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 α-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 α-Keggin ion, Xⁿ⁺W ₁₂O₄₀^(8-n)-, from Al³⁺ to Si ⁴⁺ to P⁵⁺. This results in a stepwise 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 α-Keggin heteropolytungstate anions to dioxygen, culminating in the recent discovery of a fundamentally new mechanism for electron transfer to O₂ in water.