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

Abstract

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 Ca2+ in the Mn4CaO5 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 O2− to Zn2+ facilitates the electron-transfer reduction of O2−, whereas Zn2+ ion that is bridged with Cu2+ by imidazolate accelerates the electron-transfer oxidation of O2−. 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 gzz values of EPR spectra of O2−–metal ion complexes, whereas a quantitative measure of Lewis acidity of paramagnetic redox-active metal ions such as Fe3+ and Co2+ is provided from the emission maxima of 10-methylacridone–metal ion complexes. The MCET reactivity of FeIV-oxo, MnIV-oxo, FeIII-peroxo, and CrIII-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 gzz values of EPR spectra of O2−–metal ion complexes and the emission maxima of metal ion–10-methylacridone complexes.