J. Phys. Chem. B 2003, 107, 11749-11756
11749
Ionic Liquids of Chelated Orthoborates as Model Ionic Glassformers Wu Xu, Li-Min Wang, Ronald A. Nieman, and C. Austen Angell* Department of Chemistry and Biochemistry, Arizona State UniVersity, Tempe, Arizona 85287-1604 ReceiVed: March 3, 2003; In Final Form: August 4, 2003
Ionic liquids based on various chelated orthoborate anions of different N-containing onium cations have been synthesized using an economic synthesis strategy. Most orthoborates do not crystallize. They are found to have much higher glass transition temperatures and room-temperature viscosities than those with perfluorinated anions such as TFSI-, BF4-, and CF3SO3- (Tf-), as predicted from anion polarizability arguments. The ambient conductivities of the new ionic liquids are low relative to those with perfluorinated anions. The transport properties all show that cohesion in these liquids increases, and ionic mobilities decrease, as anion size increases, implying that van der Waals interactions, not Coulomb interactions, have become the controlling influence. In view of their resistance to crystallization, the large range of temperature over which these liquids can be studied, their hydrophobic properties, and their high fragilities, these liquids may provide good model systems for fundamental liquid state investigations and interesting solvents for large-molecule dissolution.
1. Introduction Ionic liquids, also known as room-temperature molten salts, have been receiving much attention for their potential applications, not only in electrochemistry but also as alternative solution and reaction media in separation sciences and chemical syntheses. They have attractive physical properties such as negligible vapor pressure even at elevated temperatures, miscibility with organic solvents, excellent thermal and chemical stability, high conductivities, and wide electrochemical windows.1-6 The applications in electrochemistry include electroplating, batteries, electrochemical capacitors, and photoelectrochemical cells. Many new ionic liquids have been reported in recent years.7-23 A characteristic of many commonly reported ionic liquids is the presence of anions, known to be weakly coordinating, such as bis(trifluoromethanesulfonyl)imide (TFSI-), bis(perfluoroethanesulfonyl)imide (BETI-), tetrafluoroborate (BF4-), hexafluorophosphate (PF6-), trifluoromethanesulfonate (Tf-), and tetrachloroaluminate (AlCl4-), although the presence of thiocyanates (SCN-) and dicyanoamides [N(CN)2-], among most fluid examples, shows that inability to coordinate is not a requirement. Recently24 we have reported a new anion, bis(oxalato)borate (BOB-), which is also very weakly coordinating. Its lithium salt, LiBOB, showed high ionic conductivity and also electrochemical stability in nonaqueous electrolytic solutions. Practical cell studies using LiBOB in carbonate electrolytes25 showed excellent charge/discharge and cycling performance relative to the industry standard LiPF6. The advantage was especially notable at elevated temperatures. Here we report the physical properties and electrochemical properties of a series of room-temperature molten salts made by combining various organic cations with orthoborate anions. We find that orthoborate ionic liquids are too viscous for most practical applications but have many attractive properties as model systems for fundamental liquid state studies. 2. Experimental Section The chemical compounds used in this work were all from Aldrich except LiTFSI, which was obtained gratis from 3M. * Corresponding author. E-mail:
[email protected].
All were used as received. The chemical structures of the cations and anions of the synthesized ionic liquids are shown in Schemes 1 and 2, respectively. It is noticed, from the literature, that molten salts with the n-butyl side group usually have lower melting points than those with the ethyl side group.9,12,22,23 Therefore in this work the butyl group was chosen over the ethyl group despite the attendant sacrifices in viscosity (higher) and conductivity (lower). Indeed the melting points obtained were so low, relative to liquid viscosities, that crystallization was not observed at all in most cases. 2.1. Characterization. The intermediate compounds and the final products were characterized by NMR spectroscopy in deuterated dimethyl sulfoxide (DMSO-d6). 1H and 13C NMR spectra were obtained on a Varian Gemini 300 NMR spectrometer with TMS as internal reference. 11B, 7Li, 23Na, and 19F NMR spectra were collected using a Varian Inova 400, with BF3‚ Et2O, LiNO3, and NaF as external references, respectively. The melting points of onium chloride compounds prepared in this work were measured using a simple differential scanning analysis (DTA) instrument, which was described in our previous publication.26 After appropriate calibration with melting point standards this simple instrument is able to define melting points with an accuracy and precision of (1 °C. 2.2. Preparation of Li or Na Orthoborate Salt. The orthoborate salts (MBXB), such as LiBOB, sodium bis(oxalato)borate (NaBOB), lithium bis(malonato)borate (LiBMB), lithium bis(salicylato)borate (LiBScB), and lithium bis(2-methyllactato)borate (LiBMLB), were prepared by complete evaporation of water from the aqueous solutions of the corresponding organic acid (i.e., oxalic acid, malonic acid, salicylic acid, and 2-methyllactic acid, respectively), boric acid, and lithium or sodium hydroxide in a molar ratio of 2:1:1. The white crude solid products were refluxed with acetonitrile, followed by filtration and drying in a vacuum oven at 100 °C for 2 days.
where X is oxalato, malonato, salicylato, and 2-methyllactato, respectively.
