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

Journal name:
Nature Chemistry
Volume:
7,
Pages:
140–145
Year published:
DOI:
doi:10.1038/nchem.2157
Received
Accepted
Published online

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.

At a glance

Figures

  1. Structure and the possible PCET mechanisms discussed for the oxidation of W–H.
    Figure 1: Structure and the possible PCET mechanisms discussed for the oxidation of W–H.

    a, Structure of W–H. b, The possible steps include stepwise PTET (pathway a, black), stepwise ETPT (pathway b, red) and CEP (pathway in blue). The thermodynamic data given are for W–H in ​acetonitrile solvent as taken from the literature23, 24, 25, 26.

  2. Cyclic voltammograms of W–H and pyridine at different scan rates.
    Figure 2: Cyclic voltammograms of W–H and ​pyridine at different scan rates.

    These were used to determine the deprotonation rate constant in the reaction W–H+ pyridine → W + H+-pyridine. a,b, Experimental data (a) and simulations (b). The data are shown with the convention that the oxidation current is positive. Conditions: scan rates from 20 to 400 mV s−1 of 1.15 mM W–H and 20 mM ​pyridine in 0.1 M ​Bu4NPF6/​MeCN. The intensity increases as the scan rate increases.

  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 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.

    The four oxidants are given in Table 1. The good agreement with the predicted slope for a single ET step (in the region where −ΔG0λ) allows for the assignment of the observed rate constant to the ET step that forms W–H•+ (ET-b, Fig. 1). Error bars, ±s.d. from at least 10 experiments. Conditions: 0.4 mM W–H and 0.4 mM oxidant.

  4. Absorbance changes during stopped-flow oxidation of W–H by [Fe(dmbpy)3]3+ used to extract kinetic data.
    Figure 4: Absorbance changes during stopped-flow oxidation of W–H by [Fe(dmbpy)3]3+ used to extract kinetic data.

    a, Spectral changes in the presence of ​3-Cl-pyridine. The initial absorption ‘spike’ reflects the flow time of the stopped-flow instrument (t < 0). b, A comparison of kinetic traces at 530 nm with (blue curve) and without (red curve) added ​3-Cl-pyridine, with second-order kinetic fits (black lines). It is clear that even the relatively weak base ​3-Cl-pyridine accelerates the reaction rate caused by PCET. Conditions: 0.4 mM W–H and 0.4 mM [Fe(dmbpy)3]3+ with (blue curve) or without (red curve) 3 mM ​3-Cl-pyridine in 0.1 M ​Bu4NPF6/​MeCN under a N2 atmosphere. The data are shown from t = 0.

  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.
    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.

    The pyridine bases used are given in Table 1. The strong correlation of ln(kCEP) and ΔG0PCET, where ΔG0PCET depends on both the oxidant potential and the ​pyridinium pKa value ( equation (3)), gives evidence for a concerted PCET reaction with these relatively weak oxidants and bases. Error bars, ±s.d. from 6–10 experiments. Conditions: 0.4 mM W–H, 0.4 mM and 3 mM ​pyridine base.

