Tunable aryl alkyl ionic liquids (TAAILs) based on 1-aryl-3,5-dimethyl-1H-pyrazoles

A series of fifteeen new 1-aryl-2,3,5-trimethylpyrazolium salts ([Ph_RC₁pz][X], R: 4-Cl, 4-Br, 4-Me, 4-OMe, 4-NO₂; C₁: methyl, X: methanesulfonate [CH₃SO₃⁻], tetrafluoroborate [BF₄⁻] and hexafluorophosphate [PF₆⁻]) were synthesized and characterized. Thermal properties (Tm/Tg and Td) of all the salts were investigated. Five of the salts can be classed as ionic liquids since they are liquid at rate time or melt below 100°C three of which are room temperature ionic liquids (RTILs). The influence of the substituents at the para position of the aryl ring was investigated by studying the changes in the melting points of the corresponding methanesulfonate, tetrafluoroborate, and hexafluorophosphate salts. The results demonstrate that the electron-withdrawing substituents tend to higher melting points than electron-donating substituents. Thermophysical properties (density, viscosity, and refractive index) of the room temperature ionic liquids were measured as a function of temperature. The electrochemical windows (EW) of all the salts were determined experimentally. The EW values of the salts were calculated theoretically to make a comparison with the experimentally determined EW values.