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An efficient and green sonochemical synthesis of some new eco-friendly functionalized ionic liquids
*Tel.: +966 562441572 mouslim@mail.be (Mouslim Messali)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Available online 4 September 2013
Abstract
Considerable stress to replace a lot of volatile organic compounds which were used as solvents in synthetic organic chemistry has been done for many chemical industries. A suitable solution for these problems is found by using the ionic liquids as a clean medium of working and avoiding the solvent effect. The present work describes a facile and green ultrasound-assisted procedure as an environmentally friendly alternative to traditional methods for the preparation of a series of 26 new functionalized imidazolium-based ionic liquids. Their structures were characterized by FT-IR, 1H, 13C, 11B, 19F, 31P NMR and mass spectra.
Keywords
Green procedure
Imidazolium salts
Room temperature ionic liquids
Ultrasound irradiation
1 Introduction
Over the past few decades the number of publications concerning room-temperature ionic liquids (RTILs) has increased substantially (Rogers and Seddon, 2002). (RTILs) provide a new class of solvents where molecules are composed of ions. At normal temperatures, ionic liquids have essentially zero-vapor pressure and are thermally stable over a wide range of temperature. Therefore, they are considered as environmentally friendly alternatives to the volatile organic compounds (VOCs).
RTILs have been also widely investigated for a variety of applications: the use as solvents or catalysts for chemical synthesis (Liu et al., 2003; Wang et al., 2007), media for electrodeposition of metals (Lin and Sun, 1999; Takahashi et al., 1999; Endres, 2002; Ibrahim and Messali, 2011), electrolyte for electrochemical devices such as battery (Brennecke and Magin, 2001), supercapacitors (Ue et al., 2003; Balducci et al., 2004), as inhibitors of corrosion (Messali, 2011; Zarrouk et al., 2012a; Zarrouk et al., 2012b; Ibrahim et al., 2011), as fluids for thermal storage and exchange in solar concentrating power plants (Moens et al., 2003) and a wide electrochemical potential window (Ngo et al., 2000; Forsyth et al., 2004; Endres et al., 2006; Al-Ghamdi et al., 2011).
Recently, many chemists promoted to explore new methods for the clean and efficient synthesis of ILs since from the point of green chemistry, the conventional syntheses of the ILs themselves are not benign. In recent reports, several modifications have been attempted including solvent-free reactions, microwave irradiation (Anastas and Warner, 1998; Messali, 2011) ultrasound assisted procedures (Leveque et al., 2006; Messali, 2013). The use of the green technologies leads to large reductions in reaction times, enhancements in conversions, sometimes in selectivity, with several advantages of the eco-friendly approach (Deetlefs and Seddon, 2003; Loupy, 2004; Yi et al., 2005; Singh et al., 2005; Aupoix et al., 2010; Messali and Ahmed, 2011).
In continuation of our previous work dealing with the development of novel functionalized ionic liquids and our general interests in green technologies such a MW-assisted or ultrasound-assisted chemical processes (Messali, 2013; Messali et al., 2013; Messali, 2011; Messali and Asiri, 2013) we present in the following an overview of our new study of the ultrasound-assisted synthesis of new functionalized imidazolium-based ionic liquids, showing the scope of this method.
2 Materials and methods
2.1 Experimental
All new compounds were synthesized and characterized by 1H NMR, 13C NMR, 19F NMR, 11B NMR, 31P NMR, IR and LCMS. 1H NMR (400 MHz), 13C NMR (100 MHz), 19F NMR (376.5 MHz), 31P NMR (162 MHz) and 11B NMR (128 MHz) spectra were measured in DMSO, CDCl3 or D2O at room temperature. Chemical shifts (δ) were reported in ppm to a scale calibrated for tetramethylsilane (TMS), which is used as an internal standard (Taibah University, Madinah, Saudi Arabia). The LCMS spectra were measured with a Micromass, LCT mass spectrometer (Mohamed First University, Oujda, Morocco). FT-IR spectra were recorded in NaCl or KBr disk on a Schimadzu 8201 PC, FTIR spectrophotometer (νmax in cm−1) (Taibah University, Madinah, Saudi Arabia). The ultrasound-assisted reactions were performed using a controllable laboratory ultrasonic bath.
3 Synthesis
3.1 General procedure for the synthesis of imidazolium halides (4–7) using conventional method
To the solution of N-alkylimidazole (1–3) (1 eq) in toluene, was added the appropriate alkyl bromide (1.1 eq) at room temperature, followed by stirring at 80 °C for 18 h. The completion of the reaction was marked by the separation of oil or solid from the initially obtained clear and homogenous mixture of N-alkylimidazole and alkyl halide in toluene. The product was isolated by extraction or filtration to remove the unreacted starting materials and solvent. Subsequently, the imidazolium salt was washed with ethyl acetate. In each case, the IL/salt (4–7) was finally dried at a reduced pressure to get rid of all the volatile organic compounds.
3.2 General procedure for the synthesis of imidazolium halides (4–7) using under Ultrasonic irradiation
N-alkylimidazole (1–3) (1 eq) and the appropriate alkyl bromide (1 eq) were placed in a closed vessel and exposed to ultrasound irradiation for 5 h at 80 °C using a sonication bath. The product was then collected as described in the conventional procedure outlined earlier.
3.3 General procedure for the methathesis reaction of (4–7) leading to compounds (8–31) using conventional method
The quaternary salt (4–7) (1 eq) was dissolved in acetonitrile to obtain a clear solution. To this solution of quaternary halide were added solution of sodium tetrafluoroborate, potassium hexafluorophosphate, trifluoroacetic acid sodium, sodium dicyanamide, sodium thiocyanate or sodium nitrate (1.2 eq), followed by stirring at 70 °C for 3 h. The cooled reaction mixture was filtered through Celite to remove solid metal halide. The evaporation of acetonitrile led quantitatively to the desired ionic liquids.
