Conducting and magnetic properties of 1-ethyl-3-methylimidazolium (EMI) salts containing paramagnetic irons: Liquids [EMI][MIIICl 4] (M = Fe and Fe0.5Ga0.5) and solid [EMI] 2[FeIICl4]

Article abstract of DOI:10.1246/bcsj.78.1921

An EMI-based room-temperature (RT) ionic liquid containing d⁵ trivalent iron(III) ions [EMI][Fe^IIICl₄] was fully investigated, where EMI is 1-ethyl-3-methylimidazolium. The viscosity of the salt is 14 cP at 30°C, and its ionic conductivity is as high as 1.8 × 10^-2 S cm^-1 at 20°C. The high conductivity and fluidity can be attributed to the reduced interionic Coulomb attractions owing to the nephelauxetic effect. Magnetic susceptibility shows the Curie-Weiss behavior arising from S = 5/2 high-spin electronic state on iron(III) ions in both liquid and solid states, leading to the conductive-paramagnetic bifunctional RT ionic liquid. The solidified salt passes through an antiferromagnetic transition at 4.2 K. Diamagnetic [EMI][Ga^IIICl ₄] and paramagnetic [EMI][Fe^IIICl₄] _0.5[Ga^IIICl4]_0.5 are also RT ionic liquids and show similar conductivities (1.8-2.0 × 10^-2 S cm^-1 at 20°C) and viscosities (12-13 cP at 30°C). These results indicate that the influence of paramagnetic iron(III) ions upon the ionic conductivity and viscosity is negligible in the present system. A 2:1 EMI salt containing d ⁶ divalent iron(II) ions [EMI]₂[Fe^IICl ₄] melts at 86°C. Its crystal structure determined by a synchrotron X-ray powder diffraction measurement is analogous to those of the reported [EMI]₂[Co^IICl₄] and [EMI] ₂[Ni^IICl₄] with expanded lattice.

Full text of DOI:10.1246/bcsj.78.1921

Ó 2005 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 78, 1921–1928 (2005) 1921
BCSJ Award Article
Conducting and Magnetic Properties of 1-Ethyl-3-methylimidazolium
III
(
(
EMI) Salts Containing Paramagnetic Irons: Liquids [EMI][M Cl ]
4
II
M ¼ Fe and Fe Ga ) and Solid [EMI] [Fe Cl ]
0:5
0:5
2
4
Yukihiro Yoshida, Akihiro Otsuka,1;2 Gunzi Saito, Seiichi Natsume, Eiji Nishibori,
ꢀ
;1
ꢀ;1
3
3
4
3
5
5
Masaki Takata, Makoto Sakata, Masahide Takahashi, and Toshinobu Yoko
1Division of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502
2
Research Center for Low Temperature and Materials Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8502
Department of Applied Physics, Graduate School of Engineering, Nagoya University, Nagoya 464-8603
Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1 Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679-5198
3
4
5Institute for Chemical Research, Kyoto University, Gokasho, Uji 611-0011
Received May 18, 2005; E-mail: yoshiday@kuchem.kyoto-u.ac.jp
5
III
An EMI-based room-temperature (RT) ionic liquid containing d trivalent iron(III) ions [EMI][Fe Cl4] was fully
ꢁ
investigated, where EMI is 1-ethyl-3-methylimidazolium. The viscosity of the salt is 14 cP at 30 C, and its ionic con-
ꢃ2
ꢃ1
ꢁ
ductivity is as high as 1:8 ꢂ 10 S cm at 20 C. The high conductivity and fluidity can be attributed to the reduced
interionic Coulomb attractions owing to the nephelauxetic effect. Magnetic susceptibility shows the Curie–Weiss behav-
ior arising from S ¼ 5=2 high-spin electronic state on iron(III) ions in both liquid and solid states, leading to the con-
ductive–paramagnetic bifunctional RT ionic liquid. The solidified salt passes through an antiferromagnetic transition at
III
III
III
4
.2 K. Diamagnetic [EMI][Ga Cl4] and paramagnetic [EMI][Fe Cl4]0:5[Ga Cl4]0:5 are also RT ionic liquids and show
ꢃ2
ꢃ1
ꢁ
ꢁ
similar conductivities (1:8{2:0 ꢂ 10 S cm at 20 C) and viscosities (12–13 cP at 30 C). These results indicate that
the influence of paramagnetic iron(III) ions upon the ionic conductivity and viscosity is negligible in the present system.
6
II
ꢁ
A 2:1 EMI salt containing d divalent iron(II) ions [EMI]2[Fe Cl4] melts at 86 C. Its crystal structure determined by a
II
synchrotron X-ray powder diffraction measurement is analogous to those of the reported [EMI]2[Co Cl4] and
II
[
EMI]2[Ni Cl4] with expanded lattice.
One of the main challenges in condensed matter science is
the design of compounds exhibiting conductive and magnetic
bifunctionality and the control of their competition and/or co-
R
2
2
1
2
R1
Me
operation such as Kondo effect, ꢀ–d interaction, and giant
magnetoresistance (GMR). For liquid materials, some attrac-
3
1N
N3
tive properties and phenomena have been reported for inor-
ganic liquids exhibiting both conductivity and magnetism,
5
4
4
such as alkali and transition metals and alloys, colloidal mag-
5
Scheme 1. Molecular structures of 1,3-dialkylimidazolium
(EMI: R ¼ Et, R ¼ H; BMI: R ¼ n-Bu, R ¼ H;
netic fluids dispersing iron nanoparticles, and reversed Moses
6
1
2
1
2
effect of copper sulfate aqueous solution. In particular, mag-
BDMI: R1 ¼ n-Bu, R2 ¼ Me).
netic fluids dispersing magnetic particles (e.g., magnetite and
iron nitride) with a size of ca. 10 nm in diameter into water,
kerosene, or various oils have been successfully applied in
the engineering field to sealing, damping, and hydrodynamic
dialkylimidazolium (e.g., 1-ethyl-3-methylimidazolium, EMI,
Scheme 1) cations, are promising materials for such a purpose
because of their high ionic conductivities and low viscosities
as well as high electrochemical and thermal stabilities (negli-
7
bearings. For organic systems, however, only a few dithiazol-
yl or dithiadiazolyl neutral radicals have been reported as para-
9
gible vapor pressure). It is thus possible that imidazolium-
8
magnetic liquids which are unstable in air and are expected
not to show an effective electric conductivity.
