A simple and practical method for the preparation and purity determination of halide-free imidazolium ionic liquids

Article abstract of DOI:10.1002/adsc.200505295

The reaction of N-alkylimidazole with alkyl sulfonates at room temperature affords 1,3-dialkylimidazolium alkanesulfonates as crystalline solids in high yields. The alkanesulfonate anions can be easily substituted by a series of other anions [BF₄, PF₆, PF₃(CF ₂CF₃)₃, CF₃SO₃ and N(CF₃SO₂)₂] by simple reaction of anions, salts, or acids in water at room temperature. Extraction with dichloromethane, filtration through a short basic alumina column and solvent evaporation affords the desired ionic liquids in 80-95% yield. The purity (> 99.4%) of these ionic liquids can be determined by ¹H NMR spectra using the intensity of the ¹³C satellites of the imidazolium N-methyl group as internal standard.

Full text of DOI:10.1002/adsc.200505295

UPDATES
DOI: 10.1002/adsc.200505295
A Simple and Practical Method for the Preparation and Purity
Determination of Halide-Free Imidazolium Ionic Liquids
a
a
a, b
Claudia C. Cassol, Günter Ebeling, Bauer Ferrera, Jairton Duponta,*
´
a
Laboratory of Molecular Catalysis, Institute of Chemistry, UFRGS, Av. Bento GonÅalves, 9500 Porto Alegre, 91501-970
RS, Brazil
Fax: (þ55)-51-3316-7304, e-mail: dupont@iq.ufrgs.br
b
´
˜
CENPES, PETROBRAS, Cidade Universitaria Q.7, Ilha do Fundao, Rio de Janeiro, 21941-598 RJ, Brazil
Fax: (þ55)-21-3865-4399, e-mail: bauerferrera@petrobras.com.br
Received: July 27, 2005; Accepted: November 4, 2005
Supporting Information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: The reaction of N-alkylimidazole with al-
kyl sulfonates at room temperature affords 1,3-di-
alkylimidazolium alkanesulfonates as crystalline sol-
ids in high yields. The alkanesulfonate anions can be
easily substituted by a series of other anions [BF4,
PF6, PF3(CF2CF3)3, CF3SO3 and N(CF3SO2)2] by sim-
ple reaction of anions, salts, or acids in water at room
temperature. Extraction with dichloromethane, fil-
tration through a short basic alumina column and
solvent evaporation affords the desired ionic liquids
in 80–95% yield. The purity (>99.4%) of these ionic
1
liquids can be determined by H NMR spectra using
the intensity of the 13C satellites of the imidazolium
N-methyl group as internal standard.
Scheme 1.
Keywords: imidazolium salts; ionic liquids; molten
salt; NMR; synthetic methods
(limit of 1.4 mg/L), ion chromatography (below
8 ppm)[14] or by cyclic voltametry (ppb).[15,16] The water
content can be determined by Karl-Fischer titration or
by cyclic voltametry.[17] The presence and quantification
of these impurities is essential in many applications such
as in catalysis[18–21] and spectroscopic investigations[22]
Introduction
Without doubt, derivatives of the 1,3-dialkylimidazoli- since the physical-chemical properties of the ILs can
um cation associated with various anions are amongst vary significantly depending on their water and halide
the most investigated class of ionic liquids (ILs).[1–7] contents.[23] Alternatively, halide-free 1,3-dialkylimida-
This is most probably due to their ease of synthesis, sta- zolium ILs can be prepared from the five-component re-
bility and the possibility of fine-tuning their physical- action (glyoxal, formaldehyde, two different amines and
chemical properties by the simple choice of the N-alkyl acids)[24] and those containing alkyl sulfate or trifluor-
substituents and/or anions.[7] The vast majority of these omethanesulfonate anions by simple alkylation of 1-al-
ILs are usually prepared by simple N-alkylation of N-al- kylimidazole with the corresponding dialkyl sulfate[25]
kylimidazole, often employing alkyl halides as alkylat- or alkyl trifluoromethanesulfonate ester,[26] respective-
ing agents, followed by association with metal-hal- ly. Similar procedures, which use alkyl sulfonates and al-
ides[8–10] or anion metathesis[11–13] (Scheme 1).
