Physicochemical characterization of paramagnetic ionic liquids 1-vinyl-3-alkylimidazolium tetrahalogenidoferrate(III) [VRIM][FeCl mBr4 - M]

Article abstract of DOI:10.1002/poc.3291

Using microwave-assisted synthesis method, a series of paramagnetic ionic liquids comprising 1-vinyl-3-alkylimidazolium VRIM⁺ cation and tetrahalogenidoferrate (III) FeCl_mBr_{4 - m}^- anion were designed and synthesized. The structure was analyzed using ¹H NMR and Raman spectroscopy. Ultraviolet-visible absorption spectra, thermal stability, magnetic susceptibility, viscosity, ionic conductivity, and solubility were characterized. Results show that elongation of the alkyl chain leads to replacement of bromides with a small amount of chlorides in the anion, shifting of UV maximum absorption peaks to shorter wavelengths, reduction of ionic conductivity, and solubility in polar solvents, as well as increase in fluidity, magnetic susceptibility, and solubility in nonpolar solvents.

Full text of DOI:10.1002/poc.3291

Research Article
Received: 13 March 2013,
Revised: 9 December 2013,
Accepted: 12 January 2014,
Published online in Wiley Online Library: 13 February 2014
(wileyonlinelibrary.com) DOI: 10.1002/poc.3291
Physicochemical characterization of
paramagnetic ionic liquids 1-vinyl-3-
alkylimidazolium tetrahalogenidoferrate(III)
[
VRIM][FeCl Br ]
m 4 ꢀ m
a
a
a
a
a
Yimei Tang *, Xiaoling Hu , Ping Guan , Xiangping Lin and Xiaoqian Li
Using microwave-assisted synthesis method, a series of paramagnetic ionic liquids comprising 1-vinyl-3-alkylimidazolium
+
ꢀ
VRIM cation and tetrahalogenidoferrate (III) FeCl Br
anion were designed and synthesized. The structure was analyzed
m
4 ꢀ m
1
using H NMR and Raman spectroscopy. Ultraviolet–visible absorption spectra, thermal stability, magnetic susceptibility, vis-
cosity, ionic conductivity, and solubility were characterized. Results show that elongation of the alkyl chain leads to replace-
ment of bromides with a small amount of chlorides in the anion, shifting of UV maximum absorption peaks to shorter
wavelengths, reduction of ionic conductivity, and solubility in polar solvents, as well as increase in fluidity, magnetic suscep-
tibility, and solubility in nonpolar solvents. Copyright © 2014 John Wiley & Sons, Ltd.
Keywords: 1-vinyl-3-alkylimidazolium; magnetic ionic liquids; physicochemical properties; tetrahalogenidoferrate
in various polypyrrole and poly(N-methylpyrrole) nanostruc-
INTRODUCTION
[
36]
tures. In addition, interest in the polymeric forms of ILs has been
[
37,38]
A new class of solvents, which are often fluid at room tempera-
increasing.
Their potential as a new class of polymers lies in
ture and consist entirely of ionic species especially used in
their novel properties with improved mechanical durability and
dimensionality combined by polymerization. Therefore, a series
of 1-vinyl-3-alkylimidazolium tetrahalogenidoferrate (III) [VRIM]
[1–5]
[6,7]
[8–10]
chemical materials,
synthesis,
and separations,
has
elicited much attention. This interest is ascribed to their unique
material characteristics, such as low vapor pressure, high thermal
stability, high ionic conductivity, and wide electrochemical
[FeCl Br4 ꢀ m] (R= n-butyl, m = 2,3,4; R = n-pentyl, m = 3,4; R = n-
m
hexyl, m = 3,4) bifunctional materials with olefinic bonds and mag-
[
11–13]
stability window
; ability to dissolve organic, inorganic, and
netic groups were designed and synthesized.
[
12]
[15]
polymeric compounds ; and physicochemical properties that
vary extensively on structural composition (anion or cation).
