Novel ionic liquids composed of only azole ions

Article abstract of DOI:10.1246/cl.2004.1022

Novel ionic liquids (ILs) composed of azole type cation and anion were prepared. Triazole and tetrazole were coupled with 1-ethyl-3-methylimidazolium hydroxide. Obtained tetrazole salt was liquid at room temperature showing only glass transition temperature at -89°C. The ionic conductivity reached to 8.9 × 10^-3 S cm^-1 at room temperature reflecting low viscosity of 42.5 cP at 25°C. This is the first report on the ILs based on only azole type ions.

Full text of DOI:10.1246/cl.2004.1022

1022
Chemistry Letters Vol.33, No.8 (2004)
Novel Ionic Liquids Composed of Only Azole Ions
Wataru Ogihara, Masahiro Yoshizawa, and Hiroyuki OhnoÃ
Department of Biotechnology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588
(Received May 11, 2004; CL-040532)
Novel ionic liquids (ILs) composed of azole type cation and
pounds were liquid state at room temperature and their structures
1
anion were prepared. Triazole and tetrazole were coupled with
1-ethyl-3-methylimidazolium hydroxide. Obtained tetrazole salt
was liquid at room temperature showing only glass transition
temperature at À89 ꢀC. The ionic conductivity reached to
8:9 Â 10À3 S cmÀ1 at room temperature reflecting low viscosity
of 42.5 cP at 25 ꢀC. This is the first report on the ILs based on
only azole type ions.
were characterized by using H NMR spectroscopy.11
Br
OH
N
N
Amberlite 400J
N
N
EMImBr
N
N
N
or
HN
N
HN
N
ILs1 are defined molten salt at room temperature, and ex-
pected as novel solvents due to their unique characteristics such
as non-volatility, high ionic conductivity, etc. ILs have been in-
vestigated as alternative electrolyte solutions for lithium ion bat-
N
tery,2 fuel cell,3 capacitor,4 and dye-sensitized solar cell.5
A
N
N
large number of ILs was prepared and characterized in this dec-
ade, most of them were based on halogenated counter anions
such as PF6À, BF4À, and (CF3SO2)2NÀ (TFSIÀ). Halogens have
been used to reduce negative charge density by their electron
withdrawing effect. These ILs have low melting point (Tm) re-
flecting their weak electrostatic interaction attributable to the
negative charge delocalization. Although some halogen-free
anions were also used to form ILs, there were few reports on
these anions like nitrate or acetate.6 It was semi-empirically be-
lieved that organic salts based on halogen-free counter anions
were difficult to prepare IL. These salts showed poor properties
even if they formed IL.
On the other hand, azole compounds such as imidazole were
frequently used to prepare cations for excellent ILs because posi-
tive charge delocalization on the azole ring weakened electro-
static interaction in spite of small molecular weight. We expect-
ed that excellent ILs could also be obtained by using charge-
delocalized azole type anion without halogenated anion. These
salts with azole type anions had already been reported. Lithium
4,5-dicyano-1,2,3-triazolate was reported as electrolyte due to
charge delocalization and interfacial stability.7 Also similar salts
were prepared for capacitor.8 However there was no report about
ILs containing azole type anions in spite that there were lots of
ILs containing azole type cations.9 It should be noted here that
these azole type ILs contained only three kinds of element, C,
H, and N. Preparation of the ILs, those are free from halogens,
O, P, and S, is significant to prepare ‘‘green’’ materials.
Tri
or
N
N
N
N
EMIm
N
N
Tet
Scheme 1. Synthesis of the ILs based on only azole ions.
The ionic conductivity of the obtained ILs was measured
with the complex-impedance method using an impedance ana-
lyzer (Schlumberger Solartron 1260 impedance/gain-phase ana-
lyzer) with a frequency range from 10 Hz to 1 MHz. The dynam-
ic ionic conductivity measurement system was developed in our
laboratory.12 All the measurements were carried out in a glove
box filled with dry N2 atmosphere in the temperature range 10
to 60 ꢀC at a cooling rate of 2.5 ꢀC minÀ1. DSC measurement
was carried out with a DSC-6000 (Seiko Instruments Inc.) in