References

  1. Weinberg, D. R. et al. Proton-coupled electron transfer. Chem. Rev. 112, 40164093 (2012).
  2. Reece, S. Y. & Nocera, D. G. Proton-coupled electron transfer in biology: results from synergistic studies in natural and model systems. Annu. Rev. Biochem. 78, 673699 (2009).
  3. Hammes-Schiffer, S. & Stuchebrukhov, A. A. Theory of coupled electron and proton transfer reactions. Chem. Rev. 110, 69396960 (2010).
  4. Lewis, N. S. & Nocera, D. G. Powering the planet: chemical challenges in solar energy utilization. Proc. Natl Acad. Sci. 103, 1572915735 (2006).
  5. Hammes-Schiffer, S. Comparison of hydride, hydrogen atom, and proton-coupled electron transfer reactions. ChemPhysChem 3, 3342 (2002).
  6. Hammes-Schiffer, S. Proton-coupled electron transfer: classification scheme and guide to theoretical methods. Energy Environ. Sci. 5, 76967703 (2012).
  7. Small, Y. A., DuBois, D. L., Fujita, E. & Muckerman, J. T. Proton management as a design principle for hydrogenase-inspired catalysts. Energy Environ. Sci. 4, 30083020 (2011).
  8. Schneider, J., Jia, H., Muckerman, J. T. & Fujita, E. Thermodynamics and kinetics of CO2, CO, and H+ binding to the metal centre of CO2 reduction catalysts. Chem. Soc. Rev. 41, 20362051 (2012).
  9. Gloaguen, F. & Rauchfuss, T. B. Small molecule mimics of hydrogenases: hydrides and redox. Chem. Soc. Rev. 38, 100108 (2009).
  10. DuBois, D. L. Development of molecular electrocatalysts for energy storage. Inorg. Chem. 53, 39353960 (2014).
  11. Tschierlei, S., Ott, S. & Lomoth, R. Spectroscopically characterized intermediates of catalytic H2 formation by [FeFe] hydrogenase models. Energy Environ. Sci. 4, 23402352 (2011).
  12. Warren, J. J., Tronic, T. A. & Mayer, J. M. Thermochemistry of proton-coupled electron transfer reagents and its implications. Chem. Rev. 110, 69617001 (2010).
  13. Costentin, C., Robert, M., Savéant, J-M. & Tard, C. Breaking bonds with electrons and protons. Models and examples. Acc. Chem. Res. 47, 271280 (2014).
  14. Hammarström, L. & Styring, S. Proton-coupled electron transfer of tyrosines in photosystem II and model systems for artificial photosynthesis: the role of a redox-active link between catalyst and photosensitizer. Energy Environ. Sci. 4, 23792388 (2011).
  15. Johannissen, L. O., Irebo, T., Sjödin, M., Johansson, O. & Hammarström, L. The kinetic effect of internal hydrogen bonds on proton-coupled electron transfer from phenols: a theoretical analysis with modeling of experimental data. J. Phys. Chem. B 113, 1621416225 (2009).
  16. Markle, T. F., Rhile, I. J. & Mayer, J. M. Kinetic effects of increased proton transfer distance on proton-coupled oxidations of phenol–amines. J. Am. Chem. Soc. 133, 1734117352 (2011).
  17. Belkova, N. V., Shubina, E. S. & Epstein, L. M. Diverse world of unconventional hydrogen bonds. Acc. Chem. Res. 38, 624631 (2005).
  18. Besora, M., Lledós, A. & Maseras, F. Protonation of transition-metal hydrides: a not so simple process. Chem. Soc. Rev. 38, 957966 (2009).
  19. Levina, V. A. et al. Neutral transition metal hydrides as acids in hydrogen bonding and proton transfer: media polarity and specific solvation effects. J. Am. Chem. Soc. 132, 1123411246 (2010).
  20. Creutz, C., Chou, M. H., Hou, H. & Muckerman, J. T. Hydride ion transfer from ruthenium(II) complexes in water: kinetics and mechanism. Inorg. Chem. 49, 98099822 (2010).
  21. Fernandez, L. E., Horvath, S. & Hammes-Schiffer, S. Theoretical analysis of the sequential proton-coupled electron transfer mechanisms for H2 oxidation and production pathways catalyzed by nickel molecular electrocatalysts. J. Phys. Chem. C 116, 31713180 (2012).
