Aluminum Complexes of Fluorinated β-Diketonate Ligands - American

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Organometallics 2010, 29, 491–500 DOI: 10.1021/om9009312

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Aluminum Complexes of Fluorinated β-Diketonate Ligands: Syntheses, Structures, Intramolecular Reduction, and Use in Ring-Opening Polymerization of Lactide Miloud Bouyahyi, Thierry Roisnel, and Jean-Franc-ois Carpentier* Catalysis and Organometallics, UMR 6226 Sciences Chimiques de Rennes, CNRS-University of Rennes 1, 35042 Rennes Cedex, France Received October 23, 2009

Routes toward heteroleptic Al complexes supported by fluorinated β-diketonate ligands have been studied. Reactions of pro-ligands (acacR1,R2)H (R1 =R2 =CF3, “(hfacac)H”; R1 =CF3, R2 =tBu) with 0.5 equiv of AlMe3 or AlMe2Cl systematically yielded the corresponding homoleptic complexes Al(hfacac)3 (1) and Al(acacCF3,tBu)3 (2). Compound 2 exists in CD2Cl2 solution as a mixture of facand mer-isomers. Heteroleptic complexes [Al(acacR1,R2)2(OiPr)]n (R1=R2=CF3, 3; R1=CF3, R2= tBu, 4) were cleanly prepared from the reaction of the corresponding (acacR1,R2)H pro-ligands and 0.5 equiv of AlMe2(OiPr). Reaction of (hfacac)H and 0.5 equiv of Al(OiPr)3 at room temperature afforded 3 contaminated by other products, of which [κ2:μ2-(hfacac)5(OiPr)4Al3] (5) was isolated. When the same reaction was carried out at 55 °C, (S,R)-[κ,μ:κ2-(4H-hfacac)(hfacac)Al(THF)]2 (6), which contains a μ-bridging dianionic ligand (4H-hfacac)2- that arises from the reduction of one carbonyl group in (hfacac)-, was isolated in moderate yield (22%). Single-crystal X-ray diffraction studies revealed that complexes 3, 6, and 7 ([Al(hfacac)2(OH)]2, the hydrolysis product of 3) are dinuclear in the solid state with μ-bridging isopropoxide or hydroxide groups, while 5 features a symmetric trinuclear structure with the two terminal Al atoms supported by two (hfacac)- ligands and bridged via μ-isopropoxide groups to a central Al atom supported by a single (hfacac)- ligand. The Al-OiPr complexes 3 and 4 are effective initiators for the ring-opening polymerization of racemic lactide in THF or toluene solutions, giving atactic PLAs, end-capped by OiPr and OH groups, with controlled molecular weights (Mn up to 30 300 g 3 mol-1) and relatively narrow polydispersities (Mw/Mn =1.10-1.32).

Introduction Organoaluminum compounds supported by monoanionic fluorinated β-diketonate ligands (acacR1,R2)- (R1, R2=CF3, CH2F, ...), including, at the forefront, homoleptic tris(hexafluoroacetylacetonate)aluminum (Al(hfacac)3; hfacac: R1 = R2 = CF3), are well-known and long-studied compounds.1 Such aluminum complexes have been shown to feature interesting properties, often revolving around the unique fluorine element, notably for potential applications in materials science

as precursors for metal organic chemical vapor deposition (MOCVD)2 or applications related to their biological activity.3 On the other hand, the chemistry of heteroleptic complexes of the type Al(acacR1,R2)3-nXn, where X is a ligand potentially active for polymerization catalysis (e.g., X = alkyl, alkoxide, amide, ...), remains largely unexplored. Only a few reports mention the use of Al(acac)R2 compounds for the polymerization of epoxides.4,5 Aluminum alkoxide complexes modified by ancillary ligands have attracted much attention in the past decade as catalysts/initiators for the ring-opening polymerization (ROP) of cyclic esters6 such as ε-caprolactone

*Corresponding author. Fax: (þ33)(0)223-236-939. E-mail: [email protected]. (1) (a) Linck, R. G.; Sievers, R. E. Inorg. Chem. 1966, 5, 806. (b) Morris, M. Lee.; Moshier, R. W.; Sievers, R. E. Inorg. Synth. 1967, 9, 28. (c) Fortman, J. J.; Sievers, R E. Inorg. Chem. 1967, 6, 2022. (d) Eisentraut, K. J.; Sievers, R. E. J. Inorg. Nucl. Chem. 1967, 29, 1931. (e) Pinnavaia, T. J.; Case, D. A. Inorg. Chem. 1971, 10, 482. (f) Kroll, W. R.; Kuntz, I.; Birnbaum, E. J. Organomet. Chem. 1971, 26, 313. (g) Wolf, W. R.; Sievers, R. E.; Brown, G. H. Inorg. Chem. 1972, 11, 1995. (h) Pickering, M.; Jurado, B.; Springer, C. S., Jr. J. Am. Chem. Soc. 1976, 98, 4503. (i) Schildcrout, S. M. J. Phys. Chem. 1976, 80, 2834. (j) Morris, M. L.; Koob, R. D. Inorg. Chem. 1981, 20, 2737. (k) Hori, A.; Shinohe, A.; Takatani, S.; Miyamoto, T. K. Bull. Chem. Soc. Jpn. 2009, 82, 96. (2) (a) Peng, Q.; Hojo, D.; Park, K. J.; Parsons, G. N. Thin Sol. Films 2008, 516, 4997. (b) Sarhangi, A.; Power, J. M. J. Vac. Sci. Technol., A 1992, 10, 1514. (c) Pauleau, Y.; Dulac, O. Chem. Mater. 1991, 3, 280. (d) Temple, D.; Reisman, A. J. Electron. Mater. 1990, 19, 995.

(3) Kumar, S. S. Asian J. Chem. 2007, 19, 3869. (4) Kuntz, I.; Kroll, W. R. J. Polym. Sci., Polym. Chem. 1970, 8, 1601. (5) For olefin polymerization catalyst systems based on M(acac)3 (M=Mn, V), see for instance: (a) Ban, H. T.; Kase, T.; Murata, M. J. Polym. Sci., Part A 2001, 39, 3733, and references therein. (b) For metal group 4 M(acac)2Cl2 systems, see: Shmulinson, M.; Galan-Fereres, M.; Lisovskii, A.; Nelkenbaum, E.; Semiat, R.; Eisen, M. S. Organometallics 2000, 19, 1208. (6) For reviews, see: (a) Dechy-Cabaret, O.; Martin-Vaca, B.; Bourissou, D. Chem. Rev. 2004, 104, 6147. (b) O'Keefe, B. J.; Hillmyer, M. A.; Tolman, W. B. J. Chem. Soc., Dalton Trans. 2001, 2215. (c) Wu, J.; Yu, T.-L.; Chen, C.-T.; Lin, C.-C. Coord. Chem. Rev. 2006, 250, 602. (d) Stanford, M. J.; Dove, A. P. Chem. Soc. Rev. 2010, DOI: 10.1039/ b815104k.

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(ε-CL)7 and lactide (LA),8 a renewable resource. The resulting polyesters are of high topical interest due to their biodegradability/biocompatibility and the practical applications derived thereof. A major interest of these so-called single-site aluminum catalysts is the high degree of control they may exhibit over polymerization, resulting in materials with controlled molecular weight and narrow molecular weight distribution. Also, in some cases, fine control of the polymerization stereochemistry appeared feasible, e.g., formation of isotactic-enriched polylactides from racemic lactide.8d,e,l,m The nature of the ancillary ligand in the Al coordination sphere is an obvious key parameter that determines the steric and electronic properties and, in turn, the catalytic performances of these Al complexes. There is therefore a permanent search for such new ancillaries. These catalytic results, in particular the valuable activities offered by fluorinated ligand platforms,7i,8l,m and the ready availability of fluorinated β-diketonate pro-ligands have encouraged us to investigate the preparation of new heteroleptic aluminum compounds based on these ancillaries. We report herein the preparation and structural characterization of a variety of Al-(acacR1,R2) compounds (R1 = R2 = CF3, hfacac; R1 = CF3, R2 = tBu, acactBu,CF3) and their performances in the ROP of racemic lactide.

