M A T E R I A L S F O R T O M O R R O W ' S I N N O V A T I V E A N D S U S T A I N A B L E I N D U S T R Y

S C I E N T I F I C H I G H L I G H T S

5 2 H I G H L I G H T S 2 0 2 3 I

Synthesis and X-ray characterisation of a new nitrogen aromatic species: the [N6]4- hexazine ring

The novel K9N56 compound was synthesised at extreme pressure and temperature conditions and characterised by single-crystal X-ray diffraction. This incredibly complex compound features neutral N2 dimers, [N5]

- rings and hitherto unknown [N6] 4-

rings. These remarkable [N6] 4- hexazine rings are

aromatic, an electronic property associated with enhanced chemical stability.

Aromaticity plays a vital role in industrial processes, chemistry and biology. In fact, aromaticity is thought to be one of the essential components of life, allowing for the existence of key hydrocarbon species. The importance of aromaticity a peculiar electron-based feature is due to the fact that it provides chemical species increased stability, enabling them to persist in otherwise impossible environments [1]. Since it has been established that aromaticity is not exclusive to carbon-based species, many researchers have dedicated their studies to the discovery of new exotic aromatic units. In particular, nitrogen-only molecules have been targeted as they are known to be notoriously unstable, a fact that could be overturned by aromaticity to make them significantly more appealing for potential technological applications.

Hexaazabenzene, an N6 ring analogous to the most well- known aromatic species, benzene, has been shortlisted as a promising candidate. A variety of configurations and geometries have been proposed based on calculations [2], including that of the hexazine anion [N6]4-, but up until now, its experimental synthesis had not been not achieved.

In an attempt to discover novel nitrogen aromatic species, potassium azide (KN3) and molecular nitrogen (N2) were loaded in a diamond anvil cell and compressed to an extremely high pressure of 46 GPa (i.e., 460,000 times atmospheric pressure) and heated to 2000°C using high power lasers. To investigate the atomic arrangement adopted by the presumed newly formed compound, the pressurized, sub-micron-sized crystallite samples were characterised using high-quality single-crystal X-ray diffraction data taken at beamlines ID15B and ID11. With these data, a full solution of the crystal structure of the K9N56 compound, shown in Figure 35, could be obtained.

The crystal structure of K9N56 features a degree of complexity almost never observed for solids produced at such high pressures. The unit cell is comprised of a repeating arrangement of 520 atoms, 72 K and 448 N, and the nitrogen atoms are assembled in three distinct type of units: N2 dimers, planar [N5]- rings, and planar [N6]4- rings. The latter, a hexazine anion, is a previously unknown nitrogen species and of particular interest.

Fig. 35: Experimentally determined crystal structure of the K9N56 compound. a) Unit cell viewed along the a axis. b)-e) The chemical environment of the N6 ring (b), with an emphasis on the neighbouring atoms: K atoms (c), N5 rings (d) and N2 dimers (e). The green spheres represent K atoms, and all other spheres represent N atoms,

with different colours representing distinct species.