Indium Complexes of Fluorinated Dialkoxy-Diimino Salen-like Ligands

Article pubs.acs.org/Organometallics

Indium Complexes of Fluorinated Dialkoxy-Diimino Salen-like Ligands for Ring-Opening Polymerization of rac-Lactide: How Does Indium Compare to Aluminum? Mickael Normand, Evgeny Kirillov,* Thierry Roisnel, and Jean-François Carpentier* Catalysis and Organometallics, UMR 6226 Sciences Chimiques de Rennes, CNRS-Université de Rennes 1, 35042 Rennes cedex, France S Supporting Information *

ABSTRACT: The reaction between InCl3 and {ONEtNO}K2 (1a), prepared from (CF3)2(OH)CCH2C(CH3)N−R−NC(CH3)CH2C(OH)(CF3)2 ({ONEtNO}H2, R = C2H4 (a); {ONCyNO}H2, R = rac-1,2-cyclohexylene (b)) and PhCH2K, gave {ONEtNO}InCl (2a). The reaction between InCl3 and 3 equiv of MeLi led to a mixture of [InMe3] and Li[InMe4], which upon further treatment with {ONRNO}H2 proligands gave the ate complexes [{ONRNO}Li]InMe2 (4a,b); the methyl complex {ONEtNO}InMe (3a) was also isolated from this reaction. Hydrocarbyl complexes {ONRNO}In(CH2SiMe3) (5a,b) were prepared cleanly from the 1:1 reactions between In(CH2SiMe3)3 and {ONRNO}H2. The solid-state molecular structures of mononuclear 2a, 3a, 4b, and 5b and of [(1a)2-(μ-H2O)]n, a distorted cubane-like core made up of four potassium atoms and two ligands, were determined. Compounds 5a,b are moderately active initiators/catalysts for the ring-opening polymerization of rac-lactide, giving polymers with controlled molecular weights and narrow polydispersities, especially in the presence of added isopropyl alcohol (1−10 equiv) as exogenous initiator. Isotacticenriched (Pm = 0.62−0.69) PLAs were obtained from 5a, while 5b gave atactic materials. The heterobimetallic compounds 4a,b are also active and afforded slightly heterotactic-enriched PLAs (Pr = 0.57−0.62), but with broader polydispersities. Those results allowed us to discuss the initiation mechanisms according to the constitution of the systems (alkyl vs alkyl/iPrOH) and also stereoselective abilities as a function of the nature (In vs Al) and coordination environment (κ4-ONNO vs. κ2-NN) of the metal center.

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INTRODUCTION Recent advances in the synthesis of biodegradable polymers have revealed that well-defined group 13 metal complexes are highly competent initiators/catalysts for the ring-opening polymerization (ROP) of cyclic esters,1 notably for the synthesis of polylactide (PLA), a valuable bioresourced plastic.1a,b,d In particular, controlled ROP of lactides (both racemic and enantiomerically pure) with aluminum compounds is largely documented in the literature,2 and some of these aluminum catalyst systems have shown very high levels of stereocontrol in these reactions.2a,d,e,3 Within the same group of elements, discrete gallium(III)4 and indium(III)-based5−9 initiators have been more recently explored for the ROP of rac-lactide as well as of other cyclic esters (e.g., ε-caprolactone). In our previous studies, we have prepared various neutral and cationic aluminum complexes incorporating “fluorinated” alkoxy ligands (Chart 1) and shown that they promote efficiently ROP of cyclic esters, eventually affording polymers with controlled architectures.10 The nature of the ligand backbone and substituents allowed fine tuning of the intrinsic Lewis acidity of the metal center in these complexes, imparted by the strong electron-withdrawing effect of the α-CF3 groups at the alkoxide moieties.11 Interestingly, some of these © 2011 American Chemical Society

fluorinated systems based on tetradentate Salen-type scaffolds such as A and B also featured significant stereocontrol abilities in the ROP of rac-lactide, producing polylactic acids (PLAs) with highly isotactic-enriched stereoblock microstructures (Pmeso up to 0.87).10b,c,12 Changing the nature of the metal center is another option for modifying the intrinsic properties of these fluorinated alkoxide group 13 complexes, such as the Lewis acidity.13,14 Accordingly, we describe herein the synthesis and structural characterization of trivalent indium complexes supported by dianionic fluorinated dialkoxy-diimino ligands {ONRNO}2− (R = C2H4 (a), R = rac-1,2-cyclohexyl (b)).15 Preliminary studies on the catalytic performances of such discrete indium alkyls in the ROP of rac-lactide are also reported.

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RESULTS AND DISCUSSION Synthesis of Indium Chloro Complexes by Salt Metathesis Reactions. In general, aluminum fluoroalkoxSpecial Issue: Fluorinein Organometallic Chemistry Received: September 28, 2011 Published: December 6, 2011 1448

dx.doi.org/10.1021/om200906e | Organometallics 2012, 31, 1448−1457

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Chart 1. Examples of Aluminum Complexes (X = Me, Cl, OiPr) Supported by Fluorinated Alkoxide Ligands Used in ROP of Cyclic Esters10

ide10b and indium phenoxide chloro complexes5,6,8 can be prepared efficiently by salt elimination reactions starting from AlCl3 or InCl3 and the corresponding ligand alkali-metal salts. Thus, we first evaluated this approach to achieve the synthesis of indium fluorinated dialkoxide-diimino chloro compounds. As summarized in Scheme 1, the fluorinated diimino-diol proligands {ONRNO}H2 (R = Et, Cy) were combined in

H2O)]n ([(1a)2-(μ-H2O)]n). The solid-state structure of the latter compound is presented in Figure 1 (see Table S1 in the

Scheme 1. Synthesis of Dipotassium Salts 1a,b of {ONRNO}2− Ligands

Figure 1. Solid-state molecular structure of [({ONEtNO}K2)2-(μH2O)]n ([(1a)2-(μ-H2O)]n) (all hydrogen atoms, except that of the water molecule, and the second ligand {ONEtNO}2− about the K3 center are omitted for clarity). Selected bond distances (Å) and angles (deg): N(1_1)−K(1), 2.885(2); N(1_2)−K(1), 3.2458(19); K(1)− O(1_1), 2.6205(16); K(1)−O(1_2), 2.6220(17); K(1)−O(2_2), 2.7589(16); K(4)−O(2_1), 2.6673(18); K(4)−O(2_2), 2.7006(16); K(4)−O(1_2), 2.7835(16); K(4)−O(3), 2.7730(18); K(4)−F(1), 2.9463(17); K(4)−F(2), 2.8364(18); K(3)−F(3), 2.8401(16); K(3)− F(4), 3.1517(17); K(2)−F(5), 3.0791(15); O(1_1)−K(1)−O(1_2), 96.22(5); N(1_1)−K(1)−N(1_2), 57.86(5); O(2_2)−K(3)− O(2_1), 94.39(5); O(2_1)−K(4)−O(3), 108.04(5); O(2_2)− K(4)−O(2_1), 91.41(5); K(1)−O(1_2)−K(4), 95.71(5); K(1)− O(2_2)−K(4), 94.51(5).

