Electrochemical synthesis of aromatic sulfur compounds in ionic liquids

Article abstract of DOI:10.1134/S1070363216020146

Electrochemical properties of ionic liquids (pyridinium and imidazolium salts) and the effect of additives of organic solvents on the electrochemical determination of organic compounds in ionic liquids have been studied. Transformations of aromatic and aliphatic sulfur compounds in ionic liquids in the presence of aromatic substrates are discussed. A new method has been proposed for identification of organic sulfur compounds-gas chromatography on columns with ionic liquid as the active phase.

Full text of DOI:10.1134/S1070363216020146

ISSN 1070-3632, Russian Journal of General Chemistry, 2016, Vol. 86, No. 2, pp. 291–295. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © A.V. Okhlobystina, А.О. Okhlobystin, N.N. Letichevskaya, V.F. Abdulaeva, N.О. Movchan, N.Т. Berberova, 2016, published in
Zhurnal Obshchei Khimii, 2016, Vol. 86, No. 2, pp. 263–267.
Electrochemical Synthesis of Aromatic Sulfur Compounds
in Ionic Liquids
А. V. Okhlobystinaa, А. О. Okhlobystina, N. N. Letichevskayaa,
V. F. Abdulaevaa, N. О. Movchanb, and N. Т. Berberovaa
a Astrakhan State Tecnhical University, ul. Tatishcheva 16, Astrakhan, 414025 Russia
e-mail: ionradical@gmail.com
b Southern Scientific Center, Russian Academy of Sciences, Rostov-on-Don, Russia
Received June 26, 2015
Abstract—Electrochemical properties of ionic liquids (pyridinium and imidazolium salts) and the effect of
additives of organic solvents on the electrochemical determination of organic compounds in ionic liquids have
been studied. Transformations of aromatic and aliphatic sulfur compounds in ionic liquids in the presence of
aromatic substrates are discussed. A new method has been proposed for identification of organic sulfur
compounds – gas chromatography on columns with ionic liquid as the active phase.
Keywords: ionic liquid, aromatic sulfur compound, electrochemical synthesis, extraction
DOI: 10.1134/S1070363216020146
Nowadays, the interest to ionic liquids has rapidly
emerged. Ionic liquids have been recognized for a
number of attractive advantages: they are practically
non-volatile, non-flammable, readily dissolve many
inorganic, organic, and organometallic compounds
(involving gases), they have been used as efficient
extracting agent that can be easily regenerated and
reused [1]. The ionic liquids with imidazolium
[R1R2Im]+ and pyridinium [R1R2Py]+ cations are
widely used. Most of them are highly stable and possess
lyophilic properties over a wide range of temperatures.
The possibility to vary cations and anions allows
designing of ionic liquids for specific tasks. Methyl
and n-butyl groups are most often used as the
hydrocarbon substituents R1 and R2. The anionic part
can contain different anions: Cl–, Br–, I–, PF6–, BF4–, etc.
studies. Investigation of the process of electrochemical
decomposition of [1-Bu-3-MeR1Im]+Х– (Х = PF6, BF4)
has shown that these ionic liquids are not always inert
at a carbon glass electrode, but the use of tungsten
electrode improves their stability [3].
The topicality of the search for novel methods of
synthesis of aromatic sulfur compounds (thiophenols,
thiocresols, aromatic sulfides, disulfides, etc.) is
mainly due to their valuable properties. These com-
pounds, possess antioxidant properties; on top of that,
they are precursors in the synthesis of organometallic
compounds exhibiting proliferative properties [4].
Aromatic sulfur compounds have been used for metal
protection against corrosion, as antimicrobial drugs,
photosensibilizers in color and IR photography,
accelerators of vulcanization, and inhibitors of rubber
aging [5, 6]. Among the studies on transformations of
sulfur compounds in the pyridinium and imidazolium
ionic liquids medium, the report [7] on transformation
of benzylthiol catalyzed by cobalt phthalocyanine in
the medium of imidazolium ionic liquid, leading to the
corresponding disulfide in 95% yield, is remarkable.
