Syntheses and structures of lanthanide borohydrides supported by a bridged bis(amidinate) ligand and their high activity for controlled polymerization of ε-caprolactone, l-lactide and rac-lactide

Abstract

Metathesis reaction of LLnCl(THF)₂ [L = (Me₃SiNC(C₆H₅)N)₂(CH₂)₃] with NaBH₄ in a 1 : 1.5 molar ratio in THF (THF = tetrahydrofuran) at 60 °C afforded the monoborohydride LLn(BH₄)(DME) [Ln = Y (1), Nd (2), Sm(3) and Yb(4)] crystallized from DME solution (DME = dimethoxyethane). Crystal structure analyses revealed 1–4 are monomers, in which each metal is ligated by one L ligand, one η³-BH₄ group and one DME molecule in a trigonal bipyramid geometry. Complexes 1–4 were found to be very active single-site initiators for the controlled ring opening polymerization of ε-caprolactone (ε-CL) and L-lactide (L-LA) as judged by relatively narrow molecular weight distributions (M_w/M_n: 1.34–1.50) and experimental values M_n(exp) were in good agreement with theoretic values M_n(theo). The highest activity and the best control over the molecular weight for both monomers were found for the system with 2. These monoborohydride complexes can also initiate the ring opening polymerization of rac-LA to gave heterotactically enriched polyLA with Pr (heterotactic enrichment) values in a range of 0.69–0.85 depending on the lanthanide metals and the most effective heterotactic enrichment (Pr) was found for 1 (Pr = 0.85). Moreover, complex 1 initiated the polymerization of rac-LA in a living fashion.