Allylimidazolium halides as novel room temperature ionic liquids

Article abstract of DOI:10.1246/cl.2004.1360

Liquid state halide salts composed of imidazolium cations having allyl groups were prepared. Their ionic conductivity, viscosity, and thermal properties were investigated. Introduction of allyl group on N-position effectively suppressed the crystallizatio

Full text of DOI:10.1246/cl.2004.1360

1
360
Chemistry Letters Vol.33, No.10 (2004)
Allylimidazolium Halides as Novel Room Temperature Ionic Liquids
ꢀ
Tomonobu Mizumo, Edy Marwanta, Noriyoshi Matsumi, and Hiroyuki Ohno
Department of Biotechnology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588
(Received July 26, 2004; CL-040872)
Liquid state halide salts composed of imidazolium cations
group(s) on imidazolium ring to lower Tm of these halide salts.
The chemical structure of the examined compounds is summa-
rized in Table 1. Among a series of allylimidazolium halide,
1-allyl-3-methylimidazolium halides [AMIm]X (X = Cl, Br,
and I), and 1,3-diallylimidazolium bromide [AAIm]Br were re-
having allyl groups were prepared. Their ionic conductivity, vis-
cosity, and thermal properties were investigated. Introduction of
allyl group on N-position effectively suppressed the crystalliza-
tion of the corresponding salts. 1,3-diallyl imidazolium halides
therefore showed considerably lower viscosity and higher ionic
conductivity than other halide salts.
8
9
ported by Chan et al., and Jones et al., respectively about 30
years ago. Very recently, Wu et al. used [AMIm]Cl as a solvent
1
0
for cellulose. These previous papers have only mentioned that
some allylimidazolium halides could be obtained as liquid.
However, in spite of potential utility of this type of ionic liquid,
there has been no systematic study on the structure-physical
property relationship for allylimidazolium halide salts. Hence,
we have prepared a series of allylimidazolium type RTIL includ-
ing several novel salts, and clarified their physical properties for
the first time in detail.
Room temperature ionic liquids (RTIL) are collecting in-
creasing attention as a new class of ion conductive matrices
1
for electrochemical devices as well as reaction media for organ-
2
ic synthesis. Most RTILs are composed of 1,3-dialkylimidazo-
ꢁ
ꢁ
ꢁ
lium cations and large anions such as BF4 , CF3SO3 , PF6 ,
ꢁ
and (CF3SO2)2N . On the other hand, 1,3-dialkylimidazolium
halides were not used as RTIL because of their relatively high
melting points (Tm). Recently, 1,3-dialkylimidazolium halides
having relatively low Tm, such as 1-butyl-3-methylimidazolium
As a typical example of a series of allylimidazolium salts,
preparation of 1-allyl-3-ethylimidazolium bromide [AEIm]Br
was carried out as follows. To a solution of N-allylimidazole
(5.0 mL, Acros Organics) in 20 mL toluene, excess amount of
1-ethylbromide (10 mL, Kanto Chemical) was added dropwise
ꢂ
chloride (Tm ¼ 67 C), have received attentions as a novel sol-
3
vent for bio- or bio-related materials. For example, cellulose,
cyclodextrin,4 and DNA were found to be soluble in 1,3-
dialkylimidazolium halides at temperature above Tm. These hal-
ide-based ionic liquids can readily dissolve biomaterials because
of their high polarity, and proton accepting nature. Furthermore,
Kimizuka and Nakashima reported the preparation of fibers, bi-
layer membranes, and vesicles through self-assembly of compo-
nent molecules in ether containing 1,3-dialkylimidazolium hal-
5
ꢂ
at 0 C. The solution was gradually wormed up to room temper-
ature. The reaction mixture was stirred for several hours. During
the stirring, an oily phase was separated from the reaction mix-
ture. This binary solution was further stirred for 24 h. After re-
moving the solvent and unreacted 1-ethylbromide, oily product
was washed with toluene for several times. Then, the product
ꢂ
was dried in vacuo at 80 C for more than 48 h. Water content
was found to be below 0.1 wt %. In the case of chloride salt,
6
ides with a small amount of water (ꢃ2:5 wt %).
ꢂ
However, due to relatively high Tm, 1,3-dialkylimidazolium
halide salts are rarely used as solvents at room temperature. For
the improvement of this drawback, advanced molecular design is
the reaction was carried out at 80 C. The water content of chlo-
ride salts was confirmed to be lower than 0.2 wt % by Karl–
Fischer titration. Water content of chloride-containing salts
was higher than that of other salts even after drying. This should
be due to larger charge density of chloride anion.
