Exploring the physicochemical properties of para-xylyl linked DBU-based dicationic ionic liquids consist of various anions: A GD3–M06–2X study

Abstract

The roles of various anions in the formation of seven new para-xylyl linked bis-1,8-diazobicyclo [5.4.0] undec-7-ene ([p-C6H4(CH2DBU)2]2+-based [X][Y1–7]2) dicationic ionic liquids (X = [p-C6H4(CH2DBU)2]2+ and Y1–7 = [CH3CO2]−, [C6H5SO2]−, [HCO3]−, [HSO4]−, [CF3CO2]−, [BF4]− and [SCN]−) were investigated using density functional theory (DFT) at GD3-M06–2X/6-311++G(d,p) level of theory. The potential electrostatic curves, interaction thermodynamic energies, structural parameters, natural charges, charge transfer (CT), reduced gradient density (RGD) plots, topological properties and electrochemical windows (ECW) were evaluated. The consequence of dispersion corrected interaction energies (−213.88 to −238.79 kcal mol−1) is [X][Y1]2 > [X][Y2]2 > [X][Y3]2 > [X][Y5]2 > [X][Y4]2 > [X][Y6]2 > [X][Y7]2, indicating that dicationc ionic liquids (DILs) based on CH3CO2−, PhSO2− and HCO3− anions show stronger binding towards the [p-C6H4(CH2DBU)2]2+ dication. The results of the electron density analysis indicate that the CH⋯O, CH⋯F, CH⋯N and CH⋯S hydrogen bonds in [X][Y1–7]2 DILs are electrostatic in nature. To confirm the electrochemical windows (ECWs) calculated for DILs, the ECW of six mono-cationic ionic liquids (MILS) synthesized in our previous work were measured by cyclic voltammetry (CV) method and was compared with the corresponding quantum chemical calculated values. A good correlation was found between the experimental and computed ECW values. The following order for ECWs of both MILs and DILs was estimated: [PhSO2]− < [CH3CO2]− < [SCN]− < [HCO3]− < [CF3CO2]− < [HSO4]− < [BF4]−. The results show that both cathodic and anodic stability of [X][BF4]2 DIL in acetonitrile solvent are controlled by the dication and its ECW value is more than those of other DILs. The results demonstrated that intermolecular hydrogen bonding and electrostatic interactions have a significant role in the stabilizing of the DILs.