Aluminum Complexes of Fluorinated β-Diketonate Ligands: Syntheses, Structures, Intramolecular Reduction, and Use in Ring-Opening Polymerization of Lactide

Routes toward heteroleptic Al complexes supported by fluorinated beta-diketonate ligands have been studied. Reactions of pro-ligands (acac(R1,R2))H (R-1 = R-2 = CF3, "(hfacac)H"; R-1 = CF3, R-2 = tBu) with 0.5 equiv of AlMe3 or AlMe2Cl Systematically yielded the corresponding homoleptic complexes Al(hfacac)(3) (1) and Al(acac(CF3,tBu))(3) (2). Compound 2 exists in CD2Cl2 Solution as a mixture of fac- and mer-isomers. Heteroleptic complexes [Al(acac(R1,R2))(2)(OiPr)](n) (R-1 = R-2 = CF3, 3; R-1 = CF3, R-2 = tBu, 4) were cleanly prepared front the reaction of the corresponding (acac(R1,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 [kappa(2):mu(2)-(hfacac)(5)(OiPr)(4)Al-3] (5) Was isolated. When the same reaction was carried Out at 55 degrees C, (S,R)-[kappa,mu:kappa(2)-(4H-hfacac)(hfacac)Al(THF)](2) (6), which contains a mu-bridging dianionic ligand (4H-hfacac)(2-) that arises from the reduction of one carbonyl group in (hfacac)(-), was isolated ill 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 mu-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 mu-isopropoxide groups to a central Al atom supported by I 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 WIT and OH groups. with controlled molecular Weights (M-n up to 30 300 g . mol(-1)) and relatively narrow polydispersities (M-w/M-n = 1.10-1.32).