Crystal structure of hydrates of imidazolium salts

Article abstract of DOI:10.1016/j.molstruc.2011.05.032

Single crystal X-ray structures of three imidazolium salts were examined. In contrast to imidazolium salt 1 possessing an ester group which created a network of C-H?halide-anion interactions with a linear array of imidazolium and the corresponding counter

Full text of DOI:10.1016/j.molstruc.2011.05.032

Journal of Molecular Structure 998 (2011) 192–197
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Crystal structure of hydrates of imidazolium salts
a
a
a
a
Shigeo Kohmoto a, , Shinpei Okuyama , Takayuki Nakai , Masahiro Takahashi , Keiki Kishikawa ,
⇑
Hyuma Masu b, Isao Azumaya c
a Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
b Chemical Analysis Center, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
c Faculty of Pharmaceutical Sciences at Kagawa Campus, Tokushima Bunri University, 1314-1 Shido, Sanuki, Kagawa 769-2193, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 April 2011
Received in revised form 19 May 2011
Accepted 19 May 2011
Available online 25 May 2011
Single crystal X-ray structures of three imidazolium salts were examined. In contrast to imidazolium salt
1 possessing an ester group which created a network of CAHꢀ ꢀ ꢀhalide–anion interactions with a linear
array of imidazolium and the corresponding counter anions, unique network structures were observed
in imidazolium salts 2 and 3 with long alkyl chains and with carboxy moieties, respectively. Water mol-
ecules were incorporated in their H-bonding networks. Water molecules were wedged into the two
neighboring halide anions to create square units which acted as a bridge to connect the two adjacent
CAHꢀ ꢀ ꢀhalide–anion networks of imidazolium moieties and counter anions.
Keywords:
X-ray diffraction
Crystal packing
H-bonding network
Imidazolium salts
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
The ionic layer thus created is separated by the layers of aliphatic
substituents [21,22,27–29] (Fig. 1a). This microphase separation is
Recently, much attention has been paid to the chemistry of imi-
dazolium salts as ionic liquids [1–6] and ionic liquid crystals [7,8].
Imidazolium-based ionic liquids are the most commonly used ionic
liquids for extraction and reaction solvents because of their
nonflammability and nonvolatility. Imidazolium salts are also uti-
lized as ionic liquid crystalline materials. Stable liquid crystals could
be generated from imidazolium salts substituted with long alkyl
chains due to an efficient microphase separation of ionic cores
(imidazolium moieties) from flexible lipophilic alkyl chains. They
are investigated as potential ion conductive materials [9–11] and
lithium-ion batteries [12]. X-ray crystallographic studies of imi-
dazolium salts have been studied in order to design novel function-
alized ionic liquids [13–20]. We are interested in the arrangement of
counter anions for elucidation of the assembled structures in ionic
liquid crystals. However, the examples of their structural elucida-
tion are limited [21–25]. In the course of our study on imidazoli-
um-based ionic liquid crystals [26], we have found and examined
the crystal structure of the hydrated imidazolium salts. In this paper,
we report on the array of the imidazolium cations and the corre-
sponding counter anions associated with water molecules.
applied to the generation of stable ionic liquid crystals. The other
type of packing includes CAHꢀ ꢀ ꢀhalide–anion interacted networks,
the association of hydrogen atoms at the 2- and 4-positions of the
imidazolium moiety with the counter anions. Imidazoliums and
counter anions are aligned alternately in a horizontal way [17]
(Fig. 1b).
The network structure we will discuss here involves the incor-
poration of water molecules. There are a few reports on single crys-
tal X-ray structures of hydrated imidazolium halides [30–34].
Water molecules were incorporated in their H-bonding networks.
We found a unique arrangement of water molecules. Water mole-
cules were wedged into the two neighboring halide anions to cre-
ate four-membered square units which acted as bridges to connect
the two adjacent CAHꢀ ꢀ ꢀhalide–anion interacted networks of imi-
dazolium moieties and counter anions (Fig. 1c). A similar type of
the hydrate of the imidazolium salt was reported on the imidazo-
lium substituted with a long mono-alkyl chain [33]. An interesting
water clusters were reported in the crystal structures of metal
complexes of zwitterionic type imidazoliums [18,35]. Apart from
imidazolium salts, halide anions associate with water to form
halide–water clusters. Supramolecular assembly of chloride–water
tapes and bromide–water chains were reported in the crystal
structure of terpyridine salts [36]. An interesting dimeric capsule
of an arene based tripodal amide receptor was formed using fluo-
ride–water clusters as a template [37]. Chloride–water clusters
were generated in MOFs (Metal Organic Frameworks) with free
carboxylic acid sites [38].
In some cases, the way of packing in the crystal structures of
imidazolium salts involves clipping of counter anions with the
two adjacent cationic imidazolium rings. Counter anions are lo-
cated near the surface of imidazolium rings via ionic interactions.
⇑
Corresponding author. Tel.: +81 43 290 3420; fax: +81 43 290 3422.
E-mail address: kohmoto@faculty.chiba-u.jp (S. Kohmoto).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.05.032

