Gelation by supramolecular dimerization of mono(urea)s

Article abstract of DOI:10.1039/c5cc06995e

Mono-ureido salicylic acid derivatives form hydrogen bonded dimers that mimic the gelation behaviour of covalently linked bis(urea)s. The 5-ureido salicylic acid isomers are much more effective gelators because of the lowered conjugation and less planar g

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RSCPublishing
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Gelation by supramolecular dimerization of
mono(urea)s
Cite this: DOI:
10.1039/x0xx00000x
Abigail E. Hoopera, Stuart R. Kennedya, Christopher D. Jonesa and Jonathan W. Steed*a
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
homosynthons form with high fidelity, allowing effective
gelation. The approach is related to the hexagonal isophthalic
acid derived assemblies reported by Potluri and Hamilton.24
Monoꢀureido salicylic acid derivatives form hydrogen bonded
dimers that mimic the gelation behaviour of covalently linked
bis(urea)s. The 5ꢀureido salicylic acid isomers are much more
effective gelators because of the lowered conjugation and less
Results and Discussion
Gelator synthesis
planar geometry, resulting in stronger urea···urea
hydrogen bonding interactions.
αꢀtape
A series of mono(urea)s based on 4ꢀ and 5ꢀaminosalycylic acid were
synthesised from reaction of the respective aminosalycylic acid with
the appropriate isocyanate to give compounds 1a–d and 2a–d in
good yield. A search of the CSD shows that the salicylic acid
functionality is almost invariably planar because of strong
intramolecular OH···O hydrogen bonding and tends to form
hydrogen bonded carboxylic acid dimers via an ꢀꢁꢁ(8) hydrogen
bonded ring motif,36 as in the structure of 5ꢀaminosalycylic acid
itself and 17 out of 24 related derivatives.37 Based on the CSD
evidence the supramolecular synthons38 formed by the salicylic acid
and urea functional groups might be expected to be orthogonal to
one another and hence selfꢀsorting in their assembly behaviour.
Introduction
Gel phase soft materials based on supramolecular interactions
between low molecular weight gelators (LMWG) have become
extremely popular in the past decade. These versatile media
have applications1, 2 in organocatalysis,3, 4 wound healing,5 drug
delivery,6, 7 as templates and supports for nanomaterials,8ꢀ10 and
in pharmaceutical polymorph screening and control.11ꢀ15
LMWG represent a very broad class of compounds which have
in common the ability to form high aspect ratio fibres by rapid
unidirectional grown of a solid, sampleꢀspanning network. Gels
are formed in a wide range of solvents including water
(hydrogels) and both polar and nonꢀpolar organic liquids
(organogels). The reversibility of the supramolecular
interactions between the small molecule gelators means that
gels derived from LMWG are usually thermally reversible and
their materials properties, particularly their rheological
characteristics, can be manipulated by light, changes in pH or
by chemical triggers such as metal cations and hydrogen bond
acceptor anions.16ꢀ21 Common LMWG are based on chemical
functionalities capable of forming one dimensional assemblies
including nucleobases, amides and ureas.21ꢀ23 Most LMWG
incorporate multiple functional groups that associate in a
directional way in order to increase the strength of the fibres by
multivalent supramolecular interactions. In ureaꢀbased systems
most gelators (with some exceptions24ꢀ27) contain at least two
urea groups.25, 28ꢀ35 We now report a novel approach to effective
ureaꢀbased gelators in which two urea functional groups are
linked by a supramolecular hydrogen bonded motif instead of
by a covalent bond. The interactions involved in the assembly
of the mono(urea) LMWG into a dimer and the subsequent
assembly of the hydrogen bonded dimers into a fibre are
orthogonal to one another, and the different supramolecular
Gelation
Gelation tests on compounds of type 1 and 2 were carried out at 1
