Thermal stability of 1,3-disubstituted imidazolium tetrachloroferrates, magnetic ionic liquids

Article abstract of DOI:10.1134/S1070427211070068

Thermal stability of synthesized magnetic liquids, tetrachloroferrates of 1,3-disubstitued derivatives of imidazolium, was examined. An effect of the cation structure on thermal stability of ionic liquids was considered and a mechanism of thermal destruction was suggested.

Full text of DOI:10.1134/S1070427211070068

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2011, Vol. 84, No. 7, pp. 1158−1164. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © O.E. Zhuravlev, N.V. Verolainen, L.I. Voronchikhina, 2011, published in Zhurnal Prikladnoi Khimii, 2011, Vol. 84, No. 7,
pp. 1086−1092.
PHYSICOCHEMICAL STUDIES
OF SYSTEMS AND PROCESSES
Thermal Stability of 1,3-Disubstituted Imidazolium
Tetrachloroferrates, Magnetic Ionic Liquids
O. E. Zhuravlev, N. V. Verolainen, and L. I. Voronchikhina
Tver State University, Tver, Russia
Received August 20, 2010
Abstract—Thermal stability of synthesized magnetic liquids, tetrachloroferrates of 1,3-disubstitued derivatives of
imidazolium, was examined. An effect of the cation structure on thermal stability of ionic liquids was considered
and a mechanism of thermal destruction was suggested.
DOI: 10.1134/S1070427211070068
In recent years a number of reviews, publications, and
patents on various aspects of production and application
of ionic liquids (IL) [1] grows. The ionic liquids are
substances that are fluids at temperatures below 100°C
and consist of bulk organic cations and various organic
or inorganic anions. Ionic liquids are used as catalyst
media and solvents.
pyridine salts is much more stable.
Knowledge of the thermal stability of MILs enables
determination of a temperature range, in which they can
be used without any special precautions.
In the paper we examined the thermal stability
of synthesized new magnetic ionic liquids: tetra-
chloroferrates, 1,3-disubstituted derivatives of imid-
azolium.
An anion nature exerts a significant influence on IL
properties: melting temperature, viscosity, density. If
halides of transition metals (Fe, Co, Ni, Mn, etc.) are
used in the ionic liquid as anion, then such ILs respond
to magnetic field (they possess magnetic susceptibility).
This new feature of IL [2] enabled isolation of similar
compounds into group “magnetic ionic liquids”
(MIL) and based on 1-butyl-3-methylimidazolium
tetrachloroferrate their magnetic susceptibility was
studied. Liquids under study are paramagnetic owing to
the presence of stable tetrahedral anion FeCl4–.
EXPERIMENTAL
The spectra in the visible region were recorded on
a spectrophotometer SF-56 in a solution of acetone,
and IR spectra, on a Fourier spectrometer “Bruker”
Equinox 55 between two plates. Data of a differential
thermogravimetry was obtained on a device Q-1500 of
MOM company (Hungary), the temperature range was
20–500°C, heating rate was 5°C min–1. Weight loss
curves were recorded on an instrument “Perkin-Elmer”
Pyris TGA, heating rate was 5 deg min–1, an air flow
was 20 ml min–1.
Depending on the nature of the cation and anion ILs
have different thermal stability. ILs can be decomposed
under an action of other substances such as protic
solvents, e.g., water which readily decomposes ionic
liquids, and of high temperatures. In the latter case the
decomposition temperature is determined by the nature
of the organic cation. The alkylammonium salts are
least stable and can be subjected to transalkylation and
dealkylation by heating to 150°C, while imidazole and
Firstly based on the imidazole we synthesized
1-(β-cyanoethyl) imidazole by a nucleophilic addition of
acrylonitrile to the imidazole according to the scheme.
In a three-necked flask equipped with a stirrer, reflux
condenser and dropping funnel 0.5 mol of imidazole
dissolved in 50 ml of acetone were placed. Upon heating
1158

