Self-assembly of linear-shaped bi-dihydrazine derivative through intermolecular quadruple hydrogen bonding

Article abstract of DOI:10.1016/j.tet.2008.09.013

A linear-shaped bi-1,3,4-oxadiazole derivative, oxalyl acid N′,N′-di(4-(2-ethylhexyloxy)benzoyl)-hydrazide (FH-Z8) was designed and synthesized. Quadruple hydrogen bonds between bi-dihydrazide units and π-π interactions cooperatively participated in forming supramolecules in chloroform at higher concentrations of FH-Z8. The association constants (K) in chloroform were 2.2×10³ and 1.8×10³ M^-1 based on NH1 and NH2 in FH-Z8, respectively. FH-Z8 could gel dichloroethane efficiently with the critical gelation concentration (CGC) of 0.14 wt %, while spontaneously crystallized from the gel during storage.

Full text of DOI:10.1016/j.tet.2008.09.013

Tetrahedron 64 (2008) 10890–10895
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Self-assembly of linear-shaped bi-dihydrazine derivative through
intermolecular quadruple hydrogen bonding
*
Songnan Qu, Min Li
Key Laboratory of Automobile Materials (Jilin University), Ministry of Education, Institute of Materials Science and Engineering, Jilin University, Changchun 130012, PR China
a r t i c l e i n f o
a b s t r a c t
A linear-shaped bi-1,3,4-oxadiazole derivative, oxalyl acid N0,N0-di(4-(2-ethylhexyloxy)benzoyl)-hydra-
zide (FH-Z8) was designed and synthesized. Quadruple hydrogen bonds between bi-dihydrazide units
Article history:
Received 17 May 2008
Received in revised form 30 August 2008
Accepted 2 September 2008
Available online 17 September 2008
and
p–p interactions cooperatively participated in forming supramolecules in chloroform at higher
concentrations of FH-Z8. The association constants (K) in chloroform were 2.2ꢀ103 and 1.8ꢀ103 Mꢁ1
based on NH1 and NH2 in FH-Z8, respectively. FH-Z8 could gel dichloroethane efficiently with the critical
gelation concentration (CGC) of 0.14 wt %, while spontaneously crystallized from the gel during storage.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Quadruple hydrogen bonds
Self-assembly
Supramolecules
Organogel
1. Introduction
bonded building block, which can gel dichloromethane/hexane at
concentrations higher than 2 wt %.9 Li and co-workers reported
Hydrogen bonds play an important role in building super-
structures in chemical and biological systems due to their speci-
ficity and directionality.1 Since Meijer and co-workers first reported
a series of 2-ureido-4(1H)-pyrimidone derivatives that can di-
merize through intermolecular quadruple hydrogen bonds,2 qua-
druple hydrogen-bonded systems have attracted much attention
and have been used in the field of materials science to synthesize,
for the first time, supramolecular polymers through the self-asso-
ciation of self-complementary monomers.3 In the quadruple hy-
drogen-bonding motifs, the linear self-complementary H-bonding
arrays offer distinct advantages in the self-assembly of supramo-
lecular polymers.4
Low-molecular-weight organogels are the materials in which
three-dimensional networks are formed due to the self-assembling
of low-molecular-weight compounds (organogelators) through
noncovalent interactions, and the networks can absorb a large
amount of solvent therein.5 Organogelators containing amide,6
hydroxyl,7 and urea8 groups are the most studied gelators whose
driving force for the formation of one-dimensional self-associated
chains is hydrogen bonding. However, organogels based on qua-
druple hydrogen bonds were rather limited. Wan and co-workers
reported a hydrazide quinolinone-based quadruple hydrogen-
a new class of highly stable hydrazide-based ADDA–DAAD hetero-
dimers, which represented the first successful application of hy-
drazide derivatives in the self-assembly of hydrogen-mediated
supramolecular systems with well-established structures.10 Chen
and co-workers reported a new family of molecular duplex strands
from hydrazide-based oligomers with malonyl groups as linkers.11
Recently, we reported the synthesis and self-assembly of twin-ta-
pered dihydrazide derivatives, oxalyl acid N0,N0-di(3,4,5-trialkoxy-
benzoyl)-hydrazide (FH-Tn), which self-assembled in chloroform at
concentrations higher than 255 mM or in bulk through inter-
molecular quadruple H-bonding.12a The association constants (K) in
chloroform were 447.5 and 217.2 Mꢁ1 based on NH-1 and NH-2
in FH-T7, respectively. Furthermore, the achiral FH-Tns are effective
gelators in ethanol.12b The gelling ability in ethanol, the morphol-
ogies, packing structures, and intermolecular H-bonding strength
were significantly influenced by the length of the alkyl side chains.