10.1021/jp034548e CCC: $25.00 © 2003 American Chemical Society Published on Web 09/27/2003
11750 J. Phys. Chem. B, Vol. 107, No. 42, 2003 SCHEME 1: Chemical Structures of the Cations of the Ionic Liquids Synthesized in This Worka
a BMI+ is 1-n-butyl-3-methylimidazolium, RNM2E+ is alkyl dimethyl ethyl ammonium, P14+ is N-methyl-N-n-butyl pyrrolidinium, and BPy+ is N-n-butylpyridinium.
LiBOB, 1: white solid. NMR in DMSO-d6: 1H no signal except for solvent; 13C δ 158.20 ppm; 11B δ 12.20 ppm; 7Li δ 1.01 ppm. NaBOB, 2: white solid. NMR in DMSO-d6: 1H no signal except for solvent; 13C δ 158.21 ppm; 11B δ 12.25 ppm; 23Na δ 1.65 ppm. LiBMB, 3: white solid. NMR in DMSO-d6: 1H δ 3.40 ppm (s); 13C δ 166.01 and 38.67 ppm; 11B δ 8.10 ppm; 7Li δ 1.02 ppm. LiBScB, 4: white solid. NMR in DMSO-d6: 1H δ 7.71 (d, 2H, J ) 7.5 Hz), 7.40 (t, 2H, J ) 7.7 Hz), 6.84 ppm (m, 4H); 13C δ 163.76, 159.05, 134.66, 129.25, 118.82, 118.04, 115.40 ppm; 11B δ 8.29 ppm; 7Li δ 1.07 ppm. LiBMLB, 5: white solid. NMR in DMSO-d6: 1H δ 1.13 ppm (s); 13C δ 181.46, 75.07 26.84 ppm; 11B δ 13.67 ppm; 7Li δ 1.08 ppm. 2.3. Preparation of Onium Chloride Salts. The chlorides of 1-n-butyl-3-methylimidazolium (BMI), n-butylpyridinium (BPy), alkyl dimethyl ethylammonium (RNM2E), and N-methylN-n-butylpyrrolidinium (P14) were synthesized by the following reactions.
NR1R2R3 + R-Cl f RN+R1R2R3 ClBMICl, 6. BMICl was synthesized by refluxing 1-methylimidazole and excess 1-chlorobutane for 2 days, followed by decanting most of the unreacted 1-chlorobutane, evaporating the rest on a rotavapor at reduced pressure, and finally drying it in a high-vacuum oven at 90 °C for 4 days to yield a light yellow solid. Yield: 84.7%. Mp: 61.5 °C. NMR in DMSO-d6: 1H δ 9.57 (s, 1H), 7.89 (s, 1H), 7.81 (s, 1H), 4.18 (t, 2H, J ) 7.1 Hz), 3.86 (s, 3H), 1.74 (m, 2H), 1.22 (m, 2H), 0.86 ppm (t,
Xu et al. SCHEME 2: Chemical Structures of the Anions of Ionic Liquids Synthesized in This Worka
a BOB-, BMB-, BScB-, and BMLB- represent the anions of bis(oxalato)borate, bis(malonato)borate, bis(salicylato)borate, and bis(2methyllactato)borate, respectively.
3H, J ) 7.5 Hz); 13C δ 135.75, 123.52, 122.25, 48.31, 35.63, 31.33, 18.70, 13.24 ppm. BPyCl, 7. BPyCl was prepared by refluxing pyridine and excess 1-chlorobutane for 3 days. After decanting the unreacted 1-chlorobutane, the solid product was washed with diethyl ether to yield a white solid, which was dried in high vacuo at ca. 90 °C for 3 days. Yield: 37.2%. Mp: 98.0 °C. NMR in DMSOd6: 1H δ 9.20 (d, 2H, J ) 6.8 Hz), 8.61 (t, 1H, J ) 7.8 Hz), 8.16 (t, 2H, J ) 7.1 Hz), 4.65 (t, 2H, J ) 7.4 Hz), 1.89 (m, 2H), 1.27 (m, 2H), 0.90 ppm (t, 3H, J ) 7.3 Hz); 13C δ 145.40, 144.78, 128.01, 60.34, 32.61, 18.63, 13.22 ppm. P14Cl, 8. P14Cl was synthesized by refluxing a solution of N-methylpyrrolidine and excess 1-chlorobutane for 3 days. The solid product was thoroughly dried in a vacuum oven at ca. 90 °C for 2 days. Yield: 38.7%. Mp: 86.4 °C. NMR in DMSOd6: 1H δ 3.50 (m, 2 × 2H), 3.36 (m, 2H), 3.00 (s, 3H), 2.06 (s, 2 × 2H), 1.66 (m, 2H), 1.29 (m, 2H), 0.91 ppm (t, 3H, J ) 7.4 Hz); 13C δ 63.21, 62.61, 47.31, 24.84, 20.96, 19.23, 13.40 ppm. Alkyl dimethyl ethylammonium chloride (RNM2ECl) was prepared by reacting the solution of alkyl chloride and excess dimethylethylamine in tetrahydrofuran (THF) at moderate temperature for 1-4 days according to the activity of alkyl chloride. All crude products were washed with fresh THF to get solids, which were dried in high vacuo at ca. 90 °C for 4 days. MOMNM2ECl, 9. This compound was obtained from the reaction in THF at 0 °C first for 3 h and then at room temperature overnight: white solid. Yield: 91.7%. Mp: 135.0 °C. NMR in DMSO-d6: 1H δ 4.71 (s, 2H), 3.58 (s, 3H), 3.34 (m, 2H), 2.96 (s, 2 × 3H), 1.20 ppm (t, 3H, J ) 7.4 Hz); 13C δ 90.60, 60.57, 55.52, 45.84, 7.42 ppm.
