Dioxygen activation chemistry by synthetic mononuclear nonheme iron, copper and chromium complexes

The activation of dioxygen (O₂) by metalloenzymes proceeds by binding O₂ at their active sites and then generating highly reactive, thermally unstable metal-oxygen intermediates, such as metal-superoxo, -(hydro)peroxo and -oxo species, via electron and proton transfer reactions. The synthesis, characterization and reactivity studies of the chemical model compounds of the key metal-oxygen intermediates can provide vital insights into the chemistry of such enzymatic reactions, and our understanding of the biologically important metal-oxygen intermediates has improved greatly by the success of synthesizing their analogues recently. In this article, we provide a focused review on the recent advances in the dioxygen activation processes at biomimetic iron, copper and chromium centers, paying particular emphasis to the factors that control the O₂-activation reactions, such as the effects of ligands, redox potentials and spin-states of biomimetic compounds. Among the most significant findings of these studies are the use of O₂ as an oxygen source in the generation of iron-oxygen intermediates and the autocatalytic radical chain reactions involved in the iron-mediated O₂-activation processes. Similarly, new approaches to achieve less overpotential have been identified, which is more desirable for the catalytic four-electron reduction of O₂ using copper complexes. In addition, the versatility of metal-superoxo species as reactive intermediates in various oxidation reactions has been elegantly demonstrated in the recent synthesis of a mononuclear nonheme chromium(III)-superoxo complex. This review will provide clues that lesson us how synthetic and mechanistic developments in biomimetic research can advance our understanding of O₂-activation processes in enzymatic reactions.