Thus, these materials are labeled as “designer solvents” or com-
monly known as ionic liquids (ILs).
ILs are designer solvents, which implies that their properties
[
14]
can be varied to suit the requirements of a particular process. In
this study, through cation change, the relationships between struc-
ture and physicochemical properties (e.g. anionic structure, melt-
ing point, viscosity, density, and hydrophobicity) were discussed
in detail to obtain some parameters for their actual applications.
[
15]
Magnetic ILs (MILs), which comprise magnetic inorganic or
organic ions, are an exceptional subclass of ILs not only because
of their unique properties but also due to their magnetic proper-
ties. MILs containing magnetic inorganic anions, such as
III 4ꢀ
[16–20]
II 2ꢀ
[20,21]
II
3ꢀ [21]
(5 ꢀ x)ꢀ
Fe X (X = Cl, Br),
Mn X4 (X = Cl, Br),
Mn (Tf N) ,
2
MATERIALS AND METHODS
II 2ꢀ
[20,22,23]
III
Co X (X = Cl, NCS, NCSe, N(CN) ),
Dy (SCN) (H O)
8ꢀ x 2
4
2
x
[24]
III
3ꢀ,[20]
(
x= 0–2),
and Gd Cl6
, have been revealed consecutively
Materials
[
25]
[26]
since the first discovery in 2004.
Tilve et al.
studied the
The reagents used for the synthesis of ILs were 1-vinylimidazole (w> 0.990;
Alfa), C H Br (w > 0.980; Aladdin), C H Br (w > 0.970; Aladdin), C H13Br
successful function of the [BMIM]Cl/FeCl IL system as a solvent
3
4
9
5
11
6
and catalyst in stereo-controlled glycosidation of 3,4,6-tri-O-
(
3 2
w > 0.980; Aladdin), FeCl · 6H O (w > 0.990; Yaohua Reagent Factory,
[
27]
acetyl–D-glucal with different alcohols. Sun et al.
demon-
Tianjin, China), hydrochloric acid (w = 0.370; Third Chemical Reagent
strated the use of the [BMIM]Cl/FeCl system for the alkylation
Factory, Tianjin, China), acetonitrile (w > 0.995; Kemiou Reagent Tianjin,
3
of deuterated benzene with ethylene. Given their strong
response to external magnetic fields, MILs have been used as
raw materials to prepare magnetically responsive materials of
*
Correspondence to: Yimei Tang, School of Natural and Applied Science,
Northwestern Polytechnical University, The Key Laboratory of Space Applied
Physics and Chemistry, Ministry of Education, Xi’an 710072, China.
E-mail: tangymhkf@163.com
[
28]
single-walled carbon nanotubes,
electron paramagnetic res-
[
29]
onance spin probes in typical achiral diamagnetic ILs,
and
magnetic ion gels. MILs have also been applied in magnetic sep-
[30]
[31]
a Y. Tang, X. Hu, P. Guan, X. Lin, X. Li
aration of materials, in magnetic resonance imaging as contrast
School of Natural and Applied Science, Northwestern Polytechnical University,
The Key Laboratory of Space Applied Physics and Chemistry, Ministry of Education,
Xi’an 710072, China
[
32]
[33,34]
agents for medical diagnostics,
in flow batteries,
in poly
[
35]
(3,4-ethylenedioxythioxythiophere) nanosphere synthesis, and
J. Phys. Org. Chem. 2014, 27 498–503
Copyright © 2014 John Wiley & Sons, Ltd.

PHYSICOCHEMICAL CHARACTERIZATION OF PARAMAGNETIC IONIC LIQUIDS
China), and acetic ether (w > 0.995; Hedongquhongyan Reagent
Factory). All chemicals were of analytical grade and used without
any purification.
