A programmed hydrogen bonding array self-assembles into a polymeric zipper-like architecture

Article abstract of DOI:10.1039/b515559b

With two hydrazide motifs installed at one edge, a novel supramolecular system self-assembled into a polymeric zipper-like structure via two hydrogen bonds at each knot, both in solution and in the solid state. the Royal Society of Chemistry the Centre Na

Full text of DOI:10.1039/b515559b

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LETTER
www.rsc.org/njc | New Journal of Chemistry
A programmed hydrogen bonding array self-assembles into a polymeric
zipper-like architecturew
Yong Yang,ab Ya-Zhou Zhang,ab Ya-Lin Tanga and Chuan-Feng Chen*a
Received (in Durham, UK) 2nd November 2005, Accepted 22nd December 2005
First published as an Advance Article on the web 6th January 2006
DOI: 10.1039/b515559b
With two hydrazide motifs installed at one edge, a novel
supramolecular system self-assembled into a polymeric zipper-
like structure via two hydrogen bonds at each knot, both in
solution and in the solid state.
10ꢀ2 ppm Kꢀ1). The observation suggests that 1 might exist as
a dimer in solution. Unequivocal evidence for this suggestion
comes from the result of a 2D NOESY experiment which
shows that contact exists between CH3 and H3. Diluting the
solution of 1 (in chloroform-d from 200 mM to 0.5 mM)
results in a strong concentration-dependent change in the
chemical shift of H2. Nonlinear regression analysis7 of the
concentration dependent data yields a dimerization constant
Recently, Keinan et al. proposed a new kind of molecular
zipper system, based on the results of first principles DFT
calculations.1 While similar to the polymeric zipper structure
of natural DNA2 and other artificial molecular zipper sys-
tems,3,4 hydrogen bonding components and information-bear-
ing components in this kind of zipper system are located on
different moieties. So that the physical properties derived from
this kind of supramolecular structure can be modified by
alternating the information-bearing components, with the
supramolecular structure preserved. Following a similar strat-
egy, the present contribution reports a molecular zipper
system of such kind, based on a programmed hydrogen
bonding motif, from an experimental perspective. The design
employs a relatively simple monomeric building block, in
which two hydrogen bonding motifs at one edge, linked by a
rigid spacer, are responsible for assembling the infinite supra-
molecular zipper structure (Scheme 1). Interestingly, another
two supramolecular zipper-like structures, having p–p stack-
ing and van der Waals interactions as locking forces, are
observed in the solid state. All of the constituent functional
moieties in 2 are advantageously used to bring about three
noncovalent interactions which together result in a multilayer
architecture.
Hydrazide derivatives,5 like the related amides, have proven
to be good hydrogen bonding acceptors and donors. We
envisaged that this functional group might be ideally suited
for constructing new supramolecular systems, such as supra-
molecular zippers. As an initial test of this proposal, the
monofunctional hydrazide 1 was synthesized by coupling
acetohydrazide with the corresponding benzoic acid deriva-
tive. Variable-temperature (40 1C to ꢀ50 1C) 1H NMR
experiments with 1 showed that the H2 resonance (Fig. 1)
shifts downfield with a large temperature coefficient (ꢀ2.41 ꢁ
for 1 of 15.6 ꢂ 0.9 Mꢀ1
.
X-Ray crystallographic analysis also showed that 1 exists as
a dimer via two C(4)8 type intermolecular hydrogen bonds in
the solid state (Fig. 1a). Highly favorable, S(6) type8 hydrogen
bonding causes this molecule to be almost planar. Bond length
and angle parameters indicate that both the intra- and inter-
molecular hydrogen bonds are moderate to strong in the solid
state.9
We set out to determine if the hydrazide based structural
unit could be used as a reliable synthon10 for constructing
supramolecular zippers. For this purpose, the difunctional
hydrazide 2 was prepared by employing a procedure similar
to the one used to construct 1.6 The contrastingly poor
solubility11 of 2 in CDCl3 (o6 mM, 25 1C for 2 vs. 4200
mM, 25 1C for 1) gave the first indication that 2 self-assembles
to form a polymeric, zipper-like structure.12,13 The observa-
tion of contact between CH3 and H3 in the 2D NOESY
spectrum of 2 provides unequivocal evidence for its solution
polymeric structure. The results of variable-temperature 1H
NMR experiments with 2 are very similar to those of the
monofunctional counterpart 1. From 40 1C to ꢀ50 1C, the
signal of H2, which is involved in intermolecular hydrogen
bonding, shifts downfield with a large temperature coefficient
(ꢀ1.94 ꢁ 10ꢀ2 ppm Kꢀ1). In contrast, the signal of H1, which
participates in S(6) type hydrogen bonding, has a much
smaller temperature coefficient (ꢀ3.4 ꢁ 10ꢀ3 ppm Kꢀ1) over
the same temperature range. The observation that the signal
for H2 broadens and shifts upfield when a solution of 2 is
diluted from 5 to 0.5 mM, supports the proposed role it plays
in intermolecular hydrogen bonding. Nonlinear regression
analysis7 of the NMR data yields an association constant for
chain extension of the aggregate of 179.6 ꢂ 38.6 Mꢀ1. In
mixing 1H NMR experiments, H2 signals for both 1 and 2 shift
downfield, which might indicate that stronger hydrogen bonds
between 1 and 2 formed. As a result, corruption of the dimeric
and polymeric structures in solution occurs (Fig. 2).
a Laboratory of Chemical Biology, Center for Molecular Science,
Institute of Chemistry, Chinese Academy of Sciences, Beijing
100080, China. E-mail: cchen@iccas.ac.cn
b Graduate School, Chinese Academy of Sciences, Beijing 100039,
China
w Electronic supplementary information (ESI) available: Experimental
preparations, NOESY spectra, diluting and variable-temperature 1H
NMR experimental data, packing diagrams and crystallographic data
for 1 and 2 (CIF format). See DOI: 10.1039/b515559b
As expected, 2 forms an infinite, hydrogen bonding guided,
supramolecular zipper structure in the solid state. The X-ray
crystallographic structure of this substancez validates this
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140 | New J. Chem., 2006, 30, 140–142