The use of ecologically friendly reagents is one of
the basic principles of “green” chemistry [2]. The
electrochemical synthesis is practically impossible in
the absence of solvent; ionic liquids can be used as the
alternative reaction medium, since they are electric
conductors. Being electrochemically stable, they allow
performing of electrochemical experiments over a
wide range of potentials. Physico-chemical properties
of ionic liquids and the nature of the used electrodes
are important points to be considered in electrochemical
In this work we studied the transformations of
aliphatic and aromatic sulfur compounds in the medium
of ionic liquids in order to prepare sulfur compounds
valuable for medicine.
291

292
OKHLOBYSTINA et al.
value of ΔЕ allow to register the redox stages for thiols
I, А
[Еap(C4H9SH) = 1.8 V] as well as for hardly oxidizable
substrates [Еap(С6Н5СН3) = 2.75 V] in the absence of
background electrolyte at room temperature (Fig. 1). A
weak peak at 1.55 V was also present on the voltam-
perogram, corresponding to the product of C4H9SH
dimerization (C4H9SSC4H9), pointing at the pathway of
sulfur compounds transformations in the ionic liquid
[8].
E, mV
Electrochemical studies with using the [1-Bu-3-
MeIm]Br salt were performed at 80°С, since at room
temperature the salt was crystalline. The low ΔЕ value
for [1-Bu-3-MeIm]Br did not allow for the electro-
chemical oxidation at potentials higher than 1.0 V
(oxidation of bromide ion). The use of [1-Bu-3-MeIm]Br
was possible only for easily oxidizable sulfur com-
pounds, like polysulfanes.
Fig. 1. Cyclic voltamperogram for electrochemical oxidation
of C4H9SH and С6H5CH3 in [1-Bu-3-MeIm]PF6 (v =
200 mV/s, Ag/AgCl, с = 5 × 10–3 mol/L, Pt electrode).
In order to optimize the conditions of the electro-
chemical transformations of sulfur compounds in ionic
liquids, we determined the electrochemical “window”
of ionic liquids stability, elucidated the effect of the
solvent on the viscosity and electrochemical sensi-
tivity, and found the optimal ionic liquid–organic
solvent ratio, temperature, and the material of working
electrode.
One of drawbacks of ionic liquids is their high
viscosity slowing down the transport of the reagents to
the substrates. The issue can be resolved by addition of
small amount of an aprotic solvent.
According to modern studies [9], ionic liquids
based on imidazolium salts exist in the form of
supramolecular polymeric structures with high degree
of self-organization. Mixing of ionic liquids with
different solvents affords the nanostructured materials
containing both polar and nonpolar domains [10].
Addition of organic solvent to an ionic liquid leads to
solvation of its cations and anions that results in en-
hanced dissociation of the liquid, decrease of its
viscosity, and increase of the conductivity. Even though
addition of a co-solvent slightly reduces the range of
the electrochemical “window” of the ionic liquid, the
decrease of its viscosity improves the conditions of
electrolysis and facilitates the transport of the charged
species in the electroconductive medium.
N-Butylpyridinium and 1-butyl-4-methylpyridinium
tetrafluoroborates {[1-BuPy]BF4 (1) and [1-Bu-4-MePy]·
BF4 (2)} as well as salts of 1-butyl-3-methyl-
imidazolium with BF4– (3), PF6– (4), and Br– (5) anions
were used as ionic liquids. C6H5CH2SH and n-C4H9SH
were selected as the model sulfur compounds.
The values of the electrochemical “window” (ΔЕ)
for the studied ionic liquids characterizing the dif-
ference between the anodic and cathodic potentials for
the redox processes are given in the table. As followed
from the obtained results, all the studied ionic liquids
except for [1-Bu-3-MeIm]Br exhibited a wide electro-
chemical stability range ΔЕ of 5–6 V. Electro-
conducting properties of ionic liquids and the high
In the present work, acetonitrile was used as the
aprotic solvent for viscosity modification. In order to
optimize the solvent–ionic liquid ratio, electrochemical
experiments using ferrocene as reference were performed.
