Symmetric and Unsymmetric Nickel(II) Schiff Base Complexes; Metal-Localized Versus Ligand-Localized Oxidation

The oxidation of symmetric and unsymmetric nickel(II) Schiff base complexes was examined in acetonitrile by cyclic voltammetry. Unlike nickel(II) bis(salicylaldimine) complexes which undergo oxidative polymerization at the electrode surface, the complexes examined in this study contain at least one β-ketoimine chelate and are irreversibly oxidized at the electrode surface. The mixed chelate complexes are oxidized at potentials midway between those of the symmetric bis(salicylaldimine) and bis(β-ketoimine) complexes, suggesting a metal-localized rather than a ligand-localized oxidation. Oxidation of nickel(II) to nickel(III) followed by rapid intramolecular electron transfer to give reactive ligand-radical species is proposed to explain the irreversible oxidation of the nickel(II) Schiff base complexes.