Chapter 12 Electron Transfer to Dioxygen by Keggin Heteropolytungstate Cluster Anions

摘要

This chapter describes recent developments in understanding electron transfer to dioxygen (O_2) and the outer-sphere oxidation of the superoxide radical anion, O _2 ~(??), by metal complexes. The following topics, of broad spectrum value in the complex chemistry of polyoxometalate (POM) systems and quantitative electron transfer are addressed: the nature of electron self-exchange between POMs; electron self-exchange between O_2 and O _2 ~(??) (including the problem of size differences between O_2 or O _2 ~(??) and their typical metal-complex electron donors or acceptors); and reactions of the one-electron-reduced POMs with O_2. Electron transfer from Keggin POMs to O_2 occurs by an outer-sphere mechanism; the effect of POM charge on rate constants for the reduction of O_2 is significant and attributable to anion-anion repulsion within the successor-complex ion pairs formed between the negatively charged POM anions and O _2 ~(??). These findings were followed by the discovery of a concerted proton-electron (CPET) pathway for electron transfer to O_2 at lower pH values (<1).