kyl phosphates as alkylating agents, have been patent-
The anion metathesis procedures generate a large va- ed.[27,28] It is, however, quite surprising that no expedi-
riety of 1,3-dialkylimidazolium-based ILs of good qual- tious method is available for the preparation and purity
ity. The determination of the purity of these ionic liquids determination of the halide-free 1,3-dialkylimidazolium
is not a simple task. The main contaminant is usually re- cation associated with the most popular and used anions
sidual chloride that can be detected by an AgNO3 test such as PF6, BF4 and N(CF3SO2)2. In fact, ionic liquids
Adv. Synth. Catal. 2006, 348, 243 – 248
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243

´
Claudia C. Cassol et al.
UPDATES
Table 1. Yield and mp of the imidazolium alkanesulfonate
salts.
such as [C4C1Im]PF6, [C4C1Im]BF4 and [C4C1Im]N(CF3-
SO2)2 are commercially available but with relatively
high levels of chloride contaminants. It is evident that
there is a need for more simple and practical methods
for the preparation of such halide-free ionic liquids
and also a more accessible, rapid and straightforward
methodology for the determination of their purity. We
disclose herein a simple method for the preparation of
various halide-free 1,3-dialkylimidazolium and 1,2,3-tri-
alkylimidazolium ionic liquids associated with PF6, BF4,
PF3(CF2CF3)3, CF3SO3 and N(CF3SO2)2 anions and their
purity (>99.4%) can be determined by 1H NMR using
Entry R1
R2
R3
R4 Yield [%] mp [8C]
1
2
3
4
5
6
7
8
9
Me
2-Bu Me
n-Bu Me
Me
Me
Et
n-Bu Me
Me
Et
Me
n-Bu
n-Bu
n-Bu
Me
n-Bu Me 94
n-Bu
Me
H
H
H
H
96
80
83
85
77.1
76.1
63.0
93.2
109.0
57.0
Me
Me
Me
H
H
H
H
H
78
75
82
76
91
52.0
n-Bu
n-Bu
(CH2)2OMe Me
n-Bu
n-Bu
À56.0
À68.0
À86.1[a]
the 13C satellites of the N-methyl group as internal stand- 10
Et
ard.
[a]
Transition glass temperature.
Results and Discussion
od has several advantages such as: it is performed at
room temperature without the use of solvents; the prod-
As alkylating agents we have chosen alkyl sulfonate es- ucts are crystalline solids and can be purified by simple
ters due to their ease, simple accessibility and possibility recrystallization in acetone. In opposition to the classi-
to use different R1 and R2 groups and thus generate a cal procedure that employs alkyl halides and must be
large variety of 1,3-dialkylimidazolium alkanesulfonate performed at reflux temperature using generally aceto-
salts (Scheme 2). The simple treatment of alcohols with nitrile solutions, 1,3-dialkylimidazolium chlorides are
alkanesulfonyl chlorides in the presence of a base such sometimes difficult to crystallize and the recrystalliza-
as triethylamine affords the pure (>99.4%) alkylsulfo- tion necessitates the use of large amounts of acetonitrile
nate esters in high yields (>90%) after a simple distilla- and ethyl acetate.[29] Note that primary, secondary and
tion.
functionalized alkanesulfonates can be employed.
The alkanesulfonate anions can be easily substituted
by a series of other anions [BF4, PF6, PF3(CF2CF3)3,
CF3SO3 and N(CF3SO2)2] by simple reaction of anion
salts or acids (Scheme 4) in water at room temperature.