diethyl ether with 2 equiv of benzylpotassium to provide cleanly the corresponding dipotassium salts 1a,b, respectively. Compounds 1a,b were isolated as colorless microcrystalline powders which are soluble in ethers (THF, Et2O) and CH2Cl2 and insoluble in aliphatic and aromatic hydrocarbons. The 1H NMR spectrum of 1a in THF-d8 at room temperature was consistent with an average highly symmetric structure on the NMR time scale: only single resonances were observed for the NC−Me, CH2C(CF3)2, and CH2CH2 groups (see the Supporting Information, Figure S1). Also, the 19F{1H} NMR spectrum of 1a featured one sharp singlet, indicating that all the CF3 moieties are magnetically equivalent under those conditions (Figure S2). The solution structure of the cyclohexylenebridged 1b featured C2 symmetry, as judged by NMR spectroscopy. In particular, in the 1H NMR spectrum of 1b in THF-d8 at room temperature (Figure S3), only one signal was observed for the NC−Me groups, while the hydrogens of the CHHC(CF3)2 groups are diastereotopic and appeared as an AB system. Accordingly, two sharp quartets of equal intensity were observed in the 19F{1H} NMR spectrum of 1b (Figure S4). Repeated recrystallization attempts of 1a from diethyl ether solutions layered with hexanes systematically resulted in a new species, probably arising from the presence of adventitious water traces, with the structural formula [({ONEtNO}K2)2-(μ-

Supporting Information for crystallographic details). The single-molecule unit of [(1a)2-(μ-H2O)]n features a distorted cubane-like core made of four potassium centers bonded to the four oxygen atoms of two ligand units. Each tetrametallic core is linked with another one by one water molecule, eventually giving rise to a one-dimensional polymeric chain structure. The coordination environments of the potassium atoms in the tetrametallic core of [(1a)2-(μ-H2O)]n are not equivalent. In fact, two of them (K1 and K3) are each coordinated by two oxygen (O1_1, O1_2 and O2_1, O2_2, respectively) and two nitrogen atoms (N1_1, N1_2 and N2_1, N2_2, respectively) of the tetradentate {ONEtNO}2− ligands. The two other potassium atoms, namely K2 and K4, are each bonded to the 1449


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= 0.42; X = Me, τ = 0.48).10b In fact, the two oxygen atoms O1 and O2 of the ligand in 2a are virtually orthogonal, as is illustrated by the O1−In−O2 angle of 90.25(5)°, while the angle N1−In−N2 is significantly smaller (75.40(5)°) (Table 1). The In−O1 and In−O2 bond lengths of 2.067(1) and 2.052(1) Å are equivalent and expectedly longer than the corresponding Al−O bonds in {MeONEtNO}AlCl (1.754(2) and 1.780(2) Å),10b reflecting the ca. 0.25 Å increase in the ionic radius of the metal center.20 Also, the In−Cl bond length (2.361(1) Å) in 2a is longer than in {MeONEtNO}AlCl (2.184(1) Å)10b and somewhat shorter than that observed in a {NNO}InCl2·Py complex (2.4025(4) Å).8 We further strove to access alkoxy and amido derivatives by nucleophilic substitution of the chloride ligand in 2a. However, salt metathesis reactions involving 2a and KOiPr, LiN(SO2CF3)2, or NaN(SiMe3)2 failed to give clean products. These observations contrast with the selective targeted displacement in aluminum fluorinated alkoxy-imino chloro complexes;10 however, they are unexceptional for indium and are notably in line with the observations reported recently by Okuda et al. for the phenoxy-based {OSSO}InCl system.6b Synthesis of Alkyl Indium Complexes by Alkane Elimination Reactions. Among the variety of recipes being used to prepare group 13 metal species incorporating nucleophilic alkyl, amido, or alkoxy groups, the one-pot σbond metathesis approach through an alkane, amine, or alcohol elimination reaction appears to be one of the most effective in terms of selectivity. Preliminary attempts to coordinate fluorinated ligands {ONRNO}2− by this approach utilizing In[N(SiMe3)2]3 or In(OiPr)3 precursors with {ONRNO}H2 failed.21 Our next efforts were therefore focused on the preparation of alkyl derivatives. As summarized in Scheme 3, the putative [InMe 3 ] compound, generated in situ via the room-temperature reaction of InCl3 with 3.0 equiv of MeLi in diethyl ether over 30 min,22 was allowed to react with {ONRNO}H2 (R = Et, Cy). Standard workup followed by crystallization repeatedly gave the analytically pure bimetallic ate complexes [{ONEtNO}Li]InMe2 (4a) and [{ONCyNO}Li]InMe2 (4b) in 30% and 40% yields, respectively. Only in a single experiment, conducted under apparently identical conditions, were X-ray diffraction quality crystals (vide infra) of the targeted neutral methyl complex 3a isolated in 10% yield. This discrepancy in the nature of the products isolated upon following this protocol may be accounted for by the existence of a two-step synthetic pathway toward the homoleptic precursor [InMe3] (Scheme 3). Apparently, the formation of [InMe3] proceeds through generation of the stable ate complex [Me4In]Li23 (path B) rather than through direct alkylation (path A). Expectedly, ate complexes 4a,b could be also prepared selectively from the reaction of {ONRNO}H2 with [Me4In]Li.23 Colorless single crystals of complex 4b suitable for an X-ray diffraction study were grown from a diethyl ether solution at −30 °C. The molecular structure of 4b is depicted in Figure 3 and features a highly symmetric species, with a C2 axis going through the In and Li centers. While the indium center lies in a distorted-tetrahedral geometry (O(1)−In−O(2), 71.26(10)°; C−In−C, 136.09(10)°) (Table 1), the coordination environment of the lithium atom is perfectly square planar (τ = 0.00):19 that is, the Li atom lies in the ONNO mean plane. It is interesting to note that the O(1)−Li−O(2) angle in 4b (86.6(3)°) is quite similar to the O(1)−In−O(2) angles in 2a (90.25(5)°) and 3a (86.11(11)°; vide infra), despite the quite

