Self-assembly of a pyridyl derivative and its silver complex: Formation of a liquid crystalline phase and organogels

Article abstract of DOI:10.1080/154214091009914

Self-assembling behavior of N,N′-didodecyl-3,5-pyridinedicarboxamide and its silver complex has been studied. They act as gelators in solution and gelate a variety of organic solvents. Though the silver complex exhibits lower gelation abilities for halogenated and aromatic solvents, long-term stability is achieved for ethanol and acetone gels. Moreover, the silver complex exhibits a smectic A phase on cooling while a single component of N,N′-didodecyl-3-5- pyridinedicarboxamide exhibits no mesomorphic behavior. The XRD pattern for the complex shows that the layer spacing of the S_A phase is 37 A.

Full text of DOI:10.1080/154214091009914

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Self-Assembly of a Pyridyl
Derivative and its Silver
Complex: Formation of a
Liquid Crystalline Phase and
Organogels
Kazuhiro Yabuuchi a & Takashi Kato a
a Department of Chemistry and Biotechnology,
School of Engineering , The University of Tokyo ,
Tokyo, Japan
Published online: 17 Oct 2011.
To cite this article: Kazuhiro Yabuuchi & Takashi Kato (2005) Self-Assembly of a
Pyridyl Derivative and its Silver Complex: Formation of a Liquid Crystalline Phase
and Organogels, Molecular Crystals and Liquid Crystals, 441:1, 261-273, DOI:
10.1080/154214091009914
To link to this article: http://dx.doi.org/10.1080/154214091009914
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Mol. Cryst. Liq. Cryst., Vol. 441, pp. 261–273, 2005
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/154214091009914
Self-Assembly of a Pyridyl Derivative and its
Silver Complex: Formation of a Liquid Crystalline
Phase and Organogels
Kazuhiro Yabuuchi†
Takashi Kato
Department of Chemistry and Biotechnology, School of Engineering,
The University of Tokyo, Tokyo, Japan
Self-assembling behavior of N,N0-didodecyl-3,5-pyridinedicarboxamide and its sil-
ver complex has been studied. They act as gelators in solution and gelate a variety
of organic solvents. Though the silver complex exhibits lower gelation abilities for
halogenated and aromatic solvents, long-term stability is achieved for ethanol and
acetone gels. Moreover, the silver complex exhibits a smectic A phase on cooling
while a single component of N,N0-didodecyl-3,5-pyridinedicarboxamide exhibits
no mesomorphic behavior. The XRD pattern for the complex shows that the layer
˚
spacing of the SA phase is 37 A.
Keywords: hydrogen bonding; liquid crystal; metal coordination; organogel; pyridine
derivative; self-assembly
INTRODUCTION
Self-assembly is one of promising processes for the development of
functional molecular materials [1]. In this process, specific molecular
Partial financial support of Grant-in-Aid for Scientific Research on Priority Areas,
‘‘Dynamic Control of Strongly Correlated Soft Materials’’ (No. 413=13031009) (T.K.)
and The 21st Century COE Program for Frontiers in Fundamental Chemistry (T.K.)
from the Ministry of Education, Culture, Sports, Science and Technology is gratefully
acknowledged. K.Y. is thankful for financial support by the Research Fellowships of
the Japan Society for the Promotion of Science (JSPS) for Young Scientists.
yPresent address: Department of Materials Science and Engineering, Faculty
of Science and Engineering, Tokyo University of Science, Yamaguchi, Onoda-shi,
Yamaguchi 756-0884, Japan.
Address correspondence to Takashi Kato, Department of Chemistry and
Biotechnology, School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan. E-mail: kato@chiral.t.u-tokyo.ac.jp
261

262
K. Yabuuchi and T. Kato
interactions such as hydrogen bonding, p-p interactions, and dipole
interactions often play important roles. On the other hand, metal coor-
dination is also useful for self-assembly [2]. The combination of these
interactions may yield elaborate assembled structures and induce
dynamic functions. From this point of view, pyridyl derivatives are
useful components for the construction of functional molecular materi-
als because of their attractive self-assembling behavior as a hydrogen
bond acceptor [3] and a metal ligand [2b,c,4]. In the field of liquid crys-
talline materials, supramolecular liquid crystals have been developed
using a pyridyl unit as a hydrogen bond acceptor [3].
Pyridyl derivatives are also useful for the construction of self-
assembled fibers and physical gels based on low molecular weight
gelators [5]. Low molecular weight gelators have attracted much
attention due to their potential for functional molecular materials
[6]. These gelators form self-assembled fibers consisting of bundles
of one-dimensional molecular aggregates through noncovalent inter-
actions in solvents. Formation of a three-dimensional network of
these self-assembled fibers results in physical gelation of solvents.
We recently reported on a new low molecular weight gelator having
FIGURE 1 Molecular structures of pyridinedicarboxamide derivatives.

