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Well-defined, solvent-free cationic barium complexes: Synthetic strategies and

catalytic activity in the ring-opening polymerization of lactide

Bo Liu, Thierry Roisnel, Yann Sarazin ⇑

UMR CNRS 6226 Sciences Chimiques de Rennes – Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France

a r t i c l e i n f o

 Article history:

Available online 16 September 2011

Young Investigator Award Special Issue

Keywords:

BariumWell-defined cationsSecondary interactionsRing-opening polymerization

a b s t r a c t

Well-defined, solvent-free cationic barium complexes of the type [{L nX}Ba]+Á[H2N{B(C6F5)3}2]À stabilized

by multidentate amino-ether phenoxide or fluorinated amino-ether alkoxide ligands {L nX}À are availableaccording to original, general and high-yield protocols. These cations have been prepared by (i)hydrolysis of heteroleptic complexes {L nX}BaN(SiMe2H)2 stabilized by BaÁÁÁHÀSi interactions with[H(OEt2)2]+

Á[H2N{B(C6F5)3}2]À, or (ii) reaction of {Ba[N(SiMe2H)2]2}n with the doubly acidic pro-ligands[{L nX}HH]+

Á[H2N{B(C6F5)3}2]À. The solid-state structures of [{LO2}Ba(THF)2]+Á[H2N{B(C6F5)3}2]À ({LO2}

H = 2-[(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)methyl]-4,6-di-tert -butylphenol) and [{RO2}Ba]+Á

[H2N{B(C6F5)3}2]À {RO2}H = 2-[(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)methyl]-1,1,1,3,3,3-hexafluoropropan-2-ol) are described, highlighting the key role of internal secondary BaÁÁÁFÀC interactionsin thesehighly electrophilic species. In combination withan excess of an external nucleophile (chosen frombenzyl alcohol, 1,3-propanediol, benzyl amine or an hydroxyl-functionalized alkoxy-amine) as a co-initia-tor, some of these Ba cations provide extremely efficient catalysts for the immortal ring-opening polymer-ization of L -lactide in thetemperature range 0–30 °C, converting rapidly up to 5000 equiv. of monomer in acontrolled fashion and with excellent end-group fidelity.

Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction

The polymerization of enantiomerically pure L -lactide (L -LA), afully bio-resourced monomer derived from the fermentation of sugars or starch [1], has attracted a great deal of attention sincethe turn of the century, not least because poly(L -lactide) (PLLA) isa biodegradable thermoplastic with mechanical properties compa-rable to those of polystyrene [2]. A broad range of well-defined{L nX}Met–Nu complexes (where {L nX}nÀ is a bulky ancillary ligandand NuÀ is a reactive nucleophilic group such as alkyl, amide oralkoxide) have been developed for the controlled ring-openingpolymerization (ROP) of  L -LA [3,4]. Many initiators based on zinc[5], aluminum [6] or group III and lanthanide metals [7] allow

for the living ROP of L -LA, as well as that of other cyclic esters suchas rac -lactide (the equimolar mixture of the D- and L -isomers of lac-tide), e-caprolactone and b-butyrolactone [8], and a good under-standing of ROP by coordination-insertion mechanism wasgained through the use of these initiators. Magnesium complexesare also known to be competent ROP initiators [3,5b,9], but be-cause of their higher sensitivity they are comparatively less com-mon than their Zn analogues. The development of catalyticsystems for the immortal ROP (iROP) of cyclic esters, first proposedby Inoue for the ROP of epoxides [10], represents one of the persist-

ing challenges in this field: whereas a living system generates onlyone polymer chain per metal, the use of an excess of external proticco-initiator (typically an alcohol) with the metal initiator enablesthe production of hundreds of polymer chains per  metal center[11]. Besides, we and others have recently demonstrated that cat-ionic well-defined complexes, with their exacerbated Lewis acidity,promoted the polymerization of cyclic esters with great efficacyand were worth considering as a new generation of ROP catalysts[12–16].

In stark contrast with Zn or even Mg, only a handful of efficientROP single-site initiators based on the larger alkaline-earth metals(calcium, strontium and barium) have been reported to date[5f,16–18]. Chisholm et al. pioneered the first effective calcium ini-

tiators supported by highly encumbered tris(pyrazolyl)borate andb-diketiminate ancillary ligands [17c–d], while Feijen et al. showedthat Ca[N(SiMe3)2]2(THF)2 [18a] and the dimeric {Ca(THF)}2

(thmd)2(l-thmd)(l-N(SiMe3)2 (thmd-H = tetramethylheptanedi-one) [17a] also constituted moderately active initiators. Hilland co-workers reported the syntheses of stable heterolepticbis(phosphinimino)methyl derivatives of Ca and Sr which showedpromising ROP catalytic ability [17b], but surprisingly they didnot elaborate on these initial results. If two other examples of ill-defined Sr compounds able to promote the ROP of cyclic estershave been reported [19], the only molecular Ba ROP initiatorknown until very recently was the aminebis(phenolate) complexdisclosed by Davidson et al. [18d].

0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.ica.2011.09.020

⇑ Corresponding author. Fax: +33 (0)2 23 23 69 39.E-mail address: yann.sarazin@univ-rennes1.fr (Y. Sarazin).

Inorganica Chimica Acta 380 (2012) 2–13

Contents lists available at SciVerse ScienceDirect

Inorganica Chimica Acta

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / i c a

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Such paucity of well-defined Ae-based initiators (Ae = Ca, Sr,and Ba) is undoubtedly related to the difficulties encountered intaming the high reactivity of these very large (r ionic = 1.00, 1.18and 1.35 for Ca, Sr and Ba, respectively) [20] and electropositivemetals. This is especially the case with the largest element, barium.Due to their high kinetic lability, the preparation of stable hetero-leptic complexes {L nX}Ba–Nu (where {L nX}À is a monoanionic

ancillary ligand) is very troublesome, as their synthesis is oftenhampered by deleterious Schlenk-type equilibria. Starting from{L nX}Ba–Nu, these side-reactions lead to the formation of aggre-gates and poorly reactive species such as {L nX}2Ba and {BaNu2}n.As the pronounced ionic nature of the bonding increases withthe ionic radius of the element, the propensity for ligand scram-bling is most detrimental in the case of Ba complexes. For instance,heteroleptic complexes of Ca and Sr supported by bis(phosphinimi-no)methyl [17b] or b-diketiminate ligands [21] are stable, but thesame is not true of their Ba parents.

Efficient ROP initiators based on Ae metals (and especially Ba)can only be developed if convenient strategies aimed at stabilizingheteroleptic complexes against ligand scrambling are devised.Although steric bulk can sometimes impart sufficient stability tothe complexes [17b,22], it is hard to rationalize it for all ligandframeworks, especially as the size of the metal varies considerablyfrom Ca to Ba. Hill and co-workers reported an interesting strategybased on the dearomatization of bulky aromatic ligands [23].Methods relying on stabilization by internal secondary interactionsseemed to us to offer real scope as a general way to stabilize labileAe species. We have recently described Ae[N(SiMe2H)2]2(THF) xhomoleptic precursors (Ae = Ca, x = 1; Sr, x = 0.66; Ba, x = 0), andshowed that the presence of internal stabilizing b-SiÀH agosticinteractions constituted a key factor in the isolation of the hetero-leptic complexes {L nX}Ae–N(SiMe2H)2 [17h]. Besides, we have alsoshown that the extremely electrophilic cations in {RO}Ae+

ÁXÀ ionpairs (where {RO}À is a tertiary fluorinated alkoxide and XÀ is aweakly-coordinating anion) were stabilized by internal AeÁÁÁFÀCsecondary interactions in the solid-state [16c].

In the present study, the syntheses and solid-state structures of discrete cationic complexes of barium supported by bulky amino-ether phenoxide and alkoxide ligands are described. A syntheticstrategy for the isolation of such cations is detailed, and cases of BaÁÁÁHÀSi and BaÁÁÁFÀC secondary interactions in charge-neutraland cationic complexes are discussed. The remarkable catalyticactivity of these cations in the immortal ROP of  L -LA is alsopresented.

2. Experimental

  2.1. General procedures

All manipulations were performed under inert atmosphereusing standard Schlenk techniques or in a Jacomex glove-box(O2 < 1 ppm, H2O < 5 ppm) for catalyst loading.

