Anion-directed self-assembly of coordination polymer into tunable secondary structure

Article abstract of DOI:10.1021/ja049799v

A bent-shaped bipyridine ligand containing a dendritic aliphatic side chain has been synthesized as a ligand and complexed with silver ion through a self-assembling process. The resulting complexes were observed to self-assemble into supramolecular struct

Full text of DOI:10.1021/ja049799v

Published on Web 05/14/2004
Anion-Directed Self-Assembly of Coordination Polymer into
Tunable Secondary Structure
Ho-Joong Kim,† Wang-Cheol Zin,‡ and Myongsoo Lee*,†
Contribution from the Center for Supramolecular Nano-Assembly and Department of Chemistry,
Yonsei UniVersity, Seoul 120-749, Korea, and Department of Materials Science and
Engineering, Pohang UniVersity of Science and Technology, Pohang 790-784, Korea
Received January 12, 2004; E-mail: mslee@yonsei.ac.kr
Abstract: A bent-shaped bipyridine ligand containing a dendritic aliphatic side chain has been synthesized
as a ligand and complexed with silver ion through a self-assembling process. The resulting complexes
were observed to self-assemble into supramolecular structures that differ significantly as a function of the
counteranion size in the solid state, as confirmed by 1-D and 2-D X-ray diffraction experiments. The
secondary structure of a cationic coordination chain appears to be dependent on the size of the counteranion.
As the size of anion increases, the secondary structure of the coordination chain changes, from a helical
chain, via a dimeric cycle, to a zigzag chain in the solid state. Interestingly, dilute solutions of the complexes
exhibiting a columnar structure in polar solvents undergo spontaneous gelation and the resulting gels display
a significant Cotton effect in the chromophore of the aromatic unit. These results represent a significant
example that small variation in the anion size can provide a useful strategy to manipulate the secondary
structure of linear chain and thereby solid-state supramolecular structure.
Introduction
interactions into a helical object which, in turn, undergoes self-
assembly into a columnar entity.6-9 Relatively strong, directional
Self-assembly of designed molecules is a challenging topic
of research in the fields of chemical biology and materials
science because it provides the spontaneous generation of a well-
defined, discrete supramolecular architecture from molecular
components under thermodynamic equilibrium.1 In particular,
there is growing interest in the design of synthetic molecules
that are able to self-assemble into a polymeric chain with
compact helical conformations, analogous to the folded state
of natural proteins.2,3 Helical organization in synthetic self-
assembling systems has been achieved by a variety of strategies
including intramolecular hydrogen bonding,3-5 solvophobic
effects,6 and metal-ligand interactions.7 For example, discrete
oligomer chains based on the connection of aromatic moieties
in a meta geometry can fold through specific intramolecular
interactions, such as complexation between conformationally
restricted bent shaped ligands and transition metals that adopt
a linear coordination geometry, can also give rise to extended
polymeric chains with a helical secondary structure.10-12 The
crystal structure of the complex based on a bipyridyl ligand
derived from a 1,1′-binaphthyl unit and a Ni(II) ion, for example,
demonstrated the formation of an infinite helical chain in which
there are four ligands for each turn of the helix.11 Similar to
this, silver ion-mediated self-assembly of a ditopic pyridine
ligand has been reported to result in extended helical springs
with a tunable helix.12 In contrast, for a conformationally flex-
ible, bent shaped ligand, one can envisage the formation of linear
coordination chains that can adopt folded or unfolded secondary
structures, triggered by counteranion binding of cationic poly-
meric backbones through electrostatic interactions.13 Depending
on the size of the counteranion, one of these possibilities may
be favored because the internal binding site of a cationic scaffold
would be complementary in size to anionic guests. As a conse-
quence, the secondary structure of the coordination chain would
be tuned by variation in the size of the binding counteranion.
† Yonsei University.
‡ Pohang University of Science and Technology.
(1) (a) Lehn, J. M. Supramolecular Chemistry: Concepts and PerspectiVes;
VCH: Weiheim, Germany. 1995. (b) Antonietti, M.; Fo¨rster, S. AdV. Mater.
2003, 15, 1323-1333. (c) Sarikaya, M.; Tamerler, C.; Jen, A. K. Y.;
Schulten, K.; Baneyx, F. Nature Mater. 2003, 2, 577-585. (d) Lee, M.;
Cho, B.-K.; Zin, W.-C. Chem. ReV. 2001, 101, 3869-3892.
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J. AM. CHEM. SOC. 2004, 126, 7009-7014
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A R T I C L E S
Kim et al.
