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

Nature Chemistry volume 10, pages 881–887 (2018)Cite this article

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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).

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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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Authors and Affiliations

  1. Department of Chemistry, Ångström Laboratory, Uppsala University, Uppsala, Sweden

    Tianfei Liu, Meiyuan Guo, Andreas Orthaber, Reiner Lomoth, Marcus Lundberg, Sascha Ott & Leif Hammarström

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Contributions

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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