Generation of zwitterionic water channels: Biszwitterionic imidazolium carboxylates as hydrogen-bonding acceptors

Article abstract of DOI:10.1021/cg200232b

Organic crystals containing several types of water channels are generated by clipping water molecules with biszwitterionic imidazolium carboxylates as hydrogen-bonding acceptors. The types of water channels to be generated depend on the size and the shape of the aromatic spacers tethered with two imidazolium carboxylates moieties. Clusters of water tetramers and dimers were obtained from imidazolium carboxylates with phenylene (1) and naphthalene (2) linkers, respectively. The latter showed pseudopolymorphism, affording layers of water hexamers. Imidazolium carboxylate possessing biphenylene linker (3) afforded channels of discrete water tetramers.

Full text of DOI:10.1021/cg200232b

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Generation of Zwitterionic Water Channels: Biszwitterionic
Imidazolium Carboxylates as Hydrogen-Bonding Acceptors
Shigeo Kohmoto,*,† Shinpei Okuyama,† Nobuyuki, Yokota,† Masahiro Takahashi,† Keiki Kishikawa,†
Hyuma Masu,‡ and Isao Azumaya§
†Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, and ‡Chemical Analysis Center,
Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
§Faculty of Pharmaceutical Sciences at Kagawa Campus, Tokushima Bunri University, 1314-1 Shido, Sanuki, Kagawa 769-2193, Japan
S Supporting Information
b
ABSTRACT: Organic crystals containing several types of water channels are generated by
clipping water molecules with biszwitterionic imidazolium carboxylates as hydrogen-
bonding acceptors. The types of water channels to be generated depend on the size and
the shape of the aromatic spacers tethered with two imidazolium carboxylates moieties.
Clusters of water tetramers and dimers were obtained from imidazolium carboxylates with
phenylene (1) and naphthalene (2) linkers, respectively. The latter showed pseudopoly-
morphism, affording layers of water hexamers. Imidazolium carboxylate possessing
biphenylene linker (3) afforded channels of discrete water tetramers.
onsiderable effort has been devoted to design water clusters
in recent years. Investigations of small water clusters from
In the course of our study on imidazolium based ionic liquid
crystals,6 we found interesting crystal structures of hydrates of
imidazolium salts in which water molecules were wedged into the
two neighboring halide anions to act as a bridge to connect the
C
both experimental and theoretical points of view have been
performed to understand the structure and nature of bulk water,1
which are entirely dependent on H-bonding interactions and
fluctuations among water molecules. They are of fundamental
importance in biological, chemical, and physical processes. Many
pharmaceuticals exist as a form of crystalline hydrates, which is
very important to maintain their chemical and physical stabilities.
Dehydration of crystalline hydrates greatly impairs their stabi-
lities as pharmaceuticals.2 To understand the role of water,
numerous crystallographic studies on hydrates of pharmaceuti-
cals have been carried out.2b,3 Therefore, elucidation of H-bond-
ing patterns of water molecules in various surroundings is highly
demanded. One of the definite ways of understanding the
association of water molecules is the crystallographic study of
small water clusters in organic hydrates. A large number of
examples of water clusters have been reported in metal organic
frameworks (MOFs). During the assembling process of coordi-
nation polymers under hydrothermal conditions, an infinite array
of water networks can be created, owing to the coordination of
water molecules to metals as a trigger to develop water clusters.
Various types of water clusters can be generated with MOFs.4
Metal-free organic hosts also afforded a number of water
clusters.5 Cyclic compounds are the typical examples, affording
water clusters of large size. Circularly arranged heteroatom-
containing frameworks behave efficiently as the scaffold to
initiate hydrogen bonding with water molecules, leading to the
formation of water clusters.
two adjacent CꢀH halideꢀanion networks of imidazolium
3 3 3
moieties and counteranions.7 The salts were found to exhibit a
propensity to include water molecules. The results prompted us
to investigate a series of salts by systematically changing the
aromatic tether. As an effective H-bonding acceptor for water
molecules, we employed a carboxylate as a counteranion attached
to an imidazolium moiety. Two units of imidazolium carboxylate
moieties were connected by an aromatic spacer to clip water
molecules. Figure 1 illustrates an example of the method of
clipping of water molecules by imidazolium carboxylate moieties
as ionic clips. The way of sandwiching water molecules could
depend on the size and shape of the spacer. A similar approach
was reported using trans-bis(4-pyridyl)ethylene dioxide5f and
protonated phenazine-2,3-diamine cation selenate salt8 as hosts.
In addition to the strong H-bonding ability of terminal carbox-
ylate anion toward crystalline water, our zwitterionic imidazo-
lium carboxylates have the following special features: (1) the
imidazolium positive charge (CꢀH) can attract water molecule
using charge assisted strong H-bonding, and (2) stacking of
aromatic spacer groups creates a hydrophobic wall to confine
water molecules.
Received: February 21, 2011
Revised:
June 6, 2011
Published: July 01, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/cg200232b Cryst. Growth Des. 2011, 11, 3698–3702
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Figure 1. Schematic representation of the clipping of water molecules
by imidazolium carboxylate moieties as ionic clips.