Values of electrochemical “window” for ionic liquids 1–5
max
ox
max
red
Ionic liquid
E
, V
E
, V
ΔЕ, V
5.1
To increase the solubility of ferrocene in [Bu-3-
MeIm]BF4, the mixture was pre-heated to 80°С, and
then cooled to ambient. Addition of acetonitrile led to
the increase of electrochemical sensitivity of the
medium, making it less viscous and affecting the value
of the ferrocene oxidation current (Fig. 2). The highest
current of ferrocene oxidation was observed at the
ionic liquid–organic solvent ratio to 1 : 1.
[1-BuPy]BF4 (1)
3.5
–1.6
–1.7
–3.0
–3.5
–1.2
[1-Bu-4-MePy]BF4 (2)
[1-Bu-3-MeIm]BF4 (3)
[1-Bu-3-MeIm]PF6 (4)
[1-Bu-3-MeIm]Br (5)
3.7
3.0
3.4
1.0
5.4
6.0
6.7
2.2
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ELECTROCHEMICAL SYNTHESIS OF AROMATIC SULFUR COMPOUNDS
293
I, А
V, μL
1
4
3
2
5
2
1
Е, mV
ln k, μA
Fig. 2. Cyclic voltamperogram for ferrocene oxidation on
Pt electrode at room temperature in a [Bu-3-MeIm]BF4–
СН3CN mixture (v = 200 mV/s, Ag/AgCl, Pt electrode) at
[Bu-3-MeIm]BF4–СН3CN ratio: (1) 5 : 1, (2) 1.4 : 1, (3)
1.25 : 1, (4) 1.1 : 1, and (5) 1 : 1.
Fig. 3. Exponential dependence of the ferrocene oxidation
current value on the content of acetonitrile in the ionic
liquid. Volume of [1-Bu-3-MeIm]BF4 = 1 mL; volume of
acetonitrile = 0.2, 0.4, 1 mL. (1) carbon glass electrode and
(2) Pt electrode.
Comparison of electrochemical properties of
ferrocene at different electrodes nature showed that the
highest current value was reached when using
platinum electrode. In the case of carbon glass elec-
trode, the value of the ferrocene oxidation current was
almost twice lower (Fig. 3).
Electrolysis of C4H9SH in the [1-Bu-3-MeIm]BF4
medium in the presence of toluene led to the
appearance of a new peak of oxidation at the potential
of 2.4 V (Fig. 4). The values of current for the peaks of
oxidation of the substrate and the reagent during
electrolysis decreased, indicating their consumption
during the electrochemical reaction. The yield with
respect to current was of 38%. The products of
electrolysis were identified by electrochemical method,
the IR spectroscopy data, and chromatography on
capillary column with ionic liquid.
Hence, the optimal conditions for carrying out the
experiments on electrochemical transformations of
sulfur compounds in ionic liquids were found.
Further on, those conditions were used to study the
transformations of benzylthiol C6H5CH2SH and
butanethiol C4H9SH in the presence of toluene. The
electrolysis of C6H5CH2SH at the potential of 2.0 V in
the 1-BuPy]BF4–СН3CN 1 : 1 mixture on Pt electrode
at room temperature resulted in the formation of the
sulfide, dibenzyl and nanosized sulfur, on top of the
disulfide (Еap = 1.65 V) (Scheme 1).
Scheme 1.
+
H2C
SH
H2C SH
H2C
S
−e
−H+
The major product of the electrolysis was the
disulfide formed in 95% yield. In the presence of air
oxygen, the formed disulfides were further oxidized
into the corresponding thiolsulfonates, which are
biologically active compounds promising for prepara-
tion of drugs exhibiting anti-neurotic, anti-asthmatic,
and anti-allergenic activity [11] (Scheme 2).
S
S
H2C
CH2
−S
−S
S
We have earlier shown that single-electron oxida-
tion of thiols in organic aprotic solvents leads to
formation of cation-radical species, being deprotonated
to formation the radicals [12].
CH2 CH2
H2C
CH2
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 86 No. 2 2016

294
OKHLOBYSTINA et al.
synthesis of aromatic sulfur compounds could be
I, А
represented by the Scheme 3.
The scheme describes the mechanism of interaction
of a model fuel (based on heptane and butanethiol)
with ionic liquid and further involvement of the
extracted sulfur impurities in the organic synthesis.
