Is Gelation Behavior Predictable through a Crystal Engineering Approach? A Case Study in Four Similar Coordination Compounds

Article abstract of DOI:10.1021/acs.langmuir.9b00027

In this paper, a detailed study on the gelation properties of a series of terpyridine and dipyrazine-pyridine ligands in the presence of metal salts is reported. To reveal the driving forces for the self-assembly of the metallogelators, their crystal stru

Full text of DOI:10.1021/acs.langmuir.9b00027

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers
Are Gelation behaviour Predictable through Crystal Engineering
Approach? A Case Study in Four Similar Coordination Compounds
Hamid Reza Khavasi, and Maryam Esmaeili
Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00027 • Publication Date (Web): 27 Feb 2019
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Are Gelation behaviour Predictable through Crystal
Engineering Approach? A Case Study in Four
Similar Coordination Compounds
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Hamid Reza Khavasi,* and Maryam Esmaeili
Department of Inorganic Chemistry and Catalysis, Shahid Beheshti University, General Campus,
Evin, Tehran 1983963113, Iran
E-mail: h-khavasi@sbu.ac.ir
RECEIVED DATE (to be automatically inserted after your manuscript is accepted if
required according to the journal that you are submitting your paper to)
CORRESPONDING AUTHOR FOOTNOTE: Hamid Reza Khavasi, Tel No: +98 21 29903105,
Fax No: +98 21 22431661.
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ABSTRACT:
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In the present paper, a detailed study on the gelating properties of a series of terpyridine and
dipyrazine-pyridine ligands in the presence of metal salts has been reported. In order to reveal
driving forces for the self-assembling of the metallogelators, their crystal structure has been
scrutinized. Inspired by the gelation of CuCl2[Terpy-nCN], where " Terpy-nCN " is 4′-(n-
cyanophenyl)-2,2′,6′,2′′-terpyridine, in order to look into the aggregation behaviour of the related
analogues, synthesizing CuCl2[Dipyz-py-nCN] derivatives, where " Dipyz-py-nCN " is 4-(n-
cyanophenyl)-2,6-di-pyrazin-2-yl-pyridine, with the same cyano groups was performed. We then
found that the Dipyz-py counterpart forms crystal when the molecules are stacked in an
alternating way, instead of unidirectional one requested in gel formation. Crystal engineering
approach was applied to determine the interactions that are favourable for fabricating a fiber
network that is likely to be present in both crystalline and gel state and founding the interaction
that disturbs this delicate balance between gelation and crystallization in coordination compound,
then we concluded that the subtle balance of molecular shape and intermolecular interactions are
what the gelation and crystallization of the current molecular system are obtained from. This
enables us to find the mutual connection between the structure of molecules, assembly behaviour
as well as intermolecular interactions. With our experiments, a deep understanding of the balance
between the solution, gelation and crystallization with subtle molecular diversions was provided.
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Introduction
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Since not long ago, having a high potential for a wide range of advanced applications, molecular
gels have evolved as a fascinating class of materials.1-21 Despite intense research on these
materials, lack of design rules is probably the main issue that hindered forming gels by linking
molecular structure to the ability of a molecule. Even so, there are reports indicating that
molecules that are highly similar to each other in term of their structure, actually, show very
different abilities when forming a gel22-24. In these studies, libraries of molecules were formed by
changing the molecular structures. Although the reason behind this phenomenon is not very
clear, it may be due to the self-assembly resulting to gelation levitates from a fine balance of
non-covalent interactions. Hence, a slight modification of intermolecular interaction of can turn a
gelator into becoming a non-gelator25. As of now many interesting results have been gained but
the most successful method for creating new gelators is believed to be modifying known gelator
scaffolds26–28Despite the fact that great attempts are still allocated to the investigation of the
subtle structural demands and molecular motifs that control the behaviour of a given compound,
the features for a certain molecule to act as a gelator have remained as a significant challenge.
There is a hypothesis that suggests to build up macromolecule-like aggregates, having
intermolecular interactions is an important criterion for developing gelators. It can be concluded
that before unidirectional (or 1D) interactions lead to the fiber-like morphologies they should
promote 1D self-assembly first29–33. These fiber-like morphologies are typically observed in
molecular gels. Their strong propensity to self-assemble unidirectionally through a set of non-
covalent bonds is what differentiate systems like this from others. Simultaneously, this network
of interactions must not make the system to be too ordered that ends up to
precipitation/crystallization34. The dynamic nature of gel systems aggravates the complexity of
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such a picture, which exhibits anisotropically reversible sol-gel transition upon exposure to
external physical or chemical stimuli at most of the times. However, a drawback of many
supramolecular gels is that they have a great propensity to crystallize over time. This
phenomenon leads to the gel collapsing and limits its lifetime stability35-37. Crystal formation of a
supramolecular gel is based on this fact that the gel is a metastable phase 23,35. Over time, gelator
molecules assemble to generate nucleation points, decreasing the available gelator concentration,
which can cause crystallization38,39. Such phenomena decrease the gel lifetime and therefore its
usefulness. So, developing platforms to avoid small molecule supramolecular gelator
crystallization has gained the interest of many40. To this end, several design strategies have been
proposed, original work in designing long-lived supramolecular gels revolved around the design
of the gelator itself. These strategies are based on criteria to achieve supramolecular gelating
apparent in the process, first by the presence of unidirectional interactions and robust self-
complimentary to enforce one-dimensional self-assembly, and after that by the control over the
fiber-fiber interfacial interaction to control 3D network formation and to prevent crystallization.
