The Reduction of Dioxygen by Keggin Heteropolytungstates

Reversible redox chemistries are an inherent feature of numerous metal oxide cluster anions (POMs). Moreover, as discrete molecular structures with well-defined and controllable solution chemistries, POMs can be deployed as physicochemical probes for studying inorganic reaction mechanisms. In the past decade, we have used an iso-structural series of alpha-Keggin heteropolytungstate cluster anions to systematically investigate a number of fundamental topics, including electron transfer to dioxygen. The iso-structural series of cluster anions is obtained by varying the heteroatom, Xn+, in the plenary, T-d-symmetry alpha-Keggin ion, Xn+ W12O40(8-n)-, from Al3+ to Si4+ to P5+. This results in a step-wise and linear modulation of ion charge and reduction potential, whose concerted effects on reaction rates can be used to better understand electron-transfer processes. Starting from the acquisition of activation parameters associated with electron self-exchange between the POMs themselves, the studies discussed in this review provide a detailed account of electron transfer from reduced alpha-Keggin heteropolytungstate anions to dioxygen, culminating in the recent discovery of a fundamentally new mechanism for electron transfer to O-2 in water.