Chart 1. Biszwitterionic Imidazolium Salts Examined
Zwitterionic imidazoliuim carboxylates have been used as
precursors for the preparation of halogen-free carboxylic acid
functionalized imidazoliums.9 Zwitterionic imidazolium salts are
known as potential ion-conductive ionic liquids.10 A number of
crystal structures of imidazolium salts have been reported.9,11
However, little is known about the crystal structures of zwitter-
ionic imidazoliums.12 The single crystal X-ray structure of
2-(1-(carboxymethyl)-1H-imidazol-3-ium-3-yl)acetate, a rare
example of a zwitterionic imidazolium possessing a carboxylate
as a counteranion, showed the existence of strong intermolecular
O
HꢀO hydrogen bonding between CH2COOꢀ and CH2
3 3 3
COOH moieties.12a Owing to this hydrogen bonding, a linear
array of polymeric structure was created.
As spacers to connect two imidazolium carboxylates, we
choose aromatic moieties, 1,4-phenylene, naphthalene-2,6-diyl,
and biphenyl-4,40-diyl, because of their rigidity and linearity.
Herein, we report on our methodology for the formulation of
various types of water clusters employing zwitterionic imidazo-
lium carboxylates.
Imidazolium carboxylates 1ꢀ3 were prepared from hydrolysis
of the corresponding ethyl ester derivatives followed by the
treatments with triethylamine. Recrystallization from water with
diffusion of acetone vapor afforded their single crystals, which
were subjected to X-ray structural analysis.13
Figure 2a shows the packing diagram of the hydrate of 1
viewed along the b axis of the unit cell. In its crystals, water
molecules were included in the molar ratio 1/H2O = 1:5. It
showed that H-bonded water molecules were sandwiched be-
tween two neighboring carboxylate moieties via H-bonding.14
Figure 2. Packing diagram of crystal 1 5H2O and an illustration of the
3
water channels generated: (a) view along the b axis; (b) side view of the
array of the water tetramer (green circle in part a) (H-bonds are indicated
with black dotted lines); (c) side view of the isolated water molecules
(magenta circle in part a) (CH/O interactions are indicated with green
dotted lines, respectively); (d) illustration of the water channels.
was observed in the crystal. Isolated water molecules were
sandwiched between two neighboring carboxylate moieties via
H-bonding with the O O distance of ca. 2.71 Å (Figure 2c).
3 3 3
The isolated water molecules were also fixed via CH/O interac-
tion with acidic hydrogen atoms at the 2-position of imidazolium
moieties. (The C O distance is ca. 3.13 Å.)
3 3 3
The O O distances are ca. 2.75ꢀ3.17 Å (see Table S1 in the
The imidazolium carboxylate 2 showed pseudopolymorphs
affording two types of crystals: needles and plates. Single
crystal X-ray structural analysis of the needle showed that it
contained water molecules in the molar ratio 2/H2O = 1:4.14
Figure 3a shows that water dimers are sandwiched with the
two neighboring carboxylates in the crystal. They form a 1D
chain of waters. Single-file arrangements of water molecules in
confined environments such as the channels of biological
3 3 3
Supporting Information). An infinite linear array of water tetra-
mers was constructed in the direction of the b axis (Figure 2b).
Imidazolium carboxylate 1 connects the 1D chains and forms a
layer. Thus, a water channel structure was constructed via
H-bonding and a CH/O interaction network in the crystal.
Figure 2d shows an illustration of water channels created in the
crystal. In addition, another type of inclusion of water molecules
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Figure 3. Packing diagram of crystal 2 4H2O: (a) view along the a axis;
3
(b) side view of the 1D array of water dimers and isolated water
molecules (green circle in part a). H-bonds and CH/O interactions
are indicated with black and green dotted lines, respectively. The
hydrogen atoms of water molecules are omitted for clarity.
membranes,15 the interior of carbon nanotubes,16 and the
channels of hydrophobic tubular peptides17 have been paid
special attention in relation to proton transfer. The former
possesses an inner polar lining, while the latter two have
hydrophobic ones. The imidazolium carboxylate 2 provides
confined polar channels suitable for 1D water chains. Similar
zwitterionic water channels of narrow pore size were created
by MOFs containing imidazolium carboxylates. Zinc-coordi-
nated imidazolium carboxylates afforded a polymeric helical
tube encapsulating a 1D water chain.9b,18
In addition to the array of water dimers, channels exist for
arrays of isolated water molecules linked by H-bonding with
the carboxylate moieties (Figure 3b). The water channels are
along the a axis of the unit cell. Various interactions are
observed between the water molecules and 2. The O
distances for H-bonding among water molecules in the
channel are ca. 2.74 and 2.87 Å (see Table S2 in the Support-
O
3 3 3
ing Information). The O
ing between water molecules and carboxylate moieties are in
O atomic distances for H-bond-
3 3 3
the range ca. 2.70ꢀ2.83 Å. Water molecules have CH
O
3 3 3
interactions with the hydrogen atoms of the imidazolium with
the C
O distances in the range ca. 3.21ꢀ3.40 Å. In contrast
3 3 3
to a 1D array of water dimers in the needle crystal of 2, the
plate crystal of 2 has a layer structure of a water hexamer
(Figure 4a). Water molecules are included in the molar ratio
2/H2O = 1:5.5 in the crystal lattice.19 The O
O distances
Figure 4. Packing diagram of crystal 2 5.5H2O: (a) view of the stacked
3 3 3
3
water layers along the b axis; (b) side view of the stacked structure; (c)
top view of the water layers (green circle in part b) (zwitterions 2 are
colored in cyan except the representative molecules; H-bonds are
indicated with black dotted lines); (d) illustration of the sandwiched
water layers.
