A solventless route to 1-ethyl-3-methylimidazolium fluoride hydrofluoride, [C2mim][F]·xHF

Article abstract of DOI:10.1021/jo800578b

(Chemical Equation Presented) The ionic liquid 1-ethyl-3-methylimidazolium fluoride hydrofluoride, [C₂mim][F]·xHF, has been synthesized through a new, solventless route that excludes halogen metathesis. The byproducts are salts, alcohols, and carbon dioxide.

Full text of DOI:10.1021/jo800578b

fluoride. This approach, however, has a known drawback in that
the Cl- impurities are notoriously difficult to completely
remove.7
We chose to implement a recently reported synthetic approach
for formation of byproduct free imidazolium salts8–10 to
circumvent halide metathesis reactions. It involves alkylation
of an alkylimidazole with dimethyl carbonate resulting in a
carboxylated-dialkylimidazolium zwitterion and subsequent
treatment with acid to produce the desired salt and CO2.
A Solventless Route to
1-Ethyl-3-methylimidazolium Fluoride
Hydrofluoride, [C2mim][F]·xHF
Christiaan Rijksen and Robin D. Rogers*
Center for Green Manufacturing and Department of
Chemistry, The UniVersity of Alabama,
Tuscaloosa, Alabama 35487
SCHEME 1. Solventless Synthesis of Ethylimidazole11
rdrogers@bama.ua.edu
ReceiVed March 13, 2008
The synthesis started with the preparation of ethylimidazole
2 (Scheme 1). Following the procedure published by Diez-Barra
et al., a solventless route was used in which potassium tert-
butoxide and imidazole were ground together with tetrabutyl-
ammonium bromide (TBAB); ultrasound was applied to this
mixture of solids, and upon addition of ethyl iodide a new
colorless liquid was formed.11 Although no biphasic liquid/liquid
system was involved, the addition of TBAB, a substance widely
used as a phase transfer catalyst, seemed to be relevant. Under
reduced pressure the newly formed ethylimidazole 2 was directly
distilled from the reaction mixture in 41% yield.
The next step involved the building of the 1-ethyl-3-
methylimidazolium core without introducing a halide counterion.
This was performed by reaction of ethylimidazole 2 with
dimethyl carbonate 3 via a BAl2 mechanism (Scheme 2).8–10,12
The product was a colorless, very hygroscopic crystalline
substance, which was washed with dry acetone, yielding 29%.
From previous studies we assumed this compound to be
1-ethyl-3-methylimidazolium-2-carboxylate 4, a zwitterion bear-
ing the carboxyl group at the C2 position, rather than the
anticipated 1-ethyl-3-methylimidazolium methyl carbonate.8 The
reaction is known to yield the thermodynamically favored
4-carboxylate as well, which is predominantly formed at higher
temperatures (>110 °C),9,13 but was not detected. Moreover,
The ionic liquid 1-ethyl-3-methylimidazolium fluoride hy-
drofluoride, [C2mim][F]·xHF, has been synthesized through
a new, solventless route that excludes halogen metathesis.
The byproducts are salts, alcohols, and carbon dioxide.
Dialkylimidazolium halide-based ionic liquids (ILs) have
proven to exhibit the necessary basicity to dissolve cellulosic
biomass1 through the anion’s ability to effectively break the
hydrogen bonds that give strength to the cellulose macrostruc-
ture.2 The original and subsequent studies3 in this area dem-
onstrated that the ability of these salts to directly dissolve
cellulose increased with the hydrogen bond basicity of the anion
in the order Br- = SCN- < Cl-. However, at the time it was
not possible to test F-. The synthesis reported here was thus
motivated by the wish to extend this series.
Dialkylimidazolium fluoride compounds exhibit a tendency
to forming (HF)nF- clusters, were n is an integer.4 This leads
to structures of the formula [C2mim][F]·xHF where x can be a
noninteger. Hagiwara et al. first reported the synthesis of
[C2mim][F]·2.3HF (C2mim ) 1-ethyl-3-methylimidazolium),4
its unique properties,5 and applications.6 The compound was
synthesized by halogen exchange of [C2mim][Cl] with hydrogen
(5) (a) Matsumoto, K.; Hagiwara, R.; Yoshida, R.; Ito, Y.; Mazej, Z.; Benkic,
P.; Zemva, B.; Tamada, O.; Yoshino, H.; Matsubara, S. Dalton Trans. 2004,
144. (b) Matsumoto, K.; Hagiwara, R. J. Fluorine Chem. 2007, 128, 317.
(6) (a) Yoshino, H.; Nomura, K.; Matsubara, S.; Oshima, K.; Matsumoto,
K.; Hagiwara, R.; Ito, Y. J. Fluorine Chem. 2004, 125, 1127. (b) Yoshino, H.;
Matsumoto, K.; Hagiwara, R.; Ito, Y.; Oshima, K.; Matsubara, S. J. Fluorine
Chem. 2006, 127, 29.
