Stability of Ion Pairs of Bis(trifluoromethanesulfonyl)amide-Based Ionic Liquids in Dichloromethane

Abstract

The ion pair formation constants at infinite dilution (KIP0) of bis(trifluoromethanesulfonyl)amide ([NTf2]−) salts in dichloromethane were determined conductometrically at 298.2 K. The cations used were nine monovalent organic ions having similar molecular size, i.e., 1-butyl-3-methylimidazolium ([BuMeIm]+), 1-butyl-2,3-dimethylimidazolium ([BuMe2Im]+), 1-butylpyridinium ([BuPy]+), 1-propyl-2,3,5-trimethylpyrazolium ([Me3PrPyra]+), 1-butyl-1-methylpyrrolidinium ([BuMePyrr]+), 1-methyl-1-propylpiperidinium ([MePrPip]+), methyltripropylammonium ([MePr3N]+), propyltrimethylammonium ([Me3PrN]+), and tetraethylammonium ([Et4N]+), where the results for [BuMeIm]+, [BuMe2Im]+, and [Et4N]+ were cited from our previous study. It was found from the KIP0 values that the ion pair stability varies with the cation in the order [BuMeIm]+ > [Me3PrN]+ > [BuPy]+ ≥ [BuMePyrr]+ ≥ [MePrPip]+ > [MePr3N]+ ≥ [Et4N]+ ≈ [BuMe2Im]+ ≥ [Me3PrPyra]+, which was explained on the basis of the structures of the cations. For the salts used,...