Polymorphs from supramolecular gels: Four crystal forms of the same silver(i) supergelator crystallized directly from its gels

Article abstract of DOI:10.1039/c1cc10305a

A new silver(i) complex with 1-phenyl-3-(quinolin-5-yl)urea has been prepared, acting as a supramolecular supergelator in the presence of polar solvents. In open vials temperature dependent, reversible gel-to-xerogel transition is observed, while in seale

Full text of DOI:10.1039/c1cc10305a

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COMMUNICATION
Polymorphs from supramolecular gels: four crystal forms of the same
silver(I) supergelator crystallized directly from its gelsw
Dario Braga,* Simone d’Agostino, Eros D’Amen and Fabrizia Grepioni*
Received 17th January 2011, Accepted 10th March 2011
DOI: 10.1039/c1cc10305a
A new silver(I) complex with 1-phenyl-3-(quinolin-5-yl)urea has
been prepared, acting as a supramolecular supergelator in the
presence of polar solvents. In open vials temperature dependent,
reversible gel-to-xerogel transition is observed, while in sealed
vials direct crystallization of four polymorphs of the same gelling
compound is achieved, depending on the gelling solvent.
Scheme 1 The ligand 1-phenyl-3-(quinolin-5-yl)urea (PQ5U).
Gels have become commonplace in everyday life.1 They can be
supergelator complex between 1-phenyl-3-(quinolin-5-yl)urea
(PQ5U) (Scheme 1) and AgNO3 in the presence of common
obtained from small molecules as well as from oligomers and
organic solvents; (ii) the behaviour of the gels with temperature;
polymers, the vast majority being represented by purely
and (iii) the four polymorphic modifications of the supergelator
organic gels and by inorganic gels, such as silica gel, etc.,
but also organometallic and metallo-gels have been reported.2
obtained by separate crystallizations from MeOH, EtOH,
i-PrOH, or CH3CN.
We have chosen the ligand PQ5U10 because (i) it is a low
Supramolecular gels are those held together by non-covalent
interactions such as hydrogen bonds and metal–ligand inter-
actions. As pointed out by Flory already in 19743 and recalled
weight molecule, and also (ii) in its solid state structure it
by Dastidar in his recent review4 ‘‘gels are materials that are
shows the typical hydrogen bonding urea tape motif; in
addition to this, the presence of a nitrogen atom in an
easier to recognize than to define’’. Crystallization of a gelator
accessible position makes it a good candidate for the formation
compound is not an easy job especially when crystallization
of a discrete metal complex with the silver cation.
takes place from the very gelling solvent. The examples available
The ligand in itself is not a gelator, and it is only slightly
in the literature are very few, in particular when small
coordination compounds are involved.5 Steed, Clarke and
soluble in polar solvents such as MeOH, EtOH, i-PrOH and
CH3CN; suspensions are turned into clear solutions upon
on metal- and anion-binding supramolecular gels.6 Of
heating, and slow cooling to room temperature yields the
coworkers have very recently published an exhaustive review
starting solid. Upon addition of AgNO3 to the PQ5U solutions
particular interest are supergelator systems, because they are
supramolecular interactions build up between the ligand, the
effective in gel formation at very low concentration, i.e. below
1% w/v.1
silver cations and the nitrate anions, and thermo-reversible
organo-gels are formed.11
Gels find applications in almost all areas of materials
chemistry,7 and are nowadays been approached with the
Gelation is obtained (Fig. 1) when the AgNO3 : PQ5U
molar ratio is in the range from 1 : 2 to 1 : 1 (see ESIw). Using
other nitrates of transition metals such as zinc and cobalt or
objective of finding materials suitable for crystallization of
new crystal phases.8 The quest for new crystal forms of
other salts of silver, such as the acetate and the tetrafluoro-
molecular species has come to the forefront of solid state
borate, does not lead to gel formation. Table 1 lists the
chemistry investigations because of the number of potential
gelation properties of the system AgNO3 : PQ5U in different
applications of the different physico-chemical properties
organic solvents.
of different crystal forms of a same species (polymorphs,
co-crystals, hydrates, solvates, salts, etc.).8,9
In this paper we report (i) the discovery of new supra-
molecular gels, obtained via formation of the silver(I)
Dipartimento di Chimica G. Ciamician, Universita` degli Studi di
Bologna, via Selmi 2, 40126 Bologna, Italy.
E-mail: fabrizia.grepioni@unibo.it; Fax: +39 051 2099456
w Electronic supplementary information (ESI) available: Forms I–IV:
synthesis, single crystal data and measurements, packing features,
ORTEP plots. CCDC 801410–801413. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c1cc10305a
Fig. 1 The clear sol (on the left) and the ‘‘tube inversion test’’
showing gel formation (right).
