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

[Image: see text] 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.