpCr(NO)(NPh2)I8 and CpCr(NO)(N'Pr2)(CH2SiMe3),9 however, have been
demonstrated to be stable.
It was proposed that these observations are best accounted for by spin-state
considerations. Unlike the stable, diamagnetic CpMo(NO)X 2 species, CpCr(NO)Cl2
might adopt a high-spin, triplet configuration. If this configuration involves either the
population of M - N O 7t-antibonding orbitals or the removal of electrons from M - N O 71-
bonding orbitals, the resultant weakening of the metal-nitrosyl bond may render the
complex prone to NO dissociation.7 The strong Cr-N(amide) 71-bonding interaction
observed for diamagnetic CpCr(NO)(NR.2)X species may prevent this mode of
decomposition by stabilizing the singlet state with respect to the triplet.9
A density functional study was undertaken by Professor Rinaldo Poli (U. de
16
Bourgogne) in collaboration with the Legzdins group in order to address the following
four questions: (1) What is the source of the discontinuity in CpM(NO)X 2 chemistry
between Mo and Cr? (2) Does CpCr(NO)Cl 2 have an accessible high-spin state? (3) Does
adopting a S = 1 configuration labilize the nitrosyl ligand? (4) How does varying the X
ligands affect the relative energies of the singlet and triplet spin states? The geometries of
six model compounds of the formula CpM(NO)X 2 (M = Cr, Mo; X = CI, N H 2 , CH 3 ) were
optimized at the B3LYP level in both singlet and triplet electronic configurations, and the
overall energies of the two spin states were compared. The optimized geometries and
relative singlet vs triplet energies of the six compounds are shown in Figure 2.1. The
geometric parameters of the complexes CpM(NO)Cl 2 (M = Mo, Cr), CpCr(NO)(NH 2)X
and CpCr(NO)(CH 3)X (X = CI, CH 3 ) are displayed in Tables 2.1, 2.2, and 2.3,
respectively.
The E H M O study presented in this chapter is divided into five subsections. The
investigation of CpCrL 3 anions was actually conducted prior to the density functional
computations of Prof. Poli and is included to establish the bonding effects of the nitrosyl
ligand in cyclopentadienyl chromium systems. The E H M O calculations of CpM(NO)Cl 2
(M = Cr, Mo) address how adopting a triplet configuration disrupts M - N O bonding
orbitals. The effect of amide TC-donation is assessed for CpCr(NO)(NH 2)X (X = CI, CH 3 ).
The unexpected singlet stabilization that accompanies alkyl ligation is considered for
CpCr(NO)(CH 3)X (X = CI, CH 3 ) . Finally, the effect of ligands on the relative singlet vs
triplet energies in CpM(NO)X 2 species is discussed in the context of spin-pairing and
orbital-splitting energies.
17
i f I P •: MO ': MO
• X * A E = 17.62 kcal/mol
•
' 1A' -. ' V ...
AE = 8.20 kcal/mol
• -Cr
| A E = 1.52 | kcal/mol
H ^ 1A' V
. AE = 2.37 I kcal/mol
_ _ _« —»- ~~
C r f * H Cr(
| A E = 0.95 1 kcal/mol
H(b-. O60H H O N
0C3H
t AE = 9.55 ..--*" I kcal/mol
Figure 2.1. Relative energies and DFT -B3LYP optimized geometries o f singlet and
triplet C p M ( N O ) X 2 .
18
Table 2.1. Optimized geometries and energies for CpM(NO)Cl2 (M = Mo, Cr)
Structural Parameter*
CpMo(NO)Cl 2 CpCr(NO)Cl 2
S = 0 S = 1 S = 0 S = 1 C N T - M 2.123 2.101 1.957 1.969 M-C(average) 2.449 2.432 2.305 2.318 M-Cl 2.395 2.405 2.272 2.296 M-NO 1.777 1.830 1.655 1.860 N-0 1.220 1.225 1.199 1.193 CNT-M-C1 112.70 119.93 114.96 121.38 CNT-M-NO 123.10 112.11 126.42 119.22 Cl-M-Cl 114.95 94.62 108.36 98.14 Cl-M-NO 95.90 103.74 94.34 95.32 M-N-0 173.39 169.77 177.18 177.50 E(hartrees) -420.9587 -420.9306 -439.6831 -439.6961 a C N T = Cp ring centroid.
Table 2.2. Optimized geometries and energies for CpCr(NO)(NH 2)X (X = CI, CH 3 )
Structural CpCr(NO)(NH 2)Cl b CpCr(NO)(NH 2)(CH 3) c
Parameter4
S = 0 S = 1 S = 0 S= 1 CNT-Cr 1.978 1.978 1.984 2.024 Cr-C(average) 2.324 2.391 2.329 2.364 Cr-X 2.330 2.329 2.071 2.057 Cr-NO 1.648 1.811 1.641 1.773 Cr-NH 2 1.796 1.927 1.803 1.924 N-0 1.210 1.210 1.224 1.216 CNT-Cr-X 113.01 119.53 111.66 117.60 CNT-Cr-NO 126.26 126.57 127.61 127.37 CNT-Cr-NH 2 118.36 125.69 121.26 125.65 X-Cr-NO 93.43 99.77 89.86 96.69 X-Cr-NH 2 104.22 92.81 100.70 91.87 ON-Cr-NH 2 97.09 82.86 98.85 88.65 Cr-N-0 175.94 174.80 175.85 175.97 ECrNH 2
360.00 358.55 359.93 358.50 H-N-Cr-N 11.39, -74.37, 7.52, -76.66,
-169.36 91.00 -169.11 88.35 | E(hartrees) -480.6751 -480.6766 -505.5853 -505.5701
a C N T = Cp ring centroid. b X = CI. C X = C H 3 .
Table 2.3. Optimized geometries and energies for CpCr(NO)(CH 3)X (X - CI, CH 3 )
Structural CpCr(NO)(CH 3)Cl b CpCr(NO)(CH 3) 2
c 1 Parameter"
S = 0 S = 1 S = 0 S = 1 CNT-Cr 1.959 2.004 1.983 2.026 Cr-C(average) 2.307 2.347 2.328 2.365 Cr-CH 3 2.057 2.049 2.029 2.073 Cr-Cl 2.254 2.298 Cr-NO 1.647 1.825 1.640 1.792 N-0 1.212 1.201 1.223 1.215 CNT-Cr-NO 126.04 121.95 127.77 137.38 CNT-Cr-CH 3 110.78 117.89 114.40 117.36 CNT-Cr-Cl 120.06 124.82 X - C r - C H 3 105.07 94.44 110.92 95.69 ON-Cr-Cl 100.41 94.12 ON-Cr-CH 3 88.85 96.78 93.09 90.61 Cr-N-0 175.30 174.45 175.99 176.91 E(hartrees) -464.5993 -464.6017 -489.4986 -489.4948
a C N T = Cp ring centroid. b X = CI. C X = C H 3 .
20
Results and Discussion
[CpCrL 3 r
The E H M O energy levels for [CpCr(CO)3]~ [CpCrCl3]" and [CpCr(NO)Cl2]" are
shown in Figure 2.2.7'10
-9.0
-10.0
-11.0 eV -12.0
-13.0-
xy COTC'Y'^V
xz yz
Cr e C o
o
mcnb
cr | ci
mcnb—f— z2
_ | L _ x V
CpNOTi % g
Figure 2.2. E H M O energy-level diagram for [CpCr(CO)3]~ [CpCrCl3]", and
[CpCr(NO)Cl2]".
For [CpCr(CO)3]~, the alignment of the z-axis along the pseudo-3-fold rotation
axis through the Cp centroid allows for maximum overlap of two orbitals (d^ and d )̂
with the two Cp Tc-symmetry orbitals, while the remaining three orbitals (d^, dX2-y2 and
dZ2) each form rc-bonds with all three carbonyl ligands (Figure 2.3). All five orbitals are
doubly occupied, with the CpCr Tc-orbitals slightly lower in energy than the CrCO
21
orbitals. These results agree well with previous molecular-orbital calculations o f
CpM(CO )3 compounds 1 1 The same orbital orientation holds for the [CpCrCfo] - complex.
The dxz and d^ orbitals are still filled and strongly n-bonding with the Cp ligand, but the
dxy, dx2-y2, and dZ2 are now high in energy, singly occupied, and weakly rc-antibonding to
the filled p-orbitals o f the three CI ligands (Figure 2.3).
Figure 2.3. Pictorial representation o f the rc-bonding interactions o f the five Cr 3d
orbitals in [CpCrL 3 ]~ (L = C O , CI).
In the case o f [CpCr(NO)Cl2]~, the orbitals mix and reorient in order to maximize
Cp and N O 7i-bonding at the expense of any interaction with the CI ligands. 1 2 Four o f the
five Cr d-orbitals combine with the two Cp and the two N O Tt-symmetry orbitals to result
in four low-energy, filled orbitals, each o f which are 7t-bonding to both Cp and N O
(Figure 2.4). The four 7t-bonding orbitals consist o f the in-phase and out-of-phase
combinations o f Cp n and N O % orbitals both in and perpendicular to the C N T - C r - N O
plane. The remaining high-energy, singly-occupied d-orbital is nonbonding to the strong
22
Tc-ligands, although there is some rc-antibonding interaction with the weak rc-donor CI
ligands.
Figure 2.4. Pictorial representation of the Cp and NO re-bonding interactions with the ;
five Cr 3d orbitals in [CpCr(NO)Cl2r.
Reexamination of the E H M O calculations previously conducted13 on
[CpCr(NO)(NH3)2]+ reveals that this cationic species possesses an identical "four43elow-
one" splitting pattern, including two additional Cp and NO Tc-bonding orbitals lower in
energy than the two previously reported. The SOMO is around 0.8 eV lower in energy
than the corresponding orbital in [CpCr(NO)Cl2]~. While the difference in charge
undoubtedly plays an important role in this variation between the two complexes, the
nature of the ligands (7C-donating chloride compared to the purely a-bonding ammine) is
likely also a contributing factor to this energy difference and the resulting ease of
oxidation of the dichloro anion over the bis(ammine) cation.7'13
The remarkable mixing of Cp and NO 7t-bonding illustrated in Figure 2 4 is due to
23
the ability of the strong re-acceptor nitrosyl to compete with the rc-donor cyclopentadienyl
for the available metal TC-symmetry orbitals. Similar extensive orbital reorganization
resulting from competition between two strong rc-donor ligands in four-coordinate
compounds has been reported by Schrock and co-workers, who concluded that in such
cases, the two different lo, 27C ligands should be treated as a single unit.14 Research on
such compounds has since been largely restricted to high-valent d° and d 2 complexes of
the early transition metals containing strong 7C-donors such as [rj'-CsRs]-, [NR] 2 - , and
[CR] 3 - ligands.15 The E H M O calculations described here suggest that a sufficiently
strong %-acceptor ligand may compete significantly with a %-donor ligand, leading to
M(1CT,2TC)2 electronic cores analogous to those found in pseudo-tetrahedral compounds
with two 7C-donor ligands. In other words, our calculations indicate that CpCr(NO) may
be considered to be a M(lrj,27t)2 fragment, comparable to C p ^ T i , 1 6 CpV(NR), 1 7
Cr(NR) 2 , 1 8 and Cp'Cr(CR). 1 9
Comparison of the relative energies, occupancies, and bonding character of the
orbitals in Figure 2.2 suggests an appealing explanation for the ligand control of
electronic stability in these cyclopentadienyl chromium complexes. It appears that
nonbonding d-orbitals in CpCrL3~ complexes can readily accommodate single, unpaired
electrons, thereby resulting in stable paramagnetic species having fewer than 18e. This
feature accounts for the relative stability of 17e [CpCr(NO)L 2] + and [CpCr(NO)X2]~
7 1̂
complexes, ' which possess one nonbonding orbital, compared to the 17e CpCr(CO)3
metalloradical, which has no such orbital.116 The fact that the SOMO of a stable 17e
species is non-bonding should not be surprising since, to a first approximation, singly-
occupied orbitals can only mix with other orbitals that possess unpaired electrons.20
24
CpM(NO)Cl 2 (M = C r or Mo)
Singly-occupied metal-centered non-bonding orbitals are also significant for the
16e CpCr(NO)Cl 2 neutral species. Instead of being based on X-ray crystallographic
parameters, the E H M O energy levels shown in Figure 2.5 are based on the DFT-
optimized geometries of high- and low-spin CpM(NO)Cl 2 (M = Cr, Mo) complexes
(Table 2.1 for geometric parameters, Figure 2.1 for singlet vs triplet relative energies):
Comparison of the S = 0 and S = 1 geometries and energies reveals that CpCr(NO)Cl 2 has
a triplet ground state, 8.20 kcal/mol lower in energy than the singlet state, and that the
Cr-NO bond of the high-spin species is over 0.2 A longer than the low-spin distance of
1.655 A. The significance of this lengthening is underscored by the constancy of
experimentally determined Cr-NO bond lengths, which fall in the narrow range of 1.65
A to 1.69 A for CpCr mononitrosyl complexes with a variety of ancillary ligands, overall
charges, and formal oxidation states.7"9'13'21 This 0.2 A difference is interpreted as
signaling a critical weakening of the Cr-NO bond in the triplet configuration, consistent
with the nitrosyl lability proposed for high-spin CpCr(NO)Cl 2. 7
-9.0-
-10.0-
-11.0—I eV
-12.0-H
-13.0-
-14.0-
- u -- u --ti-s = o S = l s = o
-tt-
S = l
Mo cK I ^ci
N o
C l ^ | CI N O
Figure 2.5. E H M O diagram for singlet and triplet CpM(NO)Cl 2 (M = Mo, Cr):
25
The qualitative shape, energies, and bonding characteristics of both the singlet
and triplet CpCr(NO)Cl2 Extended Huckel molecular orbitals are very similar to those
obtained for the doublet CpCr(NO)Ci2 anion (Figure 2.4). This is because the partially-
disordered7 solid-state molecular structure of CpCr(NO)Cl2~ closely resembles the DFT-
optimized geometries of the neutral species, and the low-level EFfMO calculations do not
account for the variation in orbital occupancy between singlet, doublet and triplet
compounds. Comparing the S = 0 and S=l occupancies, we see that in triplet
CpCr(NO)Cl2 an electron has been promoted from an orbital with Cr-NO re-bonding
character to a metal-centered non-bonding orbital.
While this difference is undoubtedly responsible for part of the Cr-NO bond
lengthening in the high-spin case, close examination of the C l - C r - C l and C N T - C r - C l
bond angles indicates a second possible factor. The triplet geometry more closely
approximates a pseudo-octahedral geometry, a trend that carries through all the DFT-
optimized S=l geometries (Tables 2.1-2.3). These geometry changes suggest that the
orbitals are no longer mixing to form four CpCr(NO) rc-bonding orbitals for the triplet
species. This would be consistent with the inability of singly-occupied orbitals to mix
with doubly-occupied orbitals,20 resulting in separate CpCr and Cr(NO) 7t-bonds. While
this effect is too subtle to be readily apparent in the low-level E H M O calculations, it
accounts for the long triplet Cr-NO distance since there remains only one doubly-
occupied Cr-NO 7t-bond.
26
CpCr(NO)(NH 2)X (X = CI or C H 3 )
The instability of CpCr(NO)Cl 2 with respect to NO loss effectively precludes its
use as a precursor to other CpCr(NO)X 2 species by metathesis routes analogous to those
used for the congeneric Mo compounds.4'5 CpCr(NO)(NPh2)I has previously been
synthesized in unreported yield from the reaction of CpCr(NO) 2Cl and LiNPh 2 in the
presence of Mel . 8 CpCr(NO)(N'Pr2)(ri1-02CPh) and CpCr(NO)(N'Pr2)(CH2SiMe3) were
recently synthesized from Cr(NO)(N'Pr2)3.9 The solid-state molecular structures of all
three CpCr(NO)(NR 2)X complexes display short Cr-N(amide) distances of 1.83 to 1.89
A and planar Cr-NR 2 groups aligned with the Cr-NO axis, thereby suggesting a strong
Cr-N(amide) TC-interaction. Consequently, we examined the model CpCr(NO)(NH2)Cl to
gauge the role of the Cr-N(amide) Tc-bond in conferring a low-spin configuration on
these CpCr(NO)(NR 2)X species.
-9.0
-10.0-
-11.0-eV
-12.0-
-13.0-
-14.0-
•NH 27i*
.CpNOrc
- f i -NH 2Tt s = o
•NH27t*
-j— mcnb
- } — N07t,NH27r*
C p N O n
-fj-NOrt,NH 2TC
S = 1 - f f -NH 2Tc S = 0
mcnb
— f - N C b t , N H > *
IF - H -- f j - N O T i , N H 27t S = 1
Cr H 2 N | ̂ C l
N O
^ C r ^ H 2 N | ^ C H 3
N O
Figure 2.6. E H M O diagram for singlet and triplet CpCr(NO)(NH 2)X (X = CI, CH 3 ) .
27
The optimized geometry o f singlet CpCr(NO)(NH2)Cl, shown in Figure 2.1,
conforms with the known molecular structures of CpCr(NO)(NR2)X compounds. The
Cr-NFfe unit is planar, with the sum of the angles around N being 360°. The Cr-NFfe
bond length of 1.796 A is slightly shorter than those determined experimentally, perhaps
due to steric interaction between the Cp ring and the bulky NR.2 (R = Ph or 'Pr) ligands of
the amide complexes. The alignment o f the amide along the C r - N O axis indicated by the
H - N - C r - N torsion angles is consistent with the experimental results. 8 ' 9 Figure 2.6 shows
the E H M O energy levels for C p C r ( N O ) ( N H 2 ) X ( X = CI, C H 3 ) species.
Figure 2.7 illustrates why the ON-Cr -NH2 group must be planar for a full Cr-
N(amide) 7t-bond to form in CpCr(NO)(NR2)X species. The orbital perpendicular to the
C r - N O axis is empty, and can accept 7t-donation from the filled amide N p orbital only
when the amide ligand lies coplanar with the Cr-nitrosyl bond. The 7i-antibonding orbital
shown in Figure 2.7 forms the L U M O of singlet CpCr (NO) (NH 2 )C l , while the C r - N H 2
7i-bonding orbital lies below the four CpCr(NO) 7t-bonding orbitals in energy. The
Cr-NH2 7i-bonding interaction is expected to raise the energy of the empty orbital
perpendicular to the C r - N O axis in C p C r ( N O ) ( N H 2 ) C l relative to the analogous orbital in
CpCr(NO)Cl2, leading to a comparatively larger H O M O - L U M O gap and an increased
preference for the low-spin, singlet electronic configuration.
H H
H
N O
N O
N O
LUMO HOMO -5
Figure 2.7. Pictorial representation o f the N H 2 7t-bonding interactions in singlet
C p C r ( N O ) ( N H 2 ) C l .
28
The planar arrangement of amide and nitrosyl ligands in CpCr(NO)(NH2)X
species is reminiscent of the ligand orientations observed in related d 4 and d 2 complexes
10 1 ^ O O 0/1 —
which contain both 7i-donor and 7i-acceptor ligands. ' ' The ability of 7t-donor
ligands to stabilize unsaturated species has previously been noted for specific
CpM(N0)X2 species,25 and has been reviewed for organometallic complexes in general.26
The present results demonstrate that ligand 7t-bonding effects can also affect the relative
energies of the spin states available to organometallic species through their influence on
the orbital-splitting energy.
The DFT-optimized geometry of triplet CpCr(NO)(NH2)Cl (Table 2.1 and Figure
2.1) is remarkably different than that of the singlet state. As in triplet CpCr(NO)Cl 2, the
Cr-NO distance has extended beyond the range observed experimentally. The Cr-amide
distance has also lengthened, and while the Cr-NFL; group remains essentially planar, the
ligand has adopted a conformation roughly perpendicular to the Cr-NO axis.
Comparison of the total energies of singlet and triplet CpCr(NO)(NFJ.2)Cl shows
that the two configurations are nearly degenerate. Although the formal replacement of CI
in CpCr(N0)Cl2 with N H 2 has increased the relative stability of the singlet vs the triplet
spin state, the calculation still favors the triplet state by 0.95 kcal/mol. The known
CpCr(NO)(N'Pr2)X complexes (X = ri 1-0 2CPh, CH 2SiMe 3), on the other hand, are
diamagnetic. Both compounds possess planar O N - C r - N C 2 moieties in the solid state (X-
ray crystallography), and the presence of four inequivalent amide C H 3 groups in the J H
NMR spectra (room temperature, CeDe) suggest that this orientation is retained in
solution.9 While S=l complexes of the formula CpCr(NR2)L2 are known,27 the long
Cr-NO bond (1.811 A) in triplet CpCr(NO)(NH2)Cl suggests that adopting a high-spin
configuration would render CpCr(N0)(NR2)X species prone to decomposition via NO-
loss. The discrepancy between experimental observations and theoretical results is
presumably due to the differences between the actual compounds and the simplified
complex we have chosen as a model. The presence of electron-donating alkyl substituents
makes N'Pr2 a better 7i-donor than the N H 2 ligand, enhancing the stability of singlet
CpCr(NO)(N'Pr2)X complexes to a greater extent than the calculations indicate.
Cr—NE'g
N O
CI N . O
SOMOT (HOMO)
...-H . .MH
o S O M 0 2 (HOMO - 1 )
Figure 2.8. Pictorial representation of the NO and N H 2 7t-bonding interactions in triplet
CpCr(NO)(NH2)Cl.
The relevant 7t-bonding orbitals for triplet CpCr(NO)(NH2)Cl are illustrated
schematically in Figure 2.8. The highest-energy singly-occupied orbital is orthogonal to
the nitrosyl, metal-centered, and non-bonding. The other singly-occupied orbital is TT
bonding to the NO and % antibonding to the N H 2 . A fully 7t-bonding combination can be
found at slightly lower energy than the CpCr(NO) 7t-bonding orbitals (i.e. HOMO -5),
while the L U M O is n antibonding to both NO and N H 2 . The source of the lengthening of
both the C r - N H 2 and Gr-NO bonds in triplet CpCr(NO)(NH2)Cl can be traced to the
lower-energy singly-occupied orbital. Since this orbital is C r - N H 2 antibonding, the
Cr-amide bond order is decreased. Like triplet CpCr(NO)Cl 2, there are only three
electrons in the Cr-NO 7t-bonding orbitals, which again leads to a weaker bond to the
nitrosyl ligand.
The change in amide orientation that accompanies the adoption of a high-spin
configuration creates a new metal-centered non-bonding orbital. This orbital forms the
HOMO of the complex, readily accommodating the highest-energy, unpaired electron in
30
a manner directly analogous to triplet CpCr(NO)Cl2, doublet CpCr(NO)Cl2~, and quartet
CpCrCfT. As a result of the conformational change of the amide ligand, no purely
metal-ligand antibonding orbitals are populated in triplet CpCr(NO)(NH2)Cl.
In order to more closely approximate the known CpCr(NO)(N'Pr2)(CH2SiMe3),
calculations were performed on CpCr(NO)(NH2)(CH3). The singlet geometry of the
model complex closely approximated the experimentally-determined structure,9 and
singlet CpCr(NO)(NH2)(CH3) was calculated to be 9.55 kcal/mol more stable than the
triplet state, as indicted in Figure 2.1. The difference in the relative singlet stability of
over 10 kcal/mol between the two CpCr(NO)(NH2)X model complexes required further
investigation. Indeed, formal metathesis of the CI ligand in CpCr(NO)(NH 2)Cl with a
C H 3 group unexpectedly resulted in a significantly greater relative change than the initial
replacement of N H 2 for CI in CpCr(NO)Cl 2 (7.25 kcal/mol). Calculations were thus
conducted on methyl-containing compounds in order to identify the source of the
enhanced singlet stability.
31
CpCr(NO)(CH 3 )X (X = CI or C H 3 )
As shown in Figure 2.1, triplet CpCr(NO)(CH3)Cl is calculated to be 1.52
kcal/mol lower in energy than the singlet state. While this constitutes an improvement in
relative singlet stability of more than 6 kcal/mol over the CpCr(NO)Cl2 species, the
change in energies is larger between CpCr(NO)Cl2 and CpCr(NO)(NH2)Cl, or
CpCr(NO)(NH2)Cl and CpCr(NO)(NH2)(CH3). Formal substitution of a second methyl
group leads to an even smaller relative difference. CpCr(NO)(CH3)2 is calculated to have
a diamagnetic ground state, but the singlet is only 2.37 kcal/mol more stable than the
triplet state.
-9.0
-10.0-
-11.0 eV -12.0
-13.0
-14.0 s = o S= 1
C r H 3 C ^ | X I
N O
s = o S = 1
T / C r
H 3 C ^ | > C H 3
N O
Figure 2.9. EFfivIO energy-level diagram for singlet and triplet CpCr(NO)(CH 3)X (X :
C1,CH 3).
While the 7T-bonding interactions of amide ligands can be discerned in the
32
planarity and orientation of the NR 2 group, the DFT-optimized geometries of the
CpCr(NO)(CH3)X species (X = CI, CH3) provide no structural clues to help explain why
alkyl ligands also favor the diamagnetic configuration. The discrepancy between
CpCr(NO)Cl 2 (triplet state more stable by 8.20 kcal/mol) and CpCr(NO)Me 2 (singlet
statefavored by 2.37 kcal/mol) in particular is difficult to reconcile with the known
ability of chloride ligands to stabilize unsaturated compounds through TC-donation.26
Examination of the bonding character of the Extended Huckel molecular Orbitals of the
CpCr(NO)(CH3)X species (Figure 2.9) reveals unusual features which suggest a possible
interpretation for these results. While the other CpCr(NO)X 2 complexes possess four
CpCr(NO) TC-bonding orbitals (Figure 2.4), CpCr(NO)(CH3)Cl and
CpCr(NO)(CH3)(NH2) have five of these orbitals, and CpCr(NO)(CH 3) 2 has six. These
"extra" orbitals are the result of mixing of the Cp and NO TC-bonding Orbitals with CH3 a-
bonding orbitals, as illustrated schematically in Figure 2.10.
Figure 2.10. Pictorial representation of the Cp and NO Tt-bonding and C H 3 a-bonding
interactions in H O M O -5 of triplet CpCr(NO)(CH 3) 2.
The effect of the inclusion of methyl a-bonding character in the frontier orbitals
on the relative spin-state energies of organometallic complexes has been previously
investigated. Hall and co-workers conducted a theoretical investigation of TiX2(dmpe)2
complexes in order to explain why the X = CI species28 is paramagnetic while the
33
isostructural X = C H 3 compound is diamagnetic, and they concluded that the variation
was due to differences in electron-electron repulsions between the two complexes.30 The
more electronegative CI ligands increase the effective charge on the metal atom, leading
to relatively contracted frontier orbitals and strong repulsions between paired electrons
which results in a triplet ground state. The more covalent CH3 ligands cause the orbitals
to be more diffuse, thereby decreasing electron-pairing energy and favoring the singlet
state.
34
Spectrochemical and Nephelauxetic Effects
E H M O calculations based on DFT-optimized geometries and singlet vs triplet
relative energies have been used to demonstrate that while CpCr(NO)Cl2 possesses a
triplet ground state which renders the complex susceptible to decomposition, formal
metathesis of the chloride ligands with either alkyl or amide groups improves the relative
energy of the singlet spin state, thereby rendering the complexes stable with respect to
nitric oxide loss. The two ligand types accomplish this stabilization in differing manners:
the NH2 7t-donation increases the orbital-splitting energy, while the covalent CH3 a-bond
decreases the interelectron-repulsion energy. These two parameters correspond exactly to
the Spectrochemical and Nephelauxetic effects. Because the underlying principles of
orbital-splitting and spin-pairing energies remain constant, the paradigms originally
developed in the 1960's to describe and explain how ligands influence the electronic
structures of coordination compounds may be successfully applied to current problems in
organometallic chemistry.
Early work in this area was conducted on pseudo-octahedral Werner-type
coordination compounds. Magnetic susceptibility measurements allowed the ground spin
state of these species to be determined, and UV/visible spectroscopy provided
information about excited electronic configurations. The predominantly ionic bonding
present in these compounds encourages the formation of mid-valent compounds with
accessible high-spin configurations. The orbitals are essentially metal-centered, allowing
electronic transitions to be readily assigned, and rendering the complexes amenable to
theoretical treatment using relatively simple crystal field or ligand field descriptions. The
systematic investigation of a large number of compounds demonstrated the effects of
ligands on the splitting energy between the t2g and eg orbitals, Ao, and the interelectron-
repulsion energy, represented by the Racah parameters, B and C . 3 1 Common ligands were
ranked in the Spectrochemical and Nephelauxetic series, which reflected their
empirically-determined effects on Ao and the Racah parameters, respectively.
35
In contrast, application of the concepts of orbital-splitting and spin-pairing
energies to organotransition-metal complexes has generally been neglected. This is in
part due to the covalent bonding and Tt-acceptor ligands typical of organometallic
compounds which tend to enforce a low-valent, diamagnetic configuration. The increased
ligand character of the frontier orbitals hampers the assignment of the electronic
transitions of the UV/vis spectra and limits the applicability of simple ligand field theory.
Thus, magnetic susceptibility and UV/vis measurements were supplanted by IR and
NMR spectroscopy as the primary analytical tools for organometallic complexes, with
subsequent investigations proceeding down avenues of inquiry suitable to the latter
techniques.32
Recent improvements in X-ray crystallography and theoretical methods have
contributed to the current reevaluation of the relevance of spin state to organometallic
chemistry. The dramatic increase in well-characterized high-spin organometallic
species33 can be attributed in part to the increased application of X-ray crystallography as
a characterization technique,34 and the attendant shift away from the diamagnetic bias
inherent in N M R spectroscopy. Open-shell compounds have demonstrated their utility in
olefin polymerization35 and dinitrogen cleavage36 reactions. While accurate evaluation of
energetically similar electronic states has long been a formidable challenge, advances in
ab initio37 and hybrid density functional38 computational techniques have helped
theoretical chemists address these near-degeneracy problems.39 With the application of
theoretical methods to the examination of unsaturated organometallic species too
transient or unstable to be observed experimentally,40 it is becoming increasingly
apparent that the relative energies of the spin states of these intermediates constitute a
critical but previously unappreciated factor in several important reactions.41 For example,
calculations indicate that a low-lying triplet excited state assists the intermolecular C - H
bond activation reactions exhibited by CpM(CO) (M = Rh, Ir) and related complexes, yet
because CpCo(CO) possesses a triplet ground state, it neither binds nor activates
36
20,42 alkanes.
The ability of TC-bonding amide ligands to increase the H O M O - L U M O gap of
CpCr(NO)X2 species and thereby stabilize the relative energy of the low-spin, singlet
state may initially appear to contradict the fundamental principles of ligand-field theory.
For octahedral, Werner-type coordination compounds, TC-donor ligands typically decrease
the H O M O - L U M O gap, which is reflected in their empirical ranking as "weak-field"
ligands in the Spectrochemical series. The effects of TC-bonding on the H O M O - L U M O
gap are illustrated for each case in Figures 2.11 and 2.12: the filled ligand Tc-donor
orbitals are shown on the right side of the figure, the metal-based orbitals prior to TC-
donation are on the left, and the new orbital-splitting energy, A, is shown for the resulting
complexes.
*2g £bjf:[
M-LTC
,--'̂ 11 (J L TC
Figure 2.11. Qualitative energy-level diagram illustrating 7t-donor effects oh orbital-
splitting energy and spin state in d 4 ML6.
