Characterization of the ionic liquid obtained by chlorosulfonation of 1-methylimidazole: 1-methyl-3-sulfonic acid imidazolium chloride, 1-methylimidazolium chlorosulfate or a zwitterionic salt?

Article abstract of DOI:10.1016/j.molliq.2021.115276

A great number of organic reactions catalyzed by the ionic liquid product of chlorosulfonation of 1-methylimidazole have been reported recently. At the same time controversial assumptions have appeared on the real structure of the catalyst. In the present report the primarily formed chlorosulfonation product is proved to be 1-methylimidazolium chlorosulfate ([HMim]⁺[SO₃Cl]^?) instead of 1-methyl-3-sulfonic acid imidazolium chloride, reported previously. The former structure is confirmed by X-ray crystallography and NMR spectroscopy, including ¹H-, ¹³C-, ¹⁷O- and ¹⁵N–¹H HSQC measurements. ¹H and ¹⁷O NMR experiments support fast hydrolysis of [HMim] [SO₃Cl] resulting in the formation of [HMim][HSO₄] in the presence of traces of water.

Full text of DOI:10.1016/j.molliq.2021.115276

Journal of Molecular Liquids 326 (2021) 115276
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Characterization of the ionic liquid obtained by chlorosulfonation
of 1-methylimidazole: 1-methyl-3-sulfonic acid imidazolium chloride,
1-methylimidazolium chlorosulfate or a zwitterionic salt?
Béla Urbán a, Gábor Szalontai b, Máté Papp a,1, Csaba Fehér a, Attila C. Bényei c, Rita Skoda-Földes a,
⁎
a
Institute of Chemistry, Department of Organic Chemistry, University of Pannonia, Egyetem u. 10. (P.O.Box 158), H-8200 Veszprém, Hungary
NMR Laboratory, University of Pannonia, Egyetem u. 10, P.O.Box 158, H-8200 Veszprém, Hungary
b
c
Laboratory for X-ray Diffraction, University of Debrecen, 4002 Debrecen, P.O.Box 400, Hungary
a r t i c l e i n f o
a b s t r a c t
Article history:
A great number of organic reactions catalyzed by the ionic liquid product of chlorosulfonation of 1-
methylimidazole have been reported recently. At the same time controversial assumptions have appeared on
the real structure of the catalyst. In the present report the primarily formed chlorosulfonation product is proved
to be 1-methylimidazolium chlorosulfate ([HMim]+[SO3Cl]−) instead of 1-methyl-3-sulfonic acid imidazolium
chloride, reported previously. The former structure is confirmed by X-ray crystallography and NMR spectroscopy,
including 1H-, 13C-, 17O- and 15N–1H HSQC measurements. 1H and 17O NMR experiments support fast hydrolysis
of [HMim] [SO3Cl] resulting in the formation of [HMim][HSO4] in the presence of traces of water.
© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://
creativecommons.org/licenses/by-nc-nd/4.0/).
Received 1 December 2020
Accepted 3 January 2021
Available online 5 January 2021
Keywords:
Brønsted acid
Chlorosulfate
Ionic liquids
NMR spectroscopy
X-ray diffraction
1. Introduction
metal halide anions as the Lewis acidic site [5]. The first example for a
Brønsted acidic IL with an SO3H moiety attached directly to imidazolium
Acid-catalyzed reactions involve one of the most important technolo-
gies applied in the chemical industry. The most common catalysts are
mineral acids, used in homogeneous phase reactions. Although they usu-
ally show high catalytic activity, they often suffer from some drawbacks,
such as the problems of side reactions, large amounts of acidic wastes and
corrosion of the equipment. Isolation of the products can also be tedious,
which can cause environmental problems. As a result, great efforts have
been devoted to the development of new catalysts. One possibility is
the application of solid acids which, as non-volatile materials, are less
harmful than traditional liquid acids [1]. However, these heterogeneous
systems often show an inferior activity when compared to their homoge-
neous counterpart. As another approach, increasing attention has been
paid to the application of task specific ionic liquids as catalysts [2]. Espe-
cially acidic ionic liquids were found to be useful and even recyclable al-
ternatives to conventional Brønsted or Lewis acid catalysts [3]. Among
them, ionic liquids (ILs) incorporating imidazolium cations with N-alkyl
sulfonic groups serve as efficient Brønsted acids in a great variety of
organic transformations [4]. Furthermore, they can be converted into
–SO3H functionalized dual Brønsted–Lewis acidic ILs possessing complex
nitrogen, 1-methyl-3-sulfonic acid imidazolium chloride ([MSim]Cl, 1,
Fig. 1), was reported by Zolfigol et al. in 2010 [6]. Since then, the catalytic
activity of ILs incorporating the [Msim]+ cation (1) and different anions
[7] was tested in a wide range of organic reactions. At the same time,
no detailed structural data are available for these compounds. Only 1H
and 13C NMR spectra were reported [6] that do not give sufficient infor-
mation about the position of the SO3 moiety. Recently, Shiflett et al.
found that chlorosulfonation of 1-methylimidazole led to a mixture of
two compounds, an overall neutral zwitterion (2) as the main product
and another derivative that was claimed to be 1 [8]. The formation of
the former was proved by crystallography but beyond that only 1H and
13C spectra of the mixture of the two compounds were presented.
