A series of energetic metal pentazolate hydrates

Singly or doubly bonded polynitrogen compounds can decompose to dinitrogen (N₂) 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 (N₃^-) or pentazenium cation (N₅⁺) are also known, with compounds containing the pentazole anion, cyclo-N₅^-, 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-N₅ complex. Here we report the synthesis and characterization of five metal pentazolate hydrate complexes [Na(H₂O)(N₅)]·2H₂O, [M(H₂O)₄(N₅)₂]·4H₂O (M = Mn, Fe and Co) and [Mg(H₂O)₆(N₅)₂]·4H₂O 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 N₅^- 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 N₅^- ion in terms of its bonding interactions will enable the exploration of inorganic nitrogen analogues of metallocenes and other unusual polynitrogen complexes.