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

A series of aluminum and zinc complexes supported by functionalized phenolate ligands were synthesized and characterized. Reaction of 2-(3,5-R2C3N2)C6H4NH2 (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 AlR32 (R-2 = Me, Et) gave corresponding aluminum aryloxides 3a-3e, while reaction with an equiv. of ZnEt2 afforded zinc aryloxides 4a-4d. Treatment of 2c with 0.5 equiv. of ZnEt2 formed diphenolato zinc complex 5. All new compounds were characterized by H-1 and C-13 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 epsilon-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. (C) 2008 Elsevier B.V. All rights reserved.