1-Vinyl-3-pentylimidazolium tetrahalogenidoferrate (III) [VPIM]
1
[
(
(
FeCl Br
]: H NMR (ppm, DMSO-d , δ):10.70 (s, 1H, CH), 7.77
m
4 ꢀ m 6
s, 1H, CH), 7.47–7.55 (dd, 2H, CH), 5.29–6.11 (m, 2H, CH
t, 2H, CH ), 1.95–2.04 (m, 2H, CH ), 1.19–1.35 (m, 4H, CH
.89 (t, 3H, CH ).
-Vinyl-3-hexylimidazolium tetrahalogenidoferrate (III) [VHIM]
2
), 4.43
2
2
2
),
0
3
Methods
1
1
Apparatus
[FeCl
(s, 1H, CH), 7.47–7.55 (dd, 2H, CH), 5.29–6.10 (m, 2H, CH
t, 2H, CH ), 1.92–2.04 (m, 2H, CH ), 1.20–1.33 (m, 4H, CH
.89 (t, 3H, CH
m
Br4 ꢀ m]: H NMR (ppm, DMSO-d
6
, δ):10.77 (s, 1H, CH), 7.73
), 4.42
),
1
2
The structures of the samples were characterized by H NMR using a
(
0
2
2
2
Bruker AVANCE 300 MHz spectrometer and by Raman spectroscopy
using inVia Raman Microscope (RENISHAW). The absorption spectra were
recorded on a Shimadzu UV-2550 spectrometer. Thermal analysis was
performed by thermogravimetric analysis (TGA) on Q1000DSC + LNCS +
FACEQ600 SDT (FA, USA). Magnetic susceptibility was verified using
3
)
1
The results of H NMR indicate that the cation structure for the
title substance is 1-vinyl-3-alkylimidazolium.
MPMS-XL-7 SQUID (USA) at 298.15 K. The viscosities η of [VRIM][FeCl
] were determined using a rotational viscometer. Ionic conductivity was
measured using conductivity meter DDS-307.
m 4
Br
Raman spectra
m
[39]
Figure 1 shows the Raman spectra of [VRIM][FeCl Br
strong band assigned to the totally symmetric stretching vibra-
tion of Fe–Cl bond is observed at ca. 331 cm in the spectra
of the three ILs, demonstrating that [VRIM][FeCl
butyl, n-pentyl and n-hexyl; m = 4) has FeCl . The peaks ob-
served at ca. 269 cm correspond to FeCl Br in the spectra of
]. The
4 ꢀ m
m
[
40]
ꢀ1
Synthesis of 1-vinyl-3-alkylimidazolium tetrahalogenidoferrate (III)
VRIM][FeCl
[
m
Br4 ꢀ m
]
m
Br4 ꢀ m] (R = n-
ꢀ
ꢀ
4
A mixture of 100 mmol bromoalkane and 100 mmol 1-vinylimidazole was
placed in a microwave reactor. After vigorous shaking for 2 min, the mix-
ture was microwave heated at 189 W for 40 s and then cooled to room
temperature naturally. This process was repeated a few times until the
mixture was translucent. The considerably viscous liquid was collected
by rinsing with ethyl acetate, after which it was concentrated using re-
duced pressure distillation. This liquid was vacuum dried at 303.15 K for
ꢀ
1
3
[
VRIM][FeCl Br
] (R = n-butyl, n-pentyl or n-hexyl; m = 3). A
m
4 ꢀ m
ꢀ
1
band located at about 243 cm is only observed in the spectra
of [VRIM][FeCl
Br4 ꢀ m] (R = n-butyl, m = 2), corresponding to
m
ꢀ
FeBr Cl2
2
.
Under identical reaction conditions, anionic differences in
Raman spectra can be explained using electron-donating and
volume effects. All chemical reactions turn toward a relatively
4
8 h and naturally cooled to room temperature. The final product was
1
verified to be 1-vinyl-3-alkylimidazolium bromide [VRIM]Br using
H
NMR and IR before subsequent use. The respective yields of [VBIM]Br,
[
VPIM]Br, and [VHIM]Br are 84.7%, 84.6%, and 82.6%.