S. Kohmoto et al. / Journal of Molecular Structure 998 (2011) 192–197
193
2.2. Synthesis
The imidazolium salts 1 and 3 were prepared by conventional
methods described below. The known salt 2 was prepared accord-
ing to the literature [40].
2.3. 3-(2-Ethoxy-2-oxoethyl)-1-(3,4,5-trimethoxybenzyl)-1H-
imidazol-3-ium bromide (1)
A mixture of 1-(3,4,5-tris(methoxy)benzyl)imidazole (0.10 g,
0.40 mmol) and ethyl 2-bromoacetate (0.13 mL, 1.2 mmol) in ace-
tonitrile (10 mL) was refluxed for 12 h at 90 °C. The solvent was re-
moved under reduced pressure. The mixture was purified by
column chromatography on silica gel (eluent: CHCl3/MeOH = 9/1)
to give 1 (0.14 g, 0.34 mmol) in 84% yield as a white powder. Single
crystals were obtained by recrystallization from chloroform/aceto-
nitrile. Mp. 142–144 °C: 1H NMR (300 MHz, CDCl3)
d 1.32
(t, J = 7.1 Hz, 3H), 3.84 (s, 3H), 3.90 (s, 6H), 4.28 (q, J = 7.1 Hz,
2H), 5.42 (s, 2H), 5.43 (s, 2H), 6.76 (s, 2H), 7.30 (t, J = 1.4 Hz, 1H),
7.41 (t, J = 1.4, 1H), 10.57 (s, 1H): 13C NMR (75 MHz, CDCl3) d
14.42, 50.73, 54.15, 57.03, 61.21, 63.40, 106.76, 122.00, 124.07,
128.54, 138.26, 139.08, 154.27, 166.44: IR(KBr)
m 3172, 3110,
2985, 2841, 1739, 1560, 1510, 1430, 1235, 1163: HRMS (ESI) calcd
for C17H23O5N2 [MꢁBr]+ 335.1601, found 335.1595.
2.4. 1,10-(1,4-phenylenebis(methylene))bis(3-(carboxymethyl)-1H-
imidazol-3-ium) bromide (3)
A mixture of 1,10-(1,4-phenylenebis(methylene))bis(3-(2-eth-
oxy-2-oxoethyl)-1H-imidazol-3-ium) bromide (0.200 g, 0.35 mmol)
in1 M aqueousHCl solution (30 mL) was refluxed for 30 min. The sol-
vent was removed under reduced pressure and the remaining solid
was washed with acetone and THF to give 3 (0.18 g, 0.34 mmol) as
a white powder in 98% yield. Single crystals were obtained by recrys-
tallization from water with diffusion of acetone vapor. Mp. > 270 °C
(dec.): 1H NMR (300 MHz, D2O) d 4.97 (s, 4H), 5.34 (s, 4H), 7.34 (s,
4H), 8.80 (s, 2H): 13C NMR (75 MHz, D2O) d 50.53, 52.95, 122.67,
124.34, 129.78, 134.74, 137.36, 170.43: IR (KBr)
m 3160, 3140, 3120,
3026, 2767, 2681, 2575, 2485, 1733, 1567, 1396, 1199, 1170 cmꢁ1
:
Fig. 1. Schematic representation of three types of network structures of imidazo-
lium salts in their crystal structures. Counter anions and water molecules are
indicated by pink and sky-blue spheres, respectively. (a) A sandwich structure in
which counter anions are clipped by the neighboring imidazolium moieties, (b)
networks of imidazolium-counter ion pairs in a horizontal way, and (c) networks
with incorporation of water molecules via H-bonding. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of
this article.)
MS (ESI) calcd for
355.1399.
C
18H19O4N4 [Mꢁ2BrꢁH]+ 355.1401, found
3. Results and discussion
The structures of Imidazolium salts 1–3 examined are shown in
Scheme 1. The salts 1 and 2 possess phenyl group substituted with
trialkoxy groups. The latter is known to show liquid crystallinity
2. Experimental
2.1. General
Table 1
Crystallographic data for imidazolium salts 1–3.
All the reagents and solvents employed were commercially
available and used as received without further purification. X-ray
diffraction data for the crystals were measured on CCD diffractom-
eters. Data collections were carried out at low temperature [150 K
for 1 and 3ꢀ(H2O)2, and 100 K for 2ꢀH2O] by using liquid nitrogen.
All structures were solved by direct methods SHELXS-97 [39] and
the non-hydrogen atoms were refined anisotropically against F2,
with full-matrix least squares methods SHELXL-97 [39]. Hydrogen
atoms were included as their calculated positions. The position of
hydrogen atoms of a water molecule in the crystals of 2ꢀH2O were
determined based on the electronic density distribution. About a
water molecule in the crystals of 3, the position of hydrogen atoms
were not calculated. Crystal data for 1 (CCDC 819420), 2ꢀH2O
(CCDC 819422), and 3ꢀ(H2O)2 (CCDC 819421) are presented in
Table 1.
Salt
1
2ꢀH2O
3ꢀ(H2O)2
Formula
Crystal system
Space group
a (Å)
b (Å)
c (Å)
C17H23BrN2O5
Triclinic
P ꢁ 1
C47H87ClN2O4
Triclinic
P ꢁ 1
C18H24Br2N4O6
Triclinic
P ꢁ 1
8.8843(9)
10.8379(11)
11.1756(11)
104.141(1)
107.610(1)
103.817(1)
935.90(16)
1.474
7.209(4)
8.242(5)
43.03(2)
85.385(7)
87.767(7)
68.883(6)
2377(2)
1.089
7.0222(8)
8.6568(10)
9.1506(11)
75.440(1)
84.095(1)
87.955(1)
535.51(11)
1.712
a
(°)
b (°)
(°)
c
V (Å3)
Dc (Mg mꢁ3
Z
)
2
2
1
150
0.0508
0.1588
T (K)
150
100
R1 [I > 2
wR2 [I > 2
r
(I)]
(I)]
0.0300
0.0804
0.0624
0.1639
r

