Yttriumâ and AluminumâBis(phenolate)pyridine Complexes

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Yttrium− and Aluminum−Bis(phenolate)pyridine Complexes: Catalysts and Model Compounds of the Intermediates for the Stereoselective Ring-Opening Polymerization of Racemic Lactide and β‑Butyrolactone Joice S. Klitzke,†,‡ Thierry Roisnel,§ Evgeny Kirillov,*,† Osvaldo de L. Casagrande, Jr.,*,‡ and Jean-François Carpentier*,† †

Institut des Sciences Chimiques de Rennes, Organometallics: Materials and Catalysis Laboratories, UMR 6226 CNRS-Université de Rennes 1, F-35042 Rennes Cedex, France ‡ Instituto de Química, Laboratório de Catálise Molecular, Universidade Federal do Rio Grande do Sul, Avenida Bento Gonçalves, 9500, Porto Alegre, RS 90501-970, Brazil § Institut des Sciences Chimiques de Rennes, Centre de Diffraction X, UMR 6226 CNRS-Université de Rennes 1, F-35042 Rennes Cedex, France S Supporting Information *

ABSTRACT: Yttrium and aluminum complexes of an original pyridine-bis(phenolate) ligand have been prepared. Reactions of {ONOMe,Cumyl}Y[N(SiHMe2)2](THF)(Et2O) (1) with 1 equiv of methyl (R)-3-hydroxybutyrate and methyl (S,S)lactyllactate afforded respectively {ONOMe,Cumyl}Y((R)-OCH(CH3)CH2COOMe) (2) and {ONOMe,Cumyl}Y((S,S)-OCH(CH3)CO2CH(CH3)CO2Me) (3), which are rare models of the active species in the ring-opening polymerization (ROP) of β-butyrolactone (BBL) and lactide (LA), respectively. 13C NMR data for 2 and 3 indicate that, in solution, the carbonyl groups coordinate onto the yttrium centers, forming mononuclear species with six- and five-membered cycles, respectively. The aluminum compounds {ONOMe,Cumyl}Al(iPr (S)-lactate) (5), {ONOMe,Cumyl}Al((R)-OCH(CH3)CH2COOCH3) (6), and {ONOMe,Cumyl}Al((rac)-OCH(CF3)CH2CO2C2H5) (7) were prepared analogously from the parent methyl complex {ONOMe,Cumyl}AlMe (4). NMR data and the solid-state structure of 6 confirm the coordination of the carbonyl group. Yttrium compounds 1 and 2 are active initiators for the ROP of racemic LA and BBL. Their performances (activity, control of the molecular weights, tacticity) are much affected by the nature of the solvent or by the addition of just a few equivalents of pyridine. Optimal conditions are quite contrasted for the ROP of rac-LA and rac-BBL, highlighting fundamental differences between these two monomers. In the best cases, highly heterotactic PLAs (Pr up to 0.96) and syndiotactic-enriched PHB (Pr up to 0.86), with good control over the molecular weights, were obtained.

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INTRODUCTION The stereoselective ring-opening polymerization (ROP) of racemic cyclic esters such as lactide and β-butyrolactone (racLA and rac-BBL, respectively) has attracted considerable interest over the past decade, to access polyesters with controlled microstructures.1 Among the variety of catalysts that have been used for this purpose, of special note are undoubtedly aluminum and rare earth compounds.1 Indeed, some aluminum compounds, especially those based on salen,2 salan,3 or related {ONNO}2− ligand frameworks,4 do allow accessing highly isotactic polylactides; these are of high interest due to their possibility to form stereocomplexes with enhanced thermal properties. On the other hand, some rare earth compounds, with yttrium ones at the forefront,5 are highly active and stereoselective initiators for the ROP of LA6 and BBL7 (a quite reluctant monomer), enabling high levels of racemic enchainments of these monomers (hence forming © XXXX American Chemical Society

heterotactic polylactide (PLA) and syndiotactic polyhydroxybutyrate (PHB)). Stereocontrol in those ROP reactions is accounted for by enantiomorphic-site control (with chiral catalysts) or chain-end control (usually with nonchiral catalysts) mechanisms.1 However, exact parameters that govern these stereocontrol mechanisms, especially for chain-end control, and even the nature of the catalytically active species involved in those reactions, often remain unclear. In particular, little is understood in the syndiotactic control during the ROP of rac-BBL with yttrium complexes. Some of us have suggested that the presence of remote aryl substituents on phenolate ligands of some catalyst systems may play a significant role in this stereocontrol, via weak attractive C−H···π interactions Received: October 29, 2013

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dx.doi.org/10.1021/om401047r | Organometallics XXXX, XXX, XXX−XXX

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Chart 1. Examples of Group 3/13 Trivalent Metal−Lactate and −Lactyllactate Model Compounds

Scheme 1. Preparation of Proligand {ONOMe,Cumyl}H2

involving these aryl groups and methylenic moieties in the growing polymer chain.7e However, such metal-β-alkoxy-ester intermediates in the ROP of β-lactones remain thus far virtually nonexistent.8 This contrasts with the ROP of LA, for which several lactate and lactyllactate compounds of rare earths,9a,b aluminum,9c−g gallium,9h,i and other metals (Zn, Sn, ...)10 have been reported (Chart 1). Also, the subtle solvent effects in these ROP reactions, with most often opposite trends between LA and BBL, remain puzzling, nonrationalized observations. In this work, we have prepared a series of well-defined Al and Y complexes supported by a tridentate pyridine-bis(phenolate) ligand having cumyl substituents. The latter cumyl substituents have been selected since they proved earlier to have a determining influence for achieving high stereoselectivities.6l,7a,c,e,11 Our aims were to assess possible interactions between these substituents and α- or β-alkoxy ester moieties coordinated onto the metal center. The latter α- or β-alkoxy ester units have been selected to mimic the last monomer unit inserted in the growing polymer chain in the ROP of lactide and β-butyrolactone.

(see Figure S32 and Table S1 in the Supporting Information). In the solid state, {ONOMe,Cumyl}H2 adopts a near (noncrystallographic) C2 symmetry, in which the phenol groups are twisted from the pyridine plane by ca. 23−24° in opposite directions. Synthesis of Yttrium Complexes Supported by the {ONOMe,Cumyl}2− Ligand. The amine elimination reaction between the tris(amido) precursor [Y(N(SiHMe2)2)3(THF)] and 1 equiv of the pro-ligand {ONOMe,Cumyl}H2 in diethyl ether at room temperature afforded the amido complex {ONOMe,Cumyl}Y[N(SiHMe2)2](THF)(Et2O) (1), with concomitant release of 2 equiv of bis(dimethylsilyl)amine (Scheme 2). Single crystals of 1 suitable for an X-ray diffraction analysis were obtained by recrystallization from a diethyl ether solution at −30 °C. Crystallographic data are summarized in Table S1, and important bond distances and angles are given in Figure 1. In the solid state, 1 features a monomeric structure with the yttrium center in a distorted octahedral geometry, sixcoordinated by the {ONOMe,Cumyl}2− tridentate ligand, the bis(dimethylsilyl)amido group, and THF and diethyl ether

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RESULTS AND DISCUSSION The diproteo proligand {ONOMe,Cumyl}H2 was efficiently prepared following a route similar to that developed previously for 2,6-bis(naphthol)pyridine proligands (Scheme 1).12 The two-step protocol starts from the methoxymethyl-protected 2cumyl-4-methylphenol and involves (i) in situ generation of the corresponding organozinc reagent and (ii) Negishi crosscoupling of the latter intermediate with 2,6-dibromopyridine using a Pd-SPhos catalyst. {ONOMe,Cumyl}H2 was recovered in overall 64% yield as a colorless, crystalline compound readily soluble in CHCl3, CH2Cl2, and THF, and in toluene (upon heating at 80 °C). This compound was characterized by 1H and 13 C NMR spectroscopy and an X-ray crystallographic study B


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Scheme 2. Synthesis of Yttrium Complexes Supported by the {ONOMe,Cumyl}2− Ligand

two oxygen donors of the 2,6-bis(phenolate)pyridine ligand are in trans position. Because of the coordination onto the yttrium center, the phenolate groups are orientated in the same directions relative to the pyridine plane with a more pronounced twisting (ca. 46° and 52°) than in the free ligand. The Y−O(phenolate) (2.143(2) and 2.144(2) Å) and the Y− N(silylamido) (2.268(3) Å) bond distances fall into the range of distances observed in related amido-yttrium complexes supported by tridentate bis(naphtholate)pyridine and tetradentate amino-alkoxy-bis(phenolate) ligands.6a−f,j,7e The Si(1)−N(3)−Si(2) angle of 122.63(16)°, which is just slightly larger than for an ideal sp2 hybridization, and long, equivalent Y···Si (3.428(1) and 3.416(1) Å) distances exclude a β(Si− H)···Y agostic interaction in complex 1.13 Compound 1 is readily soluble in common organic solvents (THF, toluene, benzene). The 1H and 13C NMR spectra of 1 in toluene-d8 or C6D6 at room temperature all contain one set of resonances indicative of the presence of one C2-symmetric species, with one THF and one diethyl ether coordinated molecule. The resonances for the latter amido, Et2O, and THF moieties are broadened at room temperature. Upon heating, those resonances sharpen, but concomitant dissociation of the labile Et2O molecule then occurs (Figure S7). Compound 1 slowly transforms in toluene and benzene at room temperature into another, yet unidentified bis(dimethylsilyl)amido species (ca. 13% after 15 min at 25 °C) and even more so at higher temperature (27% conversion after 4 h at 90 °C) (Figure S7). In pyridine-d5, the THF and diethyl ether ligands are, as expected, displaced from the yttrium coordination sphere, and the resulting species remains intact, at least for several days, at room temperature (see the Experimental Section). The Si−H resonance in 1 (δ 4.80 ppm at 25 °C in C6D6) is only slightly shifted upfield as compared to the corresponding resonance in Y{N(SiHMe2)2}3(THF) (δ 4.99 ppm), which argues against a strong β(Si−H) agostic interaction with the yttrium center in hydrocarbon solutions;14 this is in line with the observations made in the solid state.