Ionic liquids (molten salts), especially those including 1,3-
based ionic liquids containing anions with magnetic ions ex-
hibit a thermal- and electrochemical-stable conductive-mag-
netic bifunctionality.10 A brief report on the first well-charac-
Published on the web October 31, 2005; DOI 10.1246/bcsj.78.1921

1
922 Bull. Chem. Soc. Jpn., 78, No. 11 (2005)
BCSJ AWARD ARTICLE
terized conductive–paramagnetic bifunctional RT ionic liquid
III
ing. The obtained data were analyzed by Rietveld refinement with
bond lengths and bond angles restrained. Crystallographic data for
4 have been deposited with Cambridge Crystallographic Data
Centre, CCDC No. 248622. Copies of the data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.
html (or from the Cambridge Crystallographic Data Centre, 12,
Union Road, Cambridge, CB2 1EZ, UK; Fax: +44 1223
336033; e-mail: deposit@ccdc.cam.ac.uk).
5
[
EMI][Fe Cl4] (1) containing d trivalent iron(III) ions was
11
recently presented by our group. Here, we report the full
studies on optical, electrochemical, transport, and magnetic
properties of 1 in comparison with those of liquids containing
III
III
diamagnetic gallium(III) ions [EMI][Fe Cl4]0:5[Ga Cl4]0:5
III
(
2) and [EMI][Ga Cl4] (3). Previous reports on [EMI]Cl/
III
III
Fe Cl3 and [EMI]Cl/Ga Cl3 systems have been limited to
Syntheses and Characterization of [EMI][MIIICl4] (M ¼ Fe
10a
12
cyclic voltammetry, and FAB-mass and Raman spectros-
1
(
synthesized in a similar manner to that previously reported for
1), Fe0:5Ga0:5 (2), and Ga (3)). These RT ionic liquids were
3
copy, respectively. Also a 2:1 EMI salt containing paramag-
II
6
netic d divalent iron(II) ions [EMI]2[Fe Cl4] (4) was prepared
and its magnetic and structural properties were investigated.
III
10a
[
and other ionic liquids were obtainable similarly. Equimolar
EMI][Fe Cl4]. Only the synthesis of 1 will be described here
III
[EMI]Cl (3.00 g, 20.5 mmol) and Fe Cl3 (3.32 g, 20.5 mmol)
were mixed at RT without any solvent and the dark brown liquid
1 that formed was stirred for 2 days at RT. Salt 2 containing half-
Experimental
Materials.
iron(III) chloride FeCl3 (99.99%), and gallium(III) chloride GaCl3
99.999%) were obtained from Aldrich and used without purifica-
tion. [EMI]Cl was purchased from Tokyo Kasei (>97%) and pu-
Anhydrous iron(II) chloride FeCl2 (99.99%),
10
molar gallium(III) ions with d system is light brown, and salt 3
containing only gallium(III) ions is colorless. The characteriza-
tions were thoroughly carried out, since the contamination of
the starting materials and/or water molecules substantially modify
the intrinsic thermal and physical properties. Found: C, 23.35; H,
3.44; N, 9.17; Cl, 45.94%. Calcd for C6H11Cl4N2Fe: C, 23.34; H,
3.59; N, 9.07; Cl, 45.92%. IR (KBr) ꢂmax: 3161, 3148, 3118, 3103
(
1
4
rified according to the literature procedure. Solvents (acetoni-
trile, ethyl acetate, and toluene) were distilled prior to use. All
manipulations for the preparation of 1–4 were carried out under
an inert atmosphere of helium gas in a glovebox with the rigid
exclusion of air and moisture (H2O, O2 < 1 ppm).
ꢃ1
(C–H aromatic), 2988, 2956 (C–H aliphatic), 1570, 1166 cm
(ring). UV–vis (acetonitrile) ꢃmax ("): 685 (0.52), 616 (0.37),
Methods. UV–vis–NIR spectra were measured in acetonitrile
1500–200 nm) or in KBr pellet (2600–240 nm) on a Shimadzu
4
4
4
(
UV-3100 spectrophotometer. FT-IR spectra were taken in KBr
530 (1.7), 361 (1:1 ꢂ 10 ), 312 (1:0 ꢂ 10 ), 241 (1:6 ꢂ 10 ) nm.
