CSIRO PUBLISHING
Rapid Communication
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2004, 57, 121–124
Ionic Liquids Based on Imidazolium and Pyrrolidinium Salts of the Tricyanomethanide Anion Stewart A. Forsyth,A Stuart R. Batten,A Qing Dai,A and Douglas R. MacFarlaneA,B A
School of Chemistry, Monash University, Melbourne VIC 3800, Australia. to whom correspondence should be addressed (e-mail:
[email protected]).
B Author
A novel series of tricyanomethanide ionic liquids have been prepared and characterized for potential use as ionic liquid solvents. Full thermal analyses of all salts at ambient and sub-ambient temperatures are reported (melting points −17◦ to 160◦ C). The thermal stability and decomposition temperatures are also presented (Tdecomp ≈ 300◦ C). An electrochemical window of ∼3 V has been established and the conductivity measured over a range of temperatures (20 mS cm−1 at 25◦ C). Manuscript received: 17 September 2003. Final version: 19 October 2003.
N
N C
⫺ C
O
C
O
⫺ N C
C
N
bis(trifluoromethane)sulfonamide (TFSA) anions are compared in Fig. 1. A clear trend of decreasing melting point with increasing alkyl chain length (on the cation) is present in all the salts displayed, with the exception of [P14 ][TCM]. This melting point depression is a well-known trend,[16] which is related to the crystallinity of the material. The minimum melting point for many pyrrolidinium salts is either P13 or P14 ,[17–19] increasing slightly thereafter for longer alkyl chains. The melting points for P13 , P14 , and 1-ethyl-3methylimidazolium (emIm) TCM salts are all sub-ambient, an important characteristic of any potential ionic liquid solvent.TheTCM melting points are clearly lower than theTFSA ionic liquids, although not quite as low as the DCA-based ionic liquids. Interestingly the difference in melting points for the imidazolium TCM, DCA, and TFSA ionic liquids appears to be minimal.
Temperature (°C)
The discovery of ionic liquids and the potential benefits of replacing traditional organic solvents have been extensively reviewed.[1–4] Typical properties of ionic liquids such as nonvolatility, non-flammability, and a large stable liquid range have obvious and immediate advantages over traditional solvents. Other properties such as superior solvating effects, low viscosity, high conductivity, hydrophobicity or hydrophilicity, and high thermal/electrochemical stability have also been identified and exploited in a wide range of applications. The continued search for ionic liquids that have advantageous properties led us to investigate the tricyanomethanide anion (TCM, Scheme 1). A close relative of TCM is the dicyanamide anion (DCA), which is known to afford very low-melting, low-viscosity ionic liquids.[5,6] The TCM ion and its conjugate acid cyanoform, HC(CN)3 , have been known for many years.[7–10] Recently the tricyanomethanide anion has been used as a connector ligand in metal complexes that display magnetic properties.[11–15] This paper reports the first ionic liquids to be based on the tricyanomethanide anion. Imidazolium and pyrrolidinium salts of TCM have been evaluated using differential scanning calorimetry, thermogravimetric analysis, cyclic voltammetry, and impedance spectroscopy. The melting points of various pyrrolidinium (P) and imidazolium (Im) salts of tricyanomethanide, dicyanamide, and
F3C
C N
S
⫺ N
S
CF3
O O
180 160 140 120 100 80 60 40 20 0 ⫺20 ⫺40 ⫺60 [P11]⫹
N
Scheme 1. Left to right, the tricyanomethanide (TCM), dicyanamide (DCA), and bis(trifluoromethane)sulfonamide (TFSA) anions.
[P12]⫹
[P13]⫹
[P14]⫹ [dmIm]⫹ [emIm]⫹
Fig. 1. Low-melting TCM- (), DCA- (),[6] and TFSA-based ()[17,23] ionic liquids.