Based on 1H NMR data, it was supposed that compound 2 could be con-
verted to 1 when an equimolar amount of HCl was added [8]. It should
also be mentioned that Zolfigol described the product as a viscous oil
[6] while the other group obtained a solid material and they recorded a
melting point of 325.8 °C for the zwitterionic component 2 [8].
As part of our ongoing interest in the application of acidic IL cat-
alysts in ring opening of epoxides [9] as well as oligomerization of
alkenes [10], we decided to explore the activity of ILs with N-SO3H
moieties. Before that the clarification of the somewhat contradictory
results seemed to be necessary that prompted us to carry out a
more detailed investigation of the chlorosulfonation product of
1-methylimidazole.
⁎
Corresponding author.
E-mail address: skodane@almos.uni-pannon.hu (R. Skoda-Földes).
Present address: Institute of Chemistry ELTE Eötvös Loránd University, Budapest,
1
Hungary.
https://doi.org/10.1016/j.molliq.2021.115276
0167-7322/© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

B. Urbán, G. Szalontai, M. Papp et al.
Journal of Molecular Liquids 326 (2021) 115276
integration was performed using the APEX3 (Ver. 2017.3–0, Bruker
AXS Inc., 2017.) software. Data reduction and multi-scan absorption cor-
rection was performed using SAINT (Ver. 8.38A, Bruker AXS Inc., 2017).
The structure could be solved using direct methods and refined on F2
using the SHELXL program [13] incorporated into APEX3 suite. Refine-
ment was performed anisotropically for all non‑hydrogen atoms.
Hydrogen atoms were placed into geometric positions. The structure is
stabilized by strong intermolecular N-H..O and weak C-H..O hydrogen
bonds. The CIF file was manually edited using the publCIF software
[14]. Results of X-ray diffraction structure determinations were very
good according to the Checkcif of PLATON software [15] and structural
parameters such as bond length and angle data are in the expected
range. Further crystallographic and refinement details can be seen in
the SI. Powder diffraction data were collected using the same instru-
ment applying Cu Kα (λ = 1.54178 Å) radiation. Integration and back-
ground correction was performed with Bruker's DIFFRAC.EVA 4.2.2.3
software.
N
N
N
N
SO3
SO3H Cl
[MSim]Cl)
1 (
2
Fig. 1. The stucture of the product of chlorosulfonation of 1-methylimidazole, suggested
by Zolfigol et al. [6] ([MSim]Cl, 1) and the zwitterionic structure 2 reported by Shiflett
et al. [8].
2. Experimental
2.1. Computational methods
Theoretical calculations were performed with the Gaussian 09 soft-
ware package [11]. Molecular geometries were optimized using the
long-range corrected hybrid density functional CAM-B3LYP [12]and
the standard 6–311++G** basis set. The optimizations were performed
without any symmetry constraint. The vibrational analysis yielded no
imaginary frequencies verifying that each of the optimized structures
is a true minimum on the potential energy surface. The Gibbs free ener-
gies were refined using the CPCM solvent model (with Solvent=dichlo-
romethane) built in the Gaussian 09 suite.
CCDC entry 2,023,770 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
2.4. NMR investigations
All NMR investigations were carried out on a Bruker Avance II
400 MHz spectrometer using a 5 mm BBO probe with temperature reg-
ulation. The relaxation delays applied were 1, 4 and 10 s for the 1H, 13C,
and 15N nuclei, respectively. The standard Bruker pulse sequences were
used. 2–3000 scans were sufficient to record 17O NMR spectra.
2.2. Synthetic methods
Chlorosulfonation of 1-methylimidazol was carried out based on the
procedure reported by Zolfigol [6].
Method A: Chlorosulfonic acid (1.05 ml, 15.6 mmol) was added
dropwise into a room temperature solution of 1-methylimidazole
(1.2 ml, 15 mmol) in 150 ml dry CH2Cl2. The mixture was stirred over-
night and then left to stand for one hour. After that the CH2Cl2 was
decanted. The residue was washed with dry CH2Cl2 (3 × 50 ml) and
dried in vacuum to obtain a viscous oil that solidified upon standing
two weeks under strictly inert conditions.