Results and Discussion Synthesis of Al(acacR1,R2)2X Compounds. In order to prepare heteroleptic complexes of the type Al(acacR1,R2)2X (X= alkyl, halide, alkoxide), a variety of reactions of aluminum precursors with two pro-ligands (acacR1,R2)H (R1=R2=CF3 “(hfacac)H”; R1 =CF3, R2 =tBu) were undertaken. When AlMe3 and AlMe2Cl were used as precursors, the reactions with 2 equiv of (acacR1,R2)H systematically yielded the corresponding homoleptic compounds Al(acacR1,R2)3 (1, 2), whatever the conditions used (temperature, solvent, addition order of reagents) (Scheme 1). Monitoring of some of these reactions by low-temperature 1H and 19F NMR spectroscopy showed no evidence for the transient formation (7) (a) Taden, I.; Kang, H.-C.; Massa, W.; Spaniol, T. P.; Okuda, J. Eur. J. Inorg. Chem. 2000, 441. (b) Liu, Y.-C.; Ko, B.-R.; Lin, C.-C. Macromolecules 2001, 34, 6196. (c) Liao, T.-C.; Huang, Y.-L.; Huang, B.-H.; Lin, C.-C. Macromol. Chem. Phys. 2003, 204, 885. (d) Alcazar-Roman, L. M.; O'Keefe, B. J.; Hillmyer, M. A.; Tolman, W. B. J. Chem. Soc., Dalton Trans. 2003, 3082. (e) Chen, C.-T.; Huang, C.-A.; Huang, B.-H. J. Chem. Soc., Dalton Trans. 2003, 3799. (f) Chen, C.-T.; Huang, C.-A.; Huang, B.-H. Macromolecules 2004, 37, 7968. (g) Nomura, N.; Aoyama, T.; Ishii, R.; Kondo, T. Macromolecules 2005, 38, 5363. (h) Hu, H.; Chen, E. Y.-X. Organometallics 2007, 26, 5395. (i) Amgoune, A.; Lavanant, L.; Thomas, C. M.; Chi, Y.; Welter, R.; Dagorne, S.; Carpentier, J.-F. Organometallics 2005, 24, 6279. (j) Chai, Z.-Y.; Zhang, C.; Wang, Z.-X. Organometallics 2008, 27, 1626. (8) (a) Spassky, N.; Wisniewski, M.; Pluta, C.; Le Borgne, A. Macromol. Chem. Phys. 1996, 197, 2627. (b) Ovitt, T. M.; Coates, G. W. J. Am. Chem. Soc. 1999, 121, 4072. (c) Ovitt, T. M.; Coates, G. W. J. Am. Chem. Soc. 2002, 124, 1316. (d) Zhong, Z.; Dijkstra, P. J.; Feijen, J. Angew. Chem., Int. Ed. 2002, 41, 4510. (e) Nomura, N.; Ishii, R.; Akakura, M.; Aoi, K. J. Am. Chem. Soc. 2002, 124, 5938. (f) Hormnirun, P.; Marshall, E. L.; Gibson, V. C.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 2004, 126, 2688. (g) Ma, H.; Melillo, G.; Oliva, L.; Spaniol, T. P.; Englert, U.; Okuda, J. J. Chem. Soc., Dalton Trans. 2005, 721. (h) Lewinski, J.; Horeglad, P.; Wojcik, K.; Justyniak, I. Organometallics 2005, 24, 4588. (i) Nomura, N; Ishii, R.; Yamamoto, Y.; Kondo, T. Chem.;Eur. J. 2007, 13, 4433. (j) Du, H.; Pang, X.; Yu, H.; Zhuang, X.; Chen, X.; Cui, D.; Wang, X.; Jing, X. Macromolecules 2007, 40, 1904. (k) Chisholm, M. H.; Gallucci, J. C.; Quisenberry, K. T.; Zhou, Z. Inorg. Chem. 2008, 47, 2613. (l) Bouyahyi, M.; Grunova, E.; Marquet, N.; Kirillov, E.; Thomas, C. M.; Roisnel, T.; Carpentier, J.-F. Organometallics 2008, 27, 5815. (m) Alaaeddine, A.; Thomas, C. M.; Roisnel, T.; Carpentier, J.-F. Organometallics 2009, 28, 1469.

Bouyahyi et al. Scheme 1

Chart 1. Facial and Meridional Isomers of Al(acacCF3,tBu)3 (2)

of the desired heteroleptic compounds Al(acacR1,R2)2X (X = Me, Cl). These observations indicate either that Al(acacR1,R2)2X compounds are thermodynamically unstable and undergo rapid disproportionation/ligand redistribution reactions and/or that the X functionality in Al(acacR1,R2)2X intermediates is much more reactive than that in starting precursors (i.e., AlMe3 and AlMe2Cl) toward pro-ligands (acacR1,R2)H. Expectedly, homoleptic compounds Al(acacR1,R2)3 (1, 2) were recovered in higher yields when the aluminum precursors were treated with 3 equiv of pro-ligands. The solid-state structure of the known compound Al(hfacac)3 (1)1b was determined for the first time (vide infra), and the new compound Al(acacCF3,tBu)3 (2) was characterized by elemental analysis and 1H, 19F{1H}, and 13C{1H} NMR spectroscopy. 19F NMR data for 2 in CD2Cl2 solution indicate the presence of both facial (fac) and meridional (mer) isomers (Chart 1),9 in a ca. 40:60 ratio.10 In the minor facial isomer, the three ligands surrounding the aluminum atom are magnetically equivalent due to the inherent C3 symmetry of the complex, which is confirmed by the observation of only one singlet resonance for the CF3 groups (δ -76.78). In contrast, the C1 symmetry of the mer-Al(acacCF3,tBu)3 complexes gives rise to the observation of three singlet resonances of equal relative intensity (δ -77.18, -76.95; the third one being obscured by that of fac-2). Those symmetry features were confirmed by low-temperature 1H NMR spectroscopy, with observation of one resonance for the tert-butyl and CH acac hydrogens in the fac-isomer and three resonances of equal intensity, respectively for each of these groups, in the mer-isomer (Figure 1). Attempts to separate those isomers of 2 by crystallization and to grow single crystals suitable for X-ray diffraction failed. In contrast to methyl and chloride compounds, heteroleptic complexes having alkoxide (i.e., isopropoxide) ligands proved reachable. The most effective route we found toward this class of compounds relies on the low-temperature reaction between AlMe2(OiPr) and the appropriate pro-ligand (acacR1,R2)H, which proceeds cleanly via methane elimination (Scheme 2). The desired compounds [Al(acacR1,R2)2(OiPr)]n (R1, R2=CF3, 3; R1=CF3, R2=tBu, 4; n=2 in the (9) (a) Cotton, F. A.; Wilkinson, G. In Advances in Inorganic Chemistry, 5th ed.; Wiley: New York, p 477. (b) Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.; Tsyba, I.; Ho, N. N.; Bau, R.; Thompson, M. E. J. Am. Chem. Soc. 2004, 125, 7377. (10) The fac/mer ratio did not change even after prolonged heating of a solution of 2 in toluene at 110 °C.

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Figure 1. Expansions of the 1H NMR spectrum (CD2Cl2, 500 MHz, -60 °C) of a 60:40 mixture of mer- (9) and fac-isomers (2) of Al(acacCF3,tBu)3 (2), showing the CH acac (left) and tert-butyl (right) regions. Scheme 2

Scheme 3

solid state) were thus prepared in 76-79% isolated yields, without noticeable contamination by side-products. Another route toward compounds 3 and 4, which is a priori as direct as and even more attractive than the aforementioned one considering the cheapness of the precursor used, is the alcohol (2-propanol) elimination reaction of proligands (acacR1,R2)H with Al(OiPr)3 (Scheme 3). As a matter of fact, this route, using a mixture of toluene and THF as solvent and a rather low reaction temperature, is as effective as the one starting from AlMe2(OiPr) (Scheme 2) in the case of (acacCF3,tBu)H for obtaining 4. However, when using (hfacac)H, this route turned out to be hampered by the formation of significant amounts of side-products. 1H and 19 F NMR spectroscopy of the crude product, recovered by evaporation of volatiles in vacuo, showed the formation of

