Anion tuning of the rheology, morphology and gelation of a low molecular weight salt hydrogelator

Article abstract of DOI:10.1039/c0sm00594k

1-(3-Methyl-1H-pyrazol-5-yl)-3-(3-nitrophenyl)urea (1) forms hydrogels in a range of acids at pH 1-2. The morphology and rheology of these gels are dependent on the identity of the anion and represent a way of tuning the gels' physical properties. The mag

Full text of DOI:10.1039/c0sm00594k

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PAPER
Anion tuning of the rheology, morphology and gelation of a low molecular
weight salt hydrogelator†
a
Gareth O. Lloyd and Jonathan W. Steed
b
*
*
Received 29th June 2010, Accepted 7th September 2010
DOI: 10.1039/c0sm00594k
1-(3-Methyl-1H-pyrazol-5-yl)-3-(3-nitrophenyl)urea (1) forms hydrogels in a range of acids at pH 1–2.
The morphology and rheology of these gels are dependent on the identity of the anion and represent
a way of tuning the gels’ physical properties. The magnitude of the elastic storage modulus of the gels
follows the trend according to the acid used to protonate the gelator H2SO4 > MePO3H2 > H3PO4 z
HBF4 z HPF6 > EtPO3H2. Chloride salts do not form gels and the nitrate gels are unstable with
respect to crystallisation. Salting out or addition of methanol results in the characterisation of five salts
of 1 which reveals extensive solvation in the solid state and the absence of urea/urea hydrogen
bonding in contrast to the well known bis(urea) gelators.
mechanical, thermal, electrochemical, electromagnetic or chem-
Introduction
ical. Chemical stimuli, as additives, have been reported, and
include examples of variation in pH, addition of/response to
biologically relevant compounds, the incorporation of metals or
anions.6,12 Anion tuning in particular has become a highly
topical subject within the field of tunable LMWGs because
anions act as effective hydrogen bond acceptors often in
competition with fibre formation.13,14 Even so, the effects that
anions can have on LMWG assembly are still not fully under-
stood.14–19 This uncertainty is partially due to the fact that in
most cases the LMWG only gels in the presence of a few anions
or gelation has only been studied with a limited range of salts.
Moreover, much recent activity has focused on neutral LMWGs
that bind anions in the presence of a corresponding charge-
balancing cation, further complicating the picture.6,13,14 In the
present work we target a range of salt LMWGs in which there is
the possibility of tuning the gel properties of one particular
LMWG cation by varying the identity of the anion. We have
chosen to study pyrazole-appended ureas in which the urea
group can contribute to gel fibre assembly20–24 while the basic
pyrazole group acts as a protonation site. There are very few
examples of pyrazole containing LMWGs in the literature,25
despite their resemblance to the better studied pyridyl ureas.6,26–31
The protonated pyrazole unit is also relatively sterically unen-
cumbered, enhancing potential hydrogen bond donor charac-
teristics and hence a protonated pyrazole urea may represent an
alternative to a bis(urea) gelator.
Gels are solid, jelly like materials formed from colloidal mixtures
comprising a liquid dispersed within a solid continuous gelator
phase. What makes gels so interesting is that by weight and
volume they are mostly liquid, yet they behave like a solid. Of the
many substances that can be used as gelators, low molecular
weight gelators (LMWGs) represent soft materials that can be
prepared by the bottom-up assembly of designed small mole-
cules. These small molecules assemble into nanoscale fibrils
which bunch to form micro-fibres which, in turn entangle and
cross-link via interconnected nodes to form macro-sized gels that
comprise a 3D sample-spanning network with the liquid immo-
bilised in continuous microchannels, usually as a result of surface
tension effects.1–6 Hydrogels are gels that are formed with water
as the major constituent of the solvent. The biological and
technological relevance of these gels has lead to a myriad of
applications, such as drug delivery, catalysis, scaffolds for tissue
engineering, templates for materials synthesis, lithography and
as separation materials.7–11 LMWGs are supramolecular in
nature, and as such, are easily tunable by both synthetic modi-
fication and external stimuli. The external stimuli may be
aDepartment of Chemistry, University of Cambridge, Lensfield Road,
Cambridge, CB2 1EW, UK. E-mail: gol20@cam.ac.uk; Fax: +44
(0)1223 336 017; Tel: +44 (0)1223 763 989
bDepartment of Chemistry, Durham University, South Road, Durham,
DH1 3LE, UK. E-mail: jon.steed@dur.ac.uk; Fax: +44 (0)191 384
4737; Tel: +44 (0)191 334 2085
Results and discussion
† Electronic supplementary information (ESI) available: Experimental
details of the synthesis and analytical data for 1. Additional rheology,
TEM, EELS, SEM and electron diffraction data for gel samples.
PXRD pattern of dried gel samples. Crystallographic data in CIF or
other electronic format with further figures of structures. CCDC
Synthesis
1-(3-Methyl-1H-pyrazol-5-yl)-3-(3-nitrophenyl)urea (1) was
synthesised from 3-amino-5-methylpyrazole and m-nitrophenyl
isocyanate, via tert-butyloxycarbonyl (BOC) protection of the
reference numbers
10.1039/c0sm00594k
CCDC
783190–783196.
See
DOI:
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Scheme 1 1-(3-Methyl-1H-pyrazol-5-yl)-3-(3-nitrophenyl)urea (1).
pyrazolyl group (Scheme 1). The BOC protection is necessary as
the direct reaction between an aminopyrazole and an isocyanate
produces a carboxamide.32–34 Proof of the BOC protection
occurring on the pyrazolyl ring comes in the form of analytical
data and the crystal structure of the BOC protected pyrazole in
which the BOC group is bound to the 3 nitrogen position of the
pyrazolyl ring, in agreement with the literature (ESI†).35,36
Testing for gel formation was carried out by dissolving 1 in 1–
2 mL of water at controlled pH in the presence of a variety of
acids with a view to forming salt hydrogelators of the type
(1H+)(Xꢀ) where Xꢀ is the conjugate anion of the acid in ques-
tion. Solutions were cooled to room temperature both with and
without sonication37 and were tested for gelation by performing
the ‘‘inversion test’’. Table 1 summarizes the acids used to adjust
the pH and the resultant characteristics of the materials formed.