To design these gelators including pincer type ligands, some supramolecular synthon approaches
are employed; (a) straightforward method including the identification of molecular scaffolds that
show predominant 1D intermolecular interactions in the solid-state, (b) the aggregation resulted
from the unidirectional stacking of molecules to fabricate fiber like networks, (c) attaching
functional groups with directional interactions. To create new gelators, the cyano groups have
been successfully used. In terms of directionality, cyano group has multiple forms of
interactions. Using robust supramolecular synthon which leads to one-dimensional self-assembly
of terpyridine derivatives with cyano group, producing stable and crystallization-free gels. So,
our interests next turned to investigate the potential strategies to tune the properties of
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supramolecular gels, albeit, systematically which results in enhancing the lifespan stability of
terpyridine-based hydrogels gel up to a noticeable point. A number of functionalized terpyridines
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have shown to be efficient gelators41-50
.
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Standing as an adoptable family of molecules that have been formerly employed in the study of
gel formation successfully, terpyridine compounds include terpyridine, which is functionalized at
the central ring with an aromatic group, electron withdrawing group, donating group or
heterocyclic substitution such as furan, thiophene, oxazole and etc51-57. Here, a complete survey
on the gelating properties of a series of terpyridine and dipyrazine-pyridine ligands,
CuCl2[Terpy-nCN] and CuCl2[Dipyz-py-nCN], [Terpy-nCN] is 4′-(n-cyanophenyl)-2,2′,6′,2′′-
terpyridine, and [Dipyz-py-nCN], is 4-(n-cyanophenyl)-2,6-di-pyrazin-2-yl-pyridine, Scheme 1,
in the presence of metal salts is reported. In general, establishing H-bonding interactions, π–π
stacking contacts and metal coordination by using simple and usually available ligands like this
is doable. In our studies on a series of CN-substituted terpyridine ligands and their dipyrazine-
pyridine counterparts, we observed that very subtle changes in the structure of the molecule,
though alteration like this might seem tiny, had an intense effect on the outcome of the assembly
process and the ability of the ligands to form the gel. As mentioned earlier, the structures of the
molecules and unidirectional interactions has been the main focus of scientists. One noticeable
challenge that is still unsolved is addressing the fact that small alterations to the structure of a
gelator can unpredictably lead to the disruption of the formation of the gel. Specifically here, we
will talk about the assembling process of two copper complexes including CN-functionalized
pincer ligands, CuCl2[Terpy-4CN], and CuCl2[Dipyz-py-4CN]. Notice that these complexes are
closely related to each other and they only differ only in two nitrogen atoms. As discussed above,
these complexes tend to self-assemble themselves and, in turn, form gels. Here, we try the
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underlying principles and rules responsible for the phenomenon will be described. Our attention
will mainly be on the possible formation of metallogels in terms of the connection between the
nature of the gelled component, the designed structure of the gelator and the influence of
molecular interactions such as Van der Waals interactions, π–π stacking and hydrogen bonding.
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Materials and methods
General. Starting materials and solvents were purchased from commercial sources and used as
received. Infrared spectra (4000–250 cm-1) of the solid sample were recorded on a BOMEM-
MB102 spectrometer as 1% dispersion in CsI pellets. With the help of a Heraeus CHN-O Rapid
analyzer Elemental analysis was performed. A Bamstead Electrothermal type 9200 melting point
apparatus was used which provided us to analyze melting points.
Synthesis of ligands [Terpy-3CN] and [Terpy-4CN]. Ligands [Terpy-3CN] and [Terpy-4CN],
differ only in the substitution position of CN group in terpyridine ligands. The synthesis of both
compounds is relatively straightforward. They are conveniently prepared in a one pot synthesis
procedure according to methods described previously58. 4.84 gr (40.0 mmol) amount of 2-
acetylpyridine was added into an ethanolic solution (100 mL) of 2.12 gr (20.0 mmol) of 3- or 4-
cyanobenzaldehyde. 58 mL (50.0 mmol) of a 29.3% aqueous solution of NH3 and 3.08 gr (40.0
mmol) of KOH pellets were then added to the reaction solution and then stirred at room
temperature for 4 hours. The resulted white precipitate was collected by filtration and then
washed three times with 10 mL of ethanol.