Figure 1. ORTEP drawing of {ONOMe,Cumyl}Y[N(SiHMe2)2](THF)(Et2O) (1) (thermal ellipsoids drawn at the 50% probability level; all hydrogen atoms are omitted for clarity). Selected bond distances (Å) and angles (deg): Y(1)−O(11), 2.1427(19); Y(1)−O(51), 2.144(2); Y(1)−O(61), 2.366(2); Y(1)−O(71), 2.358(2); Y(1)−N(2), 2.615(2); Y(1)−N(3), 2.268(3); Y(1)−Si(1), 3.4276(10); Y(1)− Si(2), 3.4158(11); N(2)−Y(1)−N(3), 176.57(9); O(11)−Y(1)− O(51), 153.77(7); O(71)−Y(1)−O(61), 162.75(9); O(11)−Y(1)− N(2), 76.32(7); O(11)−Y(1)−N(3), 106.16(8); O(11)−Y(1)− O(61), 90.60(8); O(11)−Y(1)−O(71), 85.44(8); O(51)−Y(1)− N(2), 77.76(7); O(51)−Y(1)−N(3), 99.91(9); O(51)−Y(1)− O(61), 88.43(8); O(51)−Y(1)−O(71), 87.77(8); O(71)−Y(1)− N(2), 83.58(8); O(71)−Y(1)−N(3), 98.90(9); O(61)−Y(1)−N(2), 79.17(7); O(61)−Y(1)−N(3), 98.33(9); Si(2)−N(3)−Y(1), 118.58(14); Si(1)−N(3)−Y(1), 118.78(14), Si(2)−N(3)−Si(1), 122.63(16).

molecules. The latter THF and diethyl ether ligands are located trans to each other and cis to the silylamido ligand, while the C


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Scheme 3

NMR spectroscopy in THF-d8 at ambient temperature using a freshly prepared sample (see the Experimental Section). The 13C{1H} NMR spectra of 3 showed that the two signals for the carbonyl groups appear at δ 190.0 (internal, OCH(CH3)C(O)OCH(CH3)CO2Me) and 169.0 (terminal, OCH(CH3)C(O)OCH(CH3)CO2Me) ppm. In comparison, in the 13C{1H} NMR spectrum of the initial methyl (S,S)lactyllactate reagent, the carbonyl carbons appear at δ 175.1 (internal) and 170.0 (terminal) ppm. Thus, the carbonyl carbon of complex 3 (δ 190.0 ppm) significantly shifted downfield from the free reagent (δ 175.1 ppm), suggesting that this internal carbonyl group is coordinated to the Y center, forming a five-membered ring, as observed for a few Al compounds bearing such a lactyllactate moiety (Chart 1; Scheme 2).2h,9c,e,f Synthesis of Aluminum Complexes Supported by the {ONOMe,Cumyl}2− Ligand. To gain a better understanding of the structure and reactivity of such putative intermediates involved in the polymerization of lactide and β-butyrolactone, we also prepared aluminum complexes supported by the tridentate ligand {ONOMe,Cumyl}2−. Although aluminum compounds typically show much lower reactivity than rare earth compounds, they do indeed exhibit structural similarities.2−7 The methyl-aluminum complex 4 was synthesized selectively by reaction of the pro-ligand {ONOMe,Cumyl}H2 with AlMe3 at 80 °C (Scheme 3). This product, isolated as colorless crystals in 77% yield, is soluble in CH2Cl2 at room temperature and in aromatic hydrocarbons (benzene, toluene) at 60 °C, and its identity was established on the basis of 1H and 13C NMR spectroscopy (see the Experimental Section). The corresponding O-lactate (5), β-alkoxy-butyrate (6), and 1,1,1-trifluoro βalkoxy-butyrate analogue (7) derivatives were prepared by methane elimination from the reaction of 4 with 1 equiv of the corresponding α- or β-hydroxy-ester at 80 °C in toluene (Scheme 3). Complexes 5−7 were recovered as air-sensitive, colorless solids in >60% yields. Complete purification of 6 proved difficult, but small amounts of analytically pure crystals were successfully isolated from a concentrated CD2Cl2 solution layered by hexane at room temperature. Complexes 5−7 were characterized by solution NMR spectroscopy, combustion analysis, and an X-ray diffraction study for 6. The 1H and 13C{1H} NMR data for the aluminum(III) O(S)-lactate (5) and (R)- and (rac)-β-alkoxy-butyrate (6, 7) complexes in toluene-d8 and CD2Cl2, at room temperature, are indicative of the existence of a single C1-symmetric species (see the Experimental Section). Of note, the 13C NMR resonance for the carbonyl group in the O-(S)-lactate and fluorinated (R)β-alkoxy-butyrate moiety of 5 and 7 was observed at δ 189.5 and 178.9 ppm, respectively; these values are, as for yttrium compounds 2 and 3, significantly shifted downfield from the

Model compounds of putative intermediates involved in the polymerization of lactide and β-butyrolactone were then sought (Scheme 2). Attempts to generate an yttrium O-lactate complex from the reaction of {ONOMe,Cumyl}Y[N(SiHMe2)2](THF)(Et2O) and 1 equiv of isopropyl (S)-lactate, under various conditions, were unsuccessful; only intractable mixtures of compounds were obtained. Gratifyingly, the β-alkoxy-butyrate yttrium derivative 2 was prepared by treatment of 1 with 1 equiv of methyl (R)-3hydroxybutyrate in toluene or benzene at room temperature and isolated in almost quantitative yield (Scheme 2). To our knowledge, compound 2 is the first example of a well-defined yttrium alkoxy-butyrate complex. It is soluble and quite stable in aromatic hydrocarbons at room temperature. Despite repeated efforts, no suitable crystal for X-ray diffraction analysis could be obtained, and this compound was therefore characterized by combustion analysis and solution NMR. The resonances in the 1H and 13C{1H} NMR spectra of 2 in toluene-d8 at room temperature are noticeably broadened, reflecting the existence of a dynamic behavior in these conditions. Decrease of the temperature to −15 °C allowed freezing this dynamic process, resulting in a set of sharp resonances, consistent with a single C1-symmetric species. The asymmetric center in the β-alkoxy-butyrate moiety makes the chelated {ONOMe,Cumyl} unit asymmetric. Thus, the 1H NMR spectrum of 2 in toluene-d8 at −15 °C displays six singlets (δ 2.31, 2.27, 2.02, 1.99, 1.66, and 1.58 ppm) for the six nonequivalent methyl groups in the {ONOMe,Cumyl} ligand unit. The 13C NMR resonance for the carbonyl group in the methyl (R)-β-alkoxy-butyrate moiety was observed at δ 180.1 ppm, significantly shifted from the corresponding resonance in methyl 3-hydroxybutyrate (δ 172.5 ppm). This is consistent with coordination of the carbonyl group onto the yttrium center and existence of a five-coordinate species (as established for the parent Al compound; vide inf ra). In addition, the DOSY NMR experiments, carried out in toluene-d8,15 allowed determining the hydrodynamic radius of 2 (5.89 Å), which suggests the latter complex to retain a mononuclear structure. Also, an yttrium-O[methyl (S,S)-lactyllactate] compound (3), which mimics the product of the first insertion of the LA molecule, was generated on the NMR scale by treatment of 1 with 1 equiv of methyl (S,S)-lactyllactate in THF at room temperature (Scheme 2). However, upon concentration of the solution, compound 3 decomposes into a mixture of unidentified species. This compound also slowly decomposes in THF-d8 solution over a few hours at room temperature, to yield complicated mixtures. We suspect that these decomposition products result from transesterification reactions within the lactyllactate moiety, but this is not clear yet. Due to the instability of 3, it was characterized by multinuclear D


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Figure 2. ORTEP drawing of the two independent molecules observed in the unit cell of {ONOMe,Cumyl}Al((R)-OCH(Me)CH2CO2Me) (6) (thermal ellipsoids drawn at the 50% probability level; all solvent molecules and hydrogen atoms are omitted for clarity). Selected bond distances (Å) and angles (deg); on the top, with C−H···π interactions: Al−O(74), 1.745(2); Al−O(51), 1.757(2); Al−O(11), 1.762(2); Al−O(71), 2.006(2); Al−N, 2.017(2); O(74)−Al−O(51), 118.16(12); O(74)−Al−O(11), 118.18(11); O(51)−Al−O(11), 122.96(11); O(71)−Al−N, 171.00(11); O(51)−Al−O(71), 84.58(10); O(74)−Al−O(71), 93.40(10); O(11)−Al−O(71), 84.05(10); O(74)−Al−N, 95.44(10); O(51)−Al−N, 92.72(10); O(11)−Al−N, 90.26(10). On the bottom, without C−H···π interactions: Al(2)−O(9), 1.743(2); Al(2)−O(8), 1.757(2); Al(2)−O(7), 1.771(2); Al(2)−N(2), 1.996(3); Al(2)−O(10), 2.057(2); O(9)−Al(2)−O(8), 120.76(12); O(9)−Al(2)−O(7), 119.58(11); O(8)−Al(2)−O(7), 119.06(11); O(9)−Al(2)−N(2), 92.25(10); O(8)−Al(2)−N(2), 92.62(11); O(7)−Al(2)−N(2), 92.83(11); O(9)−Al(2)−O(10), 91.27(10); O(8)−Al(2)−O(10), 85.07(10); O(7)−Al(2)−O(10), 85.95(10); N(2)−Al(2)−O(10), 176.44(10).

