Reactivities and Electronic Structures of μ-1,2-Peroxo and μ-1,2-Superoxo CoCo Complexes: Electrophilic Reactivity and O Release Induced by Oxidation.

Peroxo complexes are key intermediates in water oxidation catalysis (WOC). Cobalt plays an important role in WOC, either as oxides CoO or as {Co(μ1,2-peroxo)Co} complexes, which are the oldest peroxo complexes known. The oxidation of {Co(μ1,2-peroxo)Co} complexes had usually been described to form {Co(μ1,2-superoxo)Co} complexes; however, recently the formation of {Co(μ1,2-peroxo)Co} species were suggested. Using a bis(tetradentate) dinucleating ligand, we present here the synthesis and characterization of {Co(μ1,2-peroxo)(μ-OH)Co} and {Co(μ-OH)Co} complexes. Oxidation of {Co(μ1,2-peroxo)(μ-OH)Co} at -40 °C in CHCN provides the stable {Co(μ1,2-superoxo)(μ-OH)Co} species and activates electrophilic reactivity. Moreover, {Co(μ1,2-peroxo)(μ-OH)Co} catalyzes water oxidation, not molecularly but rather via CoO films. While {Co(μ1,2-peroxo)(μ-OH)Co} can be reversibly deprotonated with DBU at -40 °C in CHCN, {Co(μ1,2-superoxo)(μ-OH)Co} undergoes irreversible conversions upon reaction with bases to a new intermediate that is also the decay product of {Co(μ1,2-superoxo)(μ-OH)Co} in aqueous solution at pH > 2. Based on a combination of experimental methods, the new intermediate is proposed to have a {Co(μ-OH)Co} core formed by the release of O from {Co(μ1,2-superoxo)(μ-OH)Co} confirmed by a 100% yield of O upon photocatalytic oxidation of {Co(μ1,2-peroxo)(μ-OH)Co}. This release of O by oxidation of a peroxo intermediate corresponds to the last step in molecular WOC.