Extraction with dichloromethane, filtration through a
short basic alumina column and solvent evaporation af-
fords the desired ionic liquids in 80–95% yield. These
Scheme 2.
The alkylation of N-alkylimidazoles such as N-meth- yields are similar to those obtained from the methathesis
ylimidazole with the alkyl sulfonate can be performed using 1,3-dialkylimidazolium halides.[13,13,30]
in solventless conditions at room temperature affording
after 48–72 h the corresponding 1,3-dialkylimidazolium
alkanesulfonate salts as crystalline solids in almost
quantitative yield (Scheme 3).
Scheme 4.
The purity of these ionic liquids can be assessed by 1H
Scheme 3.
NMR spectra using the intensity of 13C satellites of the
imidazolium N-methyl group. Note that the natural
abundance of 13C is 1.11% therefore the intensity of
These salts can be further purified by simple re-crys- each satellite peak corresponds to 0.555% in the case
tallization in acetone leading to crystalline samples (pu- of the N-methyl singlet. For example, the purity of the
rity>99.4% by NMR, see below) in most of the cases [C4C1Im]PF6 is>99.4% since no signals relative to other
and in high yields (Table 1).
hydrogen-containing compounds are present except the
This procedure can be applied from small-scale (10 g) residual water peak at 2.97 ppm in the 1H NMR spectra
to 500-g scale for the preparation of the dialkylimidazo- (Figure 1) of the ionic liquid [C4C1Im]PF6 isolated from
lium salts. These salts have been characterized by DSC, the reaction of [C4C1Im]CH3SO3 with KPF6 in water.
1H, 13C NMR and C,H,N analysis. This alkylation meth- The purity of this liquid was also checked by cyclic vol-
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Adv. Synth. Catal. 2006, 348, 243 – 248

Preparation and Purity Determination of Halide-Free Imidazolium Ionic Liquids
UPDATES
Figure 1. 1H NMR spectra (500 MHz, 258C) of [C4C1Im]PF6
Figure 2. 1H NMR spectra (500 MHz, 258C) of [C4C1Im]BF4
in CD2Cl2 (top) and expansion between 2.4 and 4.6 ppm (bot-
tom) showing the signals of the N-methyl 13C satellites, resid-
ual water and of the CH3SO3 anion pertinent to the starting
material [C4C1Im]CH3SO3. (Relative intensities: one 13C sat-
in CD2Cl2 (top) and expansion between 2.3 and 4.5 ppm (bot-
tom) showing the signal of the N-methyl 13C satellites and re-
sidual water. (Relative intensities: one 13C satellite¼5.73 and
water¼1.54).
¼
ellite¼9.25, CH3SO3 23.44 and water¼1.53).
tametry and only water was detected. In opposition, in
1
the H NMR spectra of [C4C1Im]BF4 (Figure 2) the
Experimental Section
peak of the CH3SO3 anion (2.58 ppm) due to the starting
[C4C1Im]CH3SO3 salt, clearly appears at 2.58 ppm with
an intensity greater than one of the 13C N-methyl satel-
lites. The purity of this ionic liquid can be estimated
as>98.5%.
General Remarks
Solvents were dried with suitable drying agents and distilled
under argon prior to use. The alkanesulfonates were prepared
using a slightly modified procedure (see below).[31] All other
chemicals were purchased from commercial sources (Acros
or Aldrich) and used without further purification. Elemental
analyses were performed by the Analytical Central Service
of IQ-USP (Brazil). NMR spectra were recorded on a Varian
Inova 300 or a Bruker ARX500 spectrometers. Infrared spec-
tra were performed on a Bomem B-102 spectrometer. The cal-
orimetric experiments were carried out on a 12000 PL-DSC
equipment with a heating rate of 108C/min. The spectroscopic
data of the new imidazolium salts are described in the Support-
ing Information.
This purity determination method is much more sim-
ple and straightforward than the others employed to
date (AgNO3 test, ion chromatography[14] and cyclic vol-
tametry[15,16]) and can be used in any laboratory in pos-
session of an NMR apparatus.