One hundred millimoles of FeCl · 6H O and 100 mmol equivalent of
VRIM]Br were added in a flask that was stirred at 298.15 K. A yellow, vis-
3
2
[
cous liquid was formed after 8 h, which was obtained by rinsing the solu-
tion with hydrochloric acid for 6 s and concentrated using reduced
pressure distillation. The final product (Scheme 1) was further collected
by vacuum drying at 308.15 K for 48 h and then naturally cooled to room
temperature before it was analyzed. The yields (in weight) of [VBIM]FeX
VPIM]FeX , and [VHIM]FeX are 81.6%, 76.8%, and 82.3%, respectively.
4
,
[
4
4
RESULTS AND DISCUSSIONS
1
H NMR
1
-Vinyl-3-butylimidazolium tetrahalogenidoferrate (III) [VBIM]
1
[
FeCl
m
Br4 ꢀ m]: H NMR (ppm, DMSO-d
.83 (s, 1H, CH), 7.47–7.55 (dd, 2H, CH), 5.31–6.13 (m, 2H, CH
.42 (t, 2H, CH ), 1.91–2.04 (m, 2H, CH ), 1.23 (m, 2H, CH ), 0.96
).
6
, δ): 10.68 (s, 1H, CH),
7
4
2
),
2
2
2
Figure 1. Raman spectra of a, [VBIM][FeCl Br
m
4ꢀm
] (m = 2, 3, 4); b, [VPIM]
(t, 3H, CH
3
[FeCl
m
Br4ꢀm] (m = 3, 4); c, [VHIM][FeCl Br4ꢀm] (m = 3, 4)
m
Scheme 1. Formation of 1-vinyl-3-alkylimidazolium tetrahalogenidoferrate
J. Phys. Org. Chem. 2014, 27 498–503
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

Y. TANG ET AL.
stable state of resultants. Elongation of the alkyl chain increases
the electron-donating ability of alkyls, causing the electropositivity
of VRIM cation to successively decrease with the elongation of
energy of the excited levels is strengthened and the factor is
dominant in the observed spectra.
+
alkyl (n-butyl, n-pentyl, and n-hexyl) chain on cation. If the resul-
tants need to be stable, the anion must be closer to the cation
and balance positive and negative charges better. With the
TGA
The thermal stabilities of [VRIM][FeCl Br4 ꢀ m] (R = n-butyl, m = 2,
3, 4; R = n-pentyl, m = 3, 4; R = n-hexyl, m = 3, 4) in nitrogen atmo-
sphere were performed at a heating rate of 283 K · min . This
process was repeated until the weight of the sample remained
m
elongation of the alkyl chain to the 3-position of the imidazole
ꢀ
ꢀ1
ring, bromide should be replaced with chloride in FeCl
m
Br4 ꢀ m
to make the anion smaller and closer to the cation. The theoret-
ical analysis is in accordance with the Raman spectra results
constant. Figure 3 shows that [VRIM][FeCl Br4 ꢀ m] is stable up
m
(
Fig. 1, Supplemental Figs. 1 and 2). Thus, under identical reac-
to 573 K, and the loss of weight is sufficiently close to 0% below
573 K. This result indirectly indicates that the title substance has
high purity. The thermal decomposition process includes two
steps. The first one is rapid, and the weight losses are ca.
58.0%, 61.0%, and 57.0%, respectively. This degradation indi-
cates the dissociative nature of the decomposition process with-
out the participation of oxygen. The respective mass losses in the
second step are ca. 13.0%, 14.0%, and 15.0%, showing the
predominance of residual carbon oxidation. Thus, the anion for
ꢀ
tion conditions, [VBIM][FeCl
whereas [VPIM][FeCl
FeBr
m
Br4 ꢀ m] contains FeBr
2
Cl
2
anion,
m
Br4 ꢀ m] and [VHIM][FeCl
m
Br4 ꢀ m] have no
ꢀ
2
Cl
2
anion.
