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The role of uranium–arene bonding in H2O reduction catalysis

Abstract

The reactivity of uranium compounds towards small molecules typically occurs through stoichiometric rather than catalytic processes. Examples of uranium catalysts reacting with water are particularly scarce, because stable uranyl groups form that preclude the recovery of the uranium compound. Recently, however, an arene-anchored, electron-rich uranium complex has been shown to facilitate the electrocatalytic formation of H2 from H2O. Here, we present the precise role of uranium–arene δ bonding in intermediates of the catalytic cycle, as well as details of the atypical two-electron oxidative addition of H2O to the trivalent uranium catalyst. Both aspects were explored by synthesizing mid- and high-valent uranium–oxo intermediates and by performing comparative studies with a structurally related complex that cannot engage in δ bonding. The redox activity of the arene anchor and a covalent δ-bonding interaction with the uranium ion during H2 formation were supported by density functional theory analysis. Detailed insight into this catalytic system may inspire the design of ligands for new uranium catalysts.

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Figure 1: Overview of the proposed mechanism for electrocatalytic H2O reduction with 1, graphical representations of mes complex 1 and tacn analogue 3, and the synthetic scheme towards oxo complexes relevant for the catalytic cycle.
Figure 2: Molecular structures of oxo complexes 2, 2–K and 4 determined by single-crystal X-ray crystallography, and a model of the distorted mesitylene backbone of complex 2.
Figure 3: EPR spectroscopic analysis of U(V) oxo complexes 2 and 4.
Figure 4: Electronic structure analysis by SQUID magnetometry and electronic absorption spectroscopy.
Figure 5: Calculated molecular orbital maps describing the electronic structure of complex 2.
Figure 6: Calculated reaction profile and SOMOs of the first transition state for H2O reduction with complex 1.

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Acknowledgements

The authors thank H.S. La Pierre for discussions and M.E. Miehlich for assistance with EPR spectroscopy. D.P.H. acknowledges support from the Graduate School of Molecular Science (GSMS) of FAU Erlangen-Nürnberg. The Bundesministerium für Bildung und Forschung (BMBF, support codes 02NUK012C and 02NUK020C), the FAU Erlangen-Nürnberg and COST Action CM1006 are acknowledged for funding.

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D.P.H. 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. L.M. performed theoretical calculations and analyses. All authors edited and reviewed the manuscript in the context of their contributions. K.M. supervised all aspects of the project.

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Correspondence to Karsten Meyer.

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The authors declare no competing financial interests.

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Crystallographic data for compound 2-K (CIF 1526 kb)

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Crystallographic data for compound 2 (CIF 8189 kb)

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Crystallographic data for compound 3 (CIF 3005 kb)

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Crystallographic data for compound 4 (CIF 7920 kb)

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Halter, D., Heinemann, F., Maron, L. et al. The role of uranium–arene bonding in H2O reduction catalysis. Nature Chem 10, 259–267 (2018). https://doi.org/10.1038/nchem.2899

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