mg mLꢀ1 in a wide range of solvents. Samples were sonicated for 30
seconds, then heated gently to dissolve the solid, and subsequently
allowed to cool to room temperature. Most gels formed within a few
minutes while for some samples crystallisation occurred over an
approximately 24 hour period. Full results are tabulated in the
supplementary information, Table S1. The 5ꢀASA derivatives of
type 2 proved to be surprisingly efficient gelators despite the
presence of only a single urea group. The unbranched chain
members of the series 2a, c and d form gels to some degree in 1,2,4ꢀ
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trichlorobenzene, 1,2ꢀdibromoethane, 1,2ꢀdichlorobenzene, 1,3ꢀ Pa at low stress, and the low yield stress of around 20 Pa, for
dichlorobenzene and nitrobenzene (see supplementary Figure S1). example (Figure 2a). This gel exhibits weak strain overshoot
The gels form within a few minutes of cooling and are all opaque characterised by a rise on G′′ with increasing stress. This viscoelastic
suggesting a relatively large particle size. Gels formed in 1,2,4ꢀ behaviour is indicative of closeꢀpacked weakly bonded particles
trichlorobenzene collapsed after 30 min, while 1,2ꢀdibromoethane forming a structure that resists deformation just prior to breaking of
and 1,3ꢀdichlorobenzene gave somewhat stronger gels, although interactions and flow occurring.40, 41 Elastic moduli were found to be
these also collapsed after a few hours. The strongest and most highly solvent dependent and to increase with increasing chain
optically transparent gels formed in 1,2ꢀdichlorobenzene and length. Hence, 1,2ꢀdibromoethane gels of the butyl compound 2a
nitrobenzene, which proved to be stable at least over a period of proved quite robust with G′ reaching ca. 10 kPa (Figure 2b, see
weeks. In comparison with the unbranched gelators, the isopropyl supplementary information Figure S5).
derivative 2b formed only partial gels in just two solvents and these
materials collapsed immediately. It is possible that the increased
(a)
steric bulk of the isopropyl group hinders urea selfꢀassembly and
hence disfavours gel formation.
The 4ꢀASA derivatives of type 1 proved to be more soluble than
their 5 ASA counterparts and did not exhibit significant gelation
behaviour. In only one case was a gelꢀlike material obtained. Slow
cooling of 1a in nitromethane gave an opaque, white selfꢀsupporting
solid with a critical gelation concentration of 1% w/v. SEM analysis
of the resulting xerogel showed it to comprise a mixture of fibrous
and crystalline material (see supplementary information, Figures S1
and S2).
(b)
Representative gels were dried in a vacuum desiccator for 24 hours
to give xerogels and then coated with either chromium or platinum
and imaged by SEM. The microscope images showed that the
xerogels adopt the expected fibrous morphology. Fibre widths and
bunching vary from sample to sample but are consistent with other
semiꢀtransparent gels.39 A representative image of the xerogel of
nꢀbutyl compound 2a obtained from nitrobenzene is shown in Figure
1a and contrasts with the xerogel of ethyl analogue 2d, which
exhibits a mixture of shorter, blockier and more well defined
crystalline tapes and long, thin and more curved fibres (Figure 1b).
The shorter ethyl substituent may result in more facile
recrystallization of some of the sample during the drying process.
SEM images of nitromethane xerogels of 1a, the only 4ꢀASA
derived gel, show a significant difference from the 5ꢀASA versions,
with wide crystalline aggregates of fibres forming sheets 1 µm
across (supplementary information Figure S4). All the fibres are
distinctly larger than those of 1a and 1d in the same solvent. This
may account for the more crystalline, opaque appearance of the gel.
Figure 2. (a) Stress sweep rheometry for a 1% w/v gel of 2a in 1,2-
dichlorobenzene. (b) comparison of the evolution of G’ in 1,2-dibromoethane as
a function of chain length.