Chart 1
phase which disappears over a narrow temperature range upon
heating and reappeared upon cooling. Especially for the complex
based on NO3-, the heat of fusion and the melting temperature
are shown to be the lowest among the complexes.
The solid-state structure was confirmed by small- and wide-
angle X-ray measurements. The small-angle X-ray diffraction
patterns of 1 show three sharp reflections, which indicate the
existence of an ordered nanoscopic structure (Figure 1). These
reflections can be indexed as a 2-D hexagonal structure with a
lattice parameter a ) 29.6 Å. At the wide angle, only a diffuse
halo remains as evidence of the lack of any positional long range
order other than the 2-D hexagonal packing of supramolecular
units. Similar to 1, the small-angle X-ray diffraction patterns
-
of 2 based on BF4 show sharp reflections corresponding to a
In this report, a conformationally flexible, bent-shaped bipyr-
idine ligand containing a dendritic aliphatic side chain has been
synthesized as a ligand and complexed with silver ion through
a self-assembling process (Chart 1). Remarkably, the secondary
structure of the resulting coordination chain appears to be
dependent on the size of the anionic guest. As the size of the
anion increases, the secondary structure changes, from a folded
helical chain, via a dimeric cycle, to an unfolded zigzag con-
formation in the solid state. The sensitivity to anion size is
highlighted by dramatic differences in the aggregation behavior
of the coordination chains.
2-D hexagonal structure with a lattice constant a ) 31.3 Å. At
the middle angles, however, two sharp reflections appear in the
spacing ratio of 1:2. This is indicative of a modulation of the
electronic density and suggests the existence of a periodicity in
the direction of the stacks of repeating units. The periodicity of
this modulation measured along the column axis is 9.6 Å. It is
only possible to account for this modulation if we assume that
the repeating units adopt a helical stacking in the column.15,16
On the basis of these results and density considerations, the
number (n) of repeating units per pitch can be estimated
according to the following equation, where M is the molecular
weight, F is density, A is cross-sectional area of the column,
and NA is Avogadro’ number.
Results
Scheme 1 outlines the synthesis of the bent-shaped bipyridine
ligand containing a dendritic side chain and the silver complexes.
The design of a dendritic side chain was focused on the
construction of an aliphatic dendritic core based on a chiral tri-
(ethylene oxide) unit. The basic synthetic methodology to
generate such a flexible dendrimer employed a facile convergent
route reported previously,14 in order to control the dendritic
architecture precisely. The first step was performed by a
Williamson etherification reaction of an alcoholic precursor with
methallyl chloride, while the activation step of the resulting
double bond was carried out through a repetitive hydroboration-
oxidation reaction. Etherification of the dendritic ether contain-
ing tosyl group with 3,5-dibromophenol in the presence of
potassium carbonate produced a precursor of ligand molecule.
The final ligand was obtained by treating the molecule with
3-ethynylpyridine according to a Sonogashira reaction. The
resulting ligand was purified by column chromatography (silica
gel) using a mixture of methanol and ethyl acetate (1:8 v/v) as
the eluent. The pyridine ligand was complexed with silver ions
with different counteranions to prepare coordination complexes.
All of the analytical data are in full agreement with the chemical
structure presented.
In contrast to the ligand, all of the complexes show an ordered
solid-state structure that is retained up to a melting temperature,
determined by DSC heating and cooling scans (Table 1). As
shown in Table 1, only a single peak is observed in both heating
and cooling scans of 1-4. It can be attributed to a melting
transition of the aromatic coordination backbone because the
dendritic side chain is not expected to crystallize due to its
branched architecture. Between the cross polarizers, the com-
plexes show strong birefringence corresponding to a crystalline
F × pitch × A × NA
n )
(1)
M
This calculation shows that the pitch of the single helix is
composed of five repeating units, giving rise to a pentagonal
helix in which the interior is occupied by BF4- counteranions.17
This encapsulation of the anions within the cavity is further
supported by high density and aromatic π-π stacking interaction
which will be discussed later. The observed periodicity along
the column axis suggests that an aromatic stacking distance
within the column is 3.2 Å. Similar to those observed for other
silver(I)-π complexes of aromatic compounds, this small
aromatic stacking distance could be attributed to the additional
attractive force mediated by strong Ag-π interaction.18,19 In
addition, similar small inter-aromatic distances (3.13-3.20 Å)
have also been observed for discotic systems with a columnar
order.20
The small-angle X-ray scattering of 3 based on triflate anion
revealed several sharp reflections, corresponding to a 2-D
hexagonal structure with a lattice constant of 34.8 Å, indicating
that the complex self-assembles into a column (Figure 1).
