A quest for supramolecular gelators: Silver(i) complexes with quinoline-urea derivatives

Article abstract of DOI:10.1039/c3dt51873f

The quinoline urea derivatives 1,3-di(quinolin-5-yl)urea (DQ5U), 1-phenyl-3-(quinolin-6-yl)urea (PQ6U), 1-(isoquinolin-5-yl)-3-phenylurea (PiQ5U) and 1-phenyl-3-(3,5-bis(pyrid-2-yl)-1,2,4-triazol-4-yl)urea (PPT4U) have been synthesised and structurally ch

Full text of DOI:10.1039/c3dt51873f

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A quest for supramolecular gelators: silver(I) complexes
with quinoline-urea derivatives†
Cite this: Dalton Trans., 2013, 42, 16949
Dario Braga, Simone d’Agostino,* Eros D’Amen, Fabrizia Grepioni,*
Damiano Genovese, Luca Prodi and Massimo Sgarzi
The quinoline urea derivatives 1,3-di(quinolin-5-yl)urea (DQ5U), 1-phenyl-3-(quinolin-6-yl)urea (PQ6U),
1-(isoquinolin-5-yl)-3-phenylurea (PiQ5U) and 1-phenyl-3-(3,5-bis(pyrid-2-yl)-1,2,4-triazol-4-yl)urea
(PPT4U) have been synthesised and structurally characterized by powder and single crystal X-ray diffrac-
tion. Their gelator behaviour in the formation of Ag-complexes has been explored. Compound DQ5U
proved capable of gelating the mixed solvent EtOH–DMF 1 : 2 (v/v) when mixed with 1 equivalent of
AgNO3. In the case of PQ6U, two polymorphic forms of the complex [Ag(PQ6U)2]NO3, plus the solvated
form [Ag(PQ6U)2]NO3·CH3CN, were crystallized. Photophysical characterization of the ligands has been
conducted in solution, while fluorescence microscopy has been used to examine the microstructure and
photophysical properties of the gels formed by PQ5U and DQ5U with AgNO3.
Received 11th July 2013,
Accepted 13th September 2013
DOI: 10.1039/c3dt51873f
www.rsc.org/dalton
triazole derivative of urea, i.e. 1-(3,5-bis(pyrid-2-yl)-1,2,4-
Introduction
triazol-4-yl)-3-phenylurea (PPT4U), was also synthesized and its
Supramolecular gels1 are a class of soft materials that have behaviour tested (see Scheme 1).
gained extensive research interest in recent years. They can be
formed when low molecular weight gelators (LMWGs)2 self- of the synthesized molecules, together with an investigation of
assemble via non-covalent interactions such as hydrogen their ability to form Ag+-complexes, with the aim to establish
bonds, π-stacking and metal–ligand coordination.1,2 In
materials chemistry, they are good candidates for several appli-
cations, such as media for controlling the growth of nanoparti-
cles3 or crystals of pharmaceuticals,4 and for drug delivery,5
amongst others.6,7 Therefore the quest for new gelators and
gelling conditions is an extremely active field of research.8
In this paper, we report the full structural characterization
We recently reported9 that 1-phenyl-3-(quinolin-5-yl)urea
(PQ5U),10 in the presence of the salt AgNO3, behaves as
a supergelator of alcohols (such as MeOH, EtOH, n-PrOH,
i-PrOH and n-BuOH) and acetonitrile.9 This observation
prompted us to explore the possibility of “engineering” a class
of soft-materials based on the quinolinyl-urea backbone. To
this end, we synthesized the following possible candidates to
be used as ligands for supramolecular gelation with AgNO3:
two isomers of the ligand PQ5U, namely 1-(isoquinolin-5-yl)-3-
phenylurea (PiQ5U) and 1-phenyl-3-(quinolin-6-yl)urea (PQ6U),
the ligand 1,3-di(quinolin-5-yl)urea (DQ5U); a bis-pyridyl-
Dipartimento di Chimica G. Ciamician, Università di Bologna, Via Selmi 2, 40126
Bologna, Italy. E-mail: fabrizia.grepioni@unibo.it
†Electronic supplementary information (ESI) available: Comparison between
experimental and calculated XRPD patterns, Ortep drawings, 1H-NMR spectra,
map of fluorescence spectra vs. excitation wavelength for the AgNO3:DQ5U gel.