In summary, the electrochemical properties of the
pyridinium and 1-butyl-3-methylimidazolium salts as
well as electrochemical transformations of C4H9SH
and C6H5CH2SH in the ionic liquid medium [1-Bu-3-
MeIm]PF6, have been investigated in the present work,
and the electrolysis of butanethiol in [1-Bu-3-MeIm]
BF4 was performed in the presence of toluene.
Е, V
EXPERIMENTAL
Fig. 4. Cyclic voltamperograms for electrolysis of C4H9SH
in the presence of toluene in the medium [BuPy]BF4–
СН3CN (1 : 1) (v = 200 mV/s, Ag/AgCl, с = 5 × 10–3 mol/L,
Pt electrode). (1) before electrolysis, (2) after 7 min, and
(3) after 17 min.
Compound 1 was prepared via the procedure
described in [14, 15]; compounds 2–5 were com-
mercial products (Sigma Aldrich).
Preparative electrolysis was performed on
stationary platinum planar electrodes (700 mm2) in a
diaphragm-free three-electrode 100 mL cell. The
oxidation potentials were measured using the method
of cyclic voltammetry in a three-electrode cell system
using an Ametek VersaSTAT 3 potentiostat with a
working Pt electrode (S = 3.14 mm2), auxiliary Pt
electrode (S = 70 mm2), and reference Ag/AgCl/KСl
electrode, equipped with with waterproof diaphragm.
Similar reactions were slower in acetonitrile or
methylene chloride in the presence of background
electrolyte, so that 75% conversion of the thiol was
attained only after 1–2 h of electrolysis. The use of
ionic liquids reduced the time of electrolysis by
approximately 3 times. The observed acceleration of
the reaction was caused by the ability of the ionic
liquid to distribute the compound uniformly over the
whole volume, coordinating the reagent in the position
favorable for the reaction. Unlike polar molecular
solvents, ionic liquids provide a unique micro-
surrounding for the compounds, preserving their high
catalytic activity [13].
Gas chromatography analysis of the sulfur com-
pounds was performed using a Crystal Lux 4000
chromatograph equipped with a flame photometric
detector and an SLB-IL111 capillary column (15 m ×
0.1 mm, 0.08 μm) using 1,5-di(2,3-dimethylimida-
zolium)pentane-bis(trifluoromethylsulfonyl)imide as
the active phase (ionic liquid). Ionic liquids were
regenerated by stepwise or fractional re-extraction
The diversity of properties of ionic liquids allows
elaboration of the combined processes including the
extraction and subsequent reactions of the extracted
sulfur-containing components with organic substrates.
Taking into account the examined electrochemical
reactions, the proposed combined process of the
Scheme 3.
C4H9SH
C7H16 + C4H9SH
Model system
Ionic liquid [BuPy]BF4
Med: mediater
Scheme 2.
O
−e
C4H9SH
CH3
+ H+
C4H9S
S
S
S
*
S
H2C
CH2
H2C
CH2
CH3
[O]
+ C4H9S
SC4H9
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ELECTROCHEMICAL SYNTHESIS OF AROMATIC SULFUR COMPOUNDS
295
Compounds), Rostov-on-Don: Sib. Otd. Ross. Akad.
Nauk, 2009.
using 10-fold volume of acetonitrile at 70°C and
70 mBar pressure. The time of single extraction cycle
was 1–2 min.
6. Plotnikov, V.G. and Efimov, А.А., Russ. Chem. Rev.,
1990, vol. 59, no. 8, p. 792. 10.1070/
RC1990v059n08ABEH003555
ACKNOWLEDGMENTS
7. Dyson, P.J., Metal Catalyzed Reactions in Ionic Liquids,
Amsterdam: Springer, 2005.
This work was financially supported by the Russian
Foundation for Basic Research (grant no. 14-03-31930
mol_а) and the President of RF (grant no. MK-
2943.2015.3).
8. WO Patent 1999US28761, 1999.
9. Tarasova, N.P., Smetannikova, Yu.V., and Zanin, A.A.,
Russ. Chem. Rev., 2010, vol. 79, no. 6, p. 463. DOI:
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