Scheme 4

desired compound 3 (major product) but accompanied by other compounds, tentatively formulated as “Aln(hfacac)m(OiPr)p”. Attempts to isolate large amounts of analytically pure compound 3 by recrystallization of the crude product recovered following this route failed. Recrystallization from dichloromethane at -30 °C afforded only a batch of single crystals of 3 and a small amount of needle-like crystals of trinuclear compound [κ2:μ2-(hfacac)5(OiPr)4Al3] (5), which was identified by an X-ray diffraction analysis (vide infra). Interestingly, performing the same reaction between Al(OiPr)3 and (hfacac)H (2 equiv vs Al) but at a higher temperature (55 °C) for 48 h allowed us to isolate, in moderate yield (22%) after recrystallization, a quite different compound, namely, (S,R)-[κ,μ:κ2-(4H-hfacac)(hfacac)Al(THF)]2 (6) (Scheme 4).11 Compound 6 was characterized by elemental analysis, NMR spectroscopy in solution, and an X-ray diffraction study (vide infra). This product does not contain isopropoxide ligands. Rather, the Al centers in 6 are supported by a regular monoanionic ligand (hfacac)- and a bridging dianionic ligand (4H-hfacac)2- that formally arises from the reduction of one carbonyl group in (hfacac)- (Scheme 4). Indeed, this product is apparently the result of an in situ Meerwein-Pondorf-Verley (MVP) reduction12 of an ancillary (hfacac)- ligand by an active AlOiPr moiety.13 (11) Performing the same reaction from (acacCF3,tBu)H and Al(OiPr)3, at 50 °C for 48 h in toluene/THF, followed by recrystallization attempts, did not allow the isolation of the corresponding reduction product “[κ,μ:κ2-(H-acacCF3,tBu)(acacCF3,tBu)Al(THF)]2”. (12) General references on MVP reduction: (a) Surrey, A. R. Name Reactions in Organic Chemistry, 2nd ed.; Academic Press: New York, 1961. (b) Meerwein, H.; Schmidt, R. Just. Lieb. Ann. Chem. 1924, 444, 221. (c) Ponndorf, W. Angew. Chem. 1926, 39, 138. (d) Verley, A. Bull. Soc. Chim. Fr. 1925, 37, 537. (e) Jackman, L. M.; Mills, J. A. Nature 1949, 164, 789. (13) This MVP process is supposed to generate also acetone, but the latter compound could not be identified, probably because of the low amounts generated, its volatility, and possibly its reactivity in the reaction mixture as well.

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In order to assess how this reduction reaction takes place, a batch of isolated compound 3 (and, in parallel, its (acacCF3,tBu) analogue 4 as well) was treated under the same conditions ast those used for the in situ formation of 6 (toluene/THF, 55 °C, 3 days) (Scheme 5). However, the formation of 6 (or its analogue derived from 4) was never observed; compounds 3 and 4 were found to be inert under those conditions. These observations indicate that, most likely, 6 does not arise from the reduction of a coordinated (hfacac)- ligand by an isopropoxide moiety from either the same coordination sphere (intramolecular pathway) or another [Al(acacR1,R2)2(OiPr)]n (3, 4) molecule (intermolecular pathway). Rather, it suggests that the observed reduction of an (hfacac)- ligand (or in the (hfacac)H pro-ligand) occurs at an earlier stage of the reaction, where the initial precursor Al(OiPr)314 and/or intermediary species of the type “[(hfacac)Al(OiPr)2]” might still be available. Possible simplified mechanistic pathways for the production of 6 are shown in Scheme 6. This unique reaction clearly demonstrates that carbon atoms in (hfacac)- ligands (or in the (hfacac)H pro-ligand) cannot always be assumed to be completely unreactive in (or toward) aluminum complexes.15 Compound 3 is readily soluble in most organic solvents, while compounds 4-6 are readily soluble in chlorinated solvents (CH2Cl2, CHCl3) and THF, slightly soluble in aromatic hydrocarbons (toluene, benzene), and poorly soluble in aliphatic hydrocarbons (pentane, hexanes). These compounds are moderately sensitive to air but quite sensitive to moisture. In fact, during recrystallization attempts, isopropoxide complex 3 underwent adventitious hydrolysis in wet chlorinated solvents to afford the corresponding hydroxy complex [Al(hfacac)2(OH)]2 (7); compound 7 was reprepared on a larger scale by slow hydrolysis of 3 in a 10:1 mixture of wet dichloromethane and THF (Scheme 7). All these new compounds were characterized by elemental analysis, multinuclear NMR spectroscopy, and an X-ray diffraction study for 3, 5, 6, and 7 (vide infra). The 19F{1H} and 1H NMR data for 3 in CD2Cl2 at 25 °C suggest that this compound has a symmetric structure in solution (on the NMR time scale), as indicated by the observation of a single sharp resonance for the CF3 and CH hfacac groups, respectively. The signals for the CH3 and CH OiPr groups are, however, somewhat broadened under these conditions of solvent and temperature, which suggests that isomers might be present in slow exchange. Low-temperature (þ25 to -60 °C) 1H NMR spectra of 3 in CD2Cl2 were therefore recorded, but their complexity, with numerous resonances partly overlapping and showing variable (14) On the other hand, reactions of [Al(hfacac)2(OiPr)]2 (3) with Al(OiPr)3 led to complex mixtures of compounds, out of which 6 could not be isolated in pure form. (15) Certain metal-hexafluoroacetylacetonate compounds may be attacked at carbon by nucleophiles, such as hydroxyl anions; see for instance: van Eldik, R. Inorg. Chem. 1985, 24, 423.

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intensity ratios with temperature, hampered a more detailed analysis. Wengrovius et al. observed similar phenomena for the related complex [Al(OiPr)(acac)2]2 and suggested the latter compound exists in CD2Cl2 as two isomers, which were proposed to be the meso and d,l compounds, in relatively fast exchange above 30 °C (Scheme 8).16 Contrastingly, the 19F{1H} and 1H NMR spectra of 3 in THF-d8 at 25 °C each include two resonances of ca. 1:1 relative intensity for the CF3 and CH hfacac groups, respectively. Apparently, the two isomers (meso or d,l) are present in equal amounts and do not exchange under such conditions. On the other hand, the 19F{1H} and 1H NMR spectra for 7 in THF-d8 at 25 °C show only a single resonance for the CF3 and CH hfacac groups, in line with the observations made for 3 in CD2Cl2 at 25 °C. Although these phenomena were not investigated in more detail, these marked differences in the behavior of isopropoxide (3) and hydroxide (7) compounds illustrate the strong influence that the nature of the bridging group (OiPr and OH, respectively) and solvent may exert in the dynamic equilibria between the aforementioned isomers. Monitoring by 19F{1H} and 1H NMR spectroscopy of CD2Cl2 and THF-d8 solutions of 3 showed that this compound is stable at least for several days at room temperature and at least for hours up to 90 °C in toluene and up to 60 °C in THF; no formation of Al(hfacac)3 (1) and Al(OiPr)3 was observed. Further evidence of the nonformation of Al(OiPr)3 and stability of compound 3 was derived from catalytic experiments (vide infra). This stability contrasts with the instability of related [Al(OR)(acacR1,R2)2]2 complexes bearing non-fluorinated β-diketonate ligands (OR=OiPr, OMe; R1, R2 = Me, Et).16 Such compounds undergo disproportionation/ligand redistribution in CD2Cl2 or toluene-d8 within hours at 25 °C and more rapidly at higher temperatures (40-86 °C) and/or in the presence of added coordinating molecules such as Et2O, amines, and alcohols. In this study, it was also observed that the ethyl acetoacetate chelates are more stable than the β-diketonate complexes.16 Our observations confirm that the nature of the β-diketonate ligand affects significantly the stability of [Al(OR)(acacR1,R2)2]2 compounds and, more particularly, that (hfacac)- allows a kinetic stabilization of such heteroleptic complexes. The NMR spectra of compounds 4 and 6 in CD2Cl2 and THF-d8 are much more complicated, including notably several singlets in the 19F{1H} NMR spectrum (see the Experimental Section and Supporting Information). The complexity of these data likely indicates the presence of several isomers in solution. No obvious simplification was observed by lowering or increasing the temperature, and no informative data could be derived from these NMR studies. Solid-State Structures of 1, 3, 5, 6, and 7 3 (2THF). Single crystals suitable for X-ray diffraction were successfully grown for compounds 1, 3, 5, 6, and 7 3 (2THF). A summary of crystal and refinement data is given in Table 1. The crystal cell of Al(hfacac)3 (1) contains two independent molecules that are almost identical, with very similar bond distances and angles, so that only one of them is shown in Figure 2 and will be discussed. The molecular structure of this compound features a six-coordinate aluminum center in an almost perfect octahedral environment. In fact, the (16) Wengrovius, J. H.; Carbauskas, M. F.; Williams, E. A.; Going, R. C.; Donahue, P. E.; Smith, J. F. J. Am. Chem. Soc. 1986, 108, 982.