corresponding resonance in the free reagent (isopropyl lactate, δ 175.0 ppm; methyl 3-hydroxybutyrate, δ 172.5 ppm). This is again indicative of coordination of the carbonyl group onto the aluminum center and is consistent with the existence of mononuclear species in solution. The solid-state structure of 6 features a monomeric molecule with an aluminum center in a slightly distorted trigonal bipyramidal geometry (τ = 0.85−0.97),16 five-coordinated by the {ONOMe,Cumyl}2− ligand and the O,O-chelating (R)-βalkoxy-butyrate moiety (Figure 2 and Table S1). The axial positions are occupied by the pyridine nitrogen atom and the

carbonyl oxygen atom with N−Al−O angles of 171.0(1)° and 176.4(1)°. The phenolate and alkoxide oxygen atoms occupy the equatorial positions with O−Al−O angles ranging from 118.2(1)° to 123.0(1)°. The Al−O(carbonyl) bond distances (Al−O(71), 2.006(2) Å; Al(2)−O(10), 2.057(2) Å) in 6 are shorter than that (Al−O, 2.147(4) Å) observed in Lewinski’s five-coordinated neutral complex9d and similar to that (Al−O, 2.018 Å) observed in Dagorne’s cationic Al complex,9f which both contain a chelating lactate moiety; this trend likely reflects lower steric constraints within the six-membered metallacycle of 6, as compared to the five-membered metallacycle in the E


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the OMe group of the β-alkoxy-butyrate moiety to phenyl hydrogens of the cumyl substituents on the NMR time scale. Ring-Opening Polymerization of rac-Lactide and racβ-Butyrolactone with Yttrium Pyridine-Bis(phenolate) Complexes. The performance of the new yttrium pyridinebis(phenolate) complexes 1 and 2 was explored in the ringopening polymerization of racemic lactide and β-butyrolactone (Scheme 5). These two compounds have potentially active

latter two lactate compounds. These are expectedly much shorter than those observed in Yb3+-β-alkoxy-butyrate complexes (Yb−O, 2.382−2.482 Å),8 due to the much larger ionic radius of the latter metal center (six-coordinate Yb3+ = 0.86 vs five-coordinate Al3+ = 0.48 Å).17 In fact, two independent molecules are observed in the crystal unit of 6, and, despite that they share the same first coordination sphere described above, they are quite different: in one, the phenyl groups of the two cumyl substituents point in the direction of the (R)-β-alkoxybutyrate moiety, while in the second molecule those phenyl groups point in the opposite direction with respect to the (R)β-alkoxy-butyrate group (Figure 2). Thus, in the first molecule, short contacts (∼2.6−2.8 Å) between the phenyl π-system and one methylene C−H of the butyrate moiety are observed. This is reminiscent of similar attractive C−H···π interactions18 in yttrium complexes supported by tetradentate amino-alkoxybis(phenolate) with aryl-containing ortho-phenolate substituents and their possible beneficial involvement (as compared to systems with no aryl ortho substituents) in the syndioselective ROP of racemic β-butyrolactone.7e Yet, the observation of another molecule in the unit cell of 6 with no such C−H···π interactions indicates that their strength, if any, is quite low, at least in these aluminum complexes.19 In a previous study,7e we have demonstrated by means of DFT computations that, in yttrium amino-alkoxy-bis(phenolate) complexes, C−H···π interactions between methylenic hydrogens of the alkoxy-butyrate moieties and certain carbon atoms (typically, ortho and meta) of phenyl rings of the cumyl substituents can be remarkably stabilizing (by 5−12 kcal· mol−1). Herein, we have used the same computational approach to assess the stabilization ability of cumyl groups in complexes 2 and 6. Thus, the molecular structures of yttrium and aluminum complexes adopting both geometries observed for 6 in the solid state (Figure 2, Scheme 4) were optimized

Scheme 5

nucleophilic groups (i.e., amido and β-alkoxy-butyrate, respectively), and β-alkoxy-butyrate complex 2 is assumed to be an early intermediate in the ROP of β-butyrolactone or, more exactly, to mimic the active species with a growing PHB chain in such a process. Representative results are summarized in Table 1. Both 1 and 2 are active in the ROP of rac-LA at ambient temperature. The activity and degree of control are quite dependent on the nature of the solvent. Complex 1 is significantly more active in THF solution than in toluene or in pyridine; polymerizations of 1000 equiv of rac-LA in THF reached 70% conversion within only 15 min (entry 7). Yet, complex 2 proved to be even more active than 1 in toluene solution (compare entry 1 vs entry 9) but less active than 1 in THF solution (compare entry 5 vs entry 10). In toluene, we assume that this difference in apparent catalytic activities reflects the greater nucleophilicity and eventual higher initiating efficiency of the β-alkoxy-butyrate group as compared to the amido; narrower polydispersities in the former case (Mw/Mn = 1.3−1.5 for 2 vs 1.7−2.0 for 1) are consistent with this hypothesis. Actually, a much narrower polydispersity (Mw/Mn = 1.14) was observed when 2-propanol was added to 1 to act as a co-initiator (entry 8).20 The number-average molecular weight values of PLAs produced with 1, as determined by GPC (Mn,GPC), were systematically higher than the calculated Mn,calc values, indicating an incomplete initiation efficiency. Nonetheless, the Mn,GPC values varied quite linearly with the monomer conversions for different monomer loadings (compare entries 5−7). The molecular weight distributions, although unimodal, were all rather broad. These observations are probably connected to the above-mentioned instability of 1 in toluene and in THF solutions. Indeed, when reactions were performed in pyridine, a solvent in which 1 is stable, a narrower polydispersity was observed (entry 4). On the other hand, in the polymerizations of rac-LA mediated by complex 2, the experimental Mn,GPC values of the PLAs were in quite good agreement with the calculated ones and those determined by NMR (Mn,NMR), taking into account the β-alkoxy-butyrate and 2-hydroxy-ethyl polymer end-groups (Figures 3 and S26; see also Figure S27 for MS analysis); also, the polydispersities were narrower (Mw/Mn = 1.3−1.5).

Scheme 4. Model Geometries Computed to Assess Possible C−H···π Interactions in 2 and 6

(see the Experimental Section for details). Unexpectedly, the only close contacts observed in the computed structures involved hydrogens of the OMe group of the alkoxy-butyrate moiety and the meta-carbon atoms of phenyl rings of the cumyl substituents (2.74−2.77 Å) (see optimized geometries 2-I and 6-I). However, only insignificant differences were found between the total electron energy values calculated for geometries I and II of complexes 2 and 6 (0.3−0.7 kcal· mol−1), which argues against strong stabilizing interactions in these molecules. This is in line with the observation of two conformations in the solid-state structure of 6. Interestingly though, the NOESY data obtained for complex 2 in toluene-d8 at 253 K (Figure S10; see the Experimental Section) are consistent with spatial proximity of hydrogens of F


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Table 1. ROP of rac-Lactide and rac-β-Butyrolactone Initiated by Complexes 1 and 2a entry

complex

monomer

[M]0/[Y]0

solvent

time (min)

convb (%)

Mn,calcc (kDa)

Mn,GPCd (kDa)

Mn,NMRe (kDa)

Mw/Mnd

Prf

1 2 3g 4 5 6 7 8h 9 10 11 12 13 14 15 16 17g 18h 19h 20 21i 22 23i 24i

1 1 1 1 1 1 1 1 2 2 2 1 1 1 1 1 1 1 1 2 2 2 2 2

rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA rac-BBL rac-BBL rac-BBL rac-BBL rac-BBL rac-BBL rac-BBL rac-BBL rac-BBL rac-BBL rac-BBL rac-BBL rac-BBL

100 500 100 100 100 500 1000 100 100 100 500 100 250 500 100 100 200 100 500 100 100 250 250 500

toluene toluene toluene pyridine THF THF THF THF toluene THF THF toluene toluene toluene pyridine pyridine toluene toluene toluene toluene toluene toluene toluene toluene

10 50 13 30 6 10 15 13 10 90 330 45 90 230 10 130 1 45 230 45 1080 90 1080 1080

41 67 55 90 94 75 70 64 87 94 65 43 39 25 82 100 100 89 62 35 81 28 93 81

5.9 48.2 7.9 13.0 13.5 54.0 100.8 9.2 12.5 13.5 46.8 3.7 8.4 10.7 7.0 8.6 17.2 7.6 26.7 3.0 7.0 6.0 20.0 34.8

10.9 60.6 9.0 18.3 30.6 79.0 165.9 4.2 9.4 8.8 25.1 20.1 44.3 41.7 23.4 27.3 28.1 nd 27.6 6.4 16.1 11.0 35.0 66.0

nd nd nd nd nd nd nd 6.4 13.9 15.8 nd nd nd nd nd nd nd 8.1 nd 3.2 11.5 7.5 26.5 40.2

1.91 1.88 1.94 1.47 1.98 2.02 1.75 1.14 1.49 1.38 1.29 1.15 1.43 2.10 1.11 1.28 2.40 nd 1.07 1.05 1.13 1.05 1.10 1.15

0.60 0.60 0.79 0.77 0.96 0.96 0.94 0.87 0.55 0.84 0.88 0.81 nd nd nd 0.59 0.86 nd 0.81 0.80 0.80 nd nd nd

General conditions: reactions performed at [rac-LA]0 = 2.0 mol·L−1 or [rac-BBL]0 = 3.0 mol·L−1, at room temperature; the reaction time was not necessarily optimized. bConversion of monomer as determined by 1H NMR on the crude reaction mixture. cCalculated Mn values considering one polymer chain per Y center. dExperimental Mn and Mw/Mn values determined by GPC in THF vs PS standards; Mn values are corrected with a 0.58 factor for PLAs and are uncorrected for PHBs. eExperimental Mn values determined by 1H NMR analysis of the reprecipitated polymer. fPr is the probability of racemic linkage, as determined by 1H NMR homodecoupled experiments. gThe polymerization was carried out in the presence of 5 equiv of pyridine vs Y. hThe polymerization was carried out in the presence of 1 equiv of iPrOH vs Y. iThe reaction mixture turned rapidly highly viscous, hampering effective stirring. a

Figure 3. 1H NMR spectrum (500 MHz, CDCl3, 298 K) of a PLA produced from {ONOMe,Cumyl}Y((R)-OCH(CH3)CH2COOMe) (2) (Table 1, entry 10).