FH-T6 and FH-T7 showed strong gelation ability in ethanol with
critical gelation concentrations of 0.25 and 0.30 wt %, respectively.
Both left- and right-handed helical ribbons with non-uniform helical
pitch were observed in FH-Tn (n¼5, 6, 7) gels. Herein we report the
synthesis and self-assembly of a linear-shaped bi-dihydrazine de-
rivative, oxalyl acid N0,N0-di(4-(2-ethylhexyloxy)benzoyl)-hydrazide.
Quadruple hydrogen bonds between bi-dihydrazide units and
p–p
interactions between neighbor phenyl rings cooperatively partici-
pated in forming supramolecules in chloroform at concentrations
higher than 80 mM. The association constants (K) in chloroform were
* Corresponding author. Tel.: þ86 431 85168254.
E-mail address: minli@mail.jlu.edu.cn (M. Li).
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.09.013

S. Qu, M. Li / Tetrahedron 64 (2008) 10890–10895
10891
2.2ꢀ103 and 1.8ꢀ103 Mꢁ1 based on NH1 and NH2 in FH-Z8, re-
spectively, which are larger than those of FH-T7. FH-Z8 could gel
dichloroethane efficiently with the critical gelation concentrations
(CGCs) of 0.14 wt %, and spontaneously crystallization from its gel
was observed during storage.
particles were removed by filtration and successively washed with
100 mL of acetone and 100 mL of chloroform. After removal of the
solvent from the combined filtrate, the crude liquid product 1 and
excess of hydrazine hydrate in 150 mL of methanol were refluxed
for 24 h. The reaction mixture was cooled to room temperature and
the precipitate was filtered. The precipitate was washed with water
then dried under vacuum. The product 2 was obtained as a white
solid (8.5 g, 80%). FH-Z8 was prepared through the route reported
in our previous work,12,14 as shown in Scheme 1. Oxalyl chloride
(1.4 g, 11 mmol) was regularly injected into the THF solution of 6.1 g
(23 mmol) of 4-(2-ethylhexyloxy)-benzoyl hydrazine 2 with vig-
orous stirring at room temperature for 8 h, yielding the product of
oxalyl acid N0,N0-di(4-(2-ethylhexyloxy)benzoyl)-hydrazide (FH-
Z8), which was purified through washing with alcohol (4.8 g, 75%).
2. Results
2.1. Synthesis
Ethyl p-hydroxybenzoate, oxalyl chloride, and 2-ethylhexyl
bromide were used as-received. Tetrahydrofuran (THF) was
refluxed over sodium under argon and distilled before use. 4-(2-
Ethylhexyloxy)-benzoyl hydrazine 2 was prepared according to the
literature.13 A mixture of 6.6 g (40.0 mmol) of ethyl p-hydroxy-
benzoate, 15.0 g of potassium carbonate (dried under vacuum at
80 ꢂC for 12 h prior to use), and 7.7 g (40 mmol) of 2-ethylhexyl
bromide in 150 mL of acetone was stirred and refluxed for 24 h.
