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

Abstract

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 fac- and 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·mol−1) and relatively narrow polydispersities (Mw/Mn = 1.10−1.32).