Figure 2.11 demonstrates how multiple TC-donor ligands interact with the orbitals
of a pseudo-octahedral, d 4 coordination compound. The TC-symmetry t2 g orbitals are raised
37
in energy, which decreases the energy difference between the t2 g and e g orbitals and
stabilizes the high-spin, S = 2 configuration. For the pseudo-octahedral C p C r ( N O ) X 2 case
shown in Figure 2.12, the t 2 g-type orbitals have already been split by the strong 7t-
acceptor nitrosyl ligand, and so the important orbital splitting occurs "within the t 2 g set"
rather than between the t 2 g and e g orbitals. In the observed planar O N - C r - N H 2
conformation, the single-sided 7t-donor amide ligand raises the energy of the t 2 g-type
orbital that is not involved in C r - N O 7t-bonding. The resulting increase in A leads to the
relative stabilization o f the low-spin, singlet spin state.
II
mcnb—L-tf-
n - II
C r - N H 2 7 t
C r - N O =tp—-\-
H H N H 2 %
Cr-NH 27C
CpCr(NO ) (NH2^xl
Figure 2.12. Qualitative energy-level diagram illustrating N H 2 7i-donor effects on orbital-
splitting energy and spin state in C p C r ( N O ) ( N H 2 ) X .
Such an increase is indeed evident in the orbital-splitting energy between
occupied and unoccupied orbitals in singlet C p C r ( N O ) X 2 species (Table 2.4). The amide-
containing complexes exhibit the largest A values, with C p C r ( N O ) ( N H 2 ) C l and
C p C r ( N O ) ( N H 2 ) C H 3 possessing splitting energies o f 2.86 and 2.77 eV, respectively, both
38
higher than the 2.63 eV for CpCr(NO)Cl 2. While E H M O calculations are not the
computational method of choice for exact qualitative results, the trend that these A values
represent is nevertheless encouraging. There is not, however, a simple correlation
between the E H M O splitting energy and the relative energies of the singlet and triplet
states as calculated by DFT. Specifically, substitution of CI with CH3 simultaneously
decreases the orbital splitting energy yet increases the relative stability of the singlet
state due to changes in pairing energy.
Table 2.4. E H M O frontier orbital splittings for singlet CpCr(NO)X 2.
NH 2/C1 N H 2 / C H 3 Cl/Cl CH3/CI CH3/CH3
L U M O
HOMO
-9.227
-12.09
-9.507
-12.27
-9.333
-11.96
-9.622
-12.05
-9.937
-12.30
A 2.86 eV 2.77 eV 2.63 eV 2.43 eV 2.36 eV
In order to quantify the effect of the methyl ligands on the pairing energy, the
Coulomb (J) and exchange (K) integrals were evaluated for the CpM(NO)Cl 2 (M = Mo,
Cr) and CpCr(NO)(CH 3)X (X = CI, CH 3 ) complexes according to the method of Hall et
a/. 3 0 These J and K integrals were calculated by Prof. Poli with the assistance of Drs. Ivo
Cacelli and Antonio Rizzo; 4 3 they are the "spin-pairing" relationships that the Racah
parameters were originally formulated to approximate.44 The overall "pairing energy" can
be roughly described by the relation Jn - J i 2 + K i 2 , which is the difference between the
coulombic repulsions experienced by the paired (Jn) and the unpaired (Ji2)
configurations, plus the exchange energy (Ki 2) (Table 2.5).
The comparison between Mo and Cr is straightforward in that the orbital-splitting
energy is greater for CpMo(NO)Cl 2 (A = 2.92 eV) than for CpCr(NO)Cl 2 (A = 2.63 eV),
while the spin-pairing energy is less for the Mo species (94.72 kcal/mol) than for the
39
lighter Cr congener (172.84 kcal/mol). Both of these trends indicate that the singlet spin
state in the Mo complex will be favored to a greater extent than in the Cr compound, an
observation that is in accordance both with the DFT calculation results and generally
accepted periodic trends.33 For the CpCr(NO)X 2 (X = CI, Me) complexes, as CI ligands
are replaced by CH3 groups the decrease in A is more than offset, however, by the large
decrease in the spin-pairing energy, as the values for Jn - J12 + K12 indicate:
CpCr(NO)Cl 2 (172.84 kcal/mol) > CpCr(NO)(CH3)Cl (74.73 kcal/mol) >
CpCr(NO)(CH 3) 2 (-138.92 kcal/mol). The net result is that alkyl ligation enhances the
relative stability of the singlet state. This effect of covalent bonding on spin-pairing
energy precisely mirrors the intuitive picture of electron "cloud expansion" (i.e. the
Nephelauxetic effect) initially developed to explain the same phenomenon in Werner-
type coordination compounds.
Table 2.5. Interelectron repulsion energy parameters for CpM(NO)X 2
(M = Mo, X = CI; M = Cr, X = CI, CH 3 ) . a
Mo/Cl/Cl Cr/Cl/Cl Cr/Cl /CH 3 C r / C H 3 / C H 3
Jll 268.80 1240.46 976.17 531.65
J12 194.24 1134.98 966.79 697.64
K i 2 20.16 67.36 65.35 27.07
J l l _ Jl2 + K i 2 94.72 172.84 74.73 -138.92
"All energies are expressed in kcal/mol.
40
Summary
Extended Huckel molecular orbital calculations were performed on a variety of
CpCr-containing complexes, based on the structural parameters obtained from X-ray
crystallography or density functional geometry optimizations. The salient points resulting
from these calculations include:
1. The four 7c-bonding orbitals o f the CpCrfNO) fragment resemble the M(la ,27c) 2
electronic cores found in pseudo-tetrahedral complexes containing two strong TC-
donor ligands.
2. The apparently anomalous stability o f paramagnetic 17e CpCr(NO)-containing
compounds may be due to the non-bonding nature of the metal-centered L U M O .
3. The discontinuity in C p M ( N O ) X 2 chemistry between M o and C r may be due to spin-
state differences attributable to the increased interelectron repulsion present in the Cr
species.
4. Adopting a high-spin, S = 1 configuration significantly lengthens the metal-nitrosyl
bond in C p C r ( N O ) X 2 species due to promotion o f an electron from a C r - N O TC-
bonding orbital. The concomitant weakening o f the C r - N O bond provides a rationale
for the nitrosyl-loss reactivity postulated for high-spin C p C r ( N O ) C l 2 .
5. Formal replacement of CI with an amide ligand stabilizes the singlet state because the
C r - N R 2 re-bonding increases the orbital splitting energy.
6. Formal alkyl-for-chloride metathesis stabilizes the diamagnetic configuration because
of a decrease in pairing energy.
41
Computational Details
The optimized geometries and relative energies o f the singlet and triplet
C p M ( N O ) X 2 species were calculated by Pro f Rinaldo Pol i using G A U S S I A N 94 4 5 on the
DEC/Alphastation 250 at the University o f Maryland in College Park and on the SGI
Power Challenge at the Universite de Bourgogne. These calculations employed a Density
Functional Theory (DFT) approach, using the three-parameter form of the Becke, Lee,
Yang and Parr functional ( B 3 L Y P ) . 4 6 The monoelectric H F analysis o f the C p M ( N O ) X 2
30
complexes was performed according to the method o f Simpson, Hal l and Guest.
The visualization of the orbital interactions and the orbital energy diagrams was
assisted by the use o f Extended Hiickel molecular-orbital calculations, which were
performed on the crystallographically-determined or B3LYP-opt imized geometries using
the commercially available HyperChem for Windows Release 3 and ChemPlus
extensions for Hyperchem. 1 3 A n unweighted Hiickel constant of 1.75 was used.
42
References and Notes
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48
Chapter 3: Pi-Bonding and NO Loss from CpCr(NO)2Y Species
Introduction 49
Results and Discussion 52
Summary 69
Future Work 7 0
Experimental 73
Characterization Data 7̂
References and Notes 79
49
Introduction
In 1956, Piper and Wilkinson 1 reported the synthesis o f C p C r ( N O ) 2 C l , one of the
first well characterized organometallic complexes o f chromium. 2 The compound is slightly
water-soluble, and upon treatment with aqueous A g N 0 3 forms solutions of the
[CpCr (NO) 2 (H 2 0) ] + cation 3 (which was originally formulated as [CpCr(NO) 2 ] + ) . 1 The
remarkable water stability of the [CpCr(NO) 2 ] + fragment is demonstrated by the isolation
of C p C r ( N O ) 2 X ( X = halide or pseudo-halide) complexes upon addition of K X salts to in
situ generated [ C p C r ( N O ) 2 ( H 2 0 ) ] + followed by CHC1 3 extraction. 1
Subsequent metathesis studies o f C p C r ( N O ) 2 C l have been performed in anhydrous
organic solvents.4"7 Under these conditions, several alkyl complexes o f the type
C p C r ( N O ) 2 R (e.g. R = M e , 4 a E t , 4 a C H 2 P h , 4 b t i u , 4 c C H 2 S i M e 3 , 4 d and CFfcX 4 6 ) have been
synthesized. The electron-donating ability of the alkyl groups is evident in the shift o f
v(NO) from 1815 and 1710 c m - 1 for C p C r ( N O ) 2 C l to -1780 and 1670 c n f 1 for
C p C r ( N O ) 2 R as measured by solution IR spectroscopy (CH 2 C1 2 ) . 4 The results o f the
alkylation reactions contrast with the metathesis reactions o f C p C r ( N O ) 2 C l with Y sources
( Y = SR, N R 2 , or OR) . The outcome of these latter reactions depends on the electronic
nature o f the Y group. A l l known compounds o f composition C p C r ( N O ) 2 Y contain
strongly electron-withdrawing substituents (e.g. S C N , 1 S 0 2 C H 3 , 4 b S C F 3 , 5 a N ( S 0 2 F ) 2 , 5 c
OS0 2 C 6 H4Me, 5 d N C O , 5 b and N 0 2
1 5 e ) . However, i f Y contains electron-donating
substituents (e.g. V = SPh , 6 b O M e , 6 g O E t , 6 e 0 'P r , 7 b N M e 2 , 6 d and N P h 2
7 a ) , the isolated
products have lost one nitric oxide ligand.
50
N O
Scheme 3.1. N O loss reactivity postulated for C p C r ( N O ) 2 Y species
A s illustrated in Scheme 3 . 1 , the initial metathesis products CpCr (NO )2Y are
presumed to initially lose N O to form the intermediate [CpCr(NO)Y] (A) , which then
either dimerizes to [CpCr(NO)(p-Y)] 2 ( B ) , 6 or undergoes a subsequent one-electron
oxidation process to form the Cr(II) species C p C r ( N O ) ( X ) Y ( C ) . 7 The process depicted in
Scheme 3 . 1 initially seems counter-intuitive since the unexpected ejection o f a strong n-
acceptor nitrosyl ligand is triggered by increased electron donation from the Y ligand. The
known CpCr(NO )2R complexes display no propensity to lose N O despite the strong
s/g7wa-donor alkyl ligands. 4 The observed reactivity is limited to /?/-donor Y ligands
because [CpCr(NO)2] + is a 7t-loaded fragment. The cyclopentadienyl and nitrosyl ligands
each form one a and two tt interactions, leaving no empty orbital available on the
M ( 1 C T , 2 U ) 3 fragment to accept % donation from the Y ligand. 8 This unfavorable filled/filled
Tt interaction9 is relieved by loss o f nitric oxide, since intermediate A possesses an empty,
it symmetry orbital to accept donation from Y .
These observations are of potential significance for the rational design of
metallonitrosyl complexes capable of releasing N O in vivo10 Several features make
CpCr(NO )2X complexes particularly suitable as model compounds for the study of N O
release from transition metals. These Cr(0), d 6 complexes are diamagnetic with two
51
equivalents o f N O per Cr atom, and the paramagnetic Cr(I), d 5 mononitrosyl compounds
that would be the products expected upon loss of N O radical have been shown to be
unusually inert. 1 1 Given the stability of the [CpCr(NO) 2 ] + fragment in aqueous solution, 1
we hypothesized that treatment of aqueous [CpCr (NO) 2 (H 2 0) ] + with base should form
CpCr(NO) 2 (OH) . The strong 7t-donor hydroxide ligand might then induce loss of one of
the nitrosyl ligands under mild, physiologically-relevant conditions.
The investigation o f these concepts of n bonding and N O loss were performed in
collaboration with V i c k i Tong, who completed her fourth-year undergraduate research
thesis in the Legzdins group under my supervision. 1 2 This chapter describes the research I
conducted prior to and concurrent with V i c k i Tong's thesis project, and reference is made
to her studies of the aqueous [CpCr (NO) 2 (H 2 0) ] + system where appropriate. A n
integrated presentation of my work in this chapter and V i c k i Tong's thesis appeared as the
Legzdins group's contribution to a special issue of Dalton Transactions to commemorate
the first anniversary o f the death of Professor Sir Geoffrey Wilkinson. 1 3
52
Results and Discussion
Synthesis of Cp'Cr(NO) 2(OTf)
The slight water solubility o f C p C r ( N O ) 2 C l initially reported by Piper and
Wilkinson 1 is presumably due to hydrolysis of the C r - C l bond as illustrated in equation
3.1.
Similar reactions have been studied in detail for C p 2 M C l 2 complexes (M = T i , Zr,
V , or M o ) . 1 4 More recent investigations o f aqueous organometallic chemistry 1 5 have relied
on the trifluoromethanesulfonate (CF3SO3, OTf) ligand which dissociates more readily
from the metal center. 1 6 As shown in equation 3.2, CpCr(NO) 2 (OTf) (3.1) can be
synthesized by treatment o f C p C r ( N O ) 2 C l with A g O T f in a manner analogous to previous
chloride abstraction reactions with A g P F 6
1 7 and A g B F 4 . 1 8 The preparation o f 3.1 is usually
best performed in E t 2 0 , but larger scale reactions (>1 mmol) give optimum results when
conducted in a 1:1 mixture of E t 2 0 and C H 2 C 1 2 .
AgOTf ^ ' ( 3 2 )
CI CH 2 Cl 2 :Et 2 0 I ^ Q T f N
o 3-i Complex 3.1 is soluble in C H 2 C 1 2 , C H 3 C N , alcohols, water, ethers, and aromatic
solvents, and is isolated as large, air-stable, black crystals. The solubility o f 3.1 in solvents
with poor ion-supporting ability suggests that the O T f ligand is bound to the C r atom. The
single resonance in the ' H N M R spectrum at 8 5.82 (CDC1 3) corresponds to the
53
cyclopentadienyl ligand, and the higher-energy value of the IR v(NO) bands of 3.1 at 1836
and 1730 c m - 1 (CH2CI2) as compared to the CI precursor reflects the greater electron-
withdrawing properties o f the O T f ligand. 1 9 The Nujol mull IR spectrum also contains six
bands between 1000 and 1350 c m - 1 consistent with the presence o f a covalently bound
triflate ligand. 2 0
Attempts to obtain single crystals o f 3.1 from numerous solvent mixtures resulted
only in twinned crystals. In order to confirm the presence of a C r - O T f bond in 3.1, the
related Cp*Cr(NO) 2 (OTf) (3.2) was synthesized. A s expected, the
pentamethylcyclopentadienyl ligand imparted increased solubility and crystallinity to the
complex, 2 1 and a sample o f 3.2 suitable for X-ray crystallographic analysis was obtained
from diethyl ether. The solid-state molecular structure of 3.2 is illustrated in Figure 3.1,
and its intramolecular parameters are listed in Table 3.1. The O T f group is indeed
covalently bound to the metal center in 3.2; the C r - 0 distance of 2.030(2) A is consistent
with a C r - 0 single bond.
Figure 3.1. ORTEP plot of Cp*Cr(NO)2(OTf) (3.2). Thermal ellipsoids of 50%
Table 3.1. Selected Bond Lengths and Angles for Gp*Cr(NO)2(OTf) (3.2).
Bond Lengths (A) Bond Ang es O Cr-O(l) 2.030(2) Cr-N(l)-0(4) 165.7(3) Cr-N(l) 1.713(3) Cr-N(2)-0(5) 169.9(3) Cr-N(2) 1.712(3) N(l)-Cr-N(2) 95.3(2) Cr-CP 1.85 0( l ) -Gr-CP 117.3
55
Reactions of 3.1 with o-donor ligands
To test the ability of the O T f ligand to act as a leaving group, 3.1 was treated with
potential a-donor ligands. A s expected, the reactions of 3.1 with amines generated the
salts [CpCr(NO) 2 (L)][OTf] (L = N H 3 (3.3), N H M e 2 (3.4), N H 2 ' B u (3.5), N H 2 P h (3.6)) in
good to moderate yields as analytically pure, air-stable green powders. This technique was
subsequently employed by V i c k i Tong to form similar salts with the N-containing
heterocycles imidazole and 3,5-dimethylpyrazole. 1 2 This reaction is most conveniently
conducted in Et 2 0 since the resulting salts are ether insoluble and are readily isolated by
removal o f the supernatant solution via cannula. As indicated in equation 3.3, isolated
CpCr(NO) 2 (OTf) may be used in these reactions, or 3.1 may be generated in situ from
C p C r ( N O ) 2 C l and A g O T f in Et 2 0.
1— —I— O T f ® L
c:r L > J r ® O T t 3-3 N H 3 ( 3 3 )
Q N ^ I ^ O T f Et20 v f " I ^ L 3.4NHMe2
N N 3.5NH2'Bu O O 3.6NH7Ph
Compound 3.6 was treated with " B u L i or K O ' B u in T H F in an attempt to generate
the neutral dinitrosyl amide complex CpCr(NO) 2 (NHPh) at low temperatures by
deprotonation. Monitoring this reaction by solution IR spectroscopy did not reveal any
v(NO) bands consistent with CpCr(NO) 2 (NHPh) . Instead, the v(NO) bands o f 3.6 were
replaced upon slow warming with many weak bands in the region expected for
mononitrosyl products; the precise nature o f these products was not investigated further.
In contrast, the heterocycle-containing [CpCr(NO) 2 (L)][OTf] species 1 2 were
cleanly deprotonated as monitored by IR spectroscopy. The product of
[CpCr(NO) 2(imidazole)][OTf] and " B u L i or K O ' B u displayed new v(NO) bands at 1817
and 1712 c m - 1 (CH 2 C1 2 ) , but the presumed C p C r ( N O ) 2 ( N 2 C 3 H 3 ) product could not be
separated from the inorganic triflate salt. The neutral 3,5-dimethylpyrazolyl product
56
CpCr(NO )2 (N 2 C 5 H7) 3.7, however, could be isolated in a pure state due to its greater
solubility (equation 3.4).
Complex 3.7 is relatively air-stable in the solid state and is soluble in T H F , C H 2 C 1 2 ,
C 6 H 6 and E t 2 0 . The shift in v(NO) from 1839 and 1722 cm" 1 for cationic
[CpCr(NO) 2(3,5-dimethylpyrazole)][OTfJ to 1800 and 1686 cm" 1 (Nujol) for neutral 3.7
reflects the increase in electron density at the Cr center that accompanies the
deprotonation reaction. The solid-state molecular structure o f 3.7 is illustrated in Figure
3.2, and its intramolecular parameters are collected in Table 3.2. The Cr-N(pyrazolyl)
bond length is a relatively long 2.011(2) A, and the planar pyrazolyl group seems to be
oriented so as to avoid unfavorable steric interactions with the Cp ligand. This contrasts
with the short C r - N distance and the electronically dictated O N - C r - N R 2 coplanarity
recently observed for [CpCr(NO)(NR 2)]-containing species which possess a C r - N %
bond, 2 2 thereby suggesting that such an interaction is absent in 3.7.
57
Figure 3.2. ORTEP plot of CpCr(NO)2(3>Me2pyrazolyl) (3.7): Thermal ellipsoids of .
50% probability are shown for the non-hydrogen atoms.
. Table 3.2. Selected Bond Lengths and Angles for CpCr(NO)2(3,5-Me2pyrazolyl) (3.7)
Bond Lengths (A) Bond Ang es O Cr-N(3) 2.011(2) Cr-N(l)-0(1) 172.7(2) Cr-N(l) 1.707(2) Cr-N(2>-0(2) 171.8(2) Cr-N(2) 1.716(2) N(l)-Cr-N(2) 91.86(9)
58
Pi-Bonding and NO loss
The ligand-dependent reactivity observed upon treatment of [CpCr(NO)2(L)][OTf]
complexes with bases underscores the importance of TC bonding in [CpCr(NO)2]-
containing species. The stability of 3.7 is attributed to the aromatic nature of the 3,5-
dimethylpyrazolyl ring which allows the electron density of the filled N p orbital to be
delocalized over the heterocyclic N2C3 moiety. The NHPh ligand presumably formed upon
reaction of 3.6 with base could also potentially delocalize excess electron density from the
amide N to the aromatic Ph ring. However, this stabilizing interaction is apparently
insufficient to prevent the loss of NO and subsequent decomposition of the conjugate base
of 3.6, even when the neutral phenylamide complex is generated under relatively mild
conditions. This reactivity is entirely consistent with the substituent effects observed for
stable5 and unstable6,7 CpCr(NO) 2Y (Y = SR, NR 2 , or OR) complexes, as outlined in the
introduction of this chapter.23
In order to explain why CpCr(NQ)2Y complexes containing TC donors seem to be
unstable with respect to NO loss, the bonding of CpCr(NO) 2Cl was examined with the aid
of Extended Hiickel molecular orbital calculations.
Figure 3.3. Pictorial representation of the NO TC-bonding interactions in CpCr(NO) 2Cl
59
A qualitative bonding picture of the frontier orbitals of CpCr(NO)2Gl is shown in
Figure 3.3. 2 4 , 2 5 All three t2g-type orbitals are engaged in 7t-bonding to both NO ligands,
while the remaining out-of-phase combination of N - 0 TC* orbitals in the O N - C r - N O
plane forms the ligand-centered, non-bonding L U M O .
If the CI ligand in CpCr(NO) 2Cl were replaced with a N R 2 group, the filled amide
p orbital could interact with either the d „ or dyz orbital, depending on the amide
orientation. Figure 3.4 shows the orbital interactions that would be expected in
CpCr(NO) 2NR 2 if the N R 2 ligand lies in the yz plane (bisecting the O N - C r - N O angle).
The dxz orbital forms a low-energy, doubly occupied combination that is TC bonding to the
NR 2 group and to both of the NO ligands. The fully antibonding combination is high in
energy and unoccupied. Between these two molecular orbitals is a doubly occupied
combination that is TC bonding to the nitrosyl ligands, but TC antibonding to the amide. The
net result is no Cr-NR 2 TC bond and stronger Cr-NO bonds. From this analysis, it appears
that the ground state TC-bonding effects in CpCr(NO)2(NR2) do not account for the NO-
loss reactions attributed to these species.
Low Energy Medium Energy High Energy Doubly Occupied Doubly Occupied Empty Tc-Bonding to N O Tc-Bonding to NO TC-Anti-Bonding to Tc-Bonding to NR2 7i-/4«r/-Bonding to NR.2 NOandtoNR.2
Figure 3.4. Pictorial representations of the NO and NR 2 TC-bonding interactions in
CpCr(NO)2(NR2)
60
Fe Ru Re £ ^ | ^ S R M e 3 P ^ | P h 3 P ^ | Y
C P M e 3 N O O
Similar ligand loss reactions have previously been reported for the d 6 C p ' M L 2 Y
complexes shown above. 9 These complexes are qualitatively similar to the C p C r ( N O ) 2 Y
species dealt with in this chapter, and studies performed on these Group 7 and 8 species
may provide insight into the reactivity of C p C r ( N O ) 2 Y complexes. Ashby, Enemark and
Lichtenberger conducted Fenske-Hall molecular orbital calculations on CpFe(CO) 2 (SH) to
explore the interaction between the filled p orbital on the thiolate ligand and filled metal
orbitals on the [CpFe(CO) 2 ] + fragment.26 They concluded that these filled/filled repulsions
were responsible for both the enhanced nucleophilicity at sulfur observed for
CpFe(CO) 2 (SR) complexes and the tendency o f these complexes to dimerize and lose
C O . 2 7 A n excellent quantitative mechanistic study o f Y ligand n donation and ligand loss
from a series o f Cp*Ru(PMe3)2Y complexes was conducted by Bryndza, Bercaw and co
workers. 2 8 The acceleration of P M e 3 dissociation from Cp*Ru(PMe3)2(OFf) over
Cp*Ru(PMe3)2(CH3) by about six orders o f magnitude was attributed to the u-donor
properties o f the hydroxide ligand. Gladysz and co-workers demonstrated that chiral
CpRe(NO)(PPh 3 )Y ( Y = O R , 2 9 " N H R 2 9 b c ) complexes epimerize via P P h 3 dissociation. In
all of these d 6 , C p ' M L 2 Y systems, the Y group does not induce L ligand lability by any
negative influence on the M - L bond strength in the ground state, but Y n donation does
stabilize the C p ' M L Y species, thereby lowering the energy of the dissociative reaction
pathway.
61
A H
reaction coordinate Figure 3.5. Qualitative enthalpy diagram o f 7t-donor-assisted ligand dissociation.
The reaction enthalpy diagram presented in Figure 3.4 illustrates how Y 7t-
stabilization o f the unsaturated C p ' M L Y species enhances the lability o f the L l igand. 9 ' 2 8 ' 2
The top curve shows the relative energies along the dissociative pathway for C p ' M L 2 R
complexes. The unsaturated C p ' M L R moiety is a high-energy intermediate with only a
small barrier to the addition o f L to reform the starting material. The lack o f empty metal
orbitals on C p M L 2 Y prohibits 7t donation from the Y ligand, leaving the ground-state
energy o f this compound roughly equivalent to that o f the alkyl species. The process of
ligand L dissociation, however, creates an empty orbital that the lone pairs on Y can
interact with. This results in a lower energy for both the C p M L Y intermediate and the
62
dissociative transition state compared to the alkyl complex which lacks this potential for
71-donor stabilization o f the unsaturated species.9
Similar bonding considerations can be used to explain why [CpCr(NO)(u J -NHR)] 2
complexes are generated when CpCr(NO)(NH 2 R)I (R = 'Bu) or [ C p C r ( N O ) ( N H 2 R ) 2 ] + (R
= C H 2 C H = C H 2 ) complexes are treated with base. 3 0 Presumably the C p C r ( N O ) ( N H R ) L (L
= N H 2 C 3 H 5 or I") species are formed initially, but then eject L due to the amide it
stabilization o f the intermediate formed by ligand dissociation. Two equivalents o f
CpCr(NO)(NHR) then associate to yield the observed products. 3 0
The loss of nitric oxide from C p C r ( N O ) 2 Y is different from the related L
dissociations discussed above in two important respects. First, the loss o f the two-electron
donor ligands C O or P R 3 from C p ' M L 2 Y complexes does not result in a change in
oxidation state at the metal center, unlike the one-electron oxidation which formally
accompanies the loss o f N O radical. Second, the C p C r ( N O ) 2 Y complexes have the
potential to convert from a linear to a bent nitrosyl ligand. The suggestion o f 7t-donor
influence on ancillary ligand bonding does resemble the "ring slipped" (rj 3-
Cp*)Ir(PPh 3 )(Y)R intermediates invoked by Bergman and Glueck ( Y = OEt , R = H ; 3 1 a Y
= N H P h , R = M e 3 1 b ) . In the absence o f a detailed theoretical investigation o f C p C r ( N O ) 2 Y
complexes, however, the possibility of a bent nitrosyl intermediate remains purely
speculative.
Hydrogen Bonding in CpCr(NO) 2(OH) (aq)
"Speculative" initially appeared to be a remarkably charitable adjective with which
to describe all the bonding arguments presented in the preceding section once V i c k i Tong
began her investigation of CpCr(NO) 2 (OTf) in water. While 3.1 did form
[CpCr (NO) 2 (H 2 0) ] + when dissolved in water, and the aqua cation was deprotonated by
added OFT (pATa = 6.8), CpCr(NO) 2 (OH) proved to be disconcertingly stable in water; the
63
single *H N M R resonance for the Cp protons remained unchanged over weeks at room
temperature in N a O D / D 2 0 solution under N 2 . 1 2
The stability of basic aqueous solutions of 3.1 is apparently due to the interaction
o f the hydrogen atoms of the solvent water molecules with the oxygen atom of the
hydroxide ligand o f CpCr(NO) 2 (OH) . The Lewis basicity of the hydroxide oxygen in
CpCr(NO) 2 (OH) has previously been established by the isolation o f C p C r ( N O ) 2 ( H O " A )
adducts with Lewis acids ( A = B P h 3 or [CpCr(NO) 2 ] + ) 3 2 from the basic aqueous work up
of CpCr (NO) 2 (BF 4 ) . Also worthy of note is the IR monitoring of the reaction of EtO~ and
C p C r ( N O ) 2 C l in ethanol which indicates that the species CpCr(NO) 2 (OEt) is stable until
the protic solvent is removed in vacuo.6* Attempts to induce N O loss by generating
CpCr(NO) 2 (OF£) in aprotic solvents such as T H F by reaction of 3.1 with C s O H instead
leads to the formation of [(CpCr(NO) 2) 2(p--OH)][OTfJ, identified in solution by IR
spectroscopy by comparison with the known BF4~ and B P h f analogues. 3 2 In the absence
of a hydrogen-bonding solvent, the initially generated CpCr(NO) 2 (OH) species displaces
the O T f ligand in another molecule o f 3.1 instead o f inducing nitrosyl ligand dissociation.
M u c h research has been conducted recently on alcohol molecules engaged in
hydrogen bonding to the oxygen atoms o f late-transition-metal alkoxide species, several of
which have been characterized in the solid state by X-ray crystallography. 3 3 These
complexes provide an interesting comparison to the Cr nitrosyl complexes reported here,
in that in both cases no empty metal orbitals are available to accept % donation, and so
strong H-bonding interactions are established to relieve the resulting filled/filled
repulsions.9 O f particular interest are the studies o f Simpson and Bergman 3 4 who invoke
analogous hydrogen bonded species as intermediates in the exchange reactions o f
alkoxides by protonolysis.
64
Synthesis of CpCr(NO)(LX)
While hydrogen bonding prevents one potential route to nitric oxide loss by
attenuating the electron density at the hydroxide oxygen of CpCr(NO) 2 (OH) , it presents a
second possible route through protonolysis with potentially chelating ligands. When
treated with acids such as acetylacetone, salicylaldehyde, or picolinic acid ( H X L ) , in situ
generated solutions of CpCr(NO )2 (OH) react slowly to give the mononitrosyl products
C p C r ( N O ) ( L X ) ( L X = 0 2 C 5 H 7 (3.8), 0 2 C 7 H 5 (3.9), NCeHjO;, (3.10)). These compounds,
which slowly precipitate from aqueous solutions as analytically pure powders, are thought
to be formed by the reaction sequence illustrated in Scheme 3.2.
Scheme 3.2 Hydrogen-bonding, protonolysis, and chelate-assisted N O loss
Initial protonolysis o f the hydroxide ligand occurs via a H-bonded intermediate,
resulting in a dinitrosyl species with a new C r - X bond and a pendant Lewis basic
heteroatom. The subsequent chelate-assisted intramolecular N O displacement reaction
forms a water insoluble paramagnetic product with a six- (3.8 or 3.9) or five-membered
(3.10) metallacyclic ring. Equations 3.535 and 3.617 display previous examples o f
substitution of nitric oxide from CpCr(NO) 2-containing species by rj-donor ligands,
65
although these reactions were performed under thermolytic conditions in organic solvents.