1
H NMR (400 MHz, D2O) ꢄ 1.44 (t, 3H, CH3CH2), 3.84 (s, 3H,
pellet with a Perkin-Elmer 1000 Series spectrophotometer (400–
ꢃ1
CH3), 4.16 (q, 2H, CH3CH2), 7.37 (s, 1H, CH), 7.44 (s, 1H,
CH), 8.66 ppm (s, 1H, NCHN). For 2: Found: C, 23.08; H,
3.39; N, 8.85; Cl, 45.03%. Calcd for C6H11Cl4N2Fe0:5Ga0:5: C,
22.82; H, 3.51; N, 8.87; Cl, 44.91%. IR (KBr) ꢂmax: 3163,
3150, 3121, 3102 (C–H aromatic), 2989, 2957 (C–H aliphatic),
7800 cm ). Melting (Tm, onset of the endothermic peak), and de-
composition (Td) temperatures were determined by differential
ꢁ
ꢃ1
scanning calorimetry (DSC) thermograms (10 C min cooling/
heating rate) on a Shimadzu DSC-60 instrument equipped with ni-
trogen cryostatic cooling, and the temperature was calibrated by
water and indium. Density values were obtained by measuring
ꢃ1
1571, 1167 cm (ring). UV–vis (acetonitrile) ꢃmax ("): 685
3
3
(0.22), 617 (0.15), 531 (0.46), 362 (4:7 ꢂ 10 ), 312 (4:4 ꢂ 10 ),
3
3
3
1
the weight of the sample in a 1 cm pycnometer held in the glove-
ꢁ
241 (7:0 ꢂ 10 ), 211 (6:2 ꢂ 10 ) nm. H NMR (400 MHz, D2O)
ꢄ 1.42 (t, 3H, CH3CH2), 3.83 (s, 3H, CH3), 4.15 (q, 2H, CH3CH2),
7.37 (s, 1H, CH), 7.44 (s, 1H, CH), 8.65 ppm (s, 1H, NCHN). For
3: Found: C, 22.05; H, 3.28; N, 8.55; Cl, 43.94%. Calcd for
C6H11Cl4N2Ga: C, 22.33; H, 3.44; N, 8.68; Cl, 43.95%. IR
(KBr) ꢂmax: 3164, 3151, 3120, 3104 (C–H aromatic), 2990,
box at 20 C, that gave essentially the same density for [EMI]BF4
(
1.28 g cm ) as reported previously.15 Viscosities were measured
ꢃ3
using a cone-type Tokyo Keiki rotational viscometer VISCONIC
ꢁ
ED at 30 C. Ionic conductivities were measured in a two plati-
ꢃ1
num electrode conductivity cell (cell constant is 45 cm , which
ꢃ2
ꢃ1
ꢃ1
ꢃ1
was determined by 5 ꢂ 10 , 1 ꢂ 10 , and 5 ꢂ 10 M aqueous
2958 (C–H aliphatic), 1571, 1167 cm (ring). UV–vis (acetoni-
3
1
potassium chloride solutions), using an Agilent Technologies im-
pedance analyzer 4294A over the frequency range from 40 Hz and
10 MHz (20–70 C). The conductance of the samples was deter-
trile) ꢃmax ("): 209 (4:6 ꢂ 10 ) nm. H NMR (400 MHz, D2O)
ꢄ 1.47 (t, 3H, CH3CH2), 3.86 (s, 3H, CH3), 4.21 (q, 2H, CH3CH2),
7.39 (s, 1H, CH), 7.46 (s, 1H, CH), 8.68 ppm (s, 1H, NCHN).
ꢁ
1
II
mined from the first real axis touchdown point in the Cole–Cole
plot of the impedance data. For the electrochemical characteriza-
tion, cyclic voltammetry measurements were performed using
Synthesis of [EMI]2[Fe Cl4] (4). A mixture of [EMI]Cl
II
(1.01 g, 6.86 mmol) and Fe Cl2 (0.436 g, 3.44 mmol) in acetoni-
3
trile (10 cm ) was heated at reflux temperature for a day. After fil-
tration, the pink solution was reduced to a half volume under re-
ꢁ
an ALS electrochemical analyzer 650A (22 C). Working and
counter electrodes were platinum, the reference electrode was
3
duced pressure. Addition of toluene (20 cm ) affords a light pink
solid that was recovered by filtration, washed with toluene (ca. 20
cm ), and dried in vacuo (yield: 57%, 0.85 g). Found: C, 34.11; H,
ꢃ1
Ag/AgCl, and the scan rate was 50 mV s . A Quantum Design
3
MPMS-XL SQUID magnetometer was used to collect DC mag-
netic susceptibility data between 1.9 and 400 K. The samples were
sealed into quartz tubes of 5 mm diameter under ambient helium
atmosphere. Data were corrected for molecular diamagnetism
and holder contribution. High-resolution synchrotron X-ray pow-
der diffraction measurements with an imaging plate as a detector
were carried out on the beam line BL02B2 at SPring-8. The inci-
5.33; N, 13.28; Cl, 33.73%. Calcd for C12H22Cl4N4Fe: C, 34.22;
H, 5.28; N, 13.34; Cl, 33.77%. IR (KBr) ꢂmax: 3144, 3104, 3082
(C–H aromatic), 2991, 2953 (C–H aliphatic), 1574, 1172 cm
ꢃ1
4
(ring). UV–vis (acetonitrile) ꢃmax ("): 209 (2:5 ꢂ 10 ) nm.
1
H NMR (400 MHz, D2O) ꢄ 1.48 (t, 3H, CH3CH2), 3.87 (s, 3H,
CH3), 4.20 (q, 2H, CH3CH2), 7.40 (s, 1H, CH), 7.46 (s, 1H,
CH), 8.68 ppm (s, 1H, NCHN). While the liquids 1–3 are fairly
stable under atmospheric condition, the salt 4 is sensitive to aero-
bic oxidation, being instantly changed from pale pink solid to
ꢀ
dent X-ray was monochromatized at a wavelength of 1.0 A with a
Si double crystal. The sample was sealed in a quartz capillary of
0
.7 mm diameter under ambient helium atmosphere. The X-ray
ꢁ
ꢁ
powder diffraction patterns was collected in 0.01 steps in 2ꢁ from
brown liquid (glass transition at ꢃ70 C).
