Proton-Coupled Electron Transfer of an Osmium Aquo Complex on a Self-Assembled Monolayer on Gold

A one-electron, one-proton (1 e 1H) redox couple based in an osmium aquo complex is attached to a self-assembled monolayer of alkanethiol on a gold electrode. The formal potential behavior of the Os aquo complex exhibits the expected pH dependence for a 1 e 1H system. The thermodynamic parameters are a formal potential of +0.30 V vs SCE for [Os(II/III)(H 2 O)], a formal potential of −0.11 V vs SCE for [Os(II/III)(OH)], a p K a of 2.4 for [Os(III)(H 2 O/OH)], and a p K a of 9.3 for [Os(II)(H 2 O/OH)]. A stepwise electron−proton transfer model, widely used in the electrochemical literature, predicts the effects of pH and overpotential on the kinetics of charge transfer for the 1 e 1H system. The kinetic behavior of the Os aquo complex deviates substantially from the predictions of the stepwise model. In particular, the standard rate constant and the transfer coefficient at zero overpotential are nearly independent of pH over the pH range at which proton-coupled electron transfer occurs, and Tafel plots are asymmetrical (steeper anodic branch) over most of the pH range examined (1−12). Possible double-layer effects caused by the presence of ionizable carboxylate moieties in the monolayer are ruled out by the absence of any pH effects on the formal potential and kinetics of an analogous Os chloride complex. A concerted electron−proton transfer mechanism is hypothesized to explain the observed kinetic behavior.