These relatively harsh conditions contrast sharply with the room-temperature, aqueous
solution reactions reported here.
C 6 H 6 | "CI
• N O
r © P F 6 ® T , M® T
PF6
C r L - L 6^Cx L n 6\ Q N ^ I N C M e C H 3 N 0 2 Y R ^ L > K J V J
N N O O
To confirm the identity o f the paramagnetic chelate complexes, 3.8-3.10 were
independently synthesized from [CpCr(NO)(p>I)] 2
3 6 and K L X salts in T H F (equation 3.7).
Compounds 3.8-3.10 are air stable as solids, and their v (NO) (1647, 1659, 1666 cm" 1
(Nujol), respectively), solubility and other physical properties are similar to related, stable
17-valence-electron Cr(I) mononitrosyl complexes. 1 1 , 1 9 The solid-state molecular structure
of 3.8 was confirmed by X-ray crystallography, as shown in Figure 3.6. The
intramolecular parameters o f 3.8 are shown in Table 3.3. The molecule has a mirror plane
that passes through the nitrosyl ligand, the C r atom, one C atom of the Cp ring, and the
methine C atom of the acetylacetonate ligand.
? $ t r i l ^ C r „ _ ^ L ^ ^ C < * > (3.7)
g y g U M
66
Figure 3.6. ORTEP plot of CpCr(NO)(acetylacetonate) (3.8). Thermal ellipsoids of 50%
probability are shown.
Table 3 .3. Selected Bond Lengths and Angles for CpCr(NO)(acetylacetonate) (3.8).
Bond Len gths (A) Bond Ang e s H Cr-0(2) 1.959(2) Cr-N(l)-0(1) 169.5(4) Cr-N(l) 1.683(5) 0(2)-Cr-N(l) 98.6(1) N(l)-0(2) 1.201(5) N(l)-Cr-CP 122.6 Cr-CP 1.89 0(2)-Cr-CP 119.7
67
Aqueous chemistry of [CpCr(NO)(H20)2]+
Since paramagnetic compounds of formula [CpCr(NO)L 2]+ have recently been
demonstrated to be remarkably inert,11 it was sought to expand this class of complexes to
the bis(aqua) mononitrosyl cation [CpCr(NO)(H 20) 2] + which may be synthesized in two
ways, as shown in Scheme 3.3. Suspensions of [CpCr(NO)(p.-I)]2 in hot water slowly
react to give green solutions of [CpCr(NO)(H20)2]+, while [nBu4N][CpCr(NO)(OTf)2]1 9
dissolves in water at room temperature over 5 min to provide the same cationic species.
Addition of aqueous NaBPh* to concentrated solutions of [CpCr(NO)(H 20) 2] + causes the
precipitation of [CpCr(NO)(H20)2][BPri4] (3.11) as analytically pure microcrystals. The
analogous chromium bis(ammonia) tetraphenylborate salt has been structurally
characterized by X-ray crystallography.11
N O H 2
H 2 0
o s o £ o s o I ° I
C F 3 C F 3
NaBPh4
H 2 0
Scheme 3.3. Synthesis of [CpCr(NO)(H 20) 2] +
Unfortunately, the pKa of [CpCr(NO)(H 20) 2] + could not be ascertained due to the
instability of its conjugate base. At pH > 7, solutions of [CpCr(NO)(H 20) 2] + precipitate
[CpCr(NO)(p.-OH)]2 (3.12) in low yields (equation 3.8). Apparently, any hydrogen-
68
bonding interactions that may exist between the expected CpCr (NO)(H 2 0) (OH) product
and the aqueous solvent are insufficient to prevent loss o f H 2 0 and subsequent
expected upon formal substitution of a nitrosyl ligand with a water molecule; H 2 0 should
be a much better leaving group than N O . Solutions of [CpCr (NO) (H 2 0) 2 ] + also react with
acetylacetone, salicylaldehyde or picolinic acid to form compounds 3.8, 3.9, and 3.10,
respectively. These reactions proceed at a qualitatively faster rate than the dinitrosyl
reactions described above, again presumably due to the enhanced lability o f the aqua
ligand compared to the nitrosyl group.
aggregation reactions. This reactivity contrasts with the stability o f CpCr (NO )2 (OH) , 1 2 as
O N /
(3.8) N O O 3.12
69
Summary
The remarkable water stability of the [CpCr(NO)2(H20)]+ fragment has been
known since the original synthesis of this class of compounds by Wilkinson in 1956.1
Subsequent studies directed at the metathesis of the CI ligand of CpCr(NO)2Cl with TC-
donor ligands report only /wowo-nitrosyl products67 Prompted by these two observations,
it was postulated that TC donation from the hydroxide ligand of CpCr(NO)2(OH) might
trigger nitric oxide release under physiologically relevant conditions. This mode of NO
loss was not observed, as CpCr(NO)2(OH) remains unchanged for weeks in basic aqueous
solution.12 The build-up of electron density at the hydroxide oxygen is presumably relieved
through hydrogen-bonding interactions with the aqueous solvent.
While hydrogen bonding may prevent the spontaneous release of nitric oxide from
CpCr(NO)2(OH), it likely helps the loss of NO upon treatment with acetylacetone and
related acids. In reactions which likely consist of sequential hydrogen bonding,
protonolysis and chelate-assisted nitrosyl-displacement steps, NO is liberated at room
temperature in aqueous solution. The identity of the paramagnetic, water-insoluble
mononitrosyl products was confirmed by their independent synthesis from [CpCr(NO)(u.-
I)]2 in aqueous solution and organic solvents.
70
Future Work Chelate assisted N O displacement
The success of the reactions of CpCr(NO) 2 (OH) with H X L sources suggests that
the chelate effect may be helpful in other NO-liberating reactions. Robert Poe is currently
investigating analogues of CpCr(NO)2Cl containing Cp ligands with pendant Lewis basic
functionalities.3 7 Potential monoanionic bidentate X L ligands include theophyline (a
smooth muscle relaxant), 3 8 O N N ( 0 ) R ligands such as Cupferron (which are known to
generate N O in vivo)39 and amino acids (which potentially could form a single
C p C r ( N O ) ( L X ) diastereomer by stereoselective displacement of N O from the prochiral
[CpCr(NO) 2 ] + fragment).40 Also of interest would be the isolation of a C p C r ( N O ) 2 ( r i 1 - L X )
intermediate, prior to the coordination of the L group.
Generation of CpCr(NO)Y species
Designing a route to monomelic C p ' C r ( N O ) Y complexes poses an intriguing and
potentially rewarding challenge. Stabilization of such species might be achieved through a
combination o f steric and electronic factors. As discussed above, Y 7t donation would be
Q O S O Q _
expected to stabilize these complexes as unsaturated species. ' ' The preference for
unsaturation might be further enhanced by utilizing sterically demanding Y and C5R5
ligands. This strategy has been used successfully to isolate and structurally characterize
monomelic C p * R u ( P R 3 ) Y compounds (PR 3 = P 'Pr 2 Ph, P C y 3 ; Y = OR, N H P h ) . 4 1
For the C p C r ( N O ) Y species, increased steric bulk is required to prevent
dimerization to [CpCr(NO)(p.-Y)] 2 complexes. 6 A n instructive comparison may be drawn
to the isolobal, paramagnetic Ti(III) C p ' 2 T i Y complexes; C5H5 species with small O R or
N R 2 ligands form [Cp 2 Ti(u\-Y)] 2 dimers, 4 2 while the use of bulky Y ligands 4 3 or C s M e 5
groups 4 4 permits the isolation of C p ' 2 T i Y monomers. 4 5
71
Cr A M e I T r Cr *• Cr *• C r \
0 N | NPhs / \ "-CH3" v I NPhs N N NPhs N
o 0 0
HOiPr T Cr
"-H-" 'PrO^ I ^O'Pr O'Pr g
Scheme 3.4. Postulated importance of steric bulk in the generation of Cp'Cr(NO)(Y)X
species from CpCr(NO)2Y precursors
Postulating that steric bulk might stabilize Cp'Cr(NO)Y intermediates provides an
appealing explanation for the unexpected synthesis of CpCr(NO)(NPh2)I and
Cp*Cr(NO)(OTJr)2 from dinitrosyl precursors.7 As shown in Scheme 3.4, both reactions
are prevented from forming [Cp'Cr(NO)(p:-Y)]2 dimers by the steric bulk of the NPh 2
7 a or
C 5 H 5
7 b groups, and instead homolytically cleave available R-I or O-H bonds. This type of
radical abstraction chemistry is common for other paramagnetic Cr species,46 but is rarely
observed for d5 CpCr(NO)-containing complexes,11 and constitutes an intriguing reaction
motif for this class of complexes.
72
Scheme 3.5. Possible synthetic routes to Cp'Cr(NO)Y complexes
Scheme 3 . 5 demonstrates possible routes to Cp'Cr(NO)Y complexes, including (i)
metathesis of Cp'Cr(NO) 2Cl, (ii) metathesis of [Cp'Cr(NO)X]2 (X = CI or I),1 9'3 6 (iii)
reduction of CpCr(NO)(N'Pr2)X,2 2 or (iv) sequential nitrosylation and reduction of
(C 5Me 4SiMe 2N'Bu)CrCl(THF). 4 7
73
Experimental
Methods
A l l reactions and subsequent manipulations were performed under anaerobic
conditions using an atmosphere of N 2 unless otherwise noted. General procedures
routinely employed in these laboratories have been described in detail previously. 4 8 The
complexes C p C r ( N O ) 2 C l , 1 , 4 9 C p * C r ( N O ) 2 C l , 1 8 [CpCr(NO)(p-I)] 2 , 3 6 and
[ n Bu 4 N][CpCr(NO)(OTf) 2 ] 1 9 were prepared by the published procedures. A l l other
reagents were used as received from commercial suppliers. Filtrations were performed
through Celite ( 1 x 2 cm) supported on a medium porosity glass frit unless otherwise
specified.
The color, yield, and elemental analysis data for the new compounds in this chapter
are listed in Table 3.4. Infrared and mass spectral data are collected in Table 3.5. Table 3.6
shows the lH and 1 3 C {Ti} N M R data for the diamagnetic dinitrosyl compounds (3.1-3.7).
Synthesis of CpCr(NO)2(OTf) (3.1).
To C p C r ( N O ) 2 C l (0.655 g, 3.08 mmol) and A g O T f (0.793 g, 3.09 mmol) was
added C H 2 C 1 2 (15 mL) followed by E t 2 0 (15 mL). The solution was stirred for 15 min and
then filtered to remove A g C l . The Celite pad was washed with E t 2 0 ( 4 x 5 mL). The
volume of the solution was reduced slightly in vacuo and then hexanes (15 mL) were
added. The volume of the solution was again reduced slightly in vacuo, and the mixture
was then cooled to -30°C overnight, to provide 0.874 g of black crystals o f
CpCr(NO) 2 (OTf) (3.1).
Synthesis of Cp*Cr(NO)2(OTf) (3.2).
To C p * C r ( N O ) 2 C l (0.110 g, 0.38 mmol) and A g O T f (0.100 g, 0.39 mmol) was
added E t 2 0 (30 mL). The solution was stirred for 15 min, and then filtered to remove
74
AgCl. The Celite pad was washed with Et20 (3x10 mL). The volume of the solution was
reduced in vacuo to ~5 mL and the final solution was then cooled to -30°C overnight to
provide 0.125 g of dark brown crystals of Cp*Cr(NO)2(OTf) (3.2).
Synthesis of [CpCr(NO) 2L][OTf] (L = N H 3 (3.3), N H M e 2 (3.4), NH 2 'Bu (3.5), NH 2 Ph
(3.6)).
The four amine adducts 3.3-3.6 were synthesized using the same general
preparative procedure outlined here for the ammine complex 3.3.
To a stirred solution of CpCr(NO)2(OTf) (0.334 g, 1.02 mmol) in Et 2 0 (35 mL)
was added an excess of CH 2C1 2 saturated with N H 3 (~5 mL). A green powder immediately
precipitated from solution. The pale yellow solution was removed by cannulation, and the
solid was washed with pentane (2x5 mL). Drying in vacuo provided 0.206 g of
[CpCr(NO)2(NH3)][OTf] as a green powder.
The other amine adducts were prepared using either a CH2Cl2-saturated solution of
the amine (3.4) or the neat amine (3.5 and 3.6) in place of N H 3 .
Synthesis of CpCr(NO) 2 (N 2 C 5 H 7 ) (3.7).
To [CpCr(NO)2(3,5-dimethylpyrazole)][OTf] (0.232 g, 0.55 mmol) and KO'Bu
(0.068 g, 0.61 mmol) was added THF (-10 mL) via vacuum transfer. The solution was
stirred for 80 min in an acetone-dry ice bath, and then the bath was removed and the
solvent was removed in vacuo. The residue was extracted with Et 2 0 (15 mL) and filtered
to remove KOTf. The Celite pad was washed with Et 2 0 (3x5 mL). The volume of the
combined filtrates was reduced in vacuo, and the solution was cooled to -30°C overnight,
to provide 0.070 g of CpCr(NO)2(3,5-dimethylpyrazolyl) (3.7) as black-green crystals.
75
Synthesis of C p C r ( N O ) ( L X ) ( L X = 0 2 C 5 H 7 (3.8), 0 2 C 7 H 5 (3.9), N C 6 H 4 0 2 (3.10)).
Method 1. To CpCr(NO) 2 (OTf) (0.038 g, 0.117 mmol) and C s O H H 2 0 was added
water (10 mL). The solution was stirred for 30 min, and acetylacetone (0.020 mL, 0.20
mmol) was added via syringe. The solution was allowed to stir for 2 d while a green
powder precipitated from solution. The solid was collected by filtration in air, washed with
cold water, and dried in vacuo to obtain 0.012 g of CpCr(NO)(acetylacetonate) (3.8) as a
green powder.
Method 2. To a stirred solution of [ n Bu 4 N][CpCr(NO)(OTf) 2 ] (0.118 g, 0.182
mmol) in water (5 mL) was added acetylacetone (0.025 ml, 0.243 mmol). Within 1 min, a
green solid precipitated from solution. After 1 h, the solid was collected by filtration in air,
washed with water and dried in vacuo to obtain 0.089g o f a green powder which was
identified as 3.8 by IR spectroscopy and mass spectrometry.
Method 3. T H F (~5 mL) was vacuum transferred onto [CpCr(NO)(u.-I)] 2 (0.083 g,
0.151 mmol) and potassium acetylacetonate (0.052 g, 0.377 mmol). E t 2 0 (5 mL) was
added, and the solution was stirred for 12 h. The solution was filtered through alumina (1
x 2 cm), and the column was washed with E t 2 0 ( 2 x 1 0 mL). The combined filtrates were
reduced in vacuo to one-third volume, and hexanes were added (10 mL). The solution
was cooled to -30°C overnight to obtain 0.051 g o f green crystals which were identified as
3.8 by IR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction.
Compounds 3.9 and 3.10 were synthesized by the methods used for 3.8,
substituting acetylacetone with salicylaldehyde and picolinic acid, respectively.
Synthesis of [CpCr(NO)(H20)2][BPh4] 3.11.
A suspension o f [CpCr(NO)(p-I)] 2 (0.260 g, 0.474 mmol) in water (10 mL) was
stirred and heated to 70°C for 30 min. The clear green solution was filtered, and the
filtrate was treated with NaBPru (0.680 g, 1.99 mmol) in water (5 mL). A microcrystalline
76
solid immediately formed and was collected by filtration, washed with water (2x5 mL)
then with hexanes (4x5 mL) to obtain 0.280g of [CpCr(NO)(H20)2][BPh4] (3.11).
Synthesis of [CpCr(NO)(n-OH)] 2 3.12.
A suspension of [CpCr(NO)(p-I)]2 (0.260 g, 0.474 mmol) in water (10 mL) was
stirred and heated to 70°C for 30 min. The clear green solution was filtered, and the
filtrate was treated with 1 M NaOH until the pH was 8.6. Within 10 min, brown powder
started to precipitate from solution. After being stirred for 12 h, the solution was filtered
in air, and the collected solid was washed with water (2x5 mL) to obtain 0.020g of
[CpCr(NO)(u.-OH)]2 (3.12) as a gold powder.
77
Characterization Data Table 3.4. Numbering Scheme, Color, Yield and Elemental Analysis Data
Complex Cmpd #
Color (yield, %)
Elemental analysis found (calcd) Complex Cmpd #
Color (yield, %) C H N
CpCr(NO)2(OTf) 3.1 black (87) 22.28 (22.09) 1.57(1.55) 8.49 (8.59)
Cp*Cr(NO)2(OTf) 3.2 brown (82) 33.26 (33.33) 3.87 (3.81) 6.88 (7.07)
[CpCr(NO)2(NH3)][OTf] 3.3 green (60) 21.17(21.00) 2.23 (2.35) 12.21 (12.24)
[CpCr(NO)2(NHMe2)] [OTf] 3.4 green (42) 25.84 (25.88) 3.12(3.26) 11.21 (11.32)
[CpCr(NO)2(NH2'Bu)] [OTf] 3.5 green (52) 30.39 (30.08) 4.30 (4.04) 10.33 (10.52)
[CpCr(NO)2(NH2Ph)] [OTf] 3.6 green (80) 34.27 (34.37) 2.88 (2.88) 10.01 (10.02) CpCr(NO)2(N2C5H7) 3.7 green (47) 43.36(44.12) 4.33 (4.44) 19.84 (20.58) CpCr(NO)(02C5H7) 3.8 green (68) 48.49 (48.78) 4.96 (4.91) 5.57 (5.69)
CpCr(NO)(02C7H5) 3.9 brown (84) 53.64 (53.74) 3.79 (3.76) 5.15 (5.22)
CpCrCNOCNCeHtOz) 3.10 green (59) 48.92 (49.08) 3.36 (3.37) 10.41 (10.41) [CpCr(NO)(H20)2][BPh4] 3.11 green (53) 69.50 (69.34) 5.86 (5.82) 2.80 (2.80)
[CpCr(NO)(M)H)]2 3.12 gold (22) 36.35 (36.59) 3.30 (3.66) 8.38 (8.54)
Table 3.5. Infrared v(NO) and Mass Spectral Data
Complex m(cm_1) FAB/MS (m/z) Complex
Nujol CH2C12 P+ P+-NO P+-2NO
3.1 1834 1729
1836 1730
326 296 266
3.2 1794 1708
1801 1702 396 366 336
3.3 1824 1735
1830 1727
194 - -
3.4 1826 1709
1825 1723
222 192 -
3.5 1818 1700
1822 1720
250 220 190
3.6 1823 1710
1830 1728
270 240 210
3.7a 1800 1686
1808 1701
272 242 212
3.8 1647 1656 246 216 -
3.9 1659 1677 268 238 -
3.10 1666 1683 269 239 -
3.11 1692 - - - -
3.12 1595 - 328 298 268
" Solution Infrared spectrum recorded in Et 20 instead of CH2C12.
78
Table 3.6. 'H and 1 3C N M R Data
Compound (solvent) * H N M R (8) 1 3 C { !H} N M R (8)
3.1 (CDC1 3 )
5.82 (s, CsHs)
3.2 (CDCI3)
1.86 (CsMe5)
3.3 ((CD 3) 2CO)
3.72 (br s, 3 H , N// 3) 6.15 (s, 5 H , Cstfj)
104.3 (CjH 5)
3.4 ((CD 3) 2CO)
2.77 (d, 6 H , N H M ? 2 ) , 5.52 (br s, IH, N/flVfej), 5.96 (s, 5 H , CsHs)
3.5 (CDCb)
1.34 (s, 9 H , NH 2CM? 3), 4.16 (s, 2 H , N// 2CMe 3), 5.94 (s, 5 H , CsHs)
30.0 (NH2'Bu), 45.1 (NH2'£w)> 103.3 (CjH 5)
3.6 (CDCI3)
5.82 (s, 5 H , C5//5), 6.38-7.22 (m, 5 H , NH2Ph) 102.8 (CjH 5), 119.9 (NH 2 P/J) , 125.3 (NH2Ph), 129.5 (NH 2 /VJ)
3.7 (C6D6)
2.19 (s, 3 H , CH3), 2.53 (s, 3 H , CH3), 4.79 (s, 5 H , CsHs), 6.18 (s, IH, CH)
14.7 (CH 3), 15.0 (CH 3), 102.8 (C5H5), 105.3 ( C H ) ,
79
References and Notes
(1) Piper, T. S.; Wilkinson, G. J. Inorg. Nucl. Chem. 1956, 2, 38.
(2) Organometallic 7c-arene complexes of Cr(I) were synthesized as early as 1918
by the reaction of CrCl 3 and PhMgBr. While Hein and co-workers studied these products
in the 1920's and 1930's, the true nature of these species went unrecognized until more
advanced synthetic techniques, spectroscopic tools and theoretical models suitable for
organometallic compounds were developed in the wake of the discovery of ferrocene. For
an excellent historical essay describing Hein's "polyphenylchromium" complexes, see
Uhlig, E. Organometallics 1993,12, 4751.
(3) Wilkinson, G.; Cotton, F. A. Prog. Inorg. Chem. 1959, 7, 1.
(4) (a) Piper, T. S.; Wilkinson, G. J. Inorg. Nucl. Chem. 1956, 3, 104. (b) Hanna,
J. A.; Wojcicki, A. Inorg. Chim. Acta. 1974, 9, 55. (c) Hoyano, J. K.; Legzdins, P.;
Malito, J. T. J. Chem. Soc, Dalton Trans. 1975, 1022. (d) Legzdins, P.; Richter-Addo, G.
B.; Wassink, B.; Einstein, F. W. B.; Jones, R. FL; Willis, A. C. J. Am. Chem. Soc. 1989,
777, 2097. (e) Hubbard, J. L . ; McVicar, W. K. Organometallics 1990, 9, 2683.
(5) (a) King, R. B.; Welcman, N. Inorg. Chem. 1969, 8, 2540. (b) Bush, M . A.;
Sim, G. A. J. Chem. Soc. A 1970, 605. (c) Frobose, R.; Mews, R.; Glemser, O. Z.
Naturforsch. B 1976, 31, 1497. (d) Hames, B. W.; Legzdins, P. Organometallics 1982, 7,
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Sim, G. A. Acta Crystallogr., Sect. B 1979, 35, 1463.
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(12) Tong, V. B.Sc. Thesis, University of British Columbia, Apr. 1996.
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(23) Another instructive parallel may be drawn to cationic [CpCr(NO)2(=CR2)]+
complexes. While X~ abstraction reactions from CpCr(NO) 2(CH 2X) precursors failed to
generate stable methylidene (R = H) complexes,46 the heteroatom-substituted carbene
complexes [CpCr(NO)2(=C(OR)NHCH3)][PF6] are stable.17
(24) In order to simplify the discussion, the observed Cp components of the
Cr-NO TC-bonding interactions have been neglected. The E H M O calculation is based on
the more recent of the two X-ray crystallographic studies25 of CpCr(NO) 2Cl. 1
(25) (a) Carter, O. L. ; McPhail, A. T.; Sim, G. A. J. Chem. Soc. A 1966, 1095. (b)
Greenhough, T. J.; Kolthammer, B. W. S.; Legzdins, P.; Trotter, J. Acta Crystallogr.,
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(26) Ashby, M . T.; Enemark, J. H ; Lichtenberger, D. L. Inorg. Chem. 1988, 27,
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191.
(27) Many o f the known [CpFe(CO)(p.-SR)]2 complexes were synthesized by G .
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group which generated the first [CpCr(NO)(p.-Y)] 2 compounds: 6 a" d g , 7 a (a) King , R. B . ;
Bisnette, M . B . J. Am. Chem. Soc. 1964, 86, 1267. (b) Ahmad, M . ; Bruce, R.; Knox, G .
R. J. Organomet. Chem. 1966, 6, 1. (c) Dekker, M . ; Knox, G . R.; Robertson, C. G . J.
Organomet. Chem. 1969,18, 161. (d) Bladon, P.; Dekker, M . ; Knox, G . R.; Willison, D . ;
Jaffari, G . A . ; Doedens, R. J.; Muir , K . W . Organometallics 1993,12, 1725.
(28) Bryndza, H . E . ; Domaille, P. J.; Paciello, R. A . ; Bercaw, J. E .
Organometallics 1989, 8, 379.
(29) (a) Saura-Llamas, I.; Gladysz, J. A . J. Am. Chem. Soc. 1992,114, 2136. (b)
Dewey, M . A . ; Knight, D . A . ; Arif, A . ; Gladysz, J. A . Chem. Ber. 1992,125, 815. (c)
Dewey, M . A . ; Stark, G . A . ; Gladysz, J. A . Organometallic 1996,15, 4798.
(30) M c N e i l , W . S. Ph.D. Thesis, University o f British Columbia, Dec. 1995.
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1991,10, 1462. (b) Glueck, D . S.; Bergman, R. G . Organometallics 1991, 10, 1479.
(32) Legzdins, P.; Martin, D . T.; Nurse, C. R.; Wassink, B . Organometallics 1983,
2, 1238.
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Am. Chem. Soc. 1987,109, 1444. (b) Bergman, R. G Polyhedron 1995, 14, 3227. (c)
Kapteijn, G . M . ; Spee, M . P. R.; Grove, D . M . ; Koojiman, H . ; Spek, A . L . ; van Koten, G .
Organometallics 1996,15, 1405.
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83
Simpson, R. D.; Bergman, R. G. Organometallics 1993,12, 781.
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(36) Legzdins, P.; Nurse, C. R. Inorg. Chem. 1985, 24, 327.
(37) Poe, R. J.; Legzdins, P. work in progress.
(38) (a) Kistenmacher, T. J.; Szalda, D. J.; Marzilli, L. G. Inorg. Chem. 1975,14,
1686. (b) Szalda, D. J.; Kistenmacher, T. J.; Marzilli, L. G. Inorg. Chem. 1975, 14, 2783.
(c) Sorrell, T.; Marzilli, L. G.; Kistenmacher, T. J. J. Am. Chem. Soc. 1976, 98, 2181. (d)
Norris, A. R.; Taylor, S. E. ; Buncel, E. ; Belanger-Gariepy, F.; Beauchamp, A. L. Inorg.
Chim. Acta 1984, 92, 271.
(39) (a) Middleton, A. R.; Wilkinson, G. J. Chem. Soc, Dalton Trans. 1981,
1898. (b) Legzdins, P.; Rettig, S. J.; Sanchez, L. Organometallics 1988, 7, 2394. (c) Yi,
G.-B.; Khan, M . A.; Richter-Addo, G. B. Inorg. Chem. 1995, 34, 5703. (d) Schneider, J.
L.; Young, V. G., Jr.; Tolman, W. B. Inorg. Chem. 1996, 35, 5410.
(40) Amino acids have recently been used as chelating ligands for zero-valent
group-six transition metals40a and [Cp'Mo(NO)I]+:4 0 b (a) Darensbourg, D. L.; Draper, J.
D. ; Reibenspies, J. H. Inorg. Chem. 1997, 36, 3648. (b) Maurus, M . ; Aechter, B.;
Hoffmuller, W.; Polborn, K.; Beck, W. Z. Anorg. Allg. Chem. 1997, 623, 299.
(41) Johnson, T. J.; Folting, K.; Streib, W. E. ; Martin, J. D.; Huffman, J. C.;
Jackson, S. A.; Eisenstein, O.; Caulton, K. G. Inorg. Chem. 1995, 34, 488.
(42) (a) Lappert, M . A.; Sanger, A. R. J. Chem. Soc. A 1971, 1314. (b) Samuel,
E. ; Harrod, J. F.; Gourier, D.; Dromzee, Y.; Robert, F.; Jeannin, Y. Inorg. Chem. 1992,
31, 3252.
(43) Cetinkaya, B.; Hitchcock, P. B.; Lappert, M . F.; Torroni, S.; Atwood, J. L. ;
84
Hunter, W . E . ; Zaworotko, M . J. J. Organomet. Chem. 1980,188, C31.
(44) (a) Pattiasina, J. W. ; Heeres, H . J.; Van Bolhuis, F. ; Meetsma, A . ; Teuben, J.
H . ; Spek, A . L . Organometallics 1987, 6, 1004. (b) Feldman, J.; Calabrese, J. C. J. Chem.
Soc, Chem. Commun. 1991, 1042. (c) Luinstra, G . A . ; Vogelzang, J.; Teuben, J. H .
Organometallics 1992,11, 2273.
(45) Lukens, W . W. ; Smith, M . R.; Andersen, R. A . J. Am. Chem. Soc. 1996, 118,
1719 and references contained therein.
(46) (a) Kochi , J. K . ; Powers, J. W . J. Am. Chem. Soc. 1970, 92, 137. (b)
Espenson, J. H . Prog. Inorg. Chem. 1983, 30, 189. (c) Tyler, D . R. Prog. Inorg. Chem.
1988, 36, 125. (d) Baird, M . C. Chem. Rev. 1988, 88, 1217. (e) Espenson, J. H . Acc.
Chem. Res. 1992, 25, 222. (f) Huber, T. A . ; Macartney, D . H . ; Baird, M . C.
Organometallics 1995, 14, 592.
(47) Liang, Y . ; Yap, G . P. A . ; Rheingold, A . L . ; Theopold, K . H . Organometallics
1996, 15, 5284.
(48) Legzdins, P.; Rettig, S. R.; Ross, K . J.; Batchelor, R. J.; Einstein, F. W . B .
Organometallics 1995, 14, 5579.
(49) Hoyano, J. K . ; Legzdins, P.; Malito, J. T. Inorg. Synth. 1978,18, 126.
85
Chapter 4: Synthetic Utility of the [CpMo(NO)(CH2Ph)]+ Fragment
Introduction 86
Results and Discussion 90
Summary 99
Future Work 1 0 0
Experimental 105
Characterization Data 107
References and Notes 1 0 9
86
Introduction The majority o f the complexes described in this chapter were derived from
C p M o ( N O ) ( C H 2 P h ) C l . 1 The Mo(U), d 4 , benzyl chloride species was used to synthesize
new C p M o ( N O ) ( C H 2 P h ) R and [CpMo(NO)(CH 2 Ph)L][OTf] complexes. The triflate salts
were treated with Bransted bases in an attempt to generate CpMo(NO)(=CHPh)L
benzylidene compounds. In order to put these reactions in their proper context,
background information is required on (a) the use of [CpMo(NO)(CO)(al lyl)] + cations in
organic synthesis,2 and (b) C - C bond forming 3 and C - H bond activating 4 reactions o f
C p ' M ( N O ) L ( M = M o , W) species.
The development o f transition-metal-based reagents is regarded as one of the
most important areas o f current and potential progress in organic synthesis.5 Among other
applications, cationic metal complexes are commonly used to bind to unsaturated organic
moieties, activating the bound group for specific attack by nucleophilic reagents. Scheme
4.1 depicts a well-studied CpMo-based system that illustrates this type of sequential,
stoichiometric, "ligand elaboration" reactivity.6
R
[ N O ] B F 4
[O] M o R . - B F 4
R R
R' R '
Scheme 4 . 1 . Application of CpMo-containing complexes to organic synthesis
87
The initial organomolybdenum species is synthesized by treating in situ-
generated CpMo(CO) 3(FBF 3) with a diene to form [CpMo(CO)2(r|4-diene)][BF4] (A).
