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