NMR spectra were recorded on Bruker AC-300, AC-400 and AM-500 spectrometers. All chemicals shifts were determined usingresidual signals of the deuterated solvents and were calibrated ver-sus SiMe4. Assignment of the signals was carried out using 1D (1H,13C{1H}) and 2D (COSY, HMBC, and HMQC) NMR experiments. Cou-pling constants are given in Hertz. 19F{1H} chemical shifts weredetermined by external reference to an aqueous solution of NaBF4.11B chemical shifts are reported relative to BF3ÁEt2O.

Size Exclusion Chromatography (SEC) measurements were per-formed on a Polymer Laboratories PL-GPC 50 instrument equippedwith a PLgel 5 Å MIXED-C column and a refractive index detector.

The GPC column was eluted with THF at room temperature at1 mL/min and was calibrated using 11 monodisperse polystyrene

standards in the range of 580–380000 gmolÀ1. The molecularweights of all poly(lactide)s were corrected by the recommendedfactor of 0.58 [24].

MALDI-ToF-MS spectra were obtained with a Bruker DaltonicMicroFlex LT, using a nitrogen laser source (337 nm, 3 ns) in linearmode with a positive acceleration voltage of 20 kV. Samples wereprepared as follow: 1 lL of a 2:1 mixture of a saturated solution

of a-cyano-4-hydroxycinnamic acid (Bruker Care) in HPLC qualityacetonitrile and a 0.1% solution of trifluoroacetic acid in ultrapurewater was deposited on the sample plate. After total evaporation,1 lL of a 5 to 10 mgmL À1 solution of the polymers in HPLC-qualityTHF were deposited. Bruker Care Peptide Calibration Standard andProtein Calibration Standard I were used for external calibration.

Elemental analyses were performed on a Carlo Erba 1108 Ele-mental Analyser instrument at the London Metropolitan Universityby Stephen Boyer and were the average of a minimum of two inde-pendent measurements.

FTIR spectra were recorded at room temperature as Nujol mullsin KBr plates on a Shimadzu Affinity-IR spectrometer.

  2.2. Materials

Benzyl alcohol (VWR) and 1,3-propanediol (Acros) were driedand distilled over dry magnesium turnings and then stored overactivated 3 Å molecular sieves. Benzyl amine (Acros) was distilledfrom CaH2 and kept over molecular sieves. BaI2 (anhydrous beads,99.995%) was purchased from Aldrich and used as received. 1-Aza-15-crown-5 (IBC) and 3,3,3-trifluoro-2-(trifluoromethyl)-1,2-pro-penoxide (Apollo) were used without purification. HN(SiMe3)2

(Acros) and HN(SiMe2H)2 (ABCR) were dried over activated 3 Åmolecular sieves and distilled under reduced pressure prior touse. Technical grade L -lactide (provided by Total Petrochemicals)was purified by recrystallization from a hot, concentrated iPrOHsolution (80 °C), followed by two subsequent recrystallizations inhot toluene (105 °C). After purification, L -lactide was stored atÀ30 °C under the inert atmosphere of the glove-box. Toluene was

distilled under Argon from melted sodium prior to use. THF wasfirst pre-dried over sodium hydroxide and distilled under argonover CaH2, and then freshly distilled a second time under argonfrom Na/benzophenone prior to use. Et2O, dichloromethane andpentane were distilled under argon from Na/benzophenone, CaH2

and Na/benzophenone/tetraglyme, respectively. All deuterated sol-vents (Eurisotop, Saclay, France) were stored in sealed ampoulesover activated 3 Å molecular sieves and were thoroughly degassedby several freeze–thaw cycles.

The synthetic precursors [H(OEt2)2]+Á[H2N{B(C6F5)3}2]À [25],

Ba[N(SiMe3)2]2(THF)2 [26], [{LO1}HH]+Á[H2N{B(C6F5)3}2]À [16c]

and {Ba[N(SiMe2H)2]2}n [17h], the complexes [{LO2}Ba]+Á

[H2N{B(C6F5)3}2]À (5) [16a] and [{RO2}Ba]+Á [H2N{B(C6F5)3}2]À (6)

[16c], and the pro-ligands {LO1}H [5j], {LO2}H [5j] and {RO2}H

[16c] were all prepared as described elsewhere.

  2.3. Syntheses and characterization

 2.3.1. Synthesis of {LO1 }BaN(SiMe 3) 2 (1)

At room temperature, a solution of {LO1}H (0.25 g, 0.71 mmol)in Et2O (5 mL) was slowly added to a solution of Ba[N(Si-Me3)2]2(THF)2 (0.45 g, 0.75 mmol) in Et2O (10 mL). A white precip-itate formed instantly. The suspension was stirred for 2 h, and thesupernatant was eliminated by filtration to afford 1 as white pow-der after drying in vacuo. Yield 0.38 g (83%). 1H NMR (C6D6, 298 K,500.13 MHz): d 7.54 (d, 4 J HH = 2.2 Hz, 1H, m-H), 7.06 (d,4 J HH = 2.2 Hz, 1H, m-H), 3.30 (s, 2H, Ar–CH 2–N), 3.58–2.89 (br,10H, CH2–CH 2–O and 6H, O–CH 3), 2.21 (br, 4H, N–CH 2–CH2), 1.69

(s, 9H, o-C(CH 3)3), 1.42 (s, 9H, p-C(CH 3)3), 0.14 (d,4

 J HH = 3.0 Hz,18H, Si(CH 3)3) ppm. 13C{1H} NMR (C6D6, 298 K, 125.76 MHz): d

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166.7 (i-C), 138.2 (o-C), 136.8 ( p-C), 129.5 (o-C), 125.8 (m-C), 121.6(m-C), 69.6 (CH2–C H2–O), 59.8 (Ar–C H2–N), 59.2 (O–C H3), 54.6 (N–C H2–CH2), 36.1 (o-C (CH3)3), 34.8 ( p-C (CH3)3), 32.5 ( p-C(C H3)3),30.6(o-C(C H3)3), 6.3 (Si(C H3)3) ppm. Repeated attempts to obtain X-rayquality crystals of  1 by recrystallization in benzene were unsuc-cessful, but yielded instead micro-crystals of {LO1}2Ba (2). At-tempts to obtain satisfactory elemental analysis for 1 were

unsuccessful, most probably because of contamination with vari-ous amounts of  2.

 2.3.2. Synthesis of {LO1 } 2Ba ( 2)

Compound 2 was prepared in the same fashion as that de-scribed for 1 by reaction of Ba[N(SiMe3)2]2(THF)2 (0.13 g,0.21 mmol) with {LO1}H (0.15 g, 0.43 mmol). Yield 0.18 g (99%).Single-crystals of  2Á[C6H6]0.5 suitable for X-ray diffraction crystal-lography were obtained by recrystallization from a concentratedC6H6 solution at room temperature. 1H NMR (C6D6, 298 K,500.13 MHz): d 7.59 (d, 4 J HH = 2.7 Hz, 2H, m-H), 7.07 (d,4 J HH = 2.7 Hz, 2H, m-H), 3.30 (s, 4H, Ar–CH 2–N), 3.10 (m, 4H,CH2–C(H)H –O), 3.04 (m, 4H, CH2–CH (H)–O), 3.01 (s, 12H, O–CH 3),2.30 (br, 4H, N–C(H)H –CH2), 2.22 (br, 4H, N–CH (H)-CH2), 1.80 (s,18H, o-C(CH 

3)

3), 1.55 (s, 18H, p-C(CH 

3)

3) ppm. 13C{1H} NMR 

(C6D6, 298 K, 125.76 MHz): d 167.2 (i-C), 136.7 (o-C), 130.4 ( p-C),127.6 (o-C), 124.0 (m-C), 122. 6 (m-C), 69.5 (CH2–C H2–O), 59.6(Ar–C H2–N), 59.2 (O–C H3), 53.3 (N–C H2–CH2), 36.2 (o-C (CH3)3),34.5 ( p-C (CH3)3), 33.1 ( p-C(C H3)3), 30.8 (o-C(C H3)3) ppm. Anal. Calc.for C42H72BaN2O6 (838.89 g molÀ1): C, 60.2; H, 8.7; N, 3.3. Found: C,60.1; H, 8.8; N, 3.3%.

