Aluminum and zinc complexes supported by functionalized phenolate ligands: Synthesis, characterization and catalysis in the ring-opening polymerization of ε-caprolactone and rac-lactide

Abstract

A series of aluminum and zinc complexes supported by functionalized phenolate ligands were synthesized and characterized. Reaction of 2-(3,5-R 2C 3N 2)C 6H 4NH 2 (R = Me, Ph) with salicylaldehyde or 3,5-di- tert-butylsalicylaldehyde afforded 2-((2-(1H-pyrazol-1-yl)phenylimino)methyl)phenol derivatives 2a– 2d. Treatment of 2a– 2d with an equiv. of AlR 2 3 (R 2 = Me, Et) gave corresponding aluminum aryloxides 3a– 3e, while reaction with an equiv. of ZnEt 2 afforded zinc aryloxides 4a– 4d. Treatment of 2c with 0.5 equiv. of ZnEt 2 formed diphenolato zinc complex 5. All new compounds were characterized by 1H and 13C NMR spectroscopy and elemental analyses. The structures of complexes 3a, 4a and 5 were further characterized by single crystal X-ray diffraction techniques. The catalytic activity of complexes 3– 5 toward the ring-opening polymerization of ε-caprolactone was studied. The zinc complexes ( 4a– 4d) exhibited higher catalytic activity than the aluminum complexes ( 3a– 3e). The diphenolato zinc complex 5 showed lower catalytic activity than the ethylzinc complexes 4a– 4d. The aluminum complex ( 3b) is inactive to initiate the ROP of rac-lactide, while the zinc complex ( 4d) is active initiator for the ROP of rac-lactide, giving atactic polylactide.