ꢁ ꢁ
ꢀ
.0 to 40.0 , which corresponds to 1.43 A resolution in d-spac-
4

Y. Yoshida et al.
Bull. Chem. Soc. Jpn., 78, No. 11 (2005) 1923
Table 1. Thermal and Physical Properties of 1–4a)
c)
b)
b)
Salt
Tm
Td
db)
ꢅ
/cP
ꢆ
/S cm
ꢇ
/emu mol
ꢁ
ꢁ
ꢃ3
ꢃ1
ꢃ1
/
C
/ C
/g cm
d)
ꢃ2
ꢃ2
ꢃ2
ꢃ2
ꢃ3
1
2
3
4
18
15
11
86
ca. 280
ca. 190
ca. 130
ca. 270
1.42
1.46
1.53
14
12
13
—
1:8 ꢂ 10
2:0 ꢂ 10
2:0 ꢂ 10
—
1:4 ꢂ 10
7:8 ꢂ 10
0.0
d)
d)
1.419e)
1:1 ꢂ 10
ꢃ2
a) Tm = melting point, Td = decomposition temperature, d = density, ꢅ = viscosity, ꢆ = ionic
ꢁ
ꢁ
conductivity, ꢇ = magnetic susceptibility. b) At 20 C. c) At 30 C. d) By pycnometer. e) By
structural refinement.
2
1
1
0
5
0
5
0
1
2
3
2.0
1.5
1
2
3
4
1
0
0
.0
.5
.0
-10
0
10
20
30
40
T / °C
2
00
400
600
800 1000
Wavelength / nm
-100
0
100
200
300
T / °C
200
400
600
800
1000
Wavelength / nm
Fig. 1. DSC thermograms of 1–4 on heating process.
The inset is the enlargement for the melting event range
of 1–3.
Fig. 2. Electronic absorption spectra of 1 (thick line), 2
thin line), and 3 (dotted line) in acetonitrile. The inset
(
is the enlargement for molar absorptivity in the visible
range.
Results and Discussion
Thermal Properties.
of 1–4 are summarized in Table 1. Figure 1 presents DSC
thermograms of 1–4. The salts 1–3 are liquids at RT with
Thermal and physical properties
enthalpy that originates from the increased interionic Coulomb
III
interactions, in a similar fashion to that from [BDMI][Fe Cl4]
II
ꢁ
(Tm < RT) to [BDMI]2[Fe Cl4] (Tm ¼ 61 C), where BDMI is
ꢁ
ꢁ
10g
the melting points within 11–18 C. For the salt 2, an addi-
tional endothermic peak at ꢃ23 C disappears on heating at
1-n-butyl-2,3-dimethylimidazolium. Also the temperature is
II
lower than those of the isomorphous salts [EMI]2[Co Cl4]
ꢁ
ꢁ
II
ꢁ
18
1
as 280 C, leading to the wide liquid range (ca. 260 C). An
50 C for 5 min. Decomposition temperature of 1 is as high
ꢁ
(Tm ¼ 100{102 C) and [EMI]2[Ni Cl4] (Tm ¼ 92{93 C),
ꢁ
and the variation of melting points might be related to the
lattice volume (vide infra).
endothermic peak on melting of 2 is distinct from those of
1
and 3, strongly indicating that the salt 2 is homogeneous
instead of a heterogeneous mixture of 1 and 3.
Optical Properties.
strongly indicative of high-spin electronic states for Fe Cl4
Electronic absorption spectra are
III
II
On cooling, a transformation from crystalline form to glass
form passing through the supercooling liquid state was absent
anion in 1 and 2, and Fe Cl4 anions in 4. Figure 2 shows ab-
sorption spectra of 1–3 in acetonitrile. Lowest-energy charge-
ꢁ
6
6
III
down to ꢃ100 C for 1–4. Similar behavior has been reported
transfer (CT) transitions A1 ! T2 of tetrahedral Fe Cl4
anion in high-spin electronic state were observed centered at
361, 312, and 241 nm for 1 and 2.19 The very weak absorption
bands observed in the range of 500–800 nm (Inset of Fig. 2)
for [EMI]Cl/AlCl3 system, in which the melt ranging in ratio
[EMI]Cl/AlCl3 from 0.64 to 1.27 shows no glass transition
down to liquid nitrogen temperature. A previous report has
16
III
6
4
shown that the supercooled liquid [BMI][Fe Cl4], where
BMI is 1-n-butyl-3-methylimidazolium, passes through the
arise from d–d forbidden transitions: 685 nm ( A1 ! T1),
6 4 6 4 4 19c,20
616 nm ( A1 ! T2), and 530 nm ( A1 ! A1, E).
On
the other hand, the salt 4 that contains iron(II) ions shows a
ꢁ
10e
glass transition at ꢃ85 C, and recently we observed the
ꢁ
17
5
5
19b,21
glass transition of [BMI][GaCl4] at ꢃ88 C. These results
are indicative of the reduced reorientation energy for short
alkyl chains of 1–3, and could lead to the viscosities of 1–3
d–d transition E ! T2 in the near-infrared region,
which is indicative of the rather low ligand field.
The infrared spectra of 1–3 are essentially identical. The
III
being lower than that of [BMI][Fe Cl4] (vide infra).
ꢁ
two high frequency bands ranging of 3161–3164 and 3148–
ꢃ1
The 2:1 salt 4 shows a melting event at 86 C, which sub-
stantially exceeds the values of 1:1 salts 1–3. The marked dif-
ference is readily explained in terms of the increased melting
3151 cm are assigned to the stretching mode of C(4)–H or
C(5)–H bond of EMI cation, while the bands assigned to
C(2)–H stretching mode appear at lower frequency regions

1
924 Bull. Chem. Soc. Jpn., 78, No. 11 (2005)
BCSJ AWARD ARTICLE
ꢃ1
of 3118–3121 and 3102–3104 cm . The corresponding aro-
matic C–H stretchings of 4 were observed at lower frequencies
ꢃ1
ꢃ1
of 3144 and 3104 cm , respectively. For 4, an additional band
centered at 3082 cm is indicative of a significant level of C–
10
18,22
j
b
HꢄꢄꢄCl interionic interactions as expected from its melting
point being higher than those of 1–3. The spectral features
of 4 are essentially identical to those of the isomorphous
Co Cl4 and Ni Cl4 salts.
