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.