Synthesis, structure and properties of imidazolium-based energetic ionic liquids

Abstract

A series of imidazolium energetic ionic liquids (EILs) 2a, 3a–f, and 4a–f with a nitrooxyethyl or hydroxyethyl side chain in their cations were easily synthesized starting from N-methylimidazole via quaternization, nitration, and metathesis reactions. The EILs and intermediates were fully characterized by FT-IR, UV/Vis, ESI-MS, ¹H NMR, ¹³C NMR, or elemental analysis. Compounds 3a, 3b, and 4b were further studied through single crystal X-ray diffraction. Solubility experiment shows that all the EILs have good solubilities in most polar solvents. The thermal properties of the EILs were investigated via differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA). All the EILs have a very wide liquid temperature range over 100 °C. DSC data show that all the energetic compounds are typical ionic liquids with glass transition temperatures from −77 °C to −12 °C and melting temperatures below 100 °C. TGA data indicate that the new ionic liquids possess good thermal stabilities with decomposition temperatures above 165 °C, except for compounds 3d, 4c, and 4d. Generally, the introduction of a nitrooxy group lowered the melting points and decomposition temperatures, but increased the glass transition temperatures of the EILs. The energetic properties of the EILs were further estimated by Kamlet–Jacobs formula. The detonation velocities of ionic liquids 2a and 4a–f are 6.84–7.63 km s⁻¹, which are between those of TNT and RDX. The detonation pressures of ionic liquids 2a, 3e, 4a, 4b, 4d, and 4e are better than that of TNT. Besides, energetic data show that the introduction of a nitrooxy group improved the energetic properties of the new ionic liquids.