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Scheme 6. Possible Schematic Routes for the Formation of the Reduction Product 6

Scheme 7

Scheme 8. Possible meso and d,l Isomers in [Al(OiPr)(hfacac)2]2 (3)

O-Al-O bond angles deviate at most ca. 1° from the ideal values of 90° and 180°. Also, the Al-O distances are essentially identical (Al-O = 1.873(2)-1.883(2) A˚) and appear slightly smaller than those observed in the parent, non-fluorinated compound Al(acac)3 (Al-O = 1.892 A˚).17 This difference can be logically accounted for by the electron-withdrawing effect of the CF3 groups, which makes the metal center more electron-deficient. Consistently with other aluminum β-diketonate-alkoxide complexes such as [Al(acacR,R)2(OiPr)]2 (R=Me, Et),16 the solid-state structure of [Al(hfacac)2(OiPr)]2 (3) is dimeric, with μ-bridging isopropoxide ligands (Figure 3). The same type of dimeric structure is observed for [Al(hfacac)2(OH)]2 (7), with μ-bridging hydroxide groups (Figure 4). However, of the two possible isomers for a symmetric dimer, i.e., meso and d,l (Scheme 8), the molecule of 3 crystallizes selectively from CH2Cl2 at -30 °C in the d,l form, while the molecule of 7 crystallizes selectively as the meso isomer18 (as observed also in [Al(acacEt,Et)2(OiPr)]2).16 This difference in the coordination mode, simply changing the bridging group, likely reflects the significant flexibility of the (hfacac)- ligand. Apart from this difference in the nature of the isomer formed in the solid state, compounds 3 and 7 have quite similar geometrical features. Both aluminum centers lie in a (17) Hon, P. K.; Pfluger, C. E. J. Coord. Chem. 1973, 3, 67. (18) Several crystals of 3 and 7 obtained from different batches were examined and all found to be identical.

Figure 2. ORTEP view of Al(hfacac)3 (1) (ellipsoids drawn at the 50% probability level) (only one of the two independent molecules is depicted; the “#1” or “i” symbols in the atom labels indicate that these atoms are at equivalent position (-y þ 1, x y þ 1, z)). Selected bond lengths (A˚) and angles (deg): Al(1)-O(11), 1.873(2); Al(1)-O(12)#1, 1.883(2); O(11)-Al(1)-O(11)#1, 89.91(11); O(11)-Al(1)-O(12), 91.02(10); O(11)#1-Al(1)O(12), 89.61(10); O(11)-Al(1)-O(12)#1, 178.96(10); O(12)Al(1)-O(12)#1, 89.47(11).

slightly distorted octahedral environment. The spatial distance between the two metal centers in 3 (Al(1) 3 3 3 Αl(2) = 2.860 A˚) is slightly longer than that in 7 (Al(1) 3 3 3 Al(i) = 2.811 A˚), which can be simply accounted for by the larger steric bulk of bridging isopropoxide groups, as compared to hydroxides. The central cycle made by Al(1)O(25)O(15)Al(2) in 3 is perfectly planar with almost identical angles Al(2)-O(16)-Al(1) and Al(2)-O(15)-Al(1) (101.22° and 101.13°, respectively); due to the presence of a crystallographic inversion center, these angles are identical in 7 (Al(i)-O(15i)-Al(1) = Al(i)-O(15)-Al(1) = 100.35°). The Al-O distances involving the acac ligands lie in a limited range (1.906(4)-1.924(4) A˚ in 3, 1.902(4)-1.926(4) A˚ in 7) and are larger than the Al-O distances involving the bridging groups (1.836(4)-1.856(4) A˚ in 3, 1.829(17)-1.831(16) A˚ in 7). The Al-O(acac) bond distances appear larger than those observed in related complexes such as [Al(acac)(OSiMe3)2]2 (Al-O, 1.872(7)-1.886(8) A˚)19 and Al(hfacac)3 (19) Garbauskas, M. F.; Wengrovius, J. H.; Going, R. C.; Kasper, J. S. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1984, C40, 1536.

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Figure 3. ORTEP view of [Al(hfacac)2(OiPr)]2 (3) (ellipsoids drawn at the 50% probability level; all fluorine and hydrogen atoms have been omitted for clarity). Selected bond lengths (A˚) and angles (deg): Al(1)-O(25), 1.836(4); Al(1)-O(15), 1.856(4); Al(1)-O(12), 1.906(4); Al(1)-O(14), 1.914(4); Al(1)-O(13), 1.921(4); Al(1)-O(11), 1.925(4); Al(1)-Al(2), 2.860(3); O(25)-Al(1)-O(15), 79.08(17); O(25)-Al(1)-O(12), 98.04(18); O(15)-Al(1)-O(12), 176.72(18); O(25)-Al(1)-O(14), 91.14(16); O(15)Al(1)-O(14), 95.08(17); O(12)-Al(1)-O(14), 86.52(17); O(25)-Al(1)-O(13), 176.07(19); O(15)-Al(1)-O(13), 97.07(18); O(12)-Al(1)-O(13), 85.83(18); O(14)-Al(1)-O(13), 88.40(17); O(25)-Al(1)-O(11), 95.72(17); O(15)-Al(1)-O(11), 90.56(16); O(12)-Al(1)-O(11), 88.15(16); O(14)-Al(1)-O(11), 171.84(18); O(13)-Al(1)-O(11), 85.05(17).

Figure 4. ORTEP view of [Al(hfacac)2(OH)]2.(2THF) (7 3 (2THF)) (ellipsoids drawn at the 50% probability level; all fluorine atoms and the lattice THF molecule have been omitted for clarity; the “#1” or “i” symbols in the atom labels indicate that these atoms are at equivalent position (-x þ 1, -y, -z)). Selected bond lengths (A˚) and angles (deg): Al(1)-O(15), 1.8294(17); Al(1)-O(15)#1, 1.8308(16); Al(1)-O(13), 1.9023(17); Al(1)-O(12), 1.9128(17); Al(1)-O(14), 1.9219(17); Al(1)-O(11), 1.9264(17); Al(1)-Al(1)#1, 2.8111(13); O(15)-Al(1)-O(15)#1, 79.65(8); O(15)-Al(1)-O(13), 174.28(8); O(15)#1-Al(1)-O(13), 94.63(8); O(15)-Al(1)-O(12), 95.03(8); O(15)#1-Al(1)-O(12), 174.42(8); O(13)-Al(1)-O(12), 90.70(8); O(15)-Al(1)-O(14), 91.89(7); O(15)#1-Al(1)-O(14), 94.76(7); O(13)-Al(1)-O(14), 88.69(7); O(12)-Al(1)-O(14), 83.68(8); O(15)-Al(1)-O(11), 94.97(7); O(15)#1-Al(1)-O(11), 93.82(7); O(13)-Al(1)-O(11), 85.24(7); O(12)-Al(1)-O(11), 88.27(7); O(14)-Al(1)-O(11), 169.86(8); O(15)-Al(1)-Al(1)#1, 39.84(5); O(15)#1-Al(1)-Al(1)#1, 39.81(5); O(13)-Al(1)-Al(1)#1, 134.44(6); O(12)-Al(1)-Al(1)#1, 134.85(7); O(14)-Al(1)-Al(1)#1, 94.33(6); O(11)-Al(1)-Al(1)#1, 95.73(6).

Bouyahyi et al.

Figure 5. ORTEP view of [κ2:μ2-(hfacac)5(OiPr)4Al3]2 (5) (ellipsoids drawn at the 50% probability level; all fluorine and hydrogen atoms have been omitted for clarity). Selected bond lengths (A˚) and angles (deg): Al(1)-O(11), 1.941(5); Al(1)-O(12), 1.911(6); Al(1)-O(13), 1.930(5); Al(1)-O(14), 1.933(5); Al(1)-O(20), 1.848(5); Al(1)-O(23), 1.853(5); Al(2)-O(20), 1.876(5); Al(2)-O(21), 1.931(5); Al(2)-O(22), 1.957(5); Al(2)-O(23), 1.914(5); Al(2)-O(24), 1.920(6); Al(2)-O(35), 1.880(5); Al(3)-O(24), 1.845(5); Al(3)-O(31), 1.951(6); Al(3)-O(32), 1.923(5); Al(3)-O(33), 1.932(6); Al(3)-O(34), 1.939(6); Al(3)-O(35), 1.842(5); O(20)-Al(1)-O(23), 78.7(2); O(20)-Al(1)-O(12), 175.7(3); O(23)-Al(1)-O(12), 97.0(3); O(20)-Al(1)-O(13), 98.6(2); O(23)-Al(1)-O(13), 176.6(3); O(12)-Al(1)-O(13), 85.7(2); O(20)-Al(1)-O(14), 95.3(2); O(23)-Al(1)-O(14), 92.3(2); O(12)-Al(1)-O(14), 84.9(3); O(13)-Al(1)-O(14), 85.8(2); O(20)-Al(1)-O(11), 94.1(2); O(23)-Al(1)-O(11), 98.2(2); O(12)-Al(1)-O(11), 86.4(3); O(13)-Al(1)-O(11), 84.0(2); O(14)-Al(1)-O(11), 167.1(2); O(20)-Al(2)-O(35), 99.1(2); O(20)-Al(2)-O(23), 76.5(2); O(35)-Al(2)-O(23),101.9(2); O(20)-Al(2)-O(24), 99.4(2); O(35)-Al(2)-O(24), 75.9(2); O(23)-Al(2)-O(24), 175.1(2); O(20)-Al(2)-O(21), 90.8(2); O(35)-Al(2)-O(21), 164.5(2); O(23)-Al(2)-O(21), 91.9(2); O(24)-Al(2)-O(21), 90.8(2); O(20)-Al(2)-O(22), 166.5(3); O(35)-Al(2)-O(22), 87.1(2); O(23)-Al(2)-O(22), 90.5(2); O(24)-Al(2)-O(22), 93.7(2); O(21)-Al(2)-O(22), 85.8(2).