3.4 General procedure for the ultrasound-assisted methathesis reaction of (4–7) leading to compounds (8–31)
Imidazolium-halide salts (4–7) (1 eq) and NaBF4, KPF6, CF3COONa, NaN(CN)2, NaNCS or NaNO3 (1 eq) were placed in a closed vessel and exposed to ultrasound irradiation for 45 min at 70 °C using a sonication bath. The product was then collected as described in the conventional procedure outlined earlier.
3.5 Characterization
3.5.1 1-Benzyl-3-(2-ethoxy-2-oxoethyl)-1H-imidazol-3-ium chloride 4
White crystals, Mp 119–121 °C, 1H NMR (400 MHz, CDCl3) δ: 1.14 (t, J = 7.6, 3H), 4.07 (q, J = 6.8, 2H), 5.37 (s, 2H), 5.44 (s, 2H), 7.23–7.36 (m, 6H), 7.68 (d, 1H), 10.32 (s,1H), 13C NMR (100 MHz, CDCl3) δ: 13.9 (CH3), 50.1 (CH2), 53.1 (CH2), 62.6 (CH2), 121.5 (CH), 124.1 (CH), 128.7(CH), 129.2(CH), 133.1(C), 137.9(CH), 166.2(CO), IR (KBr): νmax cm−1 3132 (C–H, sp2), 1726 (C⚌O), 1562–1448 (C⚌C), 1166 (C–N), 1030 (C–O), LCMS (M–Cl) 245 found for C14H17N2O2+.
3.5.2 1-Benzyl-3-(4-phenoxybutyl)-1H-imidazol-3-ium bromide 5
White crystals, Mp 153–155 °C,1H NMR (400 MHz, CDCl3) δ: 1.77 (quint, J = 8, 2H), 2.07 (quint, J = 8, 2H), 3.91 (t, J = 8, 2H), 4.35 (t, J = 8, 2H), 5.52 (s, 2H), 6.77–6.88 (m, 3H), 7.17–7.55 (m, 9H), 10.45 (s, 1H), 13C NMR (100 MHz, CDCl3) δ: 25.7 (CH2), 26.9 (CH2), 49.7 (CH2), 53.2 (CH2), 66.7 (CH2), 114.4 (CH), 120.8 (CH), 122.2 (CH), 122.6 (CH), 128.9 (CH), 129.4 (CH), 129.5 (CH), 133.0 (C), 135.5 (CH), 158.6 (C), IR (KBr): νmax cm−1 3132 (C–H, sp2), 1599–1471 (C⚌C), 1165(C–N),1082(C–O), 815 (C–H, bending). LCMS (M–Br) 307 found for C20H23N2O+.
3.5.3 3-(3-hydroxypropyl)-1-propyl-1H-imidazol-3-ium bromide 6
1H NMR (400 MHz, CDCl3) δ: 0.82 (t, J = 7.2, Hz 3H), 1.79 (t, J = 7.2, Hz 2H), 2.01 (quint, J = 7.6 Hz, 2H), 2.18 (sixtet, J = 7.6 Hz, 2H), 4.12 (t, J = 7.6 Hz, 2H), 4.37 (t, J = 7.6 Hz, 2H), 5.11 (s, 1H), 7.48–7.50 (m, 2H), 9.90 (s, 1H), 13C NMR (100 MHz, CDCl3) δ: 8.8 (CH3), 21.6 (CH2), 29.0 (CH2), 45.1 (CH2), 49.6 (CH2), 55.2 (CH2), 119.9 (CH), 120.8 (CH), 135.1 (CH), IR (NaCl): νmax cm−1 3213 (O-H), 3161 (C–H, sp2), 1566 (C⚌C), 1161 (C–N), 1157 (C–O), LCMS (M–Br) 169 found for C9H17N2O+.
3.5.4 3-(3-Hydroxypropyl)-1-pentyl-1H-imidazol-3-ium bromide 7
1H NMR (400 MHz, D2O) δ: 0.67 (t, J = 7.6, 3H), 1.10–1.17 (m, 4H), 1.72 (quint, 2H), 1.94 (quint, J = 7.6, 2H), 3.41 (t, J = 6.8, 2H), 4.12 (t, J = 7.6, 2H), 4.32 (t, J = 6.8, 2H), 7.37 (d, 1H), 7.64 (d, 1H), 9.73 (s,1H), 13C NMR (100 MHz, D2O) δ 11.7 (CH3), 19.8 (CH2), 26.0 (CH2), 27.7 (CH2), 30.6 (CH2), 44.7 (CH2), 47.8 (CH2), 54.9 (CH2), 120.0 (CH), 121.0 (CH), 134.3 (CH), IR (NaCl): νmax cm−1 3212 (O–H), 3160 (C–H, sp2), 1565 (C⚌C), 1163 (C–N), 1158 (C–O), LCMS (M–Br 197 found for C11H21N2O+.
