Synthesis and characterization of a new nitrogen aromatic species
Extreme pressure and temperature conditions are used to synthesize K9N56 which is comprised of the aromatic hexazine unit.
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 hydrocarbons 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.
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 and 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, including that of the hexazine anion [N6]4-, but up until now, its experimental synthesis was not achieved.
Aromatic hexazine unit
Dr Dominique Laniel led a team of international collaborators in employing extreme pressure and temperature conditions to synthesize a remarkably complex K9N56 compound which is comprised of the [N6]4- hexazine ring.
To accomplish this, potassium azide (KN3) and molecular nitrogen (N2) were first squeezed to enormous pressures—more than 400,000 times atmospheric pressure—and heated using high power lasers to 2000°C. Then, to characterize the atomic arrangement adopted by the newly formed compound under these conditions, the samples were illuminated by an intense X-ray beam at two particle accelerators, the German PETRA III and the European EBS-ESRF synchrotrons.
Dr Laniel commented:
We were very surprised about the atomic arrangement of the K9N56 compound: it is of a complexity almost never observed for solids produced at such high pressures. We found that it is composed of a repeating arrangement of 520 atoms, 72 K and 448 N, and that the nitrogen atoms were assembled in three distinct type of units: N2 dimers, planar [N5]- rings, and planar [N6]4- rings. We could immediately see that the planar [N6]4- rings were fulfilling the basic requirements for aromaticity—namely Hückel’s rule—though further analysis of the experimental data and advanced calculations methods were required to verify it.
These computational analyses were performed by Dr Florian Trybel and his colleagues at Linköping University. The calculations corroborated the stability of the K9N56 compound and provided further insight establishing the aromaticity of the [N6]4- hexazine ring.
This study, which is published in Nature Chemistry, brings to a close a more-than 40 years old quest for an aromatic hexazine unit. The demonstration of its existence will give materials scientists and chemists a clear target for its synthesis at normal pressure conditions. On account of its aromaticity, and thereby of its expected increased stability, the [N6]4- ring is a prime candidate for technological applications.