After cooling the reaction mixture to room temperature, the solid
2.2. Self-assembly behaviors in chloroform
To understand the self-assembly behaviors of the linear-shaped
bi-dihydrazine derivative, 1H NMR diluting experiments were car-
ried out for FH-Z8 in CDCl3. Figure 1 shows the partial 1H NMR
spectra (500 MHz, CDCl3) of FH-Z8 at different concentrations. At
0.040 mM, peaks of NH1 (near to bi-carbonyl group, 9.92 ppm) and
NH2 (near to alkoxy phenyl, 8.56 ppm) in FH-Z8 were sharp, and
Ar-H1 (near to amide group, 7.81 ppm) and Ar-H2 (near to alkoxy,
6.97 ppm) exhibited double peaks, suggesting its monomeric fea-
ture. Increasing concentrations of FH-Z8 in CDCl3 from 0.080 to
40 mM led remarkable broadening and downfield shift of both NH1
O
C
+
HO
OCH2CH3
RO
RBr
O
C
K2CO3/Actone
80 °C, 24 hrs
OCH2CH3
1
hydrazide hydrate,
O
O O
Cl C C Cl
EtOH
(Dd¼0.92 ppm) and NH2 (Dd¼1.23 ppm), concomitance with the
+
RO
RO
C
NHNH2
broadening and upfield shift of both Ar-H1 (Dd¼0.20 ppm) and Ar-
H2 (Dd¼0.34 ppm). The dependence of the chemical shifts of NH1,
NH2, Ar-H1, and Ar-H2 in FH-Z8 on concentrations is shown in
Figure 2. The chemical shifts of NH1 and NH2 increased, while
those of Ar-H1 and Ar-H2 decreased almost linearly with the in-
crease in concentration and then slowed down and appeared to
level off at 8 mM. The above results demonstrated that quadruple
reflux 40 hrs
2
O
C
H
N
H
O
C
THF
N
C
O
OR
room temperature 8 hrs
C
O
N
H
N
H
FH-Z8: R=
Scheme 1. Synthesis of FH-Z8.
hydrogen bonds between bi-dihydrazide units and
p–p in-
teractions between neighbor phenyl rings cooperatively
Figure 1. Partial proton NMR spectra (500 MHz, CDCl3, 25 ꢂC) of FH-Z8 at different concentrations: (a) 40, (b) 3.60, (c) 1.27, (d) 0.400, (e) 0.08, (f) 0.04 mM.

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S. Qu, M. Li / Tetrahedron 64 (2008) 10890–10895
by the ‘falling drop’ method.16 The Tm increased from 13 ꢂC for the
gel at 0.14 wt % to 42 ꢂC at 1.8 wt %. The sol–gel transition can be
repeated many times through a heating–cooling process. However,
if the gel was allowed to stand more than 12 h at room tempera-
ture, it crystallizes spontaneously. The procedure for gel formation
and gel–crystal transition is outlined in Figure 4. Crystallization
from the gel was completed at 50 h.
In order to investigate the aggregation morphology of the
organogel, the xerogel FH-Z8 from dichloroethane was prepared
and subjected to the scanning electron microscope (SEM) obser-
vation. To avoid crystallization during storage, the FH-Z8 xerogel
from dichloroethane was obtained once the gel formed and then
immediately evaporating the solvent in vacuum. As shown in
Figure 4a, the FE-SEM image of FH-Z8 xerogel revealed a network
structure composed of intertwined fibers with the diameters of
200–400 nm. The formation of elongated fiber-like aggregates in-
dicates that the self-assembly of FH-Z8 is driven by strong di-
rectional intermolecular interactions. The FE-SEM images of
crystals developed from the gel in dichloroethane were board-like,
as shown in Figure 5b.
Figure 2. Concentration dependence of the chemical shifts of NH1, NH2, Ar-H1, and
Figure 6 shows the X-ray diffraction patterns of FH-Z8 xerogel,
the corresponding crystal developed from the gel, and the FH-Z8
film casted from chloroform. The XRD pattern of FH-Z8 xerogel
from dichloroethane showed one strong peak (20.7 Å), one rela-
tively weak peak (14.9 Å), and several broad weak peaks in low-
angle range, and two diffuse broad peaks with a maximum at about
4.4 and 4.6 Å, suggesting the coexistence ordered and disordered
feature. The broad peak at 4.4 Å is characteristic of the liquid-like
arrangement of the aliphatic chains, while that at 4.6 Å might be
attributed to the repeat distance of bi-dihydrazide units in stacking,
illustrated in Figure 8(a). The XRD pattern of the FH-Z8 film is al-
most the same as that of the xerogel, suggesting their similar mo-
lecular arrangement in both cases. In contrast, the XRD patterns of
FH-Z8 crystals developed from dichloroethane gel showed three
sharp strong diffractions with the ratio of 1:1/2:1/3 in low-angle
range, indicating its well defined layer structure, and several sharp
diffractions in high-angle range, suggesting its crystalline feature.