the temperature range À150 to 200 ꢀC at a heating rate of
10 ꢀC minÀ1. The thermal stability of the ILs was investigated
by TG/DTA 220 (Seiko Instruments Inc.) with heating rate of
10 ꢀC minÀ1 from 25 to 450 ꢀC. The viscosity measurement of
the ILs was carried out with DV-I + viscometer (Brookfield).
Thermal analysis revealed that EMImTri and EMImTet
were liquids showing only glass transition temperature (Tg) as
shown in Table 1. They showed very low Tg similar to those salts
containing TFSI anion.13 As shown in Table 1, Low Tg and vis-
1-Ethyl-3-methylimidazolium bromide (EMImBr) was pre-
pared by the quarternization of methylimidazole with ethyl bro-
mide as already reported.10 Bromide anion of EMImBr was con-
verted into hydroxide by anion exchange resin (Amberlite-400J),
and neutralized with 1,2,4-triazole (Tri) or 1,2,3,4-tetrazole (Tet)
as shown in Scheme 1. Each aqueous solution was concentrated
with rotary evaporator and residual liquid was mixed with dieth-
yl ether to extract the target salt. The extracted compounds were
further washed with ethyl acetate twice and dried under reduced
pressure at room temperature for two days. The obtained com-
Table 1. Thermal properties and viscosity of EMImTri and
EMImTet
Tg/ꢀC
Td/ꢀC
ꢀ/cP at 25 ꢀC
EMImTri
EMImTet
À76
À89
207
—
60.2
42.5
—; Not measured
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.8 (2004)
1023
cosity imply that azole type anions had potential to form excel-
lent ILs. EMImTri showed no weight loss upto 200 ꢀC. We did
not carry out the thermal analysis at higher temperature of
EMImTet because of explosive property.
COE program of ‘‘Future Nano-materials’’ in Tokyo University
of Agriculture and Technology. The present study was supported
by the Grant-in-Aid for Scientific Research from the Ministry of
Education, Culture, Sports, Science and Technology, Japan
(No.14205136).
-1
-2
-3
-4
References and Notes
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3.0
3.2
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1000 T-1 / K-1
´
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Figure 1. Temperature dependence of the ionic conductivity
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ˆ
The ionic conductivity of azole type ILs is shown in Figure
1. The ionic conductivity of EMImTet was five times higher than
that of EMImTri at measured temperature range. This was attrib-
utable to both low Tg and low viscosity, the conductivity (8:9 Â
10À3 S cmÀ1 at 25 ꢀC) was comparable to that of general IL
(EMImTFSI; 8:4 Â 10À3 S cmÀ1 at 26 ꢀC).4 Since the properties
of azoles change depending on the substituent group,14 it should
be effective to introduce some substituent groups such as cyano
groups expecting negative charge delocalization and improve-
ment of thermal stability. According to the strategy as mentioned
in the present work, it is easy to expect ILs containing imidazole
or pyrazole as anion species. We have already attempted to elab-
orate ILs with derivatives of imidazoles and pyrazoles, but
failed.
10 P. Bonhote, A.-P. Dias, M. Armand, N. Papageorgiou, K.
Kalyanasundaram, and M. Gratzel, Inorg. Chem., 35, 1168
(1996).
¨
1
11 EMImTri: H NMR (DMSO) ꢁ 1.34 (t, 3H), 3.79 (s, 3H),
4.12 (q, 2H), 7.57 (s, 1H), 7.68 (s, 1H), 7.77 (s, 1H), 9.34
1
(s, 1H); EMImTet: H NMR (DMSO) ꢁ 1.35 (t, 3H), 3.81
(s, 3H), 4.15 (q, 2H), 7.70 (t, 1H), 7.79 (t, 1H), 8.22 (s,
1H), 9.22 (s, 1H).
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Chem., 7, 991 (1970); R. Milcent and C. Redeuilh, J.
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Preparation of thermally or chemically stable salt based on
azole type anion was not so easy, because there are only few
acidic azole compounds just like triazole or tetrazole. These
new azole type ILs may be used as not only electrolytes or sol-
vents, but also explosives.15
15 G. Drake, T. Hawkins, A. Brand, L. Hall, and M. Mckay,
Propellants, Explos., Pyrotech., 28, 174 (2003).
The present work was carried out under the 21st Century
Published on the web (Advance View) July 12, 2004; DOI 10.1246/cl.2004.1022