The results indicate that a narrow pH range of ca. 1–2 is
required for gel formation to occur, thus making compound 1
a pH tunable LMWG.20,28,38–57 At the low pH used to induce gel
formation in 1, it is expected that the pyrazolyl group will be
protonated to give a cationic species (pyrazoles are strongly basic
with pKa ca. 2.5).58 This protonation allows systematic study of
the effect of the variation in anion on the gel properties, by
changing the acid used to acidify the water.
Visual inspection of the resultant materials shows that there are
changes in the type of materials produced, according to the
identity of the anion. Of the 13 acids tested, gels were formed by
the acids H2SO4, H3PO4, HBF4, HPF6, MePO3H2 and EtPO3H2
(Fig. 1, S1 and S2†). HNO3 gave an unstable gel, while HCl,
CH3COOH, CF3COOH, MeSO3H, CF3SO3H and p-toluene
sulfonic acid all gave precipitates (powder patterns of HNO3 and
HCl experiments, Fig. S21 and S22†). The HNO3 acidified solu-
tion formed a weak gel upon slow cooling, however, the gel breaks
down within a few minutes.59,60 The gel and precipitate formation
is reversible, in that the solution can be heated and cooled
repeatedly giving the same results each time. As has been seen in
other classes of LMWGs, the gelation ability of 1 was found to be
enhanced when hot solutions were sonicated to give more uniform
gels, especially, in the cases when HBF4 and HPF6 were used.37
Tuning of the gel state by addition of anions has become
a focused subject within the field of LMWGs. Since 1 does not gel
in the presence of HCl, chloride salts (NaCl and tetrabuty-
lammonium chloride, TBA+Clꢀ) were added to modify the strength
of gels of 1. When one equivalent of anion (in relation to 1) was
added to gels, breakdown of the gel to a precipitate was observed
irrespective of the counterion forming the gel. This precipitation
caused by the addition of Clꢀ provides the ability to tune a gel of 1
and represents a kind of salting out effect.41,51,57,61
Microscopy
Visual inspection of the gels reveals a notable difference in the
appearance and colour. The morphology of the gels was
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TEM imaging of the HBF4 and HPF6 acidified gels reveals
that the microstructure is different to that of the H2SO4 acidified
gels (Fig. 3). Both gels showed a particle-based gel structure.62–64
The images of the HBF4 acidified gels showed a porous structure
with struts consisting of small particles ranging in size from 10 to
50 nm with the gel spanning the pores of the carbon mesh holding
the samples. The images of the HPF6 gels showed a similar gel
structure with struts consisting of nano-sized particles. The
particles of the HBF4 and HPF6 acidified gels were crystalline in
nature as shown by their ability to diffract the electron beam in
diffraction mode on the TEM instrument (Fig. S8†). The d-
spacings do not much up with any of the known crystal struc-
tures or powder patterns of 1 or dried samples of gels of 1
(Fig. S17 and S18†). SEM images of gel samples acidified with
HBF4 and HPF6 that had been dried under high vacuum and
covered in a thin layer of Pt showed that the HBF4 acidified gels
form crystalline flakes upon drying (Fig. 4a). The SEM images of
the HPF6 acidified gels showed porous structures which are
similar to those seen for the H2SO4 acidified gels but with a much
smaller scale in size (Fig. 4b).
Fig. 1 Photograph of gels of 1H+ at 1% by weight in water acidified with,
from left to right, EtPO3H2; MePO3H2; H3PO4; H2SO4; HPF6 and HBF4.
Fig. 2 (a) SEM image of dried gels of 1 in water acidified with H2SO4 to
a pH of 1.0 that has been covered with a thin layer of Pt. (b) TEM image
of a gel of 1 in water acidified with H2SO4 to a pH of 1.0.
The transient nature of the nitrate-acidified gel, that turns into
a precipitate, meant that it could not be studied using rheology.
However, TEM images of a gel that had just formed allowed for
some characterisation of the gel (Fig. 5).60 Interestingly, the
transformation from gel to crystal could be observed directly, as
clusters of nanocrystals grew on what appeared to be the gel fibres.
determined using scanning electron microscopy (SEM) and
tunneling electron microscopy (TEM).
The morphology of the H2SO4 gels (which are also represen-
tative of the H3PO4 (Fig. S10†), MePO3H2 and EtPO3H2 gels) was
studied by SEM and TEM imaging (Fig. 2). Samples of gels of 1
acidified with H2SO4 to a pH of 1.0 for SEM imaging were dried
under vacuumand thencoatedwith athin layer of platinum metal.
As can be seen from Fig. 2a the imaging reveals a porous sponge-
like morphology. The Pt coating has created a smooth surface on
the gel struts with the pores of the gels appearing darker. The gel
morphology from the TEM images, as shown in Fig. 2b, appears
to be the same as that seen in the SEM images. The gel material
breaks down under the electron beam, but the porous sponge-like
morphology of the gel structure remains visible. The visible edge
of a large circular pore, which the gel transverses, is the edge of the
carbon mesh grid used to hold the gel sample. Electron energy-loss
spectroscopy (EELS) was run on the TEM samples to confirm the
presence of SO42ꢀ or HSO4ꢀ within the gel struts (Fig. S6†). The
evidence of sulfur within the sample confirms the presence of the
sulfate anion and hence a salt gel.
Rheology
The rheological characteristics of a gel can be used to identify
important changes that occur in its structure as a function of the
identity of the anion.65–71 Gels of protonated 1 were characterised
by stress sweep and frequency sweep rheometry. When a frequency
sweep was performed with a small amplitude stress (Fig. 6, S7 and
S9†), with a number of different samples varying in concentration,
the solid-like nature at 20 ꢁC was reflected in the storage modulus,
G0, typically two to five times greater than the loss modulus, G00,
thus demonstrating the elastic behaviour of the systems, albeit in
robust gels an order of magnitude difference is expected.
