Effect of Aromaticity in Anion on the Cation–Anion Interactions and Ionic Mobility in Fluorine-Free Ionic Liquids

Ionic liquids (ILs) composed of tetra( n -butyl)phosphonium [P 4444 ] + and tetra( n -butyl)ammonium [N 4444 ] + cations paired with 2-furoate [FuA] - , tetrahydo-2-furoate [HFuA] - , and thiophene-2-carboxylate [TpA] - anions are prepared to investigate the effects of electron delocalization in anion and the mutual interactions between cations and anions on their physical and electrochemical properties. The [P 4444 ] + cations-based ILs are found to be liquids, while the [N 4444 ] + cations-based ILs are semi-solids at room temperature. Thermogravimetric analysis revealed higher decomposition temperatures and differential scanning calorimetry analysis showed lower glass transition temperatures for phosphonium-based ILs than the ammonium-based counterparts. The ILs are arranged in the decreasing order of their ionic conductivities as [P 4444 ][HFuA] (0.069 mS cm -1 ) > [P 4444 ][FuA] (0.032 mS cm -1 ) > [P 4444 ][TpA] (0.028 mS cm -1 ) at 20 °C. The oxidative limit of the ILs followed the sequence of [FuA] - > [TpA] - > [HFuA] - , as measured by linear sweep voltammetry. This order can be attributed to the electrons' delocalization in [FuA] - and in [TpA] - aromatic anions, which has enhanced the oxidative limit potentials and the overall electrochemical stabilities.