The MILs were obtained by rinsing with hydrochloric acid
for 6 s. A series of supplementation experiments (Supplemental
Data) shows that a portion of the Br content has been
substituted by Cl during rinsing and a thermodynamic equilib-
rium (rinsing with HCl for different durations) has been reached
upon comparison with the Raman spectra and MS (Supplemental
Figs. 1 and 2) of the solutions.
ꢀ
ꢀ
3
or FeBrCl . Based on this
[VRIM][FeCl
assumption, the weight percentage of the cation is calculated
Table 1). Experiments indicate that the weight losses in the first
m
Br4 ꢀ m] is either FeCl
4
(
decomposition are within the range of the above-calculated
Electronic absorption spectra
value, which reveals that the first step is cationic decomposi-
[
42]
ꢀ
[29]
Figure 2 shows the absorption spectra of [VRIM][FeCl Br
in
tion and the final residue may be FeCl Br
debris.
m
4 ꢀ m]
m
4 ꢀ m
acetonitrile. For [VRIM][FeCl Br
], the observed bands with
4 ꢀ m
m
maxima at ca. 361, 312, and 234 nm, which are similar to those
[
17]
Magnetism
of [EMIM][FeCl4],
are readily assigned to the lowest energy
6
6
charge transfer transition A → T of tetrahalogenidoferrate
III) . The maximum absorption peaks move into the direction
Magnetic susceptibility (χ ) exhibits a linear field dependency over
1
2
g
[41]
(
the applied magnetic field range of ꢀ10,000 to 10,000 Oe, which
of shorter wavelength upon elongation of the alkyl chain to
the 3-position of imidazole ring. Due to the replacement of a
portion of bromides with chlorides in FeCl Br
corresponds to paramagnetic behavior (Fig. 4). Most of the ILs
ꢀ
containing FeCl4 exhibit a paramagnetic temperature depen-
ꢀ
anion
dence of χ , with only small deviations from the Curie–Weiss law
m
4 ꢀ m
g
observed in the Raman spectra (Fig. 1), the ligand field strength
Δ) known by the spectrochemical sequence, which follows the
at low temperatures, thereby showing very weak antiferromagnetic
[
43]
(
interactions.
χ
g
of [VBIM][FeCl
is 33.3 × 10
36.9× 10 cm ·g , respectively. With the elongation of alkyl
chain in the imidazole ring, the χ of the title substance increases.
The χ is approximately constant for the three compounds, whose
relationship is given by χ =χ ×M, where M is molecular weight. χ
is expected to increase with the decrease of molecular weight, where
This
result demonstrates the increase in bromide substitution in
VRIM][FeCl
m
Br4 ꢀ m], [VPIM][FeCl
m
Br4 ꢀ m],
ꢀ
ꢀ
ꢀ
ꢀ
2ꢀ
ꢀ
ꢀ6
ꢀ6
order I < Br < Cl < F < H
2
O < C
2
O
4
< NH
3
< NO
2
< CN ,
and [VHIM][FeCl
m
ꢀ1
Br4 ꢀ m
]
,
36.8 × 10
,
and
ꢀ
6
3
increases. The transition energies are also expected to increase.
Thus, a blue shift of the absorption peak can be observed (Fig. 2).
Combining the results of Raman spectra, the covalency in Fe–X
g
g
(X = Cl or Br) bonds also decreases with the replacement (Br is
M
g
g
more nephelauxetic than Cl in the expected order), so the
M
[VBIM][FeClmBr4 ꢀ m] > M[VPIM][FeClmBr4 ꢀ m] > M[VHIM][FeClmBr4 ꢀ m].
[
m
Br4 ꢀ m] with the elongation of alkyl chain in the
imidazole ring based on Raman spectra results.