 2.3.3. Synthesis of {LO1 }BaN(SiMe 2H) 2 ( 3)

A solution of {LO1}H (0.20 g, 0.57 mmol) in Et2O (5 mL) wasadded slowly at room temperature to a solution of {Ba[N(Si-Me2H)2]2}n (0.24 g, 0.60 mmol) in Et2O (10 mL). A white precipitateformed slowly upon stirring at room temperature. The suspensionwas stirred for 6 h, and the precipitate was then isolated by filtra-tion. Following drying to constant weight, 3 was obtained as a col-orless powder. Yield 0.28 g (79%). Recrystallization from aconcentrated benzene solution afforded X-ray quality crystals of 3. 1H NMR (C6D6, 298 K, 500.13 MHz): d 7.60 (d, 4 J HH = 2.7 Hz, 1H,m-H), 7.06 (d, 4 J HH = 2.7 Hz, 1H, m-H), 4.72 (m, 1 J SiH = 160 Hz, 2H,Si(CH3)2H ), 3.30 (s, 2H, Ar–CH 2–N), 3.09 (m, 2H, CH2–C(H)H –O),3.04 (m, 2H, CH2–CH (H)–O), 3.00 (s, 6H, O–CH 3), 2.30 (m, 2H, N–C(H)H –CH2), 2.22 (m, 2H, N–CH (H)–CH2), 1.80 (s, 9H, o-C(CH 3)3),1.55 (s, 9H, p-C(CH 3)3), 0.11 (d, 4 J HH = 3.0 Hz, 12H, Si(CH 3)2H)ppm. 13C{1H} NMR (C6D6, 298 K, 125.76 MHz): d 166.9 (i-C),136.5 (o-C), 130.7 ( p-C), 127.2 (o-C), 123.8 (m-C), 122.5 (m-C),69.5 (CH2–C H2–O), 59.3 (Ar–C H2–N), 59.2 (O–C H3), 53.4 (N–C H2–CH2), 36.0 (o-C (CH3)3), 34.3 ( p-C (CH3)3), 32.6 ( p-C(C H3)3), 30.9 (o-C(C H3)3), 5.3 (Si(C H3)2H) ppm. 29Si{1H} NMR (C6D6, 298 K,79.49 MHz): d À30.4 ppm. FTIR (Nujol, KBr plates): m = 1987 (br

s), 1910 (w sh), 1804 (w), 1770 (w), 1686 (w), 1604 (m) cm À1. Anal.Calc. for C25H50BaN2O3Si2 (620.20 g molÀ1): C, 48.4; H, 8.1; N, 4.5.Found: C, 48.5; H, 8.2; N, 4.6%.

 2.3.4. Synthesis of [{LO1 }Ba]+Á[H  2N{B(C 6 F 5) 3 } 2]À ( 4)

A solution of [{LO1}HH]+Á[H2N{B(C6F5)3}2]À (0.40 g, 0.29 mmol)

in Et2O (5 mL) was added dropwise at room temperature to a solu-tion of {Ba[N(SiMe2H)2]2}n (0.12 g, 0.30 mmol) in Et2O (10 mL). Theresulting colorless solution was stirred at room temperature for2 h. The volatiles were then removed under vacuum to yield an

off-white solid. Repeated washings with pentane (5 Â 20 mL) fol-lowed by stripping with CH2Cl2 (5 Â 5 mL) yielded[{LO1}Ba]+

Á[H2N{B(C6F5)3}2]À (4) as a white powder which alwayscontained residual Et2O; attempts to obtain the complex free of solvent were thwarted by the solubility properties of  4.1 Yield

0.43 g (93%). Alternatively, 4 could be obtained upon treatment of 

3 (0.12 g, 0.19 mmol) with [H(OEt2)2]+Á[H2N{B(C6F5)3}2]À (0.23 g,

0.19 mmol) in Et2O (15 mL).

1

H NMR (CD2Cl2, 298 K, 500.13 MHz):d 7.48 (d, 4 J HH = 2.5 Hz, 1H, arom. H), 7.14 (d, 4 J HH = 2.5 Hz, 1H, arom.

H), 5.70 (br, 2H, NH 2), 4.21–2.53 (m, 16H, Ar–CH 2–N, N–CH 2–CH2,

CH 2–O and O–CH 3), 1.53 (s, 9H, C(CH 3)3), 1.30 (s, 9H, C(CH 3)3)

ppm. 13C{1H} NMR (CD2Cl2, 298 K, 125.76 MHz): d 148.8, 147.0,

140.0, 138.1, 137.6, 135.6 (all C 6 F5), 157.6 (i-C ), 139.9 ( p-C ), 137.2

(o-C ), 128.5 (m-C ), 126.0 (m-C ), 124.0 (o-C ), 71.6, 62.1, 60.0, 59.1,

58.3, 51.6 (all br., weak signals, Ar–C H2–N, N–C H2–CH2, C H2–O and

O–C H3), 35.6 (C (CH3)3), 34.4 (C (CH3)3), 31.6 (br, C(C H3)3) ppm.19F{1H} NMR (CD2Cl2, 188.29 MHz, 298 K): d À132.8 (d, 3 J FF = 18.9,

12F, o-F), À160.2 (t, 3 J FF = 18.9, 6F, p-F), À165.7 (d, 3 J FF = 18.9, 12F,

m-F) ppm. 11B NMR (CD2Cl2, 96.29 MHz, 298 K): d À8.4 ppm. Satis-

factory elemental analysis for C57H38B2BaF30N2O3 (1527.84 gmolÀ1)

could not be obtained in a reproducible fashion as the samples sub-

mitted contained various amounts of residual Et2O.

 2.3.5. Synthesis of [{LO 2 }Ba(THF) 2]+Á[H  2N{B(C 6 F 5) 3 } 2]À (5Á(THF ) 2)

5Á( THF)2 was synthesized quantitatively by dissolving 5 (0.28 g,0.17 mmol) in THF (3 mL), stirring at room temperature for 10 minfollowed by removal of the solvent under vacuum. Yield 0.30 g(99%). X-ray quality crystals of  5Á( THF)2 were obtained by recrys-tallization in CH2Cl2 and pentane in the presence of a small amountof THF at room temperature. 1H NMR (CD2Cl2, 298 K, 500.13 MHz):d 7.42 (d, 4 J HH = 2.2 Hz, 1H, arom. H), 7.08 (d, 4 J HH = 2.2 Hz, 1H,arom. H), 5.70 (br, 2H, NH 2), 4.37 (d, 2 J HH = 11.3 Hz, 1H, Ar–CH (H)–N), 3.96 (m, 2H, H macrocycle), 3.83 (m, 1H, H macrocycle), 3.71(br, 8 + 3H, THF + H macrocycle), 3.58 (m, 1H, H macrocycle), 3.53–3.32(m, 6H, H macrocycle), 3.26 (d, 2 J HH = 11.5 Hz, 1H, Ar–C(H)H –N),3.21–3.08 (m, 3H, H macrocycle), 2.87 (m, 1H, H macrocycle), 2.76 (m,

1H, H macrocycle), 2.63(m, 1H, H macrocycle), 2.36 (m, 1H, H macrocycle),1.85 (br, 8H, THF), 1.52 (s, 9H, C(CH 3)3), 1.29 (s, 9H, C(CH 3)3)ppm. 13C{1H} NMR (CD2Cl2, 298 K, 125.76 MHz): d 149.5, 147.6,140.7, 138.3, 138.1, 136.3 (all C 6 F5), 158.3 (i-C ), 138.7 ( p-C ), 138.0(o-C ), 127.8 (m-C ), 126.0 (m-C ), 124.3 (o-C ), 70.8, 70.0, 69.8, 69.6,69.5, 69.5, 68.8, 67.1, 59.3 (all C macrocycle), 68.4 (O-C H2 (THF)),64.0 (Ar–C H2–N), 35.6 (C (CH3)3), 34.8 (C (CH3)3), 34.4 (C(C H3)3),31.9 (C(C H3)3), 26.1 (O–CH2–C H2 (THF)) ppm. 19F{1H} NMR (CD2Cl2,188.29 MHz, 298 K): d À133.0 (d, 3 J FF = 18.9, 12F, o-F), À160.1 (t,3 J FF = 18.9, 6F, p-F), À165.6 (d, 3 J FF = 18.9, 12F, m-F) ppm. 11BNMR (CD2Cl2, 96.29 MHz, 298 K): d À8.4 ppm. Anal. Calc. forC69H60BaF30N2O7 (1758.15 gÁmolÀ1): C, 47.1; H, 3.4; N, 1.6. Found:C, 47.4; H, 3.3; N, 1.8%.