Coordination to the cationic CpMo(CO) 2
+ fragment renders the diene prone to
nucleophilic attack, affording the CpMo(CO)2(Y|3-allyl) ( B ) species. In order to
regenerate a reactive cationic species, one of the carbonyl ligands of B is substituted with
N O + to obtain [CpMo(KO)(CO)(n3-allyl)][BF4] (C). Addition of a second nucleophile to
C creates a Mo(0), d 6 complex, CpMo(NO)(CO)(ri2-olefin) (D) . Oxidation or hydrolysis
is then employed to liberate the organic product from the metal center. Each of the
individual reactions outlined in Scheme 4.1 has been individually studied, with great
attention being paid to the isomeric possibilities, the potential scope of nucleophiles and
organic substrates, and issues of regio- and stereoselectivity. More recent work has
extended the CpMo-based chemistry to related CpW, 7 TpMo, 8 TpW, 9 and T p * W 1 0
Scheme 4.2. Potential extension of known CpMo-based organic synthesis to C - H bond
activation and C - C bond formation reactions.
Scheme 4.2 shows two additional synthetic steps that might be achieved if the
carbonyl ligand could be induced to dissociate from D to form the unsaturated, 16e
CpMo(NO)(n2-olefin) (E) moiety. Related Cp'M(NO)L (M = Mo, W) species have been
systems.
K
88
invoked to explain a wide range of C - H bond activation and C - C bond formation
reactions that initially afford complexes analogous to CpMo(NO)(alkyl) 2 (F) and
CpMo(NO)(metallacycle) (G), respectively. 1 1' 1 2 Unfortunately, the M o - C O bond is quite
strong due to the n bonding interaction with the filled, high-energy dxy orbital orthogonal
to the M o - N O axis, 1 3 and so C O loss from D is not expected to occur even under
thermolytic or photolytic conditions. In order for the desired reactivity to be realized, an
analog of D has to be synthesized that contains a less strongly bound, 7t-neutral, 2e a -
donor ligand (L) in place of the 7t-acceptor C O group. However, initial attempts to
generate the target CpMo(NO) (C 3 H 5 )L + cations by reaction of CpMo(NO) (C 3 H 5 ) (CO) +
or CpMo(NO)(C 3 Hs)(OTf) with PPh 3 were unsuccessful, presumably due to the
susceptibility of the allyl ligand towards nucleophilic attack. To avoid these difficulties, a
different precursor was required in order to explore the possibility of C - H bond
activation and C - C bond formation reactions involving CpMo(NO)-containing
complexes (Scheme 4.3).
© O T f 3
Base
Scheme 4.3. Planned reactions to investigate the synthetic utility of the
[CpMo(NO)(CH2Ph)]+ fragment
89
The CpMo(NO)(CH2Ph)Cl starting material was chosen for five reasons. First,
it is readily available in high overall yield from the [CpMo(NO)Cl(u.-Cl)]2 d imer 1 4 in two
steps, unlike other CpM(NO)(R)Cl complexes which are only accessible via
hydrogenation of CpM(NO)R.2 in N C M e , followed by treatment o f the resulting
CpM(NO)(N=C(H)Me)R species with H C I . 1 5 Second, the r| 2-benzyl interaction typically
observed in Cp 'M(NO)(CH2Ph)X complexes 1 (Figure 4.1) was expected to stabilize
CpMo(NO)(CH2Ph)(OTf), thereby allowing access to [CpMo(NO)(CH 2 Ph)(L)][OTfJ
salts in two steps from the benzyl chloride. Third, the orientation o f the Ti 2 -benzyl ligand
and the lack of substituents on the Cp ring render the CH2Ph ct-H's exposed for
intermolecular deprotonation, unlike related C p * W compounds which form "tucked-in"
Me4C5CH2 complexes upon treatment with strong bases. 1 6 Fourth, the r | 2 -benzyl ligand
was expected to stabilize the products of C - H activation. Fifth, it was anticipated that the
reaction of R~ reagents with the benzyl chloride precursor would afford the
CpMo(NO)(CH2Ph)R complexes directly. This independent synthesis would provide an
opportunity to assess the stability o f the CpMo(NO)(CH 2 Ph)R products and establish
their diagnostic spectroscopic properties prior to attempting to generate them via the
CpMo(NO)(=CHPh) (H) intermediate.
Figure 4.1.T | 2-Benzyl interaction in C p ' M ( N O ) ( C H 2 P h ) X species.
90
Results and Discussion Direct Synthesis of CpMo(NO)(CH 2Ph)R.
R 2 M g / T H F
cr | CI ' N
O
R = CH2'Bu,o4oryl, R Ph, CH2SiMe3 N
R ' 2 M g / T H F •
'C I R'= CH2'Bu, o-tolyl, R ' Ph, CH2SiMe3, Me
\v (4-1) | ^ R -
N O
Equation 4.1 illustrates the procedure that has previously been used to synthesize
Cp*W(NO)(R)R ' mixed bis(alkyl) compounds. 1 7 The initial alkylation must be
performed at low temperatures, otherwise a 50:50 mixture o f the dichloro starting
material and the symmetric bis(alkyl) complex wi l l be formed. The second alkylating
reagent can be added to isolated Cp*W(NO)(R)Cl , or the alkyl chloride intermediate can
be generated and used in situ.
This synthetic protocol unfortunately cannot be extended to the CpMo(NO)(R)R '
species due to the decreased solubility of the unsubstituted cyclopentadienyl derivatives.
Reactions of [CpMo(NO)(Cl)(p-Cl)] 2 with one-half equivalent o f R 2 Mgx(dioxane) result
in the formation o f C p M o ( N O ) R 2 , as the initially-generated CpMo(NO)(R)Cl species
reacts more readily with the Grignard reagent than the comparatively insoluble dichloro
dimer. As a result, none of the CpMo(NO)(R)R' analogues of the compounds illustrated
in equation 4.1 have yet been synthesized.
ci-M o
N O
X I
B z 2 M g
T H F
HCI
C H 2 C 1 2
(4.2)
The C p M o ( N O ) ( C H 2 P h ) C l precursor used in the current study was prepared as
shown in Equation 4.2. The dichloro dimer is treated with Mg(CH 2 Ph) 2 x(dioxane) , and
91
then the isolated bis(benzyl) product is dissolved in dichloromethane and treated with
an excess of H C I dissolved in E t 2 0 , which selectively cleaves just one Mo-CH2Ph bond. 1
R R " - xL 4.1 M e (4.3)
T H F r " I ^ R 4 ' 2 P h
4.3 C C P h 4.4 Cp
Even after the desired benzyl chloride precursor has been acquired, the decreased
solubility o f the C p M o ( N O ) ( C H 2 P h ) X complexes compared to the Cp*W(NO)(R)R ' or
Cp*M(NO)(CH 2 CMe3)X derivatives continues to hamper the synthesis o f
CpMo(NO)(CH 2 Ph)R complexes. The reaction shown in Equation 4.3 proceeds cleanly
in T H F as determined by monitoring the change in v (NO) during the course o f the
reaction by solution IR spectroscopy. The metathesis can be accomplished using a
diverse range of alkylating reagents, including organolithium and organosodium species
as well as the more typical Grignard complexes. Unfortunately, the lack o f solubility o f
the CpMo(NO)(CH2Ph)R complexes 4.1-4.4 in alkane solvents hinders their complete
separation from the ionic byproducts o f the reaction. While these impurities are not
apparent in the solution IR spectra of the crude benzyl alkyl products, they are quite
evident in their elemental analyses, which are consistently low in carbon content.
Analytically-pure, crystalline samples may be obtained by chromatography on Alumina I
using toluene as eluant, followed by recrystallization from toluene:hexanes solvent
mixtures.
The replacement of the electronegative CI ligand with more covalently-bound
hydrocarbyl groups has the expected effect on the electron density at the metal center.
This is indicated by the v(NO) bands in the Nujol-mull IR spectra of the
CpMo(NO)(CH 2 Ph)R compounds which are from 14 to 44 c m - 1 lower in frequency than
the 1620 c m - 1 value exhibited by C p M o ( N O ) ( C H 2 P h ) C l . 1 The magnitude o f this decrease
92
in v(NO) follows the trend C ^ C P h < Cp ~ Ph ~ C H 2 P h < C H 2 S i M e 3 ~ M e , which is
consistent with the electron-donating abilities o f these hydrocarbyl ligands. The IR
spectrum o f 4.3 also displays a v (C=C) band at 2091 c m - 1 . 1 8
In their T I N M R (C6D6) spectra, the C5H5 signals o f compounds 4 .1-4.4 lie in the
4.80 to 5.20 ppm region and are expected to be useful diagnostic indicators o f these
complexes. Only a single resonance for both Cp ligands is observed in the room
temperature ' H and 1 3 C N M R spectra of C p 2 M o ( N O ) ( C H 2 P h ) . Also significant is the
signal at -0.88 ppm characteristic of a M e ligand in the ! H N M R (CeDe) spectrum of 4.1.
The I 3 C N M R spectra o f complexes 4.1-4 .3 are important chiefly for the ipso C signals
of the C H 2 P h ligands which fall in the 110 to 113 ppm range indicative o f a rj 2-benzyl
interaction. 1 , 1 9
Complex 4.4 does not appear to possess a r) 2-benzyl ligand according to its ' H
and 1 3 C N M R spectra. The benzyl methylene lH N M R signal appears as a singlet at room
temperature, and the ipso C signal occurs at 152.3 ppm in the 1 3 C N M R spectrum o f 4.4.
The presence o f a V-benzyl ligand in 4.4 was confirmed by an X-ray crystallographic
analysis. The solid-state molecular structure o f C p 2 M o ( N O ) ( C H 2 P h ) is shown in Figure
4.1, and selected bond lengths and angles are collected in Table 4.1.
93
Figure 4.2. O R T E P plot of C p 2 M o ( N O ) ( C H 2 P h ) (4.4). Thermal ellipsoids of 50%
Table 4.1. Selected Bond Lengths, Bond Angles, and Torsion Angles for
C p 2 M o ( N O ) ( C H 2 P h ) (4.4).
Bond Lengths (A) Bond Angles (") Torsion An eles (°) M o - C ( 4 ) 2.700(3) M o - N ( l ) - 0 ( l ) 174.1(2) C ( l ) - C ( 2 ) -
C(3)-C(4) 7.0(4) M o - C ( 5 ) 2.667(3) M o - C ( l l ) - C ( 1 2 ) 117.4(2)
C ( l ) - C ( 2 ) -C(3)-C(4) 7.0(4)
C(4)-C(5) 1.317(5) C P ( l ) - M o - C P ( 2 ) 119.8 C ( 3 ) - C ( 2 ) -C ( l ) - C ( 5 ) 8.0(4)
M o - C ( l l ) 2.258(3) N ( l ) - M o - C ( l l ) 87.66(10) C ( 3 ) - C ( 2 ) -C ( l ) - C ( 5 ) 8.0(4)
94
The nature o f the C p - M o bonds in C p 2 M o ( N O ) X species has been the subject
of debate since their initial synthesis almost 30 years ago. 2 0 , 2 1 I f these compounds
possessed two t | 5 -Cp groups and a linear nitrosyl ligand, they would have a formal
electron count o f 20e. Early suggestions that this unfavorable situation might be avoided
by adopting an asymmetric n 5 -Cp / r | 3 -Cp binding mode 2 0 a b was not supported by
subsequent X-ray crystallographic studies. The solid state molecular structures o f
C p 2 M o ( N O ) ( r i 1 - C p ) 2 0 c , C p 2 M o ( N O ) ( C H 3 ) , 2 0 d and C p * ( C p ) M o ( N O ) ( C H 3 ) 2 0 e all exhibited
essentially planar but skewed C p ' groups with all 10 multihapto ring C atoms lying
within bonding distance o f the M o center. This contrasts with the bonding in C p 2 W ( C O ) 2 ,
which possesses distinct r | 5 -Cp and r f - C p ligands. 2 2 The rj 3-Cp group contains a
localized C - C double bond, with the two unique C atoms bent 20° away from the plane
containing the three C s bound to the W center.
The solid state molecular structure of C p 2 M o ( N O ) ( C H 2 P h ) (4.4) shown in Figure
4.1 exhibits features intermediate between C p 2 W ( C O ) 2 and the previously-reported
C p 2 M o ( N O ) R structures. A l l ten Cp C s are within 2.336 to 2.700 A (average = 2.475 A)
of the M o center, and the difference between the two Cp groups is not as marked as in the
C p 2 W ( C O ) 2 structure. However, the C( l ) -C(5) ring exhibits characteristics more typical
of a r | 3 -Cp ligand than previously observed for any other structurally-characterized
C p 2 M o ( N O ) R species. The C(4) and C(5) atoms are 2.700(3) and 2.667(3) A away from
the M o atom, respectively, and the C(4)-C(5) bond length o f 1.317(5) A is significantly
shorter than the other C - C distances in the ring (1.392(5) to 1.407(5) A, average 1.401
A). The C ( l ) - C ( 2 ) - C ( 3 ) - C ( 4 ) and C(3 ) -C(2 ) -C( l ) -C(5 ) torsion angles o f 7.0(4)° and
8.0(4)°, respectively, are also reminiscent of, i f less pronounced than, those found in
C p 2 W ( C O ) 2 . While these distortions may be attributable to some inherent difference
between 4.4 and other C p 2 M o ( N O ) R complexes, they may also simply reflect crystal
95
packing forces, or the high quality o f the current structural determination relative to the
older X-ray crystallographic studies.
Stabilizing Effects of r j 2 -CH 2 Ph.
The coordinative and electronic saturation imparted by the rj 2-benzyl ligand is
deemed to be responsible for the stability o f compounds 4.1-4.3. O f the known
C p ' M ( N O ) R 2 (Cp ' = C5H5, C 5 M e 5 ; M = M o , W) complexes, the CpMo-containing
species are the most Lewis acidic, the most prone to M - C bond hydrolysis, and the most
thermally sensitive. 2 3 For example, CpMo(NO)(CH 2CMe3) 2 spontaneously undergoes
elimination of CMe4 via a - H abstraction at room temperature,2 4 while the
C p * W ( N O ) ( C H 2 C M e 3 ) 2 analog requires heating to 70°C for 2 days to duplicate this
mode of r e a c t i v i t y . u b e 2 5 Due to this increased reactivity, the methodologies developed for
other 16e C p ' M ( N O ) R 2 complexes may be used to generate C p M o ( N O ) R 2 compounds
with relatively small R groups (e.g. R = Ph, /?-tolyl, o-tolyl) at low temperatures, but
these species decompose at ambient temperature even in the presence o f trapping
agents.2 6
Figure 4.3.; 18e CpM(NO)R-containing complexes
The fragility o f the 16e C p M o ( N O ) R 2 species is in marked contrast to the robust
18e complexes shown in Figure 4.2. These compounds achieve a saturated electronic
configuration via additional a-donor ligation, 2 7 lower oxidation state,2 8 or multihapto
ancillary ligand bonding. 1 8 ' 2 0 A l l o f the species in Figure 4.2 are thermally stable at
ambient temperature, and several are remarkably tolerant to air and/or water. The range
96
of hydrocarbyl ligands available to these 18e complexes extends to groups which are
difficult to synthesize (e.g. Me , Ph) or entirely unknown (e.g. Et, 'Bu , C = C R , CeFs) for
the 16e C p ' M ( N O ) ( R ) R ' compounds.
Synthesis of [CpMo(NO)(CH 2Ph)L][OTfJ.
The stabilizing influence o f the benzyl ligand is also o f critical importance to
C p M o ( N O ) ( C H 2 P h ) O T f (4.5), synthesized in high yields as illustrated in equation 4.4.
^ A g O T f > ^ ( 4 4 )
| CI C H 2 C I 2 : E t 2 0 f | ^ O T f
P 8 ( ^ O 4.5
Silver salts have been previously shown to abstract halide ligands from
Cp'M(NO)-containing complexes. If this reaction is performed using AgBF4 in
acetonitrile, NCMe-solvated organometallic cations can be i so la ted . 1 6 a ^ 9 , 3 0 Treatment of
C p ' M ( N O ) 2 C l with A g B F 4 in C H 2 C 1 2 generates the reactive C p ' M ( N O ) 2 ( F B F 3 ) species 3 1
which have been used to synthesize lactones 3 1 6 and pyrones. 3 1 c Halide abstraction
reactions with A g 0 2 C R (R = 2-phenylbutyrate) 2 9 d or A g 0 3 S R (R = p-tolyl, camphor) 3 2 in
C H 2 C 1 2 yields neutral complexes with covalent M - 0 bonds.
L ike the reaction of C p ' C r ( N O ) 2 C l and A g O T f described in Chapter 3, the
synthesis of 4.5 is best accomplished in a 1:1 solvent mixture of C H 2 C 1 2 and E t 2 0 The
stability of CpMo(NO)(CH 2 Ph)(OTf) , which is isolable as a orange powder that can even
be handled briefly in air without deleterious effects, 3 3 can be contrasted with that o f
CpMo(NO)(r| 3-(Z)-crotyl)(OTs) (which was not isolated, but generated at low
temperature and used in situ)32* and that o f Cp* W ( N O ) ( C H 2 S i M e 3 ) O T f (which
decomposes upon being generated in non-coordinating arene or chlorinated solvents). 3 4
The existence o f a covalent M o - O S 0 2 C F 3 bond is inferred from the solubility o f 4.5 in
97
poor ion-supporting solvents such as toluene, C e D 6 and Et20, and by its IR spectrum
(Nujol) . 3 5 The highly electron-withdrawing nature of the O T f group in 4.5 is evidenced
by the v(NO) (Nujol) value of 1654 c m - 1 , much higher than that exhibited by the
hydrocarbyl products 4.1-4.4 or the benzyl chloride starting material.
O T f C H 2 C 1 2
O 8
© O T f 3 L (4.5) v L 4.6 P P h 3
N 4.7 N C 5 H 5
A s shown in equation 4.5, the rj 2-benzyl ligand in 4.5 proved to be less
susceptible to nucleophilic attack than the rj 3-allyl group in the corresponding
CpMo(NO)(C3H 5)(OTf) complex. Reaction of isolated or in-situ generated
CpMo(NO)(CH 2 Ph)(OTf) with P P h 3 or pyridine results in the formation of complexes 4.6
and 4.7, respectively. These [CpMo(NO)(CH 2 Ph)L][OTfJ salts display decreased
solubility compared to neutral complexes 4.1-4.5; 4.6 and 4.7 are only moderately soluble
even in T H F , and dissolve readily only in good ion-supporting solvents such as CH2CI2
and CH3NO2. Solid 4.6 and 4.7 appear to be relatively stable with respect to O2 and
water, since the yellow powders may be handled briefly in air without noticeable
decomposition. The decrease in the highest-frequency v(SO) band o f the triflate group
from 1321 c m - 1 in 4.5 to -1260 c m - 1 in 4.6 and 4.7 is consistent with the displacement of
the O T f from the M o center.3 5 The lU and 1 3 C N M R spectra of 4.6 and 4.7 help
demonstrate that only one L ligand is present in each molecule and that the rj 2-benzyl
interaction is preserved.
Deprotonation of [CpMo(NO)(CH2Ph)(PPh3)][OTfJ
In an attempt to form a benzylidene complex via intermolecular deprotonation, 1 6 0
cationic 4.6 was treated with K O ' B u in T H F . While this reaction did result in a decrease
98
in the frequency o f the v (NO) band to 1614 c m - 1 (THF) consistent with the formation
of a neutral species, the identical spectroscopic properties were obtained by treatment of
C p M o ( N O ) ( C H 2 P h ) C l with K O ' B u in T H F . The product o f both reactions appeared to be
CpMo(NO)(CH 2 Ph) (0 'Bu) , as indicated by signals attributable to benzyl and butoxide
ligands in the LH N M R spectra o f the crude reaction products (C6D6).24b'36
Substitution o f P P h 3 seemed to be avoided by using a more hindered base. The
anionic amide L i N ( S i M e 3 ) 2 reacted with 4.6 in T H F or C H 2 C 1 2 to form red solutions
which displayed a broad v(NO) band at -1600 c m - 1 . The red residue that remained after
the reaction solvent was removed in vacuo was readily dissolved in E t 2 0 , providing
another indication that a neutral species had been formed. Somewhat unexpectedly,
neutral amine bases such as H N ' P r 2 , N E t 3 , and D B U also reacted with 4.6 in C H 2 C 1 2 ,
resulting in complete consumption o f starting material, as determined by solution TR
spectroscopy, and the formation o f red solutions with the same broad v(NO) band at
-1600 cm" I
© O T f 9 N R ,
C H 2 C 1 2
PPh3 (4.6)
The transformation shown in equation 4.6 is surprising, since H N R 3
+ cations are
more acidic than the neutral R O H species which have been demonstrated to protonate the
neopentylidene ligand o f C p M o ( N O ) ( = C H ' B u ) 2 4 b It is conceivable, however, that the
replacement o f the ' B u group with the more electron-withdrawing Ph moiety might
enhance the acidity o f the methylene H ' s o f 4.6. Unfortunately, conclusive evidence that
the reaction of 4.6 with potential Brensted bases actually forms the desired benzylidene
complex has yet to be obtained. The ' H N M R spectra o f the crude reaction mixtures were
devoid o f signals characteristic of alkylidene H 's . None o f the attempts to purify,
99
chromatograph, crystallize or precipitate the nitrosyl-containing product resulting from
the attempted reaction shown in equation 4.6 resulted in any tractable material.
Summary The failure o f this synthetic study to generate CpMo(NO)(=CHPh) via
intermolecular deprotonation leaves the synthetic utility o f the [CpMo(NO)(CH2Ph)]+
fragment yet to be demonstrated. Nevertheless, the following properties o f these
complexes have been successfully ascertained:
1. The 3e-donor CFfcPh ligand is capable o f stabilizing complexes containing
hydrocarbyl ligands such as M e , Ph, and C = C P h that are otherwise too
reactive to isolate as 16e CpMo(NO)R2 species.
2. The rj 2-benzyl interaction can be displaced by the introduction o f other
anionic ligands capable of multiple-electron donation, as evidenced by the
solid-state molecular structure of Cp2Mo(NO)(CH 2Ph).
3. Displacement o f the O T f group o f 4.5 can be achieved without the hindrance
o f nucleophilic attack at the 3e-donor benzyl ligand.,
100
Future Work
C a r b e n e t r a n s f e r f r o m C p M o ( N O ) ( = C H P h ) ( L 2 ) i n t e rmed ia te s .
Although the intermolecular deprotonation reactions described in this chapter
were not successful, modification of the organometallic cation, Bransted base and/or
reaction conditions may eventually provide the desired reactivity. One possible alteration
is to employ a bidentate a-donor ligand such as dppe or bipy instead of PPh3 or pyridine.
These chelating ligands would force the benzyl group to adopt a ry1 -coordination mode,
and would be expected to render the complex less prone to reduction or nucleophilic
attack at the metal center. These properties should permit the use of more potent bases
(e.g. "BuLi) without attendant reductive decomposition or L ligand substitution. If
formed, the resulting benzylidene complex (I) would be expected to be highly reactive
due to the lack of an available Tc-symmetry orbital on the Mo atom.37 One possible mode
of reactivity of I is transfer of the "CHPh" moiety to an olefin to form a cyplopropane
derivative38 and the known3 9 Mo(0), d 6 CpMo(NO)(dppe) species (Equation 4.7).
l e R e d u c t i o n o f [ C p M o ( N O ) ( C H 2 P h ) L ] [ O T f ] .
A potential mode of reactivity for [CpMo(NO)(CH2Ph)L][OTf] salts is their
reduction to paramagnetic, 17e, Mo(I), d 5 species (Equation 4.8). Analogous
CpCr(NO)(L)R complexes have been investigated,40 and the synthesis of
[Cp*Mo(NO)(CH2SiMe3)2]~ has also been recently reported.160
101
A preliminary attempt to generate the neutral CpMo(CH 2 Ph) (PPh 3 ) was
unsuccessful. The reaction of 4.6 with sodium naphthalide in T H F resulted in the
consumption o f starting material without producing any nitrosyl-containing products, as
determined by the absence o f v(NO) bands in the IR spectrum o f the final reaction
mixture.
Ligand-based reactivity of CpMo(NO)(CH 2Ph)R.
M u c h of the chemistry that has been explored for C p ' M ( N O ) R 2 complexes has
involved "metal-based reactivity". Typically, small molecules form 18e adducts by
coordination to the metal center, often followed by reaction with the M - R o bonds. 2 3 In
fact, the chemistry o f the benzyl complexes remains comparatively underdeveloped
because the r | 2 - C H 2 P h interaction effectively saturates the complex, thereby hampering
adduct formation.
The facile exchange between r\l- and rj2-coordination modes, the stabilization of
unusual hydrocarbyl ligands, and the deterrence of nucleophilic attack at the metal center
all combine to make the benzyl group an excellent ancillary ligand to assist in the
exploration o f the "ligand-based reactivity" of C p ' M ( N O ) ( C H 2 P h ) R . This mode o f
reactivity is exemplified by an impressive series of papers from Templeton and co
workers describing the chemistry of Tp*W(CO)(RC=CR')-containing species. 4 1 These
studies exploit the ability o f the alkyne ligand to donate two or four electrons to the
W(II), d 4 metal center, 4 2 thereby maintaining a saturated 18e configuration while
coordinated ligands are elaborated via sequential nucleophilic and electrophilic addition
(4.8)
102
reactions. One potential reaction o f 4.3 is thus the attack o f R~ on the B - C o f the
acetylide ligand, as shown in equation 4.9.
Cp'M(NO)(CH 2 Ph)Cl as Cp2Zr(CH3)Cl analog.
Another means o f harnessing the characteristic properties o f the benzyl ligand to
develop the utility o f the C p ' M ( N O ) fragment is suggested by the extensive synthetic
methodology developed by Buchwald and coworkers employing the Cp2Zr(CH3)Cl
precursor (Equation 4.10). 3 c The chloride ligand of the zirconocene precursor is replaced
with a hydrocarbyl group bearing 3~H's via standard salt metathesis techniques.
Thermolysis o f this species induces the elimination of methane and the unsaturated
organic moiety thus formed, stabilized by coordination to the Cp2Zr fragment, is rendered
prone to a wide range of coupling reactions.
(4.10)
Extension of this methodology to Cp'M(NO)-containing complexes has been
pioneered by Sean L u m b , 1 2 d but this technique is currently restricted to the
Cp*W(NO)(CH 2 SiMe 3 )(H2C=CPh) species. Synthesis o f C p * W ( N O ) ( C H 2 S i M e 3 ) R
complexes with other hydrocarbyl ligands containing P ~ H atoms has been effectively
precluded by the apparent thermal instability o f these species. It seems possible that the
103
rj2-benzyl group might render Cp*W(NO)(CH 2 Ph)R compounds sufficiently stable to
permit their isolation, yet reactive enough to exhibit the required B - H elimination
chemistry. Indeed, the varying thermal stability profiles exhibited by C p ' M ( N O ) -
containing species may well be utilized to selectively tune the toluene-elimination
reaction to a specific temperature range.
Synthesis and reactivity of Cp2M(NO)H.
The synthesis and structural charaterization of 4.4 may incite renewed interest in
Cp2M(NO)X species. A n intriguing target complex in this class is the hydride species,
C p 2 M ( N O ) H , which may be accessible either from C p 2 M ( N O ) I 2 0 a ' 4 3 or from the currently
unknown C p 2 M ( N O ) C l precursors (Scheme 4.4). Unlike C p * W ( N O ) ( C H 2 S i M e 3 ) H ,
which decomposes in the absence o f trapping reagents due to the reductive elimination of
S i M e 4 , 3 6 c C p 2 M o ( N O ) H species are potentially "self-trapping" as the CpM(NO)(r | 4 -
C5H6) complexes. 4 4 The possibility of a dynamic equilibrium between the M(II), d 4
hydride and the M(0) , d 6 diene compounds has exciting implications o f the reactivity o f
these species with small molecules. Likely reactivity modes include (i) ligand
coordination to form a M(II), rfVn1 complex, 1 1* (ii) insertion o f unsaturated organic
species into the M - H bond, 3 6 c (iii) intermolecular C - H activation, 1 1 (iv) coupling
reactions, 1 2 and (v) ligand coordination to form a M(0), t|5/r|2 compound. 4 4" Selectivity
between these modes may be influenced by the identity o f the metal center or the type of
cyclopentadienyl ligand used, especially the indenyl and fluorenyl derivatives.
Scheme 4.4. Possible reactivity modes of Cp2M(NO)H
105
Experimental
Methods
A l l reactions and subsequent manipulations were conducted under anaerobic
conditions using an atmosphere of N 2 . The complexes CpMo(NO)(CH2Ph)Cl 1 and
CpMo(NO)(C3Hs)I,4 5 were prepared by the published procedures. A l l other reagents were
used as received from commercial suppliers. Filtrations were performed through Celite (1
x 2 cm) supported on a medium porosity frit unless otherwise specified. For low
temperature reactions, solvents were transferred via trap-to-trap distillation from the
drying reagent directly onto the reactants contained in a flask cooled by a liquid nitrogen
bath.
The color, yield, and elemental analysis data for the new compounds in this
chapter are listed in Table 4.2. Infrared and mass spectral data are collected in Table 4.3,
and Table 4.4 shows the *H and 1 3 C { ' H } N M R data.
Synthesis of CpMo(NO)(CH2Ph)R (R = Me (4.1), Ph (4.2), C=CPh (4.3), Cp (4.4)).
The synthesis o f C p M o ( N O ) ( C H 2 P h ) M e (4.1) is described as a representative
example. T H F (-25 mL) was vacuum transferred onto C p M o ( N O ) ( C H 2 P h ) C l (0.186 g,
0.59 mmol) and Me 2Mg-x(dioxane) (0.089 g, 0.59 mmol). The solution was allowed to
warm slowly to room temperature. After -1 h, the solvent was removed from the red
solution in vacuo. The residue was extracted with a 2:1 solvent mixture o f E t 2 0 : C H 2 C l 2
and filtered. The solvent was again removed in vacuo, and the residue was triturated with
E t 2 0 twice to afford CpMo(NO)(CH 2 Ph)Me (4.1) as an orange powder (0.110 g, 63%
yield). Analytically-pure samples were obtained by extraction of the crude powder with a
1:1 solvent mixture of THF:toluene, followed by chromatography using an Alumina I
column and toluene as eluant. The resulting orange eluate was then reduced in volume in
106
vacuo, hexanes were added, and the solution was chilled to - 3 0 ° C to obtain orange
crystals.
CpMo(NO)(CH 2 Ph)Ph (4.2), C p M o ( N O ) ( C H 2 P h ) C = C P h (4.3), and
C p 2 M o ( N O ) C H 2 P h (4.4) were synthesized from C p M o ( N O ) ( C H 2 P h ) C l in an analogous
manner, using Ph 2Mg-x(dioxane), L i C = C P h , and N a ( D M E ) C p , respectively, instead of
Me 2Mg-x(dioxane) as the alkylating reagent.
Synthesis o f C p M o ( N O ) ( C H 2 P h ) O T f (4.5).
First C H 2 C 1 2 (20 mL) and then E t 2 0 (20 mL) were added via syringe to
C p M o ( N O ) ( C H 2 P h ) C l (0.433 g, 1.36 mmol) and A g O T f (0.350 g, 1.36 mmol). After -10
min, a flocculent white precipitate had formed in the orange solution. The solution was
filtered, and the Celite plug was washed with E t 2 0 ( 3 x 5 mL). The combined filtrates
were taken to dryness in vacuo, and the residue was triturated twice with E t 2 0 (10 mL) to
obtain CpMo(NO)(CH 2 Ph)(OTf) (4.5) as an orange powder (0.529 g, 83% yield).