Electrochemical Window. The electrochemical stabilities
of 1–3 were investigated at 22 C. All ionic liquids are stable
2
l
h
1
d
3
c
g
II
II
18
i
q
o
k
f
n
r
ꢁ
p
1
down to a low potential of around ꢃ2 V vs Ag/AgCl, below
which the reductive decomposition of EMI cation occurs. A re-
versible wave for Fe(III)/Fe(II) redox process was observed
for 1 and 2 at around 0.1 V as reported previously. In the
oxidation scan, 1–3 were stable up to ca. þ2 V regardless of
the anion species, and it is thus apparent the salt 3 has a wide
electrochemical window (ca. 4 V).
s
m
1
0a
-2
-1
1
0
10
-1
-1
η / cP
Fig. 3. A plot of the equivalent conductivity against the re-
ciprocal of the viscosity for simple-type [EMI]X RT ionic
liquids (X = (b) N(CN)2, (c) BF4,15,28a (d) C(CN)3,23 (f)
CF3CO2, (g) CuCl2, (h) CF3BF3,
(j) AlCl4,16 (k) NbF6, (l) C2F5BF3, (m) C3F7CO2,
Ionic Conductivity and Viscosity. Ionic conductivities
ꢃ2
ꢃ1
23
(
ꢆ) of 1–3 lie in the region of 1:8{2:0 ꢂ 10 S cm at 20
ꢁ
28b
28c
28d,e
28b
C. These values are comparable to those of 0.2 M KCl and
.5 M CuSO4 aqueous solutions, and those of the most conduc-
(i) CF3SO3,
28f
28e
28b
0
ꢃ2
ꢃ1
23
(n) SbF6,28g (o) C3F7BF3, (p) TaF6,28f (q) (CF3SO2)2N,
r) C4F9BF3,28e and (s) WF728g). A solid line is the least-
squares fit to the data. The ꢈ values are unavailable for the
28e
28a,b
tive simple-type EMI salts [EMI][AlCl4] (2:3 ꢂ 10 S cm
ꢁ
16
ꢃ2
ꢃ1
ꢁ
(
at 22 C ), [EMI][N(CN)2] (2:7 ꢂ 10 S cm at 20 C ),
ꢃ2
ꢃ1
ꢁ
23
and [EMI][C(CN)3] (1:8 ꢂ 10 S cm at 20 C ). However,
0a
30b
(
density values.
a) SCN3 and (e) SeCN salts due to the absence of
the conductivities are apparently less than that of the double
ꢃ1
ꢃ1
ꢁ
24
salt [EMI][F(HF)2:3] (1:0 ꢂ 10 S cm at 25 C ), in which
F(HF)2 and F(HF)3 anions are intermingled. Equivalent
conductivities (ꢈ), which are represented by ꢆ=M (M: molar
concentration) for 1:1 electrolytes, are estimated to 3.9, 4.3,
liquids as seen in Fig. 3.15,16,23,28 A deviation from the line ob-
tained by least-squares fit would arise from the rather high
measured temperature for viscosity (30 C). The increased
fluidity and conductivity of 1 in comparison with those of
2
ꢃ1
ꢁ
ꢁ
and 4.2 S cm mol
at 20 C for 1–3, respectively. For
[EMI][CF3SO3]/[EMI][(CF3SO2)2N] binary system, the bina-
III
ꢃ3
ꢃ1
ry mixtures give a pronounced increase in ꢈ of 40–50% in
comparison with those expected from a simple law of mix-
tures, which arises from the increase in charged carriers.25
By contrast, the ionic conductivity of 2 involving equimolar
[BMI][Fe Cl4] (ca. 30 cP and ca. 8 ꢂ 10 S cm at 25
ꢁ
10e 17
ꢁ
ꢃ3
ꢃ1
C, and 31 cP and 8:9 ꢂ 10 S cm at 25 C ) as expect-
ed from the DSC thermograms are mainly caused by the short-
ened alkyl chain, which leads to the suppression of the vdW
interactions between the chains. Similar trends of viscosity
and conductivity for EMI and BMI salts have been reported
III
III
Fe Cl4 and Ga Cl4 anions is comparable to those of the
parent salts 1 and 3, presumably due to the analogous anion
species.
2
7
16
15b
for other anion systems, such as F(HF)2:3, AlCl4, BF4,
2
8d,e
and (CF3SO2)2N.28b
Figure 4 shows the temperature dependence of ionic con-
The viscosities (ꢅ) of 1–3 are also similar to each other (12–
4 cP at 30 C), and it is thus apparent that both the ionic con-
CnF2nþ1BF3 (n ¼ 1, 2),
ꢁ
1
ꢁ
ductivity and the viscosity of the present system do not depend
on their magnetic properties. The viscosities are comparable to
those observed for the most fluid simple-type EMI salts
ductivity of 1–3 in the range of 20–70 C. As temperature
rises, the ꢆ values increase according to the Arrhenius law
ꢆ ¼ ꢆ0expðꢃ"a=kBTÞ, where the activation energy in conduc-
ꢁ
16
ꢃ1
[
2
EMI][AlCl4] (18 cP at 25 C ), [EMI][N(CN)2] (17 cP at
2
tion "a was estimated to be 14.9, 14.5, and 14.8 kJ mol
respectively. These values are significantly low in compar-
,
ꢁ
23
ꢁ
23
C ), and [EMI][C(CN)3] (18 cP at 22 C ), but still
ꢃ1
higher than the value for the double salt [EMI][F(HF)2:3]
ison with those of [EMI][BF4] (17.6 kJ mol ), [EMI]-
ꢃ1
ꢁ
24
(
4.9 cP at 25 C ).