Ionic Liquids of Chelated Orthoborates EOMNM2ECl, 10. This compound was obtained with the same mehod as MOMNM2ECl: white solid. Yield: 89.3%. Mp: 134.7 °C. NMR in DMSO-d6: 1H δ 4.84 (s, 2H), 3.84 (m, 2H), 3.39 (m, 2H), 3.01 (s, 2 × 3H), 1.19 ppm (m, 2 × 3H); 13C δ 89.09, 68.54, 55.33, 45.65, 15.05, 7.49 ppm. MOENM2ECl, 11. This compound was obtained from the reaction in an autoclave at ca. 75 °C for 2 days; white solid. Yield: 77.3%. Mp: 85.7 °C. NMR in DMSO-d6: 1H δ 3.73 (m, 2H), 3.55 (t, 2H, J ) 4.8 Hz), 3.43 (m, 2H), 3.27 (s, 3H), 3.05 (s, 2 × 3H), 1.21 ppm (t, 3H, J ) 7.3 Hz); 13C δ 65.34, 61.56, 59.43, 58.04, 49.96, 7.87 ppm. EOENM2ECl, 12. This compound was obtained from the reaction in an autoclave at 80 °C for 4 days: white solid. Yield: 47.2%. Mp: 59.4 °C. NMR in DMSO-d6: 1H δ 3.76 (m, 2H), 3.52 (t, 2H, J ) 5.0 Hz), 3.45 (m, 2H), 3.39 (m, 2H), 3.05 (s, 2 × 3H), 1.21 (t, 3H, J ) 7.3 Hz), 1.10 ppm (t, 3H, J ) 7.0 Hz); 13C δ 65.49, 63.24, 61.63, 59.46, 50.01, 14.78, 7.84 ppm. BNM2ECl, 13. This compound was obtained from the reaction in an autoclave at 80 °C for 7 days: yellowish white solid. Yield: 25.5%. Mp: 99.9 °C. NMR in DMSO-d6: 1H δ 3.32 (q, 2H), 3.19 (m, 2H), 2.97 (s, 2 × 3H), 1.61 (m, 2H), 1.31 (m, 2H), 1.23 (t, 3H, J ) 7.2 Hz), 0.90 ppm (t, 3H, J ) 7.0 Hz); 13C δ 62.38, 58.61, 49.44, 23.67, 19.17, 13.36, 7.71 ppm. 2.4. Synthesis of Orthoborate Ionic Liquids. In this work we made ionic liquids by substitution of orthoborate anions for chloride ions using an economic but effective metathesis reaction, which has been widely used for the preparations.3,27,28 The insolubility of sodium chloride in anhydrous acetonitrile, and particularly in dichloromethane, is sufficient enough that when these solvents are used in successive steps of the synthesis and refinement, the NMR spectrum of the final product in DMSO-d6 shows no sodium signal. In most cases even an AgNO3 solution test for trace chloride anion proves negative. This level of purity is quite sufficient for the purposes of the ionic liquid evaluations that we have elected to make in this study. The obvious advantage of this method is that the quantities of product needed to make additional measurements such as viscosities (often not reported in other studies) are not excluded for economic reasons. The onium chloride salt was reacted with excess orthoborate salt in anhydrous acetonitrile under refluxing for 1-3 days to allow the chloride to react completely. After cooling, the alkali halide precipitates were filtered off and the solvent in the filtrate was evaporated on a rotavapor at reduced pressure. The residue was dried in a vacuum oven at 100 °C for 1 day and then dissolved in a large amount of dichloromethane (CH2Cl2). The solution was allowed to stand at room temperature overnight in order for the dissolved lithium or sodium orthoborate salt to precipitate in CH2Cl2. After filtration and evaporation of the solvent from the filtrate, the residue was further dried in a vacuum oven at 90 °C for 2 days to yield the orthoborate ionic liquids. For comparison, the 1-n-butyl-3-methylimidazolium salts of TFSI-, BF4-, and Tf- were also prepared by reacting BMICl and the corresponding lithium (LiTFSI anf LiTf) or sodium salt (NaBF4) in acetonitrile, following the procedures as above. BMITFSI was extracted from the aqueous solution with CH2Cl2. Their properties are being reported elsewhere.26 BMI-BOB, 14: yellow viscous liquid. Yield: 76.8%. NMR in DMSO-d6: 1H δ 9.07 (s, 1H), 7.73 (s, 1H), 7.66 (s, 1H), 4.15 (t, 2H, J ) 7.2 Hz), 3.85 (s, 3H), 1.76 (m, 2H), 1.25 (m, 2H), 0.87 ppm (t, 3H, J ) 7.4 Hz); 13C δ 158.16, 136.49, 123.60, 122.23, 48.53, 35.77, 31.37, 18.78, 13.19 ppm; 11B δ 12.05 ppm;
J. Phys. Chem. B, Vol. 107, No. 42, 2003 11751 7Li