194
S. Kohmoto et al. / Journal of Molecular Structure 998 (2011) 192–197
[40,11]. The salt 3 has two imidazolium moieties. Each substituted
with a carboxymethyl group. Single crystals suitable for X-ray
analysis were obtained for all these salts. Single crystals of 1 and
3 were obtained from water pervaded with acetone vapor. Recrys-
tallization was carried out from chloroform/acetonitrile (contain-
ing trace amounts of water) for 2. The salts 2 and 3 were
obtained as monohydrate and dihydrate, respectively. Table 1
shows their crystallographic data. Fig. 2 shows the ORTEP diagram
and the H-bonding networks observed in the crystal structure of 1
in which imidazolium moieties and bromine anions are aligned
alternately. The acidic hydrogen atoms of the imidazolium create
the network of CAHꢀ ꢀ ꢀBr interaction in the direction of the a-axis.
The Cꢀ ꢀ ꢀBr atomic distances are 3.46 and 3.51 Å for the interactions
involving the hydrogen atoms at the 2- and 4-position, respec-
tively. The resulting layers created by this H-bonding are packed
in an anti-parallel manner. The packing pattern belongs to the
one as aforementioned schematically in Fig. 1a. The CAHꢀ ꢀ ꢀBr
interaction is also observed for the phenyl and methyl hydrogen
atoms. The Cꢀ ꢀ ꢀBr atomic distances are 3.81 and 3.78 Å for hydro-
gen atoms of the phenyl and methyl groups, respectively.
Even with three long alkyl chains the salt 2 gave single crystals.
In contrast to the crystal structure of 1, the salt 2 showed a differ-
ent network structure. It contained an equimolar amount of water
molecules. Water molecules were incorporated in the H-bonding
networks. Fig. 3a and b shows its ORTEP and packing diagrams,
respectively. Water molecules are wedged into the two neighbor-
ing halogen anions via the OAHꢀ ꢀ ꢀBr interaction to create a square
unit as depicted schematically in Fig. 1b.
This unit is located alternately with imidazolium to create an
ionic layer. This ionic layer was surrounded by lipophilic layers
created by long alkyl chains. Lamellar type bilayers are constructed
as a whole. Similar lamellar type crystal structure was reported in
benzimidazole substituted with two dodecyl groups [30]. It is
known that 2 is a thermotropic liquid crystalline material exhibit-
ing a hexagonal columnar liquid crystal phase [11,40]. It forms a
disc-like structure by self-assembling in a liquid crystal phase.
An expecting liquid crystal phase from the X-ray structure of 2 is
a smectic phase. This discrepancy could be originated in the ther-
mal motion of molten long alkyl chains in its liquid crystal phase.
Fig. 2. Crystal structure of 1. (a) ORTEP diagram, (b) selected short contacts
between bromide anions and CAH hydrogen atoms indicated with green capped
sticks, and (c) packing diagram. Bromide anions are shown as red-brown spheres
with arbitrary radii in (b) and (c). (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)
Scheme 1. Chemical diagrams of imidazolium salts 1–3.