Walden pointed out that the product of ꢈꢅ remains constant
[(CF3SO2)2N] (19.7 kJ mol ), and [EMI][(C2F5SO2)2N]
(26.8 kJ mol ),28a and are comparable to that of the most con-
ꢃ1
over a wide range of aqueous solution systems.26 The so-called
Walden rule is interpreted in the same manner as the combina-
ductive simple-type liquid [EMI][N(CN)2] (15.2 kJ mol
This indicates the rather low potential barrier for ionic conduc-
tion of 1–3, in support of the high RT conductivities.
For organic syntheses, ionic liquids have been applied either
as catalyst, as co-catalyst or catalyst activator, as source of
new ligand for a catalytic metal center, or just as the solvent
for the reaction.29 The important property for such reactions
is obviously to have low viscosity as well as the low vapor
ꢃ1 11b
).
2
2
tion of the Nernst–Einstein equation Di ¼ RTꢈi=zi F and the
Stokes–Einstein equation Di ¼ RT=6ꢀNAꢅri for the self-diffu-
sivity (Di) of species i with ꢈi in a medium with viscosity ꢅ,
where zi and ri are the charge and radius of species i, and F
and NA are the Faraday constant and the Avogadro’s number,
respectively. It has recently become apparent that the rule ap-
plies well to ionic liquids formed with imidazolium analogue
III
pressure, and the [BMI]Cl/Fe Cl3 systems have been success-
10c
1
0e,23,27
cations, and the present salts 1–3 also follow the rela-
tion on the basis of seventeen simple-type [EMI]X RT ionic
fully applied for Friedel–Crafts acylation
0d
and carbonyla-
tion.1 Although the viscosity of 1–3 are apparently low in

Y. Yoshida et al.
Bull. Chem. Soc. Jpn., 78, No. 11 (2005) 1925
III
comparison with that of [BMI][Fe Cl4], they might be unfa-
vorable as solvents for organic syntheses due to the melting
point being near RT.
posed of completely dissociated ions is normally proportional
to the ion mobility, which is largely governed by the interionic
interactions and molecular weight of ions. Figure 5 illustrates
the plots of ꢈ and ꢆ against the reciprocal of the molecular
Relation between Conductivity and Molecular Weight.
The high ionic conductivities of 1–3 are even more marked
when the conductivity is plotted as a function of molecular
weight of anion. The ꢈ value is represented by ionic conduc-
tivity (ꢆ) and molar concentration (M) as ꢈ ¼ ꢆ=M, where M
weight of anion (MWa) for simple-type [EMI]X RT ionic
liquids including the present three salts 1–3.1
5,16,23,28,30
Linear
relations except for ionic liquids containing the tetrahedral
anions with central metals, i.e., X ¼ AlCl4, FeCl4 (1),
Fe0:5Ga0:5Cl4 (2), and GaCl4 (3), strongly suggest that the
MWa is a factor governing the ionic conductivity in most
EMI-based ionic liquids.2 It is then likely that the relatively
high equivalent and ionic conductivities of 1–3 and the AlCl4
salt in comparison with those expected from the linear relation
are mainly caused by the reduced interionic interactions. Pre-
viously, the Xꢊ calculations predicted the charge distribution
depending on ionic size, density, and molecular weight is
closely related to the carrier concentration.2
value does not change widely in the EMI-based RT ionic liq-
8b,e
Since the M
8b,e
ꢃ3
ꢃ3 15,16,23,28
uids (3:9{6:5 ꢂ 10 mol cm ),
the ionic conductiv-
ity is a factor governing the equivalent conductivity. Accord-
ing to the Nernst–Einstein equation and the Einstein relation
Di ¼ ꢉ RT=jzijF for the relation between ion mobility (ꢉ )
i
i
ꢃ
and self-diffusivity of species i, the ꢈ value of the liquid com-
in FeCl4 anion to be þ0:916 for Fe and ꢃ0:479 for Cl as
the covalency in M–Cl (M ¼ Al, Ga, or Fe) bonds increases
due to the nephelauxetic effect.31 It thus appears that this effect
is associated with the highly ionic conductivities observed
for 1–3 and the AlCl4 salt, owing to the reduced interionic
Coulomb attractions. The nephelauxetic series has been
established in the order of covalency in the metal–ligand
T / °C
80
60
40
20
−
2.8
3.2
ꢃ
ꢃ
ꢃ
bond:
ꢃ
F
Cl < CN < Br . To account for a contribution of molec-
< H2O < NH3 < 1,2-ethanediamine < SCN
<
−
ꢃ
32
ular weight to the conductivity, tetracyanometallate mono-
III
ꢃ
anions of trivalent metals [M (CN)4] would be promising
for highly conductive ionic liquids although the significant
coordination ability of cyano groups. The ionic conductivity
−
−
−
3.6
4.0
4.4
ꢃ3
ꢃ1
of the SCN salt is extraordinary low (ca. 8 ꢂ 10 S cm at
ꢁ
30a
25 C ) in comparison with that expected from the linear
relation in Fig. 5b, although its viscosity (21 cP) is as low as
that of the highly conductive N(CN)2 salt with comparable
MWa. The peculiar feature is indicative of the reduced carriers
in the liquid, which might be caused by the ion association or
ion pairing of a significant fraction of oppositely charged ions
and/or the formation of neutral thiocyanogen (SCN)2.
2
.8
3.0
3.2
3.4
-1
3.6
-
1
1
000T / K
Fig. 4. Temperature dependence of ionic conductivity of 1
), 2 ( ), and 3 ( ).