Single crystals suitable for Xꢀray crystallography were obtained for
compounds 1a, 1c and 1d by recrystallization from ethyl acetate,
acetone and 2ꢀbutanone, respectively. The sample of 1d proved to be
of very low quality and is not reported herein but a qualitative
structure solution confirmed the packing is similar to the other two
compounds of type 1. The structures of 1a and 1c are of high
precision. The structures of the 4ꢀASA derivatives of type 1 all
exhibit the common headꢀtoꢀhead carboxylic acid dimer motif,42
with approximate coꢀplanarity of the carboxylic acid group and aryl
ring enforced by an intramolecular OH⋅⋅⋅O interaction from the
phenolic OH group to the acid carbonyl functionality, Figure 3. The
high acidity of the salicylic acid moiety results in short O⋅⋅⋅O
distances across the ꢀꢁꢁ(8) hydrogen bonded ring of 2.61 – 2.65 Å.
The urea moieties are not precisely coꢀplanar with the salicylic acid
ring although there is a relatively long intramolecular ArCH⋅⋅⋅O=C
hydrogen bond, as is the case with electron withdrawing
(b)
(a)
Figure 1. SEM images of (a) xerogel of 2a obtained from nitrobenzene and (b)
xerogel of 2d obtained from nitromethane.
Gels of 2a, 2c and 2d were characterised by oscillatory stress sweep diarylureas.43, 44 In 1a the urea groups form a twisted ꢀꢁꢂ(6) urea αꢀ
rheometry to probe their viscoelastic properties in several solvents. tape hydrogen bonded chain that is independent of the acid dimer
The solidꢀlike character of the materials was manifest in the fact that hydrogen bonding. This urea motif, particularly in an antiparallel
the G′ (storage modulus) remains well over an order of magnitude arrangement in covalently linked bis(urea)s, has been linked to the
ubiquitous gelꢀforming ability of these compounds.28, 39, 45 However,
higher than the viscous modulus G′′ up to the yield stress of the gel.
The relatively weak nature of the gels of compound 2a in 1,2ꢀ in 1a, each hydrogen bonded dimer links into two different twisted
dichlorobenzene is apparent in the fairly low value of G′ of ca. 1000
αꢀtapes to give an overall 2D hydrogen bonded network based on
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two crystallographically independent molecules. In 1c, in contrast, the competitive solvent the compound exists as a mixture of
monomer and dimer while in nonꢀpolar media it is fully dimerised.
the two urea αꢀtapes formed by each hydrogen bonded dimer are
approximately coplanar and align antiparallel to give a 1D fibre,
mimicking the arrangement found in covalently bonded bis(ureas).
Evidence for the relevance of the single crystal Xꢀray structural data
to the gels formed by compounds of type 2 was sought from Xꢀray
powder diffraction.48ꢀ50 However the XRPD patterns of the xerogels
of 2a, 2c and 2d from nitromethane and nitrobenzene all showed the
materials to be amorphous. Given the simplicity of the compounds
and the consistent structural results, however, it remains a plausible
working hypothesis that the local structure of the gel fibres is closely
related to the interactions found in the single crystals.
The 5ꢀASA derivative 2a was also characterised by single crystal Xꢀ
ray crystallography using a sample obtained by recrystallization
from methanol, Figure 3c. As with the compounds of type 1, this
compound also exhibits a headꢀtoꢀhead carboxylic acid dimer
arrangement and, as with 1c, each dimer stacks with its neighbours
to give an antiparallel pair of urea αꢀtape interactions running
throughout the crystal. Notably the urea NH···O distances of 2.80 –
2.82 Å are at the lower end of the typical range found for this
interaction44 and are considerably shorter than the in 1a and 1c (the
interactions in 1a are unsymmetrical at 2.81/3.07 and 2.88/2.98 Å,
while those in 1c are symmetrical at 2.89 – 2.91 Å). This short
urea···urea distance is correlated with the much more significant
outꢀofꢀplane orientation of the urea group relative to the salicylic
acid, which breaks the intramolecular CH···O interaction.
