Concerted proton-coupled electron transfers in aquo/hydroxo/oxo metal complexes: Electrochemistry of [Os ^II (bpy) ₂ py(OH ₂ )] ²⁺ in water

Abstract

Kinetic analysis of the successive oxidative cyclic voltammetric responses of [Os II (bpy)2py(OH2)] 2؉ in buffered water, together with determination of H/D isotope effects, has allowed the determination of the mechanisms of the successive proton-coupled electron transfers that convert the Os II -aquo complex into the Os III -hydroxo complex and the later into the Os IV -oxo complex. The stepwise pathways prevail over the concerted pathway in the first case. However, very large concentrations of a base, such as acetate, trigger the beginning of a concerted reaction. The same trend appears, but to a much larger extent, when high local concentration of carboxylates are attached close to the Os complex. The Os III -hydroxo/Os IV -oxo couple is globally much slower and concerted pathways predominate over the stepwise pathways. Water is, however, not an appropriate proton acceptor in this respect. Other bases, such as citrate or phosphate, are instead quite effective for triggering concerted pathways. Here, we suggest factors causing these contrasting behaviors, providing a practical illustration of the prediction that concerted processes are an efficient way of avoiding high-energy intermediates. Observation of a strong decelerating effect of inactive ions together with the positive role of high local concentrations of carboxylates to initiate a concerted route underscores the variety of structural and medium factors that may operate to modulate and control the occurrence of concerted pathways. These demonstrations and analyses of the occurrence of concerted pathways in an aquohydroxo-oxo series are expected to serve as guidelines for studies in term of methodology and factor analysis.concerted processes ͉ electron transfer chemistry of metal complexes M any reactions of electron transfer chemistry and radical chemistry associate single electron transfer with proton transfer. Molecular electrochemists have been interested in this as a special case of the coupling between electron transfer and acid-base reactions in a general sense (ref. 1, pp. 140 -148). In these electrochemical proton-coupled electron transfers (PCET), proton transfer and electrochemical electron transfer have been viewed for a long time as separate steps as represented by the blue pathways in Scheme 1, proton transfer first, followed by electron transfer (PET) or vice versa (EPT). The mechanisms of many important processes have thus been uncovered by observing the change induced by addition of an acid to a nonacidic solvent in reductions or, conversely, of a base to a nonbasic solvent in oxidations (1). In water, treatment of the kinetics under the assumption that the protonation step are fast and reversible, albeit not always relevant, has proved successful in quite a number of cases (2). This conceptual framework changed rapidly when it was realized that electrochemistry could contribute efficiently to the interpretation of clues indicating that proton and electron transfers could be concerted (red pathway in Scheme 1) in several natural process...

Proton-coupled electron transfer at modified electrodes by multiple pathways

“…Significant rate accelerations for RuðIII → IVÞ oxidation are also observed with OAc − added to the external solution as a proton acceptor base. As discussed below, this observation points to a third pathway for Ru III -OH 2þ → Ru IV ¼O 2þ oxidation, base-assisted, concerted EPT as reported by Savéant and coworkers in the oxidation of cis-½Os III ðbpyÞ 2 ðpyÞðOHÞ 2þ to cis-½Os IV ðbpyÞ 2 ðpyÞðOÞ 2þ (7). Illustrative data are shown in Fig.…”

“…Electrocatalysis by deliberate introduction of PCET pathways at modified electrode surfaces has been reported (6)(7)(8)(9)(10)(11)(12)(13)(14)(15). This study is important in extending solution reactivity, for example, toward water and hydrocarbon oxidation catalysis, to electrode interfaces in device configurations.…”

“…The role of PCET in electrochemical reactivity has been discussed in a series of papers (6)(7)(8)(9)(10)(11)(12)(13)(14)(15). In a cyclic voltammetry (CV) study on the Os-based couples cis-½Os IV ðbpyÞ 2 ðpyÞðOÞ 2þ ∕ cis-½Os III ðbpyÞ 2 ðpyÞðOHÞ 2þ (where bpy is 2,2′-bipyridine) and cis-½Os III ðbpyÞ 2 ðpyÞðOHÞ 2þ ∕cis-½Os II ðbpyÞ 2 ðpyÞðOH 2 Þ 2þ , Savéant and coworkers (7) concluded that oxidation of Os II Rate accelerations were observed with added proton acceptor bases, the Britton-Robinson buffer (phosphate, citrate, borate, and acetate), accompanied by the appearance of a solvent KIE of 2-2.5.…”

“…In a cyclic voltammetry (CV) study on the Os-based couples cis-½Os IV ðbpyÞ 2 ðpyÞðOÞ 2þ ∕ cis-½Os III ðbpyÞ 2 ðpyÞðOHÞ 2þ (where bpy is 2,2′-bipyridine) and cis-½Os III ðbpyÞ 2 ðpyÞðOHÞ 2þ ∕cis-½Os II ðbpyÞ 2 ðpyÞðOH 2 Þ 2þ , Savéant and coworkers (7) concluded that oxidation of Os II Rate accelerations were observed with added proton acceptor bases, the Britton-Robinson buffer (phosphate, citrate, borate, and acetate), accompanied by the appearance of a solvent KIE of 2-2.5. The latter was attributed to concerted electron-proton transfer (EPT) at the electrode with electron transfer to the electrode occurring in concert with proton transfer to the added base form of the buffer (7).…”

The electrochemical approach to concerted proton—electron transfers in the oxidation of phenols in water

“…The mechanism of the PCET processes preceding dimerization was not addressed in these cases as well as in other contributions (26)(27)(28)(29) simply because the notion of a concerted proton-electron transfer pathway was not part of the theoretical framework at the time. It should be noted, en passant, that characterization of electrochemical CPET processes are scarce even outside the domain of phenol oxidation (30). In a recent preliminary note, the phenoxyl radical produced by CPET oxidation of the phenol was protected against follow-up dimerization by ortho and para tert-butyl groups (31).…”