This viscoelastic behaviour is associated with classical gels,
and therefore supports the notion that the cooling of these
samples from a solution to a solid-like material resulted in a true
gel state.72 The effect of the concentration on the gel strength, in
the form of G0 and ‘‘yield strength’’ (Fig. S3†) values, was also
investigated using rheology to ascertain the morphology of the
gel. The two theoretical models put forward for the mechanical
properties of gels73 that are pertinent to LMWGs are the
colloidal gel description73,74 and the cellular solid model.65,73,75,76
Fig. 3 TEM images of gels of 1 acidified with (a and b) HBF4 and (c and
d) HPF6.
Fig. 4 SEM images of dried gels of 1 acidified with (a) HBF4 and (b) HPF6.
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the ‘salting-out’ anions and chaotropic behaviour applies to
‘salting-in’ anions. When the kosmotropic anions, from the acids
H2SO4, MePO3H2, H3PO4, EtPO3H2, were used, gel formation
2ꢀ
2ꢀ
was seen, with the SO4 and MePO3 based gels being the
strongest, whereas the chaotropic anions are weaker. Gelation by
nitrate is borderline. The chaotropic anion gels (BF4ꢀ, PF6ꢀ and
the nitrate material) have rather different morphologies to the
kosmotropic anion based materials. The hydration of the anions,
therefore, possibly plays a role in the morphology of the gel.
Crystallography
The transient nature of the HNO3 acidified gels and the precip-
itation upon addition of Clꢀ reveal a close connection between
the stabilities of the gel and crystalline states.80–82 Character-
isation of the crystalline materials is expected therefore to lead to
a better understanding of the nature of the anion–gel interactions
as well as fully characterising the salts of 1. The formulae of all of
the crystalline products isolated are given in Table 1 and it is
interesting that all proved to be highly solvated.
Fig. 5 TEM images of the gels from acidification of a water solution of 1
with HNO3. (a) and (b) are from 1 with HNO3 gel sample. (c) and (d) are
from a gel of 1 with HNO3 that had started to break down to a precipitate.
The crystal structure of parent compound 1 was determined
with the crystals grown from water at pH 7 (Fig. 8). The structure
exhibits an intramolecular hydrogen bond between N4 and N2,
within the asymmetric unit of 1, which results in an anti confor-
mation for the urea group. A few 2-pyridylureas and 2-pyrroly-
lurea derivatives exhibit similar conformations.83–86 The rest of the
urea group that is not involved in the intra-molecular hydrogen
bond forms a dimer with the neighbouring molecule via an R22(8)
hydrogen bond motif, in a similar way to related compounds.87,88
The dimers further aggregate to give a 1D hydrogen bonded chain
via a N1–H1/O3 hydrogen bond between the pyrazole and nitro
groups (Fig. S12†). These flat chains stack one on top of each
˚
other resulting in 2D stacks (inter-planar distance 3.3 A). These
Fig. 6 Frequency sweep rheometry of a gel formed by the sulfate salt of
protonated 1 (0.1% by weight, pH 1.0); G0 (dark grey filled >) and G00
(light grey filled ,).
stacks run perpendicular to each other (Fig. S13†). This stacking is
˚
possible due to the flatness of 1, RMS value of 0.0694 A.
The gels of 1 formed in water acidified at pH 1.0 with H2SO4
were induced to crystallise by addition of salt in the form of
Na2SO4 or by mechanical stress. Stirring with a spatula, which
creates nuclei for crystallisation to occur, resulted in crystal-
lisation after a few weeks with the gel slowly converting to the
crystal form.60 The crystals were dissolved by heating the solu-
tion, and upon cooling, the gel reformed. The structure of these
crystals (1H+)2(SO42ꢀ)$7H2O provides a ‘‘snapshot’’ into the
structural characteristics of the hydrogel.28
The dependence of the strength of the gel of 1, in water acid-
ified to pH 1 with H2SO4, and the concentration of 1 proved to fit
well to the relationships G0 f [conc]2.2 (Fig. S4†) and ‘‘yield
stress’’ f [conc]1.5 (Fig. S5†). This fit is in agreement with the
cellular solid model76 which is predicted to have a relationship of
G0 f [conc]2 and ‘‘yield stress’’ f [conc]n where n can vary
between 1 and 2, with the colloidal gel description giving n >
2.65,76 The cellular solid model describes an open-cell cellular
material which consists of load bearing struts interconnected via
crosslinks or junction points which deform by bending, which
agrees with the morphologies seen by microscopy.
The asymmetric unit comprises two molecules of protonated 1
2ꢀ
(1H+), an SO4 anion and seven water molecules (Fig. 9). The
additional proton is located on the pyrazole group resulting in
the breaking of the intra-molecular hydrogen bond seen in the
crystal structure of 1 and hence imparting a syn conformation to
the urea group. The 1H+ cations are flat (RMS deviation of 0.102
A comparison of all the gels, at a fixed 1% by weight, by stress
sweep rheometry reveals the tuning of the rheological charac-
teristics of gels formed by 1 by changing the identity of the anion
(Fig. 7). The plateau G0 value for each of the different salt gels
˚
˚
A and 0.08 A for the molecules with atoms C1 and C12,
respectively). There are intramolecular hydrogen bonds between
the carbonyl group of the urea and the pyrazole NH nearest the
urea group as well as between the carbonyl group of the urea and
the CH moiety on the nitrophenyl group. This type of CH/O
intermolecular interaction is expected for ureas with electron
withdrawing substituents on the aryl rings.89
shows the trend in strength to be protonation by H2SO4
>
MePO3H2 > H3PO4 z HBF4 z HPF6 > EtPO3H2. The ‘‘yield
stress’’ follows a similar trend H2SO4 > MePO3H2 > HPF6 >
H3PO4 > HBF4 > EtPO3H2. This ranking parallels the well-
known Hofmeister series77–79 which ranks anions from kos-
motropic (strongly hydrated anions) to chaotropic (weakly
2ꢀ
2ꢀ
2ꢀ
hydrated anions) in the order: SO4 z MePO3 z HPO4
>
The large number of water molecules form an extensive hydrogen
bonded network (ESI†). There are just three hydrogen bonds that do
EtPO3 > Clꢀ > NO3ꢀ > PF6ꢀ > BF4ꢀ. Kosmotropic relates to
2ꢀ
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Fig. 7 Anion tuning of the rheometric characterisations of gels of 1 shown by stress sweep rheometry of the hydrogels of 1 at 1% by weight acidified with
H2SO4 (blue >), H3PO43 (orange ,), MePO3H2 (green >), HBF4 (light blue O), HPF6 (brown B) and EtPO3H2 (light green ,).
