title: Concerted and stepwise proton-coupled electron transfers in aquo/hydroxo complex couples in water: oxidative electrochemistry of [Os(II)(bpy)(2)(py)(OH(2))](2+).
abstract: Successive oxidation of transition metal(II) aqua complexes (M(II)OH(2) to M(III)OH) is a domain in which proton-coupled electron transfer reactions are extremely common. The mechanism of these PCET reactions-concerted or stepwise-is an important issue in the understanding and design of natural or artificial systems catalyzing the formation of dioxygen by four-electron oxidation of water. Concerted proton-coupled electron transfer from an aqua metal(II) to a hydroxo metal(III) complex requires the close proximity of a proton-accepting group with a pK value between those of the aqua complexes. Otherwise, stepwise electron-proton or proton-electron pathways involving high-energy intermediates are followed. Concerted proton-electron pathways involving water as proton-acceptor or proton-donor group are inefficient. Cyclic voltammetry of the title complex in buffered aqueous solution and re-examination of previous results for the same complex attached to an electrode surface are used to establish these conclusions, which provide a starting point on the route to higher degrees of oxidation, such as those involved in the catalysis of water oxidation.
authors.affiliation: Laboratoire d'Electrochimie Moléculaire, UMR 7591 CNRS, Université Paris Diderot, 2 place Jussieu 75251 Paris Cedex 05, France.
authors.forename: Cyrille
authors.initials: C
authors.lastname: Costentin
authors.forename: Marc
authors.initials: M
authors.lastname: Robert
authors.forename: Jean-Michel
authors.initials: JM
authors.lastname: Savéant
authors.forename: Anne-Lucie
authors.initials: AL
authors.lastname: Teillout