Homodecoupled 1H NMR spectra showed that some of the PLAs formed with these systems had highly heterotactic microstructures (Table 1 and Figure S29). As we observed with tetradentate amino-alkoxy-bis(phenolato)lanthanide6b and lanthanide-amido complexes supported by tridentate bis(orthosilyl-substituted naphtholate)pyridine/thiophene ligands,6j the stereoselectivity strongly depended on the nature of the solvent: all the PLAs produced in toluene with 1 or 2 had

poorly heterotactic microstructures (probability of racemic linkage, Pr = 0.55−0.60), whereas those obtained in THF with complex 1 had highly heterotactic microstructures (Pr = 0.94− 0.96; Figure 4); these are improved stereoselectivities over those achieved under identical conditions with {ONOSiR3}La[N(SiHMe2)2](THF) compounds (Pr = 0.84−0.90).6j Surprisingly yet, the PLAs produced in THF with alkoxy-butyrate complex 2 were less heterotactic (Pr = 0.86−0.88). The G


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Figure 4. Methine region of the 1H NMR and 1H homodecoupled NMR spectra of a PLA obtained in THF with complex 1 (Table 1, entry 6).

Figure 5. 1H NMR spectrum (500 MHz, CDCl3, 298 K) of a PHB produced from {ONOMe,Cumyl}Y((R)-OCH(CH3)CH2COOMe) (2) (Table 1, entry 21).

higher than that in neat pyridine (entry 15). This suggests that a few equivalents of pyridine are needed to enhance the intrinsic reactivity of the propagating species but, most likely, that excess pyridine is detrimental because its coordination on the metal center then overwhelms that of rac-BBL. Spassky and co-workers have previously noted that addition of 1 equiv of pyridine to an aluminum-alkoxide initiator used in the ROP of rac-LA, besides limiting transesterification reactions,23 also greatly increases the rate of polymerization.24 One can speculate here that coordination of pyridine to the yttrium center may enhance, e.g., via its trans influence, the nucleophilicity of the propagating alkoxide. Compounds 1 and 2 grossly featured the same activity in a given solvent (compare, e.g., entries 12 vs 20 and 13 vs 22). The degree of control over the molar masses proved, however, better with 2. With this compound as initiator, the Mn,NMR values determined by 1H NMR from the PHB β-alkoxybutyrate end-groups (Figures 5 and S26) were in close agreement with the calculated ones. The Mn,GPC values (uncorrected) for PHBs produced with 1 were, for a given

presence of pyridine, either as a few equivalents added to 1 in THF solution (entry 3) or as solvent (entry 4), afforded PLAs with intermediate heterotacticities (Pr = 0.77−0.79).21 Similarly, the ROP of rac-BBL promoted by 1 and 2 allowed the formation of poorly to highly syndiotactic PHBs, with Pr values of ca. 0.60 for PHBs produced in pyridine and ca. 0.80 for those produced in toluene (Figure S30, entries 12−24). Such superiority of toluene as solvent for stereocontrol in racBBL polymerizations was previously noted.6j,7a−c,e Unexpectedly enough, when 5 equiv of pyridine was added to 1 in toluene, an increased Pr value of ca. 0.86 was observed (entry 17), while a value intermediate between those in neat pyridine and in neat toluene may have been anticipated. Those observations highlight how elusive these solvent effects are and how difficult it is to rationalize them at this moment.19 As reflected in entries 12 and 15, polymerization of rac-BBL with 1 proceeded faster in pyridine than in toluene. Yet, in line with the above comments, rac-BBL was best polymerized with 1 in the presence of 1−5 equiv of pyridine (entry 17);22 the apparent activity of this system is at least 1 order of magnitude H


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Figure 6. MALDI-ToF mass spectrum of a PHB sample produced from 2 using IAA as matrix (Table 1, entry 20). The two observed distributions correspond to a single distribution of macromolecules, i.e., MeOC(O)CH2CH(CH3)O−(BBL)n−H, ionized by Na+ and K+, respectively. Calculated m/z for n = 44 − H + Na+ = 3928.05 Da; K+ = 3944.1 Da.

number of rac-BBL equivalents converted, about 2−3 times larger than those of PHBs produced with 2. This indicates imperfect initiation efficiency with amido complex 1, in line with that noted in the ROP of rac-LA (vide supra); in contrast, excellent initiation efficiency was observed with β-alkoxybutyrate complex 2, as also attested by MALDI-TOF mass spectrometry (Figures 6 and S30). In most cases, the molecular weight distributions of the PHBs were unimodal and much narrower (Mw/Mn = 1.05−1.15) than those observed for PLAs (Table 9). The main exception to this trend appeared with the catalyst system made of 1 and 5 equiv of pyridine (Mw/Mn = 2.4, entry 17). The aluminum-O-lactate complex (S)-5 also acts as a singlesite initiator in the ROP of rac-lactide. As well-established for aluminum complexes, its reactivity proved much lower than that of parent group 3 metal complexes, and significant polymerization rates were observed only from 80 °C. Despite a fairly good control over the molecular weights (e.g., in a typical experiment, [rac-LA]0/[(S)-5] = 100, T = 80 °C, t = 17 h; conv = 20%; Mn,calc = 2,880 g·mol−1; Mn,GPC(corr) = 2740 g·mol−1; Mw/Mn = 1.15), only atactic polymers (Pr = 0.5) were recovered.

equivalents of pyridine, but surprisingly enough, this effect was not noted in the ROP of rac-LA. Overall, these results confirm the valuable contribution of ortho-cumyl substituents in tri/tetradentate bis(phenolate) ligands, when associated to group 3 metal centers, especially yttrium, for achieving the hetero/syndiotacticity in ROP of racLA and rac-BBL. The β-alkoxy-butyrate yttrium 2 and aluminum 6 and 7 complexes constitute very rare examples of models of putative active species involved in the ROP of BBL. The crystal structure of 2 could not be established to gauge the pertinence of C−H···π interactions that have been proposed to account for a part of the high syndiotacticity in the ROP of rac-BBL.7a,c,e Interestingly, such interactions appear at best quite weak in the β-alkoxy-butyrate aluminum complex 6, as revealed by an X-ray diffraction study and confirmed by DFT computations.

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EXPERIMENTAL SECTION

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) and Na/K alloy (toluene, pentane, hexanes) under argon, degassed thoroughly, and stored under argon prior to use. Deuterated solvents (>99.5% D, Eurisotop) were vacuum-transferred from Na/K alloy (benzene-d6, toluene-d8) or from CaH2 (CD2Cl2) into storage tubes. Pyridine-d5 was distilled under argon and kept over activated 4 Å molecular sieves. Isopropyl alcohol (Acros) was distilled over Mg turnings under argon atmosphere and kept over activated 4 Å molecular sieves. Y[N(SiHMe2)2]3(THF)13 was prepared using reported procedures. rac-Lactide (rac-LA) was received from Acros; its purification required a three-step procedure involving first a recrystallization from a hot, concentrated iPrOH solution (80 °C), followed by two subsequent recrystallizations in hot toluene (100 °C). After purification, rac-LA was stored at a temperature of −30 °C in the glovebox. Racemic β-butyrolactone (rac-BBL, Aldrich) was freshly distilled from CaH2 under nitrogen and degassed thoroughly by freeze−vacuum−thaw cycles prior to use. Other starting materials and reagents were purchased from Acros, Strem, Aldrich, and Alpha Aesar and used as received.