Method B: Chlorosulfonic acid (1.05 ml, 15.6 mmol) was added
dropwise into a cooled solution of 1-methylimidazole (1.2 ml,
15 mmol) in 150 ml dry CH2Cl2. The mixture was stirred for 20 min
and left to stand a further 20 min to obtain a colorless solid precipitate.
After that the CH2Cl2 was decanted. The residue was washed with dry
CH2Cl2 (3 × 50 ml) and dried in vacuum.
[HMim][HSO4](4) was prepared by the reaction of 1-methylimidazole
and H2SO4. Concentrated sulfuric acid (1.34 ml, 25.1 mmol) was added
dropwise to 2 ml (25.1 mmol) 1-methylimidazole under vigorous stirring
in an ice bath. After warming to room temperature it was stirred for 48 h
and then was washed with 5 × 5 ml diethyl ether and was dried in
vacuum at 70 °C for 10 h.
3. Results and discussion
3.1. Quantum chemical calculations
Acidities of some well-known acids and different sulfonated
imidazolium ions were compared based on their dissociation free ener-
gies (Table 1). Sulfonation of imidazole derivatives is reported to take
place in position four or five under usual conditions for electrophilic
substitutions [16]. 2-Sulfonated imidazoles were prepared by oxidation
of 2-substituted thiols [17]. These sulfonic acids were shown to exist
mainly as zwitterionic compounds (Fig. 2) according to the crystallo-
graphic data [16,17]. In the present case however, the formation of a cat-
ionic species is supposed so data for the proposed [6] N-sulfonated
Table 1
Relative dissociation free energies for different acids in CH2Cl2 solvent.
[HMim]Cl was prepared by the reaction of 1-methylimidazole and
HCl. HCl was liberated from the reaction of NaCl and H2SO4 in a
separated flask connected with the vessel containing 2 ml (25.1 mmol)
1-methylimidazole. The reaction mixture was stirred at room tempera-
ture overnight, then it was washed with 5 × 10 ml diethyl ether and
was dried in vacuum at 60 °C for 10 h.
Entry
AH
ΔGdissΔGdiss [kJ/mol]a
1
2
3
4
HCl
H2SO4
CF3SO3H
0
−34.2 (128.9)b
−59.4
−4.3
5
6
−80.6
2.3. X-ray diffraction analysis
X-ray quality crystals of 3 were grown from the neat sample ob-
tained by Method A upon standing. A suitable crystal was fixed onto a
Mitegen loop using high density oil. Diffraction intensity data collection
was carried out using a Bruker-D8 Venture diffractometer equipped
with INCOATEC IμS 3.0 dual (Cu and Mo) sealed tube micro sources
and Photon II Charge-Integrating Pixel Array detector using Mo Kα
(λ = 0.71073 Å) radiation. Low temperature data collection (150 K)
was applied to protect the sample from traces of moisture and also be-
cause of the low melting point. High multiplicity data collection and
−104.1
7
−126.7
a
ΔGdiss- ΔGdiss(HCl).
b
2nd H+ in parenthesis.
2

B. Urbán, G. Szalontai, M. Papp et al.
Journal of Molecular Liquids 326 (2021) 115276
Table 2
SO3
NH
Gibbs free energy differences between
2
+
HA and [MSim]A structures in
dichloromethanea.
N
NH
SO3
Entry
Anion
ΔG [kJ/mol]
HN
1
2
3
chloride
−85.8
−75.4
−71.9
hydrogensulfate
Et
triflate
a
G(2+HA) - G([MSim]+anion)
Fig. 2. Zwitterionic structures of sulfonated imidazoles [16,17].
hydrogensulfate ([HMim][HSO4], 4) with SOCl2 and was characterised
cation ([MSim]) (entry 7) as well as those for cationic C-4 (entry 5) and
C-2 sulfonated derivatives (entry 6) were determined.
by HRMS and 1H NMR spectroscopy [20].
The dissociation free energies defined by eq. (1) were calculated
using the cam-B3LYP density functional with the 6–311++G** basis
set. The geometry optimisations and free energy calculations were per-
formed using the conductor-like polarisable continuum model (CPCM)
with CH2Cl2 solvent. The dissociation energy data, relative to HCl are
summarized in Table 1. For the sulfonated imidazolium ions, the calcu-
lations gave surprisingly high energies (Table 1 entries 5–7) compared
to the data of common acids. The value for the [MSim]+ cation indicates
much higher acidity of this structure than that of CF3SO3H.
3.2. Preparation of ionic liquids
Chlorosulfonation of 1-methylimidazole was carried out based on
the report of Zolfigol [6]. A careful addition of chlorosulfonic acid to
the room temperature solution of 1-methylimidazole and stirring the
mixture overnight (Method A) led to the formation of a highly viscous
oil forming a separate phase. After the removal of the dichloromethane
phase, the material solidified upon standing two weeks under strictly
anhydrous conditions and even crystals suitable for X-ray investigations
could be isolated. In another procedure, chlorosulfonic acid was added
dropwise to a cooled solution of 1-methylimidazole in dichloromethane
(Method B). In this case, a colorless solid precipitated almost immedi-
ately that was separated after stirring the mixture for 20 min.