two oxygen atoms (O1_1 and O1_2, or O2_1 and O2_2, respectively) of a given ligand and the oxygen atom of the bridging water molecule. The K−O(ligand) bond lengths are in the range 2.620(2)−2.783(2) Å, which is in agreement with those reported in the literature (2.721(2)−3.203(3) Å) for [K(DNPGS)(H2O)]n (DNGPS = dinitrophloroglucinolsulfonic acid).16 The K−O distances involving K2 and K4 and the oxygen atoms of the bridging water molecules (K2−O3′ = 2.811(2) Å and K4−O3 = 2.773(2) Å, respectively) argue for a binding between water and the K4O4 cores stronger than that observed in {K[O2C−C6H4−N(H)NN(CH3)O]n·4H2O}n, in which bridging water molecules also generated a polymeric compound (K−O(water) distances: 2.700(2)−3.304(3) Å).17 It is worth mentioning that each K4O4 core in [(1a)2-(μH2O)]n is stabilized additionally by K···F interactions. Those K···F distances in [(1a)2-(μ-H2O)]n (2.836(2)−3.152(2) Å) fall into the range (2.708(9)−3.139(4) Å) observed in compounds featuring related K···F contacts.18 The one-pot salt-metathesis reaction of 1a with InCl3, carried out under regular conditions as summarized in Scheme 2, led to Scheme 2. Synthesis of Indium Chloro Complex 2a by Salt Metathesis

the isolation of the targeted complex {ONEtNO}InCl (2a) in 60% yield as a colorless microcrystalline powder. In contrast, the analogous reaction of the cyclohexyl-bridged disalt 1b gave complex mixtures of products, from which 2b could not be confirmed or isolated. An X-ray diffraction study of single crystals of 2a confirmed the five-coordinate structure (Figure 2), the latter being overall

Figure 2. Solid-state molecular structure of {ONEtNO}InCl (2a) (ellipsoids drawn at the 50% probability level; all hydrogen atoms are omitted for clarity).

similar to that previously determined for the corresponding aluminum analogues.10 The geometry at the indium center is best described as distorted square pyramidal with a calculated value for the trigonal index τ of 0.30.19 This value is rather similar to that observed in the related aluminum complexes {MeONEtNO}AlX ({MeONEtNO}H2 = (CF3)2(OH)CCMe2C(CH3)N−R−NC(CH3)CMe2C(OH)(CF3)2) (X = Cl, τ 1450


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Table 1. Key Bond Distances (Å) and Bond Angles (deg) for Complexes 2a, 3a, 4b, and 5b 2a In−C(1) In−Cl(1) In−O(1) In−O(2) In−N(1) In−N(2) Li−O(1,2) Li−N(1,2) O(1)−In−O(2) N(1)−In−N(2) O(1)−In−N(1) N(1)−In−C(1) N(1)−In−Cl(1) O(1)−Li−O(2) N(1)−Li−N(2)

2.3607(5) 2.0669(14) 2.0522(13) 2.2367(15) 2.2482(15)

90.25(5) 75.40(5) 86.70(6)

3a

4b

5b

2.154(4)

2.135(3)

2.154(2)

2.099(3) 2.126(3) 2.319(3) 2.268(4)

2.177(2) 2.177(2)

2.0736(15) 2.1013(15) 2.4320(18) 2.2705(18)

86.11(11) 73.27(12) 80.97(12) 108.64(15)

1.915(6) 1.164(6) 74.25(10)

108.56(13)

94.36(6) 68.22(6) 84.61(6) 98.71(7)

111.40(4) 86.6(3) 79.9(3)

Scheme 3. One-Pot Synthesis of Neutral and Ate Methyl Indium Complexes 3a,b and 4a,b

As summarized in Scheme 4, equimolar quantities of those reagents were combined in diethyl ether at 60 °C for 15 h in a Scheme 4. Synthesis of Trimethylsilylmethyl Indium Complexes 5a,b

Figure 3. Solid-state molecular structure of [{ONCyNO}Li]InMe2 (4b) (ellipsoids drawn at the 50% probability level; all hydrogen atoms are omitted for clarity).

sealed Schlenk flask. Workup eventually afforded the targeted complex {ONEtNO}In(CH2SiMe3) (5a) in 45% yield. The synthesis of {ONCyNO}In(CH2SiMe3) (5b) appeared to be much less selective, although the nature of the coproducts could not be established. Analytically pure 5b was obtained in 10% yield after reprecipitation of the crude material from a Et2O/pentane (3/1 v/v) mixture. Compounds 5a,b are both colorless solids which are poorly soluble in common organic

different nature of these metal centers coordinated to the ONNO ligand. The In−C and In−O bond distances of 2.135(3) and 2.177(2) Å, respectively, in 4b are in the usually observed ranges. We next sought for a more selective approach toward alkyl indium complexes and used In(CH2SiMe3)3·1/2THF6a,24 as the precursor in σ-bond/protonolysis reactions with {ONRNO}H2. 1451


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intact in the presence of 1−5 equiv of iPrOH, even upon reflux in C6F6, benzene, THF, and Et2O solvents over prolonged time periods (48 h). Solution NMR Studies of Complexes 2a, 4a,b, and 5a,b. NMR experiments were carried out to examine the solution structures of the complexes, and in particular to investigate whether the solid-state structures are retained or if any other geometric isomers could be detected. Both the 1H and 13C{1H} NMR spectra of {ONEtNO}InCl (2a) in CD2Cl2 (Figures S5 and S7, Supporting Information) exhibited a single set of sharp resonances consistent with a single (>98%) species present in solution, as observed for the analogous aluminum complex.10 The average Cs symmetry of this complex on the NMR time scale is evidenced by the following characteristic pattern: one singlet for the CH2 groups of the ethylene bridge, one singlet for the CH2C(CF3)2 moieties, and one singlet for the CH3 hydrogens. The 19 1 F{ H} NMR spectrum of 2a in CD2Cl2 (Figure S6) showed two broad signals (δ −76.15 and −79.95 ppm) of equal intensity, indicating that the two CF3 groups are nonequivalent within each C(CF3)2 moiety. The molecule of 5a is also symmetric in solution on the NMR time scale, as evidenced from the observation in both the 1 H and 13C{1H} NMR spectra of only one series of resonances for each type of nucleus (Figures S14 and S16, respectively); also, in the 19F{1H} NMR spectrum of 5a (Figure S15), two equal-intensity quartets were observed. On the other hand, the molecule of 5b in C6D6 solution at 25 °C features C1 symmetry, as revealed from the presence of four quartets of equal intensity (two of them overlapped) in the 19F{1H} NMR spectrum (Figure S18). The C1 molecular symmetry of 5b was also evident from the 1H and 13C{1H} NMR data (Figures S17 and S19). No change in the 1H NMR spectra of 5b in toluene-d8 was noticed in the temperature range 273−373 K (except a slight dependence of some chemical shifts, but these resonances did not broaden). This indicates that the asymmetry of compound 5b is intrinsically related to the chiral trans-1,2cyclohexylene bridge and does not rely on restricted motions in the five- and six-membered metallacycles. Overall, these data for complexes 2a and 5a,b are consistent with the presence of a unique species in solution, having either a trigonal-bipyramidal or square-pyramidal structure, as observed in the solid state. The 1H and 13C{1H} NMR data of both 4a and 4b in C6D6 at room temperature (Figures S8, S10, S11, and S13) were consistent with average Cs- or C2-symmetric molecular structures: each group (In−CH3, CH2C(CF3)2, CH2CN, NCH3, −(C2H4)− or −(c-C6H10)− bridges) came out as single sharp resonances for each type of nucleus. A single sharp resonance was observed in the 19F{1H} NMR spectrum of 4a (Figure S9), reflecting fast motion of the five- and sixmembered metallacycles at room temperature. On the other hand, for the more rigid 1,2-trans-cyclohexylene-bridged 4b, two quartets for nonequivalent CF3 groups within each C(CF3)2 moiety appeared in the room-temperature 19F{1H} NMR spectrum (Figure S12). Preliminary Studies on ROP of rac-Lactide. The performances of the prepared compounds 4a,b and 5a,b that possess a potentially reactive nucleophilic alkyl group to act as initiators/(pre)catalysts in the ROP of rac-lactide were evaluated. Our initial main aim was to compare the abilities of the series of {ONRNO}-indium derivatives to those of their aluminum analogues, in particular, to probe the influence of the

