Publication
640
Chem. Phys. Chem.
10, 191-198, 2009.
DOI:
10.1002/cphc.200800361 j
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Concerted and stepwise proton coupled electron transfers in water. Mechanisms of aquo/ hydroxo complex redox couples illustrated by the oxidative electrochemistry of OsII(bpy)2pyOH2]2+. |
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Cyrille Costentin, Marc Robert, Jean-Michel Savéant, and Anne-Lucie Teillout
Laboratoire d‘Electrochimie Moléculaire, UMR CNRS 7591 Université Paris Diderot, 2 place Jussieu 75251 Paris Cedex 05 (France)
Successive oxidation of transition metal(II) aqua complexes (MIIOH2 to MIIIOH) 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. |