AH → A− + H +
.
ΔGdiss ¼ GA− þ GHþ−GAH
ð1Þ
GA−: Calculated Gibbs free energy for A−.
3.3. X-ray measurements
GAH: Calculated Gibbs free energy for AH.
GH+: Gibbs free energy of H+ (GH+ = −26.28 kJ/mol) [18]
Crystals degraded rapidly upon warming so diffraction data had to
be collected at low temperature (150 K). The compound crystallized
in the P21/c monoclinic space group. Crystallographic data are summa-
rized in Table 3. The results revealed the formation of an imidazolium
salt with a ClSO−3 anion: ([HMim][ClSO3] (3) Scheme 2, Fig. 3). It should
be mentioned that to the best of our knowledge, X-ray data for only two
structures incorporating the ClSO−3 anion were reported before [21].
Comparison of the PXRD of the bulk sample shows that all the bulk
crystalline material is the same as the single crystal salt structure 3
(Fig. S3).
Geometry optimizations were also performed for the N-sulfonated
cation ([MSim]+) in the presence of chloride, hydrogensulfate or triflate
anions as well as for the deprotonated form (2) with the corresponding
acid (Scheme 1, Table 2). In each case, the zwitterion 2 had much lower
Gibbs free energy.
These results support the assumption that the equilibrium depicted
in Scheme 1 is shifted considerably to the right side for Cl−, HSO4− or
OTf− anions. That means that the N-sulfonated derivative may exist
mainly in the zwitterionic form 2 similarly to other sulfonated deriva-
tives (Fig. 2). In other words, hydrogen chloride is not a sufficiently
strong acid to protonate zwitterion 2, especially when they are present
in equimolar amounts. It should be mentioned however that sulfuryl
imidazolium salts, the ester derivatives of the proposed ILs [MSim]A,
are known compounds and their synthesis and applications were re-
ported before [19].
3.4. NMR investigations
Next, detailed NMR investigations [22] of the materials obtained by
the two different methods (A and B) were carried out.
The presence of zwitterion 2 in the two component mixture was
proved by Shiflett [8] but we felt that some more evidence was neces-
sary to be certain about the structure of the other component. Accord-
ingly, a more detailed investigation of the chlorosulfonation product of
1-methylimidazole was carried out. Besides the formation of the zwit-
terionic form 2, a simple acid-base reaction leading to the [HMim]+ cat-
ion (Scheme 2) was also taken into consideration. [HMim][SO3Cl] (3)
had been obtained before by the reaction of 1-methylimidazolium
NMR data of a freshly prepared neat sample of the ionic liquid ob-
tained by Method A supported the X-ray structure and proved the for-
mation of 3. It is noteworthy that because of the peculiar properties,
such as the extreme viscosity or high polarity or low melting point of
the ionic liquids, recording and interpretation of spectra require special
attention [23]. In the 1H NMR spectrum of the neat sample (Fig. 4) a
one-proton signal could be found at 12.7 ppm and three more with 1:
2: 3 integral ratios at 8.41 (2-H), 7.11 (4-H, 5-H), and 3.57 (N-CH3)
N
N
N
N
+
HA
SO3
A
SO3H
2
Scheme 1. Possible deprotonation the N-sulfonated cation ([MSim]+) in the presence of anions
3

B. Urbán, G. Szalontai, M. Papp et al.
Journal of Molecular Liquids 326 (2021) 115276
1
ClSO3H
N
N
SO3
N
N
CH2Cl2, rt
2
N
N
SO3Cl
H
3
Scheme 2. Possible products of the reaction of 1-methylimidazole with chlorosulfonic acid
ppm, respectively. Unlike the 4-H, 5-H proton signals, the correspond-
ing carbons are clearly resolved in the 13C{1H} NMR spectrum (Fig. 5).
The high chemical shift of the NH proton (12.7 ppm) can be ex-
plained by the existence of a hydrogen bond between this proton and
one of the oxygen atoms of the SO3Cl− anion in the neat sample, simi-
larly to the solid structure (Fig. S2).
It should also be mentioned that another set of signals can be seen in
the 1H and 13C{1H} NMR spectra with a relative integral ratio of about
1/10 (Fig. 4) that shows the presence of some minor component(s).