© CSIRO 2004
10.1071/CH03245
0004-9425/04/020121
122
S. A. Forsyth et al.
Table 1. Thermal properties of tricyanomethanide salts Compound
Abbreviation
Tg [◦ C]
Tx [◦ C]
Ts−s [◦ C]
Tm [◦ C]
Sf [J K−1 mol−1 ] (±10%)
Tdec [◦ C]
N,N-dimethylpyrrolidinium tricyanomethanide N-ethyl-N-methylpyrrolidinium tricyanomethanide N-propyl-N-methylpyrrolidinium tricyanomethanide N-butyl-N-methylpyrrolidinium tricyanomethanide 1,3-dimethylimidazolium tricyanomethanide 1-ethyl-3-methylimidazolium tricyanomethanide 1-ethyl-3-methylimidazolium dicyanamide
[P11 ][TCM]
–
–
−25
163
41
315
[P12 ][TCM]
–
–
−15
45
15
320
[P13 ][TCM]
−98
−73 and −54
−29
−17
23
320
[P14 ][TCM]
−92
−48
−15
−8
24
315
[dmIm][TCM]
–
–
–
36
52
335
[emIm][TCM]
−85
−50
–
−10
29
290
[emIm][DCA]
−104
−65
–
−21
60
275
Tx , Ts−s , and Tm are the onset of transition temperatures.
[P11][TCM]
100 90
[dmIm]⫹ [P11]⫹ [P12]⫹ [P14]⫹ [P13]⫹ [emIm]⫹
Weight (%)
[P12][TCM]
Endotherm [P13][TCM] [P14][TCM]
Exotherm
80 70 60 50
[dmIm][TCM]
40
[emIm][TCM]
30
⫺150 ⫺100
⫺50
0 50 100 Temperature (°C)
150
200
Fig. 2. Differential scanning calorimetry of the TCM-based ionic liquids.
Thermal analyses of all TCM salts are summarized in Table 1 ([emIm][DCA] is included for comparison). Figure 2 displays the thermal traces for the TCM series of salts. All the pyrrolidinium TCM salts have a solid–solid transition before melting. In the case of [P12 ][TCM] this transition consumes much of the available entropy leaving a relatively low entropy melt of just 15 J K−1 mol−1 . Salts with low entropies of fusion (less than 20 J K−1 mol−1 ) have been identified as belonging to the family of plastic crystal materials.[20] They also typically have significant ion conductivity in the plastic phase.[21,22] Interestingly all of the solid–solid transitions occur at approximately −20◦ C in the TCM family studied here; the decreasing melting point trend thus reducing the temperature range in which each salts exists in this phase. The pyrrolidinium TCM salts may be of interest as solid electrolytes at temperatures at which they are not useful as liquid solvents. The imidazolium TCM salts did not display any solid–solid transitions, nor did the imidazolium DCA salts. Figure 3 displays thermogravimetric traces of all the TCM salts. The TCM salts do not appear to be hygroscopic as indicated by the lack of mass loss (volatile H2 O) for atmosphere equilibrated samples. By comparison, similarly equilibrated
0
100
200 300 Temperature (°C)
400
500
Fig. 3. Thermogravimetric analyses of the TCM-based ionic liquids.
