The synthesis of ionic liquids with 1,4-bis methylenebenzene as a linker unit started from the reaction of commercially available α,α′-dibromo-p-xylene with 2.1 equivalents of 1-methyl/1-butyl imidazole in the presence of dry CH₃CN at room temperature with stirring for 24 hours to afford a white precipitate. The solvent was evaporated under reduced pressure and then the residue was washed several times with acetone to remove unreacted starting materials and give the imidazolium salts 2a/2e as white solids in excellent yield (92–95 %).

The ¹H-NMR spectrum of compound 2a showed the two sharp singlets at δ 3.86 and 5.49 for the methyl and methylene protons, respectively. The phenyl protons appeared as a singlet at δ 7.51. Compound 2a has three different protons in the imidazole moiety, and showed one singlet and two doublets at δ 9.42, 7.85 and 7.71, respectively. The ¹³C-NMR spectrum of compound 2a showed signals at δ 35.3 and 51.3 for methyl and methylene carbons. Aromatic and imidazole carbons appeared in their respective region. Satisfactory elemental analysis further confirmed our proposed product. Similarly, butyl dicationic ionic liquid 2e was fully characterized by spectral and analytical data.

The ionic liquids 2a/2e underwent a metathesis reaction with a slight stoichiometric excess of NaBF₄/KPF₆/LiCF₃SO₃ (MX) in ethanol/water (4:1) ratio at room temperature for 4 hours. After completion of the reaction, solvents were removed under reduced pressure. Attempted extraction with various solvents like CHCl₃, CH₂Cl₂ and EtOAc did not extract any ionic liquid from the organic layer because of their poor solubility in the the abovementioned organic solvents. Hence, a Soxhlet extractor was used to remove the inorganic salt. Using Soxhlet extraction with ethanol afforded the ionic liquids 2b-d,f-h in 82–90 % yields.

Chemicals shift for the protons of the eight synthesized dimeric imidazolium ionic salts containing methyl/butyl substitution with various anions were recorded in different deuterated solvents (Tables 1 and 2). The dimeric materials are insoluble in some of the deuterated solvents like acetone, acetonitrile, ethyl acetate and chlorinated solvents.

We recorded the NMR spectra in different deuterated solvents with the same concentrations of ionic liquids 2a-h; interestingly, we found that the chemical shift values were remarkable changed from TFA to DMF. We observed that acidic proton of the imidazolium dication shifted towards downfield depending on the deuterated solvent used as well as the anions. We employed four different anions, namely Br⁻, BF₄⁻, PF₆⁻ and CF₃SO₃⁻; chemical shift values of acidic protons appeared to follow the order Br⁻<BF₄⁻<PF₆⁻<CF₃SO₃⁻. This might be due to electronegativity as well as the bulkiness of the anions. As a result, the CF₃SO₃⁻ and PF₆⁻ ions are better hydrogen bond acceptors than the Br⁻ or BF₄⁻ salts. The imidazolium ring contains three different protons, namely H₂, H₄ and H₅. The H₂ proton is more sensitive than the other two protons because it is surrounded by two electronegative atoms; when the alkyl side chain as well as anions was changed interesting trends in the chemical shifts were observed.

α,α′-Dibromo-p-xylene, 1-methyl imidazole, 1-butyl imidazole and solvents were purchased from commercial sources and used without further purification. ¹H- and ¹³C-NMR were recorded on a Jeol 400 MHz NMR spectrometer operating at 400 and 100 MHz, respectively. Chemical shifts are reported relative to TMS as reference for proton and carbon. Elemental analyses were carried out on a ThermoFinnigan Eager 300 elemental analyzer.

Compounds 2a/2e were prepared by stirring one equivalent of α,α′-dibromo-p-xylene and 2.1 equivalents of methyl/butyl imidazole in dry CH₃CN for 24 hours at room temperature to give the respective quaternized salts. Solvent was removed under reduced pressure and the residue washed with acetone to remove the unreacted imidazole, followed by filtration to afford white precipitates of the dicationic imidazolium salts 2a/2e in quantitative yield. Compounds 2a/2e were dissolved with 4:1 (ethanol/water) along with a slight molar excess of NaBF₄/KPF₆/LiCF₃SO₃ and the mixture was stirred for 4 hours at room temperature to afford the respective anion exchanged products 2b-d, f-h. After completion of the reaction period, the mixture was concentrated under reduced pressure as much as possible, followed by addition of anhydrous MgSO₄. The solid material was extracted using Soxhlet extraction with ethanol for 3 hours to afford the ionic liquids 2b-df-h, in 82–90 % yield.

1, 1′-(1,4-Phenylenebismethylene)bis(3-methyl-1H-imidazolium-1-yl) di(bromide) (2a): Colorless solid; mp: 180 – 182 °C; Yield: 95 %; ¹H-NMR (DMSO-d₆): δ_H 9.42 (s, 2H), 7.85 (d, 2H), 7.71 (d, 2H), 7.51 (s, 4H), 5.49 (s, 4H), 3.86 (s, 6H); ¹³C-NMR: δ_C 136.3, 135.5, 128.6, 122.5, 122.0, 51.3, 35.3; Anal. Cald. (%) C₁₆H₂₀Br₂N₄ (428): C, 44.85; H, 4.67; N, 13.09. Found: C, 44.81; H, 4.62; N, 13.03.

