Highly interlocked anion-bridged supramolecular networks from interrupted imidazole-urea gels

Article abstract of DOI:10.1039/c4cc05789a

An imidazole urea displays switchable hydrogelation behaviour that is turned off by metal ion coordination to give anion-bound coordination complex arrays.

Full text of DOI:10.1039/c4cc05789a

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Highly interlocked anion-bridged supramolecular
networks from interrupted imidazole–urea gels†
Cite this: Chem. Commun., 2014,
0, 12851
5
Samuel J. James, Andrea Perrin, Christopher D. Jones, Dmitry S. Yufit and
Jonathan W. Steed*
Received 25th July 2014,
Accepted 5th September 2014
DOI: 10.1039/c4cc05789a
www.rsc.org/chemcomm
An imidazole urea displays switchable hydrogelation behaviour that is demonstrated that compounds of this type favour homomeric
7
turned off by metal ion coordination to give anion-bound coordination association of the urea and imidazole functional groups, and are
8
complex arrays.
interesting in the context of proton dipolar channel formation.
Compound 1b crystallises as an acetonitrile solvate showing distinct
The ability of urea derivatives to form supramolecular gels by means urea a-tape and imidazole NHꢀꢀꢀN interactions. Indeed, while
of extended hydrogen bonded chains has given rise to a rich field of less explored than their urea analogues, the ability of imidazole
synthetically and externally tunable soft materials which has been derivatives to form hydrogen bonded chains has been exploited in
1
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the subject of intense investigation in recent years. Interruption of several supramolecular gel-forming systems. As well as its self-
the urea a-tape hydrogen bonding pattern by hydrogen bonding to complementary hydrogen bond donor and acceptor capacity, the
strong acceptors such as anions can weaken and ultimately ‘switch basic imidazole group in compounds of type 1 makes them suitable
1a,h,2
10
off’ gelation behaviour.
In contrast, metal binding to function- as ligands for transition metal centres, offering the possibility of
ality such as a pyridyl group can ‘switch on’ gelation by removing the metal-modulated gelation behaviour and material properties. In
3
3,11
competing urea–pyridyl hydrogen bonding interaction. There are comparison to the relatively well studied pyridyl ureas, the smaller
now an increasing number of reports of metallogel formation in size of the imidazole functionality also potentially results in less steric
4
which the metal is integral to the gelation behaviour. Examples of crowding at the metal centre and hence higher metal:ligand stoi-
anion-promoted gelation have also been observed and there is chiometry, involving more hydrogen bonded cross linking and hence
5
evidence for gel fibre formation being mediated by urea–anion a more three dimensional topology compared to the pyridyl systems.
6
interactions in pyrazole functionalised ureas. In this work we focus
While 1a is a mono(urea) and gelation is generally associated with
1a,e,4c,12
on the imidazole containing urea derivative 1a, which is conveniently bis(urea)s,
prepared from histamine.
additional hydrogen bonding from the imidazole
group is expected to strengthen the hydrogen bonded chain. Com-
pound 1a was screened for gel formation in a variety of solvents.
Compound 1a does not gel a range of organic solvents, however it
proved to be an effective hydrogelator (see ESI,† Fig. S1). Complete
dissolution in water requires vigorous heating, but upon cooling at
room temperature, a thermoreversible, opaque gel forms in less than
Unlike pyridyl ureas, in which hydrogen bonding to the pyridyl three minutes in the range of 0.7–2.0 wt%. Below 0.7 wt%, weak
nitrogen atom inhibits gelation, the basic imidazole nitrogen atom partial gels or precipitates form, while above 2 wt% it becomes
in 1a has the potential to hydrogen bond with the strongly hydrogen difficult to completely dissolve the solid. Compound 1a also forms
bond donating imidazole NH group, leaving the urea moiety free to gels in mixtures of water and other polar solvents such as methanol.
