Amphoteric reactivity of metal–oxygen complexes in oxidation reactions

Mononuclear metal–oxygen species, such as metal-superoxo, -peroxo, -hydroperoxo and -oxo complexes, are key intermediates involved in dioxygen activation and oxidation reactions catalyzed by a variety of metalloenzymes and their biomimetic compounds. Dioxygen is an electrophile or electron acceptor, but not a nucleophile or electron donor. However, when dioxygen is bound to metal complexes and metal–oxygen species are formed, such as metal-superoxo, -peroxo, -hydroperoxo, alkylperoxo, and -oxo complexes, the metal–oxygen intermediates react as nucleophiles and electron donors as well as electrophiles and electron acceptors. This review is intended to focus on such an amphoteric reactivity of the metal–oxygen complexes in electrophilic and nucleophilic reactions. Both the electronic and steric effects of the ligands have finely tuned the reactivity of metal–oxygen complexes in both the electrophilic and nucleophilic reactions. The amphoteric reactivity of metal–oxygen complexes can also be tuned by binding of redox-inactive metal ions acting as Lewis acids and also by binding of Bronsted acids.