Self-Assembly of a Pyridyl Derivative
263
a pyridyl unit as a hydrogen bond acceptor [5]. For this gelator, the
incorporation of intra- and intermolecular hydrogen bonding leads to
efficient gelation. As for gelators containing ligand groups, only a
few examples have been reported [7]. Utilization of metal coordination
has not yet been developed extensively in the field of physical gels.
Here we report self-assembly of pyridyl derivatives 1–3 and metal
complex 4 (Fig. 1). We have examined the liquid crystalline properties
and gelation abilities of 1–4.
RESULTS AND DISCUSSION
Self-Assembling Behavior in Solution: Formation
of Organogels
We have found that N,N0-didodecyl-3,5-pyridinedicarboxamide 1 acts
as an efficient gelator for common organic solvents among pyridinedi-
carboxamide derivatives 1–3 (Fig. 1). Compound 1 was prepared from
3,5-pyridine dicarboxylic acid. It gelates a wide range of solvents as
shown in Table 1. For example, 4 g of 1 can gelate 1 L of toluene. A
network of self-assembled fibers is formed in the gels as shown in
Figure 2a. It should be noted that the position of amide moieties on
a pyridine ring affects self-assembling behavior of pyridinedicarboxa-
mide derivatives. No gelation ability is observed for compound 2 due to
its high solubility. As for 2,4-disubstituted compound 3, crystallization
occurs without gelation. Only 3,5-disubstituted compound 1 forms
organogels. It is expected that a nitrogen atom on the pyridyl unit is
TABLE 1 Comparison of the Gelation Ability of Pyridinedicarboxamide
Derivatives and Metal Complex 4a
Solvent
1
2
3
4
Acetone
Ethanol
Dichloromethane
Tetrachloromethane
Ethyl acetate
Hexane
Benzene
Toluene
Pyridine
G (9)
G (18)
G (14)
G (13)
G (5)
I
G (9)
G (4)
C
S
S
S
S
S
C
S
S
S
C
C
S
C
C
C
C
C
C
G (5)
G (22)
Gb
Gb
Gb
I
C
Gb
C
aMinimum gel concentrations are in parentheses (given in g Lꢀ1). The following abbre-
viations are used: C, crystallization; I, insoluble; S, solution; G, gel.
bGelation occurred below room temperature.

264
K. Yabuuchi and T. Kato
FIGURE 2 SEM images of the self-assembled fibers of 1 (a) and 4 (b) formed
in ethanol.
involved in the fibrous self-assembly process of these compounds as a
hydrogen bond acceptor. We previously reported on this behavior for
the organogels based on a 2,6-disubstituted bisurea compound [5].
We have also examined gelation abilities of the metal complexes of
pyridinedicarboxamide derivatives. Silver trifluoromethane sulfonate
was chosen as a metal salt because it is soluble in many organic sol-
vents and often used for a component of metallomesogens [2b,c,4].
Complex 4 was simply obtained in a high yield from compound 1
and silver trifluoromethane sulfonate in a molar ratio 2:1. Complex
4 also acts as a gelator as shown in Table 1. No gelation is observed
for metal complexes of other pyridinedicarboxamide derivatives. For
only ethanol and acetone, compound 4 shows good gelation abilities
at room temperature. The morphologies of self-assembled fibers are
slightly changed by the addition of silver trifluoromethane sulfonate
as shown in Figure 2b. The fibers of complex 4 seem to be rounded
and elongated compared to those of 1 as shown in Figure 2a.
Metal coordination improves thermal stability of the gels based on
1. In the case of ethanol gels containing 25 g Lꢀ1 of gelators, the gel-
sol transition temperature increases from 52ꢁC for 1 to 72ꢁC for 4.
Moreover, the addition of silver trifluoromethane sulfonate to 1 affords
long-term stability of self-assembled fibers. Though compound 1
gelates various organic solvents, some of these gels lack long-term
stability. Figure 3 shows changes in appearance of ethanol gels with
time. There is little difference in appearance between two gels shortly
after the preparation. Both gels are turbid. The gel containing 20 g Lꢀ1
of 1 collapses and crystallization occurs in ten days. On the other