form the usual hydrogen bonded a-tape motif and hence act as a For example water/methanol mixtures in ratios of up to 2 : 1 can be
gelator. This expectation is borne out by the crystal structure of the gelled. Brief sonication of the hot aqueous solutions of 1a results in
phenyl derivative 1b reported by Barboiu and co-workers who near-instantaneous gelation. Optical microscopy suggests that gels
formed by slow cooling are quite crystalline while the sonication
Department of Chemistry, Durham University, South Road, Durham DH1 3LE, UK.
induced gels appear much less crystalline and sonication allows
for gelation of considerably lower gelator concentrations, down to
E-mail: jon.steed@durham.ac.uk
†
Electronic supplementary information (ESI) available: Crystallographic information in
about 0.4 wt% (ESI,† Fig. S2). Ultrasound induced gelation has
CIF format for the structures of 1a, 2 and 3, and experimental details covering synthesis
13
been observed previously and the mechanism may well involve
of new compounds, gel rheology and SEM analysis. CCDC 1016071–1016073. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4cc05789a instantaneous dissolution and re-precipitation of gelator aggregates
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1
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weak structural interactions between large particles or fibres.
Such interactions can resist deformation at low stresses, after
which they begin to break down resulting in an increase in the
0
0
loss modulus G . The structure is then completely destroyed
upon reaching the yield stress and the material begins to flow.
This behaviour is consistent with the crystalline nature of 1a, and
the large fibres observed by SEM. Concentration dependency
0
studies show that G and yield stress increase with concentration
up to close to the solubility limit.
Colourless single crystals of 1a were isolated via slow cooling
in acetonitrile and characterised by X-ray crystallography (see
ESI,† for crystallographic details). The compound exhibits a
typical hydrogen bonded urea a-tape motif between adjacent
molecules that aggregate in alternating alignments along the
tape axis. This allows for the formation of a second ‘zigzag’
Fig. 1 SEM image of a dried 1 wt% xerogel of 1a prepared thermally hydrogen bonded tape between the imidazole nitrogen atoms.
without sonication, showing twisted ribbon-like fibres.
The combination of urea NHꢀꢀꢀO and imidazole NHꢀꢀꢀN interactions
is clearly sufficiently robust to give rise of fibre formation and hence
from a solid surface under saturation conditions, preventing the gelation in an analogous fashion to bis(urea) gelators. The urea
formation of long range ordered aggregates. SEM images of the NꢀꢀꢀO distances of 2.8937(16) Å and 2.8787(16) Å are short and
chromium coated xerogels of 1a prepared thermally show entangled, symmetrical which, along with the imidazole NHꢀꢀꢀN distance of
ribbon-like fibres typically up to about 10 mm in width. Some fibres 2.9297(18) Å suggests a robust, unhindered hydrogen bonding
1
are short and straight with well-defined edges, which is in keeping pattern (Fig. 2). H NMR spectroscopy of 1a over the concentration
6
with the relative crystalline character of the material observed in range 0.05–5.0 w/v in DMSO-d revealed significant broadening of
optical microscopy. Interestingly, some of the fibres are uniformly the imidazole NH resonance into the baseline and a small but
twisted into helical structures (Fig. 1). Helical gel fibres are commonly consistent chemical shift change Dd for the urea NH resonances of
observed for chiral gelators, since chirality is often translated from 0.04 ppm. This data supports the formation of both urea NHꢀꢀꢀO
14
the molecular level to the nanoscale. However, compound 1a is not and imidazole NHꢀꢀꢀN interactions in solution.
a chiral molecule and both left and right handed helical architectures
The X-ray crystal structure of the closely related phenyl
are observed throughout the non-sonicated xerogel in every sample derivative 1-[2-(1H-imidazol-4-yl)ethyl]-3-phenylurea (1b) was reported
7
examined.
