Hexanitrogen (N6): A Synthetic Leap Towards Neutral Nitrogen Allotropes

As the availability of renewable energy varies drastically over space and time, energy storage is a prime challenge for humankind. Current strategies include battery systems or high-energy molecules including hydrogen or ammonia for chemical energy storage. Compounds consisting only of the element nitrogen (polynitrogens or nitrogen allotropes), are deemed the cleanest and ideal energy storage materials due to their immense energy content (about five times higher than hydrogen) and because they release only harmless nitrogen (N₂) upon decomposition1. However, their extreme instability poses a significant synthetic challenge and no neutral molecular nitrogen allotrope beyond N2 has been isolated2,3. Here, we present the room temperature preparation of molecular N6 (hexanitrogen) through the gas-phase reaction of chlorine or bromine with silver azide, followed by trapping in argon matrices at 10 K. We also prepared neat N6 as a film at liquid nitrogen temperature (77 K), indicating its unexpectedly high stability (at a temperature that can be handled well) and the scalability of the synthesis. Infrared and ultraviolet spectroscopy, heavy isotope 15N labelling experiments, and ab initio computations firmly support our findings. The preparation of a metastable nitrogen allotrope beyond N₂ opens new vistas for the development of highly sought-after high-energy materials.