δ 1.01 ppm (very weak signal, when LiBOB was used) or no signal (when NaBOB was used). BMI-BMB, 15: yellow viscous liquid. Yield: 38.0%. NMR in DMSO-d6: 1H δ 9.28 (s, 1H), 7.76 (s, 1H), 7.69 (s, 1H), 4.14 (t, 2H, J ) 7.0 Hz), 3.82 (s, 3H), 3.35 (s, 2H), 1.72 (m, 2H), 1.20 (m, 2H), 0.83 ppm (t, 3H, J ) 7.2 Hz); 13C δ 165.90, 136.49, 123.47, 122.15, 48.50, 38.60, 35.77, 31.42, 18.83, 13.30 ppm; 11B δ 8.09 ppm; 7Li δ 1.08 ppm (very weak signal). BMI-BScB, 16: yellow-brown solid. Yield: 85.6%. NMR in DMSO-d6: 1H δ 9.08 (s, 1H), 7.73 (s, 1H), 7.71 (d, 2H), 7.67 (s, 1H), 7.41 (t, 2H, J ) 7.8 Hz), 6.87 (t, 2H, J ) 7.6 Hz), 6,81 (d, 2H, J ) 8.0 Hz), 4.13 (t, 2H, J ) 7.2 Hz), 3.82 (s, 3H), 3.36 (s, 2H), 1.74 (m, 2H), 1.23 (m, 2H), 0.87 ppm (t, 3H, J ) 7.2 Hz); 13C δ 163.52, 158.81, 136.62, 134.51, 129.08, 123.48, 122.13, 118.68, 117.90, 115.23, 48.50, 35.74, 31.35, 18.79, 13.29 ppm; 11B δ 8.28 ppm; 7Li δ 1.02 ppm (very weak signal). BMI-BMLB, 17: yellow viscous liquid. Yield: 51.9%. NMR in DMSO-d6: 1H δ 9.35 (s, 1H), 7.80 (s, 1H), 7.72 (s, 1H), 4.15 (t, 2H, J ) 7.0 Hz), 3.83 (s, 3H), 1.72 (m, 2H), 1.20 (m, 2H), 1.09 (d, 12H, J ) 6.0 Hz), 0.83 ppm (t, 3H, J ) 7.4 Hz); 13C δ 181.20, 136.47, 123.42, 122.12, 75.01, 48.42, 35.70, 31.38, 26.78 (d), 18.77, 13.26 ppm; 11B δ 13.67 ppm; 7Li δ 1.08 ppm (very weak signal). BMI-TFSI, 18: yellow liquid. Yield: 89.3%. NMR in DMSOd6: 1H δ 9.05 (s, 1H), 7.70 (s, 1H), 7.64 (s, 1H), 4.12 (t, 2H, J ) 7.2 Hz), 3.81 (s, 3H), 1.73 (m, 2H), 1.22 (m, 2H), 0.86 ppm (t, 3H, J ) 7.4 Hz); 13C δ 136.49, 123.58, 122.23, 121.65, 117.38, 48.53, 35.68, 31.33, 18.73, 13.11 ppm; 19F δ 46.11 ppm; 7Li δ 1.01 ppm (very weak signal). BMI-BF4, 19: yellow liquid. Yield: 83.6%. NMR in DMSOd6: 1H δ 9.04 (s, 1H), 7.72 (s, 1H), 7.66 (s, 1H), 4.15 (t, 2H, J ) 7.2 Hz), 3.84 (s, 3H), 1.76 (m, 2H), 1.25 (m, 2H), 0.89 ppm (t, 3H, J ) 7.4 Hz); 13C δ 136.46, 123.58, 122.23, 48.52, 35.68, 31.32, 18.75, 13.20 ppm; 11B δ 3.63 ppm; 19F δ -23.46 ppm; 23Na no signal. BMI-Tf, 20: yellow liquid. Yield: 82.8%. NMR in DMSOd6: 1H δ 9.19 (s, 1H), 7.77 (s, 1H), 7.70 (s, 1H), 4.16 (t, 2H, J ) 7.2 Hz), 3.85 (s, 3H), 1.75 (m, 2H), 1.24 (m, 2H), 0.89 ppm (t, 3H, J ) 7.4 Hz); 13C δ 136.58, 123.58, 122.86, 122.26, 118.59, 48.59, 35.67, 31.37, 18.76, 13.09 ppm; 19F δ 46.98 ppm; 7Li δ 0.95 ppm (very weak signal). BPy-BOB, 21: yellow viscous liquid. Yield: 75.6%. NMR in DMSO-d6: 1H δ 9.06 (d, 2H, J ) 7.0 Hz), 8.59 (t, 1H, J ) 7.8 Hz), 8.13 (t, 2H, J ) 7.4 Hz), 4.61 (t, 2H, J ) 7.5 Hz), 1.90 (m, 2H), 1.28 (m, 2H), 0.86 ppm (t, 3H, J ) 7.3 Hz); 13C δ 158.40, 145.54, 144.79, 128.19, 60.83, 32.75, 18.82, 13.23 ppm; 11B δ 12.25 ppm; 23Na no signal. P14-BOB, 22: yellow viscous liquid. Yield: 74.9%. NMR in DMSO-d6: 1H δ 3.47 (m, br, 2 × 2H), 3.29 (m, 2H), 2.98 (s, 3H), 2.09 (s, 2 × 2H), 1.68 (m, 2H), 1.30 (m, 2H), 0.89 ppm (t, 3H, J ) 7.3 Hz); 13C δ 158.35, 63.63, 63.19, 47.68, 25.00, 21.14, 19.31, 13.35 ppm; 11B δ 12.08 ppm; 23Na no signal. MOMNM2E-BOB, 23: yellow viscous liquid. Yield: 69.6%. NMR in DMSO-d6: 1H δ 4.59 (s, 2H), 3.59 (s, 3H), 3.30 (m, 2H), 2.94 (s, 2 × 3H), 1.21 ppm (m, 3H); 13C δ 158.37, 91.07, 60.67, 55.96, 46.20, 7.45 ppm; 11B δ 12.07 ppm; 23Na no signal. EOMNM2E-BOB, 24: yellow viscous liquid. Yield: 71.2%. NMR in DMSO-d6: 1H δ 4.63 (s, 2H), 3.81 (m, 2H), 3.29 (m, 2H), 2.92 (s, 2 × 3H), 1.19 ppm (m, 3H); 13C δ 158.30, 89.56, 68.83, 55.83, 46.07, 14.98, 7.50 ppm; 11B δ 12.05 ppm; 7Li δ 1.02 ppm (very weak signal). MOENM2E-BOB, 25: yellow viscous liquid. Yield: 92.0%. NMR in DMSO-d6: 1H δ 3.71 (s, 2H), 3.48 (t, 2H, J ) 4.8 Hz), 3.39 (m, 2H), 3.27 (s, 3H), 3.03 (s, 2 × 3H), 1.23 ppm (t, 23Na