  2.3.6. Re-crystallization of [{RO 2

 }Ba]+Á[H  2N{B(C 6 F 5) 3 } 2]

À

(6 )Single-crystals of  6 [16c] suitable for X-ray studies were ob-

tained by very slow diffusion (over a period of several weeks) atroom temperature of vapors of pentane into a solution of  6 inCH2Cl2 (using two Schlenk vessels connected by a long U-shapeglass tube, the former containing the solution in CH 2Cl2 whilethe latter contained dry pentane).

  2.3.7. Typical polymerization procedure

In the glove-box, the metal-based initiators and the purifiedmonomer were placed at once in a Schlenk flask. The vessel wassealed and removed from the glove-box. All subsequent operationswere carried out using standard Schlenk techniques. The requiredamount of dry, degassed solvent was added with a syringe to the

Schlenk flask containing the initiator and monomer. The nucleo-philic co-initiator was then added rapidly, the Schlenk vessel was

1 Ion pairs such as 4–6 can usually be obtained free of Et2O or THF by dissolving

them in CH2Cl2 and re-precipitating them by very slow addition of pentane upon

vigorous stirring, see Ref. [16a]. However, this method proved ineffective in the case

of 4, as the yields were then dramatically decreased (from 93% to ca. 20–30%) and theproduct still contained traces of Lewis base.

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immerged in a water or oil bath set at the desired temperature andthe polymerisation time was measured from this point. The reac-tion was terminated by addition of acidified MeOH (HCl, 1%) andthe polymer was precipitated in methanol. It was purified by re-precipitation, using dichloromethane or THF as solvent and meth-anol as a non-solvent. The polymer was then dried to constantweight under dynamic vacuum.

  2.3.8. X-ray diffraction crystallography

Suitable crystals for X-ray diffraction analysis of  2Á[C6H6]0.5, 3,5Á( THF)2 and 6 (CCDC numbers 826134–826137) were obtainedby recrystallization of the purified products. Diffraction data werecollected at 150 K on a Bruker APEX CCD diffractometer withgraphite-monochromated Mo Ka radiation (k = 0.71073 Å). A com-bination of x and U scans was carried out to obtain at least a un-ique data set. The crystal structures were solved by direct methods,remaining atoms were located from difference Fourier synthesisfollowed by full-matrix least-squares refinement based on F 2 (pro-grams SIR 97 and SHELXL -97) [27]. The hydrogen atom contributionswere calculated but not refined. All non-hydrogen atoms were re-fined with anisotropic displacement parameters. Summary of crys-

tal and refinement data for compounds 2Á[C6H6]0.5, 3, 5Á( THF)2 and6 are collected in Table 1.

3. Results and discussion

 3.1. Synthesis of neutral and cationic complexes

Discrete cationic complexes have recently emerged as a newfamily of highly active catalysts for the ROP of cyclic esters [12–16], and our group has for instance developed various cations of the large Ae metals (Ae = Ca, Sr, and Ba) [16a,c]. We have reportedthe syntheses of the Ba species [{LO2}Ba]+

Á[H2N{B(C6F5)3}2]À (5;{LO 2}H = 2-[(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)-methyl]-4,6-di-tert -butylphenol) and [{RO2}Ba]+

Á[H2N{B(C6F5)3}2]À

(6; {RO2}H = 2-[(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)-

methyl]-1,1,1,3,3,3-hexafluoropropan-2-ol), but their X-ray struc-tures remained elusive.

Both ligands contain a highly chelating aza-crown-ether frag-ment, but while {LO2}À is based on a bulky phenoxide, the{RO2}À platform is supported by a tertiary fluorinated alkoxide(Fig. 1). Because of the electron-withdrawing substituents in a po-

sition to the oxygen atom, alkoxides such as {RO

2

}

À

are far less ba-sic and p-donating, and therefore have a significantly lowerbridging ability than conventional alkoxides which readily leadto the formation of aggregated and poorly-defined species [28].However, both {LO2}À and {RO2}À are highly chelating, and thepreparation of the less hindered [{LO1}Ba]+

Á[H2N{B(C6F5)3}2]À (4;{LO1}H = 2-{[bis(2-methoxyethyl)amino]methyl}-4,6-di-tert -butylphenol) was carried out, with the rationale that 4 would dis-play higher catalytic activities than its derivatives 5 and 6

(Scheme 1) on account of the lower coordination number and high-er Lewis acidity of the metal center in 4.

The reaction of {LO1}H with Ba[N(SiMe3)2]2(THF)2 in Et2Oyielded the heteroleptic {LO1}BaN(SiMe3)2 (1) in good yield. Thecomplex was characterized by 1H and 13C{1H} NMR spectroscopy,which indicated the presence of some contamination by the homo-leptic derivative {LO1}2Ba (2), and indeed only crystals of the less

  Table 1

Crystal and refinement data for compounds 2Á[C6H6]0.5, 3, 5Á( THF)2 and 6.

2Á[C6H6]0.5 3 5Á( THF)2 6

Empirical formula C42H72BaN2O6, 0.5(C6H6) C25H49BaN2O3Si2 C69H60B2BaF30N2O7 C50H24B2BaF36N2O5

Formula weight 877.41 619.18 1758.15 1575.67Crystal system monoclinic triclinic triclinic monoclinicSpace group P 21/n P 1 P 1 P 21/c 

a (Å) 11.9756(3) 11.3434(3) 9.9479(8) 12.5284(5)b (Å) 14.7336(3) 12.5243(3) 19.7011(18 20.9798(11)c  (Å) 26.2381(5) 12.6446(3) 20.266(2) 20.9607(9)a (°) 90 62.8210(10) 89.753(6) 90b (°) 92.8460(10) 80.6770(10) 80.164(6) 92.238(2)

c (°

) 90 89.2680(10) 82.094(5) 90Volume (Å3) 4623.84(17) 1573.10(7) 3875.5(6) 5505.2(4) Z  4 2 2 4Dcalc (gcmÀ3) 1.260 1.307 1.507 1.901Abs. Coeff. (mmÀ1) 0.904 1.364 0.635 0.894F (000) 1852 642 1760 3072Crystal size, mm 0.3 Â 0.16Â 0.1 0.25 Â 0.24 Â 0.23 0.15 Â 0.15Â 0.1 0.6 Â 0.6 Â 0.6h Range (°) 1.55–27.47 1.82–27.47 1.02–27.52 1.37–27.47Limiting indices À15 < h < 10 À13 < h < 14 À12 < h < 12 À15 < h < 16

À19 < k < 16 À16 < k < 16 À25 < k < 25 À27 < k < 27À34 < l < 33 À16 < l < 16 À26 < l < 25 À27 < l < 27

Rint 0.0283 0.0241 0.0653 0.0445Reflections collected 38079 21577 42387 72870Unique reflections [I > 2r(I )] 10537 7137 17484 12562Data/restraints/parameters 10537/6/480 7137/1/309 17484/0/1010 12562/0/827Goodness-of-fit (GOF) on F 2 1.181 1.193 0.959 1.048R1 [I > 2r(I )] (all data) 0.0288 (0.0369) 0.0299 (0.0333) 0.0605 (0.1037) 0.0460 (0.0585)wR2 [I > 2r(I )] (all data) 0.0787 (0.0954) 0.0861 (0.1007) 0.1583 (0.1785) 0.1192 (0.1309)

Largest difference (eAÀ3) 1.509 and À1.059 2.883 and À1.303 0.796 and À1.270 2.058 and À1.353

Fig. 1. Pro-ligands employed in this study.