(1) (a) Swatloski, R. P.; Spear, S. K.; Holbrey, J. D.; Rogers, R. D. J. Am.
Chem. Soc. 2002, 124, 4974. (b) Fort, D. A.; Remsing, R. C.; Swatloski, R. P.;
Moyna, P.; Moyna, G.; Rogers, R. D. Green Chem. 2007, 9, 63. (c) Kilpelaeinen,
I.; Xie, H.; King, A.; Granstrom, M.; Heikkinen, S.; Argyropoulos, D. S. J.
Agric. Food. Chem. 2007, 55, 9142. (d) Honglu, X.; Tiejun, S. Holzforschung
2006, 60, 509. (e) Zhang, J.; Wu, H.; Zhang, J.; He, J. Macromolecules 2005,
38, 8272. (f) Zhu, S.; Wu, Y.; Chen, Q.; Yu, Z.; Wang, C.; Jin, S.; Ding, Y.;
Wu, G. Green Chem. 2006, 8, 325.
(7) Gallo, V.; Mastrorilli, P.; Nobile, C. F.; Romanazzi, G.; Suranna, G. P.
Dalton Trans. 2002, 4339.
(8) (a) Holbrey, J. D.; Reichert, W. M.; Tkatchenko, I.; Bouajila, E.; Walter,
O.; Tommasi, I.; Rogers, R. D. Chem. Commun. 2003, 28. (b) Tommasi, I.;
Sorrentino, F. Tetrahedron Lett. 2006, 47, 6453.
(2) (a) Youngs, T. G. A.; Hardacre, C.; Holbrey, J. D. J. Phys. Chem. B
2007, 111, 13765. (b) Youngs, T. G. A.; Holbrey, J. D.; Deetlefs, M.;
Nieuwenhuyzen, M.; Gomes, M. F. C.; Hardacre, C. ChemPhysChem 2006, 7,
2279. (c) Liu, Z. W.; Remsing, R. C.; Moore, P. B.; Moyna, G. Abstr. Pap. Am.
Chem. Soc. 2006, 231, 40-IEC.
(9) Smiglak, M.; Holbrey, J. D.; Griffin, S. T.; Reichert, W. M.; Swatloski,
R. P.; Katritzky, A. R.; Yang, H.; Zhang, D.; Kirichenko, K.; Rogers, R. D.
Green Chem. 2007, 9, 90.
(10) Bridges, N. J.; Hines, C. C.; Smiglak, M.; Rogers, R. D. Chem. Eur. J.
2007, 13, 5207.
(3) (a) Anderson, J. L.; Ding, J.; Welton, T.; Armstrong, D. W. J. Am. Chem.
Soc. 2002, 124, 14247. (b) Fukaya, Y.; Hayashi, K.; Wada, M.; Ohno, H. Green
Chem. 2008, 10, 44.
(11) Diez-Barra, E.; de la Hoz, A.; Sanchez-Migallon, A.; Tejeda, J. Synth.
Commun. 1993, 23, 1783.
(4) Hagiwara, R.; Hirashige, T.; Tsuda, T.; Ito, Y. J. Fluorine Chem. 1999,
99, 1.
(12) Douglas, J. E.; Campbell, G.; Wigfield, D. C. Can. J. Chem. 1993, 71,
1841.
5582 J. Org. Chem. 2008, 73, 5582–5584
10.1021/jo800578b CCC: $40.75 2008 American Chemical Society
Published on Web 06/13/2008

SCHEME 2. Synthesis of
1-Ethyl-3-methylimidazolium-2-carboxylate 4a
a Traces of water induce the conversion into 1-ethyl-3-methylimidazolium
hydrogen carbonate 5.
SCHEME 3. Synthesis of [C2mim][F]·HF (6)
FIGURE 1. Kinetic study of water uptake by [C2mim][F]·xHF, using
ATR FT-IR. The significant peak is at 3400 cm-1
.
the kinetic product, 1-ethyl-3-methylimidazolium-2-carboxylate
4, reacts with water to form 1-ethyl-3-methylimidazolium
hydrogen carbonate 5 crystals.9,10
NMR studies in deuterated methanol showed a mixture of
21% 1-ethyl-3-methylimidazolium-2-carboxylate 4 and 79%
1-ethyl-3-methylimidazolium hydrogen carbonate 5 formed with
water. Compound 5 showed no signal of the C-2 proton in
CD3OD due to exchange with the solvent.