Magnetic Properties. Ionic liquids 1 and 2 containing
iron(III) ions exhibit a paramagnetic behavior at around RT,
(
(
a)
(b)
6
4
2
0
30
b
j
j
b
2
1
3
20
10
0
3
2
1
d
e
d
h
h
g
l
c
o
l
q
r
o
n
g
c
k
f
q
p
r
f
i
i
k
m
n
p
a
m
s
s
0
0.005 0.010 0.015 0.020
0
0.005 0.010 0.015 0.020
MWa-1 / g mol
-1
MWa-1 / g mol
-1
Fig. 5. A plots of (a) equivalent conductivity and (b) ionic conductivity against the reciprocal of the molecular weight (MWa) of
anion for simple-type [EMI]X RT ionic liquids. Indices are identical with those in Fig. 3.15,16,23,28,30 Solid lines act as a guide to
viewing.

1
926 Bull. Chem. Soc. Jpn., 78, No. 11 (2005)
BCSJ AWARD ARTICLE
(
a)
(b)
5
4
3
2
1
0
1
1
50
00
50
0
0
100
200
300
400
0
100
200
300
400
T / K
T / K
ꢃ1
Fig. 6. Temperature dependence of (a) ꢇT and (b) ꢇ of 1 ( ), 2 ( ), 3 ( ), and 4 (ꢂ) in an applied field of 100 Oe on heating
process.
4
4
4
4
.35
.30
.25
.20
1
.5
2
1
.0
.5
1
.0
1.0
0
.5
T
f
T
m
0
0
10
H / kOe
20
30
2
75 280 285 290 295 300 305
0.5
2
4
6
8
T / K
T / K
Fig. 8. Temperature dependence of static susceptibility of 1
Fig. 7. Temperature dependence of ꢇT of 1 in the temper-
ature range of 275–305 K on heating ( ) and cooling (
processes in an applied field of 10 kOe.
)
below 8 K. Data are collected after cooling down to 4 K
ꢃ1
from 300 K by slow cooling (ꢃ0:5 K min
,
) and by
and the effective magnetic moments (ꢉ ) were found to be
eff
quenching ( ). Inset is the field dependence of magnetiza-
tion at 2 K ( ) and 8 K ( ).
5
.83ꢉ and 4.31ꢉ at 300 K (both are in liquid state), respec-
B B
tively. It is thus apparent that 1 and 2 are bifunctional liquids
with high ionic conductivities and paramagnetic behaviors.11
The estimated ꢉeff values are significantly large in comparison
with the dithiazolyl or dithiadiazolyl neutral radicals (ca.
the melting point (291 K). This observation is contrary to our
simple expectation that the solid phase has significant antifer-
romagnetic interactions between Fe Cl4 anions in comparison
III
8
,33
III
1
.3ꢉB), and are unambiguously originated from the Fe Cl4
with those in liquid phase. The thermal hysteresis is indepen-
dent of the magnetic field up to 50 kOe. Below 270 K, the ꢇT
values of 1 and 2 remain almost constant down to 30 K, where
it then begins to fall owing to the antiferromagnetic interac-
tions between iron(III) ions. Temperature dependence of the
both salts down to 10 K follows the Curie–Weiss expression
anions with S ¼ 5=2 high-spin electronic state (spin-only
values are 5.92ꢉ and 4.18ꢉ , respectively). The salt 3
B
B
containing only gallium(III) ions are diamagnetic. Figure 6a
shows the product of static susceptibility (ꢇ) and temperature
for 1–4 as a function of temperature. On cooling, the ꢇT value
ꢃ2
ꢃ1
ꢃ1
of 1 shows an abrupt jump of 5 ꢂ 10 emu K mol at 284 K
with ꢇ ¼ CðT ꢃ ꢁÞ (Fig. 6b), where the Curie constant (C)
(
Fig. 7), which is close to the freezing point of 282 K. Al-
and the Weiss temperature (ꢁ) are estimated to be 4.33
ꢃ1
ꢃ1
though we do not have information about the low-temperature
crystal structure and optical properties at present, it is un-
ambiguously evident that a solidification of 1 is related to
the upturn since the ꢇT value during the heating process falls
emu K mol and ꢃ0:9 K for 1, and 2.34 emu K mol and
ꢃ1:7 K for 2. On further cooling, however, 1 and 2 behave
quite differently. The salt 1 exhibits a susceptibility kink at
4.2 K, below which it loses a significant fraction of its suscep-
tibility (Fig. 8). Both the absence of hysteresis between zero
ꢃ2
ꢃ1
by a similar amount 5 ꢂ 10 emu K mol at 293–295 K near

Y. Yoshida et al.
Bull. Chem. Soc. Jpn., 78, No. 11 (2005) 1927
field cooling (ZFC) and field cooling (FC) processes and the
observation of spin-flop at ca. 8 kOe (inset of Fig. 8) strongly
indicate that the behavior is not due to the spin-glass forming,
but arises from the long-range antiferromagnetic ordering.
Magnetic behaviors below the transition depend on the cooling
(a)
(b)
Fe(2)
ꢃ1
rate. The slowly cooled (ꢃ0:5 K min ) sample shows a
Fe(2)
gradual decrease in susceptibility of ca. 30%, which is usual
for non-oriented antiferromagnets. For the quenched sample,
the loss of susceptibility is rather small due to either insuffi-
cient crystallinity to form the long-range magnetic ordering
or formation of paramagnetic and antiferromagnetic domains.
For the salt 2, on the other hand, the temperature dependence
follows the Curie–Weiss law down to 1.9 K, possibly owing to
Fe(1)
Fe(1)
II
Fig. 9. Interionic contacts between EMI cation and Fe Cl4
anion in the polycrystalline 4 (a) around Fe(2) ion and (b)
around EMI cation at 300 K. Short C–HꢄꢄꢄCl contacts are
shown by dashed lines.
III
the magnetic dilution by diamagnetic Ga Cl4 anions. This
finding again indicates that the solidified salt 2 is homo-
geneous instead of a heterogeneous mixture of 1 and 3.
