Direct Determination of Electron-Transfer Properties of Dicopper-Bound Reduced Dioxygen Species by a Cryo-Spectroelectrochemical Approach.

Abstract : Direct experimental determination of redox properties of superoxo (O2*-) and peroxo (O22-) embedded in dicopper complexes bearing an unsymmetrical binucleating ligand was achieved using cryo-electrochemistry and cryo-spectroelectrochemistry in dichloromethane. Cyclic voltammetry for dicopper(I) (1+) oxidation to a Cu(I)Cu(II) mixed-valent species (12+) under inert atmosphere reveals slow heterogeneous electron-transfer kinetics, indicative of a large reorganization energy. Oxygenation of the dicuprous complex 1+ gives the bridged peroxo dicopper(II) species 3+ which is reversibly oxidized to the superoxo complex 22+ at E0 = 0.11 V (vs SCE) with a small inner sphere electron-transfer reorganization energy, lambda_i = 0.54 eV, determined from variable temperature electrochemical impedance spectroscopy. The data suggest that the O2*-/O22- redox process occurs directly on the O2-derived core.