Self-Assembly of a Pyridyl Derivative
265
FIGURE 3 Comparison of the appearance of gels based on 1 and 4 shortly
after the preparation of the samples (a) and after 10 days (b).
hand, the gel containing 20 g Lꢀ1 of 1 with 5.1 g Lꢀ1 of silver trifluor-
omethane sulfonate which is equivalent to 25 g Lꢀ1 of complex 4 seems
to be unchanged. This silver-containing gel is stable even for months.
A similar trend is observed for acetone gels based on 4. Figure 4 shows
the SEM images of self-assembled fibers of 1 and 4 taken from the
samples left for two weeks after the preparation. The collapse of a net-
work of self-assembled fibers is observed for the gels based on 1, while
a network of elongated fibers is maintained for the gels based on com-
plex 4. This observation agrees with macroscopic observation as shown
in Figure 3. These results indicate that assembled structures of pyri-
dinedicarboxamides are changed by metal coordination.
1H NMR measurements have been performed to examine the effects
of metal coordination on hydrogen-bonded structures of 1 (Fig. 5).
The downfield shifts of the N-H protons are observed by metal
FIGURE 4 SEM images of the self-assembled fibers of 1 (a) and 4 (b) left for
two weeks after the preparation of gels.

266
K. Yabuuchi and T. Kato
1
FIGURE 5 Comparison of the chemical shifts of H NMR measurements in
CDCl3. (a) 1 (10 mM); (b) complex 4 (10 mM); (c) 1 (20 mM).
coordination. The range of the shifts on coordination is larger than
that on a concentration change. When the concentration of 1 increases
from 10 mM to 20 mM, the chemical shift of N-H protons moves from
6.38 to 6.56 ppm. On the other hand, the addition of silver trifluoro-
methane sulfonate to 10 mM of 1 induces downfield shifts from 6.38
to 7.25 ppm. In addition, the downfield shifts of the proton at the C4
position of a pyridine ring are also observed. These shifts are attribu-
table to ionic interactions with counter anions. A similar trend was
observed for the anion-templated rotaxane formation [9]. The incor-
poration of ionic interactions and hydrogen bonding may lead to the
stabilization of the self-assembled fibers of 4.

Self-Assembly of a Pyridyl Derivative
267
Self-Assembling Behavior in Bulk States: Mesomorphic
Behavior
The mesomorphic behavior of pyridinedicarboxamide derivatives and
their metal complexes has also been studied. As for pyridinedicarbox-
amide derivatives without metal salts, no mesomorphic behavior is
observed. Melting temperatures of 1, 2, and 3 are 153, 91, and 94ꢁC,
respectively. Compound 1 exhibits the highest melting temperature,
which also indicates that a nitrogen atom on the pyridyl unit plays
an important role for the self-assembly process of these compounds
as a hydrogen bond acceptor.
The addition of silver trifluoromethane sulfonate to 1 induces meso-
morphic behavior along with distinct changes in thermal properties.
The melting point increases from 150ꢁC for 1 to 187ꢁC for 4 by metal
complexation. DSC measurements (Fig. 6) and microscopic obser-
vation (Fig. 7) reveal that complex 4 exhibits a smectic A phase
between 173 and 160ꢁC on cooling. These results indicate that the
metal coordination affects the molecular packing of 4. Figure 8 shows
the XRD pattern for complex 4 at 160ꢁC. The layer spacing of the SA
˚
phase is 37 A. The sharp peaks are attributable to the layered struc-
ture of silver atoms. Considering that the distance between a silver
atom and the alkyl terminal group of 1 in complex 4 can be estimated
˚
about 20 A, the alkyl chains are expected to interdigitate one another
as shown in Figure 9.
FIGURE 6 DSC thermogram of complex 4.

268
K. Yabuuchi and T. Kato
FIGURE 7 Polarized optical micrograph of complex 4 at 160ꢁC.
There are only a few examples of low molecular weight gelators
which exhibit mesomorphic behavior [8]. Recently, Mori and cowor-
kers reported troponoid-based gelators with hexagonal columnar
phases [8a]. Mesomorphic properties and gelation abilities seem to
be closely related since non-mesomorphic tropones do not act as
FIGURE 8 XRD pattern of complex 4 at 160ꢁC.