Synthesis o f [ C p M o ( N O ) ( C H 2 P h ) ( L ) ] [ O T f ] (L = PPh 3 (4.6), N C 5 H 5 (4.7)).
Both triflate salts may be obtained from either isolated or in situ generated 4.5.
The two-step synthesis o f [CpMo(NO)(CH 2 Ph)(PPh 3 )][OTf] from C p M o ( N O ) ( C H 2 P h ) C l
is described as a representative example. First C H 2 C 1 2 (10 mL) and then E t 2 0 (10 mL)
were added to C p M o ( N O ) ( C H 2 P h ) C l (0.115 g, 0.36 mmol) and A g O T f (0.092 g, 0.36
mmol). After - 1 0 min, the solution was filtered through Celite onto solid P P h 3 (0.096 g,
0.36 mmol). The solution was stirred briefly, then hexanes (10 mL) were added. The total
volume o f the solution was reduced to -25 mL, and chilled to - 3 0 ° C overnight to obtain
[CpMo(NO)(CH 2 Ph)(PPh 3 )][OTf] (4.6) as a yellow powder (0.198 g, 79% yield).
107
Characterization Data
Table 4.2. Numbering Scheme, Color, Yield and Elemental Analysis.
Complex Cmpd #
Color (yield, %)a
Elemental analysis found (calcd) Complex Cmpd #
Color (yield, %)a C H N
CpMo(NO)(Bz)Me 4.1 orange (63) 52.67 (52.54) 5.11 (5.09) 4.78 (4.71)
CpMo(NO)(Bz)Ph 4.2 orange (72) 60.03 (60.18) 4.75 (4.77) 3.89(3.90)
CpMo(NO)(Bz)C=CPh 4.3 orange (68) 62.73 (62.67) 4.45 (4.47) 3.66 (3.65)
Cp2Mo(NO)Bz 4.4 dk green (65) 58.29 (58.80) 4.84 (4.93) 3.91 (4.03)
CpMo(NO)(Bz)OTf 4.5 orange (83) 36.35 (36.21) 2.81 (2.80) 3.16 (3.25)
[CpMo(NO)(Bz)(PPh3)] [OTf] 4.6 yellow (79) 53.31 (53.69) 3.95 (3.92) 1.89 (2.02)
[CpMo(NO)(Bz)(py)][OTf] 4.7 yellow (82) 42.51 (42.36) 3.29 (3.36) 5.22 (5.49)
a Yield calculated from crude isolated product
Table 4 .3 . Infrared v(NO) and Mass Spectral Data.
Complex Cmpd
#
IR(v(NO) cm"1) F A B / M S (m/z) Complex Cmpd
# Nujol C H 2 C I 2 P +
CpMo(NO)(Bz)Me 4.1 1576 1591 299
CpMo(NO)(Bz)Ph 4.2 1592 1603 361
CpMo(NO)(Bz)f>CPh 4.3 1604 1620 385
Cp2Mo(NO)Bz 4.4 1592 1607 349
CpMo(NO)(Bz)OTf 4.5 1654 1657 284
[CpMo(NO)(Bz)(PPh3)] [OTf] 4.6 1655 1655 546
[CpMo(NO)(Bz)(py)] [OTf] 4.7 1643 1651
108
Table 4.4. lH and 1 3 C NMR Data.
Compound (solvent)
*H N M R (8) 13C CB) N M R (8)
4.1 (C<P6)
7.5-6.8 (m, Ph), 4.99 (s, 5H, C5//5), 3.19 (d, IH CHH), 2.21 (d, IH, CH//), -0.88 (s, 3H, CH3)
131.0-127.5 (Ph), 112.2 (Cipso), 99.3 (C5H5), 37.8 (CH2), 1.34 (CH3)
4.2 (C6D6)
7.2-6.4 (m, Ph), 5.04 (s, 5H, C5//5), 3.12 (d, IH C//H), 2.83 (d, IH, CHH)
196.9 (C ipso Ph), 140.4-124.2 (Ph), 112.1 (C ipso Bz ) , 100.1 (C5H5),
40.1 (CH2) 4.3
(CsDs) 7.6-7.0 (m, Ph), 5.13 (s, 5H, C5//5), 3.19 (d, IH CHH), 2.66 (d, IH, CHH)
136:1-131.4 (Ph), 112.7 (C ipso Bz ) , 108.8 (C=C), 106.1 (CsHs), 42.8 (CH2)
4.4 (Q>D6)
7.6-7.0 (m, Ph), 5.17 (s, 10H, C5//5), 3.45 (s, 2H CH2)
152.3 (CipsoBz), 128.0, 127.7, 123.8 (CPh), 109.0 (C5H5), 25.1 (C772)
4.5 (C6D6)
7.5-6.8 (m, Ph), 5.04 (s, 5H, C5//5), 3.12 (d, IH C//H), 2.83 (d, IH, CHH)
137.2-127.7 (Ph), 112.4 (Cipso), 102.7 (C5H5), 50.5 (CH2)
4.6 (CD3NO2)
7.8-6.5 (m, Ph), 5.78 (s, 5H, C5H5), 3.92 (d, IH CHH), 3.63 (d, 1H, CHH)
136.3-126.8 (Ph), 110.1 (Cipso), 103.1 (C5H5), 44.7 (CH2)
4.7 (CD 3N0 2)
7.9-7.0 (m, Ph), 6.18 (s, 5H, C5H5), 3.95 (d, IH CHH), 3.78 (d, IH, CHH)
156.1-127.8 (Ph, NC5H5), 112.7 (C ipso), 104.9 (C5H5), 50.2 (CH2)
109
References and Notes
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L i u , R. -S. Organometallics 1997,16, 4232.
(8) (a) Ipaktschi, J.; Hartmann, A . J. Organomet. Chem. 1992, 431, 303. (b)
Ward, Y . D . ; Villanueva, L . A . ; Allred, G . D . ; Payne, S. C ; Semones, M . A . Liebeskind,
L . S. Organometallics 1995,14, 4132. (c) Ward, Y . D . ; Villanueva, L . A . ; Allred, G . D . ;
Liebeskind, L . S. J. Am. Chem. Soc. 1996,118, 897. (d) Villanueva, L . A . ; Ward, Y . D . ;
Lachiotte, R. ; Liebeskind, L . S. Organometallics 1996,15, 4190. (e) Ward, Y . D . ;
Villanueva, L . A . ; Allred, G . D . ; Liebeskind, L . S. Organometallics 1996,15, 4201. (f)
Pearson, A . J.; Neagu, I. B . ; Pinkerton, A . A . ; Kirschbaum, K . ; Hardie, M . J.
Organometallics 1997,16, 4346. (g) Sapunov, V . N . ; Slugov, C ; Mereiter, K . ; Schmid,
R.; Kirchner, K . J. Chem. Soc. Dalton Trans. 1997, 3599. (h) Pearson, A . E . ; Schoffers,
E . Organometallics 1997,16, 5365.
(9) (a) Mauthner, K . ; Slugovc, C ; Mereiter, K . ; Schmid, R.; Kirchner, K .
Organometallics 1996,15,181. (b) Slugovc, C ; Mauthner, K . ; Mereiter, K . ; Schmid, R.;
Kirchner, K . Organometallics 1996, 75, 2954.
I l l
(10) Frohnapfel, D . S.; White, P. S.; Templeton, J. L . ; Riiegger, H . ; Pregosin, P.
S. Organometallics 1997,16, 3737.
(11) C - H activation by C p * W ( N O ) L (L = P M e 3 , l l a C6H4, u b = C H ' B u , l l c f
H C = C P h , I l d P P h 3
U e ) : (a) Legzdins, P.; Martin, J. T.; Einstein, F W . B . ; Jones, R. H .
Organometallics 1987, 6, 1826. (b) Debad, J. D . Ph.D. Thesis, University o f British
Columbia, 1994, pp. 112-115. (c) Ross, K . J. Ph.D. Thesis, University o f British
Columbia, 1994, pp. 175-180. (d) Debad, J. D . ; Legzdins, P.; Lumb, S. A . ; Batchelor, R.
J.; Einstein, F. W . B . J. Am. Chem. Soc. 1995,117, 3288. (e) Debad, J. D . ; Legzdins, P . ;
Lumb, S. A . ; Batchelor, R. J.; Einstein, F. W . B . Organometallics 1995,14, 2543. (f)
Tran, E . ; Legzdins, P. J. Am. Chem. Soc. 1997,119, 5071.
(12) C - C formation by C p ' M ( N O ) L : (a) Christensen, N . J.; Legzdins, P.;
Einstein, F. W . B . ; Jones, R. H . Organometallics 1991, 20, 3070. (b) Christensen, N . J.;
Legzdins, P.; Trotter, J.; Yee, V . C. Organometallics 1991,10, 4021. (c) Debad, J. D . ;
Legzdins, P.; Young, M . A . ; Batchelor, R. J.; Einstein, F. W. B . J. Am. Chem. Soc. 1993,
115, 2051. (d) Legzdins, P.; Lumb, S. A . Organometallics 1997,16, 1825.
(13) (a) Schilling, B . E . R.; Hoffmann, R.; Lichtenberger, D . L . J. Am. Chem. Soc.
1979,101, 585. (b) Schilling, B . E . R.; Hoffmann, R.; Faller, J. W . J. Am. Chem. Soc.
1979,101, 592.
(14) (a) Seddon, D . ; Ki ta , W . G . ; Bray, J.; McCleverty, J. A . Inorg. Synth. 1976,
16, 24. (b) Dryden, N . H . ; Legzdins, P.; Batchelor, R. J.; Einstein, F. W . B .
Organometallics 1991,10, 2077.
(15) (a) Debad, J. D . ; Legzdins, P . ; Batchelor, R. J.; Einstein, F. W . B .
Organometallics 1992, 11, 6. (b) Brunei, N . ; Debad, J. D . ; Legzdins, P.; Trotter, J.;
Veltheer, J. E . ; Yee, V . C. Organometallics 1993,12, 4572. (c) Debad, J. D . ; Legzdins,
P.; Lumb, S. A . ; Batchelor, R. J.; Einstein, F. W. B . Organometallics 1995,14, 2543.
112
(16) (a) Debad, J. D.; Legzdins, P.; Rettig, S. J.; Veltheer, J. E . Organometallics
1993,12, 2714. (b) Legzdins, P.; Sayers, S. F. Organometallics 1996,15, 3907. (c)
Legzdins, P.; Sayers, S. F. Chem. Eur. J. 1997, 3, 1579.
(17) Debad, J. D.; Legzdins, P.; Batchelor, R. J.; Einstein, F. W. B.
Organometallics 1993,12, 2094. For the related synthesis of Cp*M(NO)(CH 2CMe 3)R
(M = Mo, W) asymmetric bis(alkyl) complexes, see ref. 16a.
(18) CpW(NO)(C 3 H 5 ) (CsCR): Ipaktschi, J.; Mirzaei, F.; Demuth-Eberle, G. J.;
Beck, J.; Serafin, M . Organometallics 1997,16, 3965.
(19) Legzdins, P.; Jones, R. H.; Phillips, E. C ; Yee, V. C.; Trotter, J.; Einstein, F.
W. B. Organometallics 1991,10, 986.
(20) Cp2Mo(NO)R: (a) King, R. B. Inorg. Chem. 1968, 7, 90. (b) Cotton, F. A.;
Legzdins, P. J. Am. Chem. Soc. 1968, 90, 6232. (c) Calderon, J. L . ; Cotton, F. A.;
Legzdins, P. J. Am. Chem. Soc. 1969, 91, 2528. (d) Cotton, F. A.; Rusholme, G. A. J. Am.
Chem. Soc. 1972, 94, 402. (e) Lauher, J. W.; Hoffmann, R. J. Am. Chem. Soc. 1976, 98,
1729. (f) de Jesus, E . ; Vazquez de Miguel, A.; Royo, P.; Lanfredi, A. M . M . ; Tiripicchio,
A. J. Chem. Soc, Dalton Trans. 1990, 2779.
(21) Cp 2Mo(NO)X: (a) Hunt, M . M . ; Kita, W. G.; Mann, B. E.; McCleverty, J. A.
J. Chem. Soc, Dalton Trans. 1978, 467. (b) Hunt, M . M . ; Kita, W. G.; McCleverty, J. A.
J. Chem. Soc, Dalton Trans. 1978, 474. (c) Hunt, M . M . ; McCleverty, J. A. J. Chem.
Soc, Dalton Trans. 1978, 480.
(22) Huttner, G.; Brintzinger, H. H.; Bell, L. G.; Friedrich, P.; Bejenke, V.;
Neugebauer, D. J. Organomet. Chem. 1978,145, 529.
(23) Legzdins, P.; Veltheer, J. Acc Chem. Res. 1993, 26,41.
113
(24) (a) Legzdins, P.; Rettig, S. J.; Veltheer, J. E . ; Batchelor, R. J.; Einstein, F. W.
B . Organometallics 1993,12, 3575. (b) Legzdins, P.; Veltheer, J. E . ; Young, M . A .
Batchelor, R. J.; Einstein, F. W . B . Organometallics 1995 ,74, 407.
(25) Thermolysis o f C p M o ( N O ) ( C H 2 P h ) 2 in C 6 D 6 at 95°C for 40 h resulted in a
slight color change (orange to red), but no difference in the IR or ' H N M R spectra of the
sample.
(26) Dryden, N . H . ; Legzdins, P.; Rettig, S. J.; Veltheer, J. E . Organometallics
1992, 11, 2583.
(27) CpMo(NO)(Me)(L)R: Alegre, B . ; de Jesus, E . ; Vazquez de Miguel , A . ;
Royo, P.; Lanfredi, A . M . M . ; Tiripicchio, A . J. Chem. Soc, Dalton Trans. 1988, 819.
(28) C p M o ( N O ) 2 R : Hoyano, J. K . ; Legzdins, P.; Mali to, J. T. J. Chem. Soc,
Dalton Trans. 1975, 1022.
(29) (a) Legzdins, P.; Nurse, C. R. Inorg. Chem. 1982, 21, 3110. (b) Chin, T. T.;
Legzdins, P.; Trotter, J.; Yee, V . C. Organometallics 1992,11, 913. (c) Legzdins, P.;
Rettig, S. J.; Sayers, S. F. J. Am. Chem. Soc. 1994,116, 12105. (d) Dryden, N . H . ;
Legzdins, P.; Sayers, S. F . ; Trotter, J.; Yee, V . C. Can. J. Chem. 1995, 73, 1035.
(30) McCleverty, J. A . ; Murray, A . J. Trans. Met. Chem. 1979, 4, 273.
(31) (a) Legzdins, P.; Martin, D . T. Organometallics 1983, 2, 1785. (b) Legzdins,
P.; Richter-Addo, G . B . ; Einstein, F . W . B . ; Jones, R. H . Organometallics 1990, 9,431.
(c) Legzdins, P.; M c N e i l , W. S.; Vessey, E . G;Batchelor , R. J.; Einstein, F. W . B .
Organometallics 1992,11, 2718.
(32) (a) Faller, J. W. ; DiVerdi , M . J.; John, J. A . Tetrahedron Lett. 1991, 32,
1271. (b) Faller, J. W. ; Nguyen, J. T.; Ell is , W ; Mazzieri , M . R. Organometallics 1993,
12, 1434. (c) Faller, F . W. ; Chase, Mazzieri , M . R. Inorg. Chim. Acta 1995, 229, 39.
114
(33) Exposure o f 4.5 to air for 5 days resulted in no visible change in the
appearance of the orange powder, but its Nujol IR spectrum changed significantly. The
v(NO) band o f the air-exposed sample had shifted from 1654 c m - 1 to 1646 c m - 1 , and the
highest-frequency O T f band shifted from 1321 c m - 1 to 1276 c m - 1 . One possible
explanation for these spectroscopic changes is the displacement of the O T f ligand o f 4.5
by atmospheric H 2 0 to form [CpMo(NO)(CH2Ph)(H 20)][OTf] in the solid state.
(34) Legzdins, P.; Sayers, S. F. unpublished results.
(35) Lawrance, G . A . Chem. Rev. 1986, 86, 17.
(36) Cp 'M(NO)(R) (OR ' ) : (a) Legzdins, P.; Lundmark, P. J.; Rettig, S. J.
Organometallics 1993,12, 3545. (b) Legzdins, P.; Rettig, S. J.; Ross, K . J.
Organometallics 1994,13, 569. (c) Debad, J. D . ; Legzdins, P.; Lumb, S. A . ; Batchelor,
R. I ; Einstein, F. W . B . Organometallics 1995,14, 2543.
(37) Bonding in CpML4 complexes: (a) Kubacek, P.; Hoffmann, R.; Havlas, Z .
Organometallics 1982, 1, 180. (b) Poli , R. Organometallics 1990, 9, 1892.
(38) Cyclopropanes from transition-metal-carbene complexes and olefins: (a)
Brookhart, M . ; Studabaker, W. B . Chem. Rev. 1987, 87, 411. (b) McVica r , W . K . ;
Hubbard, J. L . Organometallics 1990, 9, 2683. (c) Gunnoe, T. B . ; Surgan, M . ; White, P.
S.; Templeton, J. L . ; Casarrubios, L . Organometallics 1997,16, 4865.
(39) Brunner, H . J. Organomet. Chem. 1969,16, 119.
(40) (a) Legzdins, P.; Shaw, M . J. J. Am. Chem. Soc. 1994,116, 7700. (b)
Legzdins, P.; Shaw, M . J.; Batchelor, R. J.; Einstein, F. W . B . Organometallics 1995,14,
4721.
(41) Tp*W(CO)(RC=CR')-containing species: (a) Feng, S. G . ; Philipp, C. C ;
Gamble, A . S.; White, P. S.; Templeton, J. L . Organometallics 1991,10, 3504. (b)
Collins, M . A . ; Feng, S. G ; White, P. A . ; Templeton, J. L . J. Am. Chem. Soc. 1992,114,
115
3771. (c) Feng, S. G ; Templeton, J. L. Organometallics 1992,11, 1295. (d) Feng, S. G.;
Templeton, J. L. Organometallics 1992,11, 2168. (e) Feng, S. G ; White, P. S.;
Templeton, J. L. J. Am. Chem. Soc. 1992,114, 2951. (f) Caldarelli, J. L. ; White, P. S.;
Templeton, J. L . J. Am. Chem. Soc. 1992,114, 10097. (g) Feng, S. G ; White, P. S.;
Templeton, J. L. Organometallics 1993,12, 1765. (h) Feng, S. G ; White, P. S.;
Templeton, J. L. Organometallics 1993, 72, 2131. (i) Caldarelli, J. L. ; Wagner, L. E.;
White, P. S.; Templeton, J. L. J. Am. Chem. Soc. 1994,116,2%1%. (j) Feng, S. G.; White,
P. S.; Templeton, J. L . Organometallics 1995, 14, 5184. (k) Gunnoe, T . B.; White, P. S.;
Templeton, J. L . J. Am. Chem. Soc. 1996,118, 6916. (1) Francisco, L. W.; White, P. S.;
Templeton, J. L. Organometallics 1996,15, 5127. (m) Wells, M . B.; White, P. S.;
Templeton, J. L. Organometallics 1997,16, 1857. (n) Francisco, L. W.; White, P. S.;
Templeton, J. L. Organometallics 1997,16, 2547. (o) Gunnoe, T . B.; White, P. S.;
Templeton, J. L . Organometallics 1997,16, 3794.
(42) Templeton, J. L . Adv. Organomet. Chem. 1989, 29, 1.
(43) Cp2W(NO)I: Legzdins, P.; Martin, D . T . ; Nurse, C. R . Inorg. Chem. 1980,
19, 1560.
(44) While Cp'M(NO)(ri4-diene) complexes typically adopt a v^-trans
configuration, 1 2 a b ' 3 6 c ' 4 4 a c r\4-cis complexes are also known:4 4 b c (a) Hunter, A. D.;
Legzdins, P.; Nurse, C. N ; Einstein, F. W. B.; Willis, A. C. J. Am. Chem. Soc. 1985, 707,
1791. (b) Hunter, A. D . ; Legzdins, P.; Einstein, F. W. B.; Willis, A. C ; Bursten, B. E.;
Garter, M , G. J. Am. Chem. Soc. 1986,108, 3843. (c) Christensen, N. I ; Hunter, A. D.;
Legzdins, P. Organometallics 1989, 8, 930.
(45) Faller, J. W.; Chodosh, D . F.; Katahira, D . J. Organomet. Chem. 1980,187,
227.
116
Chapter 5: Towards CpCr(NO)R2
Introduct ion 117
Results and Discussion 120
Summary 136
Future W o r k 137
Exper imenta l 142
References and Notes 148
117
Introduction
Olefin polymerization is one of the most important commercial applications of
transition-metal chemistry. Industrial chemists have spent decades empirically optimizing
reaction conditions for multi-component, heterogeneous catalysts based on T i and Cr . 1
Recently, academic chemists have conducted many synthetic, mechanistic and theoretical
studies of single-site, homogeneous systems. These soluble, well-defined complexes are
more amenable to the spectroscopic monitoring, detailed analysis, and computational
modeling required to elucidate the fundamental principles which govern the
polymerization process. 2 The enormous potential for rational design of homogeneous
catalysts is a result of several factors: (a) chain growth occurs in the coordination sphere
of the metal, (b) the ligand environment exercises a powerful influence on the overall
polymer structure by directing the insertion of each monomer, (c) the physical properties
of the polymer depend heavily on the polymer structure. The combination of the low cost
of the feedstock monomers, the high activity and efficiency o f the catalysts, and the
potential commercial value of new classes o f polymers combine to justify the
considerable effort invested in the design o f elaborate ligand systems.
But while homogeneous Group 4 catalysts have received intense academic
scrutiny, comparatively little attention has been paid to homogenous, Cr-based olefin
polymerization. 4 Studies o f C r - C o bonds have been hampered by the paramagnetism of
monomeric, midvalent C r alkyl complexes. Unlike C p 2 Z r R 2 and related d°, Group 4
catalyst precursors, organometallic Cr(III) species are uniformly paramagnetic with
broad, shifted signals in their N M R spectra. 5 ' 6 A l k y l compounds o f Cr(II) are either 7 R Q 11
electronically and coordinatively saturated, dimenc, or paramagnetic.
It is in this context that the potential significance o f C p C r ( N O ) R 2 complexes can
best be appreciated. I f the synthesis o f these unsaturated, monomeric, diamagnetic Cr(II)
alkyl compounds could be attained, they would present intriguing possibilities for the
118
synthetic, spectroscopic, mechanistic and theoretical investigation of C r - C a bonds.
When the work described in this chapter was initiated, chromium alkyl nitrosyl
complexes were restricted to the +1 and 0 oxidation states. 1 2 ' 1 3 The molybdenum and
tungsten congeners of the desired Cr(II) bis(alkyl) compounds are accessible in two steps
from the zero-valent Cp 'M(NO)(CO )2 species, as shown in Scheme 5.1. 1 4 While the
analogous Cp 'Cr(NO)(CO )2 compounds are readily available, their oxidative
halogenation reactions diverge from those observed for M o and W (Scheme 5.1).
Scheme 5.1. Oxidative halogenation of C p M ( N O ) ( C O ) 2 ( M = Cr, M o , W ) and
subsequent alkylation reactions
The lack of a direct route to Cr(H) dihalo complexes required the development of
new synthetic strategies in order to obtain the desired Cp'Cr(NO)R .2 species. This chapter
presents my work towards this goal, as well as some unanticipated products and insights
that this investigation revealed. For example, while the study of the oxidative
halogenation o f Cp 'Cr(NO)(CO )2 complexes did not result in the isolation o f stable
119
C p ' C r ( N O ) X 2 products, it helped guide the synthesis o f new [Cp*Cr(NO)(u,-X)] 2 ( X
= I (5.1), CI (5.2)) dimers. The inorganic complex Cr(NO)(N 'Pr 2 ) 3 served as a precursor
to Cr(U)(NO) complexes via amine elimination and salt metathesis reactions. O f
particular interest is the highly unsaturated bis(alkyl) compound,
C r ( N O ) ( N ' P r 2 ) ( C H 2 S i M e 3 ) 2 (5.3). The final precursor species examined in this study was
C p * C r ( N O ) 2 C l , which was used to generate Cp*Cr (NO)(CH 2 Ph) 2 in two steps via a
methodology that bypasses the unstable dihalo compounds.
120
Results and Discussion
Oxidation of Cp'M(NO)(CO) 2 with Halogen Sources.
The first example of the oxidative halogenation of zero-valent C p ' M ( N O ) ( C O ) 2
complexes ( M = Cr. M o , W ) was reported in 1967 when K i n g reacted C p M o ( N O ) ( C O ) 2
with I 2 in C H 2 C 1 2 to form [CpMo(NO)I(p.-I)] 2 . 1 5 This reaction was extended to related
Group 6 compounds by the research groups of McCleverty, Royo, and Legzdins, and was
shown to be remarkably general for Cp and Cp* derivatives o f M o and W using I 2 , B r 2 ,
C l 2 and PCI5 as halogen sources. 1 6 ' 2 1 In contrast, no analogous neutral dihalo Cr(II)
complexes have been prepared. Only in the single, specific case illustrated in Scheme 5.1
(i.e. C p ' = Cp, X 2 = I 2) are any mononitrosyl chromium products isolable from these
reactions. 2 2 In all other cases, C p ' C r ( N O ) 2 X species are obtained in < 50% yield, and no
other N O - or CO-containing products are observed in the final reaction mixtures by IR
spectroscopy. 2 2 ' 2 3
The initial stages of my own investigation o f this reaction were focused on
generating Cr(II) nitrosyl dihalo species, particularly from Cp*Cr (NO)(CO) 2 which had
not been as intensely studied as the Cp analog. 2 3 The failure o f these reactions to generate
[ C p ' C r ( N O ) X 2 ] n products is consistent with the nitrosyl-ligand lability which was
subsequently attributed to these presumably high-spin Cr(II) species. 2 4 When combined
with the qualitative reaction rates that can be inferred by IR spectroscopy, the postulated
NO loss from triplet Cr(U) intermediates helps explain the lack of generality o f the
reactions o f C p ' C r ( N O ) ( C O ) 2 with X 2 sources in non-coordinating solvents.
Examination o f the published 2 2 IR monitoring of the reaction o f CpCr (NO)(CO) 2
with I 2 in C H 2 C 1 2 reveals how the relative rates of reaction o f the various chromium
species in solution with iodine favor the clean formation o f [CpCr(NO)(u,-I)] 2. The key
feature is that only after all the CpCr (NO)(CO) 2 is consumed does any excess I 2 present
react with Cr(I) species to generate absorptions attributable to C p C r ( N O ) 2 I . 2 2 In other
121
words, I 2 reacts with CpCr (NO)(CO) 2 faster than with CpCr(NO)(CO)I or
[CpCr(NO)(p-I)] 2 , and so the presence o f the dicarbonyl Cr(0) reactant prevents the
formation of CpCr (NO)I 2 and the subsequent NO-transfer reactions. Clean generation of
other [Cp 'Cr(NO)(p-X)] 2 complexes by the same synthetic methodology is not possible
because the relative rates of reaction that favor the formation of [CpCr(NO)(p>I)] 2 do not
extend to related systems. Thus, when CpCr (NO) (CO) 2 is treated with PCI5, C l 2 , or B r 2 in
C H 2 C 1 2 , 2 2 or when Cp*Cr (NO)(CO) 2 is treated with any halogen source in a non-
coordinating solvent, 2 3 IR bands due to C p ' C r ( N O ) 2 X species are observed before all o f
the Cr(0) dicarbonyl reactant has been consumed. In these cases, therefore, X 2 reacts with
one or more o f the Cr(I) species in solution at rates comparable to its reaction with
Cp 'Cr (NO)(CO) 2 , thus hindering isolation o f the Cr(I) halo-bridged dimers (Scheme 5.2).
Scheme 5.2. Oxidation of Cp 'Cr(NO)(CO )2 with halogen sources
Therefore, in order to isolate [Cp'Cr(NO)(p.-X)] 2 complexes from the reactions of
C p ' C r ( N O ) ( C O ) 2 with X 2 , the Cr(I) compounds in solution must be converted to species
more resistant to oxidation than the Cr(0) dicarbonyl reactant. This was accomplished by
performing the oxidative halogenation reactions in the coordinating solvent acetonitrile.
In N C M e , C p ' C r ( N O ) ( C O ) 2 reacts cleanly with I 2 or PCI5, with only minimal amounts of
C p ' C r ( N O ) 2 X byproducts being evident by IR spectroscopy. The coordinated N C M e is
122
easily removed by repeated washing/vacuum cycles from the C p ' C r ( N O ) ( N C M e ) X
intermediates to obtain the appropriate dimer, including the previously inaccessible
[Cp*Cr(KO)(p>X)]2 compounds ( X = I (5.1), CI (5.2)), as summarized in equation 5.1.
N X = I (5.1)
C r ^ ' ^ » C r • y7cr - ' ' l X " ' » C f / = CI (5.2)
0 ° I C G N C M E X I N C M e ' * J7 ( 5 . 1 )
O O O ^
The ability o f a-donor ligands (L) to stabilize Cr(I) nitrosyl complexes has been
previously established. 2 5 CpCr(NO)(L)I compounds display no electrochemical oxidation
features to the solvent limit in T H F . 2 3 CpCr(NO)(PR .3)(CO) complexes are cleanly
converted to CpCr(NO)(PPv3)X compounds by I 2 , B r 2 and C I 2 . 2 2 Previous attempts to
make C p * C r ( N O ) ( L ) X complexes were successful when a trapping ligand (L = P P h 3 or
pyridine) was added immediately after Cp*Cr(NO)(CO )2 was treated with I2 without an
attempt to isolate the iodo-bridged dimer. 2 3
O f the two new [Cp*Cr(NO)(u.-X)] 2 complexes, the monochloro dimer is
preparable in higher yields, is easier to obtain in an acetonitrile-free form, and is more
readily crystallized. Crystals of 5.2 suitable for X-ray crystallographic analysis were
obtained by recrystallization from toluene:hexanes solvent mixtures. The solid-state
molecular structure o f [Cp*Cr(NO)(p>Cl)]2 is shown in Figure 5.1, and selected bond
lengths and angles are collected in Table 5.1.
123
Figure 5.1. ORTEP plot of [Cp*Cr(NO)(u-Cl)]2 (5.2). Thermal ellipsoids of 50%
probability are shown.
C(6A)
Table 5.1. Selected Bond Lengths and Angles for [Cp*Cr(NO)(p:-Cl)]2 (5.2).
Bond Lengths (A) Bond Angles (°) C r - C r 3.124(5) Cl -Cr-Cl ( lA) 96.40(4) C r - N 1.709(4) Cr-Cl -Cr( lA) 83.60(4) Cr-Cl(l) 2.339(1) C r - N - 0 171.3(3) Cr-Cl( lA) 2.343(1) C l - C r - N 97.85(11)
124
The intramolecular C r - C r distance in dimer 5.2 is 3.124 A. Metal-metal
bonding interactions can rarely be definitively assigned solely on the basis o f X-ray
crystallography, so the significance o f this distance must be judged with due caution.