The FTIR spectra of the FH-Z8 film casted from chloroform and
the xerogel in dichloroethane were similar, which showed the N–H
stretching vibrations at 3245 cmꢁ1 (the absence of free N–H, a rel-
atively sharp peak with the frequency higher than 3400 cmꢁ1) and
amide I vibration bands at 1693 and 1632 cmꢁ1, indicating that the
N–H groups are associated with C]O groups via N–H/O]C hy-
drogen bonding.17 The N–H stretching vibrations of the FH-Z8
crystal located at 3383, 3314, and 3272 cmꢁ1 and amide I at 1700,
1683, 1658, and 1631 cmꢁ1, revealing that weaker H-bonding
strength compared to that in its gel or casted film. The ns(CH2) and
nas(CH2) in FH-Z8 in the casted film and the xerogel appeared at
2859 and 2930 cmꢁ1, indicating disordered alkyl chains, which was
Ar-H2 protons in the 1H NMR spectra of FH-Z8 in CDCl3.
participated in forming supramolecules at concentrations higher
than 80 mM.
The association constants for FH-Z8 were estimated using the
simplest (isodesmic) model (i.e., Kn¼K, for nꢃ2) from Eq. 1:
ꢀ
ꢁ.ꢀ
ꢁ
2
KCT ¼ ðP ꢁ PaÞ Px ꢁ Pa
Px ꢁ P
(1)
where Pa and Px are the chemical shifts for monomers and mole-
cules within a stack, P is the observed chemical shift, K is the as-
sociation constant and CT is total molar concentration of the
compounds.15 For FH-Z8, Pa was assigned to 9.92 and 8.56 ppm
based on NH1 and NH2, respectively. The data from the diluting
experiment fitted well with the calculated curves, giving associa-
tion constants (K) of 2.2ꢀ103 and 1.8ꢀ103 Mꢁ1 based on NH1 and
NH2, respectively.
2.3. Gelling behavior
We found FH-Z8 could gel dichloroethane with high efficiency
with the critical gelation concentrations (CGCs) of 0.14 wt %. Fig-
ure 3 shows the melting temperatures (Tms) of FH-Z8 gel in di-
chloroethane as a function of concentration. Tms were determined
Figure 3. Concentration-dependent melting temperature of FH-Z8 gels in
Figure 4. Photograghs showing progressive gellation and crystallization of FH-Z8 in
dichloroethane.
dichloroethane with 0.5 wt % FH-Z8 at different time periods.

S. Qu, M. Li / Tetrahedron 64 (2008) 10890–10895
10893
Figure 7. FTIR spectra of FH-Z8: (a) film casted from chloroform, (b) xerogel from
dichloroethane (0.5 wt %), and (c) crystals developed from dichloroethane gel.
Figure 5. SEM images of FH-Z8: (a) xerogel in dichloroethane (0.5 wt %), (b) crystals
developed from dichloroethane gel.
consistent with the XRD results.18 Both ns(CH2) and nas(CH2) in the
FH-Z8 crystal were detected at 2854 and 2926 cmꢁ1, respectively,
indicating increased population of the trans conformation of alkyl
chains in the FH-Z8 crystal (Fig. 7).
Figure 6. XRD patterns of FH-Z8: (a) film casted from chloroform, (b) xerogel from
Figure 8. Schematic representation of FH-Z8 and FH-Tn self-assembling to supra-
dichloroethane (0.5 wt %), and (c) crystals developed from dichloroethane gel.
molecules (the dashed line illustrates the hydrogen bonding).