Self-Assembly of a Pyridyl Derivative
269
FIGURE 9 Schematic illustration of layered structure of 4. Triflate anions
are omitted for clarity.
gelators. In our case, it is expected that a smectic layered structure is
closely related to the formation of stable self-assembled fibers in
solvents mentioned above.
CONCLUSION
We have developed a pyridine-based diamide gelator that can form
metal complexes. Metal coordination to a pyridyl unit induces the
stabilization of assembled structures leading to the exhibition of meso-
morphic behavior and the formation of organogels with long-term
stability. Here, the incorporation of hydrogen bonding and metal coor-
dination plays an important role for the self-assembly. Such molecular
design would serve a new approach not only for the development of low
molecular weight gelators and liquid crystalline materials but also for
other organic=inorganic hybrid materials.

270
K. Yabuuchi and T. Kato
EXPERIMENTAL
General Method
Unless otherwise noted, chemical reagents and solvents were used
without further purification. As for dichloromethane, a commercially
available anhydrous solvent was used for reactions. All reactions were
1
carried out under argon atmosphere. H and 13C NMR spectra were
determined with a Jeol JNM-400EX. Infrared spectra were recorded
on a Jasco FT-IR 8900 m spectrometer. Elemental analyses were per-
formed with a Perkin-Elmer 2400II CHNS=O elemental analyzer.
DSC measurements were performed with a Mettler DSC 30.
Synthesis of N,N 0-Didodecyl-3,5-Pyridinedicarboxamide (1)
Thionyl chloride 20 mL was added to 3,5-pyridine dicarboxylic acid
(0.334 g, 2.0 mmol) and the reaction mixture was stirred for 6 h under
reflux condition. Unreacted thionyl chloride was removed under
reduced pressure. The residual needle crystals were dissolved in
10 mL of dichloromethane. This solution was slowly added to a sol-
ution of dodecylamine (0.927 g, 5.0 mmol) and 1.0 mL of triethylamine
in dichloromethane (10 mL) at 0ꢁC. The reaction mixture was stirred
for 8 h at room temperature. The resultant mixture was filtered to give
a white solid. The solid was recrystallized from hexane=ethyl acetate
to afford 1 (0.685 g, 68%) as a white solid (Found: C, 74.25; H, 11.07;
N, 8.57. Calc. for C31H55N3O2: C, 74.20; H, 11.05; N, 8.37%); mp
153ꢁC (from hexane=ethyl acetate); 1H NMR (CDCl3, 400 MHz): d
9.10 (s, 2H, ArH), 8.44 (s, 1H, ArH), 6.25 (m, 2H, NH), 3.50
(q, J ¼ 6.6 Hz, 4H, NHCH2), 1.67 ꢀ 1.61 (m, 4H, NHCH2CH2),
1.35 ꢀ 1.26 (m, 36H, CH2), 0.88 (t, J ¼ 6.6 Hz, 6H, CH3); 13C NMR
(CDCl3, 100 MHz): d 164.77, 150.28, 133.35, 130.08, 40.38, 31.89,
29.63, 29.61, 29.58, 29.53, 29.34, 29.30, 26.98, 22.68, 14.12.
Synthesis of N,N 0-Didodecyl-2,6-Pyridinedicarboxamide (2)
This compound was synthesized as described above for 1, starting
from 2,6-pyridine dicarboxylic acid (3.27 g, 19.6 mmol). The reaction
mixture was extracted with chloroform. The organic layer was washed
with saturated aqueous NaHCO3 followed by drying on anhydrous
MgSO4. After filtration the solvent was removed under reduced pres-
sure. The crude product was purified by column chromatography with
chloroform as eluent followed by recrystallization from hexane=ethyl
acetate to afford 2 (7.42 g, 75%) as a white solid (Found: C, 74.76; H,
11.99; ꢁN, 8.58. Calc. for C31H55N3O2: C, 74.20; H, 11.05; N, 8.37%);
1
mp 91 C (from hexane=ethyl acetate); H NMR (CDCl3, 400 MHz): d

