Rare earth metal alkyl complexes bearing N, O, P multidentate ligands: Synthesis, characterization and catalysis on the ring-opening polymerization of l-lactide

Abstract

Alkane elimination reactions of rare earth metal tris(alkyl)s, Ln(CH 2SiMe 3) 3(THF) 2 (Ln = Y, Lu) with the multidentate ligands HL 1–4, afforded a series of new rare earth metal complexes. Yttrium complex 1 supported by flexible amino-imino phenoxide ligand HL 1 was isolated as homoleptic product. In the reaction of rigid phosphino-imino phenoxide ligand HL 2 with equimolar Ln(CH 2SiMe 3) 3(THF) 2, HL 2 was deprotonated by the metal alkyl and its imino C N group was reduced to C–N by intramolecular alkylation, generating THF-solvated mono-alkyl complexes ( 2a: Ln = Y; 2b: Ln = Lu). The di-ligand chelated yttrium complex 3 without alkyl moiety was isolated when the molar ratio of HL 2 to Y(CH 2SiMe 3) 3(THF) 2 increased to 2:1. Reaction of steric phosphino β-ketoiminato ligand HL 3 with equimolar Ln(CH 2SiMe 3) 3(THF) 2 afforded di-ligated mono-alkyl complexes ( 4a: Ln = Y; 4b: Ln = Lu) without occurrence of intramolecular alkylation or formation of homoleptic product. Treatment of tetradentate methoxy-amino phenol HL 4 with Y(CH 2SiMe 3) 3(THF) 2 afforded a monomeric yttrium bis-alkyl complex of THF-free. The resultant complexes were characterized by IR, NMR spectrum and X-ray diffraction analyses. All alkyl complexes exhibited high activity toward the ring-opening polymerization of l-lactide to give isotactic polylactide with controllable molecular weight and narrow to moderate polydispersity.