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Table 1 Results of the gelation experiments, in different solvents, for
AgNO3 : PQ5U stoichiometric ratios of 1 : 2 and 1 : 1 (S = clear
solutions; I = insolubility; G = gel formation)
Solvent
Result
Solvent
Result
EtOH
MeOH
i-PrOH
CH3CN
G
G
G
G
G
I
THF
I
1-butanol
1-hexanol
Toluene
Ethyl acetate
Nitromethane
Dioxane
G
S
I
I
S
I
(À)Ethyl-lactate
Acetone
DMF
Xa
I
Dichloromethane
Ethylene glycol
Cyclohexane
Ethanol amine
t-Butanol
S
S
S
I
a
Decomposition product.
Upon heating, gel-to-sol transition is observed, due to
disruption of the supramolecular interactions that hold the
gel-structure together; the solutions revert to the gel state upon
cooling.
Fig. 3 XRPD patterns of fresh gels (AgNO3 : PQ5U = 1 : 2)
obtained from acetonitrile, methanol, i-propanol and ethanol.
In order to evaluate the thermal stability of the gel, a plot of
the gel–sol dissociation temperature (Tgel) in EtOH versus the
gelator concentration was constructed (see ESIw) by using the
‘‘dropping ball’’ method.1b Fig. 2 clearly indicates that Tgel
increases with the gelator concentration.12 The gel–sol–gel
interconversion is fully reversible over ten to fifteen cycles of
heating and cooling. This means that the assembly of the
gelator molecules arises from cooperative non-covalent inter-
actions between the PQ5U molecules, the silver and nitrate
ions, and these interactions are broken upon heating the gel.
XRPD patterns of fresh gels, obtained directly in the sample
holder, were measured for the gels obtained from MeOH,
i-PrOH, CH3CN and EtOH (Fig. 3). All samples show a broad
peak at 2y between 31 and 41, which corresponds to a d spacing
of B2.5 nm; for gels obtained from ethanol and acetonitrile
this peak is sharper than those obtained from methanol and
i-propanol.
Fig. 4 XRPD patterns of the xerogels (AgNO3 : PQ5U = 1 : 2)
obtained from EtOH, MeOH, CH3CN and i-PrOH.
If the gels are dried in open vials xerogels are formed:
XRPD patterns of the air-dried samples (xerogels) were also
recorded (see Fig. 4): as it can be seen, the low angle peak is
Fig. 5 (a) SEM image of the xerogels obtained by drying in the air the
1 : 2 AgNO3 : PQ5U gel from EtOH. The fibrous nature of the
xerogel can be appreciated. The white spots indicate the presence of
silver(I) (Courtesy of Prof. Giuseppe Falini). (b) Crystallization of
Form IV as obtained by drying in a sealed vial the 1 : 2 AgNO3 : PQ5U
gel from MeOH.
maintained for all samples, but more and different peaks are
observed at a higher angle.
The same patterns can be observed if AgNO3 and PQ5U
are ground together in the stoichiometric ratio 1 : 2 in the
presence of a small quantity of solvent (kneading). A SEM
image (see Fig. 5a), of the xerogel obtained by air-drying the
Fig. 2 Effect of the concentration12 (expressed in % w/v) on the
gel–sol transition temperature in different solvents for an AgNO3 : PQ5U
molar ratio of 1 : 2.
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between molecular isomers and ‘‘crystal isomers’’, e.g. poly-
morphs,9,14 can be semantic and depends on the compromise
between minimization of crystal energy and of molecular
energy.
Fig. 7 shows the first coordination sphere around the Ag+
ion, which is completed by an Ourea atom and a silver cation in
Form I, by two Ourea atoms in Forms II and III and by a
nitrate anion and two C–H groups of adjacent ligands in
Form IV.
In conclusion, four different crystal forms of the same
gelator could be obtained from four different gelling solvents,
indicating that crystallization from gels is a viable route to
obtain new crystal forms; work is in progress to extend this
method to the preparation of polymorphs of co-crystals, and
to investigate (i) the combined effect of gel and solvent on the
formation of polymorphs; (ii) crystallizations of the complex
from solvents that do not form gels with it and (iii) the relative
stability of the four polymorphic forms.
Fig. 6 Images of the single crystals for the four polymorphs of the
complex [Ag(PQ5U)2]NO3, as obtained from CH3CN, Form I, EtOH,
Form II, i-PrOH, Form III and MeOH, Form IV.
We acknowledge financial support from the University of
Bologna and from MIUR (PRIN2006).
ethanol containing gel, reveals the presence of a 3D network of
branched and entangled fibres, which must be effective in
trapping the solvent molecules in the gel phase.
Notes and references
1 (a) R. G. Weiss and P. Terech, Molecular Gels Materials with
Self-Assembled Fibrillar Networks, Springer, Dordrecht, 2006;
(b) D. K. Smith, in Organic Nanostructures, ed. J. L.
Atwood and J. W. Steed, WILEY-VCH, Weinheim, 2008,
pp. 111–148.
