Aluminum complexes of bidentate phenoxy-amine ligands: Synthesis, characterization and catalysis in ring-opening polymerization of cyclic esters

A series of aluminum complexes 1a–5a bearing bidentate phenoxy-amine ligands were synthesized and characterized by NMR spectroscopy and elemental analyzes. The variation of the ortho-substituent of phenoxy moiety and the amino group of the ancillary ligands has crucial influence on the stoichiometric structure of the obtained aluminum complexes. Reaction of 2-(N-benzyl-N-R3-aminomethyl)-4-R2-6-R1-phenols (R1 = tBu, R2 = Me, R3 = ethyl, 1; R1 = R2 = cumyl, R3 = ethyl, 2; R1 = R2 = cumyl, R3 = tBu, 3) with either one or half equivalent of AlMe3 exclusively afforded monoligated complexes LBu,EtAlMe2 (1a), LCumyl,EtAlMe2 (2a) and LCumyl,BuAlMe2 (3a), whereas the treatment of 2-(N-benzyl-N-R3-aminomethyl)-4,6-dichlorophenols (R3 = ethyl, 4; R3 = tBu, 5) with 0.5 equivalent of AlMe3 resulted in the formation of (LCl,Et)2AlMe (4a) or (LCl,Bu)2AlMe(HLCl,Bu) (5a) respectively. All of these aluminum methyl complexes can efficiently polymerize rac-lactide to atactic polymers and actively initiate the ring-opening polymerization of ɛ-caprolactone.