Conclusion
In summary, we have described a simple straightforward
procedure for the preparation of halide-free imidazoli-
um based ionic liquids. Moreover, the purity of these
1
ionic liquids is easily verified through simple H NMR
Butyl Methanesulfonate
experiments.
Methanesulfonyl chloride (183.2 g, 1.60 mol) was added (over
45 min), with vigorous stirring, to a solution of n-butanol
(118.4 g, 1.60 mol) and triethylamine (161.6 g, 1.60 mol) in di-
chloromethane (1.5 L). An external water-ice bath was used
to control the reaction mixture temperature between 10–
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245

´
Claudia C. Cassol et al.
UPDATES
208C. After addition, stirring was continued for further 2 h. at
room temperature. Water (300 mL) was added, the aqueous
layer containing the triethylammonium chloride by-product
was separated; the organic layer was washed with water
(200 mL) and dried with sodium carbonate. Solvent evapora-
tion followed by reduced pressure distillation of the residue af-
forded the desired butyl methanesulfonate, as a colorless liq-
uid; yield: 227.0 g (93%); bp 81–838C/4 mmHg.
crystals of 1,2-dimethyl-3-butylimidazolium methanesulfo-
nate; yield: 399.0 g (94%); mp 109.08C.
1-Butyl-3-methylimidazolium Ethanesulfonate
Butyl ethanesulfonate (10.46 g, 63.0 mmol) was mixed with 1-
methylimidazole (5.17 g, 63.0 mmol) and the reaction mixture
was kept at room temperature for 96 h. The resulting crystal-
line mass was washed twice with ethyl acetate (20 mL) and, af-
ter vacuum drying, colorless and very hygroscopic crystals of 1-
butyl-3-methylimidazolium ethanesulfonate were obtained;
yield: 7.68 g (78%); mp 57.08C.
1-Butyl-3-methylimidazolium Methanesulfonate
Butyl methanesulfonate (241.9 g, 1.59 mol) was mixed with 1-
methylimidazole (130.5 g, 1.59 mol) and the reaction mixture
was kept at room temperature by means of an external water
bath. After 24 h, one crystal of 1-butyl-3-methyl imidazolium
methanesulfonate was added and the resulting crystalline reac-
tion mass was kept at room temperature for a further 72 h. Re-
crystallization was performed twice using acetone as solvent
(350 mL; from reflux temperature to freezer temperature over-
night). After vacuum drying, colorless and very hygroscopic
crystals of 1-butyl-3-methylimidazolium methanesulfonate
were obtained; yield: 357.1 g (96%); mp 77.18C.
1-(2-Methoxyethyl)-3-methylimidazolium
Ethanesulfonate.
2-Methoxyethyl ethanesulfonate (5.38 g, 32.0 mmol) was
mixed with 1-methylimidazole (2.62 g, 32.0 mmol) and the re-
action mixture was kept at 608C for 30 h. The resulting liquid
was washed twice with ethyl acetate (5 mL) and, after vacuum
drying, 1-(2-methoxyethyl)-3-methylimidazolium ethanesul-
fonate was obtained as colorless and very hygroscopic liquid;
yield: 7.28 g (91%); Tg À86.18C
1-Butyl-3-methylimidazolium 2-Butanesulfonate
Butyl 2-butanesulfonate (29.10 g, 150 mmol) was mixed with 1-
methylimidazole (12.30 g, 150 mmol) and the reaction mixture
was kept at room temperature for 72 h. The resulting crystal-
line mass was washed twice with ethyl acetate (20 mL) and, af-
ter vacuum drying, colorless and very hygroscopic crystals of 1-
butyl-3-methylimidazolium 2-butanesulfonate were obtained;
yield: 33.10 g (80%); mp 76.18C.
