Zinc complexes supported by multidentate aminophenolate ligands: synthesis, structure and catalysis in ring-opening polymerization of rac-lactide.

Monomeric zinc silylamido and ethyl complexes bearing tetradentate aminophenolato ligands [(DNNO)ZnR] (D = NMe(2), OMe; R = N(SiMe(3))(2) (1-5, 8), Et (6, 7, 9, 10)), were isolated from the reaction of ZnN(SiMe(3))(2) or ZnEt(2) and one equivalent of aminophenols {aryl-CH(2)N[(CH(2))(2)NMe(2)]CH(2)-phenol} in moderate to high yields. The monomeric nature of these complexes was further confirmed by X-ray diffraction studies of silylamido complexes 1, 3 and ethyl complexes 7, 9, 10. The methoxy or N,N-dimethylamino group of the aryl unit does not coordinate with the metal center in the solid state, only the remaining three donors of the ligand and silylamido or ethyl group interact with zinc center constructing a distorted tetrahedral coordination geometry at the metal center. All these zinc complexes efficiently initiated the ring-opening polymerization of rac-lactide, and the polymerization runs were better controlled in the presence of isopropanol, giving atactic PLAs end-capped with isopropyl ester and hydroxyl groups. The structure of the ancillary ligands showed some influence on the catalytic activity and selectivity of the corresponding zinc complexes. The introduction of bulky ortho-substituents on the phenoxy unit resulted in a decrease of the polymerization rate, whereas the isotactic dyad selectivity in the ROP of rac-lactide was enhanced.