Ligand-Derived Oxidase Activity. Catalytic Aerial Oxidation of Alcohols (Including Methanol) by Cu(II)-Diradical Complexes

Abstract

Seven new bis(o-iminosemiquinonato)copper(II) complexes, 1−5, 1a, 1b, derived from differently substituted N-phenyl-2-aminophenol-based ligands, are described. Their crystal structures were determined by X-ray diffraction, and their electronic structures were established by various physical methods including electron paramagnetic resonance and variable-temperature (2−290 K) susceptibility measurements. Like complex 6, which was reported recently by us, all complexes exhibit an S_t = ¹/₂ ground state, based on the “isolated” copper(II)-spin character resulting from the dominating antiferromagnetic spin coupling between the two radicals; the ground-state electronic configuration can thus be designated as (↑↑↓)[R-Cu-R]. In addition, broken spin symmetry density functional solutions have been obtained. From the set of unrestricted canonical Kohn−Sham orbitals, the magnetic orbitals have been identified. The identification procedure is based on the nonvanishing overlap integrals between the space parts of orbitals occupied by electrons of opposite spin. The theoretically determined magnetic orbitals support the spin configurations suggested by the experiments. Electrochemical measurements (cyclic voltammetry and square-wave voltammetry) indicate ligand-centered redox processes. Complex 1 is found to be the best catalyst among the Cu(II) complexes for oxidation of primary alcohols with aerial oxygen as the sole oxidant to afford aldehydes under mild conditions. Thus, the function of the copper-containing enzyme Galactose Oxidase has been mimicked. Kinetic measurements in conjunction with electron paramagnetic resonance and electronic spectral studies have been used to decipher the catalytic oxidation process. A ligand-derived redox activity has been proposed as a mechanism for the aerial oxidation of primary alcohols.