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CONCLUSIONS The amido and β-alkoxy-butyrate yttrium complexes 1 and 2 supported by the new tridentate bis(ortho-cumyl-substituted phenolate)pyridine ligand feature high performance in the stereoselective ROP of rac-LA and rac-BBL. The heterotacticity values observed in the ROP of rac-LA (Pr = 0.94−0.96) are noticeably higher than those achieved under identical conditions with {ONOSiR3}Y[N(SiHMe2)2](THF) compounds (Pr = 0.84−0.90; SiR3 = SiPh3 or SitBuMe2).6j The level of syndiotacticity in the ROP of rac-BBL is similar for these two classes of compounds (Pr = 0.80−0.86; o-cumyl or o-SiPh3). The activity and stereoselectivity of 1 in toluene toward racBBL were significantly enhanced upon addition of a few I


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NMR spectra of complexes were recorded on Bruker AC-300, Avance DRX 400, and AM-500 spectrometers in Teflon-valved NMR tubes at 25 °C unless otherwise indicated. 1H and 13C 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 complexes was made from 2D 1H−13C HMQC and HMBC NMR experiments. Elemental analyses (C, H, N) were performed using a Flash EA1112 CHNS Thermo Electron apparatus and are the average of two independent determinations. Molecular weights of PLAs were determined by size exclusion chromatography in THF at 30 °C (flow rate = 1.0 mL·min−1) on a Polymer Laboratories PL50 apparatus equipped with a refractive index detector and two ResiPore 300 × 7.5 mm columns. The polymer samples were dissolved in THF (2 mg·mL−1). 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 Mark−Houwink factor of 0.58, to account for the difference in hydrodynamic volumes between polystyrene and polylactide.25 The microstructure of PLAs was determined by homodecoupling 1 H NMR spectroscopy at 25 °C in CDCl3, with a Bruker AC-500 spectrometer. The microstructure of PHBs was determined by analyzing the carbonyl and methylene regions of 13C{1H} NMR spectra at 40 °C in CDCl3 with a Bruker AC-500 operating at 125 MHz, using ca. 0.2 mol·L−1 solutions of PHB (reprecipitated with pentane from a dichloromethane solution).7c MALDI-ToF mass spectra were recorded at the CESAMO (Bordeaux, France) on a Voyager mass spectrometer (Applied Biosystems) equipped with a pulsed N2 laser source (337 nm) and a time-delayed extracted ion source. Spectra were recorded in the positive-ion mode using the reflectron mode and with an accelerating voltage of 20 kV. A THF solution (1 mL) of the matrix (trans-3indoleacrylic acid, IAA (Aldrich, 99%); 10 mg.mL−1) and possibly a MeOH solution of the cationization agent (NaI, 10 mg·mL−1) were prepared. A fresh solution of the polymer samples in THF (10 mg· mL−1) was then prepared. The two (or three) solutions were then rapidly combined in a 1:1(:10) volume ratio of matrix-to-sample-tocationization agent. Then 1−2 μL of the resulting solution was deposited onto the sample target and vacuum-dried. 1-(Methoxymethyl)-4-methyl-2-(2-phenylpropan-2-yl)benzene. To a suspension of NaH (3.20 g, 133 mmol) in DMF (100 mL), under argon flow, was added dropwise a solution of 4-methyl-2-(2phenylpropan-2-yl)phenol26 (20.0 g, 88.4 mmol) in DMF (100 mL) at 0 °C. After stirring for 4 h at room temperature, methoxymethyl chloride (13.64 g, 141 mmol) was added slowly, and the mixture was stirred for 18 h. The reaction mixture was carefully diluted with water (ca. 1 L), and organic materials were extracted with CH2Cl2 (3 × 100 mL). The combined organic extracts were washed with water (2 × 500 mL) and brine and dried over MgSO4. The solution was evaporated to dryness at 80 °C to give a colorless, oily product (21.0 g, 88%), which was used without further purification. 1H NMR (300 MHz, CDCl3, 298 K): δ 7.34 (s, 1H, Haro), 7.22 (br s, 4H, Haro), 7.13−6.91 (m, 3H, Haro), 4.56 (s, 2H, OCH2OCH3), 3.00 (s, 3H, OCH2OCH3), 2.40 (s, 3H, CH3), 1.72 (s, 6H, CH3). {ONOMe,Cumyl}H2. A solution of sec-BuLi (30.0 mL of a 1.3 M solution in hexane, 39.0 mmol) was added dropwise over 20 min to a stirred solution of 1-(methoxymethyl)-4-methyl-2-(2-phenylpropan-2yl)benzene (9.50 g, 35.1 mmol) in THF (100 mL) at −78 °C. After stirring overnight at room temperature, a dark solution was obtained, to which was added a solution of ZnCl2 (4.79 g, 35.1 mmol) in THF (ca. 30 mL) at −78 °C over 30 min. Then, the resulting solution was gently warmed to room temperature. To the resulting organozinc solution was added a suspension of Pd2(dba)3 (0.16 g, 0.35 mmol), SPhos (0.326 g, 0.70 mmol), and 2,6-dibromopyridine (4.16 g, 17.6 mmol) in THF (10 mL). The reaction mixture was stirred for 72 h at 64 °C, then cooled to room temperature, diluted with water (200 mL), and finally extracted with CH2Cl2 (3 × 50 mL). After evaporation to dryness of the combined organic extracts, the crude material was recrystallized in ethyl acetate to give a white powder (7.00 g, 65%).

The resulting solid was dissolved in a mixture of 36% aqueous HCl (20 mL), CHCl3 (30 mL), and EtOH (40 mL), and the solution was refluxed for 24 h. The reaction mixture was cooled to 0 °C and carefully reacted with a concentrated solution of NaOH (30 mL). Then, a concentrated solution of NH4Cl was added to adjust the pH value to 7−8. The product was extracted with CH2Cl2 (3 × 20 mL), and the combined organic extracts were dried over MgSO4 and finally evaporated to afford {ONOMe,Cumyl}H2 as a white solid (5.80 g, 97% second step, 64% overall). 1H NMR (500 MHz, CDCl3, 298 K): δ 9.80 (br s, 2H, OH), 7.82 (t, 3J = 7.8 Hz, 1H, Haro), 7.55 (d, 3J = 7.65 Hz, 2H, Haro), 7.38−7.22 (m, 14H, Haro), 2.44 (s, 6H, CH3), 1.78 (s, 12H, CH3). 13C{1H} NMR (125 MHz, CDCl3, 298 K): δ 156.53, 152.53, 150.71, 139.03, 137.22, 129.65, 128.04, 127.91, 127.07, 125.73, 125.43, 122.42, 120.11 (Caro), 42.28 (C(CH3)2), 29.77 (C(CH3)2), 21.17 (CH3). Anal. Calcd for C37H37NO2: C, 84.21; H, 7.07; N, 2.65. Found: C, 84.15; H, 7.21; N, 2.58.

Chart 2. Numbering Scheme of the Bis(phenolate)pyridine (Pro)ligand

{ONOMe,Cumyl}Y[N(SiHMe2)2](THF)(Et2O) (1). A Schlenk flask was charged with {ONOMe,Cumyl}H2 (331 mg, 0.63 mmol) and Y[N(SiHMe2)2]3(THF) (350 mg, 0.63 mmol), and diethyl ether (15 mL) was vacuum transferred in. The reaction mixture was gently warmed to room temperature and stirred overnight. After filtration, the clear solution was concentrated and placed at −30 °C, which eventually afforded 1 as yellow crystals (130 mg, 23%). Anal. Calcd for C49H67N2O4Si2Y: C, 65.89; H, 7.56; N, 3.14. Found: C, 65.73; H, 7.62; N, 3.05. 1H NMR (500 MHz, toluene-d8, 298 K): δ 7.37−7.35 (m, 6H, Haro), 7.09−6.89 (m, 11H, Haro), 4.68 (br s, 2H, Si-H), 3.22 (q, 3J = 7.0 Hz, 4H, Et2O), 3.15 (br m, α-CH2, 4H, THF), 2.33 (s, 6H, Me), 1.93 (s, 12H, Me), 1.06 (br s, 6H, Et2O), 0.99 (br m, β-CH2, 4H, THF), 0.26 (br s, 12H, SiHMe2). Note that this compound is, however, not very stable in toluene-d8 and slowly transforms into another, yet unidentified species at room temperature (ca. 13% after 15 min at 25 °C) and even more so at higher temperature (27% conversion after 4 h at 90 °C). Characteristic signals for the decomposition species formed: (500 MHz, toluene-d8, 298 K) δ 7.59 (2H, aro), 7.41 (2H, Haro), 7.28 (m, 2H, Haro), 7.17 (m, 2H, Haro), 7.11 (m, 2H, Haro), 6.85 (m, 1H, Haro), 6.81 (d, 3J = 1.85 Hz, 1H, Haro), 6.76 (d, 3J = 2.3 Hz, 1H, Haro), 6.47 (dd, 3J = 2.4 Hz and 4J = 0.5 Hz, 1H, Haro), 6.39 (dd, 3J = 6.6 Hz, 4J = 2.1 Hz, 1H, Haro), 4.77 (m, 2H, Si-H), 2.42 (s, 3H, Me), 2.33 (s, 3H, Me), 1.96 (s, 3H, Me), 1.90 (s, 3H, Me), 1.79 (s, 3H, Me), 0.04 (d, 3J = 3.0 Hz, 6H, SiHMe2), −0.08 (d, 3J = 3.0 Hz, 6H, SiHMe2). Compound 1 is much more stable in pyridine-d5, in which it remains intact, at least for several hours at room temperature. 1H NMR (500 MHz, pyridine-d5, 298 K): δ 7.63 (d,3J = 7.9 Hz, 4H, H o-cumyl), 7.48 (s, 2H, H4), 7.37−7.29 (m, 5H, H m-cumyl + H3p), 7.18−7.16 (m, 2H, H p-cumyl), 7.06 (d,3J = 7.9 Hz, 2H, H2p), 6.9 (s, 2H, H6), 4.77 (br m, 2H, Si-H), 3.64 (br m, α-CH2, 4H, free THF), 3.35 (q, 3J = 7.0 Hz, 4H, free Et2O), 2.35 (s, 6H, CH3), 2.13 (s, 12H, CH3 cumyl), 1.60 (br m, β-CH2, 4H, free THF), 1.11 (t, 3J = 7.0 Hz, 4H, free Et2O), 0.20 (d,3J = 2.8 Hz, 12H, SiHMe2). 13 C{1H} NMR (125 MHz, pyridine-d5, 298 K): δ 161.74 (C−N), 158.79 (C−O), 158.77 (C−O), 153.17, 138.13, 137.51, 131.84, 130.11 (C6), 129.09 (C4), 128.11, 126.77, 125.06, 124.12, 67.84 (α-CH2, free THF), 65.79 (OCH2, free Et2O), 43.19 (C(CH3)2), 31.34 (C(CH3)2), 25.82 (β-CH2, free THF), 21.05 (CH3), 15.52 (CH3, free Et2O), 4.15 (SiHMe2). J