11752 J. Phys. Chem. B, Vol. 107, No. 42, 2003 3H, J ) 7.3 Hz); 13C δ 158.27, 65.41, 61.92, 59.93, 58.11, 50.22, 7.86 ppm; 11B δ 12.06 ppm; 7Li δ 1.02 ppm (very weak signal). EOENM2E-BOB, 26: yellow viscous liquid. Yield: 76.0%. NMR in DMSO-d6: 1H δ 3.76 (s, 2H), 3.47 (m, 2 × 2H), 3.38 (m, 2H), 3.02 (s, 2 × 3H), 1.23 (t, 3H, J ) 7.2 Hz), 1.11 ppm (t, 3H, J ) 7.0 Hz); 13C δ 158.15, 65.61, 63.25, 61.87, 59.74, 50.12, 14.79, 7.84 ppm; 11B δ 12.27 ppm; 23Na no signal. BNM2E-BOB, 27: yellow viscous liquid. Yield: 73.5%. NMR in DMSO-d6: 1H δ 3.31 (m, 2H), 3.20 (m, 2H), 2.97 (s, 2 × 3H), 1.62 (m, 2H), 1.31 (m, 2H), 1.23 (t, 3H), 0.90 ppm (t, 3H, J ) 7.3 Hz); 13C δ 158.30, 62.43, 58.68, 49.54, 23.71, 19.18, 13.38 7.76 ppm; 11B δ 12.27 ppm; 23Na no signal. 2.5. Physical Properties of the Ionic Liquids. The thermal properties of the ionic liquids were determined using a PerkinElmer DSC-7 differential scanning calorimeter in the temperature range -150 to 80 °C. The instrument temperature scale was calibrated with crystal-crystal transition of cyclopentane (-151.16 °C) and melting of indium (+156.60 °C). Samples were sealed in aluminum pans, purged with helium gas, and heated at a rate of 20 K min-1. For the orthoborate compounds only a glass transition endotherm (Tg) could be observed, unless the samples were stored for several days before scanning; melting points (Tm) could then be observed in the rescans. In the cases of TFSI- and Tf- compounds, devitrification temperature (Tc) and melting peaks (Tm) could also be observed in the initial scan. Approximate density values at different temperatures, accurate to 0.5%, were obtained simply by measuring the weight of the sample filling a 2.00 mL volumetric flask within a VAC drybox. The flasks were maintained in a heating block at the measured temperature for half an hour until the temperature was steady before measurement. Kinematic viscosities of ionic liquids were measured using Cannon-Ubbelohde viscometers of appropriate viscometer constants, in the temperature range between ambient and 130 °C. CaCl2 drying tubes were used to protect the samples from moisture in the air. A uniform temperature environment was provided by a tall aluminum temperature smoothing block with slots to permit meniscus observation. The temperature of the sample was maintained for half an hour before measurement. The precision of measurement with Cannon-Ubbelohde viscometers is controlled by the reproducibility of flow times, and accuracy is controlled by accuracy of calibration constants and by temperature measurement. Precision was limited at the highest temperatures (above 100 °C) by the short flow times ( BOB- > BMB- > BMLB-. Apart from the BScB-, the density of the orthoborate ionic liquids decreases with the increase of anion size. However, when compared with data for salts of fluorinated anions, the density at room temperature follows the order TFSI- > BOB- > Tf> BF4-. It seems that the density increases with anion size, perhaps because packing becomes more efficient as the alternating positive and negative species become more even in size. Keeping the anion constant (BOB-), the aromatic N-hetero cations (BPy+ and BMI+) show higher density than the aliphatic N-hetero cations (P14+ and BNM2E+). The densities are in the order BPy+ > BMI+ > P14+ > BNM2E+ (Table 1). For BPy+, the densest, and also the most fragile case, there is some indication of deviations from the linear temperature dependence. This will be investigated using a more precise method of measurement. For the RNM2E+-BOB- liquids, the pure alkyl groupcontaining salts show lower density than the same length of
Ionic Liquids of Chelated Orthoborates
J. Phys. Chem. B, Vol. 107, No. 42, 2003 11753 TABLE 2: Calorimetric Properties of Ionic Liquids from DSC Measurements
Figure 1. Densities of BMI+ ionic liquids with different anions, in relation to temperature. See also Table 1.