It is interesting to note that both TGA and NMR indicated
that these compounds were hygroscopic, in contrast to the
reports of Hagiwara et al.15 This suggests that the hygroscopic
nature of [C2mim][F]·xHF drastically changes with x. Because
of this, we measured the absorption rate of water by a
combination of Karl Fischer titration and IR spectroscopy in a
kinetic experiment. A single drop of the compound was placed
on an attenuated total reflection Fourier transformation infrared
(ATR FT-IR) spectrometer and the spectrum was recorded at
different time intervals while leaving the substance under
ambient atmosphere with a relative humidity of 65% (Figure 1).
The development of a signal at ca. 3400 cm-1, which is
specific for hydroxyl groups in hydrogen bonds, e.g. water,
proved its hygroscopicity (Figure 1). These data reveal that
approximately half of the water uptake occurs within the first 2
min. After 14 min the hydration was virtually complete. The
drop was then collected and the water content was found to be
13.5% by Karl Fischer titration.
Elemental analysis (CHNF) is complicated by the hygroscopic
nature of the compound; however, the data indicated a water
content between 1.5 and 2.1 mol. The lower water content (1.5
mol, 14.4%) is consistent with the Karl Fischer measurements
at saturation. The higher value of 2.1 mol corresponds to 19%
water, which might be explained by water absorption over a
longer period of time. Consideration of all of the data obtained
suggests the best value of x to be 1.5-1.6 HF.
The crystals of 5 were analyzed by single-crystal X-ray
diffraction (see the Supporting Information). The observed
structure is similar to the dimethyl analogue reported earlier.9,10
In the third step (Scheme 3), the crude mixture of 4 and 5
was placed in a PFA reactor and dissolved in liquid HF by
condensation at -195 °C. After reaching room temperature, the
excess HF was removed and the sample was dried, yielding
the product quantitatively as a dark, free flowing liquid.
The reaction was complete in 100% yield in a short reaction
time, despite the use of the crude mixture of 1-ethyl-3-
methylimidazolium-2-carboxylate 4 and 1-ethyl-3-methylimi-
dazolium hydrogen carbonate 5. The final reaction was straight-
forward, fast, and yielded neither side products nor byproducts
apart from CO2.
1
A broad peak at 13.7 ppm in the H NMR corresponds to
-
the HF2 anion, which can also be seen as a doublet at -70.2
ppm in the 19F NMR spectrum. The IR spectrum is similar to
that reported in the literature5 and indicates that hydrofluoride
is present. Unfortunately, it proved to be difficult to completely
remove the water even after overnight drying under high
1
The complete absence of solvents and the nontoxic starting
materials (except HF) and byproducts, combined with the
circumvention of halide contamination, no need for purification,
and a quantitative yield in the last reaction step, make this
synthetic approach attractive. The presence of some water in
this IL is irrelevant for biomass dissolution because of the
ubiquitous presence of water in the feedstock. Nevertheless,
uncertainties in the exact composition of the IL and the delicacy
of handling liquid HF might hinder large-scale applicability of
fluoride-based ILs in biomass application.
vacuum. Water was visible in the H NMR spectrum and was
quantified by using Karl Fischer titration, revealing an intrinsic
water content between 5.9% and 6.5% (corresponding to ca.
0.5 mol of H2O per mol of [C2mim]+). A related compound,
[C4mim][F]·H2O, a crystalline monohydrate, has been reported
to form from the hydrolysis of [C4mim][PF6].14
Thermogravimetric analysis (TGA) showed two decomposi-
tion steps and a continuous weight loss up to 200 °C. This might
be due to the permanent loss of incorporated water or HF (see
the Supporting Information).
(13) Fischer, J.; Siegel, W.; Bomm, V.; Fischer, M.; Mundinger, K. U.S.
Patent 6175019, 2001.
(14) Swatloski, R. P.; Holbrey, J. D.; Rogers, R. D. Green Chem. 2003, 5,
361.
(15) (a) Hagiwara, R.; Matsumoto, K.; Nakamori, Y.; Tsuda, T.; Ito, Y.;
Matsumoto, H.; Momota, K. J. Electrochem. Soc. 2003, 150, D195. (b) Yoshino,
H.; Matsubara, S.; Oshima, K.; Matsumoto, K.; Hagiwara, R.; Ito, Y. J. Fluorine
Chem. 2004, 125, 455.
J. Org. Chem. Vol. 73, No. 14, 2008 5583

3H), 1.48 (t, 3H, J ) 7.2 Hz); 13C NMR (90 MHz, MeOD) δ 16.5,
37,9, 46.6, 122.6, 125.1.
Experimental Section
Ethylimidazole (2). Imidazole (10.0 g, 147 mmol), KOtBu (18.2
g, 162 mmol), and tetrabutylammonium bromide (TBAB, 0.82 g,
2.94 mmol, 2 mol %) were mixed and submerged in an ultrasonic
bath for 15 min. The solid mixture was cooled to 0 °C, 22.9 g
(147 mmol) of ethyl iodide was added, and the reaction was stirred
for 3 h while being allowed to reach room temperature. A new
liquid phase formed. Distillation from the reaction mixture afforded
the product, which was dried under high vacuum to remove traces
of tBuOH. A fractionated distillation was done to purify the
compound. Yield: 5.83 g (60 mmol), 41%.
