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

Ionic liquids (ILs) composed of tetra(-butyl)phosphonium [P] and tetra(-butyl)ammonium [N] 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] cations-based ILs are found to be liquids, while the [N] 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][HFuA] (0.069 mS cm) > [P][FuA] (0.032 mS cm) > [P][TpA] (0.028 mS cm) 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.