by the Barboiu group in 2008. Compound 1b crystallises as an
Some materials are known to twist as they crystallise under the acetonitrile solvate. The solvate structure also exhibits both a hydro-
15
right conditions possibly because of variations in mechanical, gen bonded urea a-tape and an imidazole chain motif, although the
thermal and/or compositional fields within the crystal growth overall crystal packing arrangement is quite different. As with 1a,
medium. For example, the process of crystallisation possesses molecules are arranged in an anti-parallel fashion. However in the
an inherent latent heat which can create localised variations in case of 1a, the urea and imidazole hydrogen bonded tape motifs
temperature. Also as crystals form, differences in density and each lie along a single plane. For 1b on the other hand, each
composition increase between the growing crystal and the molecule in the urea tape is offset from the two molecules either
surrounding solution. Twisting can occur if these field variations side of it by approximately 601, allowing for CH–p interactions
16
are significant on either side of the propagating fibre. In addition, between the imidazole and the phenyl ring.
internal and external stresses caused by impurities or surface
tension are sometimes thought to be the cause of twisting.
Compound 1b was re-synthesised and screened for gelation
behaviour in an analogous fashion to 1a. Unlike 1a however,
The sonicated 1a xerogels were also examined using SEM compound 1b does not act as a hydrogelator, instead
see ESI,† Fig. S3). Likewise these xerogels are fibrous, but on
(
average the ribbon-like fibres tend to be smaller in width than
the non-sonicated gels (less than about 5 mm), and are generally
longer and more uniform. The twisted structures are also still
present but occur less frequently.
The rheological properties of hydrogels of 1a (prepared
thermally) confirm that the material is a true gel. A stress sweep at
0
1
wt% shows that at low stresses, G is almost two orders of
00
magnitude greater than G , indicating significant elastic behaviour
characteristic of a gel. However the marked increase in G upon
1
h
00
approaching the yield stress is more unusual. This type of behaviour
1
7
is known as ‘weak strain overshoot’, and can occur most
commonly in particulate viscoelastic materials as a result of N3ꢀꢀꢀO1 2.8937(16) Å, N4ꢀꢀꢀO1 2.8787(16) Å, imidazole N1ꢀꢀꢀN2 2.9297(18) Å.
Fig. 2 Crystal packing in 1a. Selected hydrogen bond lengths (Å): urea
1
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precipitating out of hot solution upon cooling or sonication. It is morphologies of the pure compound 1a hydrogels (see ESI,†
possible that the presence of the methyl group allows a faster- Fig. S5).
growing, more compact structure in comparison to the solvate-
In order to probe the interaction mode of the metal salts with
forming 1b. Consistent with this hypothesis is the fact that the gelator 1a, single crystal X-ray crystal structures were determined for
hydrogen bonded distances in the urea a-tape in 1b are significantly two complexes derived from the reaction of copper(II) salts with 1a.
longer at 2.92 and 3.03 Å, whereas the imidazole NHꢀꢀꢀN interaction The blue complexes were prepared by vapour diffusion of diethyl
is markedly shorter at 2.75 Å. The presence of the methyl group may ether into a solution of 1a in methanol containing 0.25 equivalents
also substantially affect the aqueous solubility of 1a.
There is considerable interest in the use of metal ions and anions containing 0.15 equivalents of Cu(NO
to tune gelation behaviour and gel material properties.
