Two New Dinuclear Sterically Crowed Cu~(II) Complexes as Catalyst Precursors for the Oxidative Coupling of 2,6-Dimethylphenol

摘要

The molecular structure and spectroscopic properties of the first two bis(μ-η~1-nitrato)-bridged, dinuclear Cu complexes containing 4,4'-biimidazole ligands are presented. ~1H NMR, EPR, and UV/Vis spectroscopy indicate that their solid-state structure is not retained in solution. Dioxygen uptake measurements show that the presence of a methyl group at the 2-position of the imidazole rings leads to a dramatic increase of the rate of the catalytic polymerization of 2,6-dimethylphenol. Detailed examination of the molecular structures of both catalyst precursors in the crystalline state suggests that this may either be caused by the difference in basicity of the biimidazole ligands or by a small but important steric effect of the 2-Me groups affecting the coplanarity of equatorial Cu~(II) ligands and the orientation of the axial nitrate ligands.