1,1′-(1,4-Phenylenebismethylene)bis(3-methyl-1H-imidazolium-1-yl) di(tetrafluoroborate) (2b): Colorless solid; mp: 68 – 71 °C; Yield: 90 %; ¹H-NMR (DMSO-d₆): δ_H 9.48 (s, 2H), 7.87 (d, 2H), 7.74 (d, 2H), 7.49 (s, 4H), 5.53 (s, 4H), 3.89 (s, 6H); ¹³C-NMR: δ_C 136.5, 135.5, 128.7, 122.3, 122.4, 51.6, 35.5; Anal. Cald. (%) C₁₆H₂₀B₂F₈N₄ (441): C, 43.53; H, 4.53; N, 12.69. Found: C, 43.49; H, 4.48; N, 12.55.

1,1′-(1,4-Phenylenebismethylene)bis(3-methyl-1H-imidazolium-1-yl) di(hexafluorophosphate) (2c): Liquid; Yield: 83 %; ¹H-NMR (DMSO-d₆): δ_H 9.55 (s, 2H), 7.86 (d, 2H), 7.75 (d, 2H), 7.50 (s, 4H), 5.48 (s, 4H), 3.87 (s, 6H); ¹³C-NMR: δ _C 136.6, 135.7, 128.3, 122.7, 122.4, 51.7, 35.7; Anal. Cald. (%) C₁₆H₂₀P₂F₁₂N₄ (557.9): C, 34.41; H, 3.58; N, 10.30. Found: C, 34.38; H, 3.52; N, 10.28.

1,1′-(1,4-Phenylenebismethylene)bis(3-methyl-1H-imidazolium-1-yl) di(trifluromethanesulphonate) (2d): Liquid; Yield: 86 %; ¹H-NMR (DMSO-d₆): δ_H 9.59 (s, 2H), 7.90 (d, 2H), 7.77 (d, 2H), 7.50 (s, 4H), 5.48 (s, 4H), 3.87 (s, 6H); ¹³C-NMR: δ_C 136.8, 135.7, 128.8, 122.7, 122.4, 51.6, 35.8; Anal. Cald. (%) C₁₈H₂₀O₆F₆N₄S₂ (565.99): C, 38.16; H, 3.53; N, 9.89. Found: C, 38.12; H, 3.50; N, 19.85.

1,1′-(1,4-Phenylenebismethylene)bis(3-butyl-1H-imidazolium-1-yl) di(bromide) (2e): Colorless solid; mp: 139 – 142 °C; Yield: 92 %; ¹H-NMR (DMSO-d₆): δ_H 9.35 (s, 2H), 7.79 (d, 2H), 7.53 (d, 2H), 7.49 (s, 4H), 5.47 (s, 4H), 4.19 (t, 4H), 1.78 (m, 4H), 1.18 (m, 4H), 0.94 (t, 6H); ¹³C-NMR: δ_C 136.1, 135.4, 128.9, 122.8, 122.4, 51.3, 48.6, 31.2, 18.7, 13.2; Anal. Cald. (%) C₂₂H₃₂Br₂N₄ (512): C, 51.56; H, 6.25; N, 10.93. Found: C, 51.51; H, 6.20; N, 10.89.

1,1′-(1,4-Phenylenebismethylene)bis(3-butyl-1H-imidazolium-1-yl) di(tetrafluroborate) (2f): Liquid; Yield: 86 %; ¹H-NMR (DMSO-d₆): δ_H 9.44 (s, 2H), 7.82 (d, 2H), 7.77 (d, 2H), 7.49 (s, 4H), 5.45 (s, 4H), 4.17 (t, 4H), 1.74 (m, 4H), 1.21 (m, 4H), 0.89 (t, 6H); ¹³C-NMR: δ_C 136.4, 135.7, 128.8, 122.7, 122.1, 51.1, 48.4, 31.1, 18.5, 13.1; Anal. Cald. (%) C₂₂H₃₂B₂F₈N₄ (525.6): C, 50.22; H, 6.08; N, 10.65. Found: C, 50.20; H, 6.03; N, 10.61.

1,1′-(1,4-Phenylenebismethylene)bis(3-butyl-1H-imidazolium-1-yl) di(hexaflurophosphate) (2g): Liquid; Yield: 84 %; ¹H-NMR (DMSO-d₆): δ_H 9.49 (s, 2H), 7.81 (d, 2H), 7.78 (d, 2H), 7.44 (s, 4H), 5.45 (s, 4H), 4.17 (t, 4H), 1.75 (m, 4H), 1.22 (m, 4H), 0.88 (t, 6H); ¹³C-NMR: δ_C 136.3, 135.7, 128.8, 122.6, 122.5, 51.5, 48.7, 31.5, 18.9, 13.7; Anal. Cald. (%) C₂₂H₃₂P₂F₁₂N₄ (641.92): C, 41.12; H, 4.98; N, 8.72. Found: C, 41.09; H, 4.94; N, 8.67.

1,1′-(1,4-Phenylenebismethylene)bis(3-butyl-1H-imidazolium-1-yl) di(trifluromethanesulphonate) (2h): Colorless solid; mp: 59 – 62 °C; Yield: 82 %; ¹H-NMR (DMSO-d₆): δ_H 9.54 (s, 2H), 7.85 (d, 2H), 7.79 (d, 2H), 7.49 (s, 4H), 5.45 (s, 4H), 4.17 (t, 4H), 1.75 (m, 4H), 1.18 (m, 4H), 0.91 (t, 6H); ¹³C-NMR: δ_C 136.2, 135.8, 128.7, 122.9, 122.6, 51.5, 48.9, 31.5, 18.4, 13.5; Anal. Cald. (%) C₂₂H₃₂O₆F₆N₄S₂ (625.99): C, 42.17; H, 5.11; N, 8.94. Found: C, 42.14; H, 5.07; N, 8.90.