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{ONOMe,Cumyl}Y((R)-OCH(CH3)CH2COOMe) (2). In the glovebox, methyl (R)-3-hydroxybutyrate (1.4 μL, 13 μmol) was added to a solution of {ONOMe,Cumyl}Y[N(SiHMe2)2](THF)(Et2O) (1, 11.6 mg, 13 μmol) in C6D6 (ca. 0.7 mL). After 10 min, the reaction was complete, as evidenced by 1H NMR. Evaporation of volatiles under vacuum left 2 as a pale yellow solid in quantitative yield (6.35 mg, 99%). Anal. Calcd for C42H44NO5Y: C, 68.94; H, 6.06; N, 1.91. Found: C, 68.6; H, 6.3; N, 2.1. 1H NMR (500 MHz, toluene-d8, 258 K): δ 7.48 (d, 3J = 7.6 Hz, 2H, Haro), 7.35 (d, 3J = 7.6 Hz, 2H, Haro), 7.20−6.93 (m, 13H, Haro), 2.96 (br m, 1H, OCH(Me)), 2.89 (s, 3H, CH2COOCH3), 2.43 (br dd, 3J = 14.4 Hz, 1H, CHHCOOMe), 2.31 (s, 3H, CH3), 2.27 (s, 3H, CH3), 2.03 (s, 3H, CH3), 1.99 (s, 3H, CH3), 1.74 (br dd, 2J = 14.4 Hz, 1H, CHHCOOMe), 1.66 (s, 3H, CH3), 1.58 (s, 3H, CH3), 0.87 (d, 3J = 5.7 Hz, 3H, OCH(CH3)). 13C{1H} NMR (100 MHz, toluene-d8, 258 K): δ 180.06 (COO), 163.02, 162.90, 159.21, 158.48, 151.99, 151.53, 138.02 (C3p), 137.37, 136.95, 136.87, 136.64, 131.63, 131.36, 131.15, 131.03, 128.92, 128.0, 127.17, 127.03, 126.79, 125.1, 125.0, 124.28, 123.47, 123.30, 123.16, 122.89, 65.07 OCH(Me), 53.37 (COOCH3), 43.46 (C(CH3)2), 43.25 (C(CH3)2), 42.37 (CH2COOMe), 31.44 (CH3), 30.94 (CH3), 29.34 (CH3), 26.53 (CH3), 23.49 (CH3), 20.98 (CH3), 20.84 (CH3). NMR Scale Synthesis of {ONOMe,Cumyl}Y((S,S)-OCH(CH3)CO2CH(CH3)CO2Me) (3). This product was prepared as described above for 2, starting from 1 (0.030 g, 33.6 μmol) and methyl (S,S)-lactyllactate (6.0 mg, 33.6 μmol) in THF-d8 (ca. 0.7 mL) for 30 min at room temperature. 1H NMR spectroscopy revealed the release of HN(SiHMe2)2 (δ 1H: 4.44 (m, 3J = 3.0 Hz, 2H, HN(SiHMe2)2), 0.09 (d, 3 J = 3.1 Hz, 12H, HN(SiHMe2)2) and Et2O (δ 1H: 3.36 (q, 3J = 7.0 Hz, 3H, OCH2CH3, free Et2O), 1.09 (t, 3J = 7.0 Hz, 3H, OCH2CH3, free Et2O)) and formation of 3 along with another species yet unidentified (33−40%). Compound 3 is not stable in solution at room temperature and transforms over 18 h into other species, probably arising from transesterification reactions. Characteristic data for 3: 1H NMR (500 MHz, THF-d8, 298 K): δ 7.63 (t, 3J = 7.9 Hz, 1H, H3p), 7.22−6.77 (m, 16H, Haro), 4.90−4.85 (two quadruplets overlapped, 3J = 6.9 Hz, 2H, −OCH(CH3)CO2CH(CH3)CO2Me), 3.67 (s, 3H, COOCH3), 2.22−2.20 (m, 6H, p-CH3 phenolate), 1.69−1.65 (m, 12 H, CH3 of cumyl), 1.56 (d, 3J = 7.0 Hz, 3H, CH(CH3)), 1.49 (d, 3J = 6.9 Hz, 3H, CH(CH3)). 13C{1H} NMR (125 MHz, THF-d8, 298 K): δ 190.0 (OCH(CH3)CO2CH(CH3)CO2Me), 169.0 (OCH(CH3)CO2CH(CH3)CO2Me), 162.1 and 162.0 (CN), 158.8 and 158.7 (CO), 154.7 and 154.6 (C quat aro), 153.5 and 153.4 (C quat aro), 136.5 (CH3p), 130.6 and 130.5 (CH), 129.15 and 129.1 (C quat aro), 128.0 and 127.8 (CH), 126.2 and 126.15 (C quat aro), 125.2 and 125.15 (CH), 125.1 and 125.0 (CH), 123.15 and 123.1 (CH), 122.5 and 122.3 (CH2p), 73.0 (OCH(CH3)CO2CH(CH3)CO2Me), 69.9 (OCH(CH3)CO2CH(CH3)CO2Me), 42.1 and 41.9 (C(CH3)2), 51.0 (OCH3), 29.1 and 29.0 (C(CH3)2), 23.2 (OCH(CH3)CO2CH(CH3)CO2Me), 20.1 and 20.3 (CH3), 16.0 (OCH(CH3)CO2CH(CH3)CO2Me). Characteristic signals for the other species observed in THF (500 MHz, THF-d8, 298 K): δ 7.73 (t, 3J = 8.0 Hz, 1H, H3p), 7.30 (d, 3 J = 8.0 Hz, 2H, H2p), 4.57 (br m, 2H, SiH), 1.81 (s, 12H, CH3 of cumyl), 0.12 (d, 3J = 3.1 Hz, 12H, N(SiHMe2)2; some other signals overlapped with those of 3. {ONOMe,Cumyl}AlMe (4). A Schlenk flask was charged with {ONOMe,Cumyl}H2 (0.300 g, 0.57 mmol), and toluene (ca. 10 mL) was vacuum transferred in at −78 °C. To this solution was added AlMe3 (0.80 mL of a 1.0 M solution in heptane, 0.80 mmol, 1.4 equiv). The reaction mixture was warmed to 80 °C and stirred overnight. The solution was cooled to room temperature, and a pale yellow microcrystalline solid precipitated. The latter solid was filtrated and washed with cold hexane (5 mL) to give 4 (0.248 g, 77%). Anal. Calcd for C38H38AlNO2: C, 80.40; H, 6.75; N, 2.47. Found: C, 80.34; H, 6.80; N, 2.19. 1H NMR (500 MHz, CD2Cl2, 298 K): δ 7.89 (t, 3J = 8.1 Hz, 1H, H3p), 7.54 (d, 3J = 8.1 Hz, 2H, H2p), 7.46 (s, 2H, H4), 7.32 (s, 2H, H6), 7.24 (m, 8H, H o- + m-cumyl), 7.14 (m, 2H, H p-cumyl), 2.39 (s, 6H, CH3 aryl), 1.70 (s, 6H, CH3 cumyl), 1.65 (s, 6H, CH3 cumyl), −1.87 (s, 3H, Al-CH3). 13C{1H} NMR (125 MHz, CD2Cl2, 298 K): δ 155.12 (C−O), 154.86 (C−N), 151.39, 141.94, 141.70, 131.81, 128.02, 127.66, 127.49, 126.11, 125.17, 122.62, 121.58 (Caro),