solvents at room temperature but highly soluble in THF at 60 °C. Single crystals of 5b suitable for X-ray diffraction studies were grown from a diethyl ether solution at −30 °C. Given the similar nature of 3a and 5b, it is worth discussing their structural features at the same time. The solid-state structures of both mononuclear alkyls 3a and 5b feature a five-coordinate indium center (Figures 4 and 5, respectively). The geometry of

Figure 4. Solid-state molecular structure of {ONEtNO}InMe (3a) (ellipsoids drawn at the 50% probability level; all hydrogen atoms are omitted for clarity).

Figure 5. Solid-state molecular structure of {ONCyNO}In(CH2SiMe3) (5b) (ellipsoids drawn at the 50% probability level; all hydrogen atoms are omitted for clarity).

3a is best described as distorted square pyramidal (τ = 0.23), while that of 5b is closer to distorted trigonal bipyramidal (τ = 0.64).19 This trend is obviously driven by the nature of the ligand backbone (ethylene vs cyclochexylene), as evidenced by the observation of asimilar distorted-trigonal-bipyramidal geometry in the series of Al-{ONCyNO} compounds.10b However, despite the different gross geometries, the bond lengths and angles in these two molecules are quite close. For instance, the In−O bonds in 3a are just somewhat longer than those in 5b (2.099(3), 2.126(3) Å vs 2.074(2), 2.101(2) Å), whereas the In−N bonds in 3a are somewhat shorter than those in 5b (2.268(4), 2.319(3) Å vs 2.271(2), 2.432(2) Å). The In−C bond lengths in both molecules are identical (2.154(4) and 2.154(2) Å, respectively). In further studies we found that neither 5a nor 5b undergoes alcoholysis reactions at the indium−alkyl bond with iPrOH under various conditions. In fact, both compounds remained 1452


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metal center on the stereoselectivity of the process.10b We were also interested in evaluating the new heterobimetallic compounds 4a,b that feature a quite different structural environment for the potentially active indium center. Those two types of compounds were used as such, that is as initiators, as well as catalysts (or catalyst precursors) when combined with an alcohol (isopropyl alcohol) cocatalyst/initiator (Scheme 5). Representative results are summarized in Table 2.

opening would proceed via nucleophilic attack of the alkyl moiety.1 Combination of 5a,b with 1−10 equiv of isopropl alcohol did not affect significantly the polymerization activities (as compared to polymerizations performed with 5a,b alone), except in THF, but those still proceeded quite slowly (Table 2, entries 2−4, 6, 7, and 9). On the other hand, a much better control over the polymerization was achieved: the experimental molecular weights, determined equally by NMR and SEC, matched well the calculated Mn values, and the molecular weight distributions were most generally quite narrow (Mw/Mn = 1.08−1.21). Also, with these binary {ONRNO}InCH2SiMe3/ iPrOH systems, the productivity could be readily increased up to 1000 equiv of lactide (entries 4 and 9); no attempt was conducted to further increase the monomer loadings, but this could be likely easily reached. All the PLAs produced in the presence of iPrOH were shown by NMR and MALDI-TOFMS techniques to be selectively end-capped with hydroxycarbonyl and isopropylcarbonyl groups. Yet, in this case, a coordination−insertion mechanism involving a putative {ONRNO}In(OiPr) species seems unlikely, since we independently showed that neither 5a nor 5b reacts with isopropyl alcohol (vide supra). Rather, an “activated-monomer” type mechanism, involving activation of lactide monomer units by the electrophilic indium center of 5a,b followed by an external nucleophilic attack of isopropyl alcohol, seems much more plausible.1 The microstructures of the PLAs were essentially dependent on the nature of the catalyst/initiator and solvent used. PLAs with a notable isotactic bias (Pm = 0.62−0.69)12 were obtained from the ethylene-bridged 5a in toluene as solvent, almost irrespective of whether isopropyl alcohol is added or not. The same systems used in THF led to slightly heterotactic-enriched PLA (Pr = 0.59).12 Quite surprisingly, the trans-1,2-cyclohexylene-bridged 5b led to essentially atactic PLAs. Those results contrast strikingly with the high stereoselectivities

Scheme 5

The alkyl complexes {ONRNO}In(CH2SiMe3) 5a,b initiate the ROP of rac-LA with moderate activity in toluene at 80 °C (entries 1 and 8, respectively); on the other hand, no activity at all was observed for 5a in THF at 60 °C (entry 5). As anticipated for poorly nucleophilic alkyl compounds, the initiation efficiency of these derivatives was low (0.22 and 0.48 for 5a,b, respectively). This was indicated by experimental molecular weights much higher than those calculated from the monomer-to-initiator ratio and unimodal, yet rather broad molecular weight distributions, especially for 5b (1.21 and 2.03, respectively). An analysis by 1H NMR spectroscopy of the PLAs produced from 5a,b revealed the presence of a trimethylsilyl resonance (δ 0.00 ppm, CDCl3; Figure S23),25 consistent with COCH2SiMe3 end groups at one terminus of the polymer chain.26 The molecular weights calculated from this resonance (and the methyl signal of main-chain lactide units) matched rather well the Mn values determined by SEC. These observations are consistent with a regular “coordination−insertion” polymerization mechanism in which initial ring

Table 2. Ring-Opening Polymerization of rac-Lactide Promoted by Indium Complexes 4a,b and 5a,ba entry

[In]

[LA]:[In]: [iPrOH]

solvent

temp (°C)

timeb (h)

conversnc (%)

Mn(calcd)d (g/mol) ×103)

Mn(exptl)e (g/mol) (×103)

Mn(NMR)f (g/mol) (×103)

Mw/ Mne

Pr

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

5a 5a 5a 5a 5a 5a 5a 5b 5b 4a 4a 4a 4a 4b 4b 4b 4b

100:1:0 100:1:1 500:1:1 1000:1:10 100:1:0 100:1:1 100:1:1 100:1:0 1000:1:10 100:1:0 100:1:1 500:1:1 1000:1:10 100:1:0 100:1:1 500:1:1 1000:1:10

toluene toluene toluene toluene THF THF THF toluene toluene toluene toluene toluene toluene toluene toluene toluene toluene

