Alternative to the Popular Imidazolium Ionic Liquids: 1,2,4-Triazolium Ionic Liquids with Enhanced Thermal and Chemical Stability

摘要

Direct quaternization of 1-methyl-1,2,4-triazole with n-alkyl methanesulfonates (alkyl = butyl, octyl, dodecyl) showed to be an atom-economic, convenient, mild, solvent- and halide-free way to obtain 1,2,4-triazolium methanesulfonate ionic liquids in high purity and yield. Subsequent metathesis with lithium bis(trifluoromethanesulfonyl)amide (LiTf2N) allows for a much desired, easy access to halide-free, bis(trifluoromethanesulfonyl)amide ionic liquids. Differential scanning calorimetry confirms that all investigated compounds qualify as ionic liquids (ILs). Moreover, it reveals for 1-methyl-4-n-dodecyl-1,2,4-triazolium methanesulfonate a rather complex thermal behavior involving formation of mesophases. Indeed, polarizing optical microscopy shows oily streaky textures that are characteristic for smectic liquid crystalline phases. Single-crystal X-ray diffraction structure analysis confirms formation of a layered structure. All compounds are photoluminescent. The color of fluorescence at room temperature can be tuned from blue to orange through the length of the alkyl side chain of the cation, the aromatic interactions between the cations, and the anion nature. In addition, at low temperatures (77 K) a close to white phosphorescence with average lifetimes in the millisecond time range can be observed for 1-methyl-4-n-butyl-triazolium methanesulfonate and all of the studied bis(trifluoromethanesulfonyl)amide ILs. All ILs show an appreciable liquidus range and thermal (up to 260-350 degrees C) and electrochemical stability. The presented set of ILs overcomes the sometimes problematic acidity and low stability of imidazolium ILs in basic environment and can be obtained easily in high purity without halide contamination. Overcoming two shortcomings of classical imidazolium ILs, they may be good alternatives for a number of applications and even enabling new ones.