Synthesis and characterization of dinuclear rare-earth complexes supported by amine-bridged bis(phenolate) ligands and their catalytic activity for the ring-opening polymerization of l-lactide

Abstract

Reactions of amine-bridged bis(phenolate) protio-ligands N,N-bis(3,5-di-tert-butyl-2-hydroxybenzyl)aminoacetic acid (L¹-H₃) and N,N-bis[3,5-bis(α,α′-dimethylbenzyl)-2-hydroxybenzyl]aminoacetic acid (L²-H₃), with 1 equiv. M[N(SiMe₃)₂]₃ (M = La, Nd, Sm, Gd, Y) in THF at room temperature yielded the neutral rare-earth complexes [M₂(L)₂(THF)₄] (L = L¹, M = La (1), Nd (2), Sm (3), Gd (4), Y (5); L = L², M = La (6), Nd (7), Sm (8), Gd (9), Y (10)). All of these complexes were characterized by single-crystal X-ray diffraction, elemental analysis and in the case of yttrium and lanthanum complexes, ¹H NMR spectroscopy. The molecular structure of 1 revealed dinuclear species in which the eight-coordinate lanthanum centers were bonded to two oxygen atoms of two THF molecules, to three oxygen atoms and one nitrogen atom of one L¹ ligand, and two oxygen atoms of the carboxyl group of another. Complexes 2–10 were also dinuclear species containing seven-coordinate metal centers similar to 1, albeit with bonding to one rather than two carboxyl group oxygens of another ligand. Further treatment of 1 with excess benzyl alcohol provided dinuclear complex [La₂(L¹)₂(BnOH)₆] (11), in which each lanthanum ion is eight-coordinate, bonded to three oxygen atoms and one nitrogen atom of one ligand, three oxygen atoms of three BnOH molecules, as well as one oxygen atom of bridging carboxyl group of the other ligand. In the presence of BnOH, complexes 1–10 efficiently catalyzed the ring-opening polymerization of L-lactide in a controlled manner and gave polymers with relatively narrow molecular weight distributions. The kinetic and mechanistic studies associated with the ROP of L-lactide using 1/BnOH initiating system have been performed.