Accelerating proton-coupled electron transfer of metal hydrides in catalyst model reactions

Abstract

Metal hydrides are key intermediates in catalytic proton reduction and dihydrogen oxidation. There is currently much interest in appending proton relays near the metal centre to accelerate catalysis by proton-coupled electron transfer (PCET). However, the elementary PCET steps and the role of the proton relays are still poorly understood, and direct kinetic studies of these processes are scarce. Here, we report a series of tungsten hydride complexes as proxy catalysts, with covalently attached pyridyl groups as proton acceptors. The rate of their PCET reaction with external oxidants is increased by several orders of magnitude compared to that of the analogous systems with external pyridine on account of facilitated proton transfer. Moreover, the mechanism of the PCET reaction is altered by the appended bases. A unique feature is that the reaction can be tuned to follow three distinct PCET mechanisms—electron-first, proton-first or a concerted reaction—with very different sensitivities to oxidant and base strength. Such knowledge is crucial for rational improvements of solar fuel catalysts.

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Fig. 1: Schematic representation of transition-metal-catalysed H2 oxidation reactions that involve metal hydrides.
Fig. 2: Structure of the W–H mother complex, as well as thermodynamic data and mechanistic principles to analyse the reaction kinetics and its dependencies on oxidant and base strengths.
Fig. 3: Synthetic routes to complexes 3a–e and selected NMR data.
Fig. 4: Cyclic voltammetry data for the first oxidation of 1 mM 3a–e and [(MeCp)WH(CO)3] in acetonitrile, showing how the potential and thus the driving force for PCET oxidation depends on the strength of the appended base.
Fig. 5: Dependence of the observed second-order PCET rate constant on the pKa value of the free pyridinium, where the distinctly different dependences for different oxidants and W–H compounds are consistent with different mechanisms (CEPT, PTET or ETPT).

References

  1. 1.

    Gray, H. B. Powering the planet with solar fuel. Nat. Chem. 1, 7 (2009).

    Article  CAS  PubMed  Google Scholar 

  2. 2.

    Graetzel, M. Artificial photosynthesis: water cleavage into hydrogen and oxygen by visible light. Acc. Chem. Res. 14, 376–384 (1981).

    Article  CAS  Google Scholar 

  3. 3.

    Gust, D., Moore, T. A. & Moore, A. L. Solar fuels via artificial photosynthesis. Acc. Chem. Res. 42, 1890–1898 (2009).

    Article  CAS  PubMed  Google Scholar 

  4. 4.

    Montoya, J. H. et al. Materials for solar fuels and chemicals. Nat. Mater. 16, 70–81 (2017).

    Article  CAS  Google Scholar 

  5. 5.

    Faunce, T. A. et al. Energy and environment policy case for a global project on artificial photosynthesis. Energy Environ. Sci. 6, 695–698 (2013).

    Article  Google Scholar 

  6. 6.

    Steele, B. C. H. & Heinzel, A. Materials for fuel-cell technologies. Nature 414, 345–352 (2001).

    Article  CAS  Google Scholar 

  7. 7.

    Debe, M. K. Electrocatalyst approaches and challenges for automotive fuel cells. Nature 486, 43–51 (2012).

    Article  CAS  PubMed  Google Scholar 

  8. 8.

    Dempsey, J. L., Winkler, J. R. & Gray, H. B. Proton-coupled electron flow in protein redox machines. Chem. Rev. 110, 7024–7039 (2010).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  9. 9.

    Hay, S. & Scrutton, N. S. Good vibrations in enzyme-catalysed reactions. Nat. Chem. 4, 161–168 (2012).

    Article  CAS  PubMed  Google Scholar 

  10. 10.

    Dogutan, D. K., McGuire, R. & Nocera, D. G. Electocatalytic water oxidation by cobalt(iii) hangman β-octafluoro corroles. J. Am. Chem. Soc. 133, 9178–9180 (2011).

    Article  CAS  PubMed  Google Scholar 

  11. 11.

    Bediako, D. K. et al. Role of pendant proton relays and proton-coupled electron transfer on the hydrogen evolution reaction by nickel hangman porphyrins. Proc. Natl Acad. Sci. USA 111, 15001–15006 (2014).

    Article  CAS  PubMed  Google Scholar 

  12. 12.

    Helm, M. L., Stewart, M. P., Bullock, R. M., DuBois, M. R. & DuBois, D. L. A synthetic nickel electrocatalyst with a turnover frequency above 100,000 s−1 for H2 production. Science 333, 863–866 (2011).

    Article  CAS  PubMed  Google Scholar 

  13. 13.

    O’Hagan, M. et al. Proton delivery and removal in [Ni(PR 2NR′ 2)2]2+ hydrogen production and oxidation catalysts. J. Am. Chem. Soc. 134, 19409–19424 (2012).