For [Terpy-3CN]: yield 65%, m.p=193°C. Anal. Calcd.: C, 79.02; H, 4.22; N, 16.76. Found: C,
78.73; H, 4.53; N, 16.98 FT-IR (cm-1): 3315, 3053, 2235 (υC≡N), 1678, 1585 (υC=N), 1471, 1388
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(υC=C), 991, 881,733. H NMR (300 MHz, DMSO-d6): δ = 8.77-8.68 (6HAr, m), 8.19 (1HAr, s),
8.16-8.12 (1HAr, m), 7.95-7.86 (1HAr, m), 7.78-7.75 (2HAr,m), 7.68-7.63 (1HAr,s) , 7.42-7.38
(2HAr,s). ; 13C NMR (400 MHz, DMSO-d6): δ = 110.93, 111.16, 118.94, 119.19, 120.82,
121.46, 122.46, 126.91, 127.41, 129.51, 130.26, 132.08, 132.89, 136.63, 137.34, 143.76, 144.08,
147.87, 148.54, 153.86, 154.60, 163.37.
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For [Terpy-4CN]: yield 73%, m.p=198°C. Anal. Calcd.: C, 79.02; H, 4.22; N, 16.76. Found: C,
79.45; H, 4.02; N, 16.81. FT-IR (cm-1): 3334, 2224 (υC≡N), 1680, 1585 (υC=N), 1469, 1388 (υC=C),
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1074, 993, 835, 787, 731. H NMR (300 MHz, DMSO-d6): δ = 8.77 (1HAr, d,J=3.0Hz), 8.72
(1HAr, s), 8.67 (1HAr, d, J = 6.0 Hz), 8.14-8.13 (1HAr, m), 8.11-8.02 (2HAr,m), 7.55-7.52 (1HAr,s).
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; C NMR (400 MHz, DMSO-d6): δ = 156.4, 155.1, 149.9, 148.3, 142.5, 142.5, 138.0, 133.7,
128.5, 125.2, 121.5, 118.7 and 112.4.
Synthesis of ligands [Dipyz-py-3CN] and [Dipyz-py-4CN]. For [Dipyz-py-3CN], and [Dipyz-
py-4CN], the synthetic method was similar to that of terpyridine ligands, except starting material
2-acetylpyrazine was used instead of 2-Acetylpyridine.
For [Dipyz-py-3CN]: yield 81%, m.p=258°C. Anal. Calcd.: C, 71.42; H, 3.60; N, 24.99. Found:
C, 71.18; H, 3.83; N, 24.83. FT-IR (cm-1): 3367, 3051, 2233 (υC≡N), 1678, 1603 (υC=N), 1471,
1369 (υC=C), 1120, 1018, 854, 810, 688.
For [Terpy-4CN]: yield 75%, m.p=250°C. Anal. Calcd.: C, 71.42; H, 3.60; N, 24.99. Found: C,
71.63; H, 3.85; N, 24.78. FT-IR (cm-1): 3043, 2225 (υC≡N), 1680, 1604 (υC=N), 1471, 1375 (υC=C),
1111, 1018, 835, 789.
Synthesis of pincer-type Cu(II) Complexes: All complexes were prepared by following
synthetic methodology. A solution of 10 mmol of ligand in 5 mL methanol was added to a
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solution of 10 mmol of copper(II)chloride in 5 mL methanol and then heated at 50 °C for 5 min.
Green precipitate was collected by filtration and purified by recrystallization in methanol.
For CuCl2[Terpy-3CN]: yield 63% based on the copper salt, m.p=decomposed at 176°C. Anal.
Calcd.: C, 56.36; H, 3.01; N, 11.95. Found: C, 56.68; H, 3.22; N, 12.25. FT-IR (cm-1): 3354,
3049, 2231 (υC≡N), 1583 (υC=N), 1402 (υC=C), 1020, 795.
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For CuCl2[Terpy-4CN]: yield 75% based on the copper salt, m.p=above 270°C. Anal. Calcd.: C,
56.36; H, 3.01; N, 11.95. Found: C, 56.42; H, 3.25; N, 11.73. FT-IR (cm-1): 3427, 3049, 2225
(υC≡N), 1603 (υC=N), 1469 (υC=C), 1248, 1016, 837, 793.
For CuCl2[Dipyz-py-3CN]: yield 81% based on the copper salt, m.p=decomposed at 260°C.
Anal. Calcd.: C, 51.02; H, 2.57; N, 17.85. Found: C, 51.16; H, 2.73; N, 17.93. FT-IR (cm-1):
3354, 3052, 2348 (υC≡N), 1581 (υC=N), 1392 (υC=C), 1176, 1039, 814.