Self-Assembly of a Pyridyl Derivative
271
8.36 (d, J ¼ 8.0 Hz, 2H, ArH ), 8.03 (t, J ¼ 7.8 Hz, 1H, ArH), 7.69 (m,
2H, NH), 3.52 (q, J ¼ 6.8 Hz, 4H, NHCH2), 1.69 ꢀ 1.62 (m, 4H,
NHCH2CH2), 1.39 ꢀ 1.26 (m, 36H, CH2), 0.88 (t, J ¼ 6.4 Hz, 6H,
CH3); 13C NMR (CDCl3, 100 MHz): d 163.37, 148.88, 138.97, 124.92,
39.65, 31.89, 29.74, 29.65, 29.62, 29.60, 29.59, 29.34, 27.06, 22.67,
14.11.
Synthesis of N,N 0-Didodecyl-2,4-Pyridinedicarboxamide (3)
This compound was synthesized as described above for 1 (2.51 g,
15 mmol), starting from 2,4-pyridine dicarboxylic acid. The crude pro-
duct was purified by recrystallization from hexane=ethyl acetate to
afford 3 (6.78 g, 90%) as a white solid (Found: C, 74.26; H, 11.13; N,
8.57. Calc. for C31H55N3O2: C, 74.20; H, 11.05; N, 8.37%); mp 94ꢁC
(from hexane=ethyl acetate); 1H NMR (CDCl3, 400 MHz): d 8.69 (d,
J ¼ 4.8 Hz, 1H, ArH), 8.35 (s, 1H, ArH), 8.06 (t, J ¼ 5.2 Hz, 1H, NH),
7.95 (d, J ¼ 4.8 Hz, 1H, ArH), 6.39 (t, 1H, J ¼ 5.0 Hz, NH), 3.50 (q,
J ¼ 5.9 Hz, 4H, NHCH2), 1.66–1.62 (m, 4H, NHCH2CH2), 1.34 ꢀ 1.26
(m, 36H, CH2), 0.88 (t, J ¼ 7.0 Hz, 6H, CH3); 13C NMR (CDCl3,
100 MHz): d 164.61, 163.60, 150.63, 149.19, 143.14, 124.65, 118.03,
40.38, 39.62, 31.91, 29.63, 29.59, 29.54, 29.45, 29.35, 29.33, 29.28,
26.99, 22.70, 14.14.
Preparation of a Silver Complex (4)
Weighed silver trifluoromethane sulfonate was dissolved in ethanol
(or acetone, THF). Weighed gelator 1 was added to this solution fol-
lowed by heating until a homogeneous solution was obtained. Molar
ratio of silver trifluoromethane sulfonate to 1 is fixed at 0.5. Then
the solvent was removed by evaporation and the resultant solid was
dried in vacuo to afford silver complex 4 as a white solid.
Gelation Test
In a typical gelation experiment, an organic solvent (0.2 mL) was
added to a weighed sample (20 mg) in a test tube. The tube was sealed
and heated until a clear solution was obtained. The resultant solution
was allowed to cool to room temperature. Then gelation was checked
visually. When the tube could be inverted without any flow, it was con-
sidered as ‘‘gel.’’ In this case, more solvent was added to the sample
and minimum gel concentration (MGC) was determined. MGC is the
minimum concentration of a gelator necessary for gelation. When
the mixture remained solution at room temperature, it was further
cooled in a refrigerator to check whether gelation occurred at lower

272
K. Yabuuchi and T. Kato
temperature. When the gel formed at lower temperature was stable
even at room temperature, it was also considered as gel.
Determination of Melting Temperatures of Organogels
For the determination of melting temperatures of organogels, ‘‘drop-
ping-ball’’ method [10] was used. A steel ball was placed on top of
the gel in a test tube. The tube was sealed and wrapped in aluminum
foil, and then itꢁwas heated on a hot stage. As the temperature was
elevated at a 10 C minꢀ1, the position of the ball was monitored. The
temperature at which the steel ball reached the bottom of the tube
was taken as the melting temperature.
Scanning Electron Microscopic (SEM) Observation
For the SEM observation of the self-assembled fibers of gelators
formed in organic solvents, a gelator was dissolved in solvents at the
concentration of MGC. A droplet of the solution was placed on a glass
slide (8 mm ꢂ 8 mm) and frozen by immersion in liquid nitrogen. Then
the solvent was removed in vacuo. The glass was attached to the SEM
sample stage. SEM observation was performed with a Hitachi S-900S.
The accelerating voltage was 10 kV. All the samples were shaded with
platinum.
Long-Term Stability Test
For the test of long-term stability of gels, fresh gels were prepared in a
sealed test tube. These gels were left in the dark under argon atmos-
phere. Changes of appearance of the gels were checked visually. After
two weeks, some of the samples were picked up and used for SEM
observation.
XRD Measurement
X-ray diffraction (WAXD) patterns were obtained using a Rigaku
RINT-2100 system with monochromated CuKa radiation (12.5 kW).
Samples were placed on a glass plate and heated to appropriate
temperatures with a PTC-20L temperature controller.
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