Cyanoborohydride (CBH)-based hypergolic coordination compounds for versatile fuels

Four novel Hypergolic coordination compounds (HCCs) were constructed based on an active cyanoborohydride (CBH) anion, using Cu²⁺ as the center and alkyl substituted imidazole as ligands. The crystal structures and physicochemical properties of HCCs 1–4 were characterized with X-ray diffraction (XRD), Differential scanning calorimetry/Thermal gravimetric analysis (DSC/TG analysis), and bomb calorimetry. The results indicated that all the four prepared HCCs have high energy density (HCC-3, E_g = 29.27 kJ·g⁻¹ and E_v = 36.67 kJ·cm⁻³) and high stability, making them insensitive to mechanical stimuli. Hypergolic testing showed that the presence of unsaturated hydrocarbons in HCCs results in ultrashort ignition delay times (HCC-3, t_id = 4 ms, HCC-4, t_id = 2 ms). A theoretical calculation of the energy gap between the highest occupied molecular orbital (E_HOMO) and the lowest unoccupied molecular orbital (E_LUMO) was performed to illustrate the relationship between the structure of HCCs and their chemical reactivity. Furthermore, the combustion performance of the energetic mixtures of HCC solid fuels and Ammonium perchlorate (AP) solid oxidants was investigated. Remarkably, the AP and HCC mixtures displayed stable and violent combustion after being ignited, along with an ideal gas being produced at about 3.20 mol per 100 g. The HCCs showed catalytic performance through the thermal decomposition of AP during this process. The promising properties exhibited by HCCs provide a good opportunity for further study, and HCCs have a broad range of potential applications, including as a component of hypergolic fuel, as a solid composite propellant, and as a gas generator.