Catalytic Activity of a Series of Zn(II) Phenoxides for the Copolymerization of Epoxides and Carbon Dioxide

A series of zinc phenoxides of the general formula (2,6-R2C6H3O)2Zn(base)2 [R = Ph, tBu, iPr, base = Et2O, THF, or propylene carbonate] and (2,4,6-Me3C6H2O)2Zn(pyridine)2 have been synthesized and characterized in the solid state by X-ray crystallography. All complexes crystallized as four-coordinate monomers with highly distorted tetrahedral geometry about the zinc center. The angles between the two sterically encumbering phenoxide ligands were found to be significantly more obtuse than the corresponding angles between the two smaller neutral base ligands, having average values of 140° and 95°, respectively. In a noninteracting solvent such as benzene or methylene chloride at ambient temperature, the ancillary base ligands are extensively dissociated from the zinc center, with the degree of dissociation being dependent on the base as well as the substituents on the phenolate ligands. That is, stronger ligand binding was found in zinc centers containing electron-donating tert-butyl substituents as opposed...