not involve water molecules; the hydrogen bonding of the urea group
2ꢀ
determines the needle morphology. This quicker growing face
was determined to be (100). The packing of the 1H+ molecules
within the structure one on top of each other through p-stacking
is in the [100] direction which correlates with the (100) quicker
growing face and hence the p-stacking interaction of the pla-
narised 1H+ drives 1D growth of the crystal and may well also do
so in gel fibre formation in an aqueous environment where
hydrophobic interactions are dominant.90 Unfortunately the
hydroscopic nature of the compound meant that no XRPD
pattern of the H2SO4 gel samples could be obtained for
comparison with the single crystal data.
(N8 and N9) to the SO4 anion in a R22(8) motif, and hydrogen
2ꢀ
bonding of the protonated pyrazole (N1) to the SO4 anion. The
overall packing of the 1H+ cations forms a one on top of each other
p-stacking arrangement with interplanar distance varying from 3.33
2ꢀ
˚
˚
A to 3.49 A to give columns that contain the SO4 and water
molecules surrounded by four stacks of 1H+ (Fig. 10). This packing is
similar to that seen by Dastidar, Das and coworkers in a bispyridyl
urea hydrogelator with occluded ethylene glycol and water solvent.
These authors suggested that the structure represents a ‘‘snapshot’’ of
the gelator interacting with the solvent it gels.28
1D growth of gel fibres due to 1D supramolecular bonds is
considered important consideration in designing LMWGs.80
Keeping this in mind, the needle-shaped crystals of
(1H+)2(SO42ꢀ)$7H2O were face indexed to determine what the
driving force is for the ‘‘quicker’’ growth of the face that
Crystallisation of the gels obtained from HBF4 and HPF6 were
attempted by inducing salt formation by adding salts such as NaCl
and Na2HPO4 to the gels and by growing single crystals from
Fig. 8 Molecular structure of 1. Atoms are shown as ellipsoids at 50%
Fig. 9 Asymmetric unit of (1H+)2(SO42ꢀ)$7H2O. Atoms are shown as
probability.
ellipsoids at 50% probability.
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Fig. 12 (a) Structure of 1H+BF4ꢀ$H2O as viewed down (100). (b)
Structure of 1H+BF4ꢀ$H2O as viewed down (010). Hydrogen bonds not
shown for clarity.
The pseudo-R22(8) hydrogen bonding to the BF4ꢀ anion through
the urea group is shifted making F1 a bifurcated acceptor
(Fig. 12). There is one water molecule that hydrogen bonds to both
ꢀ
the 1H+ (as both a donor and an acceptor) and the BF4 anion.
The intra-molecularly hydrogen bonded N2–H2N also hydrogen
ꢀ
Fig. 10 Packing diagram of (1H+)2(SO42ꢀ)$7H2O showing the stacking
bonds to the BF4 anion. Due to the twisting of the nitrophenyl
2ꢀ
of 1H+ one on top of each other resulting in a column in which the SO4
group in 1H+, the 1H+ cations do not quite pack one on top of each
other as seen in the other structures containing 1H+ (Fig. 12a). The
p–p stacking of the nitrophenyl groups down (010) is therefore
rather different in character to the sulfate salt (Fig. 12b, the c axis
and water are located. Packing viewed down [100].
MeOH solutions of 1 acidified with the appropriate acids. No
single crystals were obtained from HPF6 but the HBF4 sample
yielded block-shaped crystals of formula (1H+)(BF4ꢀ)$H2O from
a MeOH solution acidified with HBF4 that was allowed to evap-
orate for one week, as well as when a gel was stirred vigorously
with a spatula (Fig. 11). The structure comprises a 1H+ cation in
which the nitrophenyl group is twisted away from the plane
defined by the pyrazole and urea groups, the torsion angle :C5–
N4–C6–C7 ¼ ꢀ30.1ꢁ compared to less than 12ꢁ in the other
structures discussed herein, and as little as 1ꢁ in the structure of
(1H+)2(SO42ꢀ)$7H2O.
˚
length of 7.7509(4) A is double the p-stacking distance).
A second structure of a co-crystal91–95 (1H+)(BF4ꢀ)$1 was
obtained by adding Na2HPO4 (50 mg to a 3 mL 0.2% by weight
gel) to a gel in water acidified with HBF4. The structure comprises
both protonated and unprotonated 1 (Fig. 13). The acidic proton
is disordered equally between N2 and N7 of neighbouring mole-
cules of 1 with the hydrogen bonding arrangement between the
two compounds changing with the position of the proton. The two
molecules of 1 are both planar but the nitrophenyl groups are
twisted slightly due to steric hindrance arising from the interac-
+
˚
The RMS deviation from planarity of 0.308 A for the 1H
molecule within the (1H+)(BF4ꢀ)$H2O structure also highlights
this twisting. The twisted conformation of the molecule 1
disrupts the intra-molecular hydrogen bonds involving the
carbonyl group of the urea, in particularly the hydrogen bond
with the nitrophenyl group, in comparison to the structures of 1
˚
tions between the pyrazole groups, RMS deviations of 0.119 A
˚
(molecule 1 based on C1) and 0.216 A (molecule 2 based on C12).
Indeed, the nitrophenyl group of molecule 2 has a ‘‘transoid’’
orientation in relation to the pyrazole group orientation rather
than the ‘‘cisoid’’ orientation seen in all the other structures of 1.
When the proton is on N2, in addition to an N2/N7
hydrogen bonding interaction, the pyrazole group forms a R22(8)
hydrogen bond with the carbonyl oxygen atom of the neutral
molecule of 1 helping to disrupt the intramolecular hydrogen
bond seen in the crystal structure of native unprotonated 1.