3.5.5 1-Benzyl-3-(2-ethoxy-2-oxoethyl)-1H-imidazol-3-ium tetrafluoroborate 8
White crystals, Mp 110–111 °C,1H NMR (400 MHz, DMSO) δ: 1.24 (t, J = 7.6, 3H), 4.22 (q, J = 6.8, 2H), 5.24 (s, 2H), 5.51 (s, 2H),7.41–7.45 (m, 5H), 7.76 (d, 1H), 7.84 (d, 1H), 9.24 (s,1H), 13C NMR (100 MHz, DMSO) δ: 13.8 (CH3), 49.6 (CH2), 52.0 (CH2), 61.8 (CH2), 122.2 (CH), 124.2 (CH), 128.2(CH), 128.8 (CH), 129.0 (CH), 134.6(C), 137.3 (CH), 166.7 (CO). 19F NMR (376.5 MHz, DMSO) δ: −148.24, 11B NMR (128 MHz, CDCl3) δ: −1.26, IR (KBr): νmax cm−1 3131 (C–H, sp2), 1728 (C⚌O), 1561–1447 (C⚌C), 1167 (C–N), 1030 (C–O), LCMS (M–BF4) 245 found for C14H17N2O2+.
3.5.6 1-Benzyl-3-(2-ethoxy-2-oxoethyl)-1H-imidazol-3-ium hexafluorophosphate 9
White crystals, Mp 89–91 °C, 1H NMR (400 MHz, CDCl3) δ: 1.25 (t, J = 7.6, 3H), 4.22 (q, J = 6.8, 2H), 5.51 (s, 2H), 5.74 (s, 2H),7.41–7.46 (m, 5H), 7.76 (d, 1H), 7.78 (d, 1H), 9.225 (s,1H), 13C NMR (100 MHz, CDCl3) δ: 12.8 (CH3), 48.6 (CH2), 51.0 (CH2), 61.8 (CH2), 121.2 (CH), 123.2 (CH), 127.2(CH), 127.8 (CH), 128.0 (CH), 133.6(C), 136.3 (CH), 165.7 (CO). 19F NMR (376.5 MHz, CDCl3) δ: −71.10 (d, J = 711.6 Hz); 31P NMR (162 MHz, CDCl3) δ: −144.16 (sep, J = 712.8 Hz), IR (KBr): νmax cm−1 3133 (C–H, sp2), 1727 (C⚌O), 1562–1447 (C⚌C), 1165 (C–N), 1031 (C–O), LCMS (M–PF6) 245 found for C14H17N2O2+.
3.5.7 1-Benzyl-3-(2-ethoxy-2-oxoethyl)-1H-imidazol-3-ium trifluoroacetate 10
1H NMR (400 MHz, CDCl3) δ: 1.17 (t, J = 7.6, 3H), 4.14 (q, J = 6.8, 2H), 5.18 (s, 2H), 5.30 (s, 2H), 7.12 (d, 1H), 7.27–7.29 (m, 5H), 7.40 (d, 1H), 10.15 (s,1H), 13C NMR (100 MHz, CDCl3) δ: 11.9 (CH3), 48.0 (CH2), 51.5 (CH2), 51.6 (CH2), 119.4 (CH), 122.0 (CH), 126.8(CH), 127.6 (CH), 127.7 (CH), 130.8(C), 137.0 (CH), 164.4 (CO), 19F NMR (376.5 MHz, CDCl3) δ: −75,41, IR (NaCl): νmax cm−1 3133 (C–H, sp2), 1726 (C⚌O), 1563–1448 (C⚌C), 1165 (C–N), 1031 (C–O), LCMS (M–CF3CO2) 245 found for C14H17N2O2+.
3.5.8 1-Benzyl-3-(2-ethoxy-2-oxoethyl)-1H-imidazol-3-ium Dicyanoamine 11
1H NMR (400 MHz, CDCl3) δ: 1.20 (t, J = 7.6, 3H), 4.16 (q, J = 6.8, 2H), 5.05 (s, 2H), 5.30 (s, 2H), 7.30–7.33 (m, 6H), 7.47 (d, 1H), 9.13 (s,1H), 13C NMR (100 MHz, CDCl3) δ: 12.1 (CH3), 48.2 (CH2), 51.7 (CH2), 61.2 (CH2), 120.1 (CH), 122.4 (CH), 126.9 (CH), 127.7 (CH), 127.8 (CH), 130.5(C), 135.1 (CH), 164.0 (CO), IR (NaCl): νmax cm−1 3131 (C–H, sp2), 1727 (C⚌O), 1559–1448 (C⚌C), 1164 (C–N), 1030 (C–O), LCMS (M–N(CN)2) 245 found for C14H17N2O2+.
3.5.9 1-Benzyl-3-(2-ethoxy-2-oxoethyl)-1H-imidazol-3-ium thiocyanate 12
1H NMR (400 MHz, CDCl3) δ: 1.29 (t, J = 7.6, 3H), 4.16 (q, J = 6.8, 2H), 5.07 (s, 2H), 5.32 (s, 2H), 7.29–7.34 (m, 6H), 7.48 (d, 1H), 9.15 (s,1H), 13C NMR (100 MHz, CDCl3) δ: 12.2 (CH3), 48.3 (CH2), 51.8 (CH2), 61.3 (CH2), 120.2 (CH), 122.5 (CH), 127.0 (CH), 127.8 (CH), 127.9 (CH), 130.6 (C), 135.2 (CH), 164.1 (CO). IR (NaCl): νmax cm−1 3132 (C–H, sp2), 1729 (C⚌O), 1562–1449 (C⚌C), 1165 (C–N), 1032 (C–O), LCMS (M–NCS) 245 found for C14H17N2O2+.
