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-R-3-aminomethyl)-4-R-2-6-R-1-phenols (R-1 = Bu-t, R-2 = Me, R-3 = ethyl, 1; R-1 = R-2 = cumyl, R-3 = ethyl, 2; R-1 = R-2 = cumyl, R-3 = Bu-t, 3) with either one or half equivalent of AlMe3 exclusively afforded monoligated complexes (LAlMe2)-Al-Bu,Et (1a), (LAlMe2)-Al-Cumyl,Et (2a) and (LAlMe2)-Al-Cumyl,Bu (3a), whereas the treatment of 2-(N-benzyl-N-R-3-aminomethyl)-4,6-dichlorophenols (R-3 = ethyl, 4; R-3 = Bu-t, 5) with 0.5 equivalent of AlMe3 resulted in the formation of (L-Cl,L-Et)(2)AlMe (4a) or (L-Cl,L-Bu)(2)AlMe(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 epsilon-caprolactone. (C) 2013 Elsevier B.V. All rights reserved.