for H-bonding among water molecules in the layer are in the
range of ca. 2.73 and 2.80 Å (see Table S3 in the Supporting
Information). Water layers are located between the layers of 2
(Figure 4b). Imidazolium carboxylates 2 are stacked and
incline to the water layer. Each carboxylate moiety is
H-bonded with water molecules with the O
O atomic
3 3 3
distances in the range of ca. 2.70ꢀ2.91 Å (Figure 4c). The
angle between the molecular axis of 2 and the water layer is ca.
25°, and the layer distance is ca. 8 Å.
Similarly, a zwitterionic water channel is created in the hydrate
of 3. Water molecules are included in the molar ratio 3/H2O =
1:4 in the crystal lattice.19 Chiral crystals (space group P21) of
one type of the channel structure exists in the crystal. In the
channel structure, four molecules of water were linked with
H-bonding discretely (Figure 5b).
Figure 5c shows the H-bonding network of water molecules
with the zwitterions. Each water molecule is H-bonded with the
neighboring carboxylate moieties and the adjacent water mole-
3 4H2O were obtained from achiral 3. Figure 5a shows that only
cule. The O O atomic distances for H-bonding among water
3
3 3 3
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3
channels including water molecules; (b) side view of the channel
structure; (c) top view of the H-bonding network (green circle in
part b). Zwitterions 3 are colored in cyan except the representative
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As we have shown, biszwitterionic imidazolium carboxylates
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’ ASSOCIATED CONTENT
S
Supporting Information. Preparation information and spec-
b
tral data of 1, 2, and 3, and the crystallographic information of
1 5H2O, 2 4H2O, 2 5.5H2O, and 3 4H2O. This material is
3
3
3
3
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: kohmoto@faculty.chiba-u.jp.
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(13) (a) Crystal data for 1 5H2O: C18H28N4O9, M = 444.44,
3
monoclinic, space group C2/c, a = 27.636(7), b = 4.744(1), c =
19.233(5) Å, β = 122.929(2)°, V = 2116.6(9) Å3, Z = 4, Dcalcd
=
1.395 mg mꢀ3, T = 120 K, μ = 0.113 mmꢀ1, GOF on F2 = 1.028, R1 =
0.0381, wR2 = 0.0813 (I > 2σ(I)), CCDC-803465. (b) Crystal data for
2 4H2O: C22H20N4O4, 2(H2O), 2(O), M = 472.45, triclinic, space
3
group P1, a = 4.7129(13), b = 10.011(3), c = 12.908(4) Å, R =
69.426(3), β = 87.476(3), γ = 81.127(3)°, V = 563.3(3) Å3, Z = 1,
D
calcd = 1.393 mg mꢀ3, T = 100 K, μ = 0.108 mmꢀ1, GOF on F2 = 1.051,
R1 = 0.0560, wR2 = 0.1431 (I > 2σ(I)), CCDC-803467. (c) Crystal data
for 2 5.5H2O: C22H20N4O4, 5.5(O), M = 492.42, triclinic, space group
3
P1, a = 9.5978(14), b = 9.7222(14), c = 13.736(2) Å, R = 85.692(2), β =
77.323(2), γ = 73.387(2)°, V = 1198.2(3) Å3, Z = 2, Dcalcd = 1.365 mg
m
ꢀ3, T = 120 K, μ = 0.109 mmꢀ1, GOF on F2 = 1.026, R1 = 0.0592, wR2
= 0.1602 (I > 2σ(I)), CCDC-803466. (d) Crystal data for 3 4H2O:
3
C24H22N4O4, 4(O), M = 494.46, monoclinic, space group P21, a =
7.166(2), b = 16.621(5), c = 10.800(4) Å, β = 107.616(4)°, V =
1226.0(7) Å3, Z = 2, Dcalcd = 1.339 Mg mꢀ3, T = 120 K, μ = 0.102
mmꢀ1, GOF on F2 = 0.993, R1 = 0.0553, wR2 = 0.1353 (I > 2σ(I)),
CCDC-803468.
(14) The positions of hydrogen atoms of water molecules which
were well-fixed via multi H-bonding in the crystal 1 5H2O and 2 4H2O
3
3
were determined based on the electron density distribution.
(15) Agre, P. Angew. Chem., Int. Ed. 2004, 43, 4278–4290.
(16) (a) Nummer, G.; Rasaiah, J. C.; Noworyta, J. P. Nature 2001,
414, 188–190. (b) Alexiadis, A.; Kassions, S. Chem. Rev. 2008,
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3702
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