CCDC 949683–949690. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c3dt51873f
Scheme 1 The ligands 1-phenyl-3-(quinolin-5-yl)urea (PQ5U,10 previously
studied by us9), 1-phenyl-3-(quinolin-8-yl)urea (PQ8U10), 1-phenyl-3-(quinolin-6-
yl)urea (PQ6U), 1-(isoquinolin-5-yl)-3-phenylurea (PiQ5U), 1,3-di(quinolin-5-yl)-
urea (DQ5U) and 1-phenyl-3-(3,5-bis(pyrid-2-yl)-1,2,4-triazol-4-yl)urea (PPT4U).
PQ6U and PiQ5U are isomers of PQ5U.
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whether coordination compounds of these ligands are able to
form molecular gels in alcohols and/or in other solvents.
Results and discussion
The ligands: PQ6U, PiQ5U, DQ5U and PPT4U
In this section, a brief description of the molecular and crystal
structures of all the ligands discussed in this paper is reported,
as determined from single crystal or powder X-ray diffraction
(vide infra).
The structure of the compound PQ6U was solved and
refined from powder data (see the Experimental section for
details). The solid-state structure of PQ6U is characterized by
the presence of the urea head-to-tail interaction,11 which is
responsible for the formation of the well-known “urea-tape”
motif (see Fig. 1a), also detected in crystalline PQ5U and PQ8U
(Fig. 1b and c).10 As shown in Fig. 2, the bifurcated N–H⋯O
interaction [N⋯O = 2.871(7)–2.907(7) Å] involving two consecu-
tive molecules generates an infinite ribbon-like structure,
which extends in the crystal parallel to the b-axis.
The same urea-tape motif is also present in both crystalline
PiQ5U [N⋯O = 2.805(4)–2.919(4) Å] (see Fig. 3 and 4) and
DQ5U [N⋯O = 2.858(4) Å]. Weak π-stacking interactions
between the quinoline moieties within the ribbons contribute
to the stabilization of the crystalline edifice.
Fig. 2 Top: the urea tape motif extending along the b-axis direction in crystal-
line PQ6U; the van der Waals picture on the right shows the π-stacking inter-
actions along the tape. Bottom: view down the b-axis, showing the relative
arrangement of the infinite tapes.
Crystalline PPT4U was obtained in two polymorphic modifi-
cations, which we call Form I and Form II, differing only
slightly in their conformation, as can be seen in Fig. 5.
Fig. 6 shows a comparison of the main packing feature in
the two polymorphs. As can be noted, a different tape-like
arrangement, with respect to the urea-tape, is present in both
solids, formed via N–H⋯N hydrogen bonding interactions
between the N–H moieties belonging to the urea group and
two nitrogen atoms on an adjacent molecule, one belonging to
one pyridyl fragment, the other to the triazole ring [N(H)⋯Npyr
= 2.968(2) Å, N(H)⋯Ntriazole = 3.011(2) Å and N(H)⋯Npyr
=
2.992(5) Å, N(H)⋯Ntriazole = 3.150(3) Å for Form I and II,
respectively].
Fig. 3 Top: the urea tape motif extending in the b-axis direction in crystalline
PiQ5U; the van der Waals picture on the right shows the π-stacking interactions
along the tape. Bottom: view down the b-axis, showing the relative arrange-
ment of the infinite tapes.
The differences between the two polymorphic modifications
are due to the fact that in Form I, adjacent molecules are
referred by a screw-axis along the b-axis direction in the P21/c
space group, while in Form II adjacent molecules along the
tape are simply related by translation along the a-axis direction
Fig. 1 The hydrogen bonding pattern known as the “urea tape motif” present
in crystalline urea (a) and in ligands PQ5U and PQ8U (ref. codes UREAXX09,
VUKHUF and VUKJER, respectively).
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Fig. 6 The tape-like arrangement observed in crystalline Form I (left, tape
extending along the b-axis direction) and Form II (right, tape extending along
the a-axis direction). Red arrows evidence the translational repetition in the two
forms. HCH omitted for clarity.