3.5.10 1-Benzyl-3-(2-ethoxy-2-oxoethyl)-1H-imidazol-3-ium Nitrate 13
1H NMR (400 MHz, CDCl3) δ: 1.20 (t, J = 7.6, 3H), 4.15 (q, J = 6.8, 2H), 5.19 (s, 2H), 5.36 (s, 2H), 7.28–7.35 (m, 6H), 7.59 (d, 1H), 9.88 (s,1H), 13C NMR (100 MHz, CDCl3) δ: 13.9 (CH3), 49.9 (CH2), 53.3 (CH2), 62.7 (CH2), 121.7 (CH), 124.2 (CH), 128.7 (CH), 129.4 (CH), 129.5 (CH), 133.0 (CH), 138.3 (C), 166.3 (CO). IR (NaCl): νmax cm−1 3132 (C–H, sp2), 1729 (C⚌O), 1560–1446 (C⚌C), 1165 (C–N), 1031 (C–O), LCMS (M–NO3) 245 found for C14H17N2O2+.
3.5.11 1-Benzyl-3-(4-phenoxybutyl)-1H-imidazol-3-ium tetrafluoroborate 14
Brown crystals, Mp 89–92 °C, 1H NMR (400 MHz, CDCl3) δ: 1.72 (quint, J = 8, 2H), 2.01 (quint, J = 8, 2H), 3.89 (t, J = 8, 2H), 4.20 (t, J = 8, 2H), 5.28 (s, 2H), 6.80–6.91 (m, 3H), 7.19–7.37 (m, 9H), 8.97 (s, 1H), 13C NMR (100 MHz, CDCl3) δ: 25.7 (CH2), 26.9 (CH2), 49.7 (CH2), 53.2 (CH2), 66.7 (CH2), 114.4 (CH), 120.8 (CH), 122.2 (CH), 122.6 (CH), 128.9 (CH), 129.4 (CH), 129.5 (CH), 133.0 (C), 135.5 (CH), 158.6 (C), 19F NMR (376.5 MHz, CDCl3) δ: −150.08 ppm, 11B NMR (128 MHz, CDCl3) δ: −1.04 ppm, IR (KBr): νmax cm−1 3131 (C–H, sp2), 1598–1470 (C⚌C), 1160(C–N),1081(C–O), 814 (C–H, bending). LCMS (M–BF4) 307 found for C20H23N2O+.
3.5.12 1-Benzyl-3-(4-phenoxybutyl)-1H-imidazol-3-ium hexafluorophospate 15
1H NMR (400 MHz, CDCl3) δ: 1.76 (quint, J = 8, 2H), 2.01 (quint, J = 8, 2H), 3.92 (t, J = 8, 2H), 4.18 (t, J = 8, 2H), 5.31 (s, 2H), 6.82–6.93 (m, 3H), 7.13–7.37 (m, 9H), 8.60 (s, 1H), 13C NMR (100 MHz, CDCl3) δ: 25.7 (CH2), 26.9 (CH2), 49.8 (CH2), 53.4 (CH2), 66.7 (CH2), 114.4 (CH), 120.8 (CH), 122.1 (CH), 122.4 (CH), 128.9 (CH), 129.5 (CH), 129.6 (CH), 132.5 (C), 135.2 (CH), 158.6 (C), 19F NMR (376.5 MHz, CDCl3) δ: −72.54 (d, J = 711.6 Hz); 31P NMR (162 MHz, CDCl3) δ: −139.77 (sep, J = 712.8 Hz), IR (NaCl): νmax cm−1 3132 (C–H, sp2), 1599–1471 (C⚌C), 1165(C–N),1082 (C-O), 815 (C–H, bending), LCMS (M–PF6) 307 found for C20H23N2O+.
3.5.13 1-Benzyl-3-(4-phenoxybutyl)-1H-imidazol-3-ium trifluoroacetate 16
1H NMR (400 MHz, CDCl3) δ: 1.77 (quint, J = 8, 2H), 2.05 (quint, J = 8, 2H), 3.99 (t, J = 8, 2H), 4.32 (t, J = 8, 2H), 5.29 (s, 2H), 6.83–6.92 (m, 3H), 7.21–7.37 (m, 9H), 10.27 (s, 1H), 13C NMR (100 MHz, CDCl3) δ: 25.9 (CH2), 27.3 (CH2), 49.8 (CH2), 53.4 (CH2), 66.8 (CH2), 114.5 (CH), 121.0 (CH), 122.0 (CH), 122.4 (CH), 129.0 (CH), 129.5 (CH), 129.6 (CH), 129.7, (CH), 133.5 (C), 136.2 (CH), 158.7 (C), 19F NMR (376.5 MHz, CDCl3) δ: −75.50, IR (NaCl): νmax cm−1 3130 (C–H, sp2), 1560–1470 (C⚌C), 1164 (C–N), 1082(C–O), 816 (C–H, bending). LCMS (M–CF3CO2) 307 found for C20H23N2O+.
3.5.14 1-Benzyl-3-(4-phenoxybutyl)-1H-imidazol-3-ium Dicyanoamine 17
1H NMR (400 MHz, CDCl3) δ: 1.76 (quint, J = 8, 2H), 2.04 (quint, J = 8, 2H), 3.91 (t, J = 8, 2H), 4.28 (t, J = 8, 2H), 5.37 (s, 2H), 6.78–6.87 (m, 3H), 7.16–7.44 (m, 9H), 9.72 (s, 1H), 13C NMR (100 MHz, CDCl3) δ: 25.8 (CH2), 27.2 (CH2), 48.8 (CH2), 53.3 (CH2), 66.7 (CH2), 114.4 (CH), 120.8 (CH), 122.2 (CH), 128.9 (CH), 129.4 (CH), 129.5 (CH), 132.7 (C), 136.0 (CH), 158.5 (C), IR (NaCl): νmax cm−1 3132 (C–H, sp2), 1598–1472 (C⚌C), 1167 (C–N), 1080 (C–O), 815 (C–H, bending). LCMS (M–N(CN)2) 307 found for C20H23N2O+.