(a)
It is plausible that gelation by compounds of type 1 and particularly
type 2 arises from urea αꢀtape formation, and the fact that this
interaction is much shorter in 2a is consistent with the much more
effective gelation behaviour of this compound compared to
compounds of type 1. While mono(urea)s interact with one another
through a single bifurcated acceptor ꢀꢁꢂ(6) synthon, covalently
bound bis(urea)s enjoy a multivalent46 stabilization by virtue of the
ꢂ( )
(b)
double ꢀꢁ
6 interaction formed by each molecule. In the present
case we postulate that the strong nonꢀcovalent dimerization arising
from the salicylic acid groups in compounds of type 1 and 2 allows
them to behave as pseudoꢀbis(urea)s with the urea groups in
hydrogen bonded pairs held in a centrosymmetric and hence
antiparallel arrangement by the centrosymmetric ꢀꢁꢁ(8) carboxylic
acid dimer arrangement. Evidence for the persistence of the dimeric
structure in the gas phase comes from the ES+ mass spectrum of 2a
which, in addition to the molecular ion peak at m/z 252.8, exhibits a
distinct peak at 505.1 of approximately half the intensity of the
molecular ion, assigned to the dimer. Similar gas phase dimers are
readily apparent in the ES+ꢀMS spectra of all compounds of type 1
and 2. In solution the 1H NMR spectrum of 2a in DMSOꢀd6 solution
did not exhibit any change in the chemical shift of the carboxylic
acid OH resonance as a function of temperature in the range 20 –
(c)
o
100 C, nor in the concentration range 1 – 8% w/v. In contrast, a
modest chemical shift change was observed for the NH resonances,
suggesting that while aggregation by ureaꢀurea interaction is in
Figure 3. (a) Twisted 2D urea hydrogen bonding in 1a; (b) parallel packing in 1c
with relatively long NH···O interactions; (c) short NH···O interactions in the
equilibrium under the conditions used in the NMR experiment, the gelator 2a. Selected hydrogen bonding distances (Å) for compound 1a: Acid
dimer O•••O 2.606(2), 2.634(2); urea tape N•••O 2.986(3), 2.880(2), 2.814(2)
and 3.072(2), compound 1c, O•••O 2.892(3), N•••O 2.911(3), 2.892(3);
compound 2a, O•••O 2.629(6); N•••O 2.797(7), 2.816(7).
carboxylic acid groups remain fully dimerised under the conditions
studied even in a competitive solvent such as DMSO. It is also
possible that the lack of chemical shift change as a function of
concentration and temperature indicates no interaction at all and the
molecule is monomeric; however, this seems unlikely given the
strong hydrogen bonding in the salicylic acid group, the ES+ꢀMS
evidence and the contrasting behaviour of the generally more weakly
hydrogen bonded urea group.47 Definitive proof for the retention of
the dimeric structure in solution was obtained from IR spectroscopy.
The solidꢀstate ATR spectra of all compounds of type 1 and 2
closely resembled the spectra in the nonꢀcompetitive solvent 1,2ꢀ
dibromoethane with a single low frequency asymmetric carbonyl
stretching band at 1660 cmꢀ1 for 2a, for example, assigned to the
carboxylic acid dimer observed in the Xꢀray crystal structures. In the
more competitive DMSO solution, however, two carboxylic acid
carbonyl bands appeared, at 1703 and 1655 cmꢀ1 assigned to
monomeric and dimeric carboxylic acids respectively. Other
consistent changes were observed throughout the spectra. Hence in
Given that both the 4ꢀASA derivatives of type 1 and 5ꢀASA
analogues of type 2 share a common dimeric structure, it is
interesting to see that compounds of type 2 exhibit shorter urea tape
hydrogen bonding and are also significantly more effective gelators
of the solvents studied than compounds of type 1. The outꢀofꢀplane
twist of the urea group compared to the aromatic ring plane in 2a is
exemplified by a torsion angle of 50.9°, significantly larger than
those of the 4ꢀASA derivatives which are between 17.3 – 38.0°. Gasꢀ
phase DFT calculations suggest that this is not solely a crystal
packing effect, with the rotation energy barrier for the 4ꢀASA
derivatives being around 29 kJ molꢀ1, nearly double the analogous
barrier for the 5ꢀASA analogues (supplementary information Figure
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DOI: 10.1039/C5CC06995E
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DOI: 10C.O10M39M/CU5CNCI0C6A99T5IOE N
Graphical Abstract
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 5