7
required for lowering their Tm and viscosity. Although some
imidazolium halide salts having long alkyl chains (C5–C10)
are known to form super-cooled liquid, they usually show high
viscosity. In the present work, we analyzed the effect of allyl
All the allylimidazolium halide salts were obtained as vis-
cous, slightly yellowish liquid at room temperature. These salts
ꢂ
kept liquid state by storing at 5 C for over one year. When these
ꢂ
Table 1. Chemical structure of 1-allyl-3-R-imidazolium halide
salts; R represents for alkyl or allyl group
salts were kept at ꢁ18 C, [AAIm]Cl, [AMIm]Br, [APIm]Br,
and [APeIm]Br were slowly crystallized within a few weeks.
However, in any case, they could not be recrystallized from su-
persaturated solutions by simple cooling. Under the same condi-
tion, [AEIm]Br, [AAIm]Br, [AOIm] Br, and [AAIm]I remained
liquid for over 6 months. These results show very strong super-
cooling nature of allylimidazolium halide salts.
ꢁ
Ionic Liquid
AAIm]Cl
AMIm]Br
R
X
ꢁ
[
[
Allyl
Me
Et
n-Propyl
Allyl
n-Pentyl
n-Octyl
Allyl
Cl
Br
Br
Br
Br
Br
Br
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
[AEIm]Br
[APIm]Br
Table 2 shows physical properties of the obtained salts. In
the DSC measurement, each sample showed glass transition tem-
[AAIm]Br
[
APenIm]Br
AOIm]Br
AAIm]I
AAIm][TFSI]
ꢂ
perature (Tg) at around ꢁ60 C. The decomposition temperature
[
ꢂ
(
Td) was higher than 250 C. The viscosity (ꢀ) of the series of
ꢁ
[
I
allylimidazolium halide salts was lower than 1000 cP at ambient
temperature except for [AAIm]Cl and [APIm]Br. [AEIm]Br
showed the lowest viscosity among these salts.
ꢁ
(CF3SO2)2N
[
Allyl
Among a series of salts, it is noteworthy that [AAIm]X were
obtained as liquid under ambient temperature in spite of sym-
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.10 (2004)
1361
Table 2. Physical properties of a series of 1-allyl-3-R-imidazo-
lium halide salts; R represents alkyl or allyl group
We also prepared [AAIm][TFSI] salt. The salt was obtained
as less viscous but hydrophobic RTIL with a water content of
24 ppm. Similarly to other halide salts, [AAIm][TFSI] did
not show Tm in spite of symmetric structure of the cation.
ꢁ1
ꢁ
i/mS cm
Tg
Tm
Td
ꢀ/cP
Ionic Liquid
ꢂ
ꢂ
/ꢂC /ꢂCb (25 ꢂC)
a
(25 C)
/ C
ꢂ
The viscosity was 31 cP at 25 C, showing that this salt can be
13
[
[
[
AAIm]Cl
AMIm] Br
AEIm] Br
0.58
1.55
1.05
0.48
0.74
0.17
0.06
1.21
2.48
ꢁ68:3 21.5 234
ꢁ65:5 53.1 264
1198
852
723
1665
827
categorized as one of the least viscous RTILs. These data also
demonstrated the advantage of introducing allyl group to prepare
excellent RTIL.
ꢁ69:0
—
270
[
[
APIm] Br
AAIm] Br
ꢁ67:0 29.5 256
ꢁ67:9 252
In conclusion, all the N-allyl substituted imidazolium salts
obtained were RTIL showing moderate viscosity. N-allyl group
has a strong plasticizing effect to introduce amorphous nature of
the ionic liquids. From the comparison of bromide salts, 1-allyl-
3-ethylimidazolium cation was confirmed to be better for the
preparation of RTIL having low viscosity and amorphous nature.
—
[
APenIm]Br
AOIm]Br
AAIm]I
AAIm][TFSI]
ꢁ61:3 21.0 260
[
ꢁ64:5
ꢁ72:1
ꢁ91:6
—
—
—
253
249
389
[
[
a
b
31
Temperature at signal peak.
Temperature for 10% weight loss under N2 atmosphere.
The present study was carried out under the COE program of
Future Nano Materials. This work was financially supported by
the Grant-in-Aid for Scientific Research (No. 14205136, and
No. 15750152) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
metrical structure. Generally, salts with such symmetrical
imidazolium cations are easily crystallized. For example, 1,3-
dipropylimidazolium bromide was reported to show Tm at
ꢂ
11
1
34 C. More interestingly, [AAIm]Br has considerably lower
ꢂ
viscosity (827 cP at 25 C) than that of [APIm]Br as shown in
Table 2. These results demonstrated strong plasticizing effect
of N-allyl group related to extremely slow crystallization and
low viscosity of these salts.
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ꢂ
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2
3
Figure 1 shows the temperature dependence of the ionic
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ꢁ
4
ꢁ3
ꢁ1
scan. The ionic conductivity was 10 –10 S cm at ambient
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4
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Figure 1. Temperature dependence of the ionic conductivity
for 1-allyl-3-R-imidazolium halide salts.
Published on the web (Advance View) September 18, 2004; DOI 10.1246/cl.2004.1360