3.5.15 1-Benzyl-3-(4-phenoxybutyl)-1H-imidazol-3-ium thiocyanate 18
1H NMR (400 MHz, CDCl3) δ: 1.80 (quint, J = 8, 2H), 2.11 (quint, J = 8, 2H), 3.95 (t, J = 8, 2H), 4.34 (t, J = 8, 2H), 5.45 (s, 2H), 6.82–6.91 (m, 3H), 7.22–7.46 (m, 9H), 9.34 (s, 1H), 13C NMR (100 MHz, CDCl3) δ: 24.0 (CH2), 25.3 (CH2), 48.2 (CH2), 51.7 (CH2), 64.9 (CH2), 112.5 (CH), 118.9 (CH), 120.4 (CH), 120.7 (CH), 127.2 (CH), 127.6 (CH), 129.5 (CH), 131.0 (C), 134.0 (CH), 156.6 (C), IR (NaCl): νmax cm−1 3133 (C–H, sp2), 1560–1471 (C⚌C), 1164 (C–N), 1081 (C–O), 814 (C–H, bending). LCMS (M-NCS) 307 found for C20H23N2O+.
3.5.16 1-Benzyl-3-(4-phenoxybutyl)-1H-imidazol-3-ium Nitrate 19
1H NMR (400 MHz, CDCl3) δ: 1.68 (quint, J = 8, 2H), 1.99 (quint, J = 8, 2H), 3.83 (t, J = 8, 2H), 4.26 (t, J = 8, 2H), 5.43 (s, 2H), 6.71–6.81 (m, 3H), 7.10–7.54 (m, 9H), 10.29 (s, 1H), 13C NMR (100 MHz, CDCl3) δ: 25.8 (CH2), 27.2 (CH2), 49.6 (CH2), 52.9 (CH2), 66.6 (CH2), 114.3 (CH), 120.7 (CH), 122.1 (CH), 122.7 (CH), 128.9 (CH), 129.2 (CH), 129.3 (CH), 129.4 (CH), 133.0 (C), 136.2 (CH), 158.4 (C), IR (NaCl): νmax cm−1 3132 (C–H, sp2), 1597–1469 (C⚌C), 1166 (C–N),1082 (C-O), 816 (C–H, bending). LCMS (M–NO3) 307 found for C20H23N2O+.
3.5.17 3-(3-Hydroxypropyl)-1-propyl-1H-imidazol-3-ium tetrafluoroborate 20
1H NMR (400 MHz, D2O) δ: 0.98 (t, J = 7.2, Hz 3H), 1.97 (t, J = 7.2, Hz 2H), 2.20 (quint, J = 7.6 Hz, 2H), 3.72 (sixtet, J = 7.6 Hz, 2H), 4.24 (t, J = 7.6 Hz, 2H), 4.39 (t, J = 7.6 Hz, 2H), 7.58–7.60 (m, 2H), 8.83 (s, 1H), 13C NMR (100 MHz, D2O) δ: 9.9 (CH3), 22.9 (CH2), 31.8 (CH2), 46.7 (CH2), 51.3 (CH2), 58.2 (CH2), 122.5 (CH), 122.6 (CH), 135.5 (CH), 19F NMR (376.5 MHz, D2O) δ: −150.20, 11B NMR (128 MHz, D2O) δ: −1.26, IR (NaCl): νmax cm−1 3214 (O–H), 3162 (C–H, sp2), 1567 (C⚌C), 1165 (C–N), 1160 (C–O), LCMS (M–BF4) 169 found for C9H17N2O+.
3.5.18 3-(3-Hydroxypropyl)-1-propyl-1H-imidazol-3-ium hexafluorophosphate 21
1H NMR (400 MHz, D2O) δ: 0.97 (t, J = 7.2, Hz 3H), 1.95 (t, J = 7.2, Hz 2H), 2.18 (quint, J = 7.6 Hz, 2H), 3.70 (sixtet, J = 7.6 Hz, 2H), 4.22 (t, J = 7.6 Hz, 2H), 4.37 (t, J = 7.6 Hz, 2H), 7.56–7.58 (m, 2H), 8.81 (s, 1H), 13C NMR (100 MHz, D2O) δ: 9.8 (CH3), 22.8 (CH2), 31.6 (CH2), 46.5 (CH2), 51.1 (CH2), 58.0 (CH2), 122.3 (CH), 122.4 (CH), 135.4 (CH), 19F NMR (376.5 MHz, D2O) δ: −71.15 (d, J = 711.6 Hz); 31P NMR (162 MHz, D2O) δ: −144.26 (sep, J = 712.8 Hz), IR (NaCl): νmax cm−1 3211 (O–H), 3159 (C–H, sp2), 1566 (C⚌C), 1162 (C–N), 1159 (C–O), LCMS (M–PF6) 169 found for C9H17N2O+.
3.5.19 3-(3-Hydroxypropyl)-1-propyl-1H-imidazol-3-ium trifluoroacetate 22
1H NMR (400 MHz, D2O) δ: 1.05 (t, J = 7.2, Hz 3H), 2.02 (t, J = 7.2, Hz 2H), 2.21 (quint, J = 7.6 Hz, 2H), 3.77 (sixtet, J = 7.6 Hz, 2H), 4.31 (t, J = 7.6 Hz, 2H), 4.46 (t, J = 7.6 Hz, 2H), 7.65–7.67 (m, 2H), 8.89 (s, 1H), 13C NMR (100 MHz, D2O) δ: 9.0 (CH3), 22.0 (CH2), 30.9 (CH2), 45.8 (CH2), 50.3 (CH2), 57.2 (CH2), 121.7 (CH), 121.8 (CH), 134.5 (CH), 19F NMR (376.5 MHz, D2O) δ: −150.03 ppm, IR (NaCl): νmax cm−1 3211 (O–H), 3162 (C–H, sp2), 1564 (C⚌C), 1165 (C–N), 1151 (C-–O), LCMS (M–CF3CO2) 169 found for C9H17N2O+.