TABLE 1: Density Equation Parameters and Molar Volumes of Ionic Liquids
ionic liquid BMI-BOB BMI-BMB BMI-BScB BMI-BMLB BMI-TFSI BMI-BF4 BMI-Tf BPy-BOB P14-BOB MOMNM2E-BOB EOMNM2E-BOB MOENM2E-BOB EOENM2E-BOB BNM2E-BOB a
density equation d ) b - aT/g cm-3 (where T in °C) a b R2 7 × 10-4 7 × 10-4 n.m.a 5 × 10-4 7 × 10-4 5 × 10-4 6 × 10-4 7 × 10-4 5 × 10-4 7 × 10-4 7 × 10-4 7 × 10-4 8 × 10-4 5 × 10-4
1.31 1.26 n.m.a 1.12 1.41 1.21 1.24 1.33 1.25 1.32 1.29 1.32 1.27 1.23
0.994 0.972 n.m.a 0.971 0.995 0.991 0.996 0.933 0.989 0.976 0.993 0.992 0.993 0.968
Vm at 25 °C/ cm3 mol-1 254 286 322 310 302 191 237 247 265 234 251 246 267 261
n.m. means not measured.
alkoxyalkyl group-containing salts, i.e., BNM2E+ < EOMNM2E+ < MOENM2E+. On the other hand, the longer the alkoxyalkyl group R, the lower the density of the liquid, i.e., MOMNM2E+ ∼ MOENM2E+ > EOMNM2E+ > EOENM2E+. The molar volumes of orthoborate ionic liquids at ambient temperature, obtained from density data, are also presented in Table 1. 3.4. Thermal Behavior: Melting Points Tm, Glass Temperatures Tg, and the “2/3 Rule”. Table 2 summarizes the calorimetric properties of the ionic liquids according to DSC measurements. None of the orthoborate ionic liquids crystallize on fast cooling in liquid nitrogen, nor do most of them crystallize on long standing at -30 °C (in a freezer). In three cases, crystals formed after standing some days at room temperature. For these, the melting points were measured and found to be only ca. 1.4Tg compared with the normal ca. 1.5Tg for glass-formers.31 Those that do not crystallize presumably have even lower Tm/Tg values. Efforts to induce crystallization of these may be worthwhile for the purpose of demonstrating the tautological origin of the well-known “2/3” rule for glass-formers (Tg/Tm ≈ 2/3).32 It would be interesting to show that the difficult-to-crystallize cases have even lower Tg/Tm ratios. It would help establish the
ionic liquid
Tg/°C
BMI-BOB BMI-BMB BMI-BScB BMI-BMLB BMI-TFSI BMI-BF4 BMI-Tf BPy-BOB P14-BOBa MOMNM2E-BOB EOMNM2E-BOB MOENM2E-BOB EOENM2E-BOB BNM2E-BOB
-29.2 -33.1 21.5 -39.4 -85.9 -85.3 -81.6 -20.5 -37.8 -32.4b -40.2b -35.9b -44.8 -44.4
Tc/°C
Tm/°C
-16.0
-4.9
2.9
16.4 50.8b 46.7b 48.3b
a MacFarlane et al. (private communication) have succeeded in crystallizing P14-BOB. b The Tg and Tm of MOMNM2E-BOB, EOMNM2EBOB and MOENM2E-BOB cannot be measured in the same scan. The samples had to be stored at room temperature for a few days to allow the formation of crystals. The melting point was determined from an initial scan, and the glass temperature from a repeat scan. The crystallized samples were cooled and measured during upscans to 80 °C. After that they were cooled to -120 °C and once again measured during upscans until 80 °C. The first measurement gives only Tm, while the second only Tg. It is indicated that the samples are not easy to crystallize.