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Scheme 1 Schematic representation of a supramolecular zipper
structure locked by two hydrogen bonds at each knot.
conclusion (Fig. 1b and 3a). In a manner similar to 1, the bond
length and angle parameters (Fig. 1b) indicate that moderate
to strong inter- and intramolecular hydrogen bonds exist in
the solid state for 2. Moreover, two strong S(6) type hydrogen
bonds cause 2 to adopt a planar conformation. In addition,
each molecule is fused to two adjacent molecules via two
hydrogen bonds to form an infinite zipper. A dihedral angle
of 37.01 exists between two adjacent molecules in 2, whereas
two neighboring molecules in the solid state dimer of 1 exist in
a parallel orientation. Perhaps steric interactions caused by the
methyl groups in 2 are responsible for the 37.01 tilted arrange-
ment.
Fig. 2 Partial spectra of mixing 1H NMR experiments. (a) 2 (2.5
mM), (b) 1 (2.5 mM), (c) 2 (2.5 mM) þ 1 (2.5 mM), (d) 2 (2.5 mM) þ 1
(5.0 mM), (e) 2 (2.5 mM) þ 1 (7.5 mM), (f) 2 (2.5 mM) þ 1 (10.0 mM),
(g) 2 (2.5 mM) þ 1 (12.5 mM), (h) 2 (2.5 mM) þ 1 (15.0 mM).
Careful examination of the crystal structures showed that
the monofunctional molecule 1 adopts a ‘simpler’ packing
pattern, forming a multilayer architecture that is devoid of any
p–p stacking interactions.6 However, the difunctional counter-
part 2 employs a more sophisticated packing mode. Here,
adjacent zippers have interdigitated aromatic rings oriented in
an antiparallel fashion, forming a second supramolecular
zipper (Fig. 3b). Strong face-to-face, center-to-edge (offset) p
–p stacking14 interactions are observed between two adjacent
aromatic rings, with a vertical displacement of 3.516 A and a
horizontal displacement of 1.375 A. Interestingly, only one
face of the aromatic ring participates in p–p stacking. The
planar conformation of the monomer facilitates this dense
stacking. According to the results of a recent theoretical
study,15 this p–p stacking interaction should stabilize the first
zipper structure locked by hydrogen bonding. Furthermore,
the octyl side chains in 2 are also interdigitated16 in the solid
state, perhaps a consequence of van der Waals interactions.
This results in a third supramolecular zipper (Fig. 3c). Hydro-
gen bonding and van der Waals interaction function alterna-
tively, resulting in an infinite lamellar structure. Further p–p
stacking of the lamellar structure results in a multilayer
architecture. Thus, all the constituent parts of molecule 2 are
utilized advantageously in creating a highly unique architec-
ture.
In summary, we have developed a novelz supramolecular
system, based on a bis(hydrazide) platform, that contains
three types of infinite supramolecular zipper structures in the
solid state. Formation of the supramolecular system is guided
Fig. 1 Representation of intermolecular and intramolecular hydro-
gen bonds in the solid state. (a) Dimer structure of 1, (b) supramole-
cular zipper structure of 2. The octyl groups are replaced with methyl
groups for clarity. Only hydrogen bonded hydrogen atoms are shown.
Parameters of hydrogen bonds and torsion angles of C–N–N–C are
also shown and the proton-labeling scheme is indicated.
Fig. 3 Supramolecular zippers observed in the X-ray structure of 2.
(a) Zipper locked by hydrogen bonding. (b) Zipper locked by p–p
stacking. For clarity, the octyl groups are replaced with methyl groups.
(c) Zipper locked by van der Waals interaction.
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New J. Chem., 2006, 30, 140–142 | 141

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by a combination of hydrogen bonding, p–p stacking and van
der Waals interaction. We envision that this zipper system
might serve as a good scaffold for the alignment and long-
range ordering of functional entities as part of novel materials.
Moreover, the abundant noncovalent interactions found in
this supramolecular system could make it an ideal model for
theoretical studies of cooperativity among multiple interac-
tions that provide insight into understanding the polymeric
zipper structure of DNA.
Jiang and Z.-T. Li, J. Am. Chem. Soc., 2004, 126, 12386–12394; (d)
X. Zhao, X.-Z. Wang, X.-K. Jiang, Y.-Q. Chen, Z.-T. Li and G.-J.
Chen, J. Am. Chem. Soc., 2003, 125, 15128–15139.
6 See ESIw.
7 C. S. Wilcox, in Frontiers in Supramolecular Organic Chemistry and
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Bernstein, R. E. Davis, L. Shimoni and N.-L. Chang, Angew.
Chem., Int. Ed. Engl., 1995, 34, 1555–1573.
We thank the National Natural Science Foundation of
China and the Chinese Academy of Sciences for financial
support.
9 T. Steiner, Angew. Chem., Int. Ed., 2002, 41, 48–76.
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ꢃc
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142 | New J. Chem., 2006, 30, 140–142