2023 Volume 82 Pages 13-28
Metal-oxygen/water complexes such as metal-oxo, metal-hydroxo, metal-aqua, metal-peroxo, metal-hydroperoxo and metal-superoxo complexes play pivotal roles as reactive intermediates in various redox catalytic reactions including biological redox processes such as oxygen reduction in respiration and water oxidation in photosynthesis. This award article focuses on our recent progress in characterization of these metal-oxygen intermediates in terms of the structure, spectroscopic properties and redox reactivity. Ever since the X-ray crystal structure of a nonheme iron(IV)-oxo complex was reported, a number of metal-oxo, metal-hydroxo, metal-aqua, metal-peroxo, metal-hydroperoxo and metal-superoxo complexes were structurally characterized by X-ray crystallography. The reactivity of these metal-oxygen/water complexes was investigated extensively. Inspired by the pivotal role of calcium ion in the oxygen evolving complex in photosystem II, the effects of redox-inactive metal ions such as calcium ion on the redox reactivity of metal-oxygen/aqua complexes were studied together with binding of metal ions to metal-oxygen/aqua complexes. Metal-oxygen/water complexes exhibit electrophilic and nucleophilic reactivity depending on the metal, oxygen/water-derived ligand and supporting ligands. The electron-transfer reactivity of metal-oxygen/water complexes has been well evaluated in light of the Marcus theory of outer-sphere electron transfer.