C r - C r bonding in particular is notoriously complicated and controversial. For example,
the "supershort" C r - C r distance o f 1.980 A in the [L i (THF) ] 4 [Cr 2 Me 8 ] dimer 8 a was
interpreted by Cotton to imply a very strong C r - C r quadruple bond, 2 6 yet Gambarotta and
co-workers have shown that the dimer readily dissociates to [CrMe4]2~ monomers upon
addition o f T M E D A . 8 b
Modeling C r - C r bonds has also posed a significant theoretical challenge for more
than 10 years. 2 7 While contemporary D F T techniques have been successfully applied to
M 2 ( 0 2 C H ) 4 and M 2 ( H N C H N F f f l ) 4 models o f Nb, M o , Tc, Ru , and R h complexes, 2 8 the
strong electron correlation effects in C r 2 ( 0 2 C H ) 4 render these D F T calculations
inaccurate. 2 9 Even the diatomic C r 2 molecule continues to resist qualitative modeling, 3 0
although the "chromium dimer problem" is no longer considered to have broader
ramifications for theoretical transition-metal chemistry in general. 3 1 ' 3 2
Table 5.2 M - M Lengths in Group 6 Dimers.
Complex M — M (A) ref. rw 2 ci 9 i 3 - 2.41 35b [CpCr(NO)(u-NO)l2 2.615 33d fMozClgl3" 2.65 35c MCpCrfNOXu-NMe^h 2.67 33c c-rCpCr(NO)(u-NMe2)l2 2.72 33c [CpCr(NO)(u-OMe)l2 2.882 33e fCpMo(NO)(u-Cl)l2 2.9098 38
lCpCr(NO)(u-SPh)] 2 2.950 33a fCr.CM 3 - 3.12 35a [Cp*Cr(NO)(n-Cl)h 3.124 5.3 [Cp*Cr(CH3)(n-Cl)l2 3.278 34d fCpCr(CO) 3l 2
3.281 39 rCpCr(CH3)(u-Cl)l2 3.287 34b [Cp*Cr(CH 2Ph)(u-Cl)l 2 3.343 34f [CpCrCl(u-Cl)]2 3.362 34a
125
Despite the difficulties inherent in determining the exact nature of the C r - C r
interaction in 5.2 in the absence o f supporting magnetic and/or theoretical investigations,
comparison with the solid-state molecular structures of related bimetallic species suggests
that [Cp*Cr(NO)(u.-Cl)]2 does not posses a C r - C r bond. The C r - C r bond lengths in the
[CpCr(NO)(M.-Y)] 2 dimers listed in Table 5.2 range from 2.615 to 2.950 A, all
significantly shorter than the 3.124 A distance found in 5.2. The original papers that
describe the solid-state molecular structures o f these compounds contain at best only
cursory synthetic and characterization information. 3 3 However, subsequent research
demonstrated that the [CpCr(NO)(p.-NR 2)]2 complexes display sharp, unshifted *H N M R
signals, consistent with the presence of a C r - C r single bond. 2 4 a C r - C r distances of > 3.0
A are typically found for Cr(UI) dimers with bridging CI ligands and no metal-metal
bond 4 a " 3 4 The congeneric series [M 2Cl9] 3~ ( M = Cr, M o , W ) is particularly instructive. 3 5
Magnetic measurements reveal that the Cr2 species contains no metal-metal bond, while
the M02 and W2 compounds contain single and triple bonds, respectively. 3 6 This is
reflected in the M - M distances following the trend C r > M o > W , despite the increase in
M(III) ionic radius expected for the heavier Group 6 metals. 3 7 The same trend is observed
for the C p M o congener o f 5.2: [CpMo(NO)(p.-Cl)]2 has a shorter M - M distance o f
2.9098 A , consistent with a single bond. 3 8 The longer C r - C r distance o f 3.281 A
observed for the weakly-bonded [CpCr(CO)3]2 dimer is presumably due to the increased
39 steric repulsion and lack of bridging ligands in this compound as compared to 5.2.
A m i n e E l imina t ion Reactions of C r ( N O ) ( N ' P r 2 ) 3 .
Faced with chemical, electrochemical and theoretical evidence o f the instability o f
CpCr(NO)Cl2 with respect to N O loss, 2 4 Cr(NO)(N'Pr2)3 was considered as a potential
precursor to CpCr(NO)R2 complexes. While this classical inorganic 16e tris(amido)
complex may seem an unlikely stepping stone to organometallic Cr(II) nitrosyl species,
126
recent improvements in the scope and selectivity o f amine elimination reactions have
dramatically increased the utility o f metal-amide bonds to synthetic organometallic
chemists ,
Amine elimination reactions were first employed to form Group 4 4 0 and actinide 4 1
organometallic complexes over 25 years ago. More recently, the research groups Of
Teuben, 4 2 Herrmann 4 3 and Jordan 4 4 demonstrated the applicability o f these reactions to
synthesize Group 4 metallocene catalyst precursors for olefin polymerization This
methodology has since been enthusiastically embraced by other researchers interested in '
high-valeht organometallic complexes of Groups 4, 5 and 6 4 5
The study of the reactions of Cr(NO)(N'Pr 2 )3 mirrors the same pattern o f early
discovery, prolonged neglect, and recent renaissance that characterizes amine elimination
reactions generally. While several Cr(NO)(NR2)3 complexes were initially reported in
-1.970,46 with the solid-state molecular structure of Cr(NO)(N(SiMe3)2)3 appearing shortly
thereafter,47 Cr(NO)(N'Pr2)3 remains the only member of this class o f compounds to be
used as a precursor to new species. 4 8 This appears to be due to the specific steric
properties o f the N ' P r 2 l igand 4 9 and the synthesis o f the tris(amide) nitrosyl complexes, as
shown in Scheme 5.3.
R = M e Et [Cr(NR2)3] — — * [Cr(NR2)2]n + Cr(NR2)4
/ unstable 'less
r, o i ' ; bulk C r C l 3 R = /Pr v j n vrv
+ *- Cr(NR 2 ) r^— Cr(NO)(NR 2)3-^U Cr(NO)(NR2)3.nXn
L i N R 2 \ \ more
X bulk R = S i M e 3 C r ( N R 2 ) 3 - ^ ^ Cr(NO)(NR 2 ) 3 -^* no reaction •
Scheme 5.3. Steric influences on the reactivity o f C r tris(amide) complexes
The initial step o f the synthesis involves the reaction o f the LiNR2 reagent with
anhydrous, polymeric C r C l 3 . The use o f amide ligands at least as bulky as N ' P r 2 leads to
127
monomeric, 13e Cr(NR2)3 compounds, while smaller amides such as NMe2 or NEt2
yield only Cr(Bi) and Cr(IV) products of disproportionation.30 The stable Cr(NR2)3
complexes react with nitric oxide gas to form Cr(NO)(NR2)3 species.46 Complexes
containing NR 2 ligands larger than N'Pr2, however, subsequently fail to react with protic
sources, presumably due to the inaccessibility of the amide N atoms for electrophilic
attack.
Cr(NO)(N/PT2)3
HOR
V(Mes)3(THF)
HOR Cr(N)(N/Pr2)3
ref.Sla
lutH+r
Cr(N)(N/Pr2)2I
RLi
Cr(N)(N/Pr2)2R ref.Slb
[Cr(NO)(OR)3]„ R = /Bu, n = 1 r e f 4 6 4 8
R = /Pr,n = 2 J
Cr(rT)(N/Pr2)2(OR) ref.51c
HOR (xs)
Cr(N)(N/Pr2)(OR)2
2 RLi
Cr(N)(N/Pr2)R2
ref.51c
Scheme 5.4. Derivatization of Cr(NO)(N'Pr2)3
Scheme 5.4 summarizes the reactivity of Cr(NO)(N'Pr2)3 that was previously
known. Complete protonolysis of all three amide ligands with 'BuOH yields the
monomeric tris(butoxide) complex, while a dimeric product is formed when the less
sterically demanding 'PrOH is used.48 Cummins and co-workers have recently reported
selective elimination of one or two amide ligands from the nitride derivative.51
The initial investigation of Cr(NO)(N'Pr2)3 as a potential precursor to
organometallic C r ( n ) nitrosyl compounds was conducted as a fourth-year undergraduate
thesis project by Jane Kuzelka.521 designed and helped oversee this project which
resulted in the reaction sequence illustrated in equation 5.2. The treatment of the
128
tris(amide) with benzoic acid led to the protonolysis of two amide ligands to form
Cr(NO)(N'Pr2)(02CPh)2. The sequential salt metathesis reactions of the benzoate ligands
performed by Jane Kuzelka led to the first well-characterized Cr(IJ.) nitrosyl alkyl
compounds. O N
Cr jPr 2N'
^ - N / P r 2
N/Pr 2
P h C 0 2 H
O N
NaCp
Cr /Pr 2 N'
N ^ 0 2 C P h 0 2 CPh
Cr / P r 2 * r | R
N O i
R 2 M g
Cr
/ P r 2 N ^ | ^ 0 2 C P h N O
(5.2)
The solid-state molecular structures of the CpCr(NO)(N'Pr2)X (X = n 1 -0 2 CPh,
CH2SiMe3, V-C5H5) complexes all contain short Cr-N(amide) bonds53 and a planar
C r - N C 2 moiety aligned parallel to the Cr-NO axis,54 indicative of a strong
chromium-amide re-bonding interaction. The room-temperature *H NMR spectra of the
CpCr(NO)(N'Pr2)X species (CtJJe) display four distinct doublets for the four inequivalent
amide methyl groups, suggesting that the O N - C r - N R 2 alignment persists in solution.
The stability of the CpCr(NO)(N'Pr2)X complexes of Jane Kuzelka, as well as the
previously reported CpCr(NO)(NPh2)I5 3 a and Cp*Cr(NO)(0'Pr) 2, 5 4 b may be attributed to
their low-spin, S = 0 configuration resulting from the increased H O M O - L U M O gap
induced by the combination of the strong TC donor amide and TC acceptor nitrosyl
ligands.24
Synthesis of Cr(NO)(N'Pr2)(CH2SiMe3)2 (5.3).
Amide TC donation renders the CpCr(NO)(N'Pr2)R alkyl complexes electronically
saturated.55 In order to generate an unsaturated Cr(II) alkyl complex,
129
Cr(NO)(N'Pr 2 )(0 2 CPh)2 was treated with M g ( C H 2 S i M e 3 ) 2 to form
Cr(NO)(N'Pr 2)(CH 2SiMe3) 2 (5.3), as shown in equation 5.3. The diamagnetic 14e
bis(alkyl) 5.3 was isolated in high yields as a spectroscopically pure red powder after
extraction of the crude reaction residue with hexanes and filtration through Celite to
remove the M g ( 0 2 C P h ) 2 byproduct. Rectangular blocks of 5.3 suitable for X-ray
crystallography were obtained by dissolving the crude material in hexamethyldisiloxane
and cooling to -30 °C. The solid-state molecular structure o f 5.3 is shown in Figure 5.2,
and the pertinent bond angles, bond distances, and torsion angles are collected in Table
5.3.
O N
O N 5.3 M g ( C H 2 S i M e 3 ) 2
(5.3) s \ - 0 2 C P h
0 2 C P h / C ^ C H 2 S i M e 3
C H 2 S M e 3
/ P r 2 N / P r 2 N
1 3 0
Figure 5.2. ORTEP plot of Cr(NO)(N'Pr2)(CH2SiMe3)2 (5.3). Thermal ellipsoids of
5 0 % probability are shown.
C ( 3 )
C ( 7 ) f / f t C ( 4 )
O(l)
C ( 6 ) C ( 5 )
C ( 8 )
C ( 1 0 )
C ( 13 ) C ( 14 )
Table 5.3. Selected Bond Lengths, Bond Angles, and Torsion Angles for
Cr(N0)(NT-r2)(CH2SiMe3)2 (5.3).
Bond Len gths (A) Bond Angles (°) Torsion Angles (°) C r - C ( l ) 2.026(3) N ( l ) - C r - N ( 2 ) 102.91(12) N ( l ) - C r -
N(2)-C(9) -1.2(3) Cr -C(2) 1.986(3) C ( l ) - C r - C ( 2 ) 115.72(12)
N ( l ) - C r -N(2)-C(9) -1.2(3)
C r - N ( l ) 1.616(2) C r - N ( l ) - 0 ( 1 ) 179.0(3) N ( l ) - C r -N(2)-C(12) -177.8(2)
Cr -N(2) 1.763(2) N ( l ) - C r - C ( l ) 102.85(12) N ( l ) - C r -N(2)-C(12) -177.8(2)
131
Figure 5.2 shows only one of the two crystallographically independent
molecules o f 5.3 found in the unit cell. The bis(alkyl) complex adopts a flattened pseudo-
tetrahedral structure, with N(amide) -Cr -C and C-Cr-C angles >112° and
N(ni t rosy l ) -Cr-C and N(nitrosyl)-Cr-N(amide) angles <104°, due to the greater steric
bulk of the amide and alkyl ligands compared to the small N O group. The SiMe3 groups
of the two alkyl ligands are oriented away from the C r atom and "up" towards the nitrosyl
ligand, also presumably to avoid steric interactions with the bulky groups. The
Cr-N(amide) bond distance is a very short 1.763 A, 5 3 and the
N(ni t rosyl)-Cr-N(amide)-C torsion angles indicate the near planarity o f the
O N - C r - N C z group (all six atoms are within 0.1 A o f the plane). 5 4 These geometrical
parameters are consistent with the existence of a Cr-N(amide) TC-bonding interaction.
While highly unsaturated, four-coordinate, monomeric Cr(II) alkyl complexes are
not uncommon, all previously known examples o f this class of compound were square
planar and paramagnetic (5 = 2 ) . I 0 I l c d In order to explain the anomalous diamagnetic
electronic configuration and tetrahedral geometry of 5.3, a qualitative bonding analysis is
required. A
->-x
0
L R
xy N R 2 TC
2 2» : -y-rf -r - - -
xz y
yz N O TC Figure 5.3. Qualitative energy-level diagram illustrating N O and N R 2 TC-bonding
interactions in tetrahedral Cr(II) complexes.
132
The orbital diagram at the left of Figure 5.3 depicts a hypothetical, pseudo-
tetrahedral Cr(L)Pv3 anion with the z-axis lying along the Cr-L bond.56 Note that the .
small orbital-splitting energy between the degenerate d^ and dyz orbitals and the dxJdX2.y2
pair is not expected to be sufficient to overcome the interelectron repulsion energy,
resulting in a high-spin S = 2 configuration. This is typical for pseudo-tetrahedral
complexes which usually adopt a high-spin configuration whenever possible. In the
absence of strong 7i-bonding ligands, the tetrahedral geometry shown in Figure 5.3 is
higher in energy for 4-coordinate, d 4 complexes than the square planar arrangement; this
is a well-known example of the Jahn-Teller effect.57
However, when the generic 2-electron cr-donor L ligand is replaced with the
strong 7t acceptor NO+, the d^ and dyz orbitals are substantially lowered in energy. The
tetrahedral ligand-field splitting (At) and nitrosyl n bonding can be viewed as working
together to create a large HOMO-LUMO gap, and a diamagnetic 12e Cr(NO)R-3 species
results.58 This is in contrast to pseudo-octahedral Cr(U)(NO) complexes, where the NO ti-
acceptor interactions occur within the t2g set, and are therefore not additive with the large
octahedral field splitting (A0) in enforcing a low-spin electronic configuration.24,59
Replacing one R ligand with a NR2 group increases the electron count from 12e to
14e due to the formation of a Cr-NR2 TI bond. While the existence of an amide 7t-donor
interaction is confirmed by the solid-state molecular structure of 5 . 3 , the Cr-NR2 K bond
does not increase the HOMO-LUMO gap of Cr(N0)(NR2)R2 compared to a hypothetical
Cr(NO)R3 complex. Thus, the Cr-NR2 K bond is not considered to be a critical factor in
enforcing a diamagnetic configuration in 5 . 3 . Instead, a combination of tetrahedral
geometry and NO TC bonding increases the orbital splitting energy, while the covalent
chromium-alkyl bonds decrease the interelectron repulsion energy,240 leading to a S = 0
ground state.
133
O N R
R — C r C i ^ C r — R
I /Pro I R 2 N
O
O N I
C r
O N
/ P r 2 N ' < ^ R
R
"** / P r 2 N — C r : I
L
- R R (5.4)
This orbital rationale does not explain, however, the apparent reluctance of 5.3 to
decrease its coordinative and electronic unsaturation either through dimerization or
interactions with potential o-donor L ligands (Equation 5.4). Undoubtedly, the relatively
large steric bulk of the amide and alkyl ligands is largely responsible for the monomeric
nature of 5.3. Nevertheless, it is noteworthy that no highly unsaturated (<16e) Mo(II) and
W(II) alkyl complexes are currently known, and that the "spin-stabilization" invoked for
the S = 2 Cr(II) alkyl species is not applicable to the diamagnetic 5.3. 55b
4 t "• I 1 1 1 1 I i 0 2 i.
,—ip—i—r—i—i—|—l—i—r—i—|—1—l—I TT - | — i 2 0 l b 10 0 6
ppm
Figure 5.4. 1 H N M R spectrum of Cr(NO)(N/Pr2)(CH2SiMe3)2 (5.3) in C 6 D 6
Interestingly, unlike the saturated CpCr(NO)(N 'Pr 2 )X complexes, the amide ' H
N M R signals of 5.3 are very broad at room temperature (Figure 5.4). Evidence that this
must be due to rotation about the C r - N R 2 bond rather than any paramagnetic broadening
includes (a) the sharp, equivalent resonances assignable to the MesSiCHa ligands, (b) the
position o f the amide peaks at 0.92 and 1.14 ppm for the amide M e ' s and 3.28 and 4.36
ppm for the amide C - H ' s , well within the expected ranges for these signals, 4 8 ' 5 1 and (c)
the variable temperature behavior of these resonances, which sharpen slightly without
shifting up-, or down-field as the sample is cooled to - 5 0 ° C .
134
Cr(N)(N/Pr2)3 1 broad signals
hindered rotation
Cr(N)(N7Pr2)(CH2SiMe3)2 2 sharp doublets
very slow or wo rotation
z2
N
Figure 5.5. Qualitative orbital splitting and amide rotation barriers in Cr(NO) and Cr(N)
species
The amide rotation is possible because unlike the CpCr(NO)(N'Pr2)X complexes,
5.3 has two orthogonal, metal-based orbitals which are capable of accepting TC donation
from the NR 2 group (Figure 5.5). Since the dZ2 orbital is slightly higher in energy than the
degenerate d^/d^.^ pair, the planar O N - C r - N R 2 orientation is preferred (as seen in the
solid-state molecular structure, Figure 5.3), and rotation is slightly hindered. If free
rotation was possible, all the N'Pr2 methyl groups would be equivalent and give one sharp -
doublet, as is observed for Cr(NO)(N'Pr2)3. As outlined in Figure 5.5, the nitride
complexes have slower rotation than the corresponding nitrosyl compounds.51 This is
presumably due to an increased orbital splitting between the dZ2 arid d ^ / d ^ i orbitals,
since the stronger Cr=N c-bond lies along the z axis.
Alternative routes to CpCr(NO)R 2 .
This section outlines my attempts to generate CpCr(NO)R2 complexes from
derivatives of Cr(NO)(N'Pr2)3: namely Cr(NO)(0'Bu) 3, 4 6' 4 8 Cr(NO)(N'Pr 2)(0 2CPh) 2, 5 2
and Cr(NO)(N'Pr2)(CH2SiMe3)2 (5.3). Complex 5.3 did not react with C5H6 to eliminate
F£N'Pr2. Treatment of Cr(NO)(N'Pr2)3 with excess P H C 0 2 H (or isolated
Cr(NO)(N'Pr2)(02CPh)2 with one equivalent of PhC02Ff) did not yield the desired
Cr(NO)(N/Pr2)3
1 sharp doublet fast rotation
Cr(NO)(N7Pr2)(CH2SiMe3)2 2 broad signals slow rotation
O N
Cr x 2-y 2
xy
135
Cr(NO)(0 2CPh) 3 product. No absorbances in the expected v(NO) region of the
spectrum were exhibited when the solid was dissolved in CH2CI2, only broad signals
were evident in its *H NMR spectra (C6D6), and the green solid displayed a parent peak
in its mass spectrum consistent with {[(PhC02)2Cr] 3(p.-0)}+. CpCr(NO)(OR)2 complexes
are an attractive alternative to the unstable CpCr(NO)Cl2 as potential precursor
complexes for CpCr(NO)R2 species. However, the tris(butoxide) complex
Cr(NO)(0'Bu) 3 did not react with CpSiMe 3 or Na(DME)Cp, and no diamagnetic
organometallic products were evident in the reaction with CpLi (*H NMR, CeDe). No
isolable, nitrosyl-containing products were obtained from the reaction of lutidinium
iodide or P h C 0 2 H with Cr(NO)(0'Bu) 3 or 'BuOH with Cr(NO)(N'Pr2)(02CPh)2.
While a viable route to CpCr(NO)(0'Bu) 2 has yet to be devised, the Cp*
bis(propoxide) analog has previously been synthesized from Cp*Cr(NO)2Cl and Na(O'Pr)
in hot 'PrOH. 5 4 b Cp*Cr(NO)(0'Pr)2 appeared to react cleanly with Mg(CH 2Ph) 2, as
judged by solution ER. spectroscopy; over 90 min, the v(NO) of the bis(propoxide)
starting material at 1669 cm - 1 was replaced with a new absorbance at 1623 cm - 1 (THF).
The *H NMR spectrum of the crude reaction residue displayed a single Cp* resonance
(CeD6), but the presumed Cp*Cr(NO)(CH2Ph)2 product could not be isolated free of the
Mg(0'Pr) 2 byproduct (equation 5.5). Both the organometallic species and the inorganic
salt were soluble in hexanes, and only an amorphous green solid was deposited from a
cold hexamethydisiloxane:hexanes solution. Thus far, attempts to separate the two
species by chromatography, aqueous work-up, or addition of benzoic acid have been
unsuccessful.
zPrO I ^ O / P r N O
Mg(CH2Ph)2
**• THF
PhCH 2
X r CH 2 Ph
+ Mg(0/Pr) 2 (5.5)
N O
136
Summary
The initial attempts to synthesize CpCr(N0)R.2 complexes were based on the
methodology used to obtain the heavier Cp'M(NO)R.2 (M = Mo, W) congeners.
Oxidative halogenation of Cr(0) d 6 Cp'Cr(NO)(CO)2 compounds failed to yield the
required Cp'Cr(NO)X2 (X = I, CI) complexes, however, due to the instability of these
Cr(II) d 4 dihalo species with respect to loss of NO. Overoxidation of the initially-formed
Cr(I) d 5 products was avoided by conducting the reactions of Cp'Cr(NO)(CO)2 with I2 or
PCI5 in NCMe, thereby providing synthetic access of the previously unknown
[Cp*Cr(NO)(u,-X)]2 (X = I 5.1, CI 5.2) dimers. The solvent dependence of this reaction is
of general significance since controlled oxidations of transition-metal carbonyls with
halogens constitute an important route to metal-halide complexes, synthetic precursors to
a vast number of organotransition-metal compounds.60
Subsequent studies employed nitrosyl complexes which already possessed the
desired Cr(II) d 4 electronic configuration. Selective amine elimination reactions were
crucial in order to use Cr(NO)(N'Pr2)3 as a precursor to organometallic Cr(II)(NO)
compounds, including Cr(NO)(N'Pr2)(CH2SiMe3)2 (5.3). A diamagnetic, tetrahedral, 14e
bis(alkyl) complex that displays rotation about the Cr-N'Pr2 bond, 5.3 provides an
interesting example of how unsaturated organometallic species can be stabilized by a
combination of steric effects and synergic 7t-bonding interactions.
When attempts to generate CpCr(NO)(O fBu)2 from Cr(NO)(N'Pr2)3 were
unsuccessful, the known Cp*Cr(NO)(0'Pr)2 was used as a precursor to Cp*Cr(NO)R.2
complexes. While Cp*Cr(NO)(0'Pr)2 appeared to react cleanly with
Mg(CH2Ph)2x(dioxane) as judged by solution IR spectroscopy, the presumed
Cp*Cr(NO)(CH2Ph)2 product could not be separated from the Mg(0'Pr)2 byproduct.
137
Future Work
l e O x i d a t i o n o f 17e C p ' C r ( N O ) R 2 a n i o n i c complexes
While the reaction of Cp*Cr(NO)(0'Pr)2 and Mg(CH 2Ph) 2 seems to indicate that
Cp*Cr(NO)(CH 2Ph) 2 is a viable target molecule, a new synthetic route is required to
avoid byproduct separation problems. One such route involves the reaction of
[Cp'Cr(NO)(p-X)]2 with alkyl lithium reagents to afford [Li(THF)n][Cp'Cr(NO)R2]
compounds. These 17e "ate" complexes could then be oxidized by I 2 or [Cp 2Fe]+ to
provide the desired 16e Cp'Cr(NO)R 2 species (equation 5.6).
O N
•n I..,. I 2 LiR c ' r © oxidation c ' (5.6)
N fa? N N
o o ^ o vLi(THF)n
Similar "ate" complexes have recently been synthesized by Steve Sayers and
Brett Sharp, including the 17e [Li(THF)n][Cp*Mo(NO)(CH2SiMe3)2] analog of the target
Cr compounds.61 The known complexes are stabilized by isonitrosyl-lithium interactions,
THF solvation of the L i + cation, and occasionally by adopting a dimeric structure with
bridging, planar M-NO(p-Li ) 2 ON-M units. Other compounds related to the proposed
[Li(THF)n][Cp'Cr(NO)R2] species include ["Bu4N][CpCr(NO)Cl2],2 4 b
[Li][Cp*Cr(CH 2Ph) 3], 3 4 e and [Li(THF) 2][CpCrCl 3]. 6 2
D i a m a g n e t i c 12e t r i s (a lkyI ) species
The qualitative orbital diagram shown in Figure 5.3 suggests that amide 7t
donation is not required to maintain the low-spin state of Cr(NO)(N'Pr2)(CH2SiMe3)2.
This hypothesis may be tested by the synthesis of a Cr(NO)R3 complex containing very
bulky R groups. I attempted to generate such a complex by reacting Cr(NO)(0'Bu)3 With
3 equivalents of LiCH(SiMe 3 ) 2 . 6 3 While a hexanes-soluble species with a new v(NO)
138
band at 1685 cm - 1 (hexanes) was observed, no product was isolated from the dark
khaki green solution (which appeared red to transmitted light). Closer examination of the
literature revealed that the synthesis of this very compound had previously been
attempted over 20 years ago (equation 5.7).58 Lappert and co-workers treated the 9e
Cr(CH(SiMe 3) 2) 3 complex with NO gas to give Cr(NO)(CH(SiMe3)2)3 This species
decomposed in cold hexanes solutions and was only characterized by IR spectroscopy
(v(NO) = 1672 cm - 1 , no medium reported).
\ / O O \ f \{ N . N. ^ S i — ( S i —
R = CH(SiMe 3) 2 / \ T
Alternative Cr(NO)R3 target molecules might include those with alkyl groups
without a-H's, in order to avoid a-hydrogen abstraction reactions to form unstable
alkylidene intermediates (e.g. R = adamantyl, mesityl).64
Paramagnetic 16e Cr(0) compounds
The bonding rationale illustrated in Figure 5.3 shows that the Cr(U), d 4, He
Cr(NO)(N'Pr2)(CH2SiMe3)2 possesses two empty, nearly degenerate, non-bonding
orbitals. This orbital description suggests that Cr(0), d6,16e complexes of the formula
Cr(NO)(NR 2)L 2 should be paramagnetic, with one electron occupying each of the two
metal-centered, non-bonding orbitals. Such a compound could be synthesized by (a) 2e
reduction of Cr(NO)(N'Pr2)(02CPh)2 in the presence of trapping L ligands, or (b) amide-
for-halide metathesis of a suitable Cr(NO)(L)4X precursor,65 followed by L ligand loss
(assisted by amide TC donation and steric effects), as shown in Equation 5.8.
139
Initial attempts to generate Cr(NO)(N'Pr2)(dppe) by method (a) using Mg or
K/graphite as reducing agents were unsuccessful.
Q , v 0 O N (a) N 0>) N •
I -2e" I , + NR 2 ~ cU^-L R , N ^ \ ° 2 C P h R o N ^ \ L - 2 L (5.8) R 2 W 0 2 CPh R 2 N L X
Paramagnetic Cr(NO)(NR2)L2 species would be of interest for five basic reasons.
First, analogous Mo and W complexes might be accessible from known M(NO)(PMe3)4X
precursors.66 Second, the tetrahedral geometry, high-spin configuration, and coordinative
and electronic unsaturation of these species would be very unusual, since Cr(0), d 6
complexes typically contain several 7t-acceptor ligands which encourage the formation of
low-spin octahedral CrL6 compounds.67 Third, the ligand addition/substitution processes
of Cr(NO)(NR 2)L 2 compounds would provide an excellent test case to study the effect of
spin state on these fundamental reactions.55b'68 Fourth, selective tuning of the spin state of
M(NO)(NR2)L2 species may be achieved by modifying the orientation and 7t-donor
properties of the NR2 group, the 7t-acceptor abilities of the L ligands, and the identity of
the metal center. Fifth, the potential C - H bond activation reactivity of these
M(NO)(NR 2)L 2 complexes to generate M(II), d 4 M(NO)(NR2)(R)(H)L2 species could
also be evaluated.
Diamagnetic 16e M(NO)(NR 2)(L)R 2 bis(alkyls) of Cr, Mo, and W
Another potential class of organometallic nitrosyl compounds which includes all
three Group 6 transition metals are M(II), d 4, 16e M(NO)(NR 2)(L)R 2 species. The
possibility that a class of M(NO)(L)X3 complexes should be synthetically accessible was
initially forwarded by Cotton, Chisholm, and co-workers.69 A prototypal ligand system I
designed employs a bidentate amidopyridine, monoanionic ligand derived from the imine
140
product o f the condensation of H 2 N R and 2-pyridinecarboxaldehyde. The sp3-
hybridized C H 2 group separates the strong 7t-donor amide from the a-donor pyridine,
thereby preventing the Tt-donor properties of the ligand from being attenuated through
derealization.70 When the amide donor aligns cis to M - N O axis, the pendant pyridine
group should coordinate trans to the nitrosyl ligand (equation 5.9). The pyridine ligation
will thus prevent the amide from rotating (as is observed in 5.3), while the chelate effect
overcomes the trans effect of the N O group, which would otherwise labilize the pyridine
moiety.48 Initial steps towards Mo(NO)(ArNCH2C5H4N)R2 compounds have been taken
by Craig Adams. 7 1 p j R N
+ N H 2 R
N O . N R (5.9)
Alternative route to Cp'CrCNOfcCl and [Cp*Cr(NO)(n-CI)h
NCMe A
* " O C — C r — N
NCMe
Diazald
PC15 NCMe
Scheme 5.5. Current synthetic routes to Cp*Cr(NO)-containing complexes
141
Scheme 5.5 illustrates the route currently used to make the Cp*Cr (NO)-
containing complexes described in this chapter. The difficulty in this sequence lies with
Cp*Cr(NO)(CO) 2 , which is more difficult to synthesize than CpCr (NO)(CO) 2 , and is
obtained in poor overall yield from Cr(CO)6 (-20-30%). The original synthesis o f
C p C r ( N O ) 2 C l from in situ generated C p C r ( T H F ) C l 2 and N O gas suggests that the
recently reported [Cp*Cr(p>Cl)] 2 dimer 9 8 may be a useful precursor to Cr(I) mononitrosyl
and Cr(0) dinitrosyl derivatives (Equation 5.10). If such a synthetic route could be
attained, it would help encourage the development of the chemistry o f Cp*Cr (NO)-
containing species.
