Metal ion-coupled electron-transfer reactions of metal-oxygen complexes

• Metal ion-coupled electron-transfer reactivity of metal-oxo complexes. • Tuning of reactivity of metal-peroxo, -superoxo species by metal ions. • Enhanced catalytic reactivity of metal-hydroperoxo species by metal ions. • Understanding of electron-transfer driving force dependence of metal-ion coupled electron-transfer reactions by Marcus plots. Redox-inactive and -active metal ions that act as Lewis acids are essential cofactors in modulating the redox reactivity of metal–oxygen complexes and metalloenzymes, such as the manganese(V)-oxo intermediate in the oxygen-evolving complex, where Ca 2+ in the Mn 4 CaO 5 cluster is indispensable for the catalytic water oxidation in Photosystem II. Zinc ion is also an essential cofactor in Cu-Zn superoxide dismutases (Cu,Zn-SOD), in which binding of O 2 − to Zn 2+ facilitates the electron-transfer reduction of O 2 − , whereas Zn 2+ ion that is bridged with Cu 2+ by imidazolate accelerates the electron-transfer oxidation of O 2 − . This review focuses on the effects of redox-inactive and –active metal ions that act as Lewis acids on the electron-transfer and redox reactivity of metal–oxygen intermediates such as metal-oxo, metal-peroxo and metal-superoxo complexes. Such electron-transfer reactions from electron donors to acceptors enhanced by binding of metal ions to electron acceptors is referred as metal ion-coupled electron transfer (MCET). Lewis acid metal ions can bind to metal-oxo, metal-peroxo and metal-superoxo complexes to enhance the MCET reactivity of these metal–oxygen species. A quantitative measure of Lewis acidity of redox-inactive diamagnetic metal ions is obtained from the g zz values of EPR spectra of O 2 − –metal ion complexes, whereas a quantitative measure of Lewis acidity of paramagnetic redox-active metal ions such as Fe 3+ and Co 2+ is provided from the emission maxima of 10-methylacridone–metal ion complexes. The MCET reactivity of Fe IV -oxo, Mn IV -oxo, Fe III -peroxo, and Cr III -superoxo complexes is enhanced by Lewis acids and the logarithm of the enhancement of the rate constant by metal ions is correlated linearly with the quantitative measure of Lewis acidity of metal ions obtained from the g zz values of EPR spectra of O 2 − –metal ion complexes and the emission maxima of metal ion–10-methylacridone complexes.