DCA salts lose ∼15 wt-% moisture. The thermal decomposition temperatures for the TCM salts vary between 290 and 335◦ C, quite similar to the DCA salts[5] but quite a long way below the thermal decomposition temperature of ∼450◦ C for TFSA[23,24] salts. The decomposition temperatures of the TCM salts compare well with salts of other non-perfluorinated anions such as mesylate (CH3 SO3 ) and tosylate (CH3 C6 H4 SO3 ).[25] Figure 4 shows typical cyclic voltammetric behaviour for the TCM salts. This electrochemistry was carried in a nitrogen atmosphere (drybox) using a Maclab potentiostat, and Maclab software. Electrodes consisted of a glassy carbon working electrode, a platinum wire counter electrode, and a silver wire pseudo-reference electrode. Although the trace shows some minor oxidative and reductive processes (reversible) over the voltage range, the electrochemical window can be estimated to be ∼3 V versus Ag/Ag+ at 20◦ C. This electrochemical window is similar to the corresponding DCA ionic liquid,[6] where cation reduction and anion oxidation occuring at similar potentials. Figure 5 shows conductivity for [emIm][TCM] between room temperature and 70◦ C. The conductivity of [emIm][TCM] at 25◦ C is 20 mS cm−1 . This value is superior to that
Ionic Liquids Based on Tricyanomethanide
123
mass spectra were recorded using a Micromass Platform electrospray mass spectrometer on samples dissolved in methanol. Thermal analyses and temperature-dependent phase behaviour was studied in the range −120 to 200◦ C by differential scanning calorimetry (Perkin–Elmer DSC7). Accurately weighed samples were loaded into aluminium pans in an inert atmosphere. Sub-ambient scans were calibrated with the cyclohexane solid–solid transition and melting point at −87.0 and 6.5◦ C respectively. Transition temperatures were recorded at the onset of the thermal transition (±1◦ C). Entropies of fusion were determined from the calibrated thermogram peak areas and the relationship Sf = Hf /Tm , where Sf and Hf are the molar entropy and enthalpy of fusion respectively and Tm is the melting point. The thermogravimetric analyses were conducted using a STA1500 (Rheometric Scientific) in a nitrogen flow (50 mL min−1 ) between 30 and 500◦ C with a temperature ramp of 10◦ C min−1 . The instrument was calibrated using four melting points (indium, tin, lead, zinc) and aluminium pans were used in all experiments. Conductance measurements were performed using a locally designed dip cell probe consisting of two platinum wires sheathed in glass. The sample was loaded into the cell and an O-ring seal fitted between the sample vial and the probe wall. The cell constant was determined with a solution of 0.01 M KCl at 25◦ C. Conductivities were obtained by measurement of the complex impedance spectra between 1 MHz and 10 kHz on a HP 4284A impedance meter. The temperature was controlled at 5◦ C intervals (±1◦ C) using a Eurotherm 2204e interfaced to the HP 4284A and a cartridge heater set in a brass block with a cavity for the cell. A T-type thermocouple was set in the block adjacent to the cell. The conductance was determined from the first real axis touchdown point in the Cole–Cole plot of the impedance data.
2
[emIm][TCM]
i (µA)
1
0
⫺1 ⫺2 ⫺2
⫺1
0
1
⫹
E (V/Ag/Ag )
Fig. 4. Cyclic voltammetry of [emIm][TCM] at 20◦ C.
Log Cond (S cm⫺1)
⫺1.3 ⫺1.4 ⫺1.5 ⫺1.6
1,3-Dimethylimidazolium Tricyanomethanide [dmIm][TCM]
⫺1.7 ⫺1.8 20
30
40
50
60
70
Temperature (°C)
Fig. 5. Conductivity of [emIm][TCM].