THERMAL STABILITY OF 1,3-DISUBSTITUTED IMIDAZOLIUM TETRACHLOROFERRATES
Scheme of the synthesis of imidazole and magnetic ionic liquids
1159
R = CH2=CH–CN, CH3; R' = C4H9, CH2C6H5, CH2CH2CH2CN, CH2CH=CH2, CH2COOC2H5.
and stirring 0.5 mole of acrylonitrile dissolved in 50 ml
of acetone were added. The synthesis was carried out
for 7 hours Acetone was distilled off. The resulting
yellowish liquid was purified by distillation (bp =
180°C). The compound was characterized according to
IR spectroscopy and elemental analysis.
imidazole (commercial product) quaternary salts of
imidazolium were obtained by quaternization reaction
with various agents (Table 1).
In a three-necked flask equipped with a stirrer,
reflux condenser and dropping funnel 0.2 mole of
1-substituted imidazole dissolved in 50 ml, and 0.2 mole
of a quaternizing agent were placed. Depending on the
Based on 1-(β-cyanoethyl)imidazole and 1-methyl-
Table 1. Yield and physicochemical constants of 1,3-disubstituted imidasolium chlorides
Found, %
Calculated, %
Compd.
no.
R
R'
M, g mol–1
Yield, %
mp, °С
C
H
1
CH3
CH3
C4H9
CH2C6H5
174.5
208.7
185.7
158.6
213.7
243.7
85
91
78
82
55
88
66–68
–23
54.9
55.0
8.6
8.7
2
3
4
5
6
63.1
63.3
6.2
6.3
CH3
CH2CH2CH2CN
CH2CH=CH2
C4H9
50–52
Liquid
''
51.6
51.8
6.4
6.5
CH3
52.8
53.0
7.1
7.0
CH2CH2CN
CH2CH2CN
56.3
56.2
7.7
7.6
CH2COOC2H5
40–42
49.4
49.3
5.9
5.8
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 7 2011

1160
ZHURAVLEV et al.
structure of the quaternizing agent a synthesis time was
ranged from 0.5 to 10 h. The yielded white crystalline
substances were filtered, washed with dry cold ether and
recrystallized from an acetone–ethyl acetate mixture of
1 : 2 ratio, dried under a vacuum for 1 day. All salts
synthesized were crystalline substances of white or
cream color.
acetonitrile, ethanol). Physicochemical constants
tetrachloroferrates of 1,3-disubstituted imidazolium are
given in Table. 2.
Hydrated tetrachloroferric acid HFeCl4·2H2O.
FeCl3·6H2O (1 mole) was dissolved in 30 ml of 36%
HCl and extracted by ether in the form HFeCl4·2H2O.
Ether was evaporated under a vacuum. The product was
an amber-yellow crystals (mp = 46°C).
Magnetic ionic liquids were prepared by mixing hot
ethanolicsolutionsofferricchloride(III)andimidazolium
chlorides. The mixture was heated with constant stirring
for 10–15 minutes. Ethanol was evaporated under
a vacuum. Obtained MILs were washed with cold dry
ether and dried under a vacuum over P2O5 for a day. The
composition and structure of MILs were confirmed by
elemental analysis, IR spectroscopy and spectroscopy in
the visible region (Fig. 1).
The spectra in the visible region indicate the presence
in the composition of MIL of particle [FeCl4–] in the form
of the anion. Spectra of MIL in the visible spectrum are
comparable with HFeCl4·2H2O (Fig. 1) and have three
characteristic bands of the ion [FeCl4–] at 534, 619,
688 nm, that agrees with published data for [FeCl4–]
[3]. Similar spectra for MIL based on derivatives of
imidazolium and FeCl3 are presented in [4].
Synthesized magnetic ionic liquids were represented
a painted liquid of green or brown colors. They were
hydrophilic, soluble in water (they are highly hydrolyzed
since anion FeCl4– is unstable in aqueous solution)
and other polar solvents (dichloromethane, acetone,
In the IR spectra of all examined compounds there
exist absorption bands of imidazole ring (νС=С ring
=
=
νС–H
= 3100–3150; νas ring = 1460, 1165; νamide III
625 acrmom–1). Moreover, in the spectra of the derivatives
there are absorption bands of functional groups (Table 3).
All synthesized tetrachloroferrates of substituted im-
idazolium give a response to the magnetic field. Visual
observations of the system oil–MIL confirm that use of
a small permanent magnet (with magnetic field 0.2 T) is
sufficient to observe the magnetic attraction of the ionic
liquid in the cell (l = 1 cm). Figure 2 shows snapshots
visualizing the response of the synthesized 3-benzyl-
1-methylimidazole tetrachloroferrate to the magnetic
field. Oil was added to the system for better visibility.
Fig. 1. Absorption spectra in the visible region of HFeCl4·2H2O
(7) and of several ionic liquids. (D) Absorption (rel. units), (λ)
wavelength (nm). (1–3) Compounds numbers in Table 2.
(a)
(b)
Fig. 2. Action of a constant magnet on the ionic liquid. (a)
Bottom layer is an ionic liquid, upper layer is oil; (b) under the
action of the constant magnet on system (a).
Fig. 3. TG curves of 1,3-disubstituted imidasolium chlorides.
(Δm) the weight loss, %; (T) temperature, °С; the same for
Figs. 4, 5. (1–3) Compounds numbers in Table 1.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 7 2011

