Synthesis of Rare-Earth Metal Amides Bearing an Imidazolidine-Bridged Bis(phenolato) Ligand and Their Application in the Polymerization of L-Lactide

A series of neutral rare-earth metal amides supported by an imidazolidine-bridged bis(phenolato) ligand were synthesized, and their catalytic activity for the polymerization Of L-lactide was explored. The amine elimination reactions of Ln[N(TMS)(2)](3)(mu-Cl)Li(THF)(3) with H-2[ONNO] {H-2[ONNO] = 1,4-bis(2-hydroxy-3,5-di-tert-butyl-benzyl)-imidazolidine} in a 1:1 molar ratio in tetrahydrofuran (THF) gave the neutral rare-earth metal amides [ONNO]Ln[N(TMS)(2)](THF) [Ln = La (1), Pr (2), Nd (3), Sm (4), Yb (5), and Y (6)] in high isolated yields, All of these complexes are fully characterized. X-ray structural determination revealed that complexes 1-6 are isostructural and have a solvated monomeric structure. The coordination geometry around each of the rare-earth metal atoms can be best described as a distorted trigonal bipyramid. It was found that complexes 1-6 are efficient initiators for the ring-opening polymerization of L-lactide, and the ionic radii of the central metals have a significant effect on the catalytic activity, A further study revealed that these rare-earth metal amides can initiate L-lactide polymerization in a controlled manner in the presence of 1 equiv of isopropyl alcohol.