Synthesis of asymmetric [bis(imidazolyl)-BH₂]⁺-cation-based ionic liquids as potential rocket fuels

As potential hypergolic fuels, hypergolic ionic liquids have attracted much attention since their development. Herein, a series of hypergolic ionic liquids based on asymmetric [bis(imidazolyl)-BH₂]⁺ cations were synthesized. The asymmetric structure of these hypergolic ionic liquids was further confirmed by NMR, infrared (IR), and high-resolution mass spectrometry-electron spray ionization (HRMS-ESI). Moreover, these hypergolic ionic liquids possess a high density of over 1.00 g cm^-3, a comprehensive liquid range from -60 °C to 20 °C, and a density-specific impulse performance ranging from 305.4 to 357.8 s g cm^-3, which is superior to that of unsymmetrical dimethylhydrazine. Remarkably, (1-allyl-1H-imidazol-3-ium-1-yl)(1-methyl-1H-imidazol-3-ium-1-yl) dihydroboronium dicyandiamide had the best ignition-delay time (18 ms), a high density (1.114 g cm^-3), and a high value for heat of formation (400 kJ mol^-1/1.48 kJ g^-1). This work provides the possibility of a promising and green hypergolic fuel as rocket propellant.