80 80 80 80 60 60 60 80 80 80 80 80 80 80 80 80 80

18 19 17 17 5 5 19 15 15 18 16 18 18 1 0.5 18 15

83 98 98 98 0 11 26 93 88 99 99 79 95 96 98 98 98

12.0 14.1 70.1 14.1

54.7 11.3 29.0 13.8

53.9 8.80 nd 13.1

1.21 1.63 1.20 1.08

0.31 0.38 0.35 0.35

1.6 3.8 13.4 12.7 14.3 14.4 57.0 13.7 13.8 14.1 70.6 14.1

1.0 4.4 27.7 13.4 12.5 4.0 22.7 10.8 12.3 5.7 18.8 11.9

1.8 3.1 54.2 13.1 12.3 5.1 20.2 19.4 10.8 9.6 nd 12.0

1.16 1.21 2.03 1.11 1.94 1.86 2.44 1.82 2.56 2.16 2.08 1.75

nd 0.59 0.51 0.53 0.60 nd nd 0.57 0.62 nd nd 0.62

a

Polymerization conditions for reactions performed in slurry/solution: [rac-LA]0 = 2.0 M. bReaction times were not necessarily optimized. Monomer conversion determined by 1H NMR spectroscopy (CDCl3, 298 K). dTheoretical molecular weight calculated using Mn(calcd) = conversn × [rac-LA]0/[In or iPrOH] × Mrac‑LA. eExperimental molecular weight determined by SEC vs polystyrene standards and corrected by a factor of 0.58; molecular weight distribution determined by SEC. fExperimental molecular weight determined by NMR from the relative intensities of the main chain and terminal resonances. c

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observed with the analogous {ONRNO} aluminum derivatives, which generated highly isotactic-enriched stereoblock microstructures (Pm = 0.80−0.87)12 for both ethylene- and cyclohexylene-bridged systems, equally in toluene slurry and in bulk molten lactide.10b This large decrease in stereoselectivity on going from aluminum to indium systems is consistent with the well-accepted crucial assistance of organometallic compounds in chain-end-controlled ROP processes, with subtle ligand modifications resulting in very important effects on stereoselectivity.27 We assume that, in the present systems, the larger indium center led to complexes less sterically crowded than those based on the relatively smaller aluminum,20 resulting in less effective transfer of chiral information during monomer enchainment. This is in line with the trends observed with group 3 metal systems of the type {ONOO}Ln, for which small metal centers (Y(III)) are much more effective in terms of stereocontrol than larger ones (Nd(III), La(III)).26b The bimetallic compounds 4a,b also are active initiators/ catalysts for the ROP of rac-LA. Without any coactivator (i.e., no iPrOH), 100 equiv of monomer is fully converted within 1 h in toluene at 80 °C to give PLAs with experimental molecular weights that match well those calculated, assuming one macromolecular chain per InMe2 center (Table 2, entries 10 and 14). The molecular weight distributions of those polymers were all monomodal but broad (Mw/Mn = 1.94−2.56). Upon addition of 1 equiv of isopropyl alcohol to 4a or 4b, 100 equiv of rac-LA was quantitatively converted to PLAs but with molecular weights 3-fold lower than those obtained in the absence of alcohol (compare entries 11 and 15 vs 10 and 14). Increasing the lactide loading to 500 equiv eventually gave PLAs with proportionally (i.e., ca. 5-fold) larger Mn values, evidencing a certain degree of control over the molecular weights (entries 12 and 16). The 1H NMR spectra of the PLAs produced from those 4a,b/iPrOH (1:1) systems showed resonances consistent with the presence of both COOiPr (major, ≥60−64%) and COCH3 (minor, ≤36−40%; δ 2.35 ppm, CDCl3; Figure S24) end groups at one terminus and CH(OH) moieties at the other terminus. This suggests that, under these conditions, ring opening of LA may occur competitively by nucleophilic attack of iPrOH and Me groups. In the presence of an excess (10 equiv) of isopropyl alcohol, the PLA chains were selectively end-capped by COOiPr moieties and the experimental (NMR, SEC) Mn values were very close to those calculated from the [LA]/[iPrOH] ratio (entries 13 and 17). However, in all cases, the polydispersities were much broader than those achieved with the 5a,b/iPrOH (1:1) systems. All the PLAs produced from 4a,b, with or without added isopropyl alcohol, showed a modest bias toward heterotactic microstructures (Pr = 0.57−0.62). We speculate that this might arise from the lower steric hindrance in those heterobimetallic compounds in which the indium center has been rejected outside of the {ONRNO}2− chelate by the lithium center.

initiation mechanisms according to the constitution of the systems (alkyl vs alkyl/iPrOH) and, probably more interestingly, the stereoselective abilities as a function of the nature (In vs Al) and coordination environment (κ4-ONNO vs κ2-NN) of the metal center. The prepared {ONNO}-indium compounds appeared significantly less stereoselective than their aluminum analogues. This is tentatively proposed to arise from the larger size of the indium center, which generates less sterically crowded compounds and in turn a less effective influence of the latter compounds in the polymerization chain-end stereocontrol.26 To be definitively confirmed, this trend would require further data on homologous series of Al/Ga/In compounds; however, it fits well previous observations made with catalysts/initiators based on trivalent rare-earth metals associated with multidentate {ONNO}2− and {ONOO}2− ligand platforms.26b