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soluble 2 were obtained upon attempts to recrystallize 1 from ben-zene. Also, slow decomposition of 1 was observed in solution, lead-ing to the formation of  2 and an unidentified barium-amidespecies. Compound 1 was fully soluble in Et2O and THF, moder-ately so in toluene and benzene, and insoluble in aliphatic hydro-carbons. The homoleptic 2 could be independently preparedquantitatively upon treatment of Ba[N(SiMe3)2]2(THF)2 with2 equiv. of {LO1}H in Et2O. The solubility of  2 in aromatic solventswas limited, but it was fairly soluble in ethers (THF, Et2O). X-ray

quality crystals of  2Á[C6H6]0.5 were obtained by recrystallizationfrom a solution in benzene. As the potential contamination of  1

and its questionable stability in solution could prove troublesomefor subsequent reactions, the synthesis of the heteroleptic species{LO1}BaN(SiMe2H)2 (3) was carried out. By contrast to 1, the kinet-ically stable 3 was obtained free of impurity in satisfactory yield(79%) by treatment of {Ba[N(SiMe2H)2]2}n with a stoichiometricamount of {LO1}H in Et2O (Scheme 1). The solubility properties of 3 were comparable to those of  1, and single-crystals suitable forX-ray diffraction crystallography were readily grown from ben-zene. The identity and purity of  3 were confirmed by 1H, 29Si{1H}and 13C{1H} NMR spectroscopy, FTIR and elemental analysis. Itsstability in solution was established by 1H NMR, as no sign of evo-lution (and formation of 2) was observed during the monitoring of 

solution of 3 in C6D6. In comparison to 1, the greater stability of 3against Schlenk-type equilibria was attributed to the presence of internal BaÁÁÁHÀSi agostic interactions, identified both in solution(NMR) and in the solid-state (FTIR). Indeed, in the 1H NMR spec-trum of  3, the 1 J SiH coupling constant of 160 Hz was indicative of weak agostic interactions between Ba and the SiÀH moieties[17h]. In the 29Si{1H} NMR spectrum of 3, the high field resonance(dSi = À30.4) was also characteristic of BaÀN(SiMe2H)2 fragmentswith BaÁÁÁHÀSi secondary interactions [17h]. In addition, thems(SiÀH) bands at 1987 (s) and 1910 (sh) cmÀ1 in the FTIR spec-trum of  3 recorded in Nujol were diagnostic of BaÁÁÁHÀSi interac-tions of low intensity (Fig. 2); by contrast, free HN(SiMe2H)2 andnon-interacting SiÀH moieties in a metal complex typically exhibitvibrations at 2122 and ca. 2000–2080 cmÀ1, respectively.

The addition of 1 equiv. of Bochmann’s acid[H(OEt2)2]+Á[H2N{B(C6F5)3}2]À to a solution of  3 in Et2O afforded

the well-defined ion pair [{LO1}Ba]+Á[H2N{B(C6F5)3}2]À (4). Gratify-

ingly, NMR spectroscopy indicated that the cationic complex[{LO1}Ba]+ in 4 (4+) was devoid of coordinated solvent molecule,even if traces of residual Et2O could be detected. The choice of the somewhat unusual fluorinated weakly-coordinating anion(WCA) [H2N{B(C6F5)3}2]À in our protocols instead of the more tra-ditional [B(C6F5)4]À was motivated because of its far better crystal-lization properties. Indeed, the latter can be considered sphericaland very often leads to the formation of oily materials. On the

other hand, the bent [H2N{B(C6F5)3}2]À counterion exhibits a sig-nificant dipolar moment and is therefore oriented towards the cat-ion, which eventually renders crystallization processes morefavorable [16a,c,25,29]. The 1H NMR spectra of  4 recorded inCD2Cl2 at room temperature displayed very broad signals for allCH2 and CH3 groups, most likely as the result of a dynamic behav-ior in solution; attempts to freeze the fluxionality by recording thespectrum at low temperature (down to À60 °C) did not afford asubstantial improvement. In the 19F{1H} spectrum of  4, the usualresonances of the non-coordinated anion were typically locatedat À132.8, À160.2 and À165.7 ppm, while characteristically a sin-gle resonance at À8.4 ppm was found in its 11B NMR spectrum. Thenear-quantitative synthesis of  4 could also be convenientlyachieved by reacting the doubly-protonated pro-ligand

[{LO1}HH]+Á [H2N{B(C6F5)3}2]À

(prepared almost quantitatively bystoichiometric reaction of [H(OEt2)2]+

Á[H2N{B(C6F5)3}2]À and{LO1H} [16c]) with {Ba[N(SiMe2H)2]2}n in Et2O.2 This second proce-

dure offered the advantage of yielding the targeted solvent-free, dis-

crete salts 4 without having to preliminarily synthesize the

heteroleptic precursor 3, and it therefore constituted the synthetic

method of choice. The syntheses of [{LO2}Ba]+Á[H2N{B(C6F5)3}2]À

(5) and [{RO2}Ba]+Á[H2N{B(C6F5)3}2]À (6) were previously carried

out following this protocol [16a,c], but the only elucidated solid-

state structure was that of the solvent-containing {6}2ÁEtOH. Instead,

the structures of  5Á( THF)2 and 6 are reported here for the first time

Scheme 1. Syntheses of complexes 3 and 4.

2 If the reaction is performed in THF, or if the traditional Ba[N(SiMe3)2]2(THF)2 is

used instead of {Ba[N(SiMe2H)2]2}n, then the final product contains a significant

amount of THF (possibly coordinated on the metal center) which cannot be removedunder vacuum.

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(vide infra). 5Á( THF)2 was prepared quantitatively by dissolving 5 in

THF, followed by simple removal of the volatiles under vacuum.While repeated attempts to grow single-crystals of  5 suitable for

X-ray studies were unsuccessful, X-ray quality crystals of  5Á( THF)2

were isolated by recrystallization in a mixture of CH2Cl2, pentane

and THF.

  3.2. Structural characterization

The solid-state structure of 2Á[C6H6]0.5 was determined by X-raydiffraction methods. The compound crystallizes in the P 21/n spacegroup. There is no contact between the solvent molecule and thecomplex; there is some disorder in one the t Bu groups in para po-sition of the aromatic ring. The metal is 8-coordinated, and sits in aslightly distorted cubic arrangement (Fig. 3). The two nitrogen

atoms (both located at ca. 2.92 Å of the metal center) and twophenoxide O-atoms are in trans position with each other(N(1)ÀBa(1)ÀN(2) = 175.2°, O(1)ÀBa(1)ÀO(2) = 176.9°). The BaÀOdistance to the phenoxide O-atoms (2.59 Å) is much shorter thanthe lengths to the O-atoms on the chelating side-arm (in the range2.820(2)–2.847(2) Å); both are comparable to those already ob-served in related compounds [17c].

Compound 3 crystallizes in the P 1 space group. The compoundis a centro-symmetric dimer in the solid-state (Fig. 4), with a cen-tral Ba2O2 planar core where the two metal centers are bridged bythe Ophenoxide atoms. There is some disorder in one of the SiMe2Hfragments, and only the main component is depicted in Fig. 4. EachBa atom is 6-coordinated, as the ligand is bonded in j4–N,O,O,Ofashion to the metal. The BaÀOphenoxide distance of 2.65 Å is shorter

than those to the O-atoms of the ether side-arms (2.77–2.87 Å).Despite the bulkiness of the –N(SiMe2H)2 amido group, theBa(1)ÀN(30) distance of 2.66 Å is far shorter than the Ba(1)ÀN(1)one (2.93 Å). Metallophilic interactions can be ruled out on accountof the long intermetallic distance (4.25 Å). There is no clear evi-dence for the presence of BaÁÁÁHÀSi agostic interaction in the dimerof 3, as the Ba(1)ÀN(30)ÀSi(X) and Si(2)ÀN(30)ÀSi(3A) angles (ca.114° and 131°, respectively) fall in the typical range for such com-pounds; similarly, the Ba(1)ÀSi(2) and Ba(1)ÀSi(3A) distances of 3.67 Å are unexceptional. Even if for each SiMe2H group the fourBa, N, Si and H atoms are almost perfectly co-planar as anticipatedin the case of BaÁÁÁHÀSi contacts, this may simply be the outcomeof steric repulsion. This is in agreement with the FTIR data, whichsuggested weak interactions in the solid-state; on the other hand,1

H and29

Si NMR data were indicative of somewhat stronger con-tacts in solution, but direct comparison could not be established

as there was no evidence that the dimeric structure of  3 was re-tained in solution. By contrast to 3, the geometric features in thesolid-state structure of {LO2}BaN(SiMe2H)2 (narrow BaÀNÀSi angleof 102° and short BaÀSi distance of 3.45 Å in one of the SiMe 2Hmoieties) were clearly diagnostic of strong BaÁÁÁHÀSi agostic inter-actions [17h]. Note that despite the somewhat weak nature of theBaÁÁÁHÀSi contacts in 3, they nevertheless play a non negligible rolein the formation of this complex, as highlighted by its greater ki-netic stability with respect to 1.