For CuCl2[Dipyz-py-4CN]: yield 69% based on the copper salt, m.p=above 270°C. Anal. Calcd.:
C, 51.02; H, 2.57; N, 17.85. Found: C, 51.35; H, 2.73; N, 17.26. FT-IR (cm-1): 3360, 3030, 2351
(υC≡N), 1606 (υC=N), 1400 (υC=C), 1173, 1039, 854.
Preparation of the Gels: In a typical gelation test a weighed amount of the copper complex
containing [Terpy-3CN] or [Terpy-4CN] ligand was mixed with a measured volume of the
solvent such as pure water or mixed solvent. The mixture was sonicated for 30 seconds and the
heating process was performed until the solid was fully dissolved. The resulting solution was
allowed to cool down to room temperature, a more common way to bring about the necessary
supersaturated solution from which gel fibre growth is possible is slow cooling. The results were
confirmed by the tube inversion test. The minimum gelation concentration (MGC) was
determined by scaling a minimum amount of gelator needed for the formation of a stable gel. As
for measuring the gel to the sol transition temperature, Tgel, the inversion test method was used.
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Notice that, for each sample, we repeated the process three times. The gels were optically
transparent with thixotropic properties. In contrast, brief sonication of mixture led to slightly
opaque gel that did not flow under tube inversion. Both kinds of gels were quite stable for
several months.
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Crystallization of Complexes: Despite doing our best, our experiments resulted in the
crystallization of the complexes (CuCl2[Terpy-4CN], CuCl2[Dipyz-py-3CN] and CuCl2[Dipyz-
py-4CN]), by slow evaporation from a methanol solution. In the case of CuCl2[Terpy-4CN],
when the solvent of slow evaporation changed from MeOH to a mixture of water and MeOH,
different kinds of crystals of CuCl2[Terpy-4CN], were obtained. Single crystal diffraction
analysis showed that these two CuCl2[Terpy-4CN] crystals are pseudo polymorphs.
Results and Discussion
Gelation behaviour: We carried out the gelation test at 1 mg.mL-1 concentration in water. After
sonication of the samples for 30 seconds, we slow heated the samples until the solid was
completely dissolved. Afterwards, we allowed the sample to cool down to room temperature,
Figure 1. Gels formed in water are the strongest and the most optically transparent and stable
one. It must be noted that although these pincer ligands were hardly dissolved in water, by
addition of CuCl2 salt (0.1 wt%), in 1:1 molar ratio to ligand, it only took an hour for the white
solid of ligands to dissolve during the stirring process via coordination to copper ion, Figure 1.
A stable gel was formed upon standing for several months. The gel even was formed during the
stirring process with a higher gelator concentration of >0.1 wt%. The gel superstructure, if the
gel is heated above the gel-to-sol phase transition temperature, actually gets disrupted. This leads
to the reformation of the isotropic solution. It is best to be noted that Upon cooling, the sol
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actually reverts back to its former from which is a gel. This reversible process can be repeated
for a great number of cycles. In addition, the metallogel transformed into a viscous sol upon
shaking and altered into a robust gel again upon resting, which indicated thixotropic behaviour
and self-healing abilities of the gels. The gelation ability of gelator was examined in a variety of
alcohols and various aqueous media at the critical concentration of gelator listed in Table 1. In
contrast to the fact that the formation of many of the gels only occurred within a few minutes, the
process of crystallization of some of the samples actually took over an hour. As it is clear from
Table 1, the solvent structure is an important factor in the formation of the gels. Focusing on
bulk gelator solubility, gelators should not be too soluble or too insoluble in solvent systems.
Complexes can only gelate mixed solvents. The reason behind this is that the solubility in most
of the chosen pure solvents is actually lower than the mixed ones. In addition, the MGC of
complexes in pure water was also researched on; gelator concentration is lower than what we
observed in water-miscible organic solvents.
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The terpyridine–Cu(II) complexes form gels in H2O, mixture of H2O:MeOH (1mL:10mL),
H2O:EtOH (2mL:10mL), H2O:PrOH (3mL:10mL), H2O:IsoPrOH (4mL:10mL) and H2O:Aceton
(2mL:1mL), Table 1. Meanwhile, a plot of the gel–sol dissociation temperature, which is the
needed temperature for the conversion of gels into liquids (Tgel), versus the gelator concentration
in the various solvent was built. This was for the evaluation of the thermal stability of the Cu(II)
metallogels. Figure 2 clearly shows the increasing of Tgel as the concentration of the gelator also
increases. These indicate that the thermal stability depends on non-covalent interaction ability of
the gelators in various solvents. With this, the supramolecular nature of the gels network was
explained. In this network non-covalent interactions such as hydrogen bonding and π-π stacking
play important roles for gel network formation. As construction principles, tuning cooperations
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of metal and ligand is widely used for the generation of metallosupramolecular architectures.
