Back-bonding between an electron-poor, high-oxidation-state metal and poor π-acceptor ligand in a uranium(v)–dinitrogen complex

Abstract

A fundamental bonding model in coordination and organometallic chemistry is the synergic, donor–acceptor interaction between a metal and a neutral π-acceptor ligand, in which the ligand σ donates to the metal, which π back-bonds to the ligand. This interaction typically involves a metal with an electron-rich, mid-, low- or even negative oxidation state and a ligand with a π* orbital. Here, we report that treatment of a uranium–carbene complex with an organoazide produces a uranium(v)–bis(imido)–dinitrogen complex, stabilized by a lithium counterion. This complex, which was isolated in a crystalline form, involves an electron-poor, high-oxidation-state uranium(v) 5f1 ion that is π back-bonded to the poor π-acceptor ligand dinitrogen. We propose that this is made possible by a combination of cooperative heterobimetallic uranium–lithium effects and the presence of suitable ancillary ligands that render the uranium ion unusually electron rich. This electron-poor back-bonding could have implications for the field of dinitrogen activation.

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Fig. 1: The classical donor–acceptor bonding model for a transition metal (M) and a neutral diatomic E≡E (N≡N or C≡O) π-acceptor ligand.
Fig. 2: Synthesis of compound 2 from precursor 1.
Fig. 3: Molecular structure of 2 at 150 K with 40% probability ellipsoids.
Fig. 4: Variable-temperature effective magnetic moment data for 2.
Fig. 5: Computed potential energy surface scans of the relative energy (kJ mol−1) versus the U1-N1 distance (Å) of 2.
Fig. 6: The singularly occupied, α-spin HOMO (338a, −1.715 eV) of 2.

Data availability

The X-ray crystallographic data for 2 have been deposited at the Cambridge Crystallographic Data Centre (CCDC) under deposition number CCDC 1869009. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre (www.ccdc.cam.ac.uk/data_request/cif). All the other data supporting the findings of this study are available within the article and its Supplementary Information, or from the corresponding author upon reasonable request.

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Acknowledgements

We acknowledge funding and support from the UK Engineering and Physical Sciences Research Council (grants EP/M027015/ and EP/P001386/1), European Research Council (grant CoG612724), Royal Society (grant UF110005), the National EPSRC UK EPR Facility, The University of Manchester and the UK National Nuclear Laboratory. B.E.A. and N.K. also thank the University of Manchester for computational resources and associated support services from the Computational Shared Facility.

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Contributions

E.L. and J.T.B. prepared and characterized the compound and its precursors. B.E.A. and N.K. performed the energy-scan calculations and analysed the results. E.L., A.J.W., I.J.V.-Y. and G.F.S.W. collected, solved, refined and analysed the crystallographic data. E.L., L.R.D., J.D.C. and P.J.C. recorded and interpreted the Raman data. F.T. recorded and interpreted the EPR data. S.T.L. originated the central idea, supervised the work, analysed the data, performed the DFT, NBO and QTAIM calculations and analysed the results, and wrote the manuscript with contributions from all the authors.

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Correspondence to Stephen T. Liddle.

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Supplementary information

Supplementary Information

Supplementary experimental details, computational details, characterization data, computational data tables and references.

Crystallographic data

CIF for compound 2; CCDC reference: 1869009.

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Lu, E., Atkinson, B.E., Wooles, A.J. et al. Back-bonding between an electron-poor, high-oxidation-state metal and poor π-acceptor ligand in a uranium(v)–dinitrogen complex. Nat. Chem. 11, 806–811 (2019). https://doi.org/10.1038/s41557-019-0306-x

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