    Article  CAS  PubMed  Google Scholar 

  14. 14.

    Zhang, S., Appel, A. M. & Bullock, R. M. Reversible heterolytic cleavage of the H–H bond by molybdenum complexes: controlling the dynamics of exchange between proton and hydride. J. Am. Chem. Soc. 139, 7376–7387 (2017).

    Article  CAS  PubMed  Google Scholar 

  15. 15.

    Costentin, C., Passard, G., Robert, M. & Savéant, J.-M. Pendant acid–base groups in molecular catalysts: H-bond promoters or proton relays? Mechanisms of the conversion of CO2 to CO by electrogenerated iron(0)porphyrins bearing prepositioned phenol functionalities. J. Am. Chem. Soc. 136, 11821–11829 (2014).

    Article  CAS  PubMed  Google Scholar 

  16. 16.

    Lilio, A. M. et al. Incorporation of pendant bases into Rh(diphosphine)2 complexes: synthesis, thermodynamic studies, and catalytic CO2 hydrogenation activity of [Rh(P2N2)2]+ complexes. J. Am. Chem. Soc. 137, 8251–8260 (2015).

    Article  CAS  PubMed  Google Scholar 

  17. 17.

    Seu, C. S., Appel, A. M., Doud, M. D., DuBois, D. L. & Kubiak, C. P. Formate oxidation via β-deprotonation in [Ni(PR 2NR′ 2)2(CH3CN)]2+ complexes. Energy Environ. Sci. 5, 6480–6490 (2012).

    Article  CAS  Google Scholar 

  18. 18.

    Roy, S. et al. Molecular cobalt complexes with pendant amines for selective electrocatalytic reduction of carbon dioxide to formic acid. J. Am. Chem. Soc. 139, 3685–3696 (2017).

    Article  CAS  PubMed  Google Scholar 

  19. 19.

    Costentin, C., Drouet, S., Robert, M. & Savéant, J.-M. A local proton source enhances CO2 electroreduction to CO by a molecular Fe catalyst. Science 338, 90–94 (2012).

    Article  CAS  PubMed  Google Scholar 

  20. 20.

    Pegis, M. L. et al. Homogenous electrocatalytic oxygen reduction rates correlate with reaction overpotential in acidic organic solutions. ACS Cent. Sci. 2, 850–856 (2016).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  21. 21.

    Auer, B., Fernandez, L. E. & Hammes-Schiffer, S. Theoretical analysis of proton relays in electrochemical proton-coupled electron transfer. J. Am. Chem. Soc. 133, 8282–8292 (2011).

    Article  CAS  PubMed  Google Scholar 

  22. 22.

    Bourrez, M., Steinmetz, R., Ott, S., Gloaguen, F. & Hammarström, L. Concerted proton-coupled electron transfer from a metal–hydride complex. Nat. Chem. 7, 140–145 (2015).

    Article  CAS  Google Scholar 

  23. 23.

    Elgrishi, N., Kurtz, D. A. & Dempsey, J. L. Reaction parameters influencing cobalt hydride formation kinetics: implications for benchmarking H2-evolution catalysts. J. Am. Chem. Soc. 139, 239–244 (2017).

    Article  CAS  PubMed  Google Scholar 

  24. 24.

    Norskov, J. K., Bligaard, T., Rossmeisl, J. & Christensen, C. H. Towards the computational design of solid catalysts. Nat. Chem. 1, 37–46 (2009).

    Article  CAS  PubMed  Google Scholar 

  25. 25.

    Olsson, M. H. M., Siegbahn, P. E. M. & Warshel, A. Simulating large nuclear quantum mechanical corrections in hydrogen atom transfer reactions in metalloenzymes. J. Biol. Inorg. Chem. 9, 96–99 (2004).

    Article  CAS  PubMed  Google Scholar 

  26. 26.

    Hammes-Schiffer, S. & Stuchebrukhov, A. A. Theory of coupled electron and proton transfer reactions. Chem. Rev. 110, 6939–6960 (2010).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  27. 27.

    Rhile, I. J. & Mayer, J. M. One-electron oxidation of a hydrogen-bonded phenol occurs by concerted proton-coupled electron transfer. J. Am. Chem. Soc. 126, 12718–12719 (2004).

    Article  CAS  PubMed  Google Scholar 

  28. 28.

    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, 17341–17352 (2011).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  29. 29.

    Zhang, M.-T., Irebo, T., Johansson, O. & Hammarström, L. Proton-coupled electron transfer from tyrosine: a strong rate dependence on intramolecular proton transfer distance. J. Am. Chem. Soc. 133, 13224–13227 (2011).

