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Concerted proton-coupled electron transfer from a metal-hydride complex

Abstract

Metal hydrides are key intermediates in the catalytic reduction of protons and CO2 as well as in the oxidation of H2. In these reactions, electrons and protons are transferred to or from separate acceptors or donors in bidirectional proton-coupled electron transfer (PCET) steps. The mechanistic interpretation of PCET reactions of metal hydrides has focused on the stepwise transfer of electrons and protons. A concerted transfer may, however, occur with a lower reaction barrier and therefore proceed at higher catalytic rates. Here we investigate the feasibility of such a reaction by studying the oxidation–deprotonation reactions of a tungsten hydride complex. The rate dependence on the driving force for both electron transfer and proton transfer—employing different combinations of oxidants and bases—was used to establish experimentally the concerted, bidirectional PCET of a metal-hydride species. Consideration of the findings presented here in future catalyst designs may lead to more-efficient catalysts.

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Figure 1: Structure and the possible PCET mechanisms discussed for the oxidation of W–H.
Figure 2: Cyclic voltammograms of W–H and pyridine at different scan rates.
Figure 3: Marcus plot of the observed rate constant (ln(kET)) versus driving force for ETs from W–H to the four oxidants in the absence of added base.
Figure 4: Absorbance changes during stopped-flow oxidation of W–H by [Fe(dmbpy)3]3+ used to extract kinetic data.
Figure 5: Marcus plot of the rate constant ln(kCEP) versus ΔG0PCET for the PCET reaction between W–H and [Fe(MeObpy)3]3+ (black squares) or [Fe(dmbpy)3]3+ (open circles) and 7.5 equiv. base.

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Acknowledgements

M.B. acknowledges T. F. Markle and M. Mirmohades for fruitful discussions. The authors are grateful to S. Glover for input regarding revision of the text. This work was supported by The Swedish Research Council, The Knut and Alice Wallenberg Foundation and The Swedish Energy Agency, as well as the Agence Nationale de la Recherche (ANR, BLANC SIMI9/0926-1, ‘TechBioPhyp’), the Centre National de la Recherche Scientifique and the Université de Bretagne Occidentale. Correspondence and requests for materials should be addressed to L.H.

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M.B. and L.H. conceived and designed the experiments, M.B. and R.S. performed the experiments, M.B. and L.H. analysed the data, L.H. and S.O. had the global idea and directed the project, F.G. and L.H. supervised the project and the experiments and M.B., F.G., L.H. and S.O. co-wrote the paper.

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Correspondence to Leif Hammarström.

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Bourrez, M., Steinmetz, R., Ott, S. et al. Concerted proton-coupled electron transfer from a metal-hydride complex. Nature Chem 7, 140–145 (2015). https://doi.org/10.1038/nchem.2157

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