Concerted proton-electron transfer to dioxygen in water

Concerted proton-electron transfer (CPET) is documented for the homogeneous reduction of O₂ to HO₂^· in water by the one-electron-reduced heteropolytungstate anion, α-PW₁₂O ₄₀^4- (1_1e). At 0.01-0.3 M H⁺, O ₂ reduction occurs via outer-sphere electron transfer followed by proton transfer (ETPT, with rate constant k_ET). Between 0.30 and 1.9 M H⁺, rates increase linearly with [H⁺] due to a parallel CPET pathway in which H⁺ is now a reactant: (1/2)k_obs = k_ET + k_CPET[H⁺] (k_ET = 1.2 M ^-1 s^-1; k_CPET = 0.8 M^-2 s ^-1). Control experiments rule out preassociation between H ⁺ and 1_1e. Analysis of plausible rate expressions shows that the first-order dependence on [H⁺] is uniquely consistent with multisite CPET, and a deuterium kinetic isotope effect of 1.7 is observed. Reductions of O₂ by α-SiW₁₂O₄₀ ^5- confirm theoretical predictions that CPET decreases in significance as ET becomes less endergonic. Marcus analysis, including the temperature dependence of ΔG°, gives reorganization energies, λ_ET = 41.5 kcal mol^-1 and λ_CPET = 52.4 kcal mol^-1. At 1.5 M H⁺, ∼75% of the (1 _1e,O₂) encounter pairs form within 6 Å of H ⁺ ions. This value (6 ± 1 Å) is the "reaction distance" for proton diffusion and probably close to that for CPET. Even so, the 70-200 ps lifetimes of the (1_1e,O₂) pairs provide additional time for H⁺ to diffuse closer to O₂. CPET is first-order in [H⁺] because k_e for "cage escape" from (1_1e,O₂) pairs is much larger than k_CPET, such that the rate expression for CPET becomes -(1/2)d[1_1e]/dt = (k_d/k_e)k_CPET[1_1e][O ₂][H⁺], where k_d is the rate constant for (1_1e,O₂) pair formation. Overall, the findings suggest that the emergence of CPET, with hydronium ion as the proton donor, may prove a general feature of sufficiently endergonic reductions of dioxygen by otherwise "outer-sphere" complexes (or electrode reactions) at sufficiently low pH values in water.