S. Kohmoto et al. / Journal of Molecular Structure 998 (2011) 192–197
195
Fig. 3. Crystal structure of 2ꢀH2O. (a) ORTEP diagram, (b) packing diagram viewed along the a-axis showing bromine anions and water molecules in a space-filling model, and
(c) a part of the structure showing short contacts observed in the ionic layer in which chloride anions and water molecules are presented in spheres of arbitrary size. The
OAHꢀ ꢀ ꢀCl, CAHꢀ ꢀ ꢀCl, and CAHꢀ ꢀ ꢀO intereactions are indicated with sky-blue, green, and red capped sticks, respectively. Distances are indicated in Å. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of this article.)
Because of this motion, a fan-shaped structural unit will be created
instead of the rigid rectangular plate in its crystalline state. Several
short contacts are observed in the ionic layers. Fig. 3c represents a
part of the structure of 2 to show them. The OAHꢀ ꢀ ꢀCl, CAHꢀ ꢀ ꢀCl,
and CAHꢀ ꢀ ꢀO interactions are indicated with sky-blue, green and
red capped sticks, respectively. The observed distances for the
Oꢀ ꢀ ꢀCl, Cꢀ ꢀ ꢀCl, and Cꢀ ꢀ ꢀO are 3.18, 3.50–3.70, and 3.12–3.52 Å
found polymeric networks created by COOHꢀ ꢀ ꢀX interactions in the
crystals. In contrast to their work, we obtained imidazolium car-
boxylic acid 3 as dihydrate. Fig. 4a shows its ORTEP diagram. The
molecule has a zigzag shape. Two carboxy moieties direct towards
the opposite directions each other, which makes it possible to cre-
ate linear polymeric networks by linking. The salts are linked with
water–bromide–water linkage (Fig. 4b). The resulting network
showed the similar pattern as observed in the crystal structure of
2ꢀH2O. Two molecules of water and bromide anions are united to
create a square unit which behaves as linkers resulting in the array
of the salt 3ꢀ(H2O)2 via water–carboxylic acid H-bonding. The dis-
tances between bromide anions and oxygen atoms of water mole-
cules are 3.15 and 3.14 Å which are 9.5% and 9.8% shorter than the
sums of their van der Waals radii, respectively. Recently, the
respectively. The CAHꢀ ꢀ ꢀ
p interaction [41,42] is also observed be-
tween the trialkoxy substituted phenyl ring and the adjacent
methyl hydrogen atom. The edge-to-face interaction takes place
with the distance of 2.60 Å between the centroid of the phenyl ring
and the adjacent methyl hydrogen atom.
Prior to our study, single crystal X-ray structures of imidazoli-
um dicarboxylic acids were reported by Dyson et al. [17,18]. They