The two nitrogens of the imidazole ring gave completely overlap-
ping signals at 170.7 ppm in the inverse-gated 15N{1H} NMR spectrum
(Fig. 6). It is important to notice that this experiment is not direc-
tly influenced by a protonation – deprotonation exchange [22].
pH-dependence of 15N NMR shifts of 1-methylimidazole had been re-
ported earlier [24]. While an approximately 80 ppm difference was ob-
served between the 15N1 and 15N3 signals under basic conditions
(corresponding to 1-methylimidazole), a single peak had been detected
for the two nitrogens in acidic solutions at pH<5, where the 1-
methylimidazolium cation is in excess. So the overlap of the signals in
the present case, though somewhat unexpected, may provide another
proof for the formation of the [HMim]+ cation.
Table 3
Summary of crystallographic data for compound 3.
Crystal data
Chemical formula
Mr
C4H7N2·ClO3S
198.63
Crystal system, space
group
Monoclinic, P21/c
Fig. 3. X-ray structure of ionic liquid ([HMim][SO3Cl] (3) with partial numbering scheme
and selected bond distances. S-O distances are in the range of 1.433–1.439 Å.Thermal
ellipsoids drawn at 50% probability.
Temperature (K)
a, b, c (Å)
β (°)
150
5.9038 (3), 18.0587 (11), 7.4778 (5)
95.048 (2)
794.15 (8)
4
V (Å3)
Z
Radiation type
Mo Kα
0.70
μ (mm−1
)
Crystal size (mm)
Data collection
Diffractometer
0.35 × 0.26 × 0.21
Bruker D8 VENTURE
Absorption correction Numerical SADABS2016/2 - Bruker AXS area detector
scaling and absorption correction
Tmin, Tmax
0.78, 0.87
No. of measured,
independent and
observed [I > 2σ(I)]
reflections
8179, 1487, 1427
Rint
0.019
0.610
(sin θ/λ)max (Å−1
)
Refinement
R[F2 > 2σ(F2)], wR(F2), 0.041, 0.139, 1.35
S
14
12
10
8
6
4
2
0
No. of reflections
No. of parameters
H-atom treatment
Δ〉max, Δ〉min (e Å−3
1487
102
H-atom parameters constrained
0.96, −0.91
Fig. 4. 1H-400 MHz NMR spectrum of a neat sample of compound 3 (obtained by
Method A).
)
4

B. Urbán, G. Szalontai, M. Papp et al.
Journal of Molecular Liquids 326 (2021) 115276
140
150
160
170
180
190
200
14
12
10
8
6
4
2
150
100
50
0
Fig. 8. Proton-detected 15N–1H HSQC spectrum of a neat sample of compound 3 (J=11 Hz)
(obtained by Method A).
Fig. 5. 13C{1H}- 100.3 MHz NMR spectrum of a neat sample of compound 3 (obtained by
Method A).
disappears, as soon as the NH protons take part in an exchange process
fast on the 1H time scale. Note that the weaker correlation seen on Fig. 8
points to the N1 nitrogen coupled also to the 12.7 proton by a three bond
coupling. These observations correspond to the presence of the
[HMim]+ cation.
The highly concentrated neat sample made possible also the record-
ing of spectra of rare nuclei such as 17O within reasonable accumulation
time. Thus, to identify the structure of the anion, the 17O NMR spectrum
was also attempted at natural abundance. At room temperature it pre-
sented a broadened peak at 240.8 ppm (Fig. 9). This chemical shift can
be indicative of the presence of a Cl-S-O moiety [20]. Incorporation of
chlorine results in a considerable downfield shift of the 17O signal
Some literature examples are as follows: CH3SO2OCH3: 170 ppm [25],
CH3SO2Cl: 238 ppm [25], CH3OSO2OCH3: 140 ppm [26], CH3OSO2Cl:
219 ppm [26]. At the same time, δO of sulfonic acid derivatives remains
far below 200 ppm (e. g. 140 ppm for p-toluenesulfonic acid monohydrate
[27], 171 ppm for Cys-SO3H) [28].) Also, an increase in the negative charge
on oxygens around sulfur was found to lead to deshielding (e.g. δO
(H2SO4) = 140 ppm, while δO (SO24−) = 167 ppm) [25a].
300
250
200
150
100
50
Fig. 6. 15N{1H} inverse-gated- spectrum of a neat sample of compound 3 (obtained by
Method A).
The proton-detected 15N–1H through-bond (HSQC) experiment
using a typical one-bond coupling value ([1]J(1H–15N) =100 Hz) [17]
exhibited an extremely strong cross peak (Fig. 7) between the
12.7 ppm proton signal and the 170.7 ppm N3 nitrogen signal,
confirming that they are directly connected. However, this correlation
The sample, stored for a period of two months in a closed bottle,
displayed a bit different 1H NMR spectrum compared to the fresh mate-
rial (Fig. 10). Although the signals of the [HMim]+ cation remained un-
altered, some increase in the integral value of the peak at 10.8 ppm was
observed. Another marked feature is the disappearance of the signals of
140
150
160
170
180
190
200
250
200
150
100
14
12
10
8
6
4
2
Fig. 7. Proton-detected 15N–1H HSQC spectrum of a neat sample of compound 3 (J=
110 Hz) (obtained by Method A).
Fig. 9. 17O-direct detection (zg) not-proton decoupled (linewidth ~ 300 Hz) NMR
spectrum of a neat sample of compound 3 (obtained by Method A).