When the proton is on N7, N1 becomes an acceptor resulting in
an intra-molecular hydrogen bond to the urea carbonyl group as
2ꢀ
and (1H+)2(SO4 )$7H2O; N2/O1 ¼ 2.626(2) A; :N2–H2N/
˚
ꢁ
ꢁ
˚
O1 ¼ 119.0 , C7/O1 ¼ 2.905(18) A; :C7–H7/O1 ¼ 111.5 .
˚
Moreover, the H/O distance is longer at 2.42 A (compared to
the 2.24 A and 2.12 A of the SO42ꢀ structure).
˚
˚
Fig. 13 Molecular structure of (1H+)(BF4ꢀ)$1. Atoms are shown as
ellipsoids at 50% probability. (a) Disordered proton situated on N1. (b)
Alternate arrangement with disordered proton situated on N7.
Fig. 11 Molecular structure of (1H+)(BF4ꢀ)$H2O. Atoms are shown as
ellipsoids at 50% probability.
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seen in the other structures. The intra-molecular hydrogen bonds
from the nitrophenyl moiety are slightly different due to the
twisting of the nitrophenyl group described above. Hydrogen
bonding between neutral 1 species and a protonated cation via
their carbonyl and pyrazole groups results in the NH groups of
the urea moieties and one NH group of the pyrazole group, N6–
H6N, being available as hydrogen bond donors and hence the
ꢀ
urea groups hydrogen bond to the BF4 with the N3–H3N and
N4–H4N groups forming a R22(8) hydrogen bond pattern. The
other urea group (N8 and N9) hydrogen bonds to F4 on two
ꢀ
symmetry independent BF4 ions. The free NH group of the
pyrazole group hydrogen bonds to O3 of neighbouring nitro
groups which results in a hydrogen bonded thread of 1H+$1
units. These threads are stacked one on top of each other (the
stacking distance being a quarter of the a–c unit cell diagonal
distance) giving a 2D dimensional layer that alternates with
ꢀ
layers of the BF4 ions (Fig. S14†). The formation of this co-
Fig. 14 Molecular structure of (1H+)(NO3ꢀ)$MeOH. Atoms are shown
crystal may be due to the change in pH induced by the addition
of basic Na2HPO4. Simulation of the PXRD patterns of either of
these BF4ꢀ1H+ single crystal structures resulted in no confirmed
match with the d-spacings found from either the electron
diffraction of the particles of the TEM gel samples (Fig. S8†) or
the XRPD patterns measured of dried gel samples (Fig. S18†).
The dried gel PXRD patterns of the HBF4 and HPF6 acidified
gels show some similarities, but are not perfectly matched.
As no single crystals of the crystalline materials from the HNO3
and HCl acidified solutions could be obtained from acidified water,
a polar organic solvent was added to slow down crystal formation.
This resulted in the formation and characterisation of a methanol
solvate of the chloride salt of 1H+ and a methanol/water solvate of
the nitrate salt when 1 was crystallised from a MeOH solution
acidified with an equal volume of the concentrated acids.
as ellipsoids at 50% probability.
of this product, (1H+)2(Clꢀ)2$MeOH$H2O, are clear. The mate-
rial is similar to the structure of (1H+)NO3ꢀ$MeOH (Fig. 15).
The 1H+ cations are planar with the pyrazole protonated, RMS
˚
˚
deviations of 0.053 A (molecular based on C1) and 0.100 A
(molecule based on C12). The intra-molecular hydrogen bonds
between the protonated pyrazole group and the carbonyl oxygen
atom of the urea, and between the carbonyl group of the urea and
a CH unit on the nitrophenyl group contribute to the planar
conformation. As with the nitrate structure, the anion is chelated
in between the two urea groups of 1H+. The R21(6) hydrogen
bonding by both ureas is symmetrical. The N1–H1N group of the
pyrazole hydrogen bonds to the second chloride ion. The other
pyrazole group has hydrogen bonds to the MeOH and water
molecules. The MeOH hydrogen bonds to the chelated Cl1. As
seen with the other structures containing protonated 1, the 1H+
cations pack one on top of each other in an alternating fashion
similar to the nitrate analogue to give the usual stacks of 1H+
with anion and solvent in between. As a result of the smaller size
of chloride compared to nitrate, the anion column between
groups of four cation stacks is shifted resulting in a kinked
arrangement (Fig. S16†).
The (1H+)(NO3ꢀ)$MeOH structure has an asymmetric unit
comprising two 1H+ cations, two NO3ꢀ anions and two molecules
of MeOH, i.e. Z0 ¼ 2 (Fig. 14).96 Once again, the 1H+ cations are
planar with the pyrazole being protonated, RMS deviations of
˚
˚
0.105 A (molecule based on C1) and 0.040 A (molecule based on
C12). The intra-molecular hydrogen bonds between the pyrazole
and the carbonyl of the urea and the carbonyl group of the urea
and a CH unit on the nitrophenyl group contribute to the pla-
narisation of the 1H+ cation. One of the nitrate anions is asym-
metrically chelated between two urea groups in an asymmetric
R22(8) hydrogen bonded motif involving O9 as a bifurcated
acceptor.97 The R22(8) motif of the urea group containing N3 and
N4 is symmetrical, however.98 The pyrazole group containing N1
and N2 hydrogen bonds to the second nitrate anion, but due to the
divergent character of the pyrazole NH groups only the N–H/
O11 hydrogen bond is short. The MeOH molecules hydrogen
bond (as donors) to this same nitrate anion and are hydrogen
bonded (as acceptors) to the other pyrazole NH group. The planar
1H+ molecules, once again, stack one on top of each other, but in
an alternating pattern (Fig. S15†). There are four stacks of 1H+
that surround a column of methanol and nitrate anions.
The crystallization of 1 from aqueous HCl gives a fine
precipitate. Crystallisation of 1 from methanolic HCl gives
a different crystalline material to that from water that is less fine
and has large crystallites allowing for the determination of the
structure using single crystal methods. While the crystallographic
data are of low precision, the qualitative details of the structure
Fig. 15 Molecular structure of 2[(1H+)(Clꢀ)]$MeOH$H2O. Atoms are
shown as ellipsoids at 20% probability.
