Recent advances in the syntheses and properties of polynitrogen pentazolate anion cyclo-N5− and its derivatives
The pentazolate anion, or cyclo-N5−, which is a five-membered ring composed solely of nitrogen atoms, has a unique structure among polynitrogen compounds. Cyclo-N5− is receiving ever-increasing levels of attention because of its potential ability to store large amounts of energy compared to the azide ion, its environmentally friendly decomposition products, and its carbon- and hydrogen-free composition, which are promising characteristics for advancing the field of high-energy-density materials (HEDMs), that include explosives, oxidisers, and propellants in closed environments. In this review, we provide a detailed introduction to cyclo-N5− and cover the following topics: (1) substituted pentazoles as precursors of cyclo-N5−, with a focus on the syntheses and stabilities of substituted pentazole derivatives; (2) routes to cyclo-N5− through cleavage of C–N bonds in substituted pentazoles, during which competitive reactions between pentazole decomposition and C–N bond cleavage need to be considered to ensure a successful outcome; (3) complexes of cyclo-N5−, summarising recent progress toward producing cyclo-N5−-based complexes through the assembly of isolated cyclo-N5− with both metallic and nonmetallic components; and (4) interactions between cyclo-N5− and metal cations and non-metal species, as well as factors that influence the stability of these complexes; in particular, the thermal stabilities of prepared cyclo-N5− salts are discussed. This review summarises recent studies and is intended to improve the understanding of polynitrogen chemistry while supporting further research into its potential application as an efficient, safe, and environmentally friendly HEDM.