Although all attempts to obtain X-ray quality crystals of the sol-vent-free 5 were unsuccessful, single-crystals of the THF adduct5Á( THF)2 suitable for crystallographic studies were obtained by re-

crystallization in a mixture of CH2Cl2, pentane and a small amountof THF at room temperature. In the cation (Fig. 5), the Ba atom is8-coordinated as the coordination spherearound the metal includesthe multidentate amino-ether phenoxide ligand bonded in j6–N,O,O,O,O,O manner and the two solvent molecules; the metal sitsin a bicapped trigonal prismatic environment. There is considerabledisorder in one of the THF molecules, and only the main componentis displayed in Fig. 5. There is no contact between the cationic metalcenter and the fluorine atoms of the counterion; besides, the verybulky WCA (V = 538 Å3 [30]) displays the characteristic pattern of intramolecular FÁÁÁHÀN hydrogen bonding in the range 1.935–2.320 Å, i.e. well below the sum of the van der Waals radii (ca. 2.5Å) for H and F [25,29]. By contrast to the related calcium derivative[{LO2}Ca]+

Á[H2N{B(C6F5)3}2]À where the complex forms a dicationic

bimetallic species in the solid-state [16a], the Ba cation is mono-meric in 5Á( THF)2, undoubtedly owing to the presence of the two

Fig. 2. 2500–1500 cmÀ1 region of the FTIR spectrum of  3 recorded in Nujol in KBrplates.

Fig. 3. ORTEP diagram of the solid-state structure of  2Á[C6H6]0.5. Only the maincomponent of the disordered t Bu group is represented. The non-interacting solventmolecule and hydrogen atoms are omitted for clarity. Ellipsoids are drawn at the50% probability level. Selected bond lengths [Å] and angles [°]: N(1)ÀBa(1) =2.918(2), N(2)ÀBa(1) = 2.920(2), O(1)ÀBa(1) = 2.588(2), O(2)ÀBa(1) = 2.588(2),O(3)ÀBa(1) = 2.839(2), O(4)ÀBa(1) = 2.847(2), O(5)ÀBa(1) = 2.820(2), O(6)ÀBa(1) = 2.820(2); O(2)ÀBa(1)ÀO(1) = 176.94(5), O(5)ÀBa(1)ÀO(3) = 176.91(5),O(6)ÀBa(1)ÀO(4) = 175.30(5), N(1)ÀBa(1)ÀN(2) = 175.19(5), O(2)ÀBa(1)ÀO(6) =89.09(5), O(1)ÀBa(1)ÀO(6) = 88.38(5), O(2)ÀBa(1)ÀO(4) = 90.23(5), O(1)ÀBa(1)ÀO(4) = 92.43(5).

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coordinated solvent molecules. The BaÀOphenoxide distance (2.55 Å)isfarshorterthanallotherBaÀOmacrocycle (2.83–2.93 Å)andBaÀOTHF

(2.82–2.91 Å) bond lengths. While the N,O,O,O,O core is folded by80 ± 2° in [{LO2}M]+

Á[H2N{B(C6F5)3}2]À (M = Zn, Mg, Ca) [16a,c], theaza-crown-ether tether is not significantly distorted in 5Á( THF)2,since the N atom lies only 1.25 Å above the plane perfectly formedby O(4), O(7), O(11) and O(14). Similarly, the presence of solventmolecules in Itoh and Kitagawa’s seminal complexes of the smallerAe elements [{LO2}M(L)]+

Á[BPh4]À (M = Mg, Ca, Sr; L = CH3OH, H2O)

yieldedstructures where themacrocycle wasmildly or notdistorted[31].

Compound 6 crystallizes in the P 21/c space group. The complexforms a centrosymmetric bimetallic dication, with a planar Ba2O2

core where the Ba atoms are bridged by the two Oalkoxide of theancillary ligand (Fig. 6). There is no contact between the metal cen-ters in the cationic fragments and the surrounding[H2N{B(C6F5)3}2]À counterion, which is stabilized by the usualFÁÁÁHÀN intramolecular interactions (1.933–2.353 Å) between F

atoms inortho

positions of the perfluorinated aromatic rings andthe NÀH protons. The long BaÁÁÁBa distance (4.35 Å) is indicativeof the absence of metallophilic interaction. In the dication, eachmetal center exhibits one strong internal BaÁÁÁFÀC secondary inter-actions with a fluorine atom, and the coordination of the ligand isconsequently best described as l2:j7,j1. Each Ba atom is therefore8-coordinated and sits in a distorted bicapped octahedral arrange-ment. The BaÀOalkoxide bond lengths (2.63–2.66 Å) are generallysubstantially shorter than the distances between metal centersand macrocyclic O atoms (2.83–2.91 Å); however, the Ba(1)ÀO(4)distance of 2.70 Å is surprisingly short in comparison to all otherBaÀOmacrocycle bond length. The Ba(1)ÀF(23) length of 2.92 Å isfar lower than the sum of the van der Waals radii for Ba (2.00 Å)and F (1.47 Å) [32]. We have recently described similar BaÁÁÁFÀCstabilizing interactions in {[{RO2}Ba]+

Á[H2N{B(C6F5)3}2]À}2ÁEtOH(BaÁÁÁFÀC = 2.93–3.08 Å), and DFT calculations indicated that thepresence of the BaÁÁÁFÀC resulted in stabilization by ca.25 kcalmolÀ1 [16c]; nevertheless, the role of the solvent moleculein this latter species was not investigated. On the other hand, 6 isdevoid of any coordinated solvent, and it therefore represents thefirst case of a solvent-free, well-defined cationic Ba complex struc-turally characterized. In light of these results, one can legitimatelyconsider that secondary BaÁÁÁFÀC interactions represent an effec-tive mean to stabilize such highly electrophilic complexes of Ba(and more generally of the alkaline-earth metals), as recently sug-gested by Ruhlandt-Senge and co-workers [33]. Other examples of such BaÁÁÁFÀC secondary interactions, albeit in neutral homolepticcomplexes, include those found in the hexafluoroacetylacetonatocomplex Ba2{hfacac}4ÁEt2O (2.77–3.09 Å) [34], in [(THF)2Ba{N(H)-

2,6-F2C6H3}2]1 (2.87–2.90 Å) [35], or in Ba{amak}2 ({amak}H = -HOC(CF3)2CH2N(CH2CH2OMe)2, 3.13–3.21 Å) [36].

Fig. 4. ORTEP diagram of the solid-state structure of  3. Only the main component of the disordered SiMe2H group is represented. Hydrogen atoms (except those on Siatoms) are omitted for clarity. Ellipsoids are drawn at the 50% probability level.Selected bond lengths [Å] and angles [°]: Ba(1)ÀO(25) = 2.653(2), Ba(1)ÀO(25)#1 = 2.647(2), Ba(1)ÀN(30) = 2.665(3), Ba(1)ÀO(4) = 2.767(2), Ba(1)ÀO(8) =2.871(2), Ba(1)ÀN(1) = 2.930(2), Ba(1)ÀSi(2) = 3.6721(11), Ba(1)ÀSi(3A) =3.6750(13), Ba(1)ÀBa(1)#1 = 4.2487(3); Si(3A)ÀN(30)ÀSi(2) = 131.09(19), Si(3A)ÀN(30)ÀBa(1) = 114.40(17), Si(2)ÀN(30)ÀBa(1) = 113.64(17).