for [emIm][TFSA][23] (8.8 mS cm−1 ) but similar to that for [emIm][DCA] (22 mS cm−1 ).[26] To conclude, a new series of tricyanomethanide ionic liquids have been synthesized and characterized. The tricyanomethanide ionic liquids are lower melting than the bis(trifluoromethane)sulfonamide ionic liquids. Thermal analyses have revealed sub-ambient solid–solid phase transitions for the pyrrolidinium salts. The tricyanomethanide ionic liquids have similar conductivity, thermal stability, and electrochemical stability as the dicyanamide ionic liquids, but they are not as hygroscopic. Experimental Potassium tricyanomethanide and silver tricyanomethanide were prepared from the reported methods of Andersen et al.[27] and Konnert et al.[28] The 1-alkyl-3-methylimidazolium and N-alkyl-Nmethylpyrrolidinium halides were prepared using previously reported methods.[17,18,29,30] Infrared spectra were obtained in the range of 4000–650 cm−1 on a Perkin–Elmer 1600 series FT-IR spectrophotometer. Ionic liquid samples were analyzed neat or as a Nujol mull between NaCl plates. 1 H and 13 C NMR spectra were recorded on a Bruker DPX 300 spectrometer for solutions in CDCl3 . Peaks are noted below only if they were significantly resolved from neighboring peaks and/or the baseline. Tetramethylsilane was used as an internal standard. Positive and negative ion electrospray
Ag[TCM] (1 g, 5.08 mmol) was added to an aqueous solution of [dmIm]I (1.08 g, 4.84 mmol) and stirred at room temperature for 24 h. Solid silver iodide was filtered from the solution and the filtrate evaporated to dryness. The pale yellow residue was then dissolved in dichloromethane and dried over anhydrous magnesium sulfate. The solution was filtered and evaporated to dryness to afford the title compound as a pale yellow solid (0.86 g, 95%). mp 49–52◦ C. ν (neat)/cm−1 3396w, 3150s, 3121s, 2960m, 2855w, 2811w, 2472w, 2166s, 2121s, 1574s, 1470m, 1418m, 1340m, 1255w, 1230w, 1173s, 1107w, 1086w, 1022w, 965w, 841m, 750m, 715w. δH (CDCl3 ) 3.98 (6H, s, CH3 ), 7.35 (1H, s, CH), 7.45 (1H, s, CH), 8.75 (1H, d, CH). δC (CDCl3 ) 35.6 (CH3 ), 35.8 (CH3 ), 120.8 (CH), 122.9 (CH). ES-MS (ES+ ) m/z 96.7 ([dmIm]+ ); (ES− ) m/z 89.7 ([TCM]− ). 1-Ethyl-3-methylimidazolium Tricyanomethanide [emIm][TCM] Ag[TCM] (1 g, 5.08 mmol) and emImBr (0.92 g, 4.84 mmol) were metathesized using the same method used for [dmIm][TCM]. The title compound was afforded as a pale yellow liquid (0.78 g, 80%). ν (Nujol)/cm−1 3483w, 3153s, 3114s, 2987m, 2812w, 2770w, 2473w, 2164s, 2122m, 1573s, 1501w, 1450m, 1427m, 1388m, 1337m, 1295w, 1254m, 1230m, 1159s, 1088m, 1031w, 959w, 842m, 802w, 751m, 701w. δH (CDCl3 ) 1.65 (3H, t, CH3 ), 3.98 (3H, s, CH3 ), 4.30 (2H, q, CH2 ), 7.38 (2H, d, CH), 8.88 (1H, s, CH). δC (CDCl3 ) 14.2 (CH3 ), 35.7 (CH3 ), 44.7 (CH2 ), 120.4 (CH), 122.0 (CH). ES-MS (ES+ ) m/z 110.6 ([emIm]+ ); (ES− ) m/z 89.5 ([TCM]− ). N,N-Dimethylpyrrolidinium Tricyanomethanide [P11 ][TCM] Ag[TCM] (1 g, 5.08 mmol) and [P11 ]I (1.10 g, 4.84 mmol) were metathesized using the same method used for [dmIm][TCM]. The title compound was afforded as a white solid (0.91 g, 99%). mp 177–180◦ C. ν (Nujol)/cm−1 3389w, 2933s, 2480w, 2162s, 1464s, 1412w, 1377s, 1321s, 1275w, 1252w, 1232m, 1049w, 999m, 976m, 933m, 815w, 723w. δH (CDCl3 ) 2.33–2.43 (4H, m, CH2 ), 3.24 (6H, s, CH3 ), 3.60–3.68 (4H, m, CH2 ). δC (CDCl3 ) 21.2 (2 × CH2 ), 51.5 (2 × CH3 ), 65.4 (2 × CH2 ), 120.6 (CN). ES-MS (ES+ ) m/z 99.6 ([P11 ]+ ); (ES− ) m/z 89.5 ([TCM]− ).