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Soft Matter, 2011, 7, 75–84 | 81

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then coated with a thin layer of platinum of 3–5 nm using
a 308UHR Ultra High Resolution Coating System (Biology,
Durham). Samples were imaged using an S-5200 UHR FE-SEM
Hitachi Scientific instrument (Biology, Durham). TEM samples
were deposited onto a holey carbon grid and were immediately
examined in a JEOL 2100F FEG TEM (Physics, Durham) oper-
ating at 200 kV. Rheology experiments were performed using
a TA Instruments Advanced Rheometer 2000. A parallel plate
geometry (40 mm diameter) was used. Single crystal diffraction
data were collected on a Bruker SMART 6K (6000 CCD) and
solved and refined using the SHELX suite of programs100 with the
aid of X-Seed.101,102 Powder samples were dried and processed
through a 100 mm microsieve and run on silicon zero beckground
sample holders using a Siemens D5000 instrument.
Conclusions
Protonation of compound 1 resulted in the formation of hydrogels
in the pH range of 1 to 2, thus making compound 1 a hydrogelator
exhibiting pH tunable gelation.20,28,38–57 Rheological characteriza-
tion, in conjunction with TEM and SEM imaging, of the acidified
gels indicates a cellular solid gel material that consists of load
bearing struts interconnected via cross-links or junction points
which deform by bending. In addition, SEM and TEM imaging
revealed a morphological difference between the gels with chaot-
ropic anions BF4 and PF6ꢀ, which are particulate, and the gels
synthesized with kosmotropic oxyanions: SO42ꢀ, HPO42ꢀ, EtPO3
and MePO32ꢀ. The anion tuning of the gel 1 characteristics was
accomplished by varying the identity of the anion by changing the
acid used to adjust the pH of the water solutions. Gel formation
ꢀ
2ꢀ
changed to crystallisation when Clꢀ or NO3 was introduced. This
ꢀ
Synthesis
resulted in the ability to precipitate a gel by the addition of salt
ꢀ
containing these anions. The smaller NO3 and Clꢀ are bound
tert-Butyl-5-amino-3-methyl-1H-pyrazole-1-carboxylate.
3-
between pairs of 1H+ cations by the urea groups in the solid state, as
shown by the crystal structures of (1H+)(NO3ꢀ)$MeOH and
(1H+)2(Clꢀ)2$MeOH$H2O. Characterisation of the crystalline salts
of 1 indicates that the assembly of the gels may be based on the 1D
stacking of the planar 1H+ cations driven by hydrophobic effects
and p–p stacking. No urea/urea hydrogen bonding is observed, in
contrast to the well known bis(urea) class of gelators.99 The anion
induced change from gel to precipitate appears to be due to the
ability of 1H+ to coordinate through multiple hydrogen bonds to the
anion. These hydrogen bond patterns, in combination with other
less well understood factors including lattice energy, nucleation and
the thermodynamics of the systems, lead to the formation of the
crystalline state instead of gelation. The crystal structures
(1H+)(BF4ꢀ)$H2O and (1H+)2(SO42ꢀ)$7H2O reveal a slight change
in the conformation of the 1H+ cation, and the level of hydration,
respectively. In addition to the anion tuning from gel to crystal
states, the rheological characteristics of gels of 1 were markedly
influenced by the identity of the counter anion. The gels of 1 are
ranked in terms of elastic modulus in the order of acid used to
protonate the gelator: H2SO4 > MePO3H2 > HPF6 > H3PO4 >
HBF4 > EtPO3H2. Neither the Hofmeister series and/or level of
hydration of the anions conclusively explains the order of strength
of these. The change in morphology of the gels, from the oxygen
containing anions to BF4ꢀ and PF6ꢀ, may explain the inconsistency,
as the structures are seen to be different due to the hydrogen bond
propensity of these anions, as shown by the crystal structure
(1H+)(BF4ꢀ)$H2O. Additionally, the crystallisation abilities of the
salts are super-imposed onto the gelation of this series of anion
containing gels. The tuning of the gel characteristics of gels of 1 is
summarized in Table 1. This study, overall, shows that LMWG salts
can be tuned in terms of gel rheology characteristics over a large
range of gel strength, from strong gel to total loss of gel
formation, by simply varying the identity of the anion or the pH.
Amino-5-methylpyrazole (2.00 g, 20.3 mmol) was added to dry
CHCl3 (100 mL) in a three necked round bottom flask fitted with
a condenser and bubbler. To this solution was added di-tert-butyl
pyrocarbonate (4.49 g, 20.3 mmol). The solution was stirred for 24
hours. Solvent was removed using a rotary evaporator to give oil
that crystallized upon standing. The product is hygroscopic. Yield:
3.83 g; 19.4 mmol, 96%. 1H NMR: (400 MHz, CDCl3, d/ppm, J/
Hz) 1.60 (9H, s, -(CH3)3) 2.12 (3H, s, -CH3) 5.18 (1H, s, ]CH-)
5.35 (2H, br s, -NH2). 13C{1H} NMR: (100 MHz, CDCl3, d/ppm)
14.4, 28.0, 84.9, 89.3, 92.0, 150.7, 153.2. Anal. Calc. C, 54.8;ꢁH,
7.67; N, 21.3%. Found: C, 54.3; H, 7.61; N, 20.9%. Mp 120 C.
ES+-MS (m/z): 198 [100%] C9H15N3O2 + H+, 220 [5%] C9H15N3O2
+ Na+, 395 [33%] 2C9H15N3O2 + H+. FT-IR: (n, cmꢀ1) 3441 (m),
3358 (w), 3285 (w), 3212 (w), 3161 (w), 3120 (w), 2982 (m), 2933
(w), 1710 (s), 1619 (s), 1553 (m).
tert-Butyl-3-methyl-5-(3-(3-nitrophenyl)ureido)-1H-pyrazole-1-
carboxylate.
tert-Butyl-5-amino-3-methyl-1H-pyrazole-1-
carboxylate (1.50 g, 7.60 mmol) was added to dry CHCl3 (100
mL) in a three necked round bottom flask fitted with a condenser
and flowing N2. To this solution was added 3-nitrophenyl
isocyanate (1.25 g, 0.960 mL, 7.60 mmol). The mixture was
refluxed under N2 for 24 hours. The solvent was removed using
a rotary evaporator to give the product as a yellow precipitate.
