Concerted Proton−Electron Transfer to Dioxygen in Water

Abstract

Concerted proton-electron transfer (CPET) is documented for the homogeneous reduction of O(2) to HO(2)(*) in water by the one-electron-reduced heteropolytungstate anion, alpha-PW(12)O(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 O(2) by alpha-SiW(12)O(40)(5-) confirm theoretical predictions that CPET decreases in significance as ET becomes less endergonic. Marcus analysis, including the temperature dependence of DeltaG(o), gives reorganization energies, lambda(ET) = 41.5 kcal mol(-1) and lambda(CPET) = 52.4 kcal mol(-1). At 1.5 M H(+), approximately 75% of the (1(1e),O(2)) encounter pairs form within 6 A of H(+) ions. This value (6 +/- 1 A) 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),O(2)) pairs provide additional time for H(+) to diffuse closer to O(2). CPET is first-order in [H(+)] because k(e) for "cage escape" from (1(1e),O(2)) 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),O(2)) 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.