Dual Functions of Water in Stabilizing Metal-Pentazolate Hydrates [M(N₅)₂(H₂O)₄]·4H₂O (M = Mn, Fe, Co, and Zn) High-Energy-Density Materials

High-energy-density materials (HEDMs) containing the cyclo-pentazole anion (cyclo-N₅) are highly desirable due to the release of more energy and being environmentally more friendly than conventional HEDMs. However, the synthesis of stable cyclo-N₅-containing HEDMs has been a challenge. In this study, quantum mechanical calculations were employed to elucidate the stability of [M(N₅)₂(H₂O)₄]·4H₂O (M = Mn, Fe, Co, and Zn), one of the few recently reported cyclo-N₅-contained HEDMs, under ambient conditions. The results from our study indicate that the stability is due to the presence of two types of water (coordinated H₂O (c-H₂O) and hydrogen-bonded H₂O (h-H₂O)). Each type uses a unique mode to stabilize the highly reactive M(N₅)₂ cores. c-H₂O binds with M to reduce the M â†" cyclo-N₅ interaction, leading to a less activated cyclo-N₅ and higher kinetic barriers (E_as) for its decomposition. In contrast, h-H₂O takes advantage of its permanent electrostatic interactions with cyclo-N₅ to inhibit the decomposition. The stabilizing effects of the two types of water on M(N₅)₂ are similar. On the basis of the lower energy cost to remove h-H₂O from the materials and the subsequent large decrease in the E_a due to this removal, we propose that h-H₂O acts as a "safety device" that prevents the materials from becoming kinetically unstable. For future design of cyclo-N₅-contained HEDMs, we proposed the use of various molecular building blocks, such as NH₃, H₂S, and PH₃, which can tightly bind to M to reduce the M â†" cyclo-N₅ interaction and impose permanent electrostatic interactions with cyclo-N₅ to provide the similar dual functions of H₂O to suppress cyclo-N₅ decomposition.