Comparative Study of the Coordination Chemistry and Lactide Polymerization of Alkoxide and Amide Complexes of Zinc and Magnesium with a β-Diiminato Ligand Bearing Ether Substituents

A series of β-diiminato complexes of the form (BDI-3)MX where (BDI-3) = [CH(CMeNC6H4-2-OMe)2]; M = Zn, Mg; X = OiPr, OtBu, or N(SiMe3)2 has been synthesized. The (BDI-3) ligand is bidentate in (BDI-3)ZnN(SiMe3)2 and tetradentate in (BDI-3)MgN(SiMe3)2. The alkoxide complexes are shown to be active for lactide polymerization. Polymerization of rac-lactide with (BDI-3)ZnOiPr gives a moderate preference for heterotactic PLA. Polymerization of rac-lactide with [(BDI-3)MgOtBu]2 shows a slight preference for heterotactic PLA in CH2Cl2 but is highly stereoselective in THF in the production of heterotactic PLA.