Strategies toward the design of energetic ionic liquids: nitro- and nitrile-substituted N,N′-dialkylimidazolium salts

Abstract

Twelve novel 1,3-dialkylimidazolium salts containing strongly electron-withdrawing nitro- and cyano-functionalities directly appended to the cationic heterocyclic rings have been synthesized; the influences of the substituents on both formation and thermal properties of the resultant ionic liquids have been determined by DSC, TGA, and single crystal X-ray diffraction, showing that an electron-withdrawing nitro-substituent can be successfully appended and has a similar influence on the melting behaviour as that of corresponding methyl group substitution. Synthesis of di-, or trinitro-substituted 1,3-dialkylimidazolium cations was unsuccessful due to the resistance of dinitro-substituted imidazoles to undergo either N-alkylation or protonation, while 1-alkyl-4,5-dicyanoimidazoles were successfully alkylated to obtain 1,3-dialkyl-4,5-dicyanoimidazolium salts. Five crystal structures (one of each cation type) show that, in the solid state, the NO₂-group has little significant effect, beyond the steric contribution, on the crystal packing.