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A terminal neptunium(V)–mono(oxo) complex

Abstract

Neptunium was the first actinide element to be artificially synthesized, yet, compared with its more famous neighbours uranium and plutonium, is less conspicuously studied. Most neptunium chemistry involves the neptunyl di(oxo)-motif, and transuranic compounds with one metal–ligand multiple bond are rare, being found only in extended-structure oxide, fluoride or oxyhalide materials. These combinations stabilize the required high oxidation states, which are otherwise challenging to realize for transuranic ions. Here we report the synthesis, isolation and characterization of a stable molecular neptunium(V)–mono(oxo) triamidoamine complex. We describe a strong Np≡O triple bond with dominant 5f-orbital contributions and σu > πu energy ordering, akin to terminal uranium-nitrides and di(oxo)-actinyls, but not the uranium–mono(oxo) triple bonds or other actinide multiple bonds reported so far. This work demonstrates that molecular high-oxidation-state transuranic complexes with a single metal–ligand bond can be stabilized and studied in isolation.

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Fig. 1: Previous molecular high-oxidation-state Np and Pu structural motifs of relevance to this study and synthesis of complexes [(TrenTIPS)NpIVCl] (1), [(TrenTIPS)NpIII] (2) and [(TrenTIPS)NpVO] (3) from NpIVCl4 and the ligand transfer reagent [(TrenTIPS)Li3] (TrenTIPS = {N(CH2CH2NSiiPr3)3}3−).
Fig. 2: Cyclic voltammogram of [(TrenTIPS)NpIVCl] (1) in THF (2 mM) versus the ferrocene +1/0 redox couple at varying scan rates.
Fig. 3: Molecular structures of crystalline [(TrenTIPS)NpIII] (2) and [(TrenTIPS)NpVO] (3).
Fig. 4: Variable-temperature magnetic data and modelled energy level diagrams for [(TrenTIPS)NpIVCl] (1), [(TrenTIPS)NpIII] (2) and [(TrenTIPS)NpVO] (3).
Fig. 5: Computed σu and πu energy gaps for [(TrenTIPS)NpIVCl] (1), [(TrenTIPS)NpIII] (2) and [(TrenTIPS)NpVO] (3) and NBO representations of the σ and two π bonds of the Np≡O triple bond in [(TrenTIPS)NpVO] (3).

Data availability

Crystallographic data for the structures reported in this Article have been deposited at the Cambridge Crystallographic Data Centre, under deposition numbers CCDC 2055264 (2) and 2055265 (3). Copies of the data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/. All other data are presented in the main text and the Supplementary Information tables, and are also available from the corresponding authors on reasonable request.

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Acknowledgements

M.P. thanks J. Bendix (University of Copenhagen) for stimulating scientific discussions. Experimental work by M.S.D. was supported by the ActUsLab programme (AUL-2017-20-206) under contract with the European Commission. This work has been partially supported by the ENEN+ project, which has received funding from the Euratom research and training Work Programme 2016–2017–1 #755576 (M.S.D., O.W. and S.T.L.). Funding and support from the ENEN+ project for mobility support (A-9514681062; M.S.D.), the UK EPSRC (EP/T011289/1 and EP/M027015/1; S.T.L.), EU ERC (GoG612724; S.T.L.), COST Action CM1006 (S.T.L.), US DOE-BES Heavy Element Chemistry Program at Los Alamos National Laboratory (LANL; DE-AC52-06NA25396; C.A.P.G. and A.J.G.), LANL Laboratory Directed Research and Development program for a Distinguished J. R. Oppenheimer Postdoctoral Fellowship (LANL-LDRD 20180703PRD1; C.A.P.G.) and The University of Manchester (M.S.D., A.J.W., S.T.L.) is gratefully acknowledged.

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Authors and Affiliations

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Contributions

M.S.D. planned the experiments, synthesized the complexes and acquired and interpreted their characterization data. C.A.P.G. and A.J.G. prepared compounds for electrochemical experiments and performed, analysed and interpreted the electrochemical experiments. M.P., J.-C.G., E.C. and R.C. acquired, analysed, modelled and interpreted the magnetic data. A.K. conducted the multireference calculations. A.J.W. and O.W. collected and refined the crystallographic data. S.T.L. conducted the single-reference calculations. O.W. and S.T.L. conceived the research idea, coordinated the research, analysed and interpreted all the data, and wrote the manuscript with contributions from all authors.

Corresponding authors

Correspondence to Olaf Walter or Stephen T. Liddle.

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Nature Chemistry thanks the anonymous reviewers for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

Experimental and theoretical details, Supplementary Figs. 1–32 and Tables 1–8.

Supplementary Data 1

Final coordinates and energy from B3LYP calculations on NpSi3C6N4OH21.

Supplementary Data 2

Final coordinates and energy from B3LYP calculations on 3.

Supplementary Data 3

Final coordinates and energy from a DFT single-point energy calculation on geometry-optimized 3 (all-electron).

Supplementary Data 4

Final coordinates and energy from a DFT single-point energy calculation on geometry-optimized 3 (5d frozen core).

Supplementary Data 5

Final coordinates and energy from a DFT single-point energy calculation on geometry-optimized 3 (6p frozen core).

Supplementary Data 6

Final coordinates and energy from a DFT single-point energy calculation on geometry-optimized 3 with Np–O distance set to 1.84 Å.

Supplementary Data 7

Final coordinates and energy from a DFT single-point energy calculation on geometry-optimized 3 with Np–O distance set to 1.85 Å.

Supplementary Data 8

Final coordinates and energy from a DFT single-point energy calculation on geometry-optimized 3 with Np–O distance set to 1.86 Å.

Supplementary Data 9

Final coordinates and energy from a DFT single-point energy calculation on geometry-optimized 3 with Np–O distance set to 1.87 Å.

Supplementary Data 10

Final coordinates and energy from a DFT single-point energy calculation on geometry-optimized 3 with Np–O distance set to 1.88 Å.

Supplementary Data 11

Final coordinates and energy from a DFT single-point energy calculation on geometry-optimized 3 with Np–O distance set to 1.89 Å.

Supplementary Data 12

Final coordinates and energy from a DFT single-point energy calculation on geometry-optimized 3 with Np–O distance set to 1.90 Å

Supplementary Data 13

Crystallographic information file (CIF) for 2, CCDC 2055264.

Supplementary Data 14

Crystallographic information file (CIF) for 3, CCDC 2055265.

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Dutkiewicz, M.S., Goodwin, C.A.P., Perfetti, M. et al. A terminal neptunium(V)–mono(oxo) complex. Nat. Chem. 14, 342–349 (2022). https://doi.org/10.1038/s41557-021-00858-0

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