Singly or doubly bonded polynitrogen compounds can decompose to dinitrogen (N) with an extremely large energy release. This makes them attractive as potential explosives or propellants, but also challenging to produce in a stable form. Polynitrogen materials containing nitrogen as the only element exist in the form of high-pressure polymeric phases, but under ambient conditions even metastability is realized only in the presence of other elements that provide stabilization. An early example is the molecule phenylpentazole, with a five-membered all-nitrogen ring, which was first reported in the 1900s and characterized in the 1950s. Salts containing the azide anion (N) or pentazenium cation (N) are also known, with compounds containing the pentazole anion, cyclo-N, a more recent addition. Very recently, a bulk material containing this species was reported and then used to prepare the first example of a solid-state metal-N complex. Here we report the synthesis and characterization of five metal pentazolate hydrate complexes [Na(HO)(N)]·2HO, [M(HO)(N)]·4HO (M = Mn, Fe and Co) and [Mg(HO)(N)]·4HO that, with the exception of the Co complex, exhibit good thermal stability with onset decomposition temperatures greater than 100 °C. For this series we find that the N ion can coordinate to the metal cation through either ionic or covalent interactions, and is stabilized through hydrogen-bonding interactions with water. Given their energetic properties and stability, pentazole-metal complexes might potentially serve as a new class of high-energy density materials or enable the development of such materials containing only nitrogen. We also anticipate that the adaptability of the N ion in terms of its bonding interactions will enable the exploration of inorganic nitrogen analogues of metallocenes and other unusual polynitrogen complexes.

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.

A carbon-free inorganic-metal complex [Zn(HO)(N)]·4HO was synthesized by the ion metathesis of [Na(HO)(N)]·2HO solution with Zn(NO)·6HO. The complex was well characterized by IR and Raman spectroscopy, elemental analysis (EA), powder X-ray diffraction (PXRD), and differential scanning calorimetry (DSC). The structure of the complex was confirmed by single-crystal X-ray crystallography and a Zn(ii) ion is coordinated in a quadrilateral bipyramid environment in which the axial position is formed by two nitrogen atoms (N1) from two pentazolate rings (cyclo-N) and the equatorial plane is formed by four oxygen atoms (O1) from four coordinated water molecules. The thermal analysis of [Zn(HO)(N)]·4HO reveals that although water plays an important role in stabilizing cyclo-N, dehydration does not cause immediate decomposition of the anion. However, cyclo-N decomposed into N and N gas at 107.9 °C (onset). Based on its chemical compatibility and stability, the complex exhibits promising potential as a modern environmentally-friendly energetic material.

Pentazolate anion (cyclo-N ), and/or N , NO were used as the ligands to obtain a series of nitrogen-rich energetic three-dimensional (3D) frameworks [Cu(N )(N )] , [Ag(N )] , [Ba(N )(NO )(H O) ] , and [NaBa (N ) (NO )(H O) ] by self-assembly. These frameworks were characterized by single-crystal X-ray diffraction, SEM, IR and Raman spectroscopy, elemental analysis, and thermal analysis. All the frameworks exhibited regular supramolecular structures and excellent stabilities at room temperature which can be attributed to the strong coordination bonds between cyclo-N anions and metal ions. The successful stabilization of the cyclo-N in more 3D multi-ligand metal-N frameworks after Na-N frameworks has been demonstrated. This breakthrough offers new opportunities for the future of metal-pentazolate frameworks and polynitrogen chemistry.

According to previous reports, metal cations or water molecules are necessary for the stabilization of pentazolate anion (cyclo-N ) at ambient temperature and pressure. Seeking a new method to stabilize N is a big challenge. In this work, three anhydrous, metal-free energetic salts based on cyclo-N 3,9-diamino-6,7-dihydro-5 H-bis([1,2,4]triazolo)[4,3-e:3',4'-g][1,2,4,5] tetrazepine-2,10-diium, N-carbamoylguanidinium, and oxalohydrazinium (oxahy ) pentazolate were synthesized and isolated. All salts were characterized by elemental analysis, IR spectroscopy, H, C, and (in some cases) N NMR spectroscopy, thermal analysis (TGA and DSC), and single-crystal XRD analysis. Computational studies associated with heats of formation and detonation performance were performed by using Gaussian 09 and Explo5 programs, respectively. The sensitivity of the salts towards impact and friction was determined, and overall the real N explosives showed promising energetic properties.