tautological basis of the “2/3 rule” by showing that substances that would violate the rule on the low side, never get tested, because no crystals are available to determine Tm. It is already known from hyperquenching studies that substances that violate the rule in the other direction usually do not get tested because they do not vitrify in the first place. The rule should be reformulated to read “substances that may readily be observed in both glassy and crystalline states are those that have glass temperatures about 2/3 of the melting points”. The glass temperatures for orthoborate melts are much higher than for the corresponding salts of the fluorinated anions. Table 2 shows that for the same N-containing onium cation, e.g., BMI+, the Tg of the ionic liquids with different anions decreases in the order BScB- > BOB- > BMB- > BMLB- . Tf- > BF4- ∼ TFSI-. The ionic liquid of BScB- has the highest Tg among the four orthoborate anions, presumably as a consequence of the phenyl ring component of this anion structure. For the same BOB- anion, the aromatic N-hetero cations (BPy+ and BMI+) show higher Tg than the aliphatic N-hetero cations (P14+ and BNM2E+). The order is BPy+ > BMI+ > P14+ > BNM2E+. For RNM2E+-BOB ionic liquids, the longer the alkoxyalkyl group R, the lower the Tg of the ionic liquid, i.e., MOMNM2E+ > MOENM2E+ > EOMNM2E+ > EOENM2E+. However, the presence of the ether group in the side chain raises Tg relative to salts with only alkyl side groups, the order being BNM2E+ < EOMNM2E+ < MOENM2E+. In general, the glass temperatures of these liquids increase with the ion size. However, as seen in Figure 2, there is no simple correlation with the molar volumes. Since all our salts have the same charge per mole of ions, it is clear from Figure 2 that, in these ionic liquids, the cohesive energy is not being determined by the Coulomb energy since the latter must decrease with increasing volume. The orthoborate class of “ionic liquid” therefore has an interesting feature: the properties of the liquid are more under the control of the van der Waals interactions than of the electrostatic interactions. Similar behavior was found for ionic liquids with increasingly large quaternary ammonium cations by Sun et al.16 The structures of such liquids may show interesting differences from those normally associated with
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Figure 2. Variation of Tg with molar volume for orthoborate ionic liquids, compared with that for BMI+ ILs with fluorinated anions. The dashed line shows the behavior of Tg with molar volume for a large number of ionic liquids in which the anions were all of low polarizability and indicates that the dominance of coulomb interactions is limited to the salts of smaller ions.
Figure 4. Fluidities of BOB- ionic liquids of different types of cations: (a) aromatic pyridine-based, aromatic imadazole-based, cyclic but nonaromatic, and noncyclic nonaromatic; (b) different nonaromatic, noncyclic cations.
Figure 3. Fluidities of BMI+ ionic liquids, shown as Arrhenius functions of temperature, showing the much higher viscosity of the orthoborate salt BMI-BOB, notwithstanding the weakly coordinating character of BOB- established by solutions studies.
molten salts. Indeed such differences are manifested in the existence of interesting types of liquid crystals reported for ionic liquids with chlorozincate anions reported by Martin et al.33 Our findings suggest that similar complex structures, driven by van der Waals interactions between the anions, might be generated from orthoborate type ionic liquids, although we have no evidence for such structures from the present study. 3.5. Dynamic Viscosity and Fragility. As indicated by their high Tg values, the orthoborate ionic liquids are much more viscous than those with perfluorinated anions such as TFSI-, BF4-, and Tf-. Here we present the data in the inverse form, fluidity, because this gives the same shape as obtained for the conductivity, and it is this form that we will use in Walden plot comparisons below. In Figure 3 we compare the dynamic inverse viscosities and their temperature dependences for BMI-BOB with other BMI+ ionic liquids. BMI+ with the BOB- anion has a much higher viscosity than seen with the fluorinated anions. This is surpris-
ing, as BMI+ has a very low and dispersed electrostatic charge. The reason must be the same as given above for the high glass temperature, namely, the strong van der Waals interactions associated with its multiatom unfluorinated character. Much of the difference in ambient temperature viscosities is removed with the increase of temperature to 100 °C because of the much larger temperature dependence of the BMI-BOB liquid. For a given anion, BOB-, the salts with n-butyl-containing aromatic and aliphatic N-hetero cations, BPy+, BMI+, P14+, and BNM2E+, show similar viscosity especially at high temperatures. At room temperature the BPy-BOB ionic liquid is the most viscous among the four ionic liquids, as shown in Figure 4a. The pyridine ring-based cations have earlier26 been found to be associated with the highest cohesion. This must be a reflection of the importance of the van der Waals interactions in determining the fluid properties, as discussed in the previous section. For the RNM2E+-BOB ionic liquids, the viscosity shows the order MOENM2E+ > MOMNM2E+ ∼ BNM2E+ ∼ EOMNM2E+ > EOENM2E+ (Figure 4b). The reason that the MOENM2E+ is the most viscous is not clear. The lowest viscosity at room temperature is 1331 cP for EOENM2E-BOB. It was noted previously26 that BPy-BOB was outstanding among ionic liquids for its rate of change of viscosity with temperature, i.e., its high fragility, which was at the high limit
Ionic Liquids of Chelated Orthoborates
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Figure 6. Arrhenius plot of conductivities of BMI+ ionic liquids of different types of orthoborate anions, compared with those of fluorinated anions.