(1) (1.873(2)-1.883(2) A˚; vide supra). The Al-O(iPr) distances in 3 (1.836(4)-1.856(4) A˚) are smaller than those in the related but non-fluorinated congener [Al(acacEt,Et)2(OiPr)]2 (1.866(6)-1.868(6) A˚),16 a difference that again can be accounted for by the electron-withdrawing effect of the CF3 groups, which makes the aluminum center more electron-deficient. The molecular structure of [κ2:μ2-(hfacac)5(OiPr)4Al3] (5) in the solid state comprises a trinuclear compound with isopropoxide groups μ-bridging two aluminum centers (Figure 5). The two terminal aluminum centers each bear two (hfacac)- ligands, while the central aluminum center has only one (hfacac)- ligand. Each of the aluminum centers is thus in a six-coordinated environment, though the distortion of the ideal octahedral geometry is more pronounced than in compounds 1, 3, and 7. The three aluminum centers form an angle Al(1)-Al(2)-Al(3) of 134.36°. The spatial distances between these metal centers Al(1) 3 3 3 Al(2) 3 3 3 Al(3) are identical (2.914 A˚) and larger than those observed in compounds 3 and 7 (2.860 and 2.811 A˚, respectively), reflecting a more sterically crowded environment (in line with the aforementioned higher distortion). The Al-O distances involving the hfacac ligands and the bridging isopropoxide groups are in the ranges 1.911(6)-1.957(5) A˚ and 1.842(5)1.880(5) A˚, respectively. These distances can be compared to those observed in the related (although much less

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Scheme 9

Figure 6. ORTEP view of (S,R)-[κ,μ:κ2-(4H-hfacac)(hfacac)Al(THF)]2 (6) (ellipsoids drawn at the 50% probability level; all fluorine and hydrogen atoms have been omitted for clarity, except hydrogen atoms on C4 and C4i atoms; the “#1” or “i” symbols in the atom labels indicate that these atoms are at equivalent position (-x þ 1, -y, -z)). Selected bond lengths (A˚) and angles (deg): Al(1)-O(11), 1.809(3); Al(1)-O(12), 1.911(3); Al(1)-O(12)#1, 1.893(3); Al(1)-O(13), 1.894(3); Al(1)-O(14), 1.964(3); Al(1)-O(15), 1.961(3); Al(1)-Al(1)#1, 2.989(3); O(11)-C(2), 1.335(5); O(12)-C(4), 1.428(5); O(11)-Al(1)O(12)#1, 170.19(14); O(11)-Al(1)-O(13), 93.19(13); O(12)# 1-Al(1)-O(13), 96.09(13); O(11)-Al(1)-O(12), 94.25(13); O(12)#1-Al(1)-O(12), 76.40(13); O(13)-Al(1)-O(12), 172.39 (14); O(11)-Al(1)-O(15), 90.67(13); O(12)#1-Al(1)-O(15), 93.50(13); O(13)-Al(1)-O(15), 83.75(13); O(12)-Al(1)-O(15), 97.76(13); O(11)-Al(1)-O(14), 88.77(13); O(12)#1-Al(1)-O(14), 88.47(13); O(13)-Al(1)-O(14), 87.43(13); O(12)Al(1)-O(14), 91.12(13); O(15)-Al(1)-O(14), 171.12(14); O(12)-C(4)-C(3), 112.5(3); C(3)-C(2)-O(11), 127.9(4).

symmetric)20 trinuclear complex [Al(OiPr)2(acac)]3 (AlO(acac), 1.890(8)-1.919(7) A˚; Al-O(iPr), 1.873(7)1.907(7) A˚),16 evidencing again a noticeable reduction of the Al-O(iPr) bond distance when replacing the acac ligand by its fluorinated version, hfacac. The bond angles AlO(iPr)-Al in these two trinuclear compounds lie in the same ranges (101.26(2)-103.04(2)° in 5, 101.5(3)-103.6(3)° in [Al(OiPr)2(acac)]316). The oxygen atoms in the bridges are coplanar with the Al centers (Al(1)O(23)Al(2)O(20) and Al(2)O(35)O(34)Al(3)), with bond angles O-Al-O (76.5(2)-176.6(3)°) also comparable to those observed in [Al(OiPr)2(acac)]3 (74.4(3)-174.6(4)°).16 The molecular structure of (S,R)-[κ,μ:κ2-(4H-hfacac)(hfacac)Al(THF)]2 (6) is shown in Figure 6. A single diastereomer is observed in the solid state. This compound is a centrosymmetric dimer, with both Al centers in a distorted octahedral geometry (range for O-Al-O bond angles: 76.4(1)-172.4(1)°). Each aluminum atom is coordinated by one THF molecule, one κ2-hfacac ligand, and one reduced ligand {4H-hfacac}2- κ,μ-bridging via the oxygen atom (012) at the reduced carbon atom (C4). The central cycle Al(1i)O(12)O(12i)Al(1) is planar, with a spatial distance between the two Al of 2.990 A˚, even larger than that observed in dimeric compounds 3 and 7 (2.860 and 2.811 A˚, respectively) and in trinuclear complex 5 (2.914 A˚). This is not unexpected, (20) This compound features a terminal Al atom having two κ2coordinated acac ligands and one terminal Al center having only isopropoxide ligands, while the central Al center bears one κ2-coordinated acac ligand and four κ2-bridged isopropoxide groups.

considering the larger steric hindrance induced by the neighboring groups at the quaternary carbon atom (C4). The Al-O(acac) bond distance trans to the bridging O atom in the nonreduced ligand (Al(1)-O(13), 1.894(3) A˚) is shorter than the analogous one in the reduced ligand (Al(1)-O(11), 1.809(3) A˚). The Al-O bond distances involving the bridging reduced ligand (Al(1)-O(12), 1.911(3) A˚; Al(1)O(12)#1, 1.893(3) A˚) are comparable to those observed in the dimeric isopropoxide compound 3 (Al-Ο(iPr), 1.904 A˚). Finally, reduction at the C4 atom in one ligand is confirmed, for instance, by the much longer bond distance O(12)-C(4) of 1.428(5) A˚, as compared to those in a nonreduced ligand (i.e., O(13)-C(7), 1.274(5) A˚; O(14)-C(9), 1.264(5) A˚). Ring-Opening Polymerization of Racemic Lactide. The ring-opening polymerization (ROP) of racemic lactide (racLA) promoted by some of the obtained complexes was investigated under various conditions (Scheme 9). Representative results are reported in Table 1. Isopropoxide complexes [Al(hfacac)2(OiPr)]2 (3) and [Al(acactBu,CF3)2(OiPr)]2 (4) show interesting performances;comparable to those of some other aluminum catalysts/initiators8;for the ROP of rac-LA in THF or toluene solution and in solvent-free conditions, that is, in the melted monomer. Both compounds feature similar activities in toluene and in THF, with turnover frequencies (TOFs, up to 20 mol(LA) mol(Al)-1 h-1 at 100 °C) similar to those achieved with Al-{fluorinated dialkoxide-diimine} complexes.8l At 60-70 °C, those initiators require relatively long reaction times (24-48 h) to reach quantitative conversions of 200 equiv of monomer (entries 1-6). To enhance the activity, ROP experiments were therefore conducted at 100 °C, at higher monomer concentration ([rac-LA] = 5.0 M) (entries 7, 8). As anticipated, the reaction times necessary for reaching high conversions were significantly reduced under those conditions. For instance, 100% conversion of 200 equiv was achieved within 20 h (entry 7) vs 70% in 48 h at 70 °C (entry 6). On the other hand, ROP experiments conducted in molten lactide at 130 °C, equally with 3 or 4, did not lead to further increase in the activities (entries 12, 13). This observation contrasts with results obtained with {ONRNO}Al(OiPr) initiators, for which the activity was dramatically increased (TOFs = ca. 10 mol(LA) mol(Al)-1 h-1 in solution vs 1000 h-1 in melt).8l At the same time, we noticed that ROP experiments conducted in molten monomer at 130 °C led to PLAs with broad polydispersity indices (PDI = Mw/Mn = 2.2-2.5). This observation suggests the occurrence of side-reactions (notably transesterification, as actually attested by MALDI-TOF-MS analysis of PLA) and/or instability of the complexes at such a high temperature; decomposition or ligand redistribution reactions16 would lead to the concomitant formation of homoleptic Al(acacR1,R2)3 complexes and Al(OiPr)3 (which are respectively inactive and active;but poorly controlled in terms of polydispersity;in the ROP of LA, vide infra and entries 19, 20).

