A series of energetic metal pentazolate hydrates

Abstract

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^1,2,3, 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^4,5,6, 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 1900s⁷ and characterized in the 1950s^8,9. Salts containing the azide anion (N₃⁻)^10,11,12 or pentazenium cation (N₅⁺)¹³ are also known, with compounds containing the pentazole anion, cyclo-N₅⁻, a more recent addition^14,15,16. Very recently, a bulk material containing this species was reported¹⁷ and then used to prepare the first example of a solid-state metal–N₅ complex¹⁸. 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 materials¹⁹ or enable the development of such materials containing only nitrogen^20,21,22,23. 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 metallocenes²⁴ and other unusual polynitrogen complexes.