124
N-Ethyl-N-methylpyrrolidinium Tricyanomethanide [P12 ][TCM] Ag[TCM] (1 g, 5.08 mmol) and [P12 ]I (1.16 g, 4.84 mmol) were metathesized using the same method used for [dmIm][TCM]. The title compound was afforded as a white solid (0.96 g, 97%). mp 50–53◦ C. ν (Nujol)/cm−1 3391m, 2932s, 2478w, 2163s, 1464s, 1403w, 1378m, 1302m, 1258m, 1231m, 1111w, 1083w, 1034w, 996m, 959m, 913w, 879w, 808m, 722w. δH (CDCl3 ) 1.50 (3H, t, CH3 ), 2.30–2.40 (4H, m, CH2 ), 3.15 (3H, s, CH3 ), 3.48 (2H, q, CH2 ), 3.50–3.60 (4H, m, CH2 ). δC (CDCl3 ) 8.5 (CH3 ), 20.9 (2 × CH2 ), 47.8 (CH3 ), 59.9 (CH2 ), 63.5 (2 × CH2 ). ES-MS (ES+ ) m/z 113.6 ([P12 ]+ ); (ES− ) m/z 89.7 ([TCM]− ). N-Propyl-N-methylpyrrolidinium Tricyanomethanide [P13 ][TCM] Ag[TCM] (1 g, 5.08 mmol) and [P13 ]I (1.23 g, 4.84 mmol) were metathesized using the same method used for [dmIm][TCM]. The title compound was afforded as a clear liquid (1.04 g, 99%). ν (Nujol)/cm−1 3490w, 3395w, 2975w, 2883m, 2810m, 2473w, 2166s, 2121m, 1636m, 1470s, 1404w, 1370w, 1340w, 1304w, 1253m, 1231m, 1121w, 1040w, 1002m, 970m, 940m, 904m, 885w, 821w, 759w. δH (CDCl3 ) 1.11 (3H, t, CH3 ), 1.88 (2H, m, CH2 ), 2.30–2.40 (4H, m, CH2 ), 3.15 (3H, s, CH3 ), 3.30–3.38 (2H, m, CH2 ), 3.50–3.60 (4H, m, CH2 ). δC (CDCl3 ) 9.8 (CH3 ), 16.6 (CH2 ), 20.9 (2 × CH2 ), 47.8 (CH3 ), 63.9 (CH2 ), 65.5 (2 × CH2 ), 120.8 (CN). ES-MS (ES+ ) m/z 127.7 ([P13 ]+ ); (ES− ) m/z 89.5 ([TCM]− ).
S. A. Forsyth et al.
[7] [8] [9] [10] [11] [12] [13] [14]
[15] [16] [17] [18]
[19]
N-Butyl-N-methylpyrrolidinium Tricyanomethanide [P14 ][TCM] Ag[TCM] (1 g, 5.08 mmol) and [P14 ]Br (1.07 g, 4.84 mmol) were metathesized using the same method used for [dmIm][TCM]. The title compound was afforded as a clear liquid (0.91 g, 99%). ν (Nujol)/cm−1 3486m, 3395m, 2965s, 2877s, 2809m, 2640w, 2473w, 2151s, 2121m, 1634w, 1467s, 1404w, 1381w, 1348w, 1253m, 1229w, 1121w, 1061w, 1030w, 1003m, 964w, 928w, 823w, 739w. δH (CDCl3 ) 1.05 (3H, t, CH3 ), 1.48 (2H, sextet, CH2 ), 1.80 (2H, m, CH2 ), 2.28–2.35 (4H, m, CH2 ), 3.11 (3H, s, CH3 ), 3.35 (2H, m, CH2 ), 3.55 (4H, m, CH2 ). δC (CDCl3 ) 12.5 (CH3 ), 18.7 (CH2 ), 20.8 (2 × CH2 ), 24.8 (CH2 ), 47.8 (CH3 ), 63.9 (3 × CH2 ). ES-MS (ES+ ) m/z 141.7 ([P14 ]+ ); (ES− ) m/z 89.4 ([TCM]− ).
