Reactivities and Electronic Structures of μ-1,2-Peroxo and μ-1,2-Superoxo CoIIICoIII Complexes: Electrophilic Reactivity and O2 Release Induced by Oxidation

Abstract

Peroxo complexes are key intermediates in water oxidation catalysis (WOC). Cobalt plays an important role in WOC, either as oxides CoO_x or as {Co^III(μ-1,2-peroxo)Co^III} complexes, which are the oldest peroxo complexes known. The oxidation of {Co^III(μ-1,2-peroxo)Co^III} complexes had usually been described to form {Co^III(μ-1,2-superoxo)Co^III} complexes; however, recently the formation of {Co^IV(μ-1,2-peroxo)Co^III} species were suggested. Using a bis(tetradentate) dinucleating ligand, we present here the synthesis and characterization of {Co^III(μ-1,2-peroxo)(μ-OH)Co^III} and {Co^III(μ-OH)₂Co^III} complexes. Oxidation of {Co^III(μ-1,2-peroxo)(μ-OH)Co^III} at -40 °C in CH₃CN provides the stable {Co^III(μ-1,2-superoxo)(μ-OH)Co^III} species and activates electrophilic reactivity. Moreover, {Co^III(μ-1,2-peroxo)(μ-OH)Co^III} catalyzes water oxidation, not molecularly but rather via CoO_x films. While {Co^III(μ-1,2-peroxo)(μ-OH)Co^III} can be reversibly deprotonated with DBU at -40 °C in CH₃CN, {Co^III(μ-1,2-superoxo)(μ-OH)Co^III} undergoes irreversible conversions upon reaction with bases to a new intermediate that is also the decay product of {Co^III(μ-1,2-superoxo)(μ-OH)Co^III} in aqueous solution at pH > 2. Based on a combination of experimental methods, the new intermediate is proposed to have a {Co^II(μ-OH)Co^III} core formed by the release of O₂ from {Co^III(μ-1,2-superoxo)(μ-OH)Co^III} confirmed by a 100% yield of O₂ upon photocatalytic oxidation of {Co^III(μ-1,2-peroxo)(μ-OH)Co^III}. This release of O₂ by oxidation of a peroxo intermediate corresponds to the last step in molecular WOC.