Fig. 5. ORTEP diagram of the cationic fragment in 5Á( THF)2. Only the main componentof the disordered THF molecule is represented. Hydrogen atoms are omitted for

clarity. Ellipsoids are drawn at the 50% probability level. Selected bond lengths [Å]and angles [°]: Ba(1)ÀO(4) = 2.833(3), Ba(1)ÀO(7) = 2.841(3), Ba(1)ÀO(11) =2.937(3), Ba(1)ÀO(14) = 2.844(3), Ba(1)ÀO(22), 2.550(3), Ba(1)ÀO(40) = 2.915(4),Ba(1)ÀO(50) = 2.824(4), Ba(1)ÀN(1) = 2.976(4); C(22)ÀO(22)ÀBa(1) = 114.2(3).

Fig. 6. ORTEP diagram of the centrosymmetric bimetallic dication in 6. Only the maincomponent of the disordered aza-crown-ether is represented. Hydrogen atoms areomitted for clarity. Ellipsoids are drawn at the 50% probability level. Selected bondlengths [Å] and angles [°]: Ba(1)ÀO(26)#1 = 2.633(2), Ba(1)ÀO(26) = 2.662(3),Ba(1)ÀO(4) = 2.698(4), Ba(1)ÀO(1) = 2.830(4), Ba(1)ÀO(13A) = 2.832(3), Ba(1)À

O(10) = 2.885(4), Ba(1)ÀN(7) = 2.915(3), Ba(1)ÀF(23) = 2.918(2), Ba(1)ÀBa(1)#1

=4.3505(4); O(26)#1ÀBa(1)ÀO(26) = 69.52(8), Ba(1)#1

ÀO(26)ÀBa(1) = 110.48(8).

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 3.3. Catalytic activity in the iROP of L-LA

The well-defined, solvent-free barium complexes 4–6 were em-ployed to catalyze the iROP of L-LA in CH2Cl2 in combination withBnOH as an external co-initiator.3,4 Selected polymerization data

are collected in Table 2. Whereas 6, supported by the fluorinated alk-

oxide ancillary ligand, displayed moderate activity and required ele-

vated temperature (entry 2), 4 proved too active under the selected

experimental conditions (entry 1). Indeed, full conversion of 

1000 equiv. of L-LA ([L-LA]0/[4]0/[BnOH]0 = 1000:1:10) was typicallyensured within 15 min at 30 °C, with a high activity (>3800 molL -

LAÁ(molBa h)À1) but a poor control over the polymerization (M w/

M n = 1.85) which could only be marginally improved by modification

of the reaction conditions.

The bulkier, multidentate phenoxide analogue 5 offered thebest compromise, being very efficient even at temperature as lowas 0 °C. At room temperature, almost complete conversion of 1000 equiv. of monomer takes place at 30 °C upon addition of 10 equiv. of BnOH to 5 (entry 3). Although the agreement betweentheoretical and experimental molecular weights was satisfactory,the polydispersity index was somewhat large (M w/M n = 1.33), anindication that the binary system 5/BnOH was probably hamperedby transesterification side-reactions at high monomer conversion.

In THF, the reaction proceeded much more slowly than in CH2Cl2(our solvent of choice with this catalytic system), most likely as

  Table 2

Polymerization data for the iROP of  L -LA catalyzed by 4–6 and an external nucleophile.a

Entry Initiator Co-initiator NuH [LA]0/[M+]0 /[NuH]0 T re (°C) t  (h) Yieldb (%) TOF [molÁ(molh)À1] M n,theoc (gmolÀ1) M n,SEC

d (gmolÀ1) M w/M nd

1 4 BnOH 1000:1:10 30 0.25 96 3 840 12 600 8 300 1.852 6 BnOH 1000:1:10 100 6.5 50 77 7 300 5 600 1.233 5 BnOH 1000:1:10 30 0.5 91 1 820 13 300 10 200 1.334e 5 BnOH 1000:1:10 30 4 94 235 13 700 10 400 1.305 5Á( THF)2 BnOH 1000:1:10 30 0.5 34 680 5 000 3 800 1.09

6f  5 BnOH 1000:1:10 30 3.5 84 240 12 200 7 400 1.547 5 BnOH 1000:1:10 0 4.5 79 176 11 500 8 200 1.088 5 BnOH 1000:1:10 0 6.5 99 152 14 300 8 500 1.089 5 BnOH 1000:1:50 30 0.5 97 1 940 2 900 2 600 1.1510 5 BnOH 2 500:1:100 30 5.5 97 441 3 600 3 000 1.1211 5 BnOH 5000:1:100 30 24 95 198 7 000 5 300 1.0812 5 BnNH2 1000:1:50 30 6 95 158 2 800 2 900 1.0913 5 HO(CH2)3OH 1000:1:50 30 6 90 150 2 700 3 000 1.0714 5 AA-OH 1000:1:50 30 6 98 163 3 100 4 100 1.13

a Polymerizations carried out in CH2Cl2 with [L -LA]0 = 2.0 M.b Isolated yield after precipitation.c Calculated from M n,theo = [L -LA]0/[NuH]0  yield  144.13 + M NuH, with M BnOH = 108, M BnNH2 = 107, M HO(CH2)3OH = 76 and M AA-OH = 277 gmolÀ1.d Determined by size exclusion chromatography calibrated vs. polystyrene standards, and corrected by a factor of 0.58 according to literature recommendations [24].e Polymerizations carried out in THF.f  Polymerizations of rac -LA.

Fig. 7. 1H NMR spectrum (500 MHz, CDCl3, 298 K) of a low molecular weight (M n,SEC = 5300 gmolÀ1, M w/M n = 1.08) prepared with 5/BnOH ([L -LA]0/[5]0/[BnOH]0 = 5000:1:100).

3 The polymerization of  L -LA proceeds in the absence of external nucleophilic agent

such as BnOH, but in a very poorly controlled and non reproducible fashion. The

polymerization activity in this case most probably results from the presence of 

unknown and variable quantities of protic impurities such as lactic acid, water or

iPrOH.4 Please note that the term co-initiator is employed here to designate the external

nucleophile, namely BnOH. The co-initiator plays the roles of both the initiating agent

(formally, BnOÀ initiates the formation of the polymer chain and is indeed located at

the end of the polymer chain at the conclusion of the reaction) and the transfer agent

(the proton H+ in BnOH is in this instance the vehicle used for the transfer betweendormant and growing (macro)alcohols).

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the result of competitive coordination of the solvent onto the metalcenter (compare entries 3 and 4); the resulting polymers neverthe-less displayed identical macromolecular features. The detrimentalinfluence of coordinated solvent molecule was further highlightedwhen the initiator 5Á( THF)2 was utilized to promote the iROP of 1000 equiv. of  L -LA in CH2Cl2, as the presence of only two mole-cules of coordinated solvent substantially reduced the catalyticactivity (entries 3 and 5); however, at lower monomer conversion(34%), the molecular weight distribution was much narrower (en-try 5, M w/M n = 1.09) than previously at near complete conversion

(entry 3,M 

w/M 

n = 1.33).Unexpectedly, the iROP of  rac -LA (entry 6) was much slowerand less controlled than that of L -LA, an observation which we can-not rationalize so far; the resulting polymer was essentially atactic,as indicated by examination of the methine region of the homode-coupled 1H NMR spectrum [37]. Interestingly, the catalytic activitywas maintained even at 0 °C, where complete conversion of 1000 equiv. of monomer could be achieved in 6.5 h in the presenceof 10 equiv. of BnOH (entries 7 and 8); although under these con-

ditions there was a very slight discrepancy between the theoreticalmolecular weights and those determined by SEC, their distributionwas extremely narrow (M w/M n = 1.08), even at complete conver-sion. Remarkably, up to 5000 of  L -LA could be quantitatively con-verted (entry 11), and the barium cation could withstand theaddition of large excesses of BnOH without significant loss of cat-alytic activity or control (entries 9–11). Thus, complete conversionof 2500–5000 equiv. of monomer could be readily achieved atroom temperature in the presence of up to 100 equiv. of co-initia-tor; the control over the polymerization parameters remainedexcellent, and the productivity was satisfactory (ca. 200–2000molL 

-LAÁ(mol

Bah)À1). Besides, as expected for an iROP ‘‘activated

monomer’’ mechanism with fast and reversible chain transfer be-tween dormant and growing macromolecular chains [11d], at fixedmonomer loading the molecular weight of the polymers decreasesregularly with increasing BnOH contents (compare entries 3 and9).