References [1] M. Freemantle, Chem. Eng. News 2000, 78, 37. [2] T. Welton, Chem. Rev. 1999, 99, 2071. doi:10.1021/CR980032T [3] H. Olivier-Bourbigou, L. Magna, J. Mol. Catal. A: Chem. 2002, 182–183, 419. doi:10.1016/S1381-1169(01)00465-4 [4] J. D. Holbrey, K. R. Seddon, J. Chem. Soc., Dalton Trans. 1999, 2133. doi:10.1039/A902818H [5] D. R. MacFarlane, S. A. Forsyth, Green Chem. 2002, 4, 444. doi:10.1039/B205641K [6] D. R. MacFarlane, J. Golding, S. A. Forsyth, M. Forsyth, G. B. Deacon, Chem. Commun. 2001, 1430. doi:10.1039/ B103064G
[20] [21] [22] [23]
[24]
[25]
[26]
[27] [28] [29] [30]
H. Schmidtman, Ber. Dtsch. Chem. Ges. 1896, 29, 1172. A. Hantzsch, G. Oswald, Ber. Dtsch. Chem. Ges. 1899, 32, 641. L. Birkenbach, Ber. Dtsch. Chem. Ges. 1925, 58, 786. L. Birkenbach, K. Huttner, Ber. Dtsch. Chem. Ges. B 1929, 62, 2065. M. Gembický, R. Boˇca, L. Jäger, C. Wagner, Inorg. Chem. Commun. 2000, 3, 566. doi:10.1016/S1387-7003(00)00142-8 J. Kozíšek, M. Hvastijová, J. Kohout, J. Mrozinski, H. Koehler, J. Chem. Soc., Dalton Trans. 1991, 1773. J. Kohout, L. Jäger, M. Hvastijová, J. Kozíšek, J. Coord. Chem. 2000, 51, 169. S. R. Batten, B. F. Hoskins, B. Moubaraki, K. S. Murray, R. Robson, J. Chem. Soc., Dalton Trans. 1999, 2977. doi: 10.1039/A904383G J. L. Manson, C. Campana, J. S. Miller, Chem. Commun. 1998, 251. doi:10.1039/A707289I J. D. Holbrey, K. R. Seddon, J. Chem. Soc., Dalton Trans. 1999, 2133. doi:10.1039/A902818H D. R. MacFarlane, P. Meakin, J. Sun, N. Amini, M. Forsyth, J. Phys. Chem. B 1999, 103, 4164. doi:10.1021/JP984145S S. A. Forsyth, J. Golding, D. R. MacFarlane, M. Forsyth, Electrochim. Acta 2001, 46, 1753. doi:10.1016/S00134686(00)00781-7 J. Golding, N. Hamid, D. R. MacFarlane, M. Forsyth, C. Forsyth, C. Collins, J. Huang, Chem. Mater. 2001, 13, 558. doi:10.1021/CM000625W J. Timmermans, Phys. Chem. Solids 1961, 18, 1. doi:10.1016/0022-3697(61)90076-2 J. Huang, M. Forsyth, D. R. MacFarlane, Solid State Ionics 2000, 136–137, 447. doi:10.1016/S0167-2738(00)00450-1 D. R. MacFarlane, P. Meakin, N. Amini, M. Forsyth, J. Phys. Condens. Matter 2001, 13, 8257. doi:10.1088/0953-8984/13/36/303 P. Bonhôte, A.-P. Dias, N. Papageorgiou, K. Kalyanasundaram, M. Grätzel, Inorg. Chem. 1996, 35, 1168. doi:10.1021/ IC951325X H. L. Ngo, K. LeCompte, L. Hargens, A. B. McEwen, Thermochim. Acta 2000, 357–358, 97. doi:10.1016/S00406031(00)00373-7 J. Golding, S. Forsyth, D. R. MacFarlane, M. Forsyth, G. B. Deacon, Green Chem. 2002, 4, 223. doi:10.1039/ B201063A C. Tiyapiboonchaiya, D. R. MacFarlane, J. Sun, M. Forsyth, Macromol. Chem. Phys. 2002, 203, 1906. doi:10.1002/15213935(200209)203:133.0.CO;2-I P. Andersen, B. Klewe, E. Thom, Acta Chem. Scand. A 1967, 21, 1530. J. Konnert, D. Britton, Inorg. Chem. 1966, 5, 1193. B. K. M. Chan, N.-H. Chang, M. R. Grimmett, Aust. J. Chem. 1977, 30, 2005. J. S. Wilkes, J. A. Levisky, R. A. Wilson, C. L. Hussey, Inorg. Chem. 1982, 21, 1263.