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Table 1. Ring-Opening Polymerization of rac-Lactide entry

complex

[LA]/[Al]

solvent

[LA] (mol/L)

temp (°C)

time (h)

conva (%)

1 2 3 4 5 6 7 8 9e 10f 11 12 13 14 15 16 17 18 19 20

3 3 3 3 3 3 3 3 3 3 3 3 3 4 4 7 7 1 Al(OiPr)3 Al(OiPr)3

100 200 200 50 100 200 200 500 500 500 200 100 200 100 400 100 100 100 200 200

THF THF THF toluene toluene toluene toluene toluene toluene toluene toluene -g -g toluene toluene toluene THF toluene toluene toluene

2.0 2.0 2.0 2.0 2.0 2.0 5.0 5.0 5.0 5.0 2.0 -g -g 2.0 2.0 2.0 2.0 2.0 2.0 2.0

60 60 60 70 70 70 100 100 100 100 70 130 130 70 70 70 60 70 70 70

48 48 120 24 48 48 20 48 20 20 120 24 24 48 48 48 48 48 48 120

90 42 56 100 86 70 97 70 81 94 81 82 90 99 97 0 0 0 60 95

Mn,calcb (g/mol) (103)

Mn(exp)c (g/mol) (103)

Mw/Mnc

Pmd (%)

12.9 12.1 16.1 7.2 12.4 20.1 27.9 50.4 9.7 6.7 23.3 11.8 25.9 14.4 55.8

11.1 10.1 14.3 6.5 12.9 14.4 18.8 30.3 6.0 4.2 16.5 19.8 34.5 15.2 40.1

1.20 1.16 1.15 1.15 1.24 1.19 1.3 1.3 1.10 1.12 1.25 2.5 2.2 1.5 1.3

57 nd nd 50 50 nd nd nd nd nd nd 58 60 62 59

5.8 9.1

4.3 10.7

2.4 1.7

nd nd

Conversion of monomer determined by 1H NMR spectroscopy. b Mn value calculated from the relation [LA]/[Al]  conv  144 or [LA]/[iPrOH]  conv  144. c Experimental Mn (corrected by a factor of 0.58)21c,23 and Mw/Mn values determined by GPC in THF vs polystyrene standards. d Pm is the probability of forming a new meso dyad and is determined from the methine region of the homonuclear decoupled 1H NMR spectrum. e Reaction conducted in the presence of 5 equiv of iPrOH vs Al. f 10 equiv of iPrOH. g Solvent-free polymerization conducted in molten lactide at 130 °C. a

Polymerizations conducted with 3 and 4 at lower temperatures (60-100 °C) are better controlled. Independently of the [monomer]/[initiator] ratios, solvent, and temperature used, those systems gave PLAs with relatively narrow molecular weight distributions (PDI e 1.30) and experimental molecular weights generally in good agreement with theoretical values (calculated assuming the growth of one PLA chain per Al center). These results indicate a significant degree of control and further confirm the aforementioned NMR observations that heteroleptic complexes 3 and 4 do not decompose under these conditions. The catalytic performances of 3 do not appear to be affected when the ROP reactions are performed in the presence of excess 2-propanol (5 and 10 equiv vs Al, entries 9 and 10, respectively). Conversions higher than 80% of 500 equiv of monomer were observed under these conditions, leading to PLAs with narrow polydispersities (PDI e 1.12) and molecular weights in good agreement with the amount of added alcohol. This behavior corresponds to an “immortal” polymerization.21 1 H NMR analyses of a relatively low molecular weight PLA sample established that the polymer chains are selectively capped by isopropoxy and hydroxy end-groups (see the Supporting Information). On the other hand, homodecoupled 1H NMR experiments conducted on representative PLAs indicated that only modest enrichment in isotactic sequences is achieved (Pm up to 62%).22 No obvious difference in the Pm values is observed when switching from [Al(hfacac)2(OiPr)]2 (3) to [Al(acacCF3,tBu)2(OiPr)]2 (4). In light of previous literature reports,6-8 one can reasonably assume that the poor stereoselectivity afforded by these (21) (a) Aida, T.; Inoue, S. Acc. Chem. Res. 1996, 29, 39. (b) Sugimoto, H.; Inoue, S. Adv. Polym. Sci. 1999, 146, 39–119. (c) Ma, H.; Okuda, J. Macromolecules 2005, 38, 2665. (d) Amgoune, A.; Thomas, C. M.; Carpentier, J.-F. Macromol. Rapid Commun. 2007, 28, 693. (e) Amgoune, A.; Thomas, C. M.; Carpentier, J.-F. Pure Appl. Chem. 2007, 79, 2013. (22) A control reaction performed with enantiomerically pure S,Slactide (L-LA) established that pure isotactic PLLA is produced with these systems. This observation confirms that no epimerization occurs under the conditions used.

systems arises from the limited steric hindrance brought by these β-diketonate ligands around the metal center. Finally, the hydroxy dimer [Al(hfacac)2(OH)]2 (7) and homoleptic complex Al(hfacac)3 (1) were found to be completely inactive in ROP of lactide in THF or toluene solutions under the conditions used for 3 and 4 (entries 16-18). These observations were not unexpected since complexes 1 and 7 do not contain any nucleophilic group likely to imitate the ROP process, in contrast to isopropoxide complexes 3 and 4.6

Conclusions In summary, we have shown that heteroleptic compounds bearing two fluorinated β-diketonate ancillaries and an isopropoxide group can be efficiently prepared by straightforward alkane elimination from the corresponding pro-ligands (acacR1,R2)H and AlMe2(OiPr) as the precursor, under mild conditions. These compounds [Al(acacR1,R2)2(OiPr)]n are stable in the solid state and in solution up to 90 °C, without undergoing ligand redistribution reactions, as earlier reported for non-fluorinated analogous compounds.16 For the preparation of [Al(hfacac)2(OiPr)]2 (3), the alkane elimination route is more selective than the alcohol elimination route starting from (hfacac)H and Al(OiPr)3. In this case, formation of more aggregated compounds has been evidenced when the reaction is carried under mild conditions, which is likely a consequence of the high bridging propensity of isopropoxide groups. Unexpectedly, reduction of the (hfacac)- ligand occurs when the aforementioned reaction between (hfacac)H and Al(OiPr)3 is performed at higher temperature. This rare reaction provides evidence that carbon atoms in (hfacac)- ligands (or in the (hfacac)H pro-ligand) cannot always be assumed to be completely unreactive in (or toward) aluminum complexes. The preliminary results obtained in ROP of racemic lactide show that [Al(acacR1,R2)2(OiPr)]n are valuable initiators/ catalysts, especially when the polymerizations are carried out in solution. These compounds afford a significant degree of control over the polymerizations in terms of molecular

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weights and molecular weights distributions and also allow performing immortal ROP in the presence of excess free alcohol as a transfer agent. Future work shall be directed toward the evaluation of similar heteroleptic complexes based on β-diketonate ligands having more sterically encumbered substituents, in order to possibly improve the stereoselectivity when using racemic monomers.