Yield: 2.61 g, 7.27 mmol, 96%. 1H NMR: (400 MHz, d6-DMSO,
d/ppm, J/Hz) 1.58 (9H, s, -(CH3)3) 2.13 (3H, s, -CH3) 6.48 (1H, s,
]CH-) 7.56 (1H, t, J ¼ 8.4, -CH]) 7.70 (1H, ddd, J ¼ 8.4, 1.0,
2.2, -CH]) 7.83 (1H, ddd, J ¼ 8.4, 1.0, 2.2, -CH]) 8.53 (1H, t, J
¼ 2.2, -CH]) 9.72 (1H, s, -NH-) 10.50 (1H, s, -NH-). 13C{1H}
NMR: (100 MHz, d6-DMSO, d/ppm) 13.8, 27.6, 85.1, 94.0, 96.1,
112.0, 116.7, 124.2, 130.1, 140.6, 141.8, 148.1, 150.1, 151.8. Anal.
Calc. C, 53.2; H, 5.30; N, 19.4%. Found: C, 53.0; H, 5.54; N,
ꢁ
ꢀ
17.8%. Mp 176 C. ES -MS (m/z): 360.2 [51%] C11H11N5O3 ꢀ
H+, 405.9 (80%) C11H11N5O3 + Clꢀ, 720.7 [25%] 2(C11H11N5O3)
ꢀ H+, 566.2 (12%) 2(C11H11N5O3) + Clꢀ, FT-IR: (n, cmꢀ1) 3342
(w), 3272 (m), 3141 (w), 3092 (w), 3034 (w), 2989 (w), 2940 (w),
1694 (m), 1661 (m), 1608 (m), 1586 (w), 1564 (m).
Experimental
General
All reagents and chemicals were obtained from commercial
sources and used without further purification. SEM samples were
partially dried under room temperature and atmosphere, and then
dried under high vacuum on a piece of silicon wafer. They were
1-(3-Methyl-1H-pyrazol-5-yl)-3-(3-nitrophenyl)urea (1). tert-
Butyl-3-methyl-5-(3-(3-nitrophenyl)ureido)-1H-pyrazole-1-
carboxylate (2.50 g, 9.65 mmol) was added to acetic acid (25 mL)
82 | Soft Matter, 2011, 7, 75–84
This journal is ª The Royal Society of Chemistry 2011

View Online
in a round bottom flask fitted with a condenser. The mixture was
refluxed for 24 hours. Upon cooling a solid formed which was
isolated by filtration. The solid was washed with water (2 ꢂ 25
mL) and saturated sodium carbonate solution (2 ꢂ 25 mL) to
give the crude product. The product was recrystallised from
MeCN and subsequently from water or THF. Yield: 2.31 g, 8.92
mmol, 92%. 1H NMR: (400 MHz, d 3-MeCN, d/ppm, J/Hz) 2.23
(3H, s, -CH3) 5.85 (1H, s, ]CH-) 7.46 (1H, t, J ¼ 8.0, -CH])
7.71 (1H, ddd, J ¼ 8.0, 0.9, 2.2, -CH]) 7.81 (1H, ddd, J ¼ 8.0,
0.9, 2.2, -CH]) 8.13 (1H, br s, -NHpyz) 8.55 (1H, t, J ¼ 2.2,
-CH]) 10.10 (1H, br s, -NH-) 10.45 (1H, br s -NH-). 13C{1H}
NMR: (100 MHz, d6-DMSO, d/ppm) 10.8, 94.0, 112.0, 116.1,
124.2, 130.0, 141.2, 148.11, 152.0. Anal. Calc. C, 50.6; H, 4.24; N,
26.8%. Found: C, 50.4; H, 4.25; N, 26.2%. Mp 180 ꢁC. ESꢀ-MS
(m/z): 260.2 [53%] C11H11N5O3 ꢀ H+, 306.0 (100%) C11H11N5O3
reflections collected, 4594 unique (Rint ¼ 0.0481). Final GooF ¼
1.025, R1 ¼ 0.0585, wR2 ¼ 0.1402, R indices based on 2963
reflections with I > 2s(I) (refinement on F2), 235 parameters, 2
restraints. Lp and absorption corrections applied, m ¼ 0.153 mmꢀ1.
Crystal data for (1H+)(BF4ꢀ)$1. C22H23BF4N10O6, M ¼
610.31, yellow prism, 0.16 ꢂ 0.09 ꢂ 0.06 mm3, triclinic, space group
ꢀ
˚
P1 (no. 2), a ¼ 7.5596(4), b ¼ 12.6366(6), c ¼ 13.7797(6) A, a ¼
ꢁ
3
˚
100.6610(10), b ¼ 94.926(2), g ¼ 96.370(2) , V ¼ 1277.98(11) A , Z
¼ 2, Dc ¼ 1.586 g cmꢀ3, F000 ¼ 628, Smart-6K, MoKa radiation, l
¼ 0.71073 A, T ¼ 120(2) k, 2qmax ¼ 56.0ꢁ, 17 616 reflections
˚
collected, 6149 unique (Rint ¼ 0.0494). Final GooF ¼ 1.011, R1 ¼
0.0587, wR2 ¼ 0.1474, R indices based on 3877 reflections with I >
2s(I) (refinement on F2), 390 parameters, 0 restraints. Lp and
absorption corrections applied, m ¼ 0.136 mmꢀ1
.
+
Clꢀ,520.8 [25%] 2(C11H11N5O3)
ꢀ
H+, 566.2 (12%)
2(C11H11N5O3) + Clꢀ. FT-IR: (n, cmꢀ1) 3346 (m), 3042 (m), 3010
Crystal data for (1H+)(NO3ꢀ)$MeOH. C24H32N12O14, M ¼
712.62, yellow plates, 0.28 ꢂ 0.24 ꢂ 0.21 mm3, monoclinic, space
group P21/c (no. 14), a ¼ 7.6261(3), b ¼ 25.9299(11), c ¼
(w), 2936 (w), 1707 (s), 1611 (m), 1583 (m), 1544 (s), 1523 (s).