42.37 (C(CH3)2, 30.49 (C(CH3)2), 28.51 (C(CH3)2), 21.19 (CH3), −17.25 (Al-CH3)). {ONOMe,Cumyl}Al(iPr (S)-lactate) (5). A Schlenk flask was charged with {ONOMe,Cumyl}AlMe (4, 0.114 g, 0.200 mmol) and toluene (10 mL). Then, in the glovebox, isopropyl (S)-2-lactate (27 μL, 0.202 mmol) was microsyringed in. The reaction mixture was then warmed to 80 °C and stirred overnight. Evaporation of volatiles under vacuum left 5 as a pale yellow powder in essentially quantitative yield (0.136 g, 99%). Anal. Calcd for C43H46AlNO5: C, 75.53; H, 6.78; N, 2.05. Found: C, 75.37; H, 6.86; N, 1.85. 1H NMR (500 MHz, toluene-d8, 298 K): δ 7.57 (m, 4H, H o-cumyl), 7.29 (m, 6H, H4 + H m-cumyl), 7.09 (m, 2H, H p-cumyl), 6.95 (s, 1H, H6), 6.91 (s, 1H, H6′), 6.55 (t, 3 J = 8.0 Hz, 1H, H3p), 6.48 (d, 3J = 7.8 Hz, 1H, H2p), 6.42 (d, 3J = 7.8 Hz, 1H, H2p′), 5.37 (hept, 3J = 6.3 Hz, 1H, C(O)OCH(CH3)2), 4.00 (q, 3J = 6.6 Hz, 1H, OCH(CH3), 2.24 (s, 3H, CH3 aryl), 2.23 (s, 3H, CH3 aryl), 2.00 (s, 3H, CH3 cumyl), 1.99 (s, 3H, CH3 cumyl), 1.83 (s, 3H, CH3 cumyl), 1.82 (s, 3H, CH3 cumyl), 1.12 (2d overlapped, 3J = 6.3 Hz, 6H, C(O)OCH(CH3)2), 0.91 (d, 3J = 6.7 Hz, 3H, OCH(CH3)). 13C{1H} NMR (125 MHz, toluene-d8, 298 K): δ 189.50 (COO), 157.27 (Caro), 157.17 (Caro), 154.53 (CN), 154.46 (C′N), 151.49 (CO), 151.38 (C′O), 139.99 (Caro), 139.78 (Caro), 139.61 (C3p), 130.71 (C4), 130.56 (C4′), 129.18 (Caro), 128.26 (Caro), 127.56 (C6), 127.43 (C6′), 126.87 (Caro), 126.86 (Caro), 122.91 (Caro), 122.83 (Caro), 120.79 (C2p), 120.57 (C′2p), 73.79 (C(O)OCHMe2), 68.76 (OCH(CH3)), 42.64 (C(CH3)2), 42.56 (C′(CH3)2), 32.41 (CH3 cumyl), 32.39 (CH3 cumyl), 27.58 (CH3 cumyl), 27.47 (CH3 cumyl), 21.82 (OCH(CH3), 21.63 (C(O)OCHMe2), 21.57 (C(O)OCHMe2), 21.19 (CH3 aryl, 2 signals overlapping). {ONOMe,Cumyl}Al((R)-OCH(CH3)CH2COOCH3) (6). In the glovebox, a J-Young NMR tube was charged with a solution of {ONOMe,Cumyl}AlMe (4, 12.7 mg, 22.4 μmol) in toluene-d8 (ca. 0.4 mL), and a solution of methyl (R)-3-hydroxybutyrate (2.5 μL, 22.4 μmol) in toluene-d8 (ca. 0.3 mL) was added at room temperature. The tube was closed, shaken, and put in a preset oil bath at 80 °C for 15 h. 1H NMR spectroscopy revealed the release of methane and completion of the reaction. Evaporation of volatiles under vacuum left 6 as a pale yellow solid. Analytically pure crystals of 6 suitable for X-ray diffraction were obtained by recrystallization from a concentrated CH2Cl2 solution layered with hexane at room temperature. Anal. Calcd for C42H44AlNO5: C, 75.32; H, 6.62; N, 2.09. Found: C, 75.0; H, 6.9; N, 2.2. 1H NMR (500 MHz, CD2Cl2, 298 K): δ 7.83 (t, 3J = 8.15 Hz, 1H, H3p), 7.65 (m, 2H, Haro), 7.45 (m, 2H, Haro), 7.26−7.07 (m, 12H, Haro), 3.91 (s, 3H, CH2COOCH3), 3.47 (m, 1H, OCH(CH3)), 2.35 (s, 3H, CH3), 2.32 (s, 3H, CH3), 1.95 (dd, 3J = 2.9 Hz and 2J = 16.4 Hz, 1H, CHHCOOCH3), 1.85 (dd, 3J = 2.9 Hz and 2J = 16.4 Hz, 1H, CHHCOOCH3), 1.77 (s, 3H, CH3), 1.72 (s, 3H, CH3), 1.68 (s, 3H, CH3), 1.63 (s, 3H, CH3), 0.65 (d, 3J = 6.2 Hz, 3H, OCH(CH3)). 1H NMR (400 MHz, C6D6, 298 K): 7.40−6.90 (m, 17H, Haro), 3.66 (m, 1H, OCH(CH3)), 3.44 (s, 3H, CH2COOCH3), 2.28 (s, 3H, CH3), 2.25 (s, 3H, CH3), 2.05 (s, 3H, CH3), 1.98 (s, 3H, CH3), 1.84 (s, 3H, CH3), 1.81 (s, 3H, CH3), 1.75 (dd, 3J = 2.9 Hz and 2J = 16.0 Hz, 1H, CHHCOOCH3), 1.52 (dd, 3J = 8.4 Hz and 2J = 16.0 Hz, 1H, CHHCOOCH3), 0.71 (d, 3J = 6.1 Hz, 3H, OCH(CH3)). 13C{1H} NMR (100 MHz, C6D6, 258 K): δ 181.16 (COO), 158.57, 158.39, 156.20, 156.17, 152.53, 152.27, 139.60, 139.55, 138.34 (C3p), 131.99, 131.96, 129.34, 128.1, 127.95, 127.59, 127.49, 126.45, 126.35, 125.70, 125.25, 125.15, 125.05, 122.89, 122.65, 120.17, 63.48 (OCH(Me)), 53.50 (COOCH3), 43.07 (C(CH3)2), 41.76 (CH2COOMe), 32.36 (CH3), 31.50 (CH3), 29.41 (CH3), 28.85 (CH3), 25.59 (OCH(CH3)), 21.30 (CH3), 21.25 (CH3). {ONOMe,Cumyl}Al((rac)-OCH(CF3)CH2CO2C2H5) (7). This compound was prepared following the same procedure as that described above for 6, starting from {ONOMe,Cumyl}AlMe (25.6 mg, 45.1 μmol) and (±)-ethyl 3-hydroxy-4,4,4-trifluorobutyrate (9.2 mg, 45.1 μmol) in toluene-d8 (ca. 0.7 mL) for 17 h at 80 °C. 1H NMR spectroscopy revealed the release of methane gas and completion of the reaction. The product is slightly soluble in toluene, and it precipitates with progress of the reaction. The supernatant solution was then removed, and the recovered solid was dried under vacuum to give 7 as a pale yellow solid (20.0 mg, 60%). Anal. Calcd for C43H43AlF3NO5: C, K


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70.00; H, 5.87; N, 1.90. Found: C, 69.86; H, 6.05; N, 1.87. 1H NMR (500 MHz, CD2Cl2, 298 K): δ 7.81 (t, 3J = 8.3 Hz, 1H, H3p), 7.65 (d, 3 J = 8.3 Hz, 1H, H2p), 7.63 (d, 3J = 8.2 Hz, 1H, H2p′), 7.45 (2 singlets, 2H, H6), 7.31−7.07 (m, 12H, H4, H4′ and other Haro), 4.34 (2 quartets overlapping, 3J = 7.2 Hz, 2H, COOCH2CH3), 3.56 (m, 3J = 3.1 and 9.1 Hz, 1H, OCH(CF3)), 2.37 (s, 3H, CH3), 2.34 (s, 3H, CH3), 1.98 (dd, J = 3.1 and 16.5 Hz, 1H, CHHCOOEt), 1.84 (dd, J = 9.1 and 16.5 Hz, 1H, CHHCOOEt), 1.81 (s, 3H, CH3), 1.75 (s, 3H, CH3), 1.67 (s, 3H, CH 3 ), 1.63 (s, 3H, CH 3 ), 1.49 (t, 3 J = 7.2 Hz, 3H, CH2COOCH2CH3). 13C{1H} NMR (125 MHz, CD2Cl2, 298 K): δ 178.86 (COO), 157.36 and 157.05 (CO), 156.30 and 156.27 (CN), 152.40 and 152.10 (quat Caro), 139.5 (CH3p), 139.38 and 139.33 (quat Caro), 132.11 and 132.02 (CH4), 129.39 and 128.58 (C5), 128.00 (CHaro), 127.44 and 127.36 (CH6), 126.20 and 126.17 (CH cumyl), 126.14 and 125.98 (CH cumyl), 125.28 and 125.21 (CH cumyl), 122.53 and 122.01 (quat Caro), 121.02 and 120.82 (CH2p), 67.25 (q, 2J = 31.2 Hz, OCH(CF3)), 64.84 (COOCH2CH3), 42.80 and 42.70 (C(CH3)2), 33.64 (CH2COOEt), 32.26 and 30.84 (C(CH3)2), 29.87 and 28.65 (C(CH3)2′), 21.17 and 21.14 (CH3 phenyl), 13.79 (COOCH2CH3); the CF3 signal was not observed due to its low intensity. 19F{1H} NMR (185 MHz, CD2Cl2, 298 K): δ −81.87 ppm. Typical Polymerization Procedure. Polymerizations of rac-LA and rac-BBL were described as reported earlier.7c X-ray Diffraction Analyses. Suitable single crystals were mounted onto a glass fiber using the “oil-drop” method. Diffraction data were collected at 100(2) K using an APEXII Bruker-AXS diffractometer with graphite-monochromatized Mo Kα radiation (λ = 0.71073 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, and the remaining atoms were located from difference Fourier synthesis followed by full-matrix least-squares refinement based on F2 (programs SIR97 and SHELXL-97)27 with the aid of the WINGX program.28 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.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic data as Table S1 and CIF files; representative 1H, 13C{1H}, and 19F{1H} NMR spectra for ligands, complexes, and some polymers; details of DFT computations. This information is available free of charge via the Internet at http://pubs.acs.org.

