Synthesis, limitations, and thermal properties of energetically-substituted, protonated imidazolium picrate and nitrate salts and further comparison with their methylated analogs

Abstract

The possibility of forming simple energetic ionic liquids via the straightforward protonation of heterocyclic amines with nitric or picric acid was explored with 1-alkylimidazoles, 1-alkyl-2-methylimidazoles, and nitro, dinitro, and dicyano-substituted derivatives. The melting points of most of the prepared salts were lower than expected and of the 30 compounds prepared, more than half were found to melt below 100 °C. Limitations in the approach were found as a result of the use of energetic electron withdrawing substituents, such as nitro or cyano, which results in a reduction in nucleophilicity of the heterocycle and an inability to form salts with the acids studied. Interesting thermal behavior was observed with several of the new salts including supercooling and crystallization on heating. Comparison of the simple protonated imidazolium nitrate and picrate salts with their methylated analogs indicated that the protonated ionic liquids do not differ substantially in their melting points from the methylated analogs. However, the thermal stabilities of protonated imidazolium salts are much lower than their alkylated derivatives. Nitrate salts with alkylated cations tend to be more thermally stable than the corresponding picrate salts, but with protonated cations, the picrate salts tend to be approximately 70–80 °C more stable than the nitrate salts. Moreover, accelerating rate calorimetry (ARC) revealed that alkylated salts decompose much less exothermically (in some cases endothermically) than the protonated analogs, and that among all the analyzed salts, the most energetic materials found were protonated 1-methylimidazolium nitrate and 1,2-dimethylimidazolium picrate.