Energetic Metal Imidazolate-Dicyanamide Frameworks for Hypergolic Solid Fuels: Ignition Property Tuning through Construction of Diverse Structural Linking Modes

Hypergolic coordination compounds have received extensive attention in the field of propellants. However, the regulation of the spontaneous combustion performance is still in its infancy through precise structural design. In this study, we employed multiple coordination modes and high-energy dicyandiamide anions (DCA– = N(CN)2–), together with neutral 1-isopropylimidazole (ipIM) and metal centers, to synthesize energetic coordination compounds (ECCs) [M3(DCA)6(ipIM)8]n (M = Co 1, Ni 2) and [Cd3(DCA)6(ipIM)4]n 3. The experimental results indicated that 1–3 exhibit high volumetric energy densities (Ev, 30.69–37.12 kJ·cm–3), considerable specific impulse (Isp), low mechanical sensitivities, and significantly different behaviors in hypergolic characteristics. The hypergolic tests demonstrated that 1 and 2 can be ignited by red fuming nitric acid (RFNA) with ignition delay (ID) times of 81 and 227 ms, while 3 cannot be ignited by RFNA. The calculated electron density surface potential (ESP) further revealed that DCA– anions in 1 and 2 selecting the monodentate coordination mode (mode 1) exhibit stronger electronegativity to react with RFNA than that of 3 without the uncoordinated C≡N site (modes 3 and 5). This study not only identified novel spontaneous combustion materials but also provided a new insight for the development of tunable hypergolic materials.