Experimental Section General Procedures. All experiments were carried out under purified argon using standard Schlenk techniques or in a glovebox. Hydrocarbon solvents, diethyl ether, and tetrahydrofuran were distilled from Na/benzophenone; toluene and pentane were distilled from Na/K alloy under nitrogen and degassed by freeze-vacuum-thaw cycles prior to use. Chlorinated solvents were distilled from calcium hydride. Deuterated solvents (>99.5% D, Eurisotop) were freshly distilled from the appropriate drying agent under argon and degassed prior to use. Hexafluoroacetylacetone ((hfacac)H, 98%, Aldrich), 1,1,1-trifluoro-5,5-dimethyl2,4-hexanedione ((acactBu,CF3)H, 98%, Aldrich), AlMe3 (2.0 M solution in heptane, Aldrich), AlMe2Cl (1.0 M solution in hexane, Aldrich), AlMe2(OiPr) (98%, Strem Chemicals), and Al(OiPr)3 (98%, Aldrich) were purchased and used as received. Racemic lactide (Aldrich) was recrystallized twice from dry toluene and then sublimed under vacuum at 50 °C before use. NMR spectra were recorded in Teflon-valved NMR tubes on Bruker AC-200, AC-300, and AM-500 spectrometers at 20 °C unless otherwise stated. 1H and 13C NMR chemical shifts were determined using residual solvent resonances and are reported versus SiMe4. Assignment of signals was made from 2D 1H-1H COSY and 1H-13C HMQC and HMBC NMR experiments. 19F chemical shifts were determined by external reference to an aqueous solution of NaBF4. Elemental analyses (C, H, N) were performed using a Flash EA1112 CHNS Thermo Electron apparatus and are the average of two independent determinations. HR-ESI-MS spectra were obtained on a high-resolution MS/MS Micromass ZABSpecTOF (4 kV) spectrometer. Size exclusion chromatography (SEC) of PLAs was performed in THF at 20 °C using a Polymer Laboratories PL-GPC 50 Plus apparatus (PLgel 5 μm MIXED-C 300  7.5 mm, 1.0 mL/min, RI and dual angle LS (PL-LS 45/90) detectors). The number average molecular masses (Mn) and polydispersity indices (Mw/ Mn) of the polymers were calculated with reference to a universal calibration versus polystyrene standards. Mn values of PLAs were corrected with a Mark-Houwink factor of 0.58,21c,23 to account for the difference in hydrodynamic volumes with polystyrene. The microstructure of PLAs was determined by homodecoupling 1 H NMR spectroscopy at 20 °C in CDCl3 with a Bruker AC-500 spectrometer. Al(hfacac)3 (1).1b Under stirring, a solution of hexafluoroacetylacetone ((hfacac)H) (300 mg, 1.44 mmol) in toluene (0.5 mL) was added dropwise (over ca. 10 min) to a solution of AlMe3 (0.36 mL of a 2.0 M solution in heptane, 0.72 mmol) in toluene (2 mL) cooled at -78 °C. The reaction mixture was slowly warmed to room temperature and stirred for 24 h. The reaction was concentrated to ca. 2 mL and left at -30 °C. After a few days, white crystals of 1 formed, which were separated from the solution and dried under vacuum (256 mg, 55%). Crystals suitable for X-ray diffraction were selected from this batch. 1H NMR (500 MHz, CD2Cl2, 298 K): δ 6.57 (s, 3H, CH hfacac). 19 F{1H} NMR (188 MHz, CD2Cl2, 298 K): δ -77.33 (s, 18F). 13 C{1H} NMR (75 MHz, CD2Cl2, 298 K): δ 181.28 (q, 2JCF = 38 Hz, C(O) hfacac), 116.78 (q, 1JCF = 310 Hz, CF3), 93.90 (CH hfacac). Anal. Calcd for C15H3AlF18O6: C, 27.80; H, 0.47. Found: C, 28.0; H, 0.4. (23) Barakat, I.; Dubois, P.; Jerome, R.; Teyssie, P. J. Polym. Sci., A: Polym. Chem. 1993, 31, 505.

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Al(acactBu,CF3)3 (2). This compound was prepared as described above for 1, starting from a solution of AlMe3 (0.50 mL of a 2.0 M solution in heptane, 1.02 mmol) in toluene (2 mL) and a solution of (acactBu,CF3)H (600 mg, 3.06 mmol) in toluene (0.5 mL). The reaction mixture was warmed to room temperature and then stirred for 24 h at 60 °C. Removal of volatiles under vacuum, washing of the solid residue with cold hexane (5 mL), and final drying under vacuum afforded 2 as a white powder (468 mg, 75%). 1H, 19F{1H}, and 13C{1H} NMR revealed that this compound is a ca. 60:40 mixture of two isomers (mer-2, major; fac-2, minor). 1H NMR (500 MHz, CD2Cl2, 298 K): fac-2, δ 6.15 (s, 3H, CH acac), 1.15 (s, 27H, C(CH3)3); mer-2, δ 6.15 (s, 2H, CH acac), 6.14 (s, 1H, CH acac), 1.15 (s, 18H, C(CH3)3), 1.14 (s, 9H, C(CH3)3). 1H NMR (500 MHz, CD2Cl2, 213 K): fac-2, δ 6.18 (s, 3H, CH acac), 1.14 (s, 27H, C(CH3)3); mer-2, δ 6.13 (s, 1H, CH acac), 6.21 (s, 1H, CH acac), 6.22 (s, 1H, CH acac), 1.145 (s, 9H, C(CH3)3) 1.14 (s, 9H, C(CH3)3) 1.13 (s, 9H, C(CH3)3). 19F{1H} NMR (188 MHz, CD2Cl2, 298 K): fac-2, δ -76.78 (s, 9F); mer-2, δ -77.18 (s, 3F), -76.95 (s, 3F), -76.78 (s, 3F). 13C{1H} NMR (75 MHz, C6D6, 298 K): fac-2, δ 210.43 (C(O)(tBu)), 171.51 (q, 2JCF = 35 Hz, C(O)(CF3)), 118.46 (q, 1JCF = 282 Hz, CF3), 92.80 (CH acac), 41.79 (C(CH3)3), 26.83 (C(CH3)3); mer-2 (some resonances overlap), δ 210.76, 210.26 ((C(O)(tBu)), 171.65 (q, 2JCF = 36 Hz, C(O)(CF3)), 171.36 (q, 2JCF = 36 Hz, C(O)(CF3)), 128.97, 125.23 (q, 1JCF = 282 Hz, CF3), 93.13, 92.41 (CH acac), 41.83, 41.73 (C(CH3)3), 26.73, 22.66 (C(CH3)3). Anal. Calcd for C24H30AlF9O6: C, 47.07; H, 4.94. Found: C, 47.2; H, 4.9. Synthesis of [Al(hfacac)2(OiPr)]2 (3) and Isolation of [κ2:μ2-(hfacac)5(OiPr)4Al3] (5). (a) Synthesis from AlMe2(OiPr): In a Schlenk flask, a solution of AlMe2(OiPr) (200 mg, 1.72 mmol) in toluene (3 mL) was cooled at -78 °C, and a solution of (hfacac)H (756 mg, 3.64 mmol) in THF (0.6 mL) was slowly added in via syringe under magnetic stirring. The reaction mixture was slowly warmed to room temperature and stirred for 24 h. Volatiles were removed under vacuum, and the solid residue was washed with cold hexanes (2  10 mL) and finally dried under vacuum for 12 h, to leave 3 as a white powder (653 mg, 76%). Single crystals of 3 suitable for X-ray diffraction analysis were obtained from a concentrated dichloromethane solution at -30 °C. (b) Synthesis from Al(OiPr)3: This reaction was carried out as described above starting from a solution of Al(OiPr)3 (350 mg, 1.72 mmol) in toluene (10 mL) and a solution of (hfacac)H (756 mg, 3.64 mmol) in THF (1 mL). Workup afforded a white powder (653 mg), which proved by 19F NMR to be a mixture of 3 and [κ,κ:μ2-(hfacac)5(OiPr)4Al3] (5). Recrystallization of this crude product from dichloromethane at -30 °C afforded single crystals of 3 as well as a small amount of needle-like crystals of 5 that proved to be suitable for X-ray diffraction analysis. Complex 3: 1H NMR (500 MHz, CD2Cl2, 298 K): δ 6.46 (s, 4H, CH hfacac), 4.05 (m, 2H, Al-OCH), 0.90-1.20 (m, 12H, OCH(CH3)2). 1H NMR (500 MHz, THFd8, 298 K): δ 6.90 (s, 2H, CH hfacac), 6.80 (s, 2H, CH hfacac), 4.10-4.20 (m, 2H, Al-OCH), 1.05-1.14 (m, 12H, OCH(CH3)2). 19 F{1H} NMR (188 MHz, CD2Cl2, 298 K): δ -77.80 (s, 24F). 19 F{1H} NMR (188 MHz, THF-d8, 298 K): δ -80.08 (s, 12F), -79.78 (s, 12F). 13C{1H} NMR (75 MHz, THF-d8, 298 K): δ 179.01 (q, 2JCF = 38 Hz, C(O) hfacac), 115.17 (q, 1JCF = 282 Hz, CF3), 91.35 (CH hfacac), 60.78 (OCH(CH3)2), 20.59 (OCH(CH3)2). Anal. Calcd for C26H18Al2F24O10: C, 31.22; H, 1.81. Found: C, 31.0; H, 2.0. Complex 5: 19F{1H} NMR (182 MHz, CD2Cl2, 298 K): δ -77.40 (s, 24F), -75.65 (s, 6F). The 1H NMR spectrum (182 MHz, CD2Cl2, 298 K) contained many broadened resonances and proved uninformative. [Al(acactBu,CF3)2(OiPr)]2 (4). (a) Synthesis from AlMe2(OiPr): This reaction was performed as described above for 3, starting from a solution of AlMe2(OiPr) (88 mg, 0.76 mmol) in toluene (3 mL) and a solution of (acactBu,CF3)H (300 mg, 1.53 mmol) in THF (0.8 mL). Workup afforded complex 4 as a white powder (286 mg, 79%). (b) Synthesis from Al(OiPr)3: This reaction was