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General Considerations. All manipulations were performed under a purified argon atmosphere using standard Schlenk techniques or in a glovebox. Solvents were distilled from Na/benzophenone (THF, Et2O) or Na/K alloy (toluene, hexane, and pentane) under argon, degassed thoroughly, and stored under argon prior to use. Deuterated solvents were stored over Na/K alloy (benzene-d6, toluene-d8, THF-d8; >99.5% D, Eurisotop) or CaH2 (CD2Cl2) and vacuum-transferred just before use. Proligands {ONRNO}H2 (R = Et, Cy)15 and precursors In(CH2SiMe3)3,28 In{N(SiMe3)2}3,29 and In(OiPr)330 (also purchased from Alfa Aesar) were prepared using literature procedures. Other starting materials were purchased from Acros, Strem, and Aldrich and used as received. NMR spectra of complexes were recorded on Bruker AC-200, AC300, Avance DRX 400, and AM-500 spectrometers in Teflon-valved NMR tubes at 25 °C unless otherwise indicated. 1H and 13C NMR chemical shifts are reported in ppm vs SiMe4 and were determined by reference to the residual solvent peaks. 19F NMR chemical shifts were determined by external reference to an aqueous solution of NaBF4. Assignment of resonances for organometallic complexes was made from 2D 1H−13C HMQC and HMBC NMR experiments. Coupling constants are given in hertz. Elemental analyses (C, H, N) were performed using a Flash EA1112 CHNS Thermo Electron apparatus and are the average of two independent determinations. Size exclusion chromatography (SEC) analyses of PLAs were performed in THF (1.0 mL min−1) at 20 °C using a Polymer Laboratories PL-GPC 50 plus apparatus equipped with a PLgel 5 μm MIXED-C 300 × 7.5 mm column and RI and dual-angle LS (PL-LS 45/90) detectors. The number average molecular masses (Mn) and polydispersity indexes (Mw/Mn) of the polymers were calculated with reference to a universal calibration vs polystyrene standards. The Mn values of PLAs were corrected with a factor of 0.58, to account for the difference in hydrodynamic volumes with polystyrene.31 The microstructure of PLAs was determined by homodecoupling 1H NMR spectroscopy at 20 °C in CDCl3 on a Bruker AC-500 spectrometer. {ONEtNO}K2 (1a). A Schlenk flask was charged with {ONEtNO}H2 (0.400 g, 0.845 mmol) and benzylpotassium (0.220 g, 1.69 mmol), and THF (ca. 10 mL) was vacuum-transferred in at ca. −80 °C. The flask was warmed to room temperature,and the reaction mixture was stirred overnight, resulting in a clear colorless solution. Volatiles were removed in vacuo to give {ONEtNO}K2 (1a) as a white solid (0.455 g, 99%). 1H NMR (500 MHz, THF-d8, 298 K): δ 3.40 (s, 4 H, NCH2CH2N), 2.57 (s, 4 H, CH2C(CF3)2), 1.90 (s, 6 H, CH3). 19 1 F{ H} NMR (188 MHz, THF-d8, 298 K): δ −78.14 (s, 12F, CF3). Anal. Calcd for C14H14F12K2N2O2: C, 30.66; H, 2.57; N, 5.11; Found: C, 30.50; H, 2.60; N, 5.00. {ONCyNO}K2 (1b). Using a protocol similar to that described above for 1a, complex 1b was prepared from {ONCyNO}H2 (0.300 g, 0.570 mmol) and benzylpotassium (0.148 g, 1.14 mmol). 1b was isolated as a white solid (0.340 g, 99%). 1H NMR (500 MHz, THF-d8, 298 K): δ 3.49 (m, 2H, CH(cy)), 2.74 (d, 2JHH = 12.4, 2H, CHHC(CF3)2), 2.42

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CONCLUSION The coordination chemistry of tetradentate fluorinated {ONNO}2− ligands onto indium(III) and the reactivity of the corresponding compounds feature some singularities as compared to that of the related aluminum derivatives. The preparation of isopropoxide-indium compounds turned out not to be possible via the routes envisioned. The results gathered in the polymerization of rac-lactide allowed us to discuss possible 1454


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MHz, THF-d8, 298 K): δ 3.61−3.80 (m, 4H, NCH2CH2N), 3.05 (d, JHH = 14.5, 2H, CHHC(CF3)2), 2.65 (d, 2JHH = 14.5, 2H, CHHC(CF3)2), 2.28 (s, 6H, CH3), 0.04 (s, 9H, Si(CH3)3), −0.38 (s, 2H, InCH2). 19F{1H} NMR (188 MHz, THF-d8, 298 K): δ −76.71 (q, 4JFF = 10.4, 6F, CF3), −80.03 (q, 4JFF = 10.4, 6F, CF3). 13C{1H} NMR (125 MHz, THF-d8, 298 K): δ 176.9 (CN), 123.0 (q, 1JCF = 310, CF3), 79.9 (m, 2JCF = 22.1, CH2C(CF3)2O), 47.4 (N(CH2)2N), 38.7 (CH2C(CF3)2), 22.9 (CH3), 20.2 (Si(CH3)3), −0.9 (CH2SiMe3). Anal. Calcd for C18H25F12InN2O2Si: C, 32.16; H, 3.75; N, 4.17; Found: C, 32.01; H, 3.60; N, 4.00. {ONCyNO}In(CH2SiMe3) (5b). Using a protocol similar to that described above for 5a, complex 5b was prepared from In(CH2SiMe3)3·1/2THF (0.209 g, 0.510 mmol) and {ONCyNO}H2 (0.267 g, 0.510 mmol) to give 5b as a white solid (0.035 g, 10%). Crystals suitable for X-ray diffraction studies were grown from a Et2O solution at −30 °C. 1H NMR (300 MHz, C6D6, 298 K): δ 3.61 (m, 1H, CH (cy)), 2.95 (d, 2JHH = 14.1, 1H, CHHC(CF3)2), 2.76 (d, 2JHH = 14.1, 1H, CHHC(CF3)2), 2.51 (d, 2JHH = 14.1, 1H, CHHC(CF3)2), 2.47 (m, 1H, CH (cy)), 2.23 (d, 2JHH = 14.1, 1H, CHHC(CF3)2), 1.46 (s, 3H, CH3), 1.25 (s, 3H, CH3), 1.12 (m, 3H, CH2 (cy)), 0.91 (m, 2H, CH2 (cy)), 0.71 (m, 2H, CH2 (cy)), 0.33 (s, 9H, Si(CH3)3), −0.13 (d, 2JHH = 12.6, 1H, InCHH), −0.26 (d, 2JHH = 12.6, 1H, InCHH). 19F{1H} NMR (188 MHz, C6D6, 298K): δ −75.93 (q, 4JFF = 10.5, 3F, CF3), −76.32 (q, 4JFF = 10.5, 3F, CF3), −78.10 (q, 4JFF = 10.5, 6F, CF3). 13C{1H} NMR (100 MHz, C6D6, 298 K): δ 175.2 (CN), 174.6 (CN), 80.4 (m, CH2C(CF3)2O), 67.7 (CH(cy)), 64.3 (CH(cy)), 42.6 (CH2C(CF3)2), 40.7 (CH2C(CF3)2), 31.2 (CH2(cy)), 30.9 (CH2(cy)), 25.4 (CH2(cy)), 24.3 (CH3), 23.8 (CH2(cy)), 21.0 (CH3), 1.6 (Si(CH3)3); resonances for C(CF3)2 groups were not observed due to their low intensity. Anal. Calcd for C22H31F12InN2O2Si: C, 36.38; H, 4.30; N, 3.86; Found: C, 36.30; H, 4.40; N, 3.80. Crystal Structure Determination of 1a, 2a, 3a, 4b, and 5b. Diffraction data were collected at 100(2) K using a Bruker APEX CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å). A combination of ω and ϕ scans was carried out to obtain at least a unique data set. The crystal structures were solved by direct methods; remaining atoms were located from difference Fourier synthesis followed by full-matrix least-squares refinement based on F2 (programs SIR97 and SHELXL-97)32 with the aid of the WINGX program.33 Many hydrogen atoms could be found from the Fourier difference analysis. Other hydrogen atoms were placed at calculated positions and forced to ride on the attached atom. The hydrogen atom contributions were calculated but not refined. All non-hydrogen atoms were refined with anisotropic displacement parameters. The locations of the largest peaks in the final difference Fourier map calculation as well as the magnitude of the residual electron densities were of no chemical significance. Crystal data and details of data collection and structure refinement for the different compounds are given in Table S1. The crystallographic data (excluding structure factors) are available as Supporting Information, as cif files. General Procedure for Polymerization of rac-Lactide. In a typical experiment (Table 2, entry 4), in the glovebox, a Schlenk flask was charged with 5a (5.0 mg, 7.4 μmol) and rac-lactide (1.070 g, 7.40 mmol, 1000 equiv vs 5a). A solution of isopropyl alcohol (3.70 mL of a 20.0 mmol L−1 solution in toluene, 74.0 μmol, 10 equiv vs 5a) was then added with a syringe to the flask containing the initiator and the monomer. The mixture was then immediately stirred with a magnetic stir bar at 80 °C for 17 h. Aliquots of the crude material were sampled regularly by pipet for determining the monomer conversion by 1H NMR spectroscopy. The reaction was finally quenched with H2O (ca. 1 mL of a 10% H2O solution in THF), and the polymer was reprecipitated three times with excess pentane (ca. 15 mL). The polymer was then filtered and dried under vacuum to constant weight.

