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Complexes featuring a linear [N≡U≡N] core isoelectronic to the uranyl cation

Nature Chemistry volume 12, pages 962–967 (2020)Cite this article

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Abstract

The aqueous chemistry of uranium is dominated by the linear uranyl cation [UO2]2+, yet the isoelectronic nitrogen-based analogue of this ubiquitous cation, molecular [UN2], has so far only been observed in an argon matrix. Here, we present three different complexes of [UN2] obtained by the reaction of the uranium pentahalides UCl5 or UBr5 with anhydrous liquid ammonia. The [UN2] moieties are linear, with the U atoms coordinated by five additional ligands (ammonia, chloride or bromide), resulting in a pentagonal bipyramidal coordination sphere that is also commonly adopted by the uranyl cation [UO2(L)5]2+ (L, ligand). In all three cases, the nitrido ligands are further coordinated through their lone pairs by the Lewis-acidic ligands [U(NH3)8]4+ to form almost linear, trinuclear complex cations. Those were characterized by single-crystal X-ray diffraction, Raman and infrared spectroscopy, 14N/15N isotope studies and quantum chemical calculations, which support the presence of two U≡N triple bonds within the [UN2] moieties.

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Fig. 1: Representative Lewis structures and structures of cations 18+, 27+ and 36+ in their crystal structures, showing the central [N≡U≡N] units.
Fig. 2: Raman spectra of the compound 2Br7·10.5NH3 with a natural 14N/15N isotope ratio and the isotope-labelled compound with 15NH3.
Fig. 3: IBOs corresponding to the U≡N triple bonds in the [UN2] molecule and cation 18+.

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Data availability

The data supporting the findings of this study (powder X-ray patterns, Raman and infrared spectra, and details of the quantum chemical calculations) are available within this Article and its Supplementary Information. Crystallographic data are available free of charge through the Cambridge Crystallographic Data Centre (www.ccdc.cam.ac.uk), with CCDC nos. 1868200 for (1Br8·26NH3), 1984956 (2Br7·10.5NH3) and 1868199 (3Cl6·6NH3). Source data are provided with this paper.

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Acknowledgements

S.S.R. and F.K. thank the Deutsche Forschungsgemeinschaft for generous funding. A.J.K. thanks CSC, the Finnish IT Center for Science, for computational resources. We thank B. Roling for use of his Raman spectrometer. We thank U. Müller for bringing [UN2] to our attention many years ago.

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Author notes

  1. These authors contributed equally: H. Lars Deubner and Matthias Müller.

Authors and Affiliations

  1. Anorganische Chemie, Fluorchemie, Fachbereich Chemie, Philipps-Universität Marburg, Marburg, Germany

    Stefan S. Rudel, H. Lars Deubner, Matthias Müller & Florian Kraus

  2. Department of Chemistry and Materials Science, Aalto University, Aalto, Finland

    Antti J. Karttunen

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Contributions

S.S.R. conceived and designed experiments and interpreted crystal structures, powder patterns and spectra for 18+ and 36+. S.S.R. and M.M. performed experiments and analytics on 18+ and 36+. M.M. contributed to the manuscript. H.L.D. conceived, designed and performed experiments on 27+. H.L.D. and M.M. interpreted the single-crystal structure of 27+. A.J.K. conceived and designed the quantum chemical calculations, interpreted results and wrote the theoretical parts of the manuscript. F.K. designed and guided research and interpreted the single-crystal structure determinations, powder patterns and spectra. S.S.R. and F.K. wrote the manuscript. All authors discussed the results and commented on the manuscript.

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Correspondence to Florian Kraus.

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Extended data

Extended Data Fig. 1 Raman spectra of 1Br8·26NH3 and 3Cl6·6NH3 measured in fused-silica tubes at room temperature.

a, 1Br8·26NH3 in NH3 crystallized at −40 °C. b, Decomposition product of 1Br8·26NH3 in NH3 obtained at room temperature. c, Solution over 1Br8·26NH3 in NH3. d) 3Cl6·6NH3 in NH3 obtained at room temperature. Bands at 850–950 cm−1 are assigned to the U≡N stretching vibration. The abbreviation rt means room temperature, the laser wavelength was λ = 532 nm.

Extended Data Fig. 2 Simulated Raman spectra for cations 18+, 27+, and 36+ at DFT-PBE0/TZVP level of theory.

a, Cation 18+. b, Cation 27+. c, Cation 36+. The grey peaks are the raw data, the black curve has been obtained from the peak data by applying Lorenzian broadening (see computational details).

Extended Data Fig. 3 Diamond ATR-FTIR spectra of the decomposition products of 1Br8·26NH3 and 3Cl6·6NH3.

a, b, Decomposition products of 1Br8·26NH3 (a) and 3Cl6·6NH3 (b) both after removal of liquid ammonia at room temperature. Uranyl impurities would result in a band at ~911–960 cm−1. The abbreviation rt stands for room temperature.

Extended Data Fig. 4 Powder X-ray diffraction pattern of the residue of the reaction of UBr5 with ammonia after heating to 600 °C in a sealed steel ampoule under argon.

Reflection positions of UNBr are marked with red strokes, those of UBr3 with blue strokes. Reflections marked with an asterisk belong to a yet unidentified compound. Measured using CuKα1 radiation in a 0.3 mm capillary.

Extended Data Fig. 5 Valence IBOs for cation 18+. Valence IBOs for cation 18+.

a, Uter−µ-N interaction. b, U−NH3 interaction. c) N−H bond. The red and green orbitals are the IBOs; atom colour code: U, cyan; N, blue; H, white. The valence IBOs shown in Fig. 3 (main text) and the four singly-occupied IBOs with four unpaired f-electrons of the Uter atom are omitted. For further details, see the caption of Fig. 3 (main text). The Isovalue for IBO isosurface plots is 0.08 a.u.

Supplementary information

Supplementary Information

Synthesis of starting materials, Supplementary Figs. 1–4, Tables 1–4 and references.

Supplementary Data 1

Crystallographic data for compound 1Br8·26NH3 (CCDC 1868200).

Supplementary Data 2

Crystallographic data for compound 2Br7·10.5NH3 (CCDC 1984956).

Supplementary Data 3

Crystallographic data for compound 3Cl6·6NH3 (CCDC 1868199).

Source data

Source Data 1

Reference orbitals for intrinsic bond orbital (IBO) analysis.

Source Data 2

Cartesian coordinates of the studied systems in XYZ format.

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Rudel, S.S., Deubner, H.L., Müller, M. et al. Complexes featuring a linear [N≡U≡N] core isoelectronic to the uranyl cation. Nat. Chem. 12, 962–967 (2020). https://doi.org/10.1038/s41557-020-0505-5

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