Similar to 2, two sharp reflections with the spacing ratio of 1:2
appear in the middle-angle range, suggesting that the complex
(15) Levelut, A. M.; Oswald, P.; Ghanem, A.; Malthete, J. J. Phys. 1984, 745.
(16) Levelut, A. M.; Malthete, J.; Collet, A. J. Phys. 1986, 47, 351.
(17) As shown in Table 1, the calculated numbers appear to be 4.8 and 5.8 for
2 and 3, respectively, which are less than expected integer values, 5 and 6.
However, this is not unreasonable because the measured densities of the
polydomain samples are expected to be less than the ideal values that would
result from the single crystalline samples.
(18) Zheng, S.-L.; Zhang, J.-P.; Wang, W.-T.; Chen, X.-M. J. Am. Chem. Soc.
2003, 125, 6882-6883.
(19) Munakata, M.; Wu, L. P.; Kuroda-Sowa, T.; Maekawa, M.; Suenaga, Y.;
Ning, G. L.; Kojima, T. J. Am. Chem. Soc. 1998, 120, 8610-8618.
(20) Gearba, R. I.; Lehmann, M.; Levin, J.; Ivanov, D. A.; Koch, M. H. J.;
Barbera, J.; Debije, M. G.; Piris, J.; Geerts, Y. H. AdV. Mater. 2003, 15,
1614-1618.
(14) (a) Jayaraman. M.; Frechet, J. M. J. J. Am. Chem. Soc. 1998, 120, 12996-
12997. (b) Lee, M.; Jeong Y.-S.; Cho, B.-K.; Oh, N.-K.; Zin, W.-C. Chem.
Eur. J. 2002, 8, 876-883.
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Anion-Directed Self-Assembly of Coordination Polymer
A R T I C L E S
Scheme 1. Syntheis of Complexes 1-4
Table 1. Characterization of Complexes of 1-4 by X-ray Scattering Experiments and Thermal Transition
phase transition (°C) and corresponding
enthalpy changes (kJ/mol)
3
3
complex
anion vol (cm /mol)
density (g/cm )
heating
cooling
lattice constant a (Å)
pitch (Å)
nb
1
2
3
4
34
49
66
94
1.40
1.38
1.44
1.43
colh 56.9 (6.0) i
colh 165.3 (4.1) i
colh 163.1 (10.3) i
lam 89.3 (11.4) i
i 43.5 (5.1) colh
i 142.2 (3.9) colh
i 147.9 (10.1) colh
i 65.0 (11.5) lam
29.6
31.3
34.8
29.9a
9.6
9.3
4.8
5.8
b
a Inter-lamellar spacing. n ) number of repeating units per pitch. colh ) hexagonal columnar, lam ) lamellar, i ) isotropic.
adopts a helical stacking in the column with the periodicity of
the helix of 9.3 Å. Additional information on the structural
dimensionality of 3 was obtained by 2-D X-ray diffraction with
an annealed sample exhibiting macroscopic orientation. As
shown in Figure 2, the X-ray beam was directed perpendicular
to the principal axes of the hexagonally arranged columns within
the domain.
6 repeating units per pitch indicate that bulky dendritic chains
are located approximately on top of each other. This packing
structure based on a helical chain is energetically unfavorable
because they are arranged in a row along the helical chain axis.
This packing consideration, the presence of the sharp X-ray
reflections corresponding to a 2-D hexagonal columnar sym-
metry, and the absence of a wide-angle reflection corresponding
to a Ag-Ag interaction rule out the formation of the helical
chains. Inspection of CPK models in combination with X-ray
results revealed that a triflate counteranion can be closely
wrapped by two ligands and two silver ions, leading to a dimeric
cyclic species. The resulting dimeric cycles self-assemble into
a tubular structure which organizes into a 2-D hexagonal lattice.
When we compare the periodicity of the helix, measured from
the middle-angle scattering and the calculated number of ligands,
we find that one helix period contains three stacked dimeric
cycles. To avoid steric hindrance between the dendritic chains,
two neighboring cycles are mutually rotated, giving rise to a
At small angle, a pair of strong (100) reflections were dif-
fracted along the meridian. These reflections are related to a
2D hexagonal array of columns, as confirmed by the powder
X-ray diffraction pattern. At wide angle, a pair of (001)
reflections were diffracted along the equator. This 2-D X-ray
diffraction pattern demonstrates that d spacing (001) reflection
at 9.3 Å accounts for the existence of a periodicity in the
direction of the column axis.