1,3-Dimethylimidazolium n-Butanesulfonate
Methyl n-butanesulfonate (7.10 g, 50.0 mmol) was mixed with
1-methylimidazole (4.10 g, 50.0 mmol) and the reaction mix-
ture was kept at room temperature for 96 h. The resulting crys-
talline mass was washed twice with ethyl acetate (10 mL) and,
after vacuum drying, colorless and very hygroscopic crystals of
1,3-dimethylimidazolium n-butanesulfonate were obtained;
yield: 8.40 g (75%); mp 52.08C.
1-Butyl-3-methylimidazolium n-Butanesulfonate
Butyl n-butanesulfonate (6.51 g, 33.5 mmol) was mixed with 1-
methylimidazole (2.75 g, 33.5 mmol) and the reaction mixture
was kept at room temperature for 72 h. The resulting crystal-
line mass was washed twice with ethyl acetate (10 mL) and, af-
ter vacuum drying, colorless and very hygroscopic crystals of 1-
butyl-3-methylimidazolium n-butanesulfonate were obtained;
yield: 7.68 g (83%); mp 63.08C.
1,3-Dibutylimidazolium Methanesulfonate
Butyl methanesulfonate (8.58 g, 56.5 mmol) was mixed with 1-
butylimidazole (7.00 g, 56.5 mmol) and the reaction mixture
was kept at room temperature for 96 h. The resulting liquid
was washed twice with ethyl acetate (10 mL) and, after vacuum
drying, 1,3-dibutylimidazolium methanesulfonate was ob-
tained as an amber-colored liquid; yield: 12.77 g (82%); mp
À56.08C.
1,3-Dimethylimidazolium Methanesulfonate
A mixture of 1-methylimidazole (4.10 g, 50 mmol) and methyl
methanesulfonate (5.50 g, 50 mmol) was kept at room temper-
ature for 72 h. The resulting crystalline mass was recrystallized
with acetone/methanol, yielding hygroscopic colorless crystals
of 1,3-dimethylimidazolium methanesulfonate; yield: 8.15 g
(85%): mp 93.18C.
1,3-Dibutylimidazolium Ethanesulfonate
Butyl ethanesulfonate (11.53 g, 69.4 mmol) was mixed with 1-
butylimidazole (8.70 g, 69.5 mmol) and the reaction mixture
was kept at room temperature for 96 h. The resulting liquid
was washed twice with ethyl acetate (15 mL) and, after vacuum
drying, 1,3-dibutylimidazolium ethanesulfonate was obtained
as an amber-colored liquid; yield: 15.42 g (76%); mp À68.08C.
1,2-Dimethyl-3-butylimidazolium Methanesulfonate
A mixture of 1,2-dimethylimidazole (164.2 g, 1.71 mol) and bu-
tyl methanesulfonate (260.0 g, 1.71 mol) was kept at room tem-
perature for 96 h. The resulting crystalline mass was recrystal-
lized twice with acetone (3.5 L), yielding hygroscopic colorless
1-Butyl-3-methylimidazolium Tetrafluoroborate[12]
A mixture of 1-butyl-3-methylimidazolium methanesulfonate
(82.0 g, 350 mmol), sodium tetrafluoroborate (42.5 g,
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Preparation and Purity Determination of Halide-Free Imidazolium Ionic Liquids
UPDATES
387 mmol) and distilled water (75 mL) was vigorously stirred
for 30 min. The lower aqueous phase was separated and dis-
carded and, to the remaining liquid, sodium tetrafluoroborate
(3.0 g, 27.3 mmol) and distilled water (5 mL) were added. Stir-
ring was continued for 15 min and dichloromethane (200 mL)
was added. The organic phase was separated, dried with so-
dium carbonateand filteredthrough a short (3 cm) basic alumi-
na column. Solvent evaporation afforded the desired 1-butyl-3-
methylimidazolium tetrafluoroborate as a pale amber liquid;
yield: 61.3 g (79%).
