Synthesis, characterization and properties of bis(imidazole)dihydroboronium hypergolic ionic liquids

A series of bis(imidazole)dihydroboronium ionic liquids with dicyanamide and nitrocyanamide anions were prepared by two-step synthetic method using trimethylamine borane as reaction substrate. Their structures and compositions were confirmed by ¹H NMR and ¹³C NMR, infrared and high resolution mass spectrometry, and properties were measured and calculated. The results show that resulting ionic liquids have good performances: decomposition temperature up to 150℃, densities from 1.04 to 1.27 g·cm^-3 and specific impulses from 173.8 to 202.1 s. With the increase of hydrocarbon side chains in imidazolium cations, densities of ionic liquids decrease while heats of formation increase. The dicyanamide ionic liquids show shorter ignition delay times than the corresponding nitrocyanamide ionic liquids. Among the ionic liquids, bis(1-allyl-1H-imidazole-3-yl)dihydroboronium dicyanamide possesses good comprehensive properties such as viscosity 69 mPa·s, decomposition temperature 180℃, heat of formation 753.6 kJ·mol^-1, ignition delay time 18 ms and specific impulse 176.7 s, therefore exhibiting application potential as the green fuel of hypergolic bipropellants.