Bridged Bis(amidinate) Ytterbium Alkoxide and Phenoxide: Syntheses, Structures, and Their High Activity for Controlled Polymerization of l-Lactide and ε-Caprolactone

Bridged bis(amidinate) ytterbium alkoxide and phenoxide with diverse molecular structures were synthesized in high yields and confirmed by X-ray crystal structural analysis. The reaction of LYbCl(THF)2 (L = Me3SiNC(Ph)N(CH2)3NC(Ph)NSiMe3) with 1 equiv of NaOAr (ArO = 2,6-diisopropylphenoxo) afforded the mononuclear complex LYb(OAr)DME 1 with a seven-coordinated ytterbium atom surrrounded by one chelating bis(amidinate) ligand, one phenoxo group, and one DME (dimethoxyethane) molecule. The same reaction with 1 equiv of NaOiPr yielded the binuclear complex Yb(μ2-L)2(μ2-OiPr)2Yb, 2, with two equivalent six-coordinate metal centers connected by two linked bis(amidinate)s and two OiPr bridges formed via a ligand redistribution reaction that occurred during the metathesis reaction. Both 1 and 2 initiated the ring-opening polymerization of l-lactide, as well as ε-caprolactone (ε-CL), in a controlled manner with high reactivity, as indicated by a linear relationship between Mn and conversion and by narrow molecular weight distributions (PDI = 1.15−1.25) up to 100% conversion. The differences in catalytic performance between complexes 1 and 2 are discussed.