Assuming that the anion FeCl
ꢀ
4ꢀ
m
Br4 ꢀ m is either FeCl or
ꢀ
FeBrCl
3
, the effective magnetic moments (μeff) at 298.15 K are
to 5.28 μ for [VBIM]
for [VPIM][FeCl Br4 ꢀ m], and
Br4 ꢀ m]. These results agree
with the expected values from the S = 5/2 high-spin electronic
state of iron (III), where the spin-only value is 5.92 μ . Due to
estimated to be in the range 4.89 μ
FeCl Br4 ꢀ m], 5.64 μ to 5.98 μ
.76 μ to 6.09 μ for [VBIM][FeCl
B
B
[
m
B
B
m
5
B
B
m
B
the replacement of bromide with chloride, the slight increase
in μeff is explained in terms of the lower ligand field strength D
ꢀ
in FeCl Br
(here, Br appears before Cl in the
m
4 ꢀ m
spectrochemical series) as μeff = μ (1 ꢀ 2λ/D), where μ is the
0
0
spin-only value, and l is the spin–orbit coupling constant of the
tetrahalogenoferrate (III) anion. In addition, μeff for [VBIM]
[
FeCl Br
] is far above the spin-only value 5.92 μ because
m
4 ꢀ m
B
ꢀ
ꢀ
of the assumption that the anion FeCl Br
or FeBrCl3 . In fact, the anions for [VBIM][FeCl Br
FeCl4 , FeBrCl3 , and FeBr Cl2
is either FeCl4
Figure 2. Electronic absorption spectra of a, [VBIM][FeCl
, 4); b, [VPIM] [FeCl Br4 ꢀ m] (m = 3, 4); c, [VHIM] [FeCl Br4 ꢀ m] (m = 3, 4)
in acetonitrile
m
Br4 ꢀ m] (m = 2,
m
4 ꢀ m
ꢀ
3
m
m
m
] are
4 ꢀ m
ꢀ
ꢀ
ꢀ
.
2
wileyonlinelibrary.com/journal/poc
Copyright © 2014 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2014, 27 498–503

PHYSICOCHEMICAL CHARACTERIZATION OF PARAMAGNETIC IONIC LIQUIDS
Figure 3. TGA of (a) [VBIM][FeClmBr4 ꢀ m] (m = 2, 3, 4); (b) [VPIM][FeClmBr4 ꢀ m] (m = 3, 4); (c) [VHIM][FeClmBr4 ꢀ m] (m = 3, 4)
Viscosity
where η and κ are constants, and E (η) is the activation energy for
0 B a
viscous flow. E (η) is estimated from the slope of the best fit curve
a
Figure 5(a) shows the viscosities that follow the order [VPIM]
as E (η) = slope × κ . Due to hydrogen bond interaction and van
a
B
[
FeCl
m
Br4 ꢀ m] < [VHIM][FeCl
m
Br4 ꢀ m] < [VBIM][FeCl Br4 ꢀ m],
m
der Waals force, the respective activation energies E (η) for
a
with respective values of 89.0, 47.0, and 60.5 mPa · s at 298.15 K.
High viscosities for ILs are mainly derived from hydrogen bond
interaction and van der Waals force. Upon elongation of the
alkyl chain, hydrogen bond interaction and van der Waals force
[
VBIM][FeCl Br
], [VBIM][FeCl Br
], and [VBIM][FeCl Br4
m
4 ꢀ m
m
4 ꢀ m m
ꢀ
1
m] are 20.68, 23.89, and 22.64 kJ· mol
.