On the basis of the calculation according to eq 1, the number
of repeating units in each pitch is estimated to be 6.17 Assuming
that the coordination chain adopts a helical chain conformation,
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A R T I C L E S
Kim et al.
Figure 1. Small-angle X-ray diffraction patterns (a) and wide-angle X-ray
diffraction patterns (b) of 1-4.
Figure 4. Schematic representation of the self-assembly of 2-4 and their
subsequent self-organization.
otherwise occur between bulky chains of neighboring cycles.
Consequently, this rotation gives rise to a helical structure within
the column with the helical periodicity of 9.3 Å, as evidenced
by the middle-angle X-ray reflections (Figure 2). Similar to 2,
the observed periodicity within the column suggests that an
aromatic stacking distance within the column is 3.1 Å which
could be attributed to the additional attractive force mediated
by strong Ag-π interaction. Similar aromatic stacking distances
have also been observed for silver(I)-π complexes of phenan-
throline compounds, as described above.18
In contrast to the cationic scaffolds based on small counter-
anions described above, the complex 4 based on the largest
counteranion, heptafluorobutyrate, self-assembles into a lamellar
structure with in-plane 2-D order with the layer thickness of
32.2 Å, as evidenced by X-ray diffraction (Figure 1).
Figure 2. 2-D X-ray diffraction pattern of 3.
The lamellar structure of the complex indicates that the
coordination chain adopts an unfolded zigzag conformation
rather than folded helical or cyclic conformations. Considering
the size of the counteranion, this observation is not entirely
unexpected. Molecular models indicate that the heptafluorobu-
tyrate counteranion is too large to be completely encapsulated
by two ligands. Consequently, for optimum interaction of the
cationic chain with the anionic guest, the coordination chain is
likely to adopt an unfolded zigzag conformation, leading to a
layered organization. On the basis of the results described above,
the possible models responsible for the generation of the 2-D
hexagonal columnar and lamellar solid-state structures depend-
ing on the size of counteranion can be presented as shown in
Figure 4.
Figure 3. Schematic representation of the self-assembly of 3 into a helical
column.
Remarkably, dilute solutions of 1-3 in polar solvents undergo
spontaneous gelation and the resulting gels display a significant
Cotton effect in the chromophore of the aromatic unit (Figure
5), indicating the presence of elongated helical columnar
aggregates in solution.3,21 This is in sharp contrast to the solution
behavior of 4 which does not form a gel and displays the lack
of a Cotton effect. Macroscopic gelation and a Cotton effect is
considered to be the result of the presence of elongated cylin-
helical column (Figure 3). All these data are consistent with a
columnar structure in which cyclic dimers are stacked along
the column axis; however, the cycles do rotate around the normal
to their plane. Rotation of one cycle with respect to the adjacent
one within the column is the method by which these cycles fill
empty space, thus improving molecular packing within the
supramolecular packing. This process allows a better arrange-
ment of the dendritic chains around the aromatic core of the
column and thus prevents the steric hindrance that could
(21) (a) Van Gorp, J. J.; Vekemans, J. A. J. M.; Meijer, E. W. J. Am. Chem.
Soc. 2002, 124, 14759-14769. (b) Brunsveld, L.; Glasbeek, M.; Vekemans,
J. A. J. M.; Meijer, E. W. J. Am. Chem. Soc. 2000, 122, 6175-6182.
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Anion-Directed Self-Assembly of Coordination Polymer
A R T I C L E S
Figure 5. (a) UV spectra of 1-4 in solide state. The spectra have been
normalized with respect to their maximum intensity. (b) CD spectra of 1-4
at 25 °C (10 wt % in MeOH).
Figure 7. Fluorescence spectra of 2-4 in the solid state.
in the cavities of the columns is further supported by density
measurements that show high-density values for these complexes
(Table 1).
In the case of the coordination chains based on small coun-
teranions such as NO3- and BF4-, a cationic scaffold can bind
effectively an anionic guest through cisoid conformation, giving
rise to a helical chain that organizes into a 2-D hexagonal
columnar structure. In contrast to complex 2 which exhibits
X-ray reflections corresponding to an intracolumnar order,
complex 1 based on NO3- shows only a broad halo at middle-
angle X-ray range, indicative of the existence of only a liquidlike
order within the column. Nitrate anion is likely to be too small
against the cavity size formed by a pentagonal helix with an
ordered helical pitch, and thus the cavity would be contracted
for efficient binding of such a small counteranion, leading to a
disorder of the helical pitch within the column. This is further
supported by density measurements of the complexes, in which
2 shows a similar high density to those of the other complexes
(Table 1). With increasing the size of counteranion as in the
case of triflate ion of 3, the cavity of helical chains would be
enlarged to form a hexagonal helix for efficient binding of larger
anions. However, the bulky dendritic side groups in the enlarged
helical chains are located on top of each other, meaning that
they are arranged in a row along the column axis. This results
in strong repulsive interactions between the bulky side groups.
