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

Alkane elimination reactions of rare earth metal tris(alkyl)s, Ln(CH2SiMe3)3(THF)2 (Ln = Y, Lu) with the multidentate ligands HL1–4, afforded a series of new rare earth metal complexes. Yttrium complex 1 supported by flexible amino-imino phenoxide ligand HL1 was isolated as homoleptic product. In the reaction of rigid phosphino-imino phenoxide ligand HL2 with equimolar Ln(CH2SiMe3)3(THF)2, HL2 was deprotonated by the metal alkyl and its imino CN 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 HL2 to Y(CH2SiMe3)3(THF)2 increased to 2:1. Reaction of steric phosphino β-ketoiminato ligand HL3 with equimolar Ln(CH2SiMe3)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 HL4 with Y(CH2SiMe3)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.

Graphical abstractA series of organolanthanide complexes stabilized by phenoxide with phosphino, amino or methoxy amino functionalities and phosphino β-ketoiminato ligands have been prepared by treatment of rare earth metal tris(alkyl)s with the neutral ligands via alkane elimination and intramolecular alkylation in some cases. The molecular structures and catalytic activity toward polymerization of lactide of the resultant complexes have shown significant dependence on the ligand framework.Figure optionsDownload full-size imageDownload as PowerPoint slide