Org. Synth. 2005, 82, 166
DOI: 10.15227/orgsyn.082.0166
[1H-Imidazolium, 1-butyl-3-methyl, tetrafluoroborate (1−)]
Submitted by Xavier Creary and Elizabeth D. Willis1.
Checked by Gustavo Moura-Letts and Dennis P. Curran.
1. Procedure
1-Butyl-3-methylimidazolium chloride (30.00 g, 172 mmol) (Note 1) is placed in a 125 mL Erlenmeyer flask containing a stir bar and a thermometer. This salt is dissolved in 35 mL of distilled water and NaBF4 (20.00 g, 182 mmol) (Note 2) is added in portions with stirring over 10-15 min. The NaBF4 dissolves as the mixture emulsifies and cools to 14°C (Note 3). After the mixture warms back to ambient temperature, 30 mL of CH2Cl2 is added and the contents are transferred to a 125 mL separatory funnel. The bottom CH2Cl2 phase is separated (Note 4). The aqueous phase is extracted with an additional 20 mL of CH2Cl2. The combined CH2Cl2 phases are shaken in a separatory funnel with a solution of NaBF4 (10.0 g, 91 mmol) in 20 mL of water. The CH2Cl2 phase is separated and dried over a mixture of 1.0 g of Na2SO4 and 3.0 g of MgSO4. The mixture is filtered through a Büchner funnel and the salts are washed with an additional 15 mL of CH2Cl2. A short path distillation head is attached to the 250 mL round-bottom-flask containing the filtrate and most of the CH2Cl2 is removed by distillation at 30 mm pressure. Care is taken not to heat the product above 50°C. The solvent is condensed in a receiver flask cooled in an ice-water slurry (Note 5). The last traces of CH2Cl2 are removed using a rotary evaporator at 15 mm and 45 °C followed by vacuum drying at ambient temperature until the weight remains constant. The pure 1-butyl-3-methylimidazolium tetrafluoroborate (34.2–34.3 g; 89% yield) is a colorless to pale yellow, viscous liquid (Notes 6-8).
2. Notes
1. The submitters prepared 1-butyl-3-methylimidazolium chloride as previously described.2 The checkers purchased this from Aldrich Chemical Co.
2. Sodium tetrafluoroborate (98%) was purchased from Acros Organics and used as received.
3. If the emulsion is allowed to stand for 30-60 min, then two phases results. However, there is no need to do this since addition of CH2Cl2 induces rapid phase separation.
4. The product at this point is not completely chloride free. In a separate analysis by the submitters, removal of the CH2Cl2 from this solution and analysis of 1.53 g of product by titration with 0.100 M AgNO3 (Mohr titration; K2CrO4 indicator)3 required 0.6 mL of the silver nitrate solution to precipitate all of the chloride ion. This corresponds to a product containing 0.7% BMIM+ Cl and 99.3% BMIM+ BF4.
5. The distilled CH2Cl2 has droplets of water from the azeotrope with CH2Cl2. This distillation process helps to dry the product and allows recovery of most of the CH2Cl2 used in the procedure.
6. Dissolution of 1.50 g of this liquid in 3 mL of water followed by addition of 0.100 M AgNO3 gave no precipitation or cloudiness.
7. The submitters report that the water content of the product was 0.17% as determined from the 1H NMR spectrum of the neat liquid by integration of the water signal at d 2.9 and the CH2 signal at d 1.93. The product is hygroscopic and water content increases with exposure to air. The checkers could not detect water in their spectrum in CD2Cl2.
8. The product exhibits the following spectroscopic properties: IR (thin film) 3646, 3162, 3122, 2964, 2877, 1575, 1467, 1431, 1171, 1048, 850 cm−1; 1H NMR pdf (500 MHz, CD2Cl2) δ 8.66 (s, 1 H), 7.37 (s, 2 H), 4.14 (t, 2 H, J = 7.3 Hz), 3.89 (s, 3 H), 1.81 (quintet, 2 H, J = 7.4 Hz), 1.32 (sextet, 2 H, J = 7.3 Hz), 0.91 (t, 3 H, J = 7.3 Hz); 13C NMR (125 MHz, CD2Cl2) δ 136.2, 123.8, 122.5, 49.7, 36.1, 31.9, 19.3, 13.1. Anal. Calcd for C8H15BF4N2: C, 42.51; H, 6.69; N, 12.37; Found: C 41.94; H, 6.85; N, 12.37; trace analysis for Cl, 636 ppm.
Handling and Disposal of Hazardous Chemicals
The procedures in this article are intended for use only by persons with prior training in experimental organic chemistry. All hazardous materials should be handled using the standard procedures for work with chemicals described in references such as "Prudent Practices in the Laboratory" (The National Academies Press, Washington, D.C., 2011 www.nap.edu). All chemical waste should be disposed of in accordance with local regulations. For general guidelines for the management of chemical waste, see Chapter 8 of Prudent Practices.
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3. Discussion
This procedure is based on the general method recently reported in Organic Syntheses.2 The preparation of 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM+ PF6) proceeds as described.2 However, in our hands, the reported method for the preparation of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM+ BF4; the most commonly used of the ionic liquids) was not reproducible. The product was contaminated with varying (and significant) amounts of BMIM+ Cl−4. The use of the relatively insoluble KBF4 (0.44 g/100 mL) in the original preparation makes dissolution and subsequent reaction problematic. Thus, analysis of 0.716 g of a typical ionic liquid product produced from KBF4 by titration with 0.100 M AgNO3 required 11.1 mL of the silver nitrate solution to precipitate all of the chloride ion. This corresponds to a product containing 27% BMIM+ Cl and 73% BMIM+ BF4.
The procedure described herein uses the much more soluble NaBF4 (97.3 g/100 mL). After the first cycle, the chloride content is only 0.7%, and the chloride can no longer be detected after the second cycle. In addition, the tedious process of removing water by distillation under reduced pressure is eliminated. This procedure is also useful for the preparation of the ionic liquid n-butyl pyridinium tetrafluoroborate, which is subject to the same chloride contamination problems when prepared from KBF4. Finally, the amount of residual water in the product can be readily determined from the 1H NMR spectrum of the neat product.

References and Notes
  1. Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556.
  2. Dupont, J.; Consorti, C. S.; Suarez, P. A. Z.; de Sousa, R. F. Organic Syntheses 2002 79, 236.
  3. Christian, G. D. “Analytical Chemistry, Fifth Edition”, John Wiley and Sons, Inc., New York, 1994, p. 278.
  4. Contamination of BMIM+ BF4 and other ionic liquids with chloride in various preparations has been noted in the literature. See Seddon, K. R.; Stark, A.; Torres, M.-J. Pure Appl. Chem. 2000, 72, 2275.

Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)

1-Butyl-3-methylimidazolium chloride:
1H-Imidazolium, 1-butyl-3-methyl-, chloride; (79917-90-1)

NaBF4: Borate(1-), tetrafluoro-, sodium; (13755-29-8)

1-Butyl-3-methylimidazolium tetrafluoroborate:
1H-Imidazolium, 1-butyl-3-methyl, tetrafluoroborate (1−); (174501-65-6)