The salt 4 also obeys the Curie–Weiss expression with C ¼
II
ꢃ1
drogen bonds are reminiscent to those observed in the Co Cl4
II
3
:481 emu K mol and ꢁ ¼ ꢃ5:6 K regardless of its forms,
and Ni Cl4 salts, and are consistent with the infrared absorp-
i.e., liquid or solid. The effective moment of 5.22ꢉ substan-
B
tion band arising from C–HꢄꢄꢄCl interionic interactions. No
short cationꢄꢄꢄcation contacts were found in the three isostruc-
tural tetrachlorometallate salts.
tially exceeds the value expected for paramagnetic S ¼ 2 spin
system (spin-only value is 4.90ꢉ ). This is found to be the
B
II
19b,21a,c,34
case in most of the Fe Cl4 salts (5.3–5.4ꢉ ),
and re-
B
sults from the total effects of the mixing-in of the first excited
state into the ground state via spin-orbit coupling, temperature-
independent paramagnetism, and some thermal contribution
Conclusion
We obtained conductive–paramagnetic bifunctional RT ion-
III
ꢃ2
III
ꢃ1
5
34
ic liquids [EMI][Fe Cl4] showing ꢆ ¼ 1:8 ꢂ 10
S cm
and ꢇ ¼ 1:4 ꢂ 10 emu mol at RT and [EMI][Fe Cl4]0:5-
from the 3d 4s configuration.
ꢃ2
ꢃ1
Crystal Structure of 4. Since attempts to obtain the single
crystals suitable for single-crystal X-ray diffraction studies
were unsuccessful, high-resolution synchrotron X-ray powder
diffraction measurements were carried out for polycrystals 4
at 300 K and the diffraction profiles were analyzed by the
Rietveld method. The peak positions and intensities of Bragg
reflections were similar to those of the reported [EMI]2-
III
ꢃ2
ꢃ1
[
Ga Cl4]0:5 showing ꢆ ¼ 2:0 ꢂ 10 S cm and ꢇ ¼ 0:8 ꢂ
ꢃ2
ꢃ1
III
1
through an antiferromagnetic transition at 4.2 K, while
0
emu mol
at RT. On cooling [EMI][Fe Cl4] passes
III
III
[
EMI][Fe Cl4]0:5[Ga Cl4]0:5 follows the Curie–Weiss law
down to 1.9 K. The colorless liquid [EMI][Ga Cl4] is diamag-
III
ꢃ2
III
ꢃ1
ꢁ
netic, but its ionic conductivity (2:0 ꢂ 10 S cm at 20 C)
II
II
18
is comparable to those of [EMI][Fe Cl4] and [EMI]-
[
Co Cl4] and [EMI]2[Ni Cl4] salts, and thus the structure
III
III
II
[Fe Cl4]0:5[Ga Cl4]0:5. These findings strongly suggest that
the influence of paramagnetic iron(III) ions upon the ionic con-
ductivity is negligible in the present system. Possible nephe-
lauxetic effect in halogenometallate anions would be promis-
ing for future exploration of the highly conductive ionic liq-
uids, as well as the paramagnetic liquids with sizable magnetic
moment. The crystal structure of [EMI]2[Fe Cl4] was deter-
mined by synchrotron X-ray powder diffraction measurements.
It appears that its lower melting point than those of isomor-
parameters of the Co Cl4 salt were used as the starting model
for the refinement. The salt 4 belongs to the tetragonal system:
ꢀ
space group I41=a, a ¼ 14:14626ð4Þ, c ¼ 19:63368ð9Þ A, V ¼
3
ꢃ3
,
ꢀ
3
929:03ð4Þ A , Z ¼ 8, Dcalcd ¼ 1:419 g cm
RI ¼ 0:039,
Rwp ¼ 0:023, and is isostructural to those of the reported
II
II
18
[
EMI]2[Co Cl4] and [EMI]2[Ni Cl4] salts as expected. The
II
unit cell is apparently expanded compared with those of the
3
II
3
II
ꢀ
ꢀ
Co Cl4 (3890(1) A ) and Ni Cl4 (3871(2) A ) salts. The low
ꢁ
melting point of 4 (86 C) in comparison with those of the
ꢁ
II
II
II
ꢁ
phous Co Cl4 and Ni Cl4 salts is related to its expanded
lattice.
two isomorphous salts (100–102 C for Co Cl4, 92–93
II
C
for Ni Cl4) is largely related to the expanded lattice, which
gives rise to the reduced melting enthalpy.
This work was in part supported by a COE Research on
Elements Science (No. 12CE2005) and a Grant-in-Aid (21st
Century COE programs on Kyoto University Alliance for
Chemistry and on Nagoya University Alliance of Frontiers
of Computational Science) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan. The authors
also acknowledge the financial support from the Grants-in-
Aid for Scientific Research (No. 15205019) by JSPS.
The crystal contains one crystallographically independent
EMI cation and two FeCl4 anions (Fe(1)Cl(1)4 and
Fe(2)Cl(2)4). Four EMI cations surround the anionic unit with
Fe(2) in a tetrahedral fashion involving hydrogens bonded to
C(2) positions (Fig. 9a). On the other hand, the anion with
Fe(1) is surrounded tetrahedrally by eight EMI cations, where
each Cl(1) is connected with two distinct EMI cations through
C(4)–HꢄꢄꢄCl(1) and C(5)–HꢄꢄꢄCl(1) hydrogen bonding type in-
teractions. Accordingly, the EMI cation is surrounded by three
anions as seen in Fig. 9b. The shortest C(2)–HꢄꢄꢄCl(2), C(4)–
HꢄꢄꢄCl(1), and C(5)–HꢄꢄꢄCl(1) distances are found to be
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