Lanthanide Alkyl Complexes Supported by a Piperazidine-Bridged Bis(phenolato) Ligand: Synthesis, Structural Characterization, and Catalysis for the Polymerization of L-Lactide and rac-Lactide

A series of lanthanide alkyl complexes supported by a piperazidine-bridged bis(phenolato) ligand were synthesized, and their catalytic activity for the polymerization of L-lactide was explored. The alkane elimination reaction of Ln(CH2SiMe3)(3)(THF)(2) with H-2[ONNO] {H-2[ONNO] = 1,4-bis(2-hydroxy-3,5-di-tert-butylbenzyl)piperazidine} in a 1:1 molar ratio in THF gave the neutral lanthanide alkyl complexes [ONNO]Ln(CH2SiMe3)(THF) [Ln = Y (1), Li (2), Yb (3), Gd (4)] in high isolated yields. Treatment of a gadolinium tris(alkyl) complex formed in situ from the reaction of anhydrous GdCl3 with 3 equiv of Li(CH2SiMe3) in THF gave a novel "ate" gadolinium alkyl complex, {[ONNO]Gd(CH2SiMe3)(mu-Li)(mu-Cl)}(2) (5). All of these complexes are fully characterized including X-ray structural determination. Complexes 1-4 are isomorphous, monomeric, and THF-solvated. The coordination geometry around the lanthanide metals can be best described as a distorted trigonal bipyramid. Complex 5 is dimeric and unsolvated, and each [ONNO]Gd(CH2SiMe3) moiety is connected by two mu-Cl and two mu-Li to form a distorted seven-coordinated capped trigonal-prismatic geometry. It was found that complexes 1-4 are highly efficient initiators for the controlled ring-opening polymerization of L-lactide, giving polymers with high molecular weights and narrow molecular weight distributions, whereas complex 5 exhibited apparently low activity for this polymerization. Complex 1 can also initiate rac-lactide polymerization with high activity, but the stereoselectivity is poor.