Bridged bis(amidinate) ytterbium alkoxide and phenoxide: syntheses, structures, and their high activity for controlled polymerization of L-lactide and epsilon-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 = Me(3)SiNC(Ph)N(CH(2))(3)NC(Ph)NSiMe(3)) with 1 equiv of NaOAr (ArO = 2,6-diisopropylphenoxo) afforded the mononuclear complex LYb(OAr)DME 1 with a seven-coordinated ytterbium atom surrounded by one chelating bis(amidinate) ligand, one phenoxo group, and one DME (dimethoxyethane) molecule. The same reaction with 1 equiv of NaO(i)Pr yielded the binuclear complex Yb(mu(2)-L)(2)(mu(2)-O(i)Pr)(2)Yb, 2, with two equivalent six-coordinate metal centers connected by two linked bis(amidinate)s and two O(i)Pr 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 epsilon-caprolactone (epsilon-CL), in a controlled manner with high reactivity, as indicated by a linear relationship between M(n) 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.