(5.10)
15e Cp*Cr(NO)R species
In chapter 3,1 outlined the possibility o f 7t-donor stabilized Cr(I), d 5 , 17e
C p ' C r ( N O ) Y complexes. If the synthesis o f these amide and/or alkoxide species is
successful, attempts could be made to generate 15e Cp*Cr(NO)R compounds with very
sterically-demanding alkyl groups (e.g. R = mesityl, adamantyl, CH(SiMe3)2,
C H 2 C M e 2 P h ) . Similar highly unsaturated (<16e), paramagnetic complexes have been
reported for Ti(ffl) (d 1 ) , 7 2 V(m) (d 2 ) , 7 3 and Cr(UI) ( d 3 ) . 4 0 0
142
Experimental
Methods
A l l reactions and subsequent manipulations were conducted under anaerobic
conditions using an atmosphere o f N2. The complexes LiCH(SiMe3)2, 6 3 N a ( D M E ) C p , 7 4
Cr(NO)(N 'Pr2) 3 , 4 6 ' 4 8 ' M a Cr(NO)(0 'Bu) 3, 4 6 ' 4 8 Cr(NO)(N'Pr2)(02CPh)2,52 and
Cp*Cr(NO)(0 'Pr) 2
5 4 b were prepared by the published procedures. C p * C r ( N O ) ( C O ) 2
7 5
was synthesized by treatment o f C p * C r ( C O ) 3 H 7 6 with Diazald in T H F . Lutidinium iodide
was prepared by treating distilled 3,5-Me2pyridine in ' B u O H with one equivalent of
Me3SiI in an inert atmosphere glove box . 5 1 c A l l other reagents were used as received
from commercial suppliers. Filtrations were performed through Celite (1x2 cm)
supported on a medium porosity frit unless otherwise specified. For low temperature
reactions, solvents were transferred via trap-to-trap distillation from the drying reagent
directly onto the reactants contained in a flask cooled by a liquid nitrogen bath.
Synthesis of [Cp*Cr(NO)(p>I)]2 (5.1).
N C M e (-30 mL) was vacuum transferred onto Cp*Cr (NO)(CO) 2 (0.123 g, 0.450
mmol), and the orange chromium complex dissolved as the solution was warmed in an
ice bath. Iodine (0.056 g, 0.441 mmol) was added, and the solution was stirred for 6 h
while being slowly warmed to ambient temperature, after which time only peaks
attributable to Cp*Cr(NO)(CO)2 remained in the v(CO) region o f the solution's IR
spectrum. The solution was heated with a 55 °C water bath for - 35 min, and the solution
was then taken to dryness in vacuo. The remaining green residue was dissolved in T H F
(15 min) and taken to dryness four times, and then triturated with Et20 (30 mL, also
removed in vacuo) four times. The residue was washed extensively with hexanes and
then extracted into toluene, filtered, and recrystallized at -30 °C from toluene:hexanes to
afford a green powder (0.096 g, 62% yield).
143
Anal . Calcd. for C2oH3oCrI2N202: C , 34.90; H , 4.39; N , 4^07. Found: C, 35.29;
H, 4.38; N , 4.36. (reported elemental analysis is the average o f three separate runs).
IR(Nujol): 1647 cm" 1 . IR(CH 2 C1 2 ) : 1650 cm" 1 . Fast Atom Bombardment M S : m/z 688
(P+).
Synthesis of [Cp*Cr(NO)(p . -Cl) ] 2 (5.2).
N C M e (-20 mL) was vacuum transferred onto Cp*Cr (NO)(CO) 2 (1.48 g, 5.41
mmol) and the orange solution was warmed to -30 °C. PCI5 (0.550 g, 2.64 mmol) was
added, and the reaction mixture was stirred at - 30 °C for - 20 min and then allowed to
warm slowly to room temperature. After 90 min, E t 2 0 (10 mL) was added to the green
solution, and the solution was filtered. The plug of Celite was washed with E t 2 0 ( 3 x 5
mL), and the combined filtrates were taken to dryness in vacuo. The remaining residue
was triturated with E t 2 0 (10 mL) and washed extensively with pentane (7 x 5 mL).
Remaining solvent was removed under static vacuum to obtain crude [Cp*Cr(NO)(p.-
C l ) ] 2 as a green powder (1.22 g, 89 % yield). Crystalline samples suitable for elemental
analysis and X-ray diffraction were obtained by recrystallizing the powder overnight at
- 30 °C from a -2:1 toluene:hexanes solvent mixture.
Anal . Calcd. for C 2 0 H 3 o C r C l 2 N 2 0 2 : C , 47.53; H , 5.98; N , 5.54. Found: C, 47.53;
H, 6.14; N , 5.49. IR(Nujol): 1648 cm" 1 . IR(CH 2 C1 2 ) : 1645 cm" 1 . Fast Atom
Bombardment M S : m/z 474 (P + -NO) .
Synthesis of C r ( N O ) ( N ' P r 2 ) ( C H 2 S i M e 3 ) 2 (5.3).
T H F (-25 mL) was vacuum transferred onto Cr (NO)(N 'Pr 2 ) (0 2 CPh) 2 (0.344 g,
0.803 mmol) andMg(CH 2 SiMe 3 ) 2 x(d ioxane) (0.249 mg, 0.803 mmol). The solution
changed from an orange suspension to a clear, red solution as it was allowed to warm
slowly to room temperature over - 30 min. The solvent was then removed in vacuo, and
144
the red residue was extracted with hexanes ( 5 x 1 0 mL) and filtered. The solvent was
removed in vacuo to obtain a red powder (0.242 g, 85% yield). Dark red rectangular
blocks suitable for X-ray diffraction were obtained by recrystallization o f this powder
from hexamethyldisiloxane at -30 °C.
Anal . Calcd. for Ci4H 36CrN 2OSi 2: C, 47.15; H , 10.17; N , 7.86. Found: C, 46.77;
H , 9.84; N , 7.66. IRfNujol): 1670 cm" 1 . lH N M R ( C 6 D 6 ) : 8 0.20 (d, 2FL CHW), 0.32 (s,
18 H, Si(CH3h), 0.92 (br s, 3FL C H ( C / k ) 2 ) , 1.14 (br s, 3FL CH(C#02), 1.99 (d, 2H ,
CH//*), 3.28 (v br s, I H , C ^ C H ^ ) , 4.36 (v br s, I H , C / / ( C H 3 ) 2 ) . 1 3 C N M R ( C 6 D 6 ) : 8
2.54 (Si (CH 3 ) 3 ) , 21.3 (br, C H ( C H 3 ) 2 ) , 28.1 (br, C H ( C H 3 ) 2 ) , 68.06 ( C H 2 ) , 49.0 (br,
C H ( C H 3 ) 2 ) , 55.9 (br, C H ( C H 3 ) 2 ) . L o w Resolution Electron Impact M S (probe
temperature 150 °C): m/z 356 (P + ) .
Reaction of 5.3 with C5H6.
C r ( N O ) ( N ' P r 2 ) ( C H 2 S i M e 3 ) 2 (0.020 g, 0.056 mmol) was dissolved in C 6 D 6 ( ~ l m L )
and freshly "cracked" C5H6 (~0.1 mL) was added. The lH N M R spectrum of this mixture
displayed resonances attributable only to the bis(alkyl) starting material, C5H6, and the
CioHn dicyclopentadiene dimer.
Reaction of Cr(NO)(N iPr2)(02CPh)2 with P h C 0 2 H .
C H 2 C 1 2 (5 mL) was added to Cr (NO)(N 'Pr 2 ) (0 2 CPh) 2 (0.058 g, 0.15 mmol) and
P h C 0 2 H (0.019 g, 0.16 mmol). The resulting orange-red solution was monitored by
solution IR spectroscopy. Over the course of several hours, the peaks due to the
bis(benzoate) species (1720 c m - 1 , s) and benzoic acid (1695 c m - 1 , m) slowly diminished
in intensity. After -18 h, the green solution exhibited only a very weak, broad peak at
1703 c m - 1 and stronger, broad peaks at 1625, 1600, and 1548 c m - 1 in its IR spectrum.
145
The solvent was removed in vacuo to leave a pale green solid, which displayed a m/z
peak at 898, assigned to {[(PhC0 2)2Cr] 3(p>0)} + (Low Resolution Electron Impact M S
(probe temperature 350 °C)).
Reaction of Cr(NO)(0'Bu)3 with Na(DME)Cp, CpSiMe3, LiCp.
T H F (-30 mL) was vacuum transferred onto Cr (NO)(0 'Bu) 3 (0.092 g, 0.31 mmol)
and N a ( D M E ) C p (0.055 g, 0.31 mmol). The solution was allowed to warm to room
temperature and was stirred overnight. The solvent was then removed in vacuo, the
residue was extracted with E t 2 0 and filtered. A n IR spectrum of the filtered solution
displayed a single v (NO) peak at 1707 c m - 1 , due to the unreacted tris(propoxide) starting
material. The lack of reaction was also confirmed by *H N M R ( C 6 D 6 ) , which showed
only the single resonance due to Cr(NO)(0 'Bu) 3 .
C r (NO)(0 'Bu) 3 was similarly unreactive with C p S i M e 3 in E t 2 0 , even after
heating at 60 °C for 4 days in a sealed vessel. Reaction o f Cr (NO)(O f Bu) 3 with L i C p in
T H F overnight did result in the slow disappearance o f the starting material peak at 1708
c m - 1 along with the concomitant growth of a new peak at 1622 c m - 1 . However, the ! H
N M R spectra o f the residue o f this reaction dissolved in CeD6 displayed only a single
resonance at 1.502 ppm and no C5H5 signal.
Reaction of Cr(NO)(0'Bu)3 with HX (HX = PhC02H, lutidinium iodide).
Cr(NO)(0 'Bu) 3 (0.020 g, 0.066 mmol) was dissolved in E t 2 0 (5 mL), and an
initial infrared spectrum of the orange solution was taken. P h C 0 2 H (0.010 g, 0.082
mmol) was then added, and a voluminous, pale green precipitate immediately formed.
The infrared spectrum of the orange supernatant was identical to the initial spectrum,
although the intensity of the v(NO) peak of 1706 c m - 1 had decreased in intensity. The
orange solution was removed form the pale green precipitate, the E t 2 0 solution was
146
removed in vacuo to leave an orange residue which sublimed readily at - 40 °C under
vacuum onto a water-cooled probe 4 8 The green precipitate dissolved readily in T H F , and
exhibited a m/z peak at 898, consistent with {[(PhC02)2Cr]3(u,-0)}+ (Low Resolution
Electron Impact M S (probe temperature 350 °C)).
Reaction of Cr(NO)(0 'Bu) 3 with one or two equivalents of lutidinium iodide in
CH2CI2 or T H F did not yield any new nitrosyl-containing products, as judged by solution
IR spectroscopy.
Generation of C p * C r ( N O ) ( C H 2 P h ) 2 .
A cold solution o f Mg(CH 2 Ph) 2 x(dioxane) (0.432 g, 2.79 mmol) in T H F (-15
mL) was added to a frozen suspension of Cp*Cr(NO)(0 'Pr) 2 (0.466 g, 1.39 mmol) in
T H F (-35 mL). The liquid N 2 bath was removed and the solution was allowed to warm
slowly, infrared monitoring o f the reaction showed the nitrosyl band o f the
bis(propoxide) at 1669 c m - 1 being replaced by a new v(NO) peak at 1623 c m - 1 . After
-100 min, the green solution (orange to transmitted light) was reduced to dryness in
vacuo. The dark green residue was extracted with hexanes and filtered. N o ionic
byproduct was evident on the Celite filter pad. The solvent was again removed in vacuo
to afford a green oil . Attempts to extract the green oil with hexamethyldisiloxane resulted
in a green solution, a green tarry residue, and a white precipitate along the walls of the
flask. Hexanes was added to the (Me 3Si)20 suspension, and the green solution was
filtered. Cooling the dark green filtrate resulted in the deposition o f an amorphous green
material. Subsequent attempts to purify portions of the solution or the tarry residue by
aqueous extraction, addition of PI1CO2H, or chromatography were unsuccessful.
Reaction of C r ( N O ) ( 0 ' B u ) 3 wi th L i C H ( S i M e 3 ) 2 .
147
T H F (-10 mL) was vacuum transferred onto Cr(NO)(0'Bu)3 (0.110 g, 0.365
mmol) in a Schlenck vessel. The flask was warmed, the tris(butoxide) dissolved to afford
an orange solution, and the solution was then frozen using a liquid N 2 bath. A solution of
L i C H ( S i M e 3 ) 2 (0.184 g, 1.11 mmol, 3.03 equivalents) in -15 m L T H F was cannulated
onto the frozen Cr(NO)(0 'Bu) 3/THF solution. The combined solution was allowed to
warm slowly to room temperature. After ~ l h , the solvent was removed in vacuo, the red-
brown oily residue was extracted with hexanes ( 4 x 1 5 mL) and filtered to give a dark
khaki green solution that appeared bright red to transmitted light. The solution was
reduced in volume in vacuo to - 7 mL, and its infrared spectrum was taken (v(NO) at
1685 cm" 1). The solution was again removed in vacuo and the red-brown residue was
extracted with hexamethyldisiloxane (-4 mL). The solution was filtered and stored at -30
°C. Over several days, a fine brown silt precipitated from solution.
Reaction of C r ( N O ) ( N ' P r 2 ) ( 0 2 C P h ) 2 wi th dppe and M ( M = M g , K/graphite) .
T H F (-25 mL) was vacuum transferred onto Cr(NO)(N'Pr 2)(0 2CPh) 2 (0.039 g,
0.091 mmol), dppe (0.036 g, 0.090 mmol) and M g powder (0.040 g, 1.7 mmol). The
orange solution was allowed to stir and warm to room temperature. The initial TR
spectrum showed only a peak at 1721 c m - 1 , corresponding to v(NO) o f
Cr(NO)(N'Pr 2)(0 2CPh) 2. After -24 h, the solution turned from orange to bright green,
with no identifiable v(NO) bands in the TR spectrum of the green solution. A control
reaction o f dppe and Cr(NO)(N'Pr 2)(0 2CPh) 2 in T H F in the absence o f M g resulted in the
same slow color change and loss o f nitrosyl peaks.
T H F (-10 mL) was vacuum transferred onto Cr(NO)(N'Pr 2)(0 2CPh) 2 (0.094 g,
0.22 mmol), dppe (0.088 g, 0.22 mmol) and K C 8 (0.075 g, 0.54 mmol). A s the solution
thawed, an immediate reaction occurred to produce a dark brown solution. The TR
spectrum of this solution showed only broad lumps in the v (NO) region.
148
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(49) While the N ' P r 2 ligand could still be described as "extremely bulky" in the
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157
Appendix
C r y s t a l l o g r a p h i c D a t a 1 5 8
D F T - O p t i m i z e d G e o m e t r i e s 1 6 0
F r a c t i o n a l C o o r d i n a t e s 1 6 6
171 B o n d L e n g t h s 1 ' J
179 B o n d A n g l e s
158
Table A l . Crystallographic Data for Complexes 3.2, 3.7, and 3.8.
3.2 3.7 3.8
formula CnHs^rFsNjOsS CioHi 2CrN 402 CioH 1 2 CrN0 3
formula weight 396.30 272.24 246.21
crystal color brown green green
cryst. size (mm) 0.20x0.40x0.40 0.45x0.42x0.30 0.07x0.30x0.30
crystal system monoclinic monoclinic orthorhombic
space group P2i/n C2/c Pnma
a (A) 8.575(1) 18.3382(4) 17.373(2)
b(A) 8.642(1) 11.7774(3) 8.833(1)
c(A) 21.8626(9) 12.91020(10) 6.926(2)
a(degrees) 90 90 90
P (degrees) 91.760(7) 121.3860(10) 90
y(degrees) 90 90 90
V (A 3 ) 1619.4(3) 2380.31(8) 1062.8(7)
z 4 8 4
D c ai c(g/cm 3) 1.625 1.519 1.539
Fooo 808 1120 508
diffractometer Rigaku AFC6S Siemens SMART Platform CCD
Rigaku AFC6S
X (Mo K a , A) 0.71096 0.71073 0.71069
temperature (K) 294(1) 173(2) 294(1)
no. observations 2546 2072 787
no. variables 209 168 102
goodness of fit 2.27 1.021 1.59
R 0.039 0.0318 0.037
Rw 0.039 0.0815 0.033
159
Table A2. Crystallographic Data for Complexes 5.2 and 5.3.
4.4 5.2 5.3
formula CnHnMoNO C2oH3oCr2N202 Ci4H36CrN2OSi2
formula weight 347.27 505.36 356.62
crystal color green/black green red
cryst. size (mm) 0.50x0.40x0.10 0.25x0.10x0.10 0.20x0.45x0.60
crystal system orthorhombic monoclinic triclinic
space group Pbca P2x/n PI
a (A) 11.120(2) 7.7192(2) 11.2004(12)
b(A) 15.2821(3) 15.0848(4) 12.8210(9)
c(A) 16.8371(6) 10.0264(3) 16.979(2)
a(degrees) 90 90 91.035(7)
B (degrees) 90 106.132(2) 91.743(3)
y(degrees) 90 90 113.6640(8)
V(A3) 2861.4(3) 1121.53(5) 2231.0(3)
z 8 2 4
Dcaic(g/cm3) 1.612 1.496 1.062
Fooo 1408.00 524 776
diffractometer Rigaku/ ADSC C C D
Siemens SMART Platform CCD
Rigaku/ ADSC C C D
X (Mo Kct, A) 0.71069 0.71073 0.71069
temperature (K) 180(1) 173(2) 180(1)
no. observations 3639 1963 4520
no. variables 181 132 361
goodness of fit 1.53 1.030 2.25
R 0.030 0.0464 0.056
Rw 0.021 0.0930 0.047
160
Table A3. Fractional coordinates of DFT-optimized geometry of CpMo(NO)Cl 2.
CpMo(NO)Cl 2 (S = 0) CpMo(NO)Cl 2 (S = 1)
X y z X y z
Mo 0.000000 0.190450 0.000000 0.000000 0.163021 0.000000
CP 0.513256 -1.9311888 0.000000 -0.2589802 -1.9222444 0.000000
CI 1.268246 -2.060042 0.000000 0.957479 -2.045700 0.000000
C2 -0.935807 -1.826917 0.724666 0.113673 -1.960797 1.167768
C3 -0.935807 -1.826917 -0.724666 -1.239863 -1.821964 0.725643
C4 0.429998 -1.972781 -1.157595 -1.239863 -1.821964 -0.725643
C5 0.429998 -1.972781 1.157595 0.113673 -1.960797 -1.167768
HI 2.345722 -2.148656 0.000000 2.033854 -2.151079 0.000000
m. 0.765832 -1.977373 2.184814 0.450458 -1.984371 2.194003
H3 -1.809573 -1.792316 1.360190 -2.111234 -1.742680 1.361070
H4 -1.809573 -1.792316 -1.360190 -2.111234 -1.742680 -1.361070
H5 0.765832 -1.977373 -2.184814 0.450458 -1.984371 -2.194003
Cl l 0.874001 1.136367 2.019498 1.243732 1.217996 1.768110
C12 0.874001 1.136367 -2.019498 1.243732 1.217996 -1.768110
N -1.511760 1.24549 0.000000 -1.597493 1.055392 0.000000
0 -2.616468 1.642179 0.000000 -2.755916 1.453255 0.000000
161
Table A4. Fractional coordinates of DFT-optimized geometry of CpCr(NO)Cl2.
CpCr(NO)Cl 2 (S = 0) CpCr(NO)Cl 2 (S = 1)
X y z X y z
Cr 0.000000 0.149609 0.000000 0.000000 0.089812 0.000000
CP -0.407645 -1.7641532 0.000000 0.2902544 -1.8572206 0.000000
CI 0.751141 -2.150810 0.000000 1.493576 -1.651475 0.000000
C2 -0.048100 -1.888680 1.156117 0.660878 -1.807838 1.165255
C3 -1.346583 -1.446298 0.721814 -0.682030 -2.009476 0.727084
C4 -1.346583 -1.446298 -0.721814 -0.682030 -2.009476 -0.727084
C5 -0.048100 -1.888680 -1.156117 0.660878 -1.807838 -1.165255
HI 1.789523 -2.450725 0.000000 2.559484 -1.472351 0.000000
H2 0.279905 -1.961445 2.182599 0.989197 -1.728526 2.191053
H3 -2.185661 -1.209217 1.360492 -1.541494 -2.171345 1.363395
H4 -2.185661 -1.209217 -1.360492 -1.541494 -2.171345 -1.363395
H5 0.279905 -1.961445 -2.182599 0.989197 -1.728526 -2.191053
Cl l 1.100844 0.895396 1.842417 0.726568 1.406943 1.734847
C12 1.100844 0.895396 -1.842417 0.726568 1.406943 -1.734847
N -1.098043 1.388438 0.000000 -1.739598 0.748655 0.000000
0 -1.936381 2.245438 0.000000 -2.836078 1.219570 0.000000
162
Table A5. Fractional coordinates of DFT-optimized geometry of CpCr(NO)(NH2)Cl.
CpCr(NO)(NH2)Cl (S = 0) CpCr(NO)(NH2)Cl (S - 1)
X y z X y z
Cr 0.179940 -0.120375 0.258956 0.155841 -0.025860 0.126545
CP -1.68665 -0.090923 -0.3958494 -1.7945818 -0.1983268 -0.1523216
CI -2.063535 0.729330 0.427830 -1.964778 0.914908 0.318149
C2 -1.570631 1.011361 -0.891307 -1.744352 0.602914 -1.071777
C3 -1.239701 -0.222758 -1.529443 -1.590178 -0.810933 -1.191199
C4 -1.510431 -1.285569 -0.590850 -1.704051 -1.383852 0.131669
C5 -2.048952 -0.686979 0.604523 -1.969550 -0.314671 1.051550
HI -2.381499 1.465814 1.153071 -2.097568 1.905669 0.728543
H2 -1.422703 1.995688 -1.309704 -1.671689 1.323773 -1.873523
H3 -0.846316 -0.340498 -2.529160 -1.413874 -1.358298 -2.107304
H4 -1.405503 -2.344718 -0.780720 -1.658725 -2.437761 0.371030
H5 -2.305364 -1.227888 1.491003 -2.100153 -0.411941 2.120533
CI 1.322775 1.866161 -0.158810 1.207600 1.998265 -0.341661
N l 1.402034 -1.061370 -0.321940 1.433612 -1.175760 -0.444796
0 2.251912 -1.765224 -0.818629 2.26852 -2.006294 -0.739494
N2 0.467073 -0.436167 2.003794 1.072334 -0.221351 1.810266
H6 1.292471 -0.899210 2.382315 1.041972 -1.029439 2.434499
H7 -0.191384 -0.152688 2.729551 1.823676 0.438050 2.024705
163
Table A6. Fractional coordinates of DFT-optimized geometry of
CpCr(NO)(NH2)(CH3).
CpCr(NO)(NH2)(CH3) (S = 0) CpCr(NO)(NH 2)(CH 3) (S = 1)
X y z X y z
Cr 0.276935 -0.253682 -0.090082 0.260177 0.210833 0.051365
CP -1.5590238 0.49401 -0.018047 -1.6641252 -0.4092824 -0.0419234
CI -2.080923 -0.599806 0.152022 -2.017809 0.649412 0.453032
C2 -1.632990 0.286060 1.191474 -1.851198 0.390194 -0.946299
C3 -1.090316 1.461193 0.583538 -1.4028880 -0.964264 -1.098595
C4 -1.177758 1.293709 -0.844229 -1.339070 -1.558098 0.215438
C5 -1.813132 0.028894 -1.100852 -1.709669 -0.563656 1.166807
HI -2.537865 -1.569498 0.299231 -2.336909 1.582909 0.896139
H2 -1.688341 0.091984 2.253121 -2.007922 1.099846 -1.746446
H3 -0.678803 2.318461 1.097952 -1.206343 -1.470024 -2.034482
H4 -0.866681 2.012896 -1.589002 -1.046530 -2.576336 0.435832
H5 -2.016326 -0.381636 -2.080780 -1.744071 -0.689617 2.241023
N l 1.691961 0.572418 -0.174880 1.718721 -0.761915 -0.215391
0 2.707884 1.254216 -0.193440 2.711528 -1.456423 -0.317665
N2 0.687675 -1.620891 -1.190551 1.114818 0.987615 1.590634
H6 1.610786 -1.798451 -1.585162 1.219422 0.517810 2.492472
H7 -0.008865 -2.285496 -1.529809 1.701867 1.819885 1.515106
C6 0.853132 -1.043244 1.735861 0.809129 1.797014 -1.137656
H8 0.122548 -1.810560 2.014320 0.177586 2.659212 -0.883793
H9 1.840873 -1.488146 1.584686 1.865300 2.032649 -0.955460
H10 0.907632 -0.256438 2.496064 0.675333 1.531546 -2.194901
164
Table A7. Fractional coordinates of DFT-optimized geometry of CpCr(NO)(CH 3) 2.
CpCr(NO)(CH 3) 2 (S = 0) CpCr(NO)(CH 3) 2 (S = 1)
X y z X y z
Cr 0.000000 3.81150 0.000000 0.000000 0.281549 0.000000
CP 0.70922 -1.4710862 0.000000 0.3175912 -1.7198644 0.000000
CI 1.883182 -1.139789 0.000000 1.515850 -1.481532 0.000000
C2 1.074130 -1.369926 1.157525 -0.651080 -1.909265 0.717461
C3 -0.242671 -1.737895 0.719960 -0.651080 -1.909265 -0.717461
C4 -0.242671 -1.737895 -0.719960 0.687133 -1.649630 -1.160128
C5 1.074130 -1.369926 -1.157525 0.687133 -1.649630 1.160128
HI 2.918678 -0.825391 0.000000 2.578372 -1.280918 0.000000
H2 1.389776 -1.266015 2.186123 1.014485 -1.581168 2.188364
H3 -1.079161 -1.995703 1.354291 -1.510335 -2.077877 1.353372
H4 -1.079161 -1.995703 -1.354291 -1.510335 -2.077877 -1.353372
H5 1.389776 -1.266015 -2.186123 1.014485 -1.581168 -2.188364
N -1.569566 0.855582 0.000000 -1.405353 1.393943 0.000000
0 -2.762181 1.126724 0.000000 -2.316012 2.198245 0.000000
C6 0.436044 1.445636 -1.671323 0.905041 1.389661 -1.499485
H6 1.502113 1.343492 -1.905142 1.962124 1.097733 -1.527345
H7 -0.177790 1.170493 -2.535251 0.420615 1.154101 -2.457336
H8 0.193362 2.477667 -1.377559 0.812597 2.457295 -1.278459
C7 0.436044 1.445636 1.671323 0.905041 1.389661 1.499485
H9 1.502113 1.343492 1.905142 1.962124 1.097733 1.527345
H10 -0.177790 1.170493 2.535251 0.420615 1.154101 2.457336
H l l 0.193362 2.477667 1.377559 0.812597 2.457295 1.278459
165
Table A8. Fractional coordinates of DFT-optimized geometry of CpCr(NO)(CH3)Cl.
CpCr(NO)(CH 3)Cl (S = 0) CpCr(NO)(CH 3)Cl (S = 1)
X y z X y z
Cr -0.211457 -0.014328 0.105076 0.155932 -0.014571 0.182793
CP 1.7029544 -0.1303132 -0.2956746 -1.792489 0.0718936 -0.2776332
CI 2.047656 0.710178 0.519777 -2.034280 0.032015 0.923950
C2 1.917450 -0.700164 0.758324 -1.801195 1.216156 0.148109
C3 1.497781 -1.327673 -0.462453 -1.594803 0.812861 -1.230860
C4 1.346614 -0.292988 -1.453915 -1.640480 -0.604375 -1.282732
C5 1.705271 0.959081 -0.840106 -1.891687 -1.097189 0.053367
HI 2.317913 1.457300 1.253426 -2.241042 -0.002826 1.983656
H2 2.109755 -1.205932 1.693346 -1.866033 2.234639 0.507657
H3 1.346043 -2.386299 -0.619376 -1.414646 1.477660 -2.065014
H4 1.060374 -0.437882 -2.486045 -1.473143 -1.217405 -2.157311
H5 1.659859 1.928287 -1.316040 -1.989477 -2.136828 0.334335
N -1.376291 -1.051998 -0.422105 1.287554 1.379374 -0.141715
0 -2.161320 -1.881166 -0.829349 2.022366 2.272032 -0.468429
CI -1.292038 1.952441 -0.105964 1.620704 -1.602511 -0.601989
C6 -0.592630 -0.630246 2.030642 0.665025 -0.286869 2.148684
H6 0.049023 -0.044905 2.695842 0.233151 -1.239911 2.476285
H7 -1.645908 -0.349846 2.142799 1.757733 -0.335854 2.184994
H8 -0.465687 -1.704170 2.191526 0.291843 0.545466 2.758515
166
Table A9. Fractional Coordinates and B e q for 3.2.
atom X y
Cr(l) 0.62171(6) 0.32891(6)
S(l) 0.30990(10) 0.12690(10)
F(l) 0.1361(3) -0.0776(3)
F(2) . 0.1278(3) 0.1452(3)
F(3) 0.3261(3) 0.0014(3)
0(1) 0.3990(2) 0.2564(2)
0(2) 0.1813(3) 0.1771(3)
0(3) 0.4049(3) 0.0055(3)
0(4) 0.7496(4) 0.3830(4)
0(5) 0.7996(3) 0.0524(3)
N(l) 0.6823(4) 0.3516(3) .
N(2) 0.7159(3) 0.1565(3)
C(l) 0.7798(3) 0.5096(4).
C(2) 0.6377(4) 0.5841(3)
C(3) 0.5170(3) 0.5308(3)
C(4) 0.5821(4) 0.4224(3)
C(5) 0.7461(3) 0.4145(4)
C(6) 0.9381(4) 0.5388(5)
C(7) 0.6246(5) 0.7051(4)
C(8) 0.3516(4) 0.5817(4)
C(9) 0.4968(4) 0.3432(4)
C(10) 0.8603(4) 0.3280(5)
C(l l) 0.2213(5) 0.0450(5)
z B e ?
0.38638(2) 2.94(1)
0.36910(4) 3.72(2)
0.4194(1) 7.62(7)
0.4615(1) 7.71(8)
0.4769(1) 7.90(8)
0.3968(1) .4.11(5)
0.3318(1) 6.39(8)
0.3462(1) . 5.89(7)
0.5068(1) 8.56(10)
0.3684(2) 7.86(10)
0.4612(1) 4.93(8)
0.3762(1) 4.53(7)
0.3573(1) 3.29(7)
0.3744(1) 3.19(7)
0.3345(1) 2.96(7)
0.2931(1) 3.07(7)
0.3060(1) . 3.18(7)
0.3866(2) 5.30(10)
0.4230(2) 5.2(1)
0.3337(2) 4.54(9)
0.2414(1) 4.66(9)
0.2689(2) 5.5(1)
0.4358(2) 4.9(1)
Table A10. Fractional Coordinates and Ueq for 3.7.