5

B. Urbán, G. Szalontai, M. Papp et al.
Journal of Molecular Liquids 326 (2021) 115276
accordance with the behaviour of the other sample (Fig. 11). Signals
corresponding to
a minor compound incorporating the 1-
methylimidazole moiety could be distinguished again in all of these
spectra, similarly to the sample prepared by Method A.
a)
A broadening of the most de-shielded two signals (~14 and
~12 ppm) was observed when the temperature was increased to
323 K and they collapsed into a single broad peak upon further heating
(Fig. S7), supporting an exchange between them. These signals disap-
peared when D2O was added to the mixture and a new broad H2O
peak was detected at 4.4 ppm indicating a deuterium-proton exchange
(Fig. 12). It is noteworthy that the singlets corresponding to the minor
component were absent from the spectrum obtained by heating this
sample to 323 K. Only four signals with the ratio of 1:1:1:3 could be dis-
tinguished at 8.63 ppm, 7.40 ppm, 7.38 ppm and 3.79 ppm, respectively,
supporting the presence of a single compound.
b)
14
12
10
8
6
4
2
Fast hydrolysis of the chlorosulfate anion [29] of [HMim][SO3Cl] (3)
may lead to the formation of protons and different anions (Scheme 3).
To prove the hypothesis of the formation of HSO−4 (or SO42−) anions
in the partially hydrolysed samples, the spectra were compared with
those of [HMim][HSO4] (4), prepared by the reaction of 1-
methylimidazole and H2SO4 (Figs. S8-S10).
Fig. 10. a) 400 MHz 1H NMR spectra of a freshly made neat sample prepared by Method B
(300K); b) 1H NMR spectrum of the same sample after a two-month storage (304 K).
The signals in the 1H NMR spectrum of a neat sample of [HMim]
[HSO4] (4) (Fig. S8) is very similar to those of the main components of
the partially hydrolysed samples of substances obtained by Methods
A (Fig. 11b) and B (Fig. S5). An exchange between the two protons
exhibiting the broad signals at 13.2 ppm and 11.0 ppm was proved by
a change in the temperature of the 1H NMR experiment (Fig. S10).
This behaviour is also similar to that of the hydrolysed material (Fig. S7).
The presence of HSO−4 (or SO24−) anions in a hydrolysed sample
could also be proved by 17O NMR measurements. The chemical shift at
165.5 ppm observed for the substance obtained by Method B in the
the 1-methylimidazole moiety of the minor component. This proves
that the proton with the peak at 10.8 ppm does not belong to the latter.
It can be assumed that ionic liquid 3 is extremely sensitive to mois-
ture, similarly to chlorosulfonic acid [29] and the changes in the 1H NMR
spectrum (Fig. 10) can be attributed to partial hydrolysis. The deliberate
addition of DMSO‑d6 containing traces of water to the fresh sample re-
sulted in a considerable change in the integral value of the signal around
11 ppm (Fig. 11b). In a sample with higher dilution (Fig. 11c), signals
corresponding to protons of the methyl group and the imidazole ring
became sharper. Besides, two broad signals could clearly be distin-
guished from the narrow ones: one above 14 ppm, corresponding to ap-
proximately one proton, and another one at 7.5 ppm with a larger
integral value (Fig. 11c). NOESY spectra indicate slow exchange be-
tween the two (Fig. S4).
Essentially the same spectra were obtained in DMSO‑d6 for the ma-
terial prepared by Method B (Fig. S5) showing that the two methods
led to the formation of the same major product. The integral value of
the peak around 11 ppm decreased in the spectrum of a more concen-
trated sample (probably due to the lower water content) together
with some downfield shift of the signal (Fig. S6). The broad singlet
with the highest chemical shift moved to the other direction, in
presence of water (and D2O) (Fig. 13b) corresponded well to the 17
O
signal of [Hmim][HSO4] (4) (Fig. 13c), as well as to that of KHSO4
(163 ppm) [30], supporting the hydrolysis of the anion of 3− to form
HSO−4 ions. A similar behaviour could be noticed on comparing the
17O NMR spectra of chlorosulfonic acid and a chlorosulfonic acid /
water mixture. The ClSO−3 ions immediately and fiercely form HSO4−
(and HCl) and practically remove water from the mixture (see spectrum
at Fig. S11).