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S. Kohmoto et al. / Journal of Molecular Structure 998 (2011) 192–197
Fig. 4. Crystal structure of 3ꢀ(H2O)2. (a) ORTEP diagram, (b) H-bonded linear network structure, and (c) a part of the crystal structure showing short contacts. The Oꢀ ꢀ ꢀBr,
CAHꢀ ꢀ ꢀBr, OAHꢀ ꢀ ꢀO, C@Oꢀ ꢀ ꢀH interactions are indicated with sky-blue, green, blue, and red capped sticks, respectively. Oxygen atom and bromide anion are indicated by a red
and a red-brown sphere of arbitrary sizes, respectively. The distances are indicated in Å unit. (For interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.)
importance of a new manner of interaction by which water affects
biomolecular behaviors, called halogen–water–hydrogen bridge
(XWH bridge), has been emphasized [43,44]. In this bridge one
H-bonding in water mediated H-bond bridge is replaced by halo-
gen bonding (X-bonding). The XWH bridges stabilize biomolecular
conformations and in mediation of protein–protein, protein–nu-
cleic acid, and receptor–ligand recognition and binding. Since
water molecules are H-bonded with neighboring carboxy moieties
in 3ꢀ(H2O)2, such XWH bridges could play a significant role in the
assembling of imidazolium salts although bromide anions were in-
volved instead of bromine atoms. Fig. 4c presents a part of the crys-
tal structure to show the short contacts observed in the ionic
region. In the figure, the Oꢀ ꢀ ꢀBr, CAHꢀ ꢀ ꢀBr, OAHꢀ ꢀ ꢀO, and C@Oꢀ ꢀ ꢀH
interactions are indicated with sky-blue, green, blue, and red
capped sticks, respectively. The resulting H-bonded chains as
shown in Fig. 4b are interwoven via CAHꢀ ꢀ ꢀBr interactions creating
the network orthogonal to the chains. The C@Oꢀ ꢀ ꢀH interactions
between the carbonyl and the benzylic and one of the phenylene
hydrogen atoms also create the network in the same direction as
for the CAHꢀ ꢀ ꢀBr interactions.
4. Conclusion
In order to elucidate the arrangement of the counter anions in
the crystal structure of imidazolium salts we have examined
X-ray structures of three types of imidazolium salts and found a

S. Kohmoto et al. / Journal of Molecular Structure 998 (2011) 192–197
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CCDC 819420, 819422, and 819421 for compounds 1, 2ꢀH2O,
and 3ꢀ(H2O)2 respectively, contain the supplementary crystallo-
graphic data for this paper. These 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 1233 336 033; or e-mail:
deposit@ccdc.cam.ac.uk.
Acknowledgement
This work was partly supported by a Grant-in-Aid for Scientific
Research (C) (No. 22550118) from the Japan Society for the Promo-
tion of Science.
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