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*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Fax: (+33) (0)223-236-939. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the Brazilian CAPES, French MESR, and CNRS. The authors are grateful to CAPESCOFECUB for joined action Ph556-07_2007-2010 and CAPES-CNRS for joined action PICS05923. L


Organometallics

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Chem. 2011, 2, 2758. (p) Buffet, J.-C.; Okuda, J. Dalton Trans. 2011, 40, 7748. (q) Platel, R. H.; White, A. J. P.; Williams, C. K. Inorg. Chem. 2011, 50, 7718. (r) Kapelski, A.; Buffet, J.-C.; Spaniol, T. P.; Okuda, J. Chem. Asian J. 2012, 7, 1320. (s) Nie, K.; Fang, L.; Yao, Y.; Zhang, Y.; Shen, Q.; Wang, Y. Inorg. Chem. 2012, 51, 11133. (t) Li, G.; Lamberti, M.; Mazzeo, M.; Pappalardo, D.; Roviello, G.; Pellecchia, C. Organometallics 2012, 31, 1180. (u) Bakewell, C.; Cao, T.-P.-A.; Long, N. J.; Le Goff, X. F.; Auffrant, A.; Williams, C. K. J. Am. Chem. Soc. 2012, 134, 20577. (v) Nie, K.; Gu, W.; Yao, Y.; Zhang, Y.; Shen, Q. Organometallics 2013, 32, 2608. (w) Bakewell, C.; Cao, T.-P.-A.; Le Goff, X. F.; Long, N. J.; Auffrant, A.; Williams, C. K. Organometallics 2013, 32, 1475. For isotactic enrichment from enantiopure initiators, see: (x) Heck, R.; Schulz, E.; Collin, J.; Carpentier, J. F. J. Mol. Catal. A: Chem. 2007, 268, 163. (y) Arnold, P. L.; Buffet, J.-C.; Blaudeck, R. P.; Sujecki, S.; Blake, A. J.; Wilson, C. Angew. Chem., Int. Ed. 2008, 47, 6033. (z) Otero, A.; Fernández-Baeza, J.; Lara-Sánchez, A.; AlonsoMoreno, C.; Márquez-Segovia, I.; Sánchez-Barba, L. F.; Rodríguez, A. M. Angew. Chem., Int. Ed. 2009, 48, 2176. (7) (a) Amgoune, A.; Thomas, C. M.; Ilinca, S.; Roisnel, T.; Carpentier, J.-F. Angew. Chem., Int. Ed. 2006, 45, 2782. (b) Ajellal, N.; Lyubov, D. M.; Sinenkov, M. A.; Fukin, G. K.; Cherkasov, A. V.; Thomas, C. M.; Carpentier, J.-F.; Trifonov, A. A. Chem.Eur. J. 2008, 14, 5440. (c) Ajellal, N.; Bouyahyi, M.; Amgoune, A.; Thomas, C. M.; Bondon, A.; Pillin, I.; Grohens, Y.; Carpentier, J.-F. Macromolecules 2009, 42, 987. (d) Kramer, J. W.; Treiter, D. S.; Dunn, E. W.; Castro, P. M.; Roisnel, T.; Thomas, C. M.; Coates, G. W. J. Am. Chem. Soc. 2009, 131, 16042. (e) Bouyahyi, M.; Ajellal, N.; Kirillov, E.; Thomas, C. M.; Carpentier, J. F. Chem.Eur. J. 2011, 17, 1872. (f) Pappalardo, D.; Bruno, M.; Lamberti, M.; Pellecchia, C. Macromol. Chem. Phys. 2013, 214, 1965. For an example of isoselective ROP of rac-BBL, see: (g) Ajellal, N.; Durieux, G.; Delevoye, L.; Tricot, G.; Dujardin, C.; Thomas, C. M.; Gauvin, R. M. Chem. Commun. 2010, 46, 1032. (8) Steudel, A.; Stehr, J.; Siebel, E.; Fischer, R. D. J. Organomet. Chem. 1998, 570, 89. (9) (a) Scandium: ref 6c. (b) Yttrium: Grunova, E.; Kirillov, E.; Roisnel, T.; Carpentier, J.-F. Organometallics 2008, 27, 5691. Aluminum: (c) Ref 2h. (d) Lewinski, J.; Horeglad, P.; Wójcik, K.; Justyniak, I. Organometallics 2005, 24, 4588. (e) Phomphrai, K.; Chumsaeng, P.; Sangtrirutnugul, P.; Kongsaeree, P.; Pohmakotr, M. Dalton Trans. 2010, 39, 1865. (f) Dagorne, S.; Le Bideau, F.; Welter, R.; Bellemin-Laponnaz, S.; Maisse-François, A. Chem.Eur. J. 2007, 13, 3202. (g) Normand, M.; Dorcet, V.; Kirillov, E.; Carpentier, J.-F. Organometallics 2013, 32, 1694. Gallium: (h) Horeglad, P.; Kruk, P.; Pécaut, J. Organometallics 2010, 29, 3729. (i) Horeglad, P.; Szczepaniak, G.; Dranka, M.; Zachara, J. Chem. Commun. 2012, 48, 1171. (10) For other examples of discrete alkyl lactate metal complexes used as initiators for/models of ROP of lactide, see the following. Zinc: (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. (b) Sn: Dove, A. D.; Gibson, V. C.; Marshall, E. L.; Rzepa, H. S.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 2006, 128, 9834. (c) Sn: Wang, L.; Kefalidis, C. E.; Sinbandhit, S.; Dorcet, V.; Carpentier, J.-F.; Maron, L.; Sarazin, Y. Chem.Eur. J. 2013, 19, 13463. (11) (a) Chen, H.-L.; Dutta, S.; Huang, P.-Y.; Lin, C.-C. Organometallics 2012, 31, 2016. (b) Wang, L.; Ma, H. Macromolecules 2010, 43, 6535. (12) Kirillov, E.; Roisnel, T.; Razavi, A.; Carpentier, J.-F. Organometallics 2009, 28, 5036. (13) Hieringer, W.; Eppinger, J.; Anwander, R.; Herrmann, W. A. J. Am. Chem. Soc. 2000, 122, 11983−11994. (14) Anwander, R.; Runte, O.; Eppinger, J.; Gerstberger, G.; Herdtweck, E.; Spiegler, M. J. Chem. Soc., Dalton Trans. 1998, 847. (15) The PGSE NMR measurements were performed on a 50 mM solution of complex 2 in toluene-d8 at room temperature using tetrakis(trimethylsilyl)silane as internal reference. The average value of the translational diffusion coefficient (Dt), determined for three separate signals in the 1H PGSE NMR spectrum, is 6.82 × 10−10 m2·s. The values a (7.22 Å) and b (6.15 Å) (the major and the minor

semiaxes of the prolate ellipsoid, respectively), estimated from the solid-state structure of the parent complex 1, were used for calculation of the hydrodynamic radius (rH,PGSE). The corresponding value of rH,DRX for 1 is 6.84 Å. (16) Calculated following the equation τ = (β − α)/60, where α and β are the O−Al−O and O(carbonyl)−Al−N angles in the present case. The τ value ranges from 0 (perfectly square pyramidal) to 1 (perfectly trigonal bipyramidal); see: Addison, A. W.; Rao, T. N.; Reedjik, J.; Van Rijn, L.; Verschoor, G. C. J. Chem. Soc., Dalton. Trans. 1984, 1349. (17) Shannon, R. D. Acta Crystallogr. A 1976, A32, 751. (18) Umezawa, Y.; Tsuboyama, S.; Takahashi, H.; Uzawa, J.; Nishio, M. Tetrahedron 1999, 55, 10047. (19) DFT computations performed on yttrium-β-alkoxybutyrate complexes supported by an amino-alkoxy-bis(phenolate) ligand bearing cumyl ortho-substituents suggested a stabilization of ca. 10− 12 kcal mol−1 for these C−H···π interactions. See ref 7e. (20) In this experiment, it is assumed that iPrOH reacts fast with amido complex 1 to release inert HN(SiMe3)2 and generate concomitantly the corresponding isopropoxy species, which is the real initiator of the polymerization, as we earlier established for closely related complexes (see ref 6b). The observed slightly lower experimental Mn values (as compared to the calculated one) likely arise from the use of a slightly larger amount (than strictly 1 equiv) of iPrOH, which in this case acts as a reversible chain-transfer agent in a so-called immortal polymerization (see ref 1g). The lower Mn values (as compared to other PLAs prepared in this study) and hence the larger proportion of end-groups relative to monomer repeat units in the main chain likely account for the apparent decrease in heterotacticity. (21) The presence of a molecule of a coordinating solvent within the coordination sphere of the metal center in the active species during the course of a ROP may affect the stereochemistry of monomer coordination/insertion; see: Chisholm, M. H.; Choojun, K.; Chow, A. S.; Fraenkel, G. Angew. Chem., Int. Ed. 2013, 52, 3264. (22) Essentially the same results as those of entry 17 were obtained when 1 equiv of pyridine was used. (23) (a) LeBorgne, A.; Vincens, V.; Jouglard, M.; Spassky, N. Makromol.Chem., Macromol. Symp. 1993, 173, 37. See also: (b) Dubois, P.; Jérôme, R.; Teyssié, P. Polym. Prepr. Div. Polym. Sci., Am. Chem. Soc. 1994, 35, 596. (24) Montaudo, G.; Montaudo, M. S.; Puglisi, C.; Samperi, F.; Spassky, N.; LeBorgne, A.; Wisniewski, M. Macromolecules 1996, 29, 6461. (25) Barakat, I.; Dubois, P.; Jerome, R.; Teyssie, P. J. Polym. Sci., A: Polym. Chem. 1993, 31, 505. (26) Kochnev, A. I.; Oleynik, I. I.; Oleynik, I. V.; Ivanchev, S. S.; Tolstikov, G. A. Russ. Chem. Bull., Int. Ed. 2007, 56, 1125. (27) (a) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115. (b) Sheldrick, G. M. SHELXS-97, Program for the Determination of Crystal Structures; University of Goettingen: Germany, 1997. (c) Sheldrick, G. M. SHELXL-97, Program for the Refinement of Crystal Structures; University of Goettingen: Germany, 1997. (d) Sheldrick, G. M. Acta Crystallogr. 2008, 64, 112. (28) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.

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