The controlled nature of the polymerization was illustrated bythe anticipated increase of the molecular weight of the materialsupon increasing the monomer contents at constant concentration

Fig. 8. MALDI-ToF MS spectrum (major population: Na+; minor population, K+) of aPLLA sample (M n,SEC = 8 500 gmolÀ1, M w/M n = 1.08) prepared with the binarycatalytic system 5/BnOH (M n,calc = 144.13 Â DP + 108.14 + 22.99, where DP is thedegree of polymerization, M BnOH = 108.14 gmolÀ1, M Na = 22.99 gmolÀ1 and M L -

LA = 144.13 gmolÀ

1). Note that the slight discrepancies between M n,calc and M n,obs

are due to the set of standards used for external calibration.

Fig. 10. MALDI-ToF MS spectrum of a PLLA sample ( M n,SEC = 2 900 gmolÀ1, M w/M n = 1.09) prepared with the binary catalytic system 5/BnNH2

(M n,calc = 144.13 Â DP + 107.16 + 39.01, where DP is the degree of polymerization,

M BnNH2 = 107.16 gmolÀ1, M K = 39.01 gmolÀ1 and M L -LA = 144.13 gmolÀ1).

Fig. 9. 1H NMR spectrum (500 MHz, CDCl3, 298 K) of a low molecular weight (M n,SEC = 2 900 gmolÀ

1, M w/M n = 1.09) prepared with 5/BnNH2 ([L -LA]0/[5]0/[BnNH2]0 = 1000:1:50).

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of co-initiator (entries 10 versus 11). This was also substantiatedby NMR and MALDI-ToF MS analyses of the resulting low to med-ium molecular weight materials. Indeed, end-group fidelity wasestablished by 1H NMR spectroscopy, which indicated that all poly-mer chains contained the expected –CH(CH3)OH and BnO–C(O)–CH(CH3)– termini (Fig. 7); this was further corroborated byMALDI-ToF MS (Fig. 8), where the presence of a single populationof macromolecules corresponding to BnO–(C6H8O4)n–H (often witha significant amount of product of transesterification side-reactions) was detected.5

A variety of nucleophilic co-initiators could be used in combina-tion with 5, and BnOH was for instance replaced by benzyl amine(entry 12), 1,3-propanediol (entry 13) or Hawker’s [38] hydroxylfunctionalized alkoxy-amine 1-hydroxy-2-phenyl-2-(20,20,60,60-tet-ramethyl-10-piperidinyloxy)-ethane (AA-OH, entry 14) withoutsignificant detrimental effect. If the polymerizations clearly pro-ceeded more slowly in these cases (6 h, non-optimized reactiontime) than with BnOH (0.5 h, entry 9), both the control (M w/

M n = 1.07–1.13) and end-group fidelity remained excellent duringthe iROP of 1000 equiv. of L -LA in the presence of 50 equiv. of theseexternal nucleophiles. Upon addition of benzyl amine (entry 12),quantitative end-functionalization of the resulting monodispersedPLLA chains by the expected Bn–NH–C(O) groups was attested byNMR spectroscopy (Fig. 9) and MALDI-ToF MS (Fig. 10) analyses.With 1,3-propanediol (entry 13), an a,x-dihydroxy-telechelic PLLAcapped by a –CH(CH3)OH terminal group at both extremities of themacromolecule was obtained, and its identity was also fullyauthenticated by NMR spectroscopy (Fig. 11) and MALDI-ToF MS(Fig. 12); such materials represent for instance convenient buildingblocks for the preparation of poly(ester-urethane)s containing bio-degradable segments [39]. In the presence of AA-OH (entry 14),PLLA end-capped by a reactive alkoxy-amine moiety was obtained

with a good level of control; this type of end-functionalized PLLAscan be employed as macro-initiators for the controlled, nitroxy-mediated polymerization of styrene, yielding poly(L-LA-b-styrene)block copolymers with tunable macromolecular features [11c,38].

4. Conclusion

Efficient and general procedures for the preparation of well-de-fined, solvent-free cationic complexes [{L nX}Ba]+

Á[H2N{B(C6F5)3}2]À

of the very large and electrophilic barium have been devised,involving either the synthesis of stable heteroleptic, neutral com-plexes, or the use of doubly-protonated cationic pro-ligands asso-ciated to the weakly-coordinating counterion [H2N{B(C6F5)3}2]À.

While heteroleptic neutral complexes can be stabilized against det-rimental Schlenk-type equilibria by internal BaÁÁÁHÀSi contacts, the

Fig. 11. 1H NMR spectrum (500 MHz, CDCl3, 298 K) of a low molecular weight (M n,SEC = 3000 gmolÀ1, M w/M n = 1.07) prepared with 5/HO(CH2)2OH ([L -LA]0/[5]0/[HO(CH2)3OH]0 = 1000:1:50).

Fig. 12. MALDI-ToF MS spectrum (major population: Na+; minor population, K+) of a PLLA sample (M n,SEC = 3 000 gmolÀ1, M w/M n = 1.07) prepared with the binarycatalytic system 5/HO(CH2)3OH (M n,calc = 144.13Â DP + 76.09 + 22.99, where DP isthe degree of polymerization, M HO(CH2)3OH = 76.09 gmolÀ1, M Na = 22.99 gmolÀ1 andM L -LA = 144.13 gmolÀ1).

5 MALDI-ToF MS analyses showed that transesterification reactions occurred to a

significant extent with the binary catalytic system 5 or 6 and BnOH, as a second

distribution of usually minor intensity (separated from the main one by an incrementof 72 Da, i.e. half a lactide unit) was often detected.

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solvent-free cation [{RO2}Ba]+ supported by the fluorinated alkox-ide could be successfully isolated and structurally characterized,showing the presence of strong BaÁÁÁFÀC secondary interactions.The role of these interactions in stabilizing the cation is crucial;by contrast to [{RO2}Ba]+, the solid-state structure of the putative[{LO2}Ba]+ cation bearing the non- fluorinated phenoxide analogueof {RO2}À could not be determined, and crystallization of a Ba cat-

ion supported by the {LO

2

}

À

ancillary ligand, namely [{LO

2

}-Ba(THF)2]+, could only be achieved upon deliberate addition of THF.As anticipated, in the presence of alcohol (up to 100 equiv.) as

an external nucleophile, the resulting highly electrophilic Ba cat-ions displayed excellent activity in the controlled iROP of L -LA, con-verting up to 5000 equiv. of monomer at room temperature, andhigh catalytic activity was maintained even at 0 °C. In particular,the catalytic efficiency of the binary catalyst 5/BnOH outclassesthat of any other cationic system reported to date [12–16]. Thehigh activity of 4 in comparison to 5 confirmed the initial assump-tion that excessive coordination of donor atoms (from the ligandframework) hampered the catalytic efficacy. This was in agreementwith the observation that the ROP of  L -LA promoted by 5Á( THF)2

was slower than with the simple 5; similarly, catalysis by 5/BnOHproceeded much faster in CH2Cl2 than in THF. A broad range of protic co-initiators (alcohols, amines) can be used with these bar-ium complexes to yield quantitatively end-functionalized PLLAchains; since these binary catalytic systems can produce severaldozen polymer chains per  metal center with excellent end-groupfidelity, they are of obvious interest to the synthetic organometallicand polymer chemists.

We are currently exploring the syntheses of similar well-de-fined cations of the alkaline-earth metals stabilized by internal sec-ondary interactions, and are investigating the influence of theligand scaffold in a variety of transformations catalyzed by theseAe cations.

 Acknowledgments

Financial support from the CNRS is gratefully acknowledged.Y.S. and B.L. are also thankful to Prof. J.-F. Carpentier (Universitéde Rennes 1) for his generous assistance. The authors thank Ste-phen Boyer (London Metropolitan University) for the elementalanalyses and Total Petrochemicals for the gift of  L -lactide.

 Appendix A. Supplementary material

CCDC 826134, 826135, 826136 and 826137 contain the supple-mentary crystallographic data for 2Á[C6H6]0.5, 3, 5Á( THF)2 and 6,respectively. These data can be obtained free of charge from TheCambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif . Supplementary data associated with this articlecan be found, in the online version, at doi:10.1016/

 j.ica.2011.09.020.

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