Publication 630

Chem. Rev. 108, 2145– 2179, 2008. DOI: 10.1021/cr0680787
Electrochemical approach to the mechanistic study of proton-coupled electron transfer

Laboratoire d’Electrochimie Moléculaire, Université Paris Diderot, UMR CNRS 7591, 2 place Jussieu, 75251 Paris Cedex 05, France,

Introduction

The coupling between electron and proton transfers has a long experimental and theoretical history in chemistry and biochemistry. To take just one example, the fact that acceptance of an electron triggers the addition of an acid or the removal of a base and vice versa for oxidations towers over all understanding of organic electrochemistry. Proton-coupled electron transfer (PCET) reactions also play a critical role in a wide range of biological processes, including enzyme reactions, photosynthesis, and respiration. A recent impressive review describes PCET reactions and phenomena.

PCET is employed here as a general term for reactions in which both an electron and a proton are transferred, either in two separate steps or in a single step. Reactions in which the electron and proton transfer between the same donor and acceptor, that is, hydrogen atom transfer, are, of course, not considered here because we consider electrochemical PCET reactions in which electrons are flowing into or from an electrode while protons are transferred between acid and base.

Molecular electrochemistry, through nondestructive techniques such as cyclic voltammetry, has proved to be very useful in characterizing electron transfers and deciphering mechanisms in which chemical reactions are associated with electron transfer. Therefore, it has been a convenient tool for the mechanistic study of reactions in which electron transfer is coupled to proton transfer, that is, in which an electron leaves or enters an electrode while a proton is transferred from or to the redox species. Until recently, PCET has been mostly thought of as stepwise electron and proton transfer (ET-PT or PT-ET). We thus review in an initial section (section 2) the analysis of such stepwise mechanisms both in aprotic media and in water. In aprotic media, hydrogen bonding often precedes proton transfer. Therefore, characterization of the dichotomy between hydrogen bonding and proton transfer as associated with electron transfer is necessary to fully describe PCET processes and is thus presented first (section 2.1). In water, specific mechanistic issues on PCET arise because water itself may act as both a donor and acceptor of protons. This role may also be played by OH^- (or H₃O⁺) and by the basic (or acidic) components of the buffers in which the experiments are often carried out. Moreover, proton transfers are fast and often assumed to be at equilibrium in water. Therefore, PCET in water is presented in section 2.2. Because redox couples tethered to an electrode afford an excellent means of observing heterogeneous electron transfer kinetics with no complications caused by mass transfer effects, such systems have been used to analyze the effects of proton transfer preceding or following an electron transfer (section 2.3). The vision of PCET as stepwise processes has, however, been recently questioned, and extensive work has been done on both the theoretical and experimental aspects of a competitive concerted pathway, that is, a one-step mechanism in which proton and electron transfer are concerted. We term the latter mechanism concerted proton and electron transfer (CPET). Other terms have been used in the literature to describe the same mechanism: electron transfer-proton transfer (ETPT), electron-proton transfer (EPT), or multiple-site electron-proton transfer (MS-EPT). The specific electrochemical approach to the analysis of CPET is reviewed in section 3.

Proton transfer and its coupling to electron transfer in most biological systems is fundamental. Electrochemistry, through protein film voltammetry (PFV), has contributed widely to the establishment of how individual proton transfers occur at the molecular level and how they are coupled to electron transfer. This issue is reviewed in section 4.