Synthesis and Properties of Hydroxyl-Containing Ionic Liquids

Article abstract of DOI:10.1134/S1070428018010153

Hydroxyl-containing ionic liquids were synthesized by quaternization of 1,2-dimethyl-1H-imidazole, N-methylpyrrolidine, and pyridine with 2-chloroethanol or 6-chlorohexan-1-ol, followed by exchange of chloride ion for bis(trifluoromethanesulfonyl)azanide, and their properties were studied.

Full text of DOI:10.1134/S1070428018010153

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2018, Vol. 54, No. 1, pp. 143–145. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © V.G. Krasovskiy, E.A. Chernikova, L.M. Glukhov, G.I. Kapustin, A.A. Koroteev, L.M. Kustov, 2018, published in Zhurnal
Organicheskoi Khimii, 2018, Vol. 54, No. 1, pp. 142–144.
SHORT
COMMUNICATIONS
Synthesis and Properties of Hydroxyl-Containing Ionic Liquids
V. G. Krasovskiya,* E. A. Chernikovaa, L. M. Glukhova, G. I. Kapustina,
A. A. Koroteevb, and L. M. Kustova
a Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences,
Leninskii pr. 47, Moscow, 119991 Russia
*e-mail: miyusha@mail.ru
b Moscow Aviation Institute, Volokolamskoe shosse 4, Moscow, 125993 Russia
Received August 30, 2017
Abstract—Hydroxyl-containing ionic liquids were synthesized by quaternization of 1,2-dimethyl-1H-imid-
azole, N-methylpyrrolidine, and pyridine with 2-chloroethanol or 6-chlorohexan-1-ol, followed by exchange of
chloride ion for bis(trifluoromethanesulfonyl)azanide, and their properties were studied.
DOI: 10.1134/S1070428018010153
Ionic liquids (ILs) are organic quaternary salts
melting in the range from room temperature to 100°C
[1, 2]. Specific properties of ionic liquids, such as high
polarity, low saturated vapor pressure, high thermal
stability, and low viscosity, have determined their wide
application as polar media in organic synthesis [3, 4],
catalysis [5, 6], and electrochemistry [7–9]. The use of
ILs in various chemical processes conforms to the
main “green chemistry” principles. Herein we describe
the synthesis of ionic liquids whose volatility was
reduced by an order of magnitude via introduction of
polar groups capable of being involved in additional
intermolecular interactions.
Quaternization of 1,2-dimethyl-1H-imidazole, N-meth-
ylpyrrolidine and pyridine with 2-chloroethanol and
6-chlorohexan-1-ol afforded the corresponding chlo-
rides. Regardless of the side chain length, the products
were solids under normal conditions. In order to obtain
ILs 1–4, the chloride ion was replaced by bulky bis-
(trifluoromethanesulfonyl)azanide ion. The latter was
selected due to its high thermal stability and the ability
to ensure low viscosity.
The thermal stability of ILs 1–4 was studied by
thermogravimetric analysis. Thermal decomposition of
quaternary ammonium salts with liberation of tertiary
amines follows two paths, retro-Menshutkin reaction
and Hofmann elimination [10]. Thermal decomposi-
tion of imidazolium cations and other cyclic ammo-
nium ions is also accompanied by liberation of amine
The simplest and most convenient method for the
preparation of ionic liquids is based on alkylation of
nitrogen-containing compounds with alkyl halides.
OH
( )n
OH
( )n
Cl(CH2)nOH
LiN(SO2CF3)2
–LiCl
N
N
N
N
N
N
Me
Me
Me
Cl–
(CF3SO2)2N–
Me
N
Me
N
Me
1, 2
OH
(CF3SO2)2N–
OH
Cl–
ClCH2CH2OH
ClCH2CH2OH
LiN(SO2CF3)2
–LiCl
Me
N
Me
Me
3
Cl–
LiN(SO2CF3)2
–LiCl
(CF3SO2)2N–
N
N
N
OH
OH
4
143