It should also be mentioned that spectra depicted in Fig. 11a and
Fig. S5 are very similar to that reported by Shiflett [8], except that a
greater difference (major / minor ratio = 10 and 8, respectively) was
observed between the amount of the major and minor components.
a)
b)
c)
a)
b)
16
14
12
10
8
6
4
2
Fig. 11. a) 400 MHz 1H NMR spectra of a neat sample prepared by Method A (300 K); b) 1
H
14
12
10
8
6
4
2
0
NMR spectrum of the same sample after addition of 30% DMSO‑d6 (300 K); c) 1H NMR
spectrum of a diluted sample (70 mg/ 0.4 ml DMSO‑d6 (300 K) (The shift of the methyl
singlet in spectrum a) is adjusted to the same signals of the spectra obtained in
DMSO‑d6 b).)
Fig. 12. 400 MHz 1H NMR spectra of the material obtained by Method B in a mixture of
DMSO‑d6 (0.3 ml) and D2O (0.1 ml) a) 313 K; b) 323 K.
6

B. Urbán, G. Szalontai, M. Papp et al.
Journal of Molecular Liquids 326 (2021) 115276
H2O
2
SO3Cl
N
N
+
H
Cl
+
SO4
H
N
N
H
H
3
(or: HSO4
)
Scheme 3. Hydrolysis of [HMim][SO3Cl] (3) in the presence of added water
Shiflett proved the formation of zwitterion 2 as one of the two prod-
ucts by X-ray investigations [8]. The other derivative was supposed to
be [MSim]Cl (1), although the explanation for the appearance of two
different peaks in the region of acidic protons in the 1H NMR spectrum
remained somewhat vague.
In our case, the integral values of the high-frequency shifted two
protons (~13–14 ppm and ~11–12 ppm) suggests that they do not be-
long to the minor product.
Based on the above results, it can be supposed that the minor com-
ponent in the substances prepared by Methods A or B is zwitterion 2.
For this compound, only signals corresponding to the methylimidazole
core can be distinguished either in the 1H- or in the 13C NMR spectra.
It was prepared before [31] by the reaction of 1-methylimidazole and
SO3 and was found to loose SO3 in the presence of water. It was also
shown that it could convert alcohols to alkyl sulfates. Consequently, its
hydrolysis may result in the formation of [HMim][HSO4] (4), similarly
to that of the main component of the material obtained by Method A,
[HMim][ClSO3] (3) (Scheme 3). This may give an explanation for the
presence of one set of signals in the region of imidazolium protons in
samples contaminated with water, corresponding to the [HMim]+
cation. The signal around 14 ppm can be attributed to the NH proton
of the same cation, while the peak with altering intensity around
11 ppm can be assigned to the HSO−4 anion. That means that wet sam-
ples contain mixtures of [HMim][ClSO3] (3), [HMim][HSO4] (4) and,
eventually, zwitterion 2. NMR investigations of the substance obtained
by Method B (Figs. S5-S7) strongly suggest that it is also a mixture of
zwitterion 2, and ionic liquids 3 and 4.
It should be mentioned that the experiments discussed above prove
that the zwitterion 2 is much less prone to hydrolysis than 3 and is con-
verted to 4 only on heating (Fig. 12), or on prolonged interaction with
water (Fig. 10).
It should be emphasized that high chemical shift of the NH proton of
the [HMim]+ cation in neat samples can be due to the existence of hy-
drogen bonds, e.g. between N3H of the cation and O1 of the anion in
case of 3, as indicated by the X-ray structure (Fig. S2). Also, the chemical
shifts of NH protons of 3, 4 and [HMim]Cl in the neat samples (Fig. S12)
change in a reversed order to the acidity of the conjugate acid of the
anion. This phenomenon was observed before for diethylmethylamine
based protic ionic liquids with the chemical shift of NH protons ranging
from 14.35 ppm to 7 ppm with increasing acid strength of the acid used
during the protonation of the amine. This tendency was explained by
the increase in the strength of hydrogen bond formed between the
anion and the protonated base, causing a deshielding of the exchange-
able proton [32].
4. Conclusions
a)
b)
c)
The results presented above show that the reaction of chlorosulfonic
acid and 1-methylimidazole can lead to a mixture of [HMim][SO3Cl] (3)
and a minor component 2 at room temperature. The primarily formed
ionic liquid is very sensitive to moisture and readily hydrolyses to
[HMim][HSO4] (4). A slower reaction of zwitterion 2 with water leads
to the same product (4). The formation of [MSim]Cl as a stable product
of the present reaction remains to be proved. 1H and 13C NMR data are
not sufficient for this as NH chemical shifts (and even 13C and 15N
NMR chemical shifts [24]) of 1-methylimidazole are subject to changes
depending on the conditions (dilution, pH, etc.). Also, it can be seen that
the chlorosulfonation reaction is extremely sensitive not only to mois-
ture but also to other experimental details. The original report describes
the formation of a single product [6], Shiflett obtained a mixture with
two components of comparable amounts [8]. The present reactions led
to the same two products but in a different ratio. Chlorosulfonic acid is
prepared by the reaction of SO3 and HCl. The reaction is reversible
[33], so chlorosulfonic acid may contain some SO3 that can lead to the
formation of zwitterion 2 in an amount corresponding to the SO3 con-
tent of the reagent.