follow the order [VBIM][FeCl
VHIM][FeCl Br4ꢀ m] and [VBIM][FeCl
[VHIM][FeCl Br4ꢀ m]. Theoretical analysis and experimental
results show that the change in viscosity behavior given by
VPIM][FeCl Br4 ꢀ m] < [VHIM][FeCl Br4 ꢀ m] < [VBIM][FeCl
m
Br4 ꢀ m] > [VPIM][FeCl
m
m
Br4 ꢀ m] >
m
Ionic conductivity
[
<
m
Br4ꢀ m]< [VPIM][FeCl
Br4ꢀ m
]
Ionic conductivity (σ) varies with the cationic species, as ob-
served for viscosity. Figure 6(a) shows that σ increases with rising
temperature and decreases with elongating alkyl chain in the
m
[
m
m
m
Br4 ꢀ m]
cation. In particular, the respective σ values for [VBIM][FeCl Br4ꢀ m],
m
is caused by the combined effect of hydrogen bonding and van
der Waals force. As the temperature rises, ionic kinetic energy
increases, which enables the ion to overcome the effect of assem-
bly and hydrogen bonding interaction; the internal structure of ILs
[
44]
is similar to that of a macromolecule.
Arrhenius plot of η for [VRIM][FeCl Br4 ꢀ m] in the temperature
range of 293.15 K to 253.15 K. The trend of function ln(η) is almost
Figure 5(b) shows the
m
ꢀ
1
2
linear with 1000 · T , with all linear correlation coefficients R
greater than 0.99. The temperature dependence of η is given by
the Arrhenius equation
η ¼ η expðEaðηÞ=κBTÞ
0
Table 1. The cationic percentage of weight under the supposition
Anion
VBIM+ (w%)
VPIM+ (w%)
VHIM+ (w%)
4ꢀ
FeCl
FeBrCl
59.34
38.45
60.92
40.89
62.37
42.55
Figure 4. Relationship between the magnetization of [VRIM][FeCl
and the applied magnetic field. Experimental point: ■, [VBIM][FeCl
m
Br4 ꢀ m
]
a
3ꢀ
m
a
Br4
] (m = 2, 3, 4); ○, [VPIM][FeCl Br
4 ꢀ m
] (m = 3, 4); ▲, [VHIM]
ꢀ m
m
2 2
FeBr Cl
[
FeCl
m
Br4 ꢀ m] (m = 3, 4)
J. Phys. Org. Chem. 2014, 27 498–503
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

Y. TANG ET AL.
ꢀ1
Figure 5. Temperature dependence of viscosity for [VRIM][FeCl
m
Br4 ꢀ m], (a) η vs. T plot and (b) ln η vs. 1000 · T plot. Experimental point: ■, [VBIM]
Br4 ꢀ m] (m = 3, 4)
[
FeCl Br4 ꢀ m] (m = 2, 3, 4); ●, [VPIM][FeCl Br4 ꢀ m] (m = 3, 4); ▲, [VHIM][FeCl
m
m
m
ꢀ1
Figure 6. Temperature dependence of ionic conductivity for [VRIM][FeCl
m
Br4 ꢀ m], (a) σ vs. T plot and (b) lnσ vs.1000 · T plot. Experimental point: ■,
m
[VBIM][FeCl Br4 ꢀ m] (m = 2, 3, 4); ●, [VPIM][FeCl
m
Br4 ꢀ m] (m = 3, 4); ▲, [VHIM][FeCl Br4 ꢀ m] (m = 3, 4)
m
[
3
VPIM][FeCl Br
], and [VHIM][FeCl Br
] are 5.1, 3.8, and
best fit curve as E (σ) = slope × κ , which is found to be 20.80,
m
4 ꢀ m
m
4 ꢀ m
a
B
ꢀ
1
ꢀ3
ꢀ1
.5 S · m × 10 [ at 298.15 K, which are nearly similar to those
22.21, and 22.95 kJ · mol
for [VBIM][FeCl Br
], [VBIM]
m
4 ꢀ m
16]
of [BMI][FeCl4].