2 (a) P. Sahoo, K. D. Krishna, D. R. Trivedi and P. Dastidar,
Tetrahedron Lett., 2008, 49, 3052–3055; (b) S. T. Lam,
G. Wang and V. W. W. Yam, Organometallics, 2008, 27,
4545–4548; (c) P. Byrne, G. O. Lloyd, L. Applegarth,
K. M. Anderson, N. Clarke and J. W. Steed, New J. Chem.,
2010, 34, 2261–2274.
3 P. J. Flory, Faraday Discuss. Chem. Soc., 1974, 57, 7–18.
4 P. Dastidar, Chem. Soc. Rev., 2008, 37, 2699–2715.
5 M. M. Piepenbrock, N. Clarke and J. W. Steed, Langmuir, 2009,
25, 8451–8456; N. N. Adarsh, P. Sahoo and P. Dastidar, Cryst.
Growth Des., 2010, 10, 4976–4986.
6 M. M. Piepenbrock, G. O. Lloyd, N. Clarke and J. W. Steed,
Chem. Rev., 2010, 110, 1960–2004.
7 A. R. Hirst, B. Escuder, J. F. Miravet and D. K. Smith, Angew.
Chem., Int. Ed., 2008, 47, 8002–8018.
8 J. A. Foster, M. M. Piepenbrock, G. O. Lloyd, N. Clarke,
J. A. K. Howard and J. W. Steed, Nat. Chem., 2010, 2, 1037–1043.
9 (a) D. Braga, F. Grepioni and L. Maini, Chem. Commun., 2010, 46,
6232–6242; (b) J. Bernstein, Polymorphism in Molecular Crystals,
Clarendon Press, Oxford, 2002; (c) U. Griesser, Polymorphism in
the Pharmaceutical Industry, ed. R. Hilfiker, Wiley VCH,
Weinheim, 2006, pp. 211–234; (d) S. L. Childs and
M. J. Zaworotko, Cryst. Growth Des., 2009, 9, 4208–4211;
(e) G. R. Desiraju, Cryst. Growth Des., 2008, 8, 3–5.
Crystallizing a gelator complex system is normally a difficult
task,6,10a,13 and more so if crystallization is attempted from the
gelling solvent. If our gels are dried in sealed vials, phase
separation takes place, resulting in clear solution and single
crystals (see Fig. 5b and ESIw). Interestingly and—to the best
of our knowledge—unprecedentedly, different crystal forms
are obtained from different gelling solvents. We report here
the polymorphs of the complex of formula [Ag(PQ5U)2]NO3
(see Fig. 6) obtained from CH3CN, Form I, EtOH, Form II,
i-PrOH, Form III and MeOH, Form IV, which were
characterized by single-crystal X-ray diffraction (see ESIw).
In terms of gross structural features the [Ag(PQ5U)2]+ units
(see ESIw) for Forms I–III are very similar, consisting of
linearly coordinated ligand–Ag+ systems, while Form IV
contains a nitrate anion participating in the coordination
sphere of the Ag+ cation to which it is directly bound.
Since the relative ligand conformation in Forms I to III
(transoid) and that in Form IV (cisoid) is different, one might
be tempted to consider this latter compound as a different
isomer of the complex. However, we reckon that these
examples lend further support to the idea that the difference
10 (a) D. Kalita, R. Sarma and J. B. Baruah, CrystEngComm, 2009,
11, 803–810; (b) We have also tested the 1-phenyl-3-(quinolin-
8-yl)urea molecule,10a but no gel formation/complexation with
Ag+ has been observed.
11 Work is in progress to photochemically characterize the lumines-
cence properties of these gels, as both solid PQ5U, its solutions and
the organo-gels described in the present communication show a
blue luminescence (471 nm) at an excitation wavelength of 400 nm.
12 Critical gelator concentrations (% w/v) are 0.08, 0.16, 0.20, 0.30
and 0.36 for 1-butanol, ethanol, i-propanol, methanol and
acetonitrile, respectively. The gel–sol transition for CH3CN is at
room temperature, irrespective of the gelator concentration. For
this reason the corresponding plot was not included in Fig. 1.
13 K. D. Krishna, J. D. Amilan, A. Das and P. Dastidar, Chem.
Commun., 2005, 4059–4061; N. N. Adarsh, P. Sahoo and
P. Dastidar, Cryst. Growth Des., 2010, 10, 4976–4986.
14 B. Moulton and M. J. Zaworotko, Chem. Rev., 2001, 101,
1629–1658.
Fig. 7 The first coordinatioÀn sphere around the Ag+ ion in Forms I
to IV. In Form IV the NO3 anion directly interacts with the silver
cation, and two (C–H)Á Á ÁAg+ interactions can also be observed
[Portions of the PQ5U ligands in Forms I and IV and most H atoms
omitted for clarity].
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5156 Chem. Commun., 2011, 47, 5154–5156
This journal is The Royal Society of Chemistry 2011