of CuCl
2
ꢀ2H
2
O, or on standing of a methanol solution of 1a
ꢀ2.5H O. Elemental analysis
In order of the crystalline products gave the formulations [Cu(1a) Cl]Cl (2)
to probe the importance of the imidazole NHꢀꢀꢀN interaction in and [Cu(1a) ](NO ) ꢀ2MeOH (3), confirmed by X-ray crystallographic
3
)
2
2
1h,4a,g
4
6
3 2
the gelation behaviour of 1a we examined its gelation formation analysis. Both structures exhibit unidentate coordination of the
properties in the presence of copper(II) and zinc(II); metal ions likely imidazole moieties to the metal centres accompanied by hydrogen
to be strongly bound by the imidazole nitrogen atom. Such com- bonding of the pendant urea groups to the counter anions. While it
plexation would interrupt the NHꢀꢀꢀN hydrogen bonding seen in the has not been established that these crystalline complexes isolated
structure of 1a, while the interactions of the urea functionality to from methanol are identical to those formed upon addition of
counter anions is expected to disrupt the urea a-tape formation, copper(II) salts to the hydrogels, they nevertheless provide a highly
since inorganic anions are significantly stronger hydrogen bond plausible mechanism by which the NHꢀꢀꢀO and NHꢀꢀꢀN hydrogen
1a
bases than the urea carbonyl oxygen atom. Rheological experi- bonding observed in the structure of 1a can be interrupted by metal
ments were conducted on hydrogels of 1a in the presence of coordination and anion hydrogen bonding.
Cu(NO ) and ZnCl (0.1 eq.), using the same method as for the
In the structure of 2 the Cu(II) centre is 5-coordinate, and
3
2
2
pure 1a gels. The gelator was dissolved in warm water, and an forms an elongated square based pyramid. The four 1a ligands
aqueous solution of the metal compound added to give an overall coordinate through the N1 imidazole nitrogen atom and sit in
1
wt% solution of gelator with further heating to dissolve the the slightly distorted equatorial positions, with Cu–N bond
resulting precipitate. Fig. 3 shows stress sweep rheometry data for lengths of 1.9970(17) Å. The axial position is occupied by a
these metallogels after cooling to room temperature in comparison chloride anion, Cu–Cl distance 2.6410(12) Å. The four organic
to a 1 wt% pure compound 1a hydrogel. In both cases, the presence ligands form a concave ‘bowl’ and these bowl-shaped structures
0
of the metal salt dramatically lowers the storage modulus G by at form a one dimensional stack via hydrogen bonds between the
least an order of magnitude compared to the pure 1a hydrogel. The imidazole NH group and the urea oxygen atoms of an adjacent
yield point also drops from 44.7 Pa for the pure gel to 3.9 and 1.2 Pa complex, to give an infinite hydrogen bonded polymer (Fig. 4)
in the presence of Zn(II) and Cu(II), respectively. Evidently coordina- with short Nꢀ ꢀ ꢀO distances of 2.735(2) Å. Each stacks interacts
tion of 1a to a metal centre substantially weakens the gel, in with three other adjacent stacks via four hydrogen bonds from
accordance with the hypothesis that the imidazole hydrogen bonding the urea NH to the uncoordinated chloride anions, Nꢀ ꢀ ꢀCl
is required for gelation. Further addition of metal compound distance 3.2983(18) Å.
(4ca. 0.2 molar equivalent) prevents gelation completely, and
The structure of 3 is based on a Jahn–Teller distorted octahedral
amorphous precipitates instantly form from the hot solution.
copper(II) centre with three crystallographically unique Cu–N bonds
Examination of a hydrogel formed in the presence of 0.1 eq. to the six 1a ligands; 2.049(2), 2.072(2) and 2.402(2) Å. Each
Cu(NO O using SEM revealed a plate shaped crystalline
3
)
2
ꢀ2.5H
2
morphology, which is a significant deviation from the fibrous
Fig. 4 X-ray crystal structure of 2 (a) stacking via imidazole NHꢀ ꢀ ꢀO
hydrogen bonding, (b) hydrogen bonding of four urea moieties to the
non-coordinated chloride anion. Selected hydrogen bond lengths (Å):
N(2)ꢀ ꢀ ꢀO(1) 2.735(2), N(4)ꢀ ꢀ ꢀCl(2) 3.2983(18) Å.
Fig. 3 Stress sweep rheometry data for a 1 wt% hydrogen of compound 1a
in the presence of Cu(NO ) and ZnCl compared to the pure compound.
3 2 2
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1
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1
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2854 | Chem. Commun., 2014, 50, 12851--12854
This journal is ©The Royal Society of Chemistry 2014