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.

Pentazolate anion (cyclo-N₅ ⁻)-containing compounds represent a unique class of energetic materials, which have attracted significant research attention. Since the bulk synthesis of cyclo-N₅ ⁻ has been achieved, significant progress has been made in pairing cyclo-N₅ ⁻ with different cations to synthesize nitrogen-rich energetic salts. However, to date, no rapid and efficient synthetic route has been developed. Herein, we investigated cyclo-N₅ ⁻ in detail as a potential all-nitrogen, room-temperature stable anion for the rapid and efficient synthesis of energetic ionic derivatives (2–10) for use as environmentally benign explosives. High yields (>90%) of potassium (2), ammonium (3), hydroxylammonium (4), hydrazinium (5), aminoguanidinium (6), diaminoguanidinium (7), biguanidinium (8), 3,4-diamino-1,2,4-triazolium (9), and 3,6,7-triamino-7H-[1,2,4]triazolo[4,3-b][1,2,4]triazol-2-ium (10) pentazolates were obtained by metathesis reactions of AgN₅ (1) and the respective chloride salts driven by the precipitation of AgCl. All the compounds were thoroughly characterized via infrared (IR) and multinuclear NMR (¹H and ¹³C) spectroscopy and elemental analysis. The solid-state structural features of the synthesized compounds were also investigated via single-crystal X-ray diffraction and several theoretical techniques. Moreover, their thermal stabilities were investigated by thermogravimetric (TG) and differential scanning calorimetry (DSC). Their heats of formation and detonation performance were calculated using the Born–Haber energy cycle and EXPLO5 v6.01 program, respectively. The data based on impact and friction tests show that these compounds range from being sensitive to insensitive. Based on the experimental and theoretical data, the biguanidinium pentazolate 8 (N = 81.36%, T_d = 124.8 °C, ΔH_f = 1362.0 kJ mol⁻¹, D = 9.257 km s⁻¹, P = 33.0 GPa, IS = 35 J, and FS = 300 N) shows potential as a high-performance energetic material.

A carbon-free inorganic–metal complex [Zn(H₂O)₄(N₅)₂]·4H₂O was synthesized in very good yield by the ion metathesis of [Na(H₂O)(N₅)]·2H₂O solution with Zn(NO₃)₂·6H₂O.