THERMAL STABILITY OF 1,3-DISUBSTITUTED IMIDAZOLIUM TETRACHLOROFERRATES
1161
Table 2. Yield and physicochemical constants of 1,3-disubstituted imidasolium chlorides
Found, %
Calculated, %
Compd.
no.
Density ρ,
g cm–3
R
R'
M, g mol–1
Appearance
N
Fe
1
2
CH3
CH3
C4H9
336.7
370.8
Greenish liquid
1.40
1.39
8.2
8.3
16.4
16.6
CH2C6H5
Greenish-brown
liquid
7.5
7.6
14.8
15.0
3
4
CH3
CH3
CH2CH2CH2CN
CH2CH=CH2
347.7
321.0
The same
1.45
1.46
12.2
12.1
16.0
16.1
Brownish liquid
8.8
8.7
17.3
17.4
5
6
CH2CH2CN
CH2CH2CN
C4H9
375.9
405.9
Brown liquid
The same
1.45
1.58
11.3
11.2
14.8
14.9
CH2COOC2H5
10.5
10.4
13.5
13.7
For comparison we have investigated the thermal
stability of the original chlorides of 1,3-disubstituted
imidazolium (Fig. 3). It is evident that the nature of
the processes occurring during heating is similar: to
150–170°C practically all the salts are stable, then rapid
weight loss occurs in one stage at a constant speed and
at a temperature of 250–270°C the 100% weight loss
occurs.
In contrast to the original quaternary imidazolium
salts their tetrachloroferrates are characterized by
much greater heat stability and thermal decomposition
process is a multi-stage (Fig. 4). All investigated
tetrachloroferrates of substituted imidazolium are stable
up to 400°C and gradually decompose in a range of
440–500°C. In all cases formation of undecomposed
residue was found which was not identified.
Accounting for that all examined compounds in a salt
structure have the same anion [FeCl4 ], differences found
Table 3.
synthesized magnetic ionic liquids
Characteristics of vibration spectra of the
–
Compd. no.
(Table 2)
Characteristic absorption bands, cm–1
1
2
ν
= 2960, 2880
–(CH2)3–CH3
ν
ν
= 1650, 1580, 1400;
= 3030, 3010
C–С
C–С
arom
arom
3
4
5
ν
ν
–CH2– = 2958, 2877;
CN
= 2256
ν
ν
= 1647;
= 3093
C=С
=C–H
ν
ν
= 2955, 2889;
–(CH2)3–CH3
= 2262
CN
6
ν
ν
= 2958, 2877;
–CH2–
Fig. 4. TG curves of 1,3-disubstituted imidasolium tetra-
chloroferrates. (1–6) Compounds numbers in Table 2.
= 1747
–CH2–COOH
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 7 2011