ꢁ
3
˚
˚
16.0689(7) A, b ¼ 98.908(2) , V ¼ 3139.2(2) A , Z ¼ 4, Dc ¼ 1.508
Crystallography
g cmꢀ3, F000 ¼ 1488, Smart-6K, MoKa radiation, l ¼ 0.71073 A,
˚
Crystal data for tert-butyl-5-amino-3-methyl-1H-pyrazole-1-
carboxylate: C9H15N3O2, M ¼ 197.24, colourless prism, 0.24 ꢂ
0.19 ꢂ 0.13 mm3, monoclinic, space group P21/c (no. 14), a ¼
T ¼ 120(2) k, 2qmax ¼ 61.1ꢁ, 30 666 reflections collected, 9546
unique (Rint ¼ 0.0692). Final GooF ¼ 1.008, R1 ¼ 0.0613, wR2 ¼
0.1458, R indices based on 5157 reflections with I > 2s(I) (refine-
ment on F2), 457 parameters, 0 restraints. Lp and absorption
corrections applied, m ¼ 0.126 mmꢀ1.
ꢁ
˚
9.8180(3), b ¼ 9.6472(3), c ¼ 11.6088(4) A, b ¼ 106.9080(10) , V ¼
3
ꢀ3
˚
1052.01(6) A , Z ¼ 4, Dc ¼ 1.245 g cm , F000 ¼ 424, Smart-6K,
MoKa radiation, l ¼ 0.71073 A, T ¼ 120(2) k, 2qmax ¼ 61.0ꢁ, 9721
˚
reflections collected, 3199 unique (Rint ¼ 0.0403). Final GooF ¼
1.024, R1 ¼ 0.0479, wR2 ¼ 0.1228, R indices based on 2223
reflections with I > 2s(I) (refinement on F2), 135 parameters, 4
restraints. Lp and absorption corrections applied, m ¼ 0.090 mmꢀ1.
Crystal data for 2[(1H+)(Clꢀ)]$MeOH$H2O. C23H30Cl2N10O8,
M ¼ 643.45, yellow needle, 0.25 ꢂ 0.08 ꢂ 0.02 mm3, triclinic, space
˚
ꢀ
group P1 (no. 2), a ¼ 7.557(8), b ¼ 14.027(16), c ¼ 15.689(17) A,
ꢁ
3
˚
a ¼ 70.74(2), b ¼ 77.71(2), g ¼ 87.03(2) , V ¼ 1534(3) A , Z ¼ 2,
Dc ¼ 1.393 g cmꢀ3, F000 ¼ 668, Smart-6K, MoKa radiation, l ¼
0.71073 A, T ¼ 120(2) k, 2qmax ¼ 50.0ꢁ, 15 248 reflections collected,
˚
Crystal data for 1. C22H22N10O6, M ¼ 522.50, yellow prism, 0.21
ꢂ 0.18 ꢂ 0.10 mm3, monoclinic, space group P21/n (no. 14), a ¼
5366 unique (Rint ¼ 0.2267). Final GooF ¼ 1.812, R1 ¼ 0.2848,
wR2 ¼ 0.5981, R indices based on 2474 reflections with I > 2s(I)
(refinement on F2), 383 parameters, 280 restraints. Lp and
absorption corrections applied, m ¼ 0.273 mmꢀ1. While the crys-
tallographic data are of low precision on this sample due to the
extremely poor crystal quality the overall nature of the structure is
apparent. As a result the limited structural data available are
provided for completeness. The model obtained is sufficient to give
evidence for the structural details that are of interest.
ꢁ
˚
10.4699(10), b ¼ 5.4899(5), c ¼ 20.581(2) A, b ¼ 103.507(4) , V ¼
3
ꢀ3
˚
1150.26(19) A , Z ¼ 2, Dc ¼ 1.509 g cm , F000 ¼ 544, Smart-6K,
MoKa radiation, l ¼ 0.71073 A, T ¼ 120(2) k, 2qmax ¼ 61.0ꢁ,
˚
10 993 reflections collected, 3450 unique (Rint ¼ 0.0388). Final
GooF ¼ 1.031, R1 ¼ 0.0486, wR2 ¼ 0.1254, R indices based on 2428
reflections with I > 2s(I) (refinement on F2), 173 parameters,
0 restraints. Lp and absorption corrections applied, m ¼ 0.114 mmꢀ1.
Crystal data for (1H+)2(SO42ꢀ)$7H2O. C22H38N10O17S, M ¼
746.68, light yellow needle, 0.38 ꢂ 0.28 ꢂ 0.24 mm3, monoclinic,
space group P21/c (no. 14), a ¼ 8.60190(10), b ¼ 16.2504(2), c ¼
Acknowledgements
ꢁ
3
˚
˚
We thank Dr Nigel Clarke for assistance in interpreting the
rheological results and Dr Budhika Mendis for assistance in
collecting the TEM data. GOL would like to thank the
Commonwealth Scholarship Commission and the Herchel Smith
Fellowship Fund for financial support.
23.2724(3) A, b ¼ 91.1000(10) , V ¼ 3252.52(7) A , Z ¼ 4, Dc ¼
1.525 g cmꢀ3, F000 ¼ 1568, Smart-6K, MoKa radiation, l ¼
0.71073 A, T ¼ 120(2) k, 2qmax ¼ 61.0ꢁ, 33 366 reflections
˚
collected, 9934 unique (Rint ¼ 0.0401). Final GooF ¼ 1.053,
R1 ¼ 0.0523, wR2 ¼ 0.1414, R indices based on 7653 reflections
with I > 2s(I) (refinement on F2), 503 parameters, 14 restraints.
Lp and absorption corrections applied, m ¼ 0.192 mmꢀ1
.
Notes and references
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˚
A, a ¼ 94.481(2), b ¼ 104.643(2), g ¼ 110.579(2) , V ¼ 753.54(7)
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84 | Soft Matter, 2011, 7, 75–84
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