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Aromatic hexazine [N6]4− anion featured in the complex structure of the high-pressure potassium nitrogen compound K9N56

Nature Chemistry volume 15, pages 641–646 (2023)Cite this article

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Abstract

The recent high-pressure synthesis of pentazolates and the subsequent stabilization of the aromatic [N5]− anion at atmospheric pressure have had an immense impact on nitrogen chemistry. Other aromatic nitrogen species have also been actively sought, including the hexaazabenzene N6 ring. Although a variety of configurations and geometries have been proposed based on ab initio calculations, one that stands out as a likely candidate is the aromatic hexazine anion [N6]4−. Here we present the synthesis of this species, realized in the high-pressure potassium nitrogen compound K9N56 formed at high pressures (46 and 61 GPa) and high temperature (estimated to be above 2,000 K) by direct reaction between nitrogen and KN3 in a laser-heated diamond anvil cell. The complex structure of K9N56—composed of 520 atoms per unit cell—was solved based on synchrotron single-crystal X-ray diffraction and corroborated by density functional theory calculations. The observed hexazine anion [N6]4− is planar and proposed to be aromatic.

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Fig. 1: Experimentally determined crystal structure of the K9N56 compound at 61 GPa by SC-XRDp.
Fig. 2: Visualization of the all-electron charge density of the [N6]4− ring in K9N56 from DFT-based calculations at 61 GPa.
Fig. 3: PXRD patterns of K9N56 and unit cell volume of K9N56 with pressure.

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Data availability

The details of the crystal structure investigations may be obtained from FIZ Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.de) on quoting the deposition numbers CSD 2127463 (K9N56 at 61 GPa, obtained the Bayerisches Geoinstitut) and CSD 2166620 (K9N56 at 58 GPa, obtained at the ID18 beamline of the EBS-ESRF). A full dataset collected at 61 GPa on a sample containing the K9N56 compound, as well as reciprocal space unwarps, are available for download through the link https://figshare.com/s/587f623b762bdccb4308 (ref. 57). Datasets generated during and/or analysed during the current study, namely the raw SC-XRDp data of K9N56 at 58 GPa and the raw PXRD data collected during the decompression of K9N56, are available from the corresponding author on reasonable request. Source data are provided with this paper.

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Acknowledgements

We acknowledge the Deutsches Elektronen-Synchrotron (DESY, PETRA III) and the European Synchrotron Radiation Facility (ESRF) for provision of beamtime at the P02.2 and, ID15b and ID11 beamlines, respectively. D.L. thanks the Alexander von Humboldt Foundation, the Deutsche Forschungsgemeinschaft (DFG, project LA-4916/1-1) and the UKRI Future Leaders Fellowship (MR/V025724/1) for financial support. N.D. and L.D. thank the Federal Ministry of Education and Research, Germany (BMBF, grant no. 05K19WC1) and the Deutsche Forschungsgemeinschaft (DFG projects DU 954–11/1, DU 393–9/2 and DU 393–13/1) for financial support. Support from the Swedish Research Council (VR) grant no. 2019-05600, the Swedish Government Strategic Research Areas in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU no. 2009 00971) and SeRC, and the Knut and Alice Wallenberg Foundation (Wallenberg Scholar grant no. KAW-2018.0194) is gratefully acknowledged. Computations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at the PDC Centre for High Performance Computing (PDC-HPC) and the National Supercomputer Center (NSC). The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript. For the purpose of open access, we have applied a Creative Commons Attribution (CC BY) licence to any author accepted manuscript version arising from this submission.

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Authors and Affiliations

  1. Material Physics and Technology at Extreme Conditions, Laboratory of Crystallography, University of Bayreuth, Bayreuth, Germany

    Dominique Laniel, Yuqing Yin, Timofey Fedotenko, Andrey Aslandukov & Natalia Dubrovinskaia

  2. Centre for Science at Extreme Conditions and School of Physics and Astronomy, University of Edinburgh, Edinburgh, UK

    Dominique Laniel

  3. Department of Physics, Chemistry and Biology (IFM), Linköping University, Linköping, Sweden

    Florian Trybel, Igor A. Abrikosov & Natalia Dubrovinskaia

  4. State Key Laboratory of Crystal Materials, Shandong University, Jinan, China

    Yuqing Yin & Leonid Dubrovinsky

  5. Bayerisches Geoinstitut, University of Bayreuth, Bayreuth, Germany

    Saiana Khandarkhaeva

  6. European Synchrotron Radiation Facility, Grenoble, France

    Georgios Aprilis, Carlotta Giacobbe, Eleanor Lawrence Bright, Michael Hanfland & Jonathan Wright

  7. Department of Science and Technology (ITN), Linköping University, Norrköping, Sweden

    Alexei I. Abrikosov, Talha Bin Masood & Ingrid Hotz

  8. Photon Science, Deutsches Elektronen-Synchrotron, Hamburg, Germany

    Konstantin Glazyrin

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  1. Dominique Laniel
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Contributions

D.L., L.D. and N.D. designed the work. D.L. prepared the high-pressure experiments, and T.F. and G.A. performed the sample laser-heating. D.L., Y.Y., T.F., S.K., A.A., C.G., E.L.B., K.G., M.H. and J.W. contributed to the synchrotron XRD experiments. D.L. and L.D. processed the synchrotron XRD data. F.T. and I.A.A. performed the theoretical calculations. A.I.A, T.B.M. and I.H. carried out the analysis and provided the visualization of the charge densities. D.L., F.T. and L.D. contextualized the data interpretation. D.L., F.T., L.D., N.D. and I.A.A. prepared the first draft of the manuscript with contributions from all other authors. All authors commented on successive drafts and have given approval to the final version of the manuscript.