1162
ZHURAVLEV et al.
in the thermal stability can be caused by differences in
the cation structure, namely, in the structure of a radical
in position 3 of imidazole ring. The thermal stability
of the hydtated tetrachloroferric acid was studied as
reference in examination of an effect of the cation
structure on the thermal stability of the synthesized
ionic liquids (Fig. 5).
liquids are a certain structure, not only ion pair, in which
apart from the Coulomb interactions, the formation of
hydrogen bonds is possible with protonated hydrogen
atoms in position 2 of the imidazole ring that leads to
a strengthening of the structure. Also the specific order
of arrangement and alternation of cations and anions
should be accounted for in the crystal lattice of the salt
[5–7].
It is seen that this compound is stable at 80°C, then
in a range of 80–200°C (Tmax = 165°C) there is a rapid
weight loss (50%) in one stage accompanied by an
endothermic effect; in a range of 280–400°C, the small
weight loss (6%); unburned residue (47%) remains after
500°C.
In the case of halides of 1,3-disubstituted imidazolium
the process of salts decomposition starts at 150–170°C
and is associated mainly with the decomposition of the
cation. According to literature data [8–11] the initial
stages of decomposition of the cation of the quaternary
salts start with the rupture of alkyl-N+, and suggest two
potentially possible reaction paths: β-elimination (E2) and
nucleophilic substitution at the quaternary nitrogen (SN):
When replacing a hydrogen atom in the organic
cation HFeCl4 by an organic cation the thermal stability
of salts significantly increased, practically all the
compounds are stable up to 400°C and lose 50% of the
weight at 440–480°C. In all cases, the undecomposed
residue is found at temperatures above 500°C.
These results indicate that the organic cation in the
ionic liquid composition participates in the formation of
special crystal structure of IL. Depending on its nature
the structure of a salt (domains, chains, lattices) may be
different, as reflected, inter alia, on the thermal stability.
Table 4 shows the thermal stability of the investigated
magnetic ionic liquids. It should be noted that the
investigated ionic liquids are characterized by multi-
stage process of decomposition, with the exception
of 1-methyl-3-butylimidazolium tetrachloroferrate
(compound 1), which is characterized by a rapid
decomposition in a single stage in a range of 400–500°C
at a constant speed. The reason for increasing the thermal
stability of these compounds should be considered as
a result of strong Coulomb interactions between the
cation and anion in comparison with an interaction of
halide anions. It should also be noted that the ionic
In the case of halides of a substituted imidazolium we
can assume that thermodestruction of the salts proceeds
by a mechanism of intramolecular β-elimination with
formation of a five center transition state [8]:
In this case, the anion Cl– acts as a nucleophile
and simultaneously a heterolytic rupture of the Cβ–H
and Cα–N+ occurs that takes place in the nucleophilic
β-elimination. In the case of halide ions, the transition
state is easily implemented, and, as a rule, chlorides (Cl–
0.181 Å) are less thermally stable in an order Cl–, Br–,
I–. When counterions are bulky, such as ClO4 (2.36 Å),
NO3– (2.53 Å), thermal stability of salts increases, due to
the fact that they hardly take three-dimensional structure
and impede the implementation of the transition state
[12–14]. Similar results were obtained when studying
the thermal stability of the quaternary ammonium
tetrafluoroborate [15]. Slowing of the reaction of
thermal decomposition of tetrafluoroborate is due
to destabilization of the transition state due to steric
repulsion of a more bulky anion. The presence in the
DTG
TG
Fig. 5. Thermogram of HFeCl4·2H2O in air.
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THERMAL STABILITY OF 1,3-DISUBSTITUTED IMIDAZOLIUM TETRACHLOROFERRATES
1163
Table 4. Thermal stability of tetrachloroferrates of substituted imidasolium
Weight loss, %, at T, °C
Residue after
800°C
Residue
Comnd.
no.
R
R'
T50, °C
Fecalc, %
200
300
400
500
%
1
2
3
4
5
6
7
CH3
CH3
C4H9
CH2C6H5
480
480
500
440
400
400
200
3
3
3
6
3
55
52
50
66
72
80
53
45
48
50
34
28
20
47
20
30
16
21
16
17
28
17
15
18
20
14
15
24
28
24
16
50
50
53
CH3
CH2CH2CH2CN
CH2CH=CH2
C4H9
0
0
CH3
0
0
CH2CH2CN
2
20
18
50
CH2CH2CN CH2COOC2H5
HFeCl4·2H2O
4
50
anion structure of the alkyl substituents leads primarily
to the decomposition of the anion [16]. In all cases, the
study of the thermal stability of 1,3-dialkylimidazolium
alkylsulfates we observed a gradual weight loss (7–
16 wt%) in a range of 50–350°C, followed by a sharp
loss of the weight at 350–400°C associated with
decomposition of the cation of the ionic liquids.
CONCLUSIONS
(1) The ionic liquids were synthesized. They
contained anion [FeCl4–] and various cations based on
1.3-disubstituted imidasolium.
(2) It was established that all studied magnetic
ionic liquids were stable up to 380–400°С, in air the
decomposition were carried out in several stages with
formation of the undecomposed residue.
In the case of the investigated 1,3-disubstituted
imidazolium tetrachloroferrates the presence in the
structure of the bulky anion [FeCl4–] (3.7 Å) even in
the larger extent complicates the implementation of
the transition state that leads to a significant increase
in the thermal stability compared to the initial halides.
1-Methyl-3-butylimidazoliumtetrachloroferrateshould
be considered as the most thermally stable among the
compounds (Table 4, compound 1), which up to 450°C
is stable and decomposes in one step. Compounds 2–4
are stable up to 450°C, their decomposition occurs in
two stages. It should be noted that for these compounds
we observed decomposition also after 500°C, and the
undecomposed residue after 800°C corresponds to the
theoretically calculated content of the iron in the salt.
Compounds 5, 6 in the series of investigated MILs are
the least stable; replacement of methyl group in the
structure of imidazole by β-cyanoethyl group leads to
a decrease in the thermal stability, the decomposition
of salt starts after 250°C, and the process is multi-stage
and corresponds to the gradual destruction of the bonds
C–N+.
(3) We demonstrated that the cation nature is of
a key factor in the thermal stability of the studied salts.
(4) It was established that the hydrated tetra-
chloroferric acid were thermally stable up to 80°С,
at 200°С it lost 50% of weight, the decomposition
proceeded in one stage. Replacement of hydrogen atom
by an organic cation leads to a significant increase in the
thermal stability of the compound.
ACKNOWLEDGMENTS
This work was supported by the Foundation for
Assistance to Small Innovative Enterprises in Science
and Technology, project no. 6972r/9585.
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