Fig. 4 Top: the urea tape motif extending along the a-axis in crystalline DQ5U;
the van der Waals picture on the right shows the π-stacking interactions
along the tape. Bottom: view down the a-axis, showing the relative arrange-
ment of the infinite tapes.
Fig. 7 Projection along the tape direction (left) and overall packing (right, here
the tapes shown on the left are coloured blue) in crystalline Form I (top) and
Form II (bottom).
Fig. 5 Superimposition of Form I and Form II molecular structures for crystal-
line PPT4U, showing that the two conformations are almost exactly the same in
the two polymorphs.
in the P21 space group. Translational repetition in the two
cases is evidenced by red arrows in Fig. 6.
Fig. 7 shows how the different arrangements of the hydro-
gen bonded molecules along the tapes affect the resulting
crystal packing in the two forms. In Form I, the presence of
C–H⋯π interactions is also evident, as shown in Fig. 8.
The two polymorphs differ in their morphology also: Fig. 9
shows the rod-like crystals of Form I and the plate-like crystals
of Form II, as observed with an optical microscope in polarized
light.
Fig. 8 CH⋯π interactions in crystalline Form I, involving a pyridyl CH terminus
and the phenyl ring on neighbouring molecules belonging to adjacent tapes.
3-phenylurea (PPT4U) and 1-(isoquinolin-5-yl)-3-phenylurea
(PiQ5U) were all treated with AgNO3 in order to explore their
gelling behavior. When crystals were recovered from the experi-
ments, the crystal structure was also determined (vide infra).
In a typical gelation experiment, 10–15 mg of a finely
ground ligand–AgNO3 mixture in 1 : 2 (for PiQ5U, PQ6U and
Gelation tests and gel characterization
The four compounds 1,3-di(quinolin-5-yl)urea (DQ5U), 1-phenyl-3-
(quinolin-6-yl)urea (PQ6U), 1-(3,5-bis(pyrid-2-yl)-1,2,4-triazol-4-yl)-
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Fig. 9 The different morphology observed for crystals of Form I (top, rods) and
Form II (bottom, plates).
PPT4U), or 1 : 1 (for DQ5U) molar ratio was suspended in 5 mL
of pure solvent or solvent mixture in a 2 cm diameter vial, and
the vial was heated to boiling point, until a clear solution or a
fine suspension was obtained, which was allowed to cool to
room temperature. The formation of a suspension indicates
limited or total insolubility of the ligand even at high tempera-
tures. Table 1 lists the results of the tests.
As can be seen from Table 1, only compound DQ5U was
able to act, in association with AgNO3, as a gelator in the pres-
ence of the solvent mixture EtOH–DMF 1 : 2 (v/v) when mixed
with 1 equivalent of AgNO3. As evidenced by the “tube-inver-
sion” test1 (see Fig. 10a), the critical gelation concentration1
(CGC, i.e. the lowest concentration at which gelation occurs) at
room temperature was found to be 0.17% (w/v). Therefore, for
this class of compounds, the most highly performing super-
gelator1 remains the previously reported complex of AgNO3
with 1-phenyl-3-(quinolin-5-yl)urea (PQ5U).10
Fig. 10 Top: The AgNO3 : DQ5U system in the solvent mixture DMF–EtOH 1 : 2
v/v. A clear sol is obtained at high temperature (left), while the “tube-inversion
test” at room temperature shows the presence of a gel (right); bottom: a plot of
gel to sol transition temperature vs. gelator concentration.
thermoreversible, with gel to sol transition temperatures
ranging from 24 °C for 0.17% (w/v) to 37 °C for a 0.325% (w/v);
after 3–4 cycles of heating (gel → sol transition) and cooling
(sol → gel transition), degradation of the system is observed: a
brown solution is obtained at higher temperature that does
not gelify upon cooling. The degradation process is presum-
ably due to the reduction of the silver cation.