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carried out as described above for 3, starting from a solution of Al(OiPr)3 (155 mg, 0.76 mmol) in toluene (5 mL) and a solution of (acactBu,CF3)H (300 mg, 1.53 mmol) in THF (1 mL). Workup afforded complex 4 as a white powder (286 mg, 79%). NMR revealed that this compound is a mixture of different isomers. 1H NMR (500 MHz, CD2Cl2, 298 K): δ 6.02-6.15 (m, 4H, CH acac), 4.05-4.25 (m, 2H, Al-OCH), 1.19-1.24 (m, 12H, OCH(CH3)2), 1.08-1.15 (several singlets, 36H, C(CH3)3). 19F{1H} NMR (188 MHz, CD2Cl2, 298 K) (major resonances): δ -77.20 (s), -77.11 (s), -77.06 (s), -76.96 (s), -75.99 (s), -75.85 (s), -75.74 (s), -75.66 (s). 13C{1H} NMR (75 MHz, CD2Cl2, 298 K): δ 210.01, 209.51, 207.81, 207.40 (C(O)C(CH3)3), 174.09, 173.57, 173.13, 169.64 (C(O)(CF3)), 121.60 (q, 1JCF = 280 Hz, CF3), 117.95 (q, 1JCF = 280 Hz, CF3), 92.75, 92.48, 91.74, 91.61 (CH acac), 64.81, 64.70, 64.59, 64.39 (OCH(CH3)2), 41.76, 41.35, 41.26, 41.22 (C(CH3)3), 27.11, 26.89, 26.72, 26.65 (C(CH3)3), 23.46, 23.30, 22.80, 22.37 (OCH(CH3)2). Anal. Calcd for C38H54Al2F12O10: C, 47.90; H, 5.71. Found: C, 47.8; H, 5.8. (S,R)-[K,μ:K2-(4H-hfacac)(hfacac)Al(THF)]2 (6). A solution of (hfacac)H (500 mg, 2.40 mmol) in THF (0.5 mL) was added dropwise (over ca. 10 min) to a solution of Al(OiPr)3 (245 mg, 1.20 mmol) in toluene (2.5 mL) at room temperature. The reaction mixture was stirred at 55 °C for 72 h. Volatiles were removed under vacuum, and the residue was redissolved in THF (5 mL). Recrystallization of this solution at -30 °C for 8 days afforded colorless crystals of 6, which were separated from the solution and dried under vacuum (135 mg, 22%). Crystals suitable for X-ray diffraction were selected from this batch. 1 H NMR of those crystals in CD2Cl2 and THF-d8 showed many overlapping resonances. 19F{1H} NMR (188 MHz, CD2Cl2, 298 K): δ -77.83 (s, 6F), -77.53 (s, 18F) (only the two major resonances are listed). 19F{1H} NMR (188 MHz, THF-d8, 298 K): δ -68.26 (s, 18F), -67.99 (s, 6H). Anal. Calcd for C28H22Al2F24O10: C, 32.70; H, 2.16. Found: C, 32.8; H, 2.2. [Al(hfacac)2(OH)]2 (7). This compound was initially obtained by adventitious hydrolysis of 3. It was reprepared on a larger scale by slow hydrolysis of 3 (350 mg, 0.35 mmol) in a 10:1 mixture of wet dichloromethane and THF (5 mL). Recrystallization of this solution at -30 °C afforded colorless crystals of 7 3 (THF)2 (240 mg, 73%). Crystals suitable for X-ray diffraction were selected from this batch. 1H NMR (300 MHz, THF-d8, 298 K): δ 6.55 (s, 4H, CH hfacac); in addition, signals for free THF were observed at δ 1.78 (m, 4H, β-CH2 THF) and 3.62 (m, 4 H, R-CH2 THF). 19F{1H} NMR (188 MHz, THF-d8, 298 K): δ -77.51 (s, 24F). 13C{1H} NMR (125 MHz, THF-d8, 298 K): δ 178.96 (q, 2JCF = 38 Hz, C(O) hfacac), 118.17 (q, 1JCF = 310 Hz, CF3), 93.12 (CH hfacac); in addition, signals for free THF were observed at δ 68.22 (R-CH2 THF) and 26.39 (β-CH2 THF). Anal. Calcd for C28H22Al2F24O12 (7 3 (2THF)): C, 31.71; H, 2.09. Found: C, 31.9; H, 2.0. Crystal Structure Determination of 1, 3, 5, 6, and 7 3 (2THF). Suitable crystals for X-ray diffraction analysis of 1, 3, 5, and 6 were obtained by recrystallization of purified products; crystals of 7 3 (2THF) were obtained upon attempting recrystallization of complex 3 (vide supra). Diffraction data were collected at 100 K using a Bruker APEX CCD diffractometer with graphite-monochromatized Mo KR radiation (λ=0.71073 A˚). 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 remaining atoms were located from difference Fourier synthesis followed by full matrix least-squares refinement based on F2 (24) (a) Sheldrick, G. M. SHELXS-97, Program for the Determination of Crystal Structures; University of Goettingen: Germany, 1997. (b) Sheldrick, G. M. SHELXL-97, Program for the Refinement of Crystal Structures; University of Goettingen: Germany, 1997.

Bouyahyi et al. (programs SHELXS-97 and SHELXL-97).24 Many hydrogen atoms could be found from the Fourier difference analysis. Carbon- and oxygen-bound 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. In 1, 3, and 7 3 (2THF), some trifluoromethyl groups were found to be disordered and accordingly modelized. In the case of 7 3 (2THF), the crystal structure has been solved in the P3c1 space group by direct methods using the SIR2004 program,25 which revealed the molecular skeleton; this raw crystal structure has been further completed using successive difference Fourier map analysis and refinement runs with SHELXL97,26 but still remained at high Bragg factors (R1 ∼18%). Finally, a twinning law (1 1 0/0 -1 0/0 0 1) has been found using the TwinRotMat routine included in the PLATON package27 and introduced in the refinement procedure, leading to a drastic drop in the reliability refinement factors and a fractional contribution of second twin domain of 0.22. Typical Procedure for the ROP of Racemic Lactide in Toluene Slurry or THF Solution. A Schlenk flask was charged with a solution of a given Al complex in toluene or THF. To this solution was rapidly added under stirring a solution of rac-LA in the appropriate ratio in toluene or THF, respectively, to reach a final lactide concentration of 2.0 or 5.0 M. The reaction mixture was stirred at the desired temperature for a given reaction time. After an aliquot of the crude solution was removed for analytical purposes, the reaction was quenched with acidic methanol (0.5 mL), and the polymer was precipitated with excess methanol. The polymer was then dried in vacuo to constant weight. Typical Procedure for Racemic Lactide Polymerization in Melt. In the glovebox, a ca. 10 mL glass lump was charged with complex 3, rac-LA (to set up the appropriate [LA]/[Al] ratio), and a magnetic stir bar. The lump was sealed under vacuum and immerged in an oil bath at 130 °C with magnetic stirring. After 72 h, the lump was taken off, cooled to room temperature, and opened in air. An aliquot was analyzed by 1H NMR spectrometry for determining the conversion. Acidic methanol (ca. 0.5 mL of a 1.2 M HCl solution in CH3OH) was added in the lump, and the polymer was precipitated with excess methanol (ca. 3 mL). Then, the supernatant solution was removed with a pipet and the polymer was dried under vacuum to constant weight. Supporting Information Available: Tabulated summary of crystal and refinement data, and crystallographic data as a combined cif file for complexes 1, 3, 5, 6, and 7 3 (2THF). 1H NMR spectrum of a PLA sample showing the terminal groups. This material is available free of charge via the Internet at http:// pubs.acs.org.

Acknowledgment. M.B. thanks the Ministere de l’Enseignement Superieur et de la Recherche for a Ph.D. fellowship. J.F.C. gratefully thanks the Institut Universitaire de France for a Junior IUF fellowship (2005-2009). (25) Burla, M. C.; Caliandro, R.; Camalli, M.; Carrozzini, B.; Cascarano, G. L.; De Caro, L.; Giacovazzo, C.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 2005, 38, 381. (26) SHELXL97; Universit€at G€ottingen: Germany, 1998 (27) (a) Spek, A. L. Acta Crystallogr., Sect. A 1990, 46, C34. (b) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool; Utrecht University: Utrecht, The Netherlands, 1998.