X Y Z U(eq)
C r ( l ) '2018 (1) 4 (1) 2303 (1) 23(1) N ( l ) 2556 (1) -134 (2) 1557 (2) 34(1) N(2) 2881 (1) -476 (2) 3635 (2) 27 (1) 0(1) 2965 (2) -118 (2) 1104 (2) . 62 (1) 0(2) 3522 (1) -690 (2) 4536 (2) 42 (1)
C ( l ) 2029 (2) 1711 (2) 2999 (3) ; 46(1) C(2) 1406 (2) 1067 (2) 3083 (2) . 34(1) C(3) 774 (1) 767 (2) 1896 (2) 32(1) C (4) 1000 (2) 1205 (2) 1093 (2) 40(1) C(5) 1771 (2) 1797 (2) 1779 (3) 48 (1) N(3) 1349 (1) -1445 (2) 1665 (2) 25(1) N(4) 1108 (1) -1778 (2) 502 (2) 26 (1) C(6) 696 (1) -2767 (2) 318 (2) 29(1) C(7) 667 (2) -3077 (2) 1338 (2) 32(1) C(8) 1084 (1) -2219 (2) 2170 (2) 29(1) C(9) 340 (2) -3370 (2) -876 (2) 42(1) C(10) 1244 (2) -2085 (2) 3426 (2) 40(1)
Table A l l . Fractional Coordinates and B
atom X y
Cr(l) 0:45374(4) 0.2500
0(1) 0.4035(2) 0.2500
0(2) 0.3885(1) 0.0918(2)
N(l) 0.4174(2) 0.2500
C(l) 0.5500(3) 0.2500
C(2) 0.5573(2) 0.3763(5)
C(3) 0.5688(2) 0.3288(5)
C(4) 0.3187(2) 0.1102(3)
C(5) . 0.2825(3) 0.2500
C(6) 0.2755(3) -0.0313(5)
for 3.8.
z occ
0.1753(1) 2.83(2) 1/2
-0.2198(6) 5.3(1) 1/2
0.2874(3) 3.20(5)
-0.0499(7) 3.4(1) 1/2
0.3938(9) 4.9(2) 1/2
0.2769(7) 4.4(1).
0.0890(8) 4.26(10)
0.3392(5) 2.70(7)
0.3560(8) 3.2(1) 1/2
0.3841(7) 4.1(1).
169
Table A12. Fractional Coordinates and B e q for 4.4.
atom y z Be,
Mo(l) 0.08242(2) 0.174875(14) 0.10364(2) 1.677(5)
0(1) 0.2723(2) 0.04584(12) 0.06261(13) 3.26(6)
N(l) : 0.1903(2) 0.09618(13) 0.07689(14) 1.91(6)
C(l) 0.0288(3), 0.2235(3) -0.0257(2) 4.54(11)
C(2) -0.0204(4) 0.1401(2) -0.0150(3) 4.80(12)
C(3) -0.1140(4) 0.1501(2) 0.0392(3) - 4.26(11)
C(4) -0.1299(3) 0.2401(2) 0.0532(2) 3.94(10)
C(5) -0.0468(4) 0.2833(2) 0.0135(2) 3.82(10)
C(6) 0.2373(3) 0.2644(2) 0.1512(2) 2.64(8)
C(7) 0.1964(3) 0.2139(2) 0.2146(2) 2.23(7)
C(8) 0.0807(3) 0.2434(2) 0.2347(2) 2.25(7)
C(9) 0.0539(3) 0.3156(2) 0.1863(2) 2.35(7)
C(10) 0.1470(3) 0.3265(2) 0.1333(2) 2.71(7)
C(H) -0.0068(3) 0.0705(2) 0.1781(2) 2.00(7)
C(12) 0.0635(3) 0.03364(15) 0.2457(2) 1.79(7)
C(13) 0.1632(3) -0.0209(2) 0.2334(2) 1.93(7)
C(14) 0.2284(3) -0.0541(2) 0.2959(2) 2.31(8)
C(15) 0.1970(3) -0.0354(2) 0.3735(2) 2.47(8)
C(16) 0.0975(3) 0.0163(2) 0.3869(2) 2.66(7)
C(17) 0.0320(3) 0.0494(2) 0.3248(2) 2.45(8)
Table A13. Fractional Coordinates and Ueq for 5.2.
X y z U(eq)
C r (1) 268 (1) 4258(1) 6124 (1) 20(1)
CI (1) 2192 (1) 5373 (1) 5761 (1) 26(1)
C ( l ) -1315 (5) 4726(3) 7590 (4) 27 (1)
0(1) 1728 (5) 2777 (2) 5033 (3) 54.(1)
N ( l ) 1115 (5) . 3404 (2) 5375 (3) 32(1)
C(2) -1517 (5) 3783(3) 7358 (4) 29(1)
C(3) 201 (6) 3399(2) 7877 (4) 29 (1)
C(4) 1461 (5) 4089(3) 8362 (4) 26 (1)
C(5) 491 (5) .4898 (2) 8219 (4) 24(1)
C(6) -2823 (6) 5392 (3) 7246 (5) 49(1)
C(7) -3260 (6) 3297 (3) 6784 (5) 52(1)
C(8) 604 (7) 2418 (3) 7954 (5) 53(1)
C(9) 3457 (6) 3975 (3) 9015 (4) 47(1)
C"(10) 1291 (7) 5801(3) 8700 (4) 44 (1)
171
Table A14. Fractional Coordinates and B e q for 5.3.
atom X y
Cr ( l ) 0.14618(4) 0.54593(4)
Cr(2) 0.19977(4) 0.06207(4)
Si( l ) 0.23795(8) 0.58170(7)
Si(2) 0.39461(9) 0.61244(8)
Si(3) 0.27887(13) 0.06298(9)
Si(4) -0.10192(9) -0.03322(8)
0(1) 0.2372(2) 0.7855(2)
0(2) 0.1152(2) 0.2358(2)
N ( l ) 0.1972(2) 0.6824(2)
N(2) -0.0011(2) 0.5097(2)
N(3) 0.1517(2) 0.1624(2)
N(4) 0.3511(2) 0.1395(2)
C ( l ) 0.1212(3) 0.4901(2)
C(2) 0.2898(3) 0.5270(2)
C(3) 0.4080(3) 0.6475(3)
C(4) 0.1887(4) 0.6979(3)
•C(5) 0.2391(3) 0.4959(3)
C(6) 0.5369(4) 0.5721(5)
C(7) 0.4527(5) 0.7669(3)
C(8) 0.3020(4) 0.5796(4)
C(9) -0.0807(3) 0.5666(3)
C(10) -0.0056(4) 0.6517(3)
C ( l l ) -0.1229(4) 0.6249(3)
C(12) -0.0515(3) 0.3833(2)
z B e ,
0.26346(3) 2.190(10)
0.26731(3) 2.494(11)
0:45335(5) 3.12(2)
0.13013(5) 3.49(2)
0.07143(6) 5.44(3)
0.33496(6) 3.91(2)
0.28680(14) 4.78(6)
0.2249(2) 5.37(7)
0.27623(14) 2.93(6)
0.20772(13) 2.53(5)
0.2429(2) 3.36(6)
0.31820(14) . 2.75(6)
0.3751(2) 2.78(7)
0.2075(2) 2.85(7)
0.4181(2) 4.13(8)
0.4855(2) 4.85(10)
0.5409(2) 4.44(9)
0.1191(3) 7.97(14)
0.1562(3) 7.65(13)
0.0349(2) 5.79(11)
0.1749(2) 3.37(8)
0.1140(2) 4.92(10)
0.2412(2) 5.00(10)
0.1985(2) 2.83(7)
172
Table A14. Fractional Coordinates and B e q for 5.3 (continued).
atom
C(13)
C(H)
C(15)
C(16)
C(17) '
C(18)
C(19)
C(20)
C(21)
C(22) •
C(23)
C(24)
C(25)
C(26)
C(27)
C(28)
-0.0559(4)
-0.1799(3)
0.2075(3)
0.0610(3)
0.2314(6)
0.4604(5)
0.2263(7)
-0.2196(4)
-0.0952(5)
-0.1655(4)
0.4289(3)
0.3522(4)
0.4755(3)
0.3923(3)
0.3915(4)
0.5201(3)
y z
0.3451(3) 0.1127(2) 4.35(9)
0.3232(3) 0.2378(2) 4.63(9)
-0.0081(3) 0.1630(2) 3.82(8)
-0.0356(3) 0.3393(2) 4.03(9)
-0.0438(4) -0.0122(3) 9.5(2)
0.1224(5) 0.0831(3) 9.6(2)
0.1799(5) 0.0482(3) 10.7(2)
-0.1589(4) 0.3870(3) 7.30(13)
0.1004(4) 0.3842(3) . 6.94(13)
-0:0415(4) 0.2320(3) 6.45(12)
. 0.2581(2) 0.3425(2) 3.17(7)
0.3024(3) ' 0.3958(2) 4.43(9)
0.3312(3) 0.2727(2) 3.90(8)
0.0500(3) 0.3460(2) 3.60(8)
0.0413(3) 0.4351(3) 5.74(11)
0.0592(3) 0.3124(3) 5.51(11)
173
Table A15. Bond Lengths for 3.2.
atom atom distance atom atom distance
Cr(l') . 0(1) 2.030(2) Gr(l) N(l) 1.713(3)
Cr(l) N(2) 1.712(3) Cr(l) C(l) 2.176(3)
Cr(l) C(2) 2.226(3) CT(1) C(3) 2.253(3)
Cr(l) C(4) 2.211(3) CT(1) C(5) 2.211(3)
Cr(l) CP 1.85 S(l) 0(1) 1.475(2)
S(l) : 0(2) 1.419(3) S(l) 0(3) 1.428(3)
S(l) C(l l ) 1.810(4) C(l l) 1.330(4)
F(2) C(H) 1.317(4) F(3) ' C(l l ) 1.306(4)
0(4) •N(l) 1.169(3) 0(5) N(2). 1.167(3)
C(l) C(2). 1.438(4) C(l) . C(5) 1.413(4)
C(l) : C(6) 1.504(4) C(2) C(3) 1.409(4)
C(2) C(7) 1.498(4) C(3) C(4) . , 1.429(4)
C(3) C(8) 1.485(4) C(4) C(5) 1.428(4)
c(4): •. C(9). 1.493(4) C(5) C(10) 1.492(4)
*CP refers to the unweighted centroid of the C(l-5) ring.
Table A16. Bond Lengths for 3.7.
C r ( 1 ) - N ( l ) C r ( l ) - N ( 3 ) C r ( 1 ) - C ( l ) C r ( 1 ) - C ( 2 ) N ( l ) -0(1) C ( l ) -C(5) C(2) -C<(3) C(4) -C(5) . N(3) -N(4) C(6) -C(7) C(7) -C(8)
1.707 (2) 2.011(2) 2.198 (3) 2.240(2) 1.168 (3) 1.392 (4) 1.402 (3) 1.401 (4) 1.383 (3) 1.393 (3) 1.380 (3)
C r ( l ) - N ( 2 ) C r ( l ) - C ( 5 ) C r ( l ) - C ( 4 ) C r ( 1 ) - C ( 3 ) N(2) -0(2) C ( l ) - C ( 2 ) C(3) -C(4) N(3) -C(8) N(4) -C(6) C(6)-C(9) C(8) -C(10)
1.716 (2) 2.192 (3) 2.212 (2) 2.245 (2) 1.173 (2) 1.423 (4) 1.399(4) 1.350(3) 1.340 (3) 1.503(3) 1.498(3)
175
Table A17. Bond Lengths for 3.8.
atom atom . distance
Cr(l) 0(2) 1.959(2)
Cr(l) • C(l) 2.255(6)
Cr(l) . C(3) 2.199(4)
0(1) N(l) 1.201(5)
C(l) C(2) 1.384(5)
C(3) C(3)* 1.393(8)
C(4) C(6) 1.490(5)
atom atom distance
Cr(l) N(l) 1.683(5)
Cr(l) ..' C(2) 2.231(4)
Cr(l) CP 1.89
0(2) C(4) 1.275(3)
C(2) C(3) 1.381(6)
C(4) C(5) 1.391(3)
176
Table A18. Bond Lengths for 4.4.
atom atom distance atom atom distance
Mo( l ) N ( l ) 1.757(2) Mo( l ) C ( l ) 2.377(4)
M o ( l ) C(2) 2.362(4) Mo( l ) C(3) 2.469(3)
M o ( l ) C(4) 2.700(3) M o ( l ) C(5) 2.667(3)
M o ( l ) C(6) 2.340(3) Mo('l) C(7) 2.336(3)
Mo( l ) C(8) 2.442(3) Mo( l ) C(9) 2.581(3)
Mo( l ) C(10) 2.477(3) Mo( l ) C ( l l ) . 2.258(3)
Mo( l ) CP(1) 2.23 Mo( l ) CP(1) 2.12
0(1) N ( l ) 1.217(3) C( l ) C(2) 1.398(5)
C( l ) C(5) . 1.407(5) C(2) C(3) 1.392(5)
C(3) . C(4) 1.407(5) C(4) C(5) 1.317(5)
C(6) C(7) 1.394(4) C(6) C(10) 1.414(4)
C(7) C(8j 1.404(4) C(8) C(9) 1.404(4)
C(9) C(10) 1.377(4) C ( l l ) C(12) 1.492(4)
C(12) C(13) 1.402(4) C(12) C(17) 1.398(4)
C(13) . ' C(14) 1.377(4) C(14) C(15) 1.382(4)
C(15) C(16) 1.378(4) C(16) C(17) 1.372(4)
* Here and elsewhere, CP(1,2) refer to the unweighted centroids of the
C(l-5) and C(6-10) rings, respectively.
Table A19. Bond Lengths for 5.2.
C r ( 1 ) - N ( l ) . ' - : 1 .709(4) C r ( 1 ) - C ( 3 ) 2 .196(4) C r ( 1 ) - C ( l ) - . 2 .269(4)
. C r ( 1 ) - C I ( 1 ) 2 .3386(11) C l ( 1 ) - C r ( 1 ) # 1 2 .3434(11) C ( l ) -C(2 ) " 1.442 (5) 0(1) - N ( l ) 1.152 (4) C(2) -C. (7 ) 1 .499(5) 6 ( 3 ) - C ( 8 ) 1.508 (5) C ( 4 ) - C ( 9 ) 1 .507(6) ,
C r ( l ) - C ( 4 ) 2 191(4) C r ( l ) - C ( 2 ) • 2 212 (4) C r ( l ) - C ( 5 ) 2 274(3) C r ( l ) - C l ( l ) # l : 2 3434(11) C ( l ) - C ( 5 ) . 1 386 (5) C ( l ) - C ( 6 ) 1 504(5) C(2) -CU) 1 408 (5) C ( 3 ) - C ( 4 ) 1 415(5) C(4) -C(5 ) 1 418(5) C ( 5 ) - C ( 1 0 ) 1 518 (5),
178
Table A20. Bond Lengths for 5.3.
atom atom distance
Cr(l) N(l) 1.616(2)
Cr(l) C(l). 2.026(3)
Cr(2) N(3) 1.637(3)
Cr'(2) C(15) 1.992(3)
Si(l) C(l) 1.859(3).
Si(l) G(4) 1.863(3)
Si(2) C(2) . 1.849(3)
Si(2) C(7) 1.859(4)
Si(3).' C(15) 1.852(4)
Si(3) C(18) 1.866(6)
Si(4) C(16) 1.836(3)
Si(4) • C(21) 1.865(4)
0(1)' N(l) 1.221(3)
..N(2) C(9) 1.464(4)
•N(4) C(23) 1.461(4)
C(9) C(10) 1.525(5)
C(12) . '' C(13) 1.520(4)
C(23) C(24) 1.516(4)
C(26) C(27) 1.520(5)
atom atom distance
Cr(l) N(2) 1.763(2)
Cr(l) C(2) 1.986(3)
Cr(2) N(4) 1.770(2)
Cr(2) C(16) 2.022(3)
Si(D C(3) 1.869(3)
Si(l) C(5) 1.869(4)
Si(2) • C(6) l'.876(4)
Si(2) C(8) 1.842(4)
Si(3) C(17) 1.863(5)
Si(3) C(19) 1.861(5)
Si(4) C(20) 1.879(4)
Si(4) C(22) 1.853(4)
0(2) N(3) • 1.207(3)
N(2) C(12) 1.491(3)
N(4) C(26) 1.477(4)
C(9) C(ll) 1.529(4)
C(12) C(14) • 1.516(4)
C(23) C(25) 1.498(4)
C(26) C(28) 1.518(5)
179
Table A21. Bond Angles for 3.2.
atom atom atom angle
0(1) Cr(l) N(l) 100.8(1)
0(1) Cr(l) CP 117.3
N(l) Cr(l) CP 118.0
0(1)'-. S(l) 0(2) . 112.8(2)
0(1) S(l) G(H) ; . 101.0(2)
0(2) S(l) 'C( l l ) • 104.2(2)
Cr(l) 0(1) S(l) 131.7(1)
Cr(l) N(2) 0(5) 169.9(3)
C(2) C(l) C(6) 125.1(3)
C(l) C(2) C(3) 107.8(3)
C(3) C(2) C(7) 126.7(3)
C(2) • C(3) ' • C(8) 126.4(3)
C(3) C(4) C(5)- 107.9(3)
C(5) '• C(4) C(9) 125.8(3)
C(l) • . C(5) C(10) 127.0(3)
S(D- 'C( l l ) F(i) '• '• 1.09.5(3)
S(l) C(l l) F(3) 111.7(3)
F(l) C (H) ' : F(3) 108.5(3)
atom atom atom angle
0(1) Cr(l) N(2) 101.3(1)
N(l) Cr(l) N(2) 95.3(2)
N(2) . Cr(l) CP : .120.2
0(1) S(l) 0(3) ,114.1(1)
0(2) S(l) 0(3) 1.17.7(2)
0(3) S(l) C(H) 104.7(2)
Cr(l) N(l) 0(4) 165.7(3)
C(2) C(l) C(5) 108.2(3)
C(5) C(l) C(6) 126.5(3)
C(l) C(2) C(7) . 125.4(3)
C(2) C(3) C(4) 108.2(3)
C(4) C(3) C(8) 125.3(3)
C(3) C(4) C(9) 126.1(3)
C(l) C(5) C(4) • 107.8(3)
C(4) C(5) C(10) 125.1(3)
S(l) C(l l) F(2) 111.3(3)
F(l) C(l l ) F(2) 107.6(3)
F(2) C(l l) F(3) 108.1(3)
180
Table A22. Bond Angles for 3.7.
N ( l ) -Cr (1) -N(2) N(2) -Cr(1) -N(3) N(2) -Cr (1 -C(5) N ( l ) -Cr (1 - C ( l ) N(3) -Cr (1 - C ( l ) N ( l ) -Cr (1 -C (4) N(3) -Cr (1 -C(4) C ( l ) - Cr (1 -C(4) N(2) -Cr (1 -C (2) C(5) -Cr (1 ) -C(2) C (4) -Cr (1 )-C(2) N(2) -Cr (1 ) -C(3) C(5) -Cr (1 ) -C(3) C (4) -Cr (1 )-C(3) 0(1) - N ( l ) -Cr(1) C(5) - C ( l ) -C(2) C (2) - C ( l ) -Cr(1) C(3) -C(2) -Cr (1) C (4) -C(3) -C(2) C(2) -C(3) -Cr(1) C(3 -C (4) -Cr(1) C ( l -C(5) -C(4) C(4 -C(5) -Cr(1) C(8 -N(3) -Cr(1) C(6 -N(4) -N(3) N(4 -C(6) -C(9) C(8 -C (7) -C(6) N(3 ) -C(8) -C(10)
91.86(9) N ( l ) 100.29(8) N ( l ) 124.14(11)N(3) 115.61(11)N(2) 144.83 (9) C(5) 100.71(10)N(2) 98.21(9) C(5) 6 1 . 8 4 ( l l ) N ( l ) 98.61(9) N(3) 6 1 . 8 7 ( l l ) C ( l ) 61.52(10)N(1)
131.31(9) N(3) 61.33(10)C(1) 36.59(9) C(2)
172.7(2) 0(2) 108.1(2) C(5) 72.92(14)C(3) 71.98(13)C(1)
108.7(2) C(4) 71.60(13)C(3) 73.01(14)C(5)
108.4 (2) 72.2 (2)
132.2 (2) 105.7 (2) 119.8 (2) 105.4 (2) 122.3 (2)
C ( l ) C(8) N(4) N(4) C(7) N(3) C(7)
-Cr(1) -Cr(1) -Cr(1) -Cr(1) -Cr(1) -Cr(1) -Cr(1) -Cr(1) -Cr(1) -Cr(1) -Cr(1) -Cr(1) -Cr(1) -Cr(1) -N(2)--C ( D -- C ( 2 ) -- C ( 2 ) -- C ( 3 ) -- C ( 4 ) -- C ( 4 ) -- C ( 5 ) --N(3)--N(3)--C(6) --C(6) --C ( 8 ) -- C ( 8 ) -
-N(3) -C(5) -C(5) - C ( l ) - C ( l ) -C(4) -C(4) -C(2) -C(2) -C(2) -C(3) -C(3) -C(3) -C(3) Cr (1) Cr (1) C ( l ) Cr (1) C r ( l ) C(5) C r ( l ) Cr (1) N(4) C r ( l ) C(7) C(9) C(7) C(10)
95. 90.
134. 94 . 36.
156 . 37.
151. 108. 37.
136 . 85. 61. 36 .
171. 71.
106 . 69. 70.
107. 70, 71
109 117 .110 129 108 129
57(9) 28 (11) 99 (10) 81 (10) 98 (12) 47(10) 11(11) 45(10) ,44(9) ,38(10) .13(10) .10 (8) .42(9) .43(9) .8(2) . .3(2) .9(2) .70(14) .40 (13) .8(2) .7(2) .8(2) .9(2) .87 (13) .8 (2) .4 (2) .1(2) .6(2)
181
Table A23. Bond Angles for 3.8.
atom atom atom angle
0(2) . Cr(l) 0(2)* 91.0(1)
0(2) . , .Gr(l) CP 119.7
Cr(l) ,' . 0 ( 2 ) • C(4) - 124.8(2)
C(2) C(l) . C(2) 107.4(6)
C(2) C(3) C(3)* 107.7(3)
0(2) C(4) C(6) . 115.5(3)
C(4) C(5) C(4)* 125.2(4)
atom atom atom angle
0(2) Cr(l) • N(l) 98.6(1)
N(l) Cr(l) CP 122.6
Cr(l) N(l) 0(1) 169.5(4)
C(l) C(2) C(3) 108.7(4)
0(2) C(4) C(5) 124.5(3)
C(5) C(4) C(6) 119.9(3)
* Symmetry operation: x, 1/2-y, z
182
Table A24. Bond Angles for 4.4.
atom atom atom angle
N ( l ) Mo( l ) C ( H ) 87.66(10)
N ( l ) M o ( l ) CP(2) 115.3
C ( U ) Mo( l ) CP (2) 106.6
Mo( l ) . N ( l ) 0(1) 174.1(2)
C ( l ) C(2) C(3) 106.1(3)
C(3) C(4) C(5) 108.5(4)
C(7) C(6) C(10) . 107.7(3)
C(7) C(8) C(9) 107.9(3)
C(6) C(10) C(9) 108.3(3)
C ( l l ) C(12) C(13) 121.8(3)
C(13) C(12) C(17) 116.2(3)
C(13) C(14) C(15) 120.9(3)
C(15) C(16) C(17) 120.8(3)
atom atom atom angle
N ( l ) Mo( l ) CP(1) 116.8
C ( H ) Mo( l ) CP(1) 103.5
CP(1) Mo( l ) CP(2) 119.8
C(2) C ( l ) C(5) 107.3(3)
C(2) C(3) G(4) 108.1(3)
C ( l ) C(5) C(4) 109.3(3)
C(6) C(7) C(8) 107.8(3)
C(8) C(9) C(10) 108.1(3)
Mo( l ) C ( H ) C(12) 117.4(2)
C ( H ) C(12) C(17) 122.0(3)
C(12) C(13) C(14) 121.5(3)
C(14) C(15) C(16) 118.5(3)
C(12) C(17) C(16) 122.0(3)
183
Table A25. Bond Angles for 5.2.
N ( l ) - C r ( 1 ) -C(4 ) 105. 0(2) N ( l ) - C r (1 ) - C ( 3 ) 90 . 18(14) C (4) - C r ( 1 ) - C ( 3 ) 37. 64(14)N(1) - C r (1 ) - C ( 2 ) 112. 1(2) C (4) - C r ( 1 ) - C ( 2 ) 62. 55(14)C(3) - C r (1 ) - C ( 2 ) 37 . 27(14) N ( l ) - C r ( 1 ) - C ( l ) 149 2(2) C(4) - C r (1 ) - C ( l ) 6 1 . 41(14) C(3) - C r ( 1 ) - C ( l ) 61 87 (14)C(2) - C r (1 ) - C ( l ) 37 . 52(14) N ( l ) - C r ( 1 ) -C(5 ) 141 7(2) C(4) - C r (1 ) - C ( 5 ) 36. 98(13) C(3) - C r ( 1 ) - C ( 5 ) 61 58(13)C(2) - C r (1 ) - C ( 5 ) 61 . 30(13) C ( l ) - C r ( 1 ) - C ( 5 ) 35 52 (13)N(1) - C r (1 ) - C l ( l ) 97. 85(11) C (4) - C r ( 1 ) - C l ( l ) 97 97 (11)C(3) - C r (1 ) - C l ( l ) 134 92(11) C (2) - C r ( 1 ) - C I ( 1 ) 147 30(11)C(1) - C r (1 ) - C l ( l ) 110 91(11) C(5) - C r ( 1 ) - C I ( 1 ) 87 13(10)N(1) - C r (1 ) - C l ( l ) # l 98 51(12) C (4) - C r ( 1 ) - C I ( 1 ) # 1 150 40 (11)C(3) - C r (1 ) - C l ( l ) # l 126 27(11) C(2) - C r ( 1 ) - C I ( 1 ) # 1 92 12 ( l l ) C ( l ) - C r (1 ) - C l ( l ) # l 89 28(10) C(5) - C r ( 1 ) - C l ( l ) # l 118 7 4 ( 1 0 ) C 1 ( 1 ) - C r ( 1 ) -C1(1 )#1 96 40 (4) C r ( l ) - C l ( l ) - C r ( l ) # l 83 60 (4) C(5) - C ( l ) -C(2 ) 108 0(3) C(5) - C ( l ) - C(6) 126 6 (4) C(2) - C ( l ) -C(6 ) 125 4(4) C(5) - C ( l ) - C r (1) 72 4 (2) C(2) - C ( l ) - C r ( l ) 69 1(2) C(6) - C ( l ) - C r (1) 125 5(3) 0(1) - N ( l ) - C r ( 1 ) 171 3(3) C(3) - C ( 2 ) - C ( l ) 107 3 (3) C(3) -C(2) -C(7) 126 4 (4) C ( l ) - C (2 ) - C(7) 126 1(4) C(3) - C (2) - C r ( 1 ) 70 8 (2) C ( l ) - C ( 2 ) - C r (1) 73 4 (2) C(7) -C(2 ) - C r ( 1 ) 125 1(3) C (2) -C(3 ) - C(4) 108 .1(3) C(2) -C(3) -C(8) 125 6.(4) C (4) - C { 3 ) - C(8) 126 .3 (4) C(2) -C(3 ) - C r ( l ) 72 0(2) C(4 - C ( 3 ) - C r (1) 71 .0(2) C(8] -C(3) - C r ( l ) 124 .7(3) C(3 - C ( 4 ) - C(5) 107 .8 (3) C(3 -C(4) -C(9 ) 126 .0(4) C(5 - C ( 4 ) - C(9) 126 .1(4) C(3 - C (4) - C r ( l ) 71 .4 (2) C(5 - C ( 4 ) - C r ( l ) 74 .7(2) C(9 -C(4) - C r ( l ) 123 .1(3) C ( l - C ( 5 ) - C(4) 108 .7(3) C ( l -C(5 ) -C(10) 125 .6 (4) C (4 - C ( 5 ) - C(10) 125 .7(4) C ( l -C(5 ) - C r ( l ) 72 .0 (2) C (4 - C ( 5 ) - C r ( l ) 68 .3 (2) C ( 1 0 ) - C ( 5 ) - C r ( 1 ) 126 .5 (3)
184
Table A26. Bond Angles for 5.3.
atom atom atom angle
N(l) Cr(l) . N(2) 102.91(12)
N(l) Cr(l) C(2) . 103.46(12)
N(2) Cr(l) C(2) 115.68(11)
N(3) Cr(2) N(4) 102.60(11)
N(3) Cr(2) • €(16) 103.73(13)
N(4) Cr(2) C(16) 112.57(13)
C(l) Sid) . C(3) •. 111.69(14)
C(l) Si(l) C(5) 110.33(14)
C(3) Si (!) C(5) 107.9(2)
C(2) Si(2) C(6) 109.5(2)
C(2) Si(2) C(8) 109.55(15)
C(6) Si(2) .' C(8) 107.2(2)
C(15> ' •Si(3) C(17) 109.7(2)
C(lo) Si(3) C(19) 111.7(2)
C(17) Si(3) C(19) 110.0(3)
C(.16) Si(4). C(20). 109.8(2)
C(16) Si(4) •C(22) 1117(2)
C(20). Si(4) C(22) 107.6(2)
Cr(l) N(l) 0(1) 179.0(3)
Cr(l) N(2) C(12) 103.2(2)
Cr(2) N(3) 0(2)- 179.5(2)
Cr(2) N(4) • C(26) 103.8(2)
Cr(l) C(l) . Si(l) 117.39(14)
N(2) C(9) C(10) 110.4(3)
atom atom atom angle
N(l) Cr(l) C(l) 102.85(12)
N(2) Cr(l) 0(1) 113.72(11)
C(l) Cr(l) C(2) 115.72(12)
N(3) Cr(2) C(15) , 102.47(14)
N(4) Cr(2) C(15) 116.33(12)
0(15) Cr(2) C(16) 116.52(13)
C(l) Si(l) C(4) 110.0(2)
C(3) Si(l) C(4) 108.55(15)
C(4) Si(l) C(5) 108.3(2)
C(2) Si(2) C(7) 110.1(2)
C(6) Si(2) C(7) •'. 110.3(2)
C(7) Si(2) C(8) 110.2(2)
C(15) Si(3) C(18) 109.2(2)
C(17) Si(3) C(18) 106.9(3)
C(18) Si(3) C(19) . 109.3(3)
C(16) ' Si(4) C(21) 109.7(2)
C(20) Si(4) C(21) 108.9(2)
C(21) Si(4) C(22) 109.1(2)
Cr(l) N(2) C(9) 138.4(2)
C(9) N(2) C(12) 118.3(2)
Cr(2) . N(4) C(23) 136.8(2)
C(23) N(4) C(26) 119.1(2)
Cr(l) C(2) Si(2) 130.6(2)
N(2) C(9) C(l l) 109.9(3)
185
Table A26. Bond Angles for 5.3 (continued).
atom atom atom angle atom atom atom angle
C(10) C(9) C(ll) 111.2(3) N(2) C(12) C(13) 111.7(3)
N(2) C(i'2) C(14). 112.6(2) C(13) C(12) C(14) 113.0(3)
Cr(2) , ' C(15) Si(3) . 128.8(2) Cr(2) C(16) Si(4) 120.2(2)
N(4) C(23) C(24) 110.4(3) N(4) C(23) C(25) 111.3(3)
C(24) C(23) C(25) 111.5(3) N(4) C(26) C(27) 112.7(3)
N(4) • C(26) C(28) . 112.8(3) C(27) C(26) C(28) 112.8(3)