(d, 2JHH = 12.4, 2H, CHHC(CF3)2), 1.94 (s, 6H, CH3), 1.65 (m, 4H, CH2(cy)), 1.39 (m, 2H, CH2(cy)), 1.19 (m, 2H, CH2(cy)). 19F{1H} NMR (188 MHz, THF-d8, 298 K): δ −82.50 (q, 4JFF = 10.2, 6F, CF3), −77.41 (q, 4JFF = 10.2, 6F, CF3). Anal. Calcd for C19H22F12K2N2O2: C, 37.01; H, 3.60; N, 4.54; Found: C, 37.20; H, 3.80; N, 4.40. {ONEtNO}InCl (2a). A solution of {ONEtNO}H2 (0.400 g, 0.850 mmol) in Et2O (10 mL) was added at room temperature to a stirred suspension of benzylpotassium (0.220 g, 1.70 mmol) in Et2O (5 mL). The reaction mixture was stirred at room temperature for 20 h. Volatiles were evaporated in vacuo,and solid InCl3 (0.187 g, 0.850 mmol) was introduced in. Et2O (ca. 10 mL) was vacuum-condensed, and the reaction mixture was stirred at room temperature overnight. Volatiles were removed in vacuo, and the resulting material was dissolved in CH2Cl2 (ca. 10 mL). The solution was filtered and concentrated to dryness, and the solid residue was dried in vacuo to give 2a as a colorless solid (0.317 g, 60%). 1H NMR (200 MHz, CD2Cl2, 298 K): δ 3.77 (s, 4H, NCH2CH2N), 3.05 (s, 4H, CH2C(CF3)2), 2.36 (s, 6H, CH3). 19F{1H} NMR (188 MHz, CD2Cl2, 298 K): δ −76.15 (br m, 6F, CF3), −79.95 (br m, 6F, CF3). 13C{1H} NMR (125 MHz, CD2Cl2, 298 K): δ 182.5 (CN), 48.2 (NCH2CH2N), 40.0 (CH2C(CF3)2), 24.0 (CH3); resonances for C(CF3)2 groups were not observed due to their low intensity. Anal. Calcd for C14H14ClF12InN2O2: C, 27.10; H, 2.27; N, 4.51; Found: C, 27.50; H, 2.50; N, 4.10. {ONEtNO}InMe (3a) and [{ONEtNO}Li]InMe2 (4a). A solution of MeLi (0.793 mL of a 1.6 M solution in Et2O, 1.270 mmol, 3 equiv vs In) was added at room temperature to a stirred suspension of InCl3 (0.094 g, 0.423 mmol) in Et2O (ca. 5 mL). The reaction mixture was stirred at room temperature for 30 min. Then, a solution of {ONEtNO}H2 (0.200 g, 0.423 mmol) in toluene (ca. 5 mL) was added to the previous solution. The resulting reaction mixture was stirred at room temperature for 20 h and the solution filtered off. Volatiles were removed in vacuo, and the crude solid residue was washed with hexane (ca. 10 mL) to give 4a as a white solid (0.078 g, 30%). 1H NMR (500 MHz, C6D6, 298 K): δ 2.67 (s, 4H, NCH2CH2N), 2.43 (s, 4H, CH2C(CF3)2), 1.27 (s, 6H, CH3), 0.28 (s, 6H, In(CH3)). 19F{1H} NMR (188 MHz, C6D6, 298 K): δ −76.51 (s, 12F, CF3). 13C{1H} (125 MHz, C6D6, 298 K): δ 170.2 (CN), 124.9 (q, 1JCF = 291, CF3), 79.3 (m, 2JCF = 26.9, CH2C(CF3)2O), 49.9 (N(CH2)2N), 41.9 (CH2C(CF3)2), 20.4 (CH3), −5.4 (In(CH3)2). Anal. Calcd for C16H20F12InLiN2O2: C, 30.89; H, 3.24; N, 4.50; Found: C, 31.10; H, 3.40; N, 4.10. Upon attempts to recrystallize 4a from Et2O/hexane mixtures at room temperature, small amounts (ca. 10%) of single crystals of {ONEtNO}InMe (3a) suitable for X-ray diffraction studies were isolated. [{ONCyNO}Li]InMe2 (4b). Using a protocol similar to that described above for 4a, complex 4b was prepared from InCl3 (0.042 g, 0.190 mmol), MeLi (0.356 mL of a 1.6 M solution in diethyl ether, 0.570 mmol, 3 equiv vs In), and {ONCyNO}H2 (0.100 g, 0.190 mmol) to give [{ONCyNO}Li]InMe2 (4b) as a white solid (0.510 g, 40%). Crystals suitable for X-ray diffraction studies were grown from a saturated Et2O solution layered with hexane at room temperature. 1H NMR (500 MHz, C6D6, 298 K): δ 2.98 (m, 2H, CH(cy)), 2.45 (s, 4H, CH2C(CF3)2), 1.47 (s, 6H, CH3), 1.41 (m, 2H, CH2(cy)), 1.01 (m, 2H, CH2(cy)), 0.91 (m, 2H, CH2(cy)), −0.29 (s, 6H, In(CH3)2). 19 1 F{ H} NMR (188 MHz, C6D6, 298 K): δ −74.40 (q, 4JFF = 10.5, 6F, CF3), −77.87 (q, 4JFF = 10.5, 6F, CF3). 13C{1H} NMR (125 MHz, C6D6, 298 K): δ 173.4 (CN), 124.8 (q, 1JCF = 289, CF3), 79.2 (m, 2 JCF = 27.5, CH2C(CF3)2O), 65.8 (CH(cy)), 43.3 (CH2(cy)), 30.5 (CH2(cy)), 24.8 (CH2(cy)), 21.8 (CH3), −5.2 (In(CH3)2). Anal. Calcd for C20H26F12InLiN2O2: C, 35.53; H, 3.88; N, 4.14; Found: C, 35.30; H, 3.40; N, 4.40. {ONEtNO}In(CH2SiMe3) (5a). A solution of {ONEtNO}H2 (0.159 g, 0.340 mmol) in Et2O (ca. 5 mL) was added at room temperature to a solution of In(CH2SiMe3)3·1/2THF (0.139 g, 0.340 mmol) in Et2O (ca. 5 mL). The reaction mixture was stirred at 60 °C for 15 h. Then, volatiles were evaporated in vacuo, THF (ca. 5 mL) was vacuumcondensed in, and the resulting solution was filtered. The filtrate was concentrated in vacuo and the resulting solid residue was dried to give 5a as a white powder (0.103 g, 0.153 mmol, 45%). 1H NMR (300