Corresponding author

Correspondence to Dominique Laniel.

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Nature Chemistry thanks Karl Christe and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Microphotographs of the samples in diamond anvil cells.

Microphotographs of the K-N samples in diamond anvil cells. a) One sample of KN3 embedded in N2 at 46 GPa before laser-heating and b) after laser-heating. c) The second sample KN3 embedded in N2 at 58 GPa before laser-heating and d) after laser-heating. For both sample, the opaque portion grew substantially after laser-heating, a sign of nitrogen diffusion into the KN3 precursor and thus the formation of a nitrogen-rich compound.

Extended Data Fig. 2 Raman spectroscopy measurements.

a) Raman spectroscopy measurements done on a KN3 + N2 sample. Spectra collected upon loading (at 12 GPa), as well as before and after laser-heating at 46 GPa. The inset shows an enlargement of the spectra for frequencies between 1600 and 2900 cm−1. b) An enlargement of the spectra between 1200 and 1550 cm−1. After laser-heating, the intense mode characteristic of the N3 azide (~ 1470 cm−1 at 46.0 GPa) disappears indicating the chemical reaction of KN3 into another compound, that is K9N56. Broad modes centered around 1930 and 2160 cm−1 appear (see inset in a)), which could belong to the N2 molecules trapped in K9N56, as previously identified in the N2(H2)2 (ref. 77), (N2)6(H2)7 (ref. 78) and Xe(N2)2 (ref. 79) compounds. Alternatively, they could be due to fluorescence produced by the quenched sample.

Source data

Extended Data Fig. 3 Interatomic distance and stress during molecular dynamics calculations.

Average interatomic distances and stress in the K9N56 compound at 61 GPa obtained by AIMD simulations at 300 K as a function of simulation time. The AIMD simulations were started from the structure relaxed at 61 GPa. Pressure equilibrated to about 70 GPa at finite temperature. The first 800 steps were treated as equlillibration steps and are not shown. a) Average interatomic distance for the N5 rings, N6 rings and N2 dimers in K9N56. b) Components of the stress tensor σ as a function of the simulation time. These calculations both show i) the dynamical stability of the K9N56 compound at 300 K; ii) interatomic values in agreement with those observed experimentally; iii) the substantially larger thermal motion of the N2 and N6 species, in accordance with the thermal displacement parameters found experimentally.

Source data

Extended Data Fig. 4 Electronic density of states (eDOS) of K9N56 at 61 GPa.

Electronic density of states (eDOS) of K9N56 at 61 GPa calculated for the relaxed structures obtained in static calculations with an electronic temperature of Tel = 800 K (green line) and Tel = 6000 K (black line). Both sets of calculations suggest K9N56 to be a metal, having filled states at the Fermi energy. The comparison between the two electronic temperatures shows the smearing effect at higher temperatures.

Source data

Extended Data Fig. 5 Calculated electronic density of states of metallic K9N56 at 61 GPa.

Calculated partial electronic density of states (eDOS) of metallic K9N56 at 61 GPa obtained in static calculations with an electronic temperature of Tel = 6000 K. a) eDOS of K9N56 for the relaxed structure. b) Enlargement of the partial eDOS around the Fermi energy. The contribution from the electronic states of the potassium atoms’ at the Fermi energy is seen to be essentially null, with the nitrogen atoms found to be driving the compound’s metallicity (that is anion-driven metallicity).

Source data

Supplementary information

Supplementary Information

Supplementary Figs. 1–9 and Supplementary Tables 1–4.

Supplementary Data 1

Crystallographic data for K9N56 at 61 GPa.

Supplementary Data 2

Crystallographic data for K9N56 at 61 GPa, structure factors.

Supplementary Data 3

Crystallographic data for K9N56 at 58 GPa.

Supplementary Data 4

Crystallographic data for K9N56 at 58 GPa, structure factors.

Supplementary Data 5

Source data for Supplementary Figs. 3 and 9.

Supplementary Data 6

Molecular dynamics first and last steps.

Supplementary Data 7

Electronic structure calculations atomic coordinates of the optimized computational models.

Source data

Source Data Fig. 2

Raman spectra collected on KN3 during compression and on K9N56 resulting from laser-heating.

Source Data Fig. 3

Data include powder X-ray diffraction patterns as well as the volume of the unit cell of the K9N56 compound at various pressures.

Source Data Extended Data Fig./Table 2

Raman spectra collected on the K-N samples at 12 GPa, 46 GPa (before laser-heating) and 46 GPa (after laser-heating).

Source Data Extended Data Fig./Table 3

AIMD simulations at 300 K, the interatomic distances are provided as well as the stress, both as a function of the number of steps.

Source Data Extended Data Fig./Table 4

Raw data of the electronic density of states (eDOS) of K9N56 at 61 GPa.

Source Data Extended Data Fig./Table 5

Raw data of the calculated electronic density of states of metallic K9N56 at 61 GPa obtained in static calculations with an electronic temperature of Tel = 6000 K.

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Laniel, D., Trybel, F., Yin, Y. et al. Aromatic hexazine [N6]4− anion featured in the complex structure of the high-pressure potassium nitrogen compound K9N56. Nat. Chem. 15, 641–646 (2023). https://doi.org/10.1038/s41557-023-01148-7

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