    Article  CAS  PubMed  Google Scholar 

  30. 30.

    Glover, S. D., Parada, G. A., Markle, T. F., Ott, S. & Hammarström, L. Isolating the effects of the proton tunneling distance on proton-coupled electron transfer in a series of homologous tyrosine-base model compounds. J. Am. Chem. Soc. 139, 2090–2101 (2017).

    Article  CAS  PubMed  Google Scholar 

  31. 31.

    Belkova, N. V., Epstein, L. M., Filippov, O. A. & Shubina, E. S. Hydrogen and dihydrogen bonds in the reactions of metal hydrides. Chem. Rev. 116, 8545–8587 (2016).

    Article  CAS  PubMed  Google Scholar 

  32. 32.

    Eigen, M. Proton transfer, acid-base catalysis, and enzymatic hydrolysis. Angew. Chem. Int. Ed. 3, 1–19 (1964).

    Article  Google Scholar 

  33. 33.

    Marcus, R. A. & Sutin, N. Electron transfers in chemistry and biology. Biochim. Biophys. Acta - Rev. Bioenerg. 811, 265–322 (1985).

    Article  CAS  Google Scholar 

  34. 34.

    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)3 2+–tyrosine complex. J. Am. Chem. Soc. 134, 16247–16254 (2012).

    Article  CAS  PubMed  Google Scholar 

  35. 35.

    Tilset, M. & Parker, V. D. Solution homolytic bond dissociation energies of organotransition-metal hydrides. J. Am. Chem. Soc. 111, 6711–6717 (1989).

    Article  CAS  Google Scholar 

  36. 36.

    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, 2618–2626 (1990).

    Article  CAS  Google Scholar 

  37. 37.

    Meyer, T. J. & Caspar, J. V. Photochemistry of metal-metal bonds. Chem. Rev. 85, 187–218 (1985).

    Article  CAS  Google Scholar 

  38. 38.

    Parker, V. D., Handoo, K. L., Roness, F. & Tilset, M. Electrode potentials and the thermodynamics of isodesmic reactions. J. Am. Chem. Soc. 113, 7493–7498 (1991).

    Article  CAS  Google Scholar 

  39. 39.

    Jordan, R. F. & Norton, J. R. Kinetic and thermodynamic acidity of hydrido transition-metal complexes. 1. Periodic trends in Group VI complexes and substituent effects in osmium complexes. J. Am. Chem. Soc. 104, 1255–1263 (1982).

    Article  CAS  Google Scholar 

  40. 40.

    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, 3945–3953 (1987).

    Article  CAS  Google Scholar 

  41. 41.

    Kaljurand, I. et al. Extension of the self-consistent spectrophotometric basicity scale in acetonitrile to a full span of 28 pK a units: unification of different basicity scales. J. Org. Chem. 70, 1019–1028 (2005).

    Article  CAS  PubMed  Google Scholar 

  42. 42.

    Markle, T. F., Tronic, T. A., DiPasquale, A. G., Kaminsky, W. & Mayer, J. M. Effect of basic site substituents on concerted proton–electron transfer in hydrogen-bonded pyridyl–phenols. J. Phys. Chem. A. 116, 12249–12259 (2012).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  43. 43.

    Rhile, I. J. et al. Concerted proton–electron transfer in the oxidation of hydrogen-bonded phenols. J. Am. Chem. Soc. 128, 6075–6088 (2006).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

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Acknowledgements

T.L. acknowledges S.D. Glover, R. Fernandez-Teran and S. Wang for fruitful discussions. This work was supported by The Swedish Research Council (grant no. 2016-04271) and The Knut and Alice Wallenberg Foundation (grant no. 2011.0067). Correspondence and requests for materials should be addressed to L.H.

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T.L., S.O. and L.H. planned the research and prepared the manuscript. T.L. performed the experiments. R.L. supervised the electrochemistry experiments. A.O. conducted the X-ray diffraction analyses and refined structures. M.G. and M.L. performed theoretical calculations and analyses. All authors edited and reviewed the manuscript in the context of their contributions.

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Correspondence to Leif Hammarström.

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Supplementary information

Supplementary information

Supplementary Methods, Supplementary Figures 1–46, Supplementary Tables 1–7, Supplementary Experimental Data, Supplementary Theoretical Data

Crystallographic data

CIF for compound 3a; CCDC reference: 1552847

Crystallographic data

CIF for compound 3d; CCDC reference: 1552848

Crystallographic data

CIF for compound 3e; CCDC reference: 1552849

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Liu, T., Guo, M., Orthaber, A. et al. Accelerating proton-coupled electron transfer of metal hydrides in catalyst model reactions. Nature Chem 10, 881–887 (2018). https://doi.org/10.1038/s41557-018-0076-x

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