Reactivities and Electronic Structures of μ-1,2-Peroxo and μ-1,2-Superoxo Co III Co III Complexes: Electrophilic Reactivity and O 2 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 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) 2 Co III } complexes. Oxidation of {Co III (μ - 1,2-peroxo)(μ-OH)Co III } at -40 °C in CH 3 CN 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 CH 3 CN, {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 O 2 from {Co III (μ - 1,2-superoxo)(μ-OH)Co III } confirmed by a 100% yield of O 2 upon photocatalytic oxidation of {Co III (μ - 1,2-peroxo)(μ-OH)Co III }. This release of O 2 by oxidation of a peroxo intermediate corresponds to the last step in molecular WOC.