Investigation of physical and electrochemical behaviour of N-sulfonic acid functionalized cyclic-ammonium based Brӧnsted acidic ionic liquids in molecular solvents

Abstract

Two types of six Brӧnsted acidic ionic liquids of N-SO₃H tethered cyclic ammonium cations (five, six and seven membered) with four anions namely [AcO]⁻, [HSO₄]⁻, [BF₄]⁻, [PF₆]⁻ were synthesized to evaluate their physical and electrochemical behavior in polar solvents (water, MeOH & CH₃CN) using conductivity and Cyclic Voltammetry techniques. Their comparative Brönsted acidic orders were determined using UV–visible Hammett plots. Except the ionic liquid of [PF₆]⁻ anion with N,N-disulfopiperidinium cation, the thermal stabilities of other ionic liquids found within 230–250 °C. Solvent dependent aggregative nature was observed in polar solvents for the ionic liquids at a particular temperature and concentration. Their aggregative properties were also evidenced by comparison of experimental and computed chemical shift values of additional acidic protons of ¹H NMR spectra. Kamlet-Taft solvatochromic parameters of molecular solvents (E_T^N, α, β) were correlated with their observed conductivities. Piperidinium ionic liquids showed wider electrochemical windows (2.99 V to 2.55 V) in acetonitrile as compared to other cyclic ammonium ionic liquids in polar solvents.