KRASOVSKIY et al.
144
via cleavage of the aromatic ring by the anion [11, 12].
The presence of a hydroxy group usually reduces
thermal stability. However, all ILs 1–4 were stable up
to 390–400°C (Table 1); i.e., their thermal stability was
similar to the thermal stability of analogous ILs
containing no hydroxy group [11, 13]. This may be
rationalized by reduced reactivity of OH groups in
thermal decomposition processes due to their interac-
tions with polar fragments of ionic liquids, cations and
anions. The decomposition temperature of pyrrolidine-
based IL (3) almost did not differ from those of ILs 1,
2, and 4 based on imidazole and pyridine (Table 1),
though cations derived from aromatic compounds are
known to be more stable [10, 14].
action of the OH group with charged structural frag-
ments of the IL due to conformational factors, which
leads to increased viscosity. The temperature depen-
dences of the viscosities of hydroxyl-containing ILs, as
well as of other ionic liquids, are well described by the
Vogel–Tamman–Fulcher equation [16] for melts.
The volatility of ILs 1–4 was evaluated using
a McBain quartz spring balance [17]. The vapor pres-
sure increases in the series Pyr (3) < Py (4) < Im (1)
for ILs with a 2-hydroxyethyl group in the cation and
is determined by ion pair interactions. The volatilities
of imidazolium derivatives 1 and 2 differ by a factor
of 8, which may be due not only to increase of the
molecular weight but also enhanced intermolecular
interactions facilitated by increased length of the sub-
stituent. The volatility of ILs containing a hydroxy
group in the cation is lower by more than an order of
magnitude than that of analogous ionic liquids having
no hydroxy group (30–50 mg cm–2 h) [18].
The decomposition point determines the upper limit
of the working temperature range of a ionic liquid,
while the lower limit is determined by its crystalliza-
tion temperature. The data on the aggregate state of
ionic liquids at temperatures below ambient were
obtained by differential scanning calorimetry (DSC,
Table 1). For ILs 1, 2, and 4 with aromatic cations we
succeeded in measuring experimentally only glass-
transition temperature (ca. –70°C). The state of IL 3
with an aliphatic cation was characterized by two
melting peaks at –21 and 14°C. This ionic liquid is
likely to form a mesophase in addition to the crys-
talline phase [15].
Ionic liquids 1–4 have a fairly high density (1.5–
1.6 g/mL; Table 1). The presence of a hexyl substituent
in the cation of 2 is likely to be responsible for the
lower density of this ionic liquid.
Thus, hydroxyl-containing ionic liquids are charac-
terized by low viscosity, high thermal stability, and low
volatility which is lower by an order of magnitude than
the volatility of classical (i.e., containing no OH
groups) ionic liquids.
The kinematic viscosity of ILs 1–4 was measured
with an Ostwald viscometer (Table 1). All ILs had
a viscosity lower than 200 cSt at 30°C, and the
viscosity increased in the series 4 < 3 < 1 for ILs with
the same hydroxyl-containing substituent in the cation.
Change of the length of the side chain in imidazole
derivatives from n = 2 (1) to n = 6 (2) increases the
viscosity almost 2.5 times. This is difficult to ration-
alize taking into account only increase of the molecular
weight of IL due to elongation of the substituent.
Presumably, the presence of a 6-hydroxyhexyl substit-
uent in the cation favors stronger intermolecular inter-
Compounds 1-4 (general procedure). Quaterniza-
tion of 1,2-dimethyl-1H-imidazole, 1-methylpyrroli-
dine, and pyridine with an equimolar amount of
2-chloroethanol or 6-chlorohexan-1-ol was carried out
in acetonitrile (50% solution) under reflux for 48 h.
A 50% solution of lithium bis(trifluoromethanesul-
fonyl)azanide (1.1 equiv) in acetonitrile was then
added, and the mixture was stirred for 30 min. The
solvent was distilled off under reduced pressure, and
the residue was washed with water to remove lithium
salts. The product was dried under reduced pressure at
100°C for 10 h.
Table 1. Properties of hydroxyl-containing ionic liquids
1-(2-Hydroxyethyl)-2,3-dimethylimidazolium
Volatility
Ionic tglass, tdecomp, Viscosity
Density at
25°C, g/mL
(220°C),a
bis(trifluoromethanesulfonyl)azanide (1). Yield
1
liquid °C
°C at 30°C, cSt
mg cm–2 h–1
91%. H NMR spectrum, δ, ppm: 2.58 s (3H, 2-CH3),
3.70 m (2H, CH2O), 3.76 s (3H, 3-CH3), 4.18 m (2H,
CH2N), 5.08 br.s (1H, OH), 7.60 m (2H, 4-H, 5-H).
13C NMR spectrum, δC, ppm: 9.65, 34.90, 50.71,
60.04, 113.53, 117.79, 121.61, 122.05, 122.48, 126.31.
Found, %: C 25.57; H 3.04; F 27.15; N 10.01; S 15.05.
C9H13F6N3O5S2. Calculated, %: C 25.66; H 3.11;
F 27.05; N 9.97; S 15.22.
1
2
3
4
–68 431
084.5
8.01
1.07
2.93
4.20
1.542
1.416
1.520
1.592
–63 401
–b14b 402
186.4
052.1
051.0
–71 387
a
In a vacuum (~0.013 Pa).
Melting point for the highest temperature peak.
b
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 54 No. 1 2018

SYNTHESIS AND PROPERTIES OF HYDROXYL-CONTAINING IONIC LIQUIDS
145
spring was measured with an accuracy of ±0.02 mm
using a KM-8 cathetometer.
1-(6-Hydroxyhexyl)-2,3-dimethylimidazolium
bis(trifluoromethanesulfonyl)azanide (2). Yield
1
91%. H NMR spectrum, δ, ppm: 1.29 m (4H, CH2),
This study was performed under financial support
by the Russian Science Foundation (project no. 14-
19-00503).
1.39 m (2H, CH2), 1.70 m (2H, CH2), 2.57 s (3H,
2-CH3), 3.38 m (2H, CH2O), 3.74 s (3H, 1-CH3),
4.09 m (2H, CH2N), 4.35 br.s (1H, OH), 7.62 m (2H,
4-H, 5-H). 13C NMR spectrum, δC, ppm: 9.25, 25.32,
25.81, 29.55, 32.61, 34.85, 48.00, 61.01, 113.52,
117.78, 121.13, 122.04, 122.60, 126.31, 144.49.
Found, %: C 32.47; H 4.28; F 23.94; N 9.01; S 13.48.
C13H21F6N3O5S2. Calculated, %: C 32.70; H 4.43;
F 23.88; N 8.80; S 13.43.
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