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ASSOCIATED CONTENT S Supporting Information * A table, figures, and CIF files giving X-ray crystallographic data for 1a, 2a, 3a, 4b, and 5b as and representative 1H, 19F{1H} and 1455


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C{1H} NMR spectra for certain complexes and some polymers. This material is available free of charge via the Internet at http://pubs.acs.org.

(c) Alaaeddine, A.; Thomas, C. M.; Roisnel, T.; Carpentier, J.-F. Organometallics 2009, 28, 1469. (d) Dagorne, S.; Bouyahyi, M.; Vergnaud, J.; Roisnel, T.; Carpentier, J.-F. Organometallics 2010, 29, 1865. (e) Bouyahyi, M.; Roisnel, T.; Carpentier, J.-F. Organometallics 2010, 29, 491. (11) (a) Lavanant, L.; Chou, T.-Y.; Chi, Y.; Yun, C.; Lehmann, C. W.; Toupet, L.; Carpentier, J.-F. Organometallics 2004, 23, 5450. (b) For a general perspective on complexes of oxophilic metals incorporating multidentate fluorinated alkoxide ligands, see: Carpentier, J.-F. Dalton Trans. 2010, 39, 37. (12) The parameters Pm and Pr are respectively the probabilities of forming new meso and racemic linkages, resulting in the case of lactide to isotactic- and heterotactic-enriched PLAs; note that Pm = 1 − Pr. See: Kasperczyk, J. E. Macromolecules 1995, 28, 3937. (13) Acid Catalysis in Modern Organic Synthesis; Hisashi, Y., Kazuaki, I., Eds.; Wiley-VCH: Weinheim, Germany, 2008. (14) The trend of relative Lewis acidities within group 13 trihalogenides was determined to be B > Ga > Al > In; see: (a) Lappert, M. F. J. Chem. Soc. 1962, 542. (b) Branch, C. S.; Bott, S. G.; Barron, A. R. J. Organomet. Chem. 2003, 666, 23. (15) Marquet, N.; Grunova, E.; Kirillov, E.; Bouyahyi, M.; Thomas, C. M.; Carpentier, J.-F. Tetrahedron 2008, 64, 75. (16) Chen, H.-Y.; Zhang, T.-L.; Zhang, J.-G.; Yang, Li.; Guo, J.-Y. Struct. Chem. 2006, 17, 445. (17) Höner, M.; Manzoni de Oliveira, G.; Rolina Wohlmuth Alves Dos Santos, A. J. Z. Anorg. Allg. Chem. 2007, 633, 971. (18) A few complexes that feature short nonvalent K···F interactions were reported in the literature. (a) For [KInCl2(OCH(CF3)2)2(THF)3]n(THF) and [K3In(OCH(CF3)2)6]n, see: Andrews, P. C.; Forsyth, C. M.; Junk, P. C.; Nuzhnaya, I.; Spiccia, L. J. Organomet. Chem. 2009, 694, 373. (b) For [K(C6F5O)2(H2O)2], see: Baillargeon, P.; Dory, Y. L.; Decken, A. J. Chem. Cryst. 2009, 39, 568. (19) Calculated following the equation τ = (β − α)/60. See: Addison, A. W.; Rao, T. N.; Reedjik, J.; Van Rijn, L.; Verschoor, G. C. J. Chem. Soc., Dalton Trans. 1984, 1349 The τ value ranges from 0 (perfectly square pyramidal) to 1 (perfectly trigonal bipyramidal). α and β are the angles that are opposite each other in the xy plane (with the In−X bond oriented along the z axis).. (20) Effective ionic radii for six-coordinate metal centers: Al3+, 0.535 Å; In3+, 0.80 Å: Shannon, R. D. Acta Crystallogr., Sect. A 1976, A32, 751. (21) Reactions between In[N(SiMe3)2]3 and both proligands {ONRNO}H2 (a,b) under various conditions systematically afforded mixtures of products which could not be separated or unambiguously identified. Also, the use of “[In(OiPr)3]” (either commercially purchased from Alfa Aesar or prepared independently from InCl3; see ref 29) led to products that were insoluble in usual solvents. (22) Runge, F.; Zimmermann, W.; Pfeiffer, H.; Pfeiffer, I. Z. Anorg. Allg. Chem. 1951, 267, 39. (23) The discrete and stable ate complex [Me4In]Li can be selectively obtained by the reaction of InCl3 with 4 equiv of MeLi in Et2O; see: (a) Hoffmann, K.; Weiss, E. J. Organomet. Chem. 1972, 37, 1. (b) Hallock, R. B.; Manzik, S. J.; Mitchell, T.; Hui, B. C. U.S. Patent 4 847 399, 1989. (24) (a) Kopasz, J. P.; Hallock, R. B.; Beachley, O. T. Inorg. Synth 1986, 24, 89−91. (b) Beachley, O. T.; MacRae, D. J.; Churchill, M. R.; Kovalevsky, A. Y.; Robirds, E. S. Organometallics 2003, 22, 3991. (25) Liu, X.; Shang, X.; Tang, T.; Hu, N.; Pei, F.; Cui, D.; Chen, X.; Jing, X. Organometallics 2007, 26, 2747. (26) The COCH2SiMe3 1H NMR resonance was not observed, overlapping with the methyl resonances of the PLA main chain.25 Attempts to obtain MALDI-ToF mass spectra of these PLAs prepared from In-CH2SiMe3 initiators, which would unambiguously support the presence of the COCH2SiMe3 terminal groups, were unsuccessful (no signal could be detected), in sharp contrast to PLAs produced in the presence of In/iPrOH systems, which readily allowed recording goodquality MS spectra evidencing COOiPr end groups. (27) (a) Chamberlain, B. M.; Cheng, M.; Moore, D. R.; Ovitt, T. M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001, 123, 3229.

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AUTHOR INFORMATION Corresponding Author *Fax: (+33)(0)223-236-939. E-mail: [email protected] (E.K.); [email protected] (J.F.C.).

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ACKNOWLEDGMENTS This work was supported by the Ministère de L’Enseignement Supérieur et de La Recherche (Ph.D. fellowship to M.N.). REFERENCES

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