Once prepared, the gel is stable for one week at room temp-
erature in the presence of air. The dropping-ball method1 was
used to evaluate the gel to sol transition temperature as a func-
tion of the gelator concentration (Fig. 10b). Gel formation is
Photophysical properties
The photophysical characterization of PQ8U, PQ6U, PiQ5U,
PQ5U and DQ5U 1 × 10−5 M in ethanol was performed. The
absorption spectra of these compounds present an intense
band between 242 and 258 nm (ε = 32 500–71 000 M−1 cm−1
)
Table 1 Results of the gelling tests with the new ligands and AgNO3 (ligand :
AgNO3 molar ratio in parenthesis). I = insoluble ligand, C = single crystals for-
mation, P = precipitate, S = stable solution, G = gel formation
and a smaller and broader band with a maximum between 307
and 332 nm (ε = 7000–9300 M−1 cm−1) (see Table 2). The
former can be attributed to a π–π* transition, while the latter is
due to an n–π* transition.12 The emission spectra are charac-
terized by a non-structured band with wavelengths at peaks
ranging from 386 to 437 nm. These compounds present very
different quantum yields, ranging from 6.5 × 10−4 for PQ8U to
1.08 × 10−1 for PQ5U, as shown in Table 2.
A systematic study of these ligands was then performed in
order to explore their affinity and photochemical behaviour in
the presence of Mg2+, Cu2+, Ag+, Cd2+, Hg2+ and NO3−, F−, Cl−,
Br−, H2PO4−, CH3COO−. It was checked that triflate and per-
chlorate anions are not complexed by the ureic moiety of these
compounds, and similarly, the tetrabutylammonium cation is
not complexed by quinolinic nitrogen: we thus chose them as
PQ6U
(2 : 1)
PiQ5U
(2 : 1)
DQ5U
(1 : 1)
PPT4U
(1 : 2)
Solvent
Water
Methanol
Ethanol
n-Propanol
i-Propanol
n-Butanol
Acetonitrile
DMF
I
C
I
I
I
I
I
I
I
I
I
P
S
I
I
I
S
S
I
I
I
I
I
I
I
I
I
I
I
I
G
I
S
S
S
S
S
S
S
I
I
I
S
S
I
C
S
I
I
I
S
S
Toluene
Chloroform
Butyl-benzoate
DMF–EtOH 1 : 2
DMF–EtOH 2 : 1
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Table 2 Photophysical properties of PQ8U, PQ6U, PiQ5U, PQ5U and DQ5U and their complexes in EtOH
Fluorescence
Absorption
Compound
Logarithm of association constant, log Ka
λmax/nm (ε/M1 cm−1
)
λmax/nm
Φ
τ/ns
PQ8U
—
249 (58 000); 331 (8900)
270 (42 000); 397 (4000)
424
422
428
386
386
386
386
396
396
396
437
437
437
434
434
434
6.5 × 10−4
3.2 × 10−4
9.6 × 10−4
2.9 × 10−2
2.6 × 10−3
8.7 × 10−3
1.6 × 10−2
9.1 × 10−2
1.0 × 10−2
2.3 × 10−2
1.08 × 10−1
7.6 × 10−3
5.4 × 10−2
6.8 × 10−2
4.8 × 10−3
3.6 × 10−2
<0.1
<0.1
<0.1
0.5
<0.1
<0.1
<0.1
1.2
<0.1
<0.1
2.5
<0.1
<0.1
1.3
PQ8U·Cu2+
PQ8U·F−
PQ6U
ML 5.53 0.08
ML <2
—
ML 4.74 0.05
ML 3.96 0.02
ML 2.66 0.02
—
ML 4.55 0.02
ML 3.47 0.02
—
ML 4.34 0.04
ML 3.69 0.04
—
ML 4.47 0.01
ML 3.53 0.02
258 (71 000); 332 (6900)
370 (7300)
375 (4400)
260 (76 000); 340 (7500)
246 (32 500); 307 (9300)
326 (5900); 355 (4700)
350 (7300)
245 (46 500); 316 (7000)
317 (3100); 380 (5100)
350 (5900)
PQ6U·Hg2+
PQ6U·Cu2+
PQ6U·Ag+
PiQ5U
PiQ5U·Hg2+
PiQ5U·Cu2+
PQ5U
PQ5U·Hg2+
PQ5U·Cu2+
DQ5U
242 (46 000); 317 (8700)
317 