  22. Fernandez, L. E., Horvath, S. & Hammes-Schiffer, S. Theoretical design of molecular electrocatalysts with flexible pendant amines for hydrogen production and oxidation. J. Phys. Chem. Lett. 4, 542546 (2013).
  23. Moore, E. J., Sullivan, J. M. & Norton, J. R. Kinetic and thermodynamic acidity of hydrido transition-metal complexes. 3. Thermodynamic acidity of common mononuclear carbonyl hydrides. J. Am. Chem. Soc. 108, 22572263 (1986).
  24. Edidin, R. T., Sullivan, J. M. & Norton, J. R. Kinetic and thermodynamic acidity of hydrido transition-metal complexes. 4. Kinetic acidities toward aniline and their use in identifying proton-transfer mechanisms. J. Am. Chem. Soc. 109, 39453953 (1987).
  25. Tilset, M. & Parker, V. D. Solution homolytic bond dissociation energies of organotransition-metal hydrides. J. Am. Chem. Soc. 111, 67116717 (1989).
  26. Ryan, O. B., Tilset, M. & Parker, V. D. Chemical and electrochemical oxidation of group 6 cyclopentadienylmetal hydrides. First estimates of 17-electron metal-hydride cation-radical thermodynamic acidities and their decomposition of 17-electron neutral radicals. J. Am. Chem. Soc. 112, 26182626 (1990).
  27. Waidmann, C. R. et al. Using combinations of oxidants and bases as PCET reactants: thermochemical and practical considerations. Energy Environ. Sci. 5, 77717780 (2012).
  28. Marcus, R. A. & Sutin, N. Electron transfers in chemistry and biology. Biochim. Biophys. Acta Bioenerg. 811, 265322 (1985).
  29. Irebo, T., Zhang, M-T., Markle, T. F., Scott, A. M. & Hammarström, L. Spanning four mechanistic regions of intramolecular proton-coupled electron transfer in a Ru(bpy)32+–tyrosine complex. J. Am. Chem. Soc. 134, 1624716254 (2012).
  30. Zhang, M-T. & Hammarström, L. Proton-coupled electron transfer from tryptophan: a concerted mechanism with water as proton acceptor. J. Am. Chem. Soc. 133, 88068809 (2011).
  31. Kadish, K. M., Lacombe, D. A. & Anderson, J. E. Electrochemistry of molybdenum and tungsten cyclopentadienyl carbonyl complexes, [M(CO)3Cp]2, [M(CO)3Cp]+, [M(CO)3Cp], and M(CO)3Cp where M = Mo and W. Inorg. Chem. 25, 22462250 (1986).
  32. Kaljurand, I. et al. Extension of the self-consistent spectrophotometric basicity scale in acetonitrile to a full span of 28 pKa units: unification of different basicity scales. J. Org. Chem. 70, 10191028 (2005).
  33. Savéant, J-M. Elements of Molecular and Biomolecular Electrochemistry (Wiley-VCH, 2006).
  34. Rudolph, M. Digital simulations on unequally spaced grids. J. Electroanal. Chem. 543, 2339 (2003).
  35. Mabrouk, P. A. & Wrighton, M. S. Resonance Raman spectroscopy of the lowest excited state of derivatives of tris(2,2′-bipyridine)ruthenium(II): substituent effects on electron localization in mixed-ligand complexes. Inorg. Chem. 25, 526531 (1986).
  36. Prasad, R. & Scaife, D. B. Electro-oxidation and electro-reduction of some iron (II), cobalt(II) and nickel(II) polypyridyl complexes in acetonitrile. J. Electroanal. Chem. Interfacial Electrochem. 84, 373386 (1977).

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Affiliations

  1. UMR 6521, Centre National de la Recherche Scientifique, Université de Bretagne Occidentale, 6 Avenue Le Gorgeu, 29238 Brest, France

    • Marc Bourrez,
    • Romain Steinmetz &
    • Frederic Gloaguen
  2. Photochemistry and Molecular Science, Department of Chemistry, Ångström Laboratory, Uppsala University, Box 532, SE-751 20 Uppsala, Sweden

    • Marc Bourrez,
    • Sascha Ott &
    • Leif Hammarström

Contributions

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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