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S. Qu, M. Li / Tetrahedron 64 (2008) 10890–10895
3. Discussion
Oxalyl acid N0,N0-di(4-(2-ethylhexyloxy)benzoyl)-hydrazide. 1H
NMR: (500 MHz, CDCl3), (ppm, from TMS): 10.0 (s, 2H); 8.7 (s, 2H);
7.81 (d, 4H, J¼8.8 Hz); 6.95 (d, 4H, J¼8.8 Hz); 3.91–3.89 (m, 4H);
1.77–1.73 (m, 2H); 1.51–1.39 (m, 8H); 1.34–1.31 (m, 8H); 0.95–0.90
(m, 12H).
Based on the results above, a packing model of FH-Z8 in chloro-
form at high concentration and the gel in dichloroethane was
proposed in Figure 8(a). Considering that FH-Z8 molecules are
linear shaped, self-assembly of the FH-Z8 molecules through
quadruple hydrogen bonding would cause the approach of the
FTIR (KBr disc, cmꢁ1): 3383, 3314, 3272, 2959, 2927, 2870, 2854,
1700, 1683, 1658, 1631, 1606, 1573, 1559, 1490, 1464, 1441, 1417,
1390, 1314, 1292, 1259, 1244, 1177, 1119, 1080, 1029, 1014, 967, 920,
840, 816, 779, 762, 752, 726, 694, 674, 654, 645, 632.
Anal. Calcd for C32H46N4O6: C, 65.96; H, 7.96; N, 9.61. Found C,
65.75; H, 8.12; N, 9.70.
neighboring molecules and induce
p–p interactions between
phenyl rings, which were confirmed by the 1H NMR diluting
experiment. The broad weak diffraction at 4.6 Å in the XRD
patterns of FH-Z8 film and the xerogel was attributed to the
repeat distance of bi-dihydrazide units of the stacks. The asso-
ciation constants (K) of FH-Z8 in chloroform were 2.2ꢀ103 and
1.8ꢀ103 Mꢁ1 based on NH1 and NH2, which are bigger than
those of FH-T7, which are measured to be 447.5 and 217.2 Mꢁ1
based on NH-1 and NH-2, respectively. The relative small
association constants in FH-T7 was attributed to the energy
required for the out-of-plane rotation of the phenyl rings for
MALDI-TOF MS: m/z: calcd for: 582.3, found: 582.6.
Acknowledgements
The authors are grateful to the National Science Foundation
Committee of China (Project no. 50373016), Program for New
Century Excellent Talents in Universities of China Ministry of Ed-
ucation, Special Foundation for PhD Program in Universities of
China Ministry of Education (Project no. 20050183057), and Project
985-Automotive Engineering of Jilin University for their financial
support of this work.
stacking and no
p–p interactions between neighboring phenyl
rings cooperating in forming supramolecules of the twin-
tapered FH-T7. In the present case, through decreasing the
number of terminal alkyl chains to reduce the packing hin-
drance, linear-shaped FH-Z8 with branched terminal chains was
designed and demonstrated to self-assemble through both hy-
drogen bonding and
p–
p
interactions and to give relatively
Supplementary data
large association constant.
We noticed that the layer spacing from XRD of FH-Z8 in crystals
developed from its gel (17.7 Å) was much smaller than the length of
full extended molecular length of FH-Z8 (35.4 Å). It can be deduced
that FH-Z8 molecules might be large angle tilted in layers or might
not be in linear conformation and thus cause less rigid in-
termolecular H-bonding sites between bi-dihydrazide units, which
is consistent with weaker intermolecular H-bonds as measured by
the FTIR spectroscopy. The exact packing of FH-Z8 in the crystal
needs to be examined further.
It includes gelation properties of FH-Z8 in different solvents.
Supplementary data associated with this article can be found in the
online version, at doi:10.1016/j.tet.2008.09.013.
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A linear-shaped bi-1,3,4-oxadiazole derivative (FH-Z8) with
branched terminal alkyl chains was designed and synthesized.
Quadruple hydrogen bonds between bi-dihydrazide units and p–p
interactions cooperatively participated in forming supramolecules
in chloroform at higher concentrations of FH-Z8. The association
constants (K) in chloroform were 2.2ꢀ103 and 1.8ꢀ103 Mꢁ1 based
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