Metallogelators are systems in which hierarchical self-assembly process affect with both the
ligand and metal ion exploit as complementary elements59. The gelation ability of ligands with
different chloride salts of metal ions such as Co(II), Mn(II), Ni(II), Fe(II), Cd(II) and Zn(II) was
further analyzed. In our reaction condition, no obvious gelation was observed. The anion-
selectivity of the gel formation was also examined for other selected Cu(II) salts which are
CuBr2, CuI2, Cu(NO3)2, and Cu(OAc)2. As the results didn’t show any gel formation, it was
concluded that counter anions actually play an important role in the assembly of gels. Upon
metal complexation of other salts, maybe the 1D network becomes linked intramolecularly,
which hinders the formation of fiber networks and consequently gelation. On the other hand, the
solubility of other complexes is different so leads to disturbing the balance between the solubility
and further aggregation. The 4CN derivatives of terpyridine type complexes proved to be more
soluble than their 3CN counterparts in general solvents such as water, methanol and ethanol and
did not exhibit a significant difference in gelation behaviour, but 4CN derivatives formed more
stable gel than 3CN and had higher Tgel. In comparison with the CuCl2[Terpy-nCN] complex
gelators, for the CuCl2[Dipyz-py-nCN] homologous complexes, no gels were formed in any
solvents and these materials turned to the crystalline state immediately. It is possible that the
increased electron withdrawing nitrogen atom in CuCl2[Dipyz-py-nCN] complexes led to self-
assembly in the 3D network and hence disfavored gel formation. It revealed that these materials
have large tendency to aggregate and crystallize in the 3D network. Despite this, an active
surface on which substrate nucleation can happen in such a way that the periodicity of the gel
fiber is underlined and transferred to a growing crystal is offered by the local one-dimensional
ordering of gel fibers. This is a process of heterogeneous secondary nucleation. Formation of a
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gel in contrast to crystalline materials is governed by a largely unpredictable balance of non-
covalent interactions. Perturbation of this balance may lead to the collapse of gel states to
precipitate or crystalline species. In our cases when the metallogels were allowed to stand for
several days, the gel did not transform into precipitate or crystalline forms. After several days,
the gels only removed the solvent to form xerogels that cannot convert into single crystals in situ,
so, are called crystallization-free gels. This observation highlights the lack of suitable interaction
for secondary nucleation to the 3D network.
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Crystallization Behaviour. Monitoring crystal formation under conditions that favour gelation
is not very common as there have been efforts made to correlate the mechanistic basis for
crystallization and gelation but none were really convincing and were most confusing and
unclear60,61. In our studies complexe CuCl2[Terpy-4CN] and CuCl2[Terpy-3CN] are soluble in
water, but CuCl2[Terpy-4CN] was hardly soluble in all selected alcohols, and in the case of
CuCl2[Terpy-3CN] was even worse. Gelators have a high solubility in a water-miscible organic
solvent, but a low solubility in water. In contrast to similar gelation behaviour, these two ligands
have significant differences in crystallization behaviour. We found that CuCl2[Terpy-3CN] was
not only able to form an stable crystallization-free gel but also was not able to crystallize in
lower gelator concentration in any of solution observed over the course of a few months. So, our
efforts for obtaining a suitable crystal of CuCl2[Terpy-3CN] were inconclusive. In order to
examine the effect of the solubility on the formation of single crystals of CuCl2[Terpy-3CN],
different pure and protic/aprotic mixed solvents with various ratios have been used as the growth
media. As no crystallization process observed in any of cases, it can be concluded that for
CuCl2[Terpy-3CN], the interaction of gelator and solvent doesn’t play a critical role in the 3D
formation of crystals. Crystallization of a gelator complex system is normally a difficult task,
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although it is hard to obtain suitable crystal from terpyridine base complexes, dipyrazine-
pyridine base complex readily forms single crystals upon resolving in MeOH and subsequently
cools to room temperature within a few minutes.
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Crystal Structure. Using X-ray crystallography analysis, we obtained a deeper understanding of
the nature of intermolecular interactions in the gel-phase aggregation by determining single
crystals of these complexes. A helpful starting point in order to understand the likely
supramolecular arrangements and the way they are likely to influence molecular aggregation in
gel formation could be the study of crystal structures23,34.We hypothesized that the gel formation
may be similar to self-assembly processes in crystallization, at least in one-direction to formation
of fibers60. Interestingly, different crystal forms of CuCl2[Terpy-4CN] are obtained from
different solvents. We report here two structures for the polymorphs of this complex obtained
from MeOH, Form I, a mixture of water and MeOH, Form II, the structure for the complex of
CuCl2[Dipyz-py-3CN] and CuCl2[Dipyz-py-4CN], which were characterized by single-crystal
X-ray diffraction. As shown in Figure 3, the coordination environment around the metal centers
in all complexes is identical and can be defined in 1:1 metal to ligand complex. A distorted
square pyramidal geometry can be best considered for defining of coordination geometry around
each Cu atom. The five coordination sites of Cu(II) ion, are occupied by three nitrogen atoms of
pincer-type ligand and two chloride ions. Despite the similarities in conformation of each related
homologous terpyridine/Dipyrazinepyridine ligands, the 3D formation of each compound is
surprisingly varied. Extended packing schemes are summarized in Figure S1. The packings are
highlighted by a number of interactions. Some are relatively strong non-classical hydrogen bonds
(C-H···N-C) and some are weak non-classical hydrogen bonds (C-H···Cl), Table S2, and
moderately weak intermolecular slipped π-π stacking interactions (Cphen···Cphen and Cphen···Cpyr),
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Table S3. Apart from the strong and weak bonds that were discussed above, as a matter of fact,
there are multiple very weak ones too.
