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Uranium-mediated electrocatalytic dihydrogen production from water

Abstract

Depleted uranium is a mildly radioactive waste product that is stockpiled worldwide. The chemical reactivity of uranium complexes is well documented, including the stoichiometric activation of small molecules of biological and industrial interest such as H2O, CO2, CO, or N2 (refs 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11), but catalytic transformations with actinides remain underexplored in comparison to transition-metal catalysis12,13,14. For reduction of water to H2, complexes of low-valent uranium show the highest potential, but are known to react violently and uncontrollably forming stable bridging oxo or uranyl species15. As a result, only a few oxidations of uranium with water have been reported so far; all stoichiometric2,3,16,17. Catalytic H2 production, however, requires the reductive recovery of the catalyst via a challenging cleavage of the uranium-bound oxygen-containing ligand. Here we report the electrocatalytic water reduction observed with a trisaryloxide U(iii) complex [((Ad,MeArO)3mes)U] (refs 18 and 19)—the first homogeneous uranium catalyst for H2 production from H2O. The catalytic cycle involves rare terminal U(iv)–OH and U(v)=O complexes, which have been isolated, characterized, and proven to be integral parts of the catalytic mechanism. The recognition of uranium compounds as potentially useful catalysts suggests new applications for such light actinides. The development of uranium-based catalysts provides new perspectives on nuclear waste management strategies, by suggesting that mildly radioactive depleted uranium—an abundant waste product of the nuclear power industry—could be a valuable resource.

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Figure 1: Electrochemical characterization of catalyst 1.
Figure 2: Independent synthesis and characterization of the uranium(IV) hydroxo complex [((Ad,MeArO)3mes)U–OH] (2–OH).
Figure 3: Postulated mechanism for the reduction of H2O by the U(iii) complex 1, based on EPR results.
Figure 4: X-band EPR spectrum of a frozen 10 mM toluene solution of 1 with a sub-stoichiometric amount of H2O.
Figure 5: Postulated electrocatalytic cycle for H2 generation from H2O in the presence of the homogeneous U(iii) catalyst [((Ad,MeArO)3mes)U] (1).

Accession codes

Data deposits

Atomic coordinates and structure factors for the reported crystal structures have been deposited in the Cambridge Crystallographic Data Centre under the accession code CCDC-1413741 (for 2–OH from THF/n-pentane), CCDC-1401838 (for 2–OH from THF), and CCDC-1437872 (for [((Ad,tBuArO)3tacn)U(OH)]).

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Acknowledgements

We thank Y. Wu for support with the electrochemical impedance experiments. We also thank J. F. Berry and M. P. Bullock for discussions. For reference experiments, J. R. Long, C. J. Chang and D. Zee are acknowledged for the synthesis and donation of their Mo-based catalyst [PY5Me2MoO](B(C6H3(CF3)2)4)2 and D. J. Mindiola for the synthesis and crystallization of [((Ad,tBuArO)3tacn)U(OH)]. We acknowledge the Bundesministerium für Bildung und Forschung (BMBF, support codes 02NUK012C and 02NUK020C), the FAU Erlangen-Nürnberg, and COST Action CM1006 for financial support.

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D.P.H., J.B., and K.M. planned the research and prepared the manuscript. D.P.H. performed the experiments. F.W.H conducted the XRD analyses and refined structures. K.M. supervised the project in all aspects.

Corresponding author

Correspondence to Karsten Meyer.

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Halter, D., Heinemann, F., Bachmann, J. et al. Uranium-mediated electrocatalytic dihydrogen production from water. Nature 530, 317–321 (2016). https://doi.org/10.1038/nature16530

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