Energetic isostructural metal imidazolate frameworks with a bridging dicyanamide linker towards high-performance hypergolic fuels

Abstract

The design and development of advanced hypergolic metal–organic frameworks (HMOFs) for space exploration have attracted widespread attention in recent years. However, enhancing the energy densities and elucidating the relationship between the structure and hypergolic properties of HMOFs are still in their infancy. In this work, high energy anions N(CN)₂⁻ were successfully introduced to build two isostructural HMOFs [M(DCA)₂(1-VIM)₂]_n (M = Cu 1, Cd 2; 1-VIM = 1-vinylimidazole, DCA⁻ = N(CN)₂⁻) by the molecular design of imidazole. The experimental results demonstrate that 1 and 2 exhibit spontaneous combustion properties, high volumetric energy density (E_v) and reliable stabilities. The ignition delay (ID) time of Cu-based 1 was shorter than that of Cd-based 2 upon coming in contact with white fuming nitric acid (WFNA). Theoretical calculations reveal that the different spontaneous combustion properties of 1 and 2 might be attributed to their metal-induced distinct electronic structures with different energy gaps, ΔE_L–H. In particular, because of implantation of the high energy anions N(CN)₂⁻, the E_v values of 1 (E_v = 42.91 kJ cm⁻³) and 2 (E_v = 41.98 kJ cm⁻³) are significantly greater than those of ZIF-based Zn(2-VIM)₂ (E_v = 18.90 kJ cm⁻³), Cd(2-VIM)₂ (E_v = 15.80 kJ cm⁻³), Co(2-VIM)₂ (E_v = 18.40 kJ cm⁻³) and Cu(2-VIM)₂ (E_v = 34.10 kJ cm⁻³). The two energetic isostructural HMOFs 1 and 2 can act as candidates for bipropellants with enhanced properties.