for [VRIM][FeCl Br
linear with 1000 · T
2
Figure 6(b) shows the Arrhenius plots of σ
[FeCl Br ], and [VBIM][FeCl Br
4 ꢀ m
], respectively. The key in-
4 ꢀ m
m
m
], in which the trend of ln(σ) is almost
(R > 0.99) for the temperature range
fluential factors of electrical conductivity are liquid density, mo-
lecular weight, viscosity, and ionic size. Higher viscosity implies
poorer electrical conductivity, but the behavior of liquid density
on ionic conductivity is opposite that of viscosity on ionic con-
ductivity. For ILs with similar viscosities and densities, molecular
weight and ionic size are important in ionic conductivity. Gener-
ally, a smaller ion implies better ionic conductivity. The viscosity
m
4 ꢀ m
ꢀ
1
2
93.15 K to 353.15 K. The conductivity can be expressed using
the Arrhenius equation
σ ¼ σ0 expðꢀEaðσÞ=κBTÞ
for [VRIM][FeCl
m
Br4 ꢀ m] follows the order η[VBIM][FeClmBr4 ꢀ m]
>
where σ
for ionic conduction. E
0 B a
and κ are constants, and E (σ) is the activation energy
η[VHIM][FeClmBr4 ꢀ m] > η[VPIM][FeClmBr4 ꢀ m], and the density follows the
a
(σ) is estimated from the slope of the
a
Table 2. The solubility of MILs in different solvents
MILs
[
VBIM][FeClmBr4 ꢀ m]
[VPIM][FeClmBr4 ꢀ m]
[VHIM][FeClmBr4 ꢀ m]
Solvent mass/10 g
Solvent
(
m = 2, 3, 4)
(m = 3,4)
(m = 3,4)
Water
Ethanol
Ethyl acetate
Ethyl ether
Benzene
0.26 g(s)
0.14 g(s)
0.09 g(sl)
0.003 g(i)
0.07 g(sl)
0.22 g(s)
0.13 g(s)
0.26 g(sl)
0.006 g(i)
0.06 g(sl)
0.13 g(s)
0.12 g(s)
0.27 g(sl)
0.005 g(i)
0.05 g(sl)
a
solubility of MILs at 298.15 K, sl—slightly soluble; s—soluble; i—indissolvable
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J. Phys. Org. Chem. 2014, 27 498–503

PHYSICOCHEMICAL CHARACTERIZATION OF PARAMAGNETIC IONIC LIQUIDS
order ρ[
> ρ
> ρ . In
[VHIM][FeClmBr4 ꢀ m]
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VBIM][FeClmBr4 ꢀ m]
[VPIM][FeClmBr4 ꢀ m]
1
particular, the respective densities for [VBIM][ FeCl Br
], [VPIM]
m
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[
1
FeCl
m
Br4ꢀ m], and [VHIM][FeCl Br4ꢀ m] are 1.607, 1.464, and
m
ꢀ1
.456 g · cm at 298.15 K. Thus, ionic conductivity follows the order
[
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[
[
elongation of the alkyl chain.
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Solubility
[
Good solubility of title compounds in polar and nonpolar solvents
is remarkable. [VRIM][FeCl
m
2
Br4 ꢀ m] is also soluble in H O and alco-
hol, slightly soluble in ethyl acetate and benzene, and insoluble in
diethyl ether. Generally, salts with imidazolium cation are insoluble
in water and do not hydrolyze in water. However, the trend of the
title substance solubility is significantly similar to that of already
known MILs, which contain tetrahalogenidoferrate (III) anions
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2
and show high solubility in polar solvents, especially in H O. Due
[
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salts with imidazolium cations are insoluble in water and do not
hydrolyze in it. However, tetrahalogenidoferrate (III) anion is
associated with water molecules through hydrogen bonding inter-
actions, causing solubility of the title substances in H
hol. The foregoing causes the H…π bond between benzene and
ionic liquids to increase solubility in benzene. Due to the differ-
ent electron-donating abilities of alkyl, the elongation of the alkyl
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The absorption peak shifts toward lower wavelengths. Magnetic
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This work is supported by the Special Funds of the National Natural
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J. Phys. Org. Chem. 2014, 27 498–503
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