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Supramolecular Synthons. The correlate between the packing similarity of Low Molecular
Weight Gelators (LMWGs) in the gel state as well as in the crystalline state23,62,63 has been
discussed repeatedly in the literature. Clear evidence has shown that an ordered fibrous matrix is
formed by Low Molecular Weight Gelator. The ordered 1D fibers resulted from the assembly of
molecules through directional and strong intermolecular interactions64. It must be noted that
forming of ordered fibrous must be done in priority to crystallization moieties. It must be noted
that regardless of the presence of self-willed structures of a fibrous matrix in the gel state, the
deduction drawn from crystal states must be handled somehow cautiously. The presence of the
mentioned anisotropic interactions are necessary for extending in only one direction through the
crystal most likely leading to fast growth in only one direction and the fibers’ growth.
The relative lack of such contacts in the other two dimensions barricades sidelong growth and
thus crystallization. So, additional attempts have focused on clarifying the correlation between
supramolecular aggregation patterns of a molecule in single-crystalline form and its gelling
behaviour62,65,66. Understanding the correlation between gelation and crystallization processes is
obligatory to furnish rational approaches to the design of non-covalent, self-assembling gel soft
matters. Thus, finding the rational design of relevant gel states have been challenging due to the
particular correlation between self-assembly into either crystalline or fibril states27.
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In order to investigate the property-structure relationships, the inspecting of the supramolecular
synthons in the crystal structures of the gelator is very important. Since recognition of
intermolecular interactions is a key factor to correlate molecular structure and crystal packing,
crystal engineering attempted to identify robust and repeatable supramolecular synthons.
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Repetition of synthons actually occurs in molecules that have the same chemical skeleton with
different functional groups67-69. In fact, synthon repetivity shows specific robust synthons in
related compound series.
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All associations in the first instance through the formation of hydrogen bonding extended
linearly and infinitely a one dimensional (1D) line. Close inspection of the crystal structures of
Cu(II) complexes of this pincer ligands revealed that, pincer complexes assemble to form an
infinite one dimensional (1D) array, which are held together by three kinds of interaction: (1)
the hydrogen bonding between nitrogen atoms of cyano group and hydrogen atoms of the
pyridine rings of neighbouring molecule (2) intermolecular C-H···Cl hydrogen-bonding
interactions (3) π-π stacking interactions between phenyl ring and the other rings in the nearest
adjacent complex. These three kinds of interactions are robust enough to order the assembled
units into linear fiber-type aggregates. It might worth to notice that notwithstanding the
presentment of an intercalary hydrogen bonding functionality (Npyrazine) in the dipyrazine-
pyridine counterpart of terpyridine ligands, the one-dimensional synthon stayed intact; which led
to the formation of the 1D chain along the main direction of the molecule. However, the
additional electron withdrawing nitrogen atoms participated in hydrogen bonding and engaged in
connecting the two neighbouring 1D chains, resulting in an overall 3D network, Figure 4. It
indicates that these interactions may play crucial roles during the formation of the gel.
The control of the self-assembling mode at the molecular level must be considered as a key
factor in the rational design of LMWGs. For the efficient growth of fibers, one-dimensional
assembly is necessary, so, strong and highly directional self-assembling groups are selected. All
of these interactions have the same orientation with the main direction of the molecule. In the
case of pincer terpyridine ligands there aren’t any functional group in the perpendicular
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orientation with the main direction of the molecule but in the dipyrazine-pyridine ligands the
second nitrogen atoms in pyrazine ring have perpendicular orientation and play a vital role in the
further linkage of the resulting chains to each other to form a 3D network. The interactions
between the two chains are strong enough, which make it hard to further assemblies elongation
in a one-dimensional way, and to generate extended fibers, resulting in the easy formation of the
crystalline state. These witnessings are steadfast with the fibrillar contacts and interatomic
separations promulgating 1D assembly whereas, in the crystalline state, molecules are aligned
along multiple axes, allowing three-dimensional growth which has an impact on the using of
crystal structures to specify supramolecular synthons for gelators. Syllogizing of interactions in
the hydrogel fibrils by using crystal structure data must be done carefully. Mostly, packing
arrangements between the gel and crystalline states are different. In hydrogelators studied herein,
several factors suggest that although the packing modes are not identical but are similar. First,
for all structures studied here, there is intramolecular hydrogen bonding robust enough to be
repeated. Next, the diversity observed in both gel and crystalline state. So, it is presumed that the
basic aggregation architecture is retained between the two states and that the crystals solely
contain extra interactions that extend the growth of the 1D fibrils in 3D dimensions.