To reduce the repulsive interactions, the helical chains would
transform into dimeric cycles which stack atop of one another
with mutual rotation to avoid steric hindrance between the
dendritic side chains, giving rise to a helical column, as shown
in Figure 3. These results indicate that the cationic coordination
chains based on small counteranions can bind efficiently anionic
guests through cisoid conformation.
Figure 6. SEM image of a dried gel of 2.
drical aggregates with a chiral superstructure in polar solutions
of complexes 1, 2, and 3, as deduced from the X-ray experiments
discussed above. A scanning electron microscopy image of the
dried methanol gel of 2 shows a fibrous nanostructure with a
diameter of approximately 10 nm, suggesting that the fiber
consists of a bundle of helical columns (Figure 6).22,23
Discussion
The results described here demonstrate that, as the size of
counteranion increases, the secondary structure of the coordina-
tion chain changes significantly from a folded helix, a dimeric
cycle, to an unfolded zigzag chain in the solid state. This
variation in the secondary structure of the coordination chain
can be rationalized by considering the size of the binding
counteranion. This binding of the counteranions by the coor-
dination chains is reflected in the systematic increment in the
lattice constant of a columnar structure with increasing the size
of counteranion (Table 1). This result implies that the counter-
anions are located in the cavities of the columns rather than
between the columns. The encapsulation of the counteranion
However, heptafluorobutyrate anion as in the case of 4 is
too large against the cavity size formed by cisoid conformation,
and thus the chain conformation would adopt a transoid state
(22) (a) Wu¨rthner, F.; Yao, S.; Beginn, U. Angew. Chem., Int. Ed. 2003, 42,
3247-3250. (b) Jang, W.-D.; Jiang, D.-L.; Aida, T. J. Am. Chem. Soc.
2000, 122, 3232-3233.
(23) Cuccia, L. A.; Lehn, J.-M.; Homo, J.-C.; Schmutz, M. Angew. Chem., Int.
Ed. 2000, 39, 233-237.
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A R T I C L E S
Kim et al.
for efficient binding of counteranion, leading to an unfolded
zigzag conformation. This conformational change of the coor-
dination chain to a transoid state is further supported by fluor-
escence measurements which show a 15 nm red shift of the
emission maximum in 4 relative to those of 2-3 (Figure 6).
These results represent a significant example that a small
change of the guest size in self-assembling system can provide
a useful strategy to control the secondary structure and thereby
solid-state supramolecular structure.
heptafluorobutyrate, organizes into a lamellar structure. These
results demonstrate that a systematic variation in the size of
the counteranion can regulate the secondary structure of the
coordination chain, from folded helical, cyclic, to unfolded linear
chain conformations in the solid state. Interestingly, dilute
solutions of the complexes exhibiting a columnar structure in
polar solvents undergo spontaneous gelation and the resulting
gels display a significant Cotton effect in the chromophore of
the aromatic unit. This unique self-assembling behavior can be
explained by considering the size of the counteranion and
consequent chain conformation to maximize the electrostatic
interactions.
Conclusions
A bent-shaped bipyridine ligand containing a dendritic
aliphatic side chain has been synthesized as a ligand and
complexed with silver ion through a self-assembling process.
The resulting complexes were observed to self-assemble into
ordered structures that differ as a function of the counteranion
size in the solid state. The coordination chains based on small
anions such as nitrate and tetrafluoroborate self-assemble into
helical chains that organize into a 2-D hexagonal lattice. The
complex based on triflate anion forms dimeric cycles stacking
atop one another to make columns that laterally assemble in a
hexagonal fashion. In contrast to the complexes based on small
counteranions, the coordination chain based on a larger anion,
Acknowledgment. This wok was supported by the National
Creative Research Initiative Program of the MOST, Korea. We
thank Pohang Accelerator Laboratory for use of the synchrotron
radiation source.
Supporting Information Available: Detailed synthetic pro-
cedures, characterization, and DSC traces. This material is
available free of charge via the Internet at http//pubs.acs.org.
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