It should be mentioned that the formation of 3 cannot be due to the
lack of proper inertness of our procedure as it was shown to decompose
in the presence of moisture soon. At the same time, under sufficiently
inert conditions it can survive a two-month long storage.
Good catalytic activity of the chlorosulfonation product is also not a
proper proof for the existence of [MSim]Cl and can also be explained by
the formation of ionic liquid 3. Bao et al. investigated the acidity of
[HMim][SO3Cl] (3), obtained by the reaction of SO2Cl and [Hmim]
[HSO4], and showed that the ionic liquid 3 had dual Brønsted—Lewis
acidity [20]. Moreover, much superior catalytic activity of the latter
salt compared to that of [Hmim][HSO4] was observed in esterification
200
100
0
Fig. 13. 54.2 MHz 17O NMR spectra without proton decoupling. a) neat [HMim][SO3Cl] (3)
δ = 240.8 ppm, b) substance obtained by Method B in the presence of DMSO‑d6 and D2O
(50 μL D2O, 298 K) δ = 165.5 ppm (HSO−4 ), δ = 13.1 ppm (DMSO), and δ = 0.0 ppm
(excess H2O was added as reference) and c) [HMim][HSO4] (4) in the presence of
DMSO‑d6 (50 μl) δ = 163.6 ppm.
7

B. Urbán, G. Szalontai, M. Papp et al.
Journal of Molecular Liquids 326 (2021) 115276
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(f) A. Khazaei, A. R. Moosavi-Zare, S. Firoozmand, M. R. Khodadadian, Synthesis,
characterization and application of 3-methyl-1-sulfonic acid imidazolium
tetrachloroferrate as nanostructured catalyst for the tandem reaction of β-
naphthol with aromatic aldehydes and amide derivatives. Appl Organometal
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reactions. It should be mentioned, that the great majority of the catalytic
processes carried out with’[MSim]’ Ils [6,7] are condensations resulting
in the formation of water, so hydrolysis or partial hydrolysis may take
place even under strictly inert conditions in the course of the reactions.
The hydrolysis product (Scheme 3) can be considered a mixture of a
protic ionic liquid and an inorganic acid. Such mixtures were shown to
exert improved catalytic activity over the single components [34], that
could be awaited in the present case, too.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
(g) N.G. Khaligh, Preparation, characterization and use of 3-methyl-1-sulfonic acid
imidazolium hydrogen sulfate as an eco-benign, efficient and reusable ionic liq-
uid catalyst for the chemoselective trimethylsilyl protection of hydroxyl groups,
J. Mol. Catal. A 349 (2011) 63–70, https://doi.org/10.1016/j.molcata.2011.08.
021;
Acknowledgement
The authors acknowledge the financial support of the Economic
Development and Innovation Operative Program of Hungary (GINOP-
2.3.2-15-2016-00053) and the National Research, Development and In-
novation Office (K120014). A. C. Bényei thanks for the support of pro-
jects GINOP-2.3.2-15-2016-00008 and GINOP-2.3.3-15-2016-00004
financed by the EU and co-financed by the European Regional
Development Fund.
(h) N.G. Khaligh, Synthesis of coumarins via Pechmann reaction catalyzed by 3-
methyl-1-sulfonic acid imidazolium hydrogen sulfate as an efficient, halogen-
free and reusable acidic ionic liquid, Catal. Sci. Technol. 2 (2012) 1633–1636,
https://doi.org/10.1039/C2CY20196H;
(i) N.G. Khaligh, Four-component one-pot synthesis of unsymmetrical
polyhydroquinoline derivatives using 3-methyl-1-sulfonic acid imidazolium hy-
drogen sulfate as a catalyst, Chin. J. Catal. 35 (2014) 1036–1042, https://doi.org/
10.1016/S1872-2067(14)60038-3;
(j) M.A. Zolfigol, A. Khazaei, A.R. Moosavi-Zare, A. Zare, H.G. Kruger, Z. Asgari, V.
Khakyzadeh, M. Kazem-Rostami, Design of ionic liquid 3-methyl-1-sulfonic acid
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situ generation of NO2 in acidic media, J. Org. Chem. 77 (2012) 3640–3645,
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Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.molliq.2021.115276.
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