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Morphological studies of the metallogels in different solvents. To better understand the
gelation mechanism and get a deeper insight into the morphological features of metallogels with
the respective solvents in the nanoscale dimension, SEM studies were carried out on their
xerogels. To the understanding of the mechanism of gel formation, analyses of its structure are
very important. The properties and behaviours of materials are rationalized by their structures.
Study of structure-property correlation is a critical role of the gelator/solvent interaction and the
fact that solvent polarity can significantly influence the morphologies. Solvent is incumbent for
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gelation to be objective. On the other hand, solvent has a critical role in the fine tuning of the
molecular architecture during the self-assembling process as well as the self-aggregated
properties of the resulted gel phase. Different morphologies in the gels can be tuned by the
controlling the conditions, resulting in special properties of the supramolecular gels70,71
.
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Metallogels obtained from the different solvent were dried in a vacuum desiccator to give
xerogels and then coated with gold and imaged by SEM, Figure 5. By simply changing the
solvent in the case of CuCl2[Terpy-3CN], the nanoscale morphology underwent a change from a
large pores and plate like aggregates (water) to high pores network (10:1 MeOH: H2O) to micro
thin plate type morphology (10:2 EtOH: H2O) to overlapping rugate layer structure for (10:3
PrOH: H2O) and (10:3 IsoPrOH: H2O) and densely packed fibrous morphology (10:3 Acetone:
H2O). The same results were obtained when the gelation behaviour of CuCl2[Terpy-4CN] was
tested, change in the solvent accompanied the similar morphological changes from a high-
density network of fibers (water) to branched networks (10:1 MeOH: H2O) to overlapping rugate
layer structure (10:2 EtOH: H2O) to layer structure (10:3 PrOH: H2O) to densely packed fibrous
and layer structure (10:3 IsoPrOH: H2O) and finally to 3D network resulted from bundles of
fibers and large pores and plate like aggregates (10:3 Acetone: H2O), Figure 5. These results
clearly show the critical role of solvents in the final gel morphology. The gel resulted from the
different solvent with different polarity was distinctly different morphology suggesting that the
gel microstructure was very sensitive and the nature of the resulted fibrillar depends on the
solvent used for the gelation as well as the gelator. A more scrupulously look at the competing
gelator/solvent and gelator/gelator contacts show that the solvent can lead to substantial changes
in both solubility and gelation ability. Significantly, when we focus on the crystal structure of
two pseudopolymorphs of CuCl2[Terpy-4CN], in molecular point of view the gelator/gelator
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interactions are similar, while solvent/gelator interactions are different. For these particular
compounds, the difference in solvent/gelator interactions leads to fabricate different morphology
of the gels. These results highlight the solvent role of in the gel morphology and crystal growth.
Finding the two polymorphs resulted from terpyridine copper complexes not only supports our
hypothesis about the mechanism of assembly but also confirms that variety in metallogels
morphologies from different solvents actually originate from the same fiber aggregates. It means
that for different solvent/gelator interactions, aggregated fibers could further aggregate together
to form different morphologies.
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The SEM studies in gel state show that tuning of the gel morphology can be done by the solvent
combination. These results clearly show the delicate lines exist between insolubility,
crystallization, gelation, and solubilization. The formation of fibers can be thought of as a
consequence of inhibited crystallization or precipitation and, as such, may not represent the
thermodynamic minimum, but rather a kinetically trapped structure. Furthermore, for kinetic
reasons, thermodynamically unstable forms named metastable forms polymorphism in the crystal
can be acquired. Since polymorph and gel state are both metastable forms72–75 maybe there is a
relation between these two states. As a result, the crystal self-assembly and gel forming are
essentially orthogonal or only very weakly coupled but observing any differences in
polymorphism can increase the probability of diversity in gel morphology. There is a suggestion
of the gelators being designed to imitate the 1D formation of crystal lattice and diversity in the
crystal structure of a different polymorph, induces to obtain gels that have no similarities in term
of their structures. A convenient way to alter the physical properties of a component that sustain
crystal structures and gel morphology is controlling the process by which the self-assembly of
the component is carried out to give the crystal or gel. This includes changing solubility by
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changing solvent, temperature and so one. Accordingly, the intermolecular interaction between
the solvent and gelator is partly influenced as the gel stays maintained. However, the strength of
these interactions influences the strength and morphology of the gel for the respective gelators as
estimated by the lower Tgel values. Notice the difference in the network formation of the fibril
structure is evident in the SEM. As other SEM images were carefully analyzed, we understood
that not only the gelator but also the solvent used for the gelation plays an important role in the
structure and the nature of the fibrillar networks.