1H NMR (360 MHz, DMSO) δ 7.62 (s, 1H), 7.17 (s, 1H), 6.87
(s, 1H), 3.97 (q, 2H, J ) 7.3 Hz), 1.31 (t, 3H, J ) 7.3 Hz); 13C
NMR (90 MHz, DMSO) δ 16.1, 40.6, 118.6, 128.1, 136.5.
1-Ethyl-3-methylimidazoliumcarboxylate (4). Ethylimidazole
(5.8 g, 60 mmol) and dimethyl carbonate (21.6 g, 240 mmol) were
put into a glass pressure tube and sealed tightly. The tube was heated
in an oven for 7 days at 70 °C. The reaction mixture was cooled to
room temperature and the solvent removed under a stream of
nitrogen. Colorless crystals of product precipitated from a brown
solution. Washing with dry acetone yielded off-white crystals. Yield:
2.66 g (17 mmol), 29%.
1H NMR [1-ethyl-3-methylimidazolium hydrogen carbonate
(major product in CD3OD)] (360 MHz, CD3OD) δ 7.66 (d, 1H, J
) 1.9 Hz), 7.58 (d, 1H, J ) 1.9 Hz), 4.27 (q, 2H, J ) 7.3 Hz),
3.93 (s, 3H), 1.53 (t, 3H, J ) 7.3 Hz); 13C NMR (90 MHz, MeOD)
δ 15.7, 36.5, 46.1, 123.4, 124.5, 161.5.
1H NMR (360 MHz, DMSO) δ 9.47 (s, 1H), 7.82 (s, 1H), 7.73
(s, 1H), 4.20 (q, 2H, J ) 7.3 Hz), 3.86 (s, 3H), 1.40 (t, 3H, J ) 7.3
Hz); 13C NMR (90 MHz, DMSO) δ 15.1, 35.6, 44.1, 122.0, 123.5.
1H NMR (360 MHz, DMSO) δ 7.65 (d, 1H, J ) 2.0 Hz), 7.58
(d, 1H, J ) 1.9 Hz), 4.46 (q, 2H, J ) 7.2 Hz), 3.95 (s, 3H), 1.33
(t, 3H, J ) 7.2 Hz); 13C NMR (90 MHz, DMSO) δ 15.9, 36.4,
44.0, 120.4, 122.5, 141.8, 154.1.
[C2mim][F] ·xHF (6). 1-Ethyl-3-methylimidazoliumcarboxylate
(0.22 g, 1.43 mmol) was three times evacuated and put under inert
gas atmosphere within a glovebox. The droplets dried out during
this procedure leaving an off-white solid, which was then transferred
into a PFA reactor and weighted. The reactor was cooled to -195
°C with liquid nitrogen and an excess of HF was condensed into
the tube, which was allowed to reach room temperature during 1 h.
The excess of HF was removed and the sample was dried under
high vacuum overnight. The product was a free flowing liquid.
Yield: 100%.
1H NMR (360 MHz, DMSO) δ 13.72 (s, 1.4 H), 9.17 (s, 1H),
7.78 (s, 1H), 7.70 (s, 1H), 4.18 (q, 2H, J ) 7.3 Hz), 3.84 (s, 3H),
1.41 (t, 3H, J ) 7.3 Hz); 13C NMR (90 MHz, MeOD) δ 15.8, 36.6,
46.1, 123.4, 125.1, 126.4; 19F NMR (470 MHz, DMSO) δ -70.95,
-69.44.CHNFelementalAnal.Calcdfor[C2mim][F]·1.5HF·1.5H2O:
C 38.5, H 8.4, N 15.0, F 25.4. Calcd for [C2mim][F] ·1.6HF ·2.1H2O:
C 36.0, H 8.5, N 14.0, F 24.7; Found: C 36.0, H 6.2, N 13.5, F
24.9.
Acknowledgment. This work was supported by BASF, North
America. We would like to thank Dr. Alfred Waterfeld and Ms.
Monica Khural for the use of the HF equipment and Dr. David
Cordes for the crystallographic data.
Supporting Information Available: TGA and crystal-
lographic data. This material is available free of charge via the
Internet at http://pubs.acs.org.
1H NMR [1-ethyl-3-methylimidazolium-2-carboxylate 4 (major
product in DMSO)] (360 MHz, CD3OD) δ 7.56 (d, 1H, J ) 2.0
Hz), 7.49 (d, 1H, J ) 1.9 Hz), 4.57 (q, 2H, J ) 7.2 Hz), 4.07 (s,
JO800578B
5584 J. Org. Chem. Vol. 73, No. 14, 2008