1,2-Dimethyl-3-butylimidazolium
Hexafluorophosphate[33,34]
A mixture of 1,2-dimethyl-3-butylimidazolium methanesulfo-
nate (389.0 g, 1.57 mol), potassium hexafluorophosphate
(304.0 g, 1.65 mol) and distilled water (840 mL) was vigorously
stirred for 30 min. The upper aqueous phase was separated and
discarded and, to the remaining liquid, potassium hexafluoro-
phosphate (9.0 g, 0.05 mol) and distilled water (130 mL) were
added. Stirring was continued for 15 min and dichloromethane
(1 L) was added. The organic phase was separated, dried with
sodium carbonate and filtered through a short (4 cm) basic alu-
mina column. Solvent evaporation afforded the desired 1,2-di-
methyl-3-butylimidazolium hexafluorophosphate as a color-
less solid; yield: 445.0 g (95%); mp 44.08C.
1-Butyl-3-methylimidazolium Hexafluorophosphate[12]
A mixture of 1-butyl-3-methylimidazolium methanesulfonate
(109.9 g, 470 mmol), potassium hexafluorophosphate (90.7 g,
493 mmol) and distilled water (250 mL) was vigorously stirred
for 30 min. The upper aqueous phase was separated and dis-
carded and, to the remaining liquid, potassium hexafluoro-
phosphate (4.3 g; 23 mmol) and distilled water (40 mL) were
added. Stirring was continued for 15 min and dichloromethane
(250 mL) was added. The organic phase was separated, dried
with sodium carbonate and filtered through a short (3 cm) ba-
sic alumina column. Solvent evaporation afforded the desired
1-butyl-3-methylimidazolium hexafluorophosphate as a color-
less liquid; yield: 126.9 g (95%).
1-Butyl-3-methylimidazolium
Tris(pentafluoroethyl)trifluorophosphate[35]
1-Butyl-3-methylimidazolium methanesulfonate (39.5 g,
169 mmol) was dissolved in water (50 mL) and, with stirring
and temperature control (108C<T<208C), tris(pentafluor-
oethyl)trifluorophosphoric acid (99.0 g, 177.5 mmol) was add-
ed. Stirring was continued for further 30 min. The upper aque-
ous layer was removed and the remaining liquid was washed
(5ꢀ) with small portions (10 mL) of water. Dichloromethane
(250 mL) was added and the solution was dried with sodium
carbonate. Solvent evaporation afforded the 1-butyl-3-methyl-
imidazolium tris(pentafluoroethyl)trifluorophosphate, as a
pale amber liquid; yield: 96.80 g (98%).
1-Butyl-3-methylimidazolium
Trifluoromethanesulfonate[32]
An aqueous sodium trifluoromethanesulfonate solution was
prepared by slow trifluoromethanesulfonic acid addition
(34.5 g, 230 mmol) to a cold aqueous sodium hydroxide solu-
tion (9.2 g, 230 mmol; dissolved in 28 mL of water). The pH
of the solution was adjusted to 6–7 with trifluoromethanesul-
fonic acid or sodium hydroxide, 1-butyl-3-methylimidazolium
methanesulfonate (47.0 g; 200 mmol) was added and the re-
sulting mixture was vigorously stirred for 30 min. The water
was evaporated under reduced pressure (708C, 40 mmHg)
and dichloromethane (200 mL) was added to the semi-solid
residue. The organic phase was separated, dried with sodium
carbonate and filtered through a short (3 cm) basic alumina
column. Solvent evaporation afforded the desired 1-butyl-3-
methylimidazolium trifluoromethanesulfonate as a pale am-
ber liquid; yield: 53.0 g (92%).
Acknowledgements
Thanks are due to CNPq, FAPERGS and PETROBRAS for
partial financial support and to NMR service of the ULP-Stras-
bourg (France) for the use of the 500-MHz NMR spectrometer.
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