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Solvent-dependent UV-vis absorption spectra of CuCl2[Terpy-nCN] complexes.
The UV-vis studies of the synthesized metallogelator were carried out in a series of solvents with
5 different polarities including H2O, MeOH, EtOH, PrOH and isoPrOH, For CuCl2[Terpy-3CN]
in general the absorption spectra were not significantly changed by the solvent medium. As can
be seen in Figure 6a, almost no shift in solvent-dependent UV-vis absorption spectra is found,
this being explained due to the presence of similar solute-solvent interactions in various solvents
used here. While in H2O solvent a small red-shift is observed. The irregular red-shifted
absorption peaks in H2O related to those in other alcoholic solvents may be produced by
increasing the dihedral angle between the fourth ring and terpyridine motif, which reduced the π
conjugation. For CuCl2[Terpy-4CN], Figure 6b, a correlation between λmax and polarity of the
solvent is observed. This can be mainly attributed to the fact that solvation by polar solvents
leading to the generation of hydrogen bonding with water induced the solvated molecule to
absorb at a lower wavelength compared to other solvents examined. This blue-shift shows
relatively strong solute-solvent interaction between the water molecules and CuCl2[Terpy-4CN]
complex. The maximum absorption peak at 280 nm can be assigned to the intramolecular charge
transfer state between the electron acceptor (cyano) group and the electron donor (terpyridine)
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moiety, showed the solvent-dependency due to its strong hydrogen bonding. The weak
absorption band resembling at 340 nm, which did not shift with increasing solvents polarity, was
originated from π–π* transitions and it indicating that the formation of self-assembled aggregates
by compound CuCl2[Terpy-4CN] was cooperative. The self-assembled aggregates may be
attributed to the synergistic operation of hydrogen bonding and π stacking interactions in the
self-assembly aggregates of the compound.
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Powder X-ray diffraction studies of CuCl2[Terpy-4CN], and CuCl2[Terpy-4CN].H2O.
Powder X-ray diffraction pattern of the xerogel of CuCl2[Terpy-4CN], is recorded to achieve a
better and deeper understanding regarding the self-assembly process, Figure 7. As it is clear from
this Figure, no good fitting is observed between experimental PXRD patterns and simulated one
from single crystal X-ray diffraction related to crystalline phase changes during the vacuum
drying. It must be noted that since monitoring the formation of crystals under conditions favoring
gelation was uncommon, the complex was dissolved in methanol and under slow evaporation to
yield green block single crystals. In the PXRD pattern of the xerogel of CuCl2[Terpy-4CN], only
one big diffraction peak at 2θ= 26.31° was observed, which based on Bragg’s equation
corresponded to a d-spacing of 3.4 Å. These diffractions resulted from the corresponding π-
stacking interactions of pyridine rings of the ligand as well as π-Cl interactions between adjacent
molecules in the chains. Figure 7(c) also shows the PXRD pattern of the xerogel of CuCl2[Terpy-
3CN]. As it is clear from this figure, similar to 4-cyano homologous, the presence of one big
diffraction peak at 2θ= 22.50° and d-spacing of 3.9 Å shows the π-stacking interactions of
pyridine rings of ligand.
Conclusion
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In conclusion, we have presented a simple terpyridine ligand decorated with the cyano
substituent that leads to metallosupramolecular gels in aqueous media. To show the driving
forces for the self-assembling of the metallogelators, their crystal structures have been
scrutinized. The gelation mechanism involves the formation of one-dimensional (1D) nanofibrils
in a straight line that is unlimitedly continued. In the next step, dipyrazine-pyridine counterparts
were synthesized in an attempt to find a design strategy for gelators. All of the synthesized
dipyrazine-pyridine compounds were either too soluble or crystalline in a wide range of solvent,
and we were unable to access the gel state by them. What this observation has an effect on is
mainly the determination of supramolecular synthons for gelator.
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ACKNOWLEDGMENT: We would like to thank the Graduate Study Councils of Shahid
Beheshti University, General Campus, and the Iran National Science Foundation (Grant No.
940090) for financial support.
SUPPORTING INFORMATION
Electronic Supplementary Information (ESI) available: The packing diagrams, hydrogen
bonding and π…π interaction data and X-ray crystallographic files in CIF format for structural
determination of CuCl2[Terpy-4CN], (CCDC No. 1510958), CuCl2[Terpy-4CN].H2O, (CCDC
No. 1510957), CuCl2[Dipyz-py-4CN].H2O, (CCDC No. 1510959), and CuCl2[Dipyz-py-3CN]
(CCDC No. 1511109). This material is free of charge via Internet at http://pubs.acs.org.
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