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PRINCIPAL PUBLICATION AND AUTHORS

Aromatic hexazine [N6]4− anion featured in the complex structure of the high-pressure potassium nitrogen compound K9N56, D. Laniel (a,b), F. Trybel (c), Y. Yin (a,d), T. Fedotenko (a), S. Khandarkhaeva (e), A. Aslandukov (a), G. Aprilis (f), A.I. Abrikosov (c), T. Bin Masood (g), C. Giacobbe (f), E.L. Bright (f), K. Glazyrin (h), M. Hanfland (f), J. Wright (f), I. Hotz (g), I.A. Abrikosov (c), L. Dubrovinsky (e), N. Dubrovinskaia (a,c), Nat. Chem. 15, 641-646 (2023); https:/doi.org/10.1038/s41557-023-01148-7 (a) Material Physics and Technology at Extreme Conditions, Laboratory of Crystallography, University of Bayreuth (Germany) (b) Centre for Science at Extreme Conditions and School of Physics and Astronomy, University of Edinburgh (UK) (c) Department of Physics, Chemistry and Biology (IFM), Linköping University (Sweden) (d) State Key Laboratory of Crystal Materials, Shandong University, Jinan (China) (e) Bayerisches Geoinstitut, University of Bayreuth (Germany) (f) ESRF (g) Department of Science and Technology (ITN), Linköping University (Sweden) (h) Photon Science, Deutsches Elektronen-Synchrotron, Hamburg (Germany)

REFERENCES

[1] T.M. Krygowski et al., ChemTexts 1, 12 (2015). [2] T. Ha et al., Chem. Phys. Lett. 195, 179-183 (1992).

Indeed, being planar, cyclic and featuring a 10π-electron system, it fulfils Hückel s rule the basic criteria for aromaticity. Its aromaticity was also verified through density functional theory (DFT) calculations using more advanced criterion. Indeed, another characteristic of aromaticity is to have a homogeneous electronic distribution across the planar ring. As evidenced through the DFT calculations, and shown in Figure 36, the [N6]4- anion of the K9N56 compound was also found have this feature.

The stability of the K9N56 compound was studied upon progressive pressure release. Powder X-ray diffraction data were collected at beamline ID15B down to a pressure of 22 GPa. Between pressures of 32 and 22 GPa, the diffraction signal of the K9N56 solid disappears, suggesting its decomposition. It is hypothesised that the K9N56 solid breaks down due to the loosely van der Waals- bonded neutral N2 dimers easily being able to flow out of the material, rather than due to the lack of stability of the hexazine anion.

Fig. 36: Visualisation of the all-electron charge density of the [N6]4− ring in K9N56 from DFT-based calculations. a,b) Two-dimensional slice through the all-electron charge density in the plane defined by the N6 ring, with (a) and without (b) the nitrogen atoms, drawn as blue spheres. c) The same N6 ring as

in (b), but with an isoelectronic surface of 0.2 e− per Å3.

These results emphasise that single-crystal X-ray diffraction of polycrystalline aggregates performed with the extreme-brilliance radiation of fourth-generation synchrotrons is crucial for solving very complex structures of solids at high pressures. This study also provides a striking example contradicting the trope of structural simplicity at high densities. The synthesis of the novel [N6]4- aromatic hexazine anion should stimulate further exploration of nitrogen chemistry in the search for novel nitrogen-based technological materials.