Ionic-strength dependence of electron-transfer reactions of Keggin heteropolytungstates: Mechanistic probes of O₂ activation in water

Kinetic data needed to use α-AlW₁₂O₄₀^6- (α-2_1e) as well-defined probe for investigating electron-transfer processes in water is evaluated using the extended Debye-Hückel equation. First, rate constants, as a function of ionic strength in water, are determined for electron self-exchange reactions between a series of AlW₁₂O₄₀^n- ions (α and β isomers of fully oxidized, one- and two-electron reduced ions). The kinetic data are plotted using the extended Debye-Hückel equation, in which the distance of closest contact is set equal to twice the hydrodynamic radius of a Keggin anion. In all cases, slopes of these plots give charge-product (z₁z₂) values within error of those anticipated based on the charges of the reacting ions. The aqueous solution chemistry of α-AlW₁₂O₄₀^5- (α-2_ox) and α-AlW₁₂O₄₀^6- (α-2_1e) is then fully defined, and the reduction potential of the α-2_ox/α-2_1e couple is determined. These studies provide the information needed to use α-2_1e, a powerful reducing agent, to obtain detailed information about the first steps in reduction of O₂ to H₂O, a process of considerable importance to chemistry and biology.