3.5.20 3-(3-Hydroxypropyl)-1-propyl-1H-imidazol-3-ium Dicyanoamine 23
1H NMR (400 MHz, D2O) δ: 0.97 (t, J = 7.2, Hz 3H), 1.96 (t, J = 7.2, Hz 2H), 2.18 (quint, J = 7.6 Hz, 2H), 3.70 (sixtet, J = 7.6 Hz, 2H), 4.24 (t, J = 7.6 Hz, 2H), 4.38 (t, J = 7.6 Hz, 2H), 7.58–7.60 (m, 2H), 8.87 (s, 1H), 13C NMR (100 MHz, D2O) δ: 10.1 (CH3), 23.1 (CH2), 31.8 (CH2), 46.7 (CH2), 51.3 (CH2), 58.2 (CH2), 122.5 (CH), 122.6 (CH), 135.5 (CH), IR (NaCl): νmax cm−1 3214 (O–H), 3158 (C–H, sp2), 1567 (C⚌C), 1162 (C–N), 1160 (C–O), LCMS (M–N(CN)2) 169 found for C9H17N2O+.
3.5.21 3-(3-Hydroxypropyl)-1-propyl-1H-imidazol-3-ium thiocyanate 24
1H NMR (400 MHz, D2O) δ: 1.12 (t, J = 7.2, Hz 3H), 2.11 (t, J = 7.2, Hz 2H), 2.34 (quint, J = 7.6 Hz, 2H), 3.85 (sixtet, J = 7.6 Hz, 2H), 4.43 (t, J = 7.6 Hz, 2H), 4.56 (t, J = 7.6 Hz, 2H), 7.75–7.77 (m, 2H), 8.03 (s, 1H), 13C NMR (100 MHz, D2O) δ 10.5 (CH3), 23.3 (CH2), 32.1 (CH2), 47.0 (CH2), 51.6 (CH2), 58.4 (CH2), 122.7 (CH), 122.8 (CH), 135.6 (CH), IR (NaCl): νmax cm−1 3216 (O–H), 3164 (C–H, sp2), 1569 (C⚌C), 1168 (C–N), 1162 (C–O), LCMS (M–NCS) 169 found for C9H17N2O+.
3.5.22 3-(3-Hydroxypropyl)-1-propyl-1H-imidazol-3-ium Nitrate 25
1H NMR (400 MHz, D2O) δ: 1.14 (t, J = 7.2, Hz 3H), 2.13 (t, J = 7.2, Hz 2H), 2.36 (quint, J = 7.6 Hz, 2H), 3.87 (sixtet, J = 7.6 Hz, 2H), 4.45 (t, J = 7.6 Hz, 2H), 4.58 (t, J = 7.6 Hz, 2H), 7.77–7.79 (m, 2H), 8.05 (s, 1H), 13C NMR (100 MHz, D2O) δ: 10.8 (CH3), 23.6 (CH2), 32.4 (CH2), 47.3 (CH2), 51.9 (CH2), 58.7 (CH2), 123.0 (CH), 123.1 (CH), 135.9 (CH), IR (NaCl): νmax cm−1 3210 (O–H), 3161 (C–H, sp2), 1567 (C⚌C), 1165 (C–N), 1160 (C–O), LCMS (M–NO3) 169 found for C9H17N2O+.
3.5.23 3-(3-Hydroxypropyl)-1-pentyl-1H-imidazol-3-ium tetrafluoroborate 26
1H NMR (400 MHz, D2O) δ: 0.89 (t, J = 7.6, 3H), 1.29–1.36 (m, 4H), 1.91 (quint, 2H), 2.14 (quint, J = 7.6, 2H), 3.65 (t, J = 6.8, 2H), 4.22 (t, J = 7.6, 2H), 4.33 (t, J = 6.8, 2H), 7.54–7.55 (dd, 2H), 8.79 (s,1H), 13C NMR (100 MHz, D2O) δ: 13.1 (CH3), 21.4 (CH2), 27.6 (CH2), 28.9 (CH2), 31.7 (CH2), 46.6 (CH2), 49.7 (CH2), 58.1 (CH2), 122.5 (CH), 122.6 (CH), 135.3 (CH), 19F NMR (376.5 MHz, D2O) δ: −150.28, 11B NMR (128 MHz, D2O) δ: −1.31, IR (NaCl): νmax cm−1 3210 (O-H), 3159 (C–H, sp2), 1563 (C⚌C), 1162 (C–N), 1158 (C–O), LCMS (M-BF4) 197 found for C11H21N2O+.
3.5.24 3-(3-Hydroxypropyl)-1-pentyl -1H-imidazol-3-ium hexafluorophosphate 27
1H NMR (400 MHz, D2O) δ: 0.88 (t, J = 7.6, 3H), 1.27–1.33 (m, 4H), 1.88 (quint, J = 7.6, 2H), 2.12 (quint, J = 7.6, 2H), 3.64 (t, J = 6.8, 2H), 4.20 (t, J = 7.6, 2H), 4.32 (t, J = 6.8, 2H), 7.51–7.52 (dd, 2H), 8.78 (s,1H), 13C NMR (100 MHz, D2O) δ 13.1 (CH3), 21.4 (CH2), 27.5 (CH2), 28.8 (CH2), 31.6 (CH2), 46.6 (CH2), 49.7 (CH2), 58.0 (CH2), 122.4 (CH), 122.5 (CH), 135.3 (CH), 19F NMR (376.5 MHz, D2O) δ: −72.96 (d, J = 711.6 Hz); 31P NMR (162 MHz, D2O) δ: −144.96 (sep, J = 712.8 Hz) ppm, IR (NaCl): νmax cm−1 3212 (O–H), 3162 (C–H, sp2), 1565 (C⚌C), 1164 (C–N), 1158 (C-O), LCMS (M-PF6) 197 found for C11H21N2O+.