Figure 5. Tg-scaled Arrhenius plot of fluidities of BOB- ionic liquids of different types of cations, compared with that for the least fragile case in ref 26, i.e., MOMNM2E-BF4. The plot shows that the BOBanion leads to more fragile liquids and appears to distinguish aromatic from nonaromatic cations.
among recorded liquids. A comparison is made in Figure 5 using the usual Tg-reduced Arrhenius plot.34 Here we note that the high fragility seems to be characteristic of all the orthoborate anion cases. For orthoborate ionic liquids with the same cation, the orthoborate with the largest viscosity is the one with the largest anion, i.e., BScB- > BMLB- > BMB- > BOB-. This confirms the increasing importance of van der Waals over Coulomb attractions since the latter are diminishing with increasing size of ions. With a glass temperature near room temperature, an exceptionally high fragility, no propensity for crystallization, and only weak sensitivity to moisture, the BPy+BScB- salt could become a good model system for future studies of glass-former phenomenology. 3.6. Conductivity. A comparison of the temperature dependence of ionic conductivity of the BMI+ ionic liquids is made in Figure 6. As expected from their high glass temperatures and viscosities, the orthoborate ionic liquids are much less conductive than their perfluorinated anion analogues. Of the orthoborates the most conductive is the smallest one with the anion BOB-. Interestingly, BOB- has a higher conductivity than the BMB- or BMLB- salts, even though it has the higher Tg. This must be a consequence of a higher fragility32 for the BOB-
salt. Indeed, BOB- salts were found to provide the most fragile of a group of ionic liquids involving eight different anions.26 Considering the effect of different cations with the BOBanion, BMI+, P14+, and BNM2E+ all have similar conductivities, while BPy+, the most viscous, has a lower value (Figure 7a). Within the quaternary ammonium cation series the order of conductivity with BOB- is MOENM2+ < MOMNM2E+ < BNM2E+ ∼ EOENM2E+ < EOMNM2E+. However, the latter is only 8.7 × 10-5 S cm-1 at room temperature. Because of the unusual rate, per K, at which BOB- salts gain fluidity with increasing temperature, it is possible that, if paired with a perfluorinated cation that will lower Tg, BOB-based ionic liquids could be highly fluid and conductive at ambient temperatures. However such cations have yet to be described. 3.7. General: Conductivity-Viscosity Relations and the Walden Plot. It was found that the equivalent conductivity could generally be predicted from the relative viscosities of ionic liquids,26 but with some interesting and unexpected exceptions. For example, a specific “docking” interaction could occur in some compounds, leading to a loss of conductivity and an increase in vapor pressure.26 To determine if such exceptions exist among the ionic liquids with orthoborate anions, we present the data in Walden plot form in Figure 8. Figure 8 shows that, while the majority of orthoborate ionic liquids conform to the normal pattern, there are indeed some cases in which the degree of association is high. One of them involves the same quaternary ammonium cation MOMNM2E+ that was found exceptional when combined with the BF4- anion. Thus the association leading to low conductivity seems to be cation-driven. The fact that the secondmost associated case should also involve a symmetrical oxy side chain (EOE in EOENM2E+) may be significant, but the mechanism is not clear to us. 4. Concluding Remarks Since the molecular weights of the orthoborate anions are comparable with, or smaller than, that of TFSI-, and since the charge dispersion across the anion is also comparable (according to the extent of dissociation in dilute solutions of lower dielectric constant solvents24), it seems that the source of superior fluidities of the TFSI-containing salts must lie in the weak van der Waals interactions that are characteristic of molecules with outermost fluorine atoms. If this is indeed the case, then the only way to
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Xu et al. recently described.26 The possible benefit of incorporating smaller anions is exemplified by the low viscosity of salts of the dicyanamide anion reported by MacFarlane et al. recently.23 We will report other examples of the success of this approach in future publications. A possible advantage of the orthoborate type systems may lie in their strong van der Waals interactions. It is possible that these will empower them to serve as solvents for high molecular weight organic substances while remaining, themselves, soluble in polar solvents. Investigations of their utility for such purposes will be reported elsewhere. Acknowledgment. This work was supported by Mitsubishi Chemical Corporation of Japan. We also acknowledge the U.S. NSF Grant CHE-9808678 for NMR measurements. References and Notes
Figure 7. Arrhenius plot of ionic conductivities of ionic liquids combining BOB- with cations of different type: (a) different aromaticities and cyclicities; (b) different quaternary ammonium cations.
Figure 8. Walden plot of ionic liquids combining BOB- with different cations. There are striking deviations from the ideal Walden behavior for two of the four salts of quaternary ammonium cations, which imply extensive ion association in these cases, increasing with increasing temperature.
make ionic liquids without the expense of fluorination will be to use small anions so that the molar volume of the salt falls near that of the minimum in the Tg versus volume relation
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