Reduction of O₂ to superoxide anion (O₂^.-) in water by heteropolytungstate cluster-anions

Fundamental information concerning the mechanism of electron transfer from reduced heteropolytungstates (POM_red) to O₂, and the effect of donor-ion charge on reduction of O₂ to superoxide anion (O₂^.-), is obtained using an isostructural series of 1e^--reduced donors: α-Xⁿ⁺W₁₂O ₄₀^(9-n)-, Xⁿ⁺ = Al³⁺, Si ⁴⁺, P⁵⁺. For all three, a single rate expression is observed: -d[POM_red]/dt = 2k₁₂[POM_red][O ₂], where k₁₂ is for the rate-limiting electron transfer from POM_red to O₂. At pH 2 (175 mM ionic strength), k ₁₂ increases from 1.4 ± 0.2 to 8.5 ± 1 to 24 ± 2 M^-1s^-1 as Xⁿ⁺ is varied from P⁵⁺ (3_red) to Si⁴⁺ (2_red) to Al³⁺ (1_red). Variable-pH data (for 1_red) and solvent-kinetic isotope (KIE = k_H/k_D) data (all three ions) indicate that protonated superoxide (HO₂•) is formed in two steps-electron transfer, followed by proton transfer (ET-PT mechanism)-rather than via simultaneous proton-coupled electron transfer (PCET). Support for an outersphere mechanism is provided by agreement between experimental k₁₂ values and those calculated using the Marcus cross relation. Further evidence is provided by the small variation in k₁₂ observed when Xⁿ⁺ is changed from P⁵⁺ to Si⁴⁺ to Al³⁺, and the driving force for formation of O₂^.- (aq), which increases as cluster-anion charge becomes more negative, increases by nearly +0.4 V (a decrease of >9 kcal mol^-1 in ΔG°). The weak dependence of k₁₂ on POM reduction potentials reflects the outersphere ET-PT mechanism: as the anions become more negatively charged, the "successor-complex" ion pairs are subject to larger anion-anion repulsions, in the order [(3_OX^3-)(O₂ ^.-)]^4- < [(2_OX^4-)(O ₂^.-)]^5- < [(1_OX ^5-)(O₂^.-)]^6-. This reveals an inherent limitation to the use of heteropolytungstate charge and reduction potential to control rates of electron transfer to O₂ under turnover conditions in catalysis.