3.5.25 3-(3-Hydroxypropyl)-1-pentyl -1H-imidazol-3-ium trifluoroacetate 28
1H NMR (400 MHz, D2O) δ: 0.98 (t, J = 7.6, 3H), 1.36–1.44 (m, 4H), 2.03 (quint, J = 7.6, 2H), 2.28 (quint, J = 7.6, 2H), 3.78 (t, J = 6.8, 2H), 4.38 (t, J = 7.6, 2H), 4.49 (t, J = 6.8, 2H), 7.69–7.70 (dd, 2H), 8.99 (s,1H), 13C NMR (100 MHz, D2O) δ: 13.4 (CH3), 21.7 (CH2), 27.9 (CH2), 29.5 (CH2), 32.3 (CH2), 46.7 (CH2), 49.8 (CH2), 57.2 (CH2), 121.9 (CH), 122.8 (CH), 136.3 (CH), 19F NMR (376.5 MHz, D2O) δ: −75.72, IR (NaCl): νmax cm−1 3211 (O–H), 3161 (C–H, sp2), 1564 (C⚌C), 1163 (C–N), 1158 (C–O), LCMS (M–CF3CO2) 197 found for C11H21N2O+.
3.5.26 3-(3-Hydroxypropyl)-1-pentyl-1H-imidazol-3-ium Dicyanoamine 29
1H NMR (400 MHz, D2O) δ: 0.96 (t, J = 7.6, 3H), 1.38–1.44 (m, 4H), 2.01 (quint, J = 7.6, 2H), 2.23 (quint, J = 7.6, 2H), 3.74 (t, J = 6.8, 2H), 4.33 (t, J = 7.6, 2H), 4.44 (t, J = 6.8, 2H), 7.66–7.67 (dd, 2H), 8.96 (s,1H), 13C NMR (100 MHz, D2O) δ: 13.5 (CH3), 21.7 (CH2), 27.8 (CH2), 29.2 (CH2), 31.9 (CH2), 46.8 (CH2), 49.9 (CH2), 58.1 (CH2), 122.7 (CH), 122.8 (CH), 135.5 (CH), IR (NaCl): νmax cm−1 3210 (O-H), 3160 (C–H, sp2), 1562 (C⚌C), 1163 (C–N), 1160 (C-O), LCMS (M–N(CN)2) 197 found for C11H21N2O+.
3.5.27 3-(3-hydroxypropyl)-1-pentyl-1H-imidazol-3-ium thiocyanate 30
1H NMR (400 MHz, D2O) δ: 0.98 (t, J = 7.6, 3H), 1.37–1.44 (m, 4H), 2.03 (quint, J = 7.6, 2H), 2.28 (quint, J = 7.6, 2H), 3.78 (t, J = 6.8, 2H), 4.39 (t, J = 7.6, 2H), 4.49 (t, J = 6.8, 2H), 7.69–7.70 (dd, 2H), 8.99 (s,1H), 13C NMR (100 MHz, D2O) δ 13.6 (CH3), 21.8 (CH2), 28.0 (CH2), 29.4 (CH2), 32.1 (CH2), 47.0 (CH2), 50.0 (CH2), 58.3 (CH2), 122.7 (CH), 122.8 (CH), 135.5 (CH), IR (NaCl): νmax cm−1 3213 (O–H), 3163 (C–H, sp2), 1565 (C⚌C), 1166 (C–N), 1163 (C–O), LCMS (M–NCS) 197 found for C11H21N2O+.
3.5.28 3-(3-Hydroxypropyl)-1-pentyl-1H-imidazol-3-ium Nitrate 31
1H NMR (400 MHz, D2O) δ: 0.91 (t, J = 7.6, 3H), 1.32–1.38 (m, 4H), 1.93 (quint, J = 7.6, 2H), 2.18 (quint, J = 7.6, 2H), 3.68 (t, J = 6.8, 2H), 4.27 (t, J = 7.6, 2H), 4.38 (t, J = 6.8, 2H), 7.59–7.60 (dd, 2H), 8.89 (s,1H), 13C NMR (100 MHz, D2O) δ: 13.3 (CH3), 21.5 (CH2), 27.7 (CH2), 29.0 (CH2), 31.8 (CH2), 46.7 (CH2), 49.8 (CH2), 58.1 (CH2), 122.6 (CH), 122.7 (CH), 135.4 (CH), IR (NaCl): νmax cm−1 3211 (O–H), 3160 (C–H, sp2), 1563 (C⚌C), 1163 (C–N), 1161 (C–O), LCMS (M–NO3) 197 found for C11H21N2O+.
4 Results and discussion
The reason for popularity of imidazolium as an IL cation is due to the features which can be achieved. It is easy to modify, stable under acidic conditions, and its charge density is low due to the aromatic system. The low charge density means that it is relatively easy to construct low melting salts from imidazolium.
The universal method of preparing ionic liquids is the alkylation of a suitable heteroatom containing organic molecule to form the cation. After the alkylation the anion is changed to one desired. The chemistry has been quite simple, which makes it economic, but usually long reaction times are needed to complete the alkylation step, this is why the exploration of the sonochemistry will be beneficial.
To the best of our knowledge, only compounds 5 and 14 have been previously reported by our team (Messali, 2013).
Initially, the known and commercially available N-alkylimidazoles (1–3) were easily prepared by treatment of imidazole with appropriate alkyl halides under basic conditions.
The quaternization reaction proceeds by an SN2 mechanism. Therefore, both the nucleophile and the alkylating reagent have a strong influence on the reaction rate. The nucleophile in all the quaternization reactions was N-alkylimidazoles (1–3). In this way, the synthesis of imidazolium halides (4–7) is carried out by using different alkyl halides under conventional preparation (CP1: toluene, 80 °C, 18 h) (Scheme 1).
(i) N-alkylation of imidazole: RBr, (KOH/K2CO3), acetonitrile, 80 °C, 24 h. R = CH2Ph; (CH2)2CH3; (CH2)4CH3. (ii) N-alkylation of N-alkylimidazole: conventional preparation (CP1) and ultrasonic irradiation conditions.(CP1) : R’Br, toluene, 80 °C, 18 h; toluene, 80 °C, 5 h. R′ = CH2CO2Et; (CH2)4OPh; (CH2)3OH.
On the other hand, the ultrasound-assisted preparation of imidazolium-based ionic liquids (4–7), already synthesized by conventional method, was explored further with the objective of shortening the reaction time. The reaction conditions were optimized by using a sonication bath with a strict control of temperature during the reaction process. The comparative results and the optimum reaction conditions determined for the synthesis of these unknown ionic liquids are summarized in Table 1.
Ionic liquid
R
R’
Yield (%) of the Quaternization step
CP1a
))))b
4
CH2Ph
CH2CO2Et
79
88
5
CH2Ph
(CH2)4OPh
82
89
6
(CH2)2CH3
(CH2)3OH
83
90
7
(CH2)4CH3
(CH2)3OH
81
89
Furthermore, ILs do not evaporate like volatile organic compounds do, but they will decompose at high temperatures. The decomposition temperature depends on the IL, and particularly on the anion. The low melting point and negligible vapor pressure lead to a wide liquid range, often more than 300–400 °C (Holbrey and Seddon, 1999).
In This way, several alternative anions were subsequently introduced by a metathesis reaction (CP2: Acetonitrile, 70 °C, 3 h) (Huddleston et al., 2001), with a slight excess of anions namely, sodium tetrafluoroborate, potassium hexafluorophosphate, trifluoroacetic acid sodium, sodium dicyanamide, sodium thiocyanate or sodium nitrate (Scheme 2). The pure metathesis products (8–31) were obtained after filtration of the salts (metal halides), then followed by filtrate evaporation and washing the residue with dichloromethane followed by further filtration to remove the excess of anion salts (NaBF4, KPF6, CF3COONa, NaN(CN)2, NaNCS, NaNO3). Finally, evaporation of the filtrate afforded the desired ionic liquids in good yields.
Anion metathesis using conventional preparation (CP2) and ultrasonic irradiation conditions. (CP2): MY, acetonitrile, 70 °C, 3 h;)))): acetonitrile, 70 °C, 45 min. M = Na, K.
The anion methathesis was easily carried out under ultrasound irradiation. The data in Table 2 indicated that very good yields were obtained within short reaction times. As observed, the anion nature of exchange agents did not affect the yields.
Ionic liquid
R
R′
X−
MY
Yield (%) for the anion metathesis
CP2a
))))b
8
CH2Ph
CH2CO2Et
Cl−
NaBF4
95
98
9
KPF6
93
96
10
NaOOCCF3
95
97
11
NaN(CN)2
94
97
12
NaNCS
94
95
13
NaNO3
94
98
14
CH2Ph
(CH2)4OPh
Br−
NaBF4
95
96
15
KPF6
95
97
16
NaOOCCF3
92
98
17
NaN(CN)2
94
97
18
NaNCS
92
97
19
NaNO3
94
95
20
(CH2)2CH3
(CH2)3OH
Br−
NaBF4
94
97
21
KPF6
93
96
22
NaOOCCF3
95
97
23
NaN(CN)2
92
96
24
NaNCS
93
95
25
NaNO3
94
97
26
(CH2)4CH3
(CH2)3OH
Br−
NaBF4
94
98
27
KPF6
93
97
28
NaOOCCF3
93
97
29
NaN(CN)2
92
96
30
NaNCS
93
95
31
NaNO3
92
96
The preparations of the ionic liquids were monitored with 1H NMR, 13C NMR, 11B NMR, 19F NMR, 31P NMR, FT-IR, and LCMS. NMR proved to be generally the most efficient method for monitoring the quaternization and methathesis reactions. The exchange of an anion from Cl− or Br− to BF4−, PF6− or other anions caused only slight changes in the NMR spectrum. The biggest difference in the 1H NMR spectra is the chemical shift of the most acidic proton of the imidazolium ring. Furthermore the 11B NMR, 19F NMR, 31P NMR spectra proved without ambiguity that the methathesis reactions were carried out successfully. Spectroscopic data of all newly synthesized room temperature ionic liquids are given in the experimental part.
5 Conclusion
In summary, new environmentally friendly functionalized imidazolium-based ionic liquids were prepared by using ultrasound irradiation. We indeed found that the simple, convenient and efficient sonochemistry methods could significantly enhance the yield of RTILs and decrease the reaction time. Further investigations on these new synthesized ionic-liquids as corrosion inhibitors will be discussed in detail in the forthcoming paper.
Acknowledgement
We gratefully acknowledge the financial support from Taibah University (Grant 430/417).
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Appendix A
Supplementary data
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.arabjc.2013.08.023.
Appendix A
Supplementary data
