Preparation and structures of enantiomeric dinuclear zirconium and hafnium complexes containing two homochiral N atoms, and their catalytic property for polymerization of rac-lactide

Abstract

The hydroxy- and phenoxy-bridged dinuclear zirconium and hafnium complexes 2 and 3 of an amine-pyridine-bis(phenolate) ligand (H₂L) were prepared from the reaction of Na₂L and ZrCl₄ in the presence of 0.5 equiv of water in THF, while the reaction of H₂L with NaH and then with ZrCl₄ in the freshly distilled THF gave an eight-coordinate bis(homoleptic) mononuclear complex L₂Zr (1). Two homochiral N atoms were created in each complex upon coordination of the unsymmetric tertiary amine ligands to the metal centers. Enantiomeric dinuclear complexes 2 and 3 both undergo spontaneous resolution during crystallization. The molecular structure of each enantiomer of 2 and 3, as well as the structure of 1, was determined by X-ray analysis and the circular dichroism spectra of N(R)N(R)-2 and N(S)N(S)-2 were studied. The racemates of 2 and 3 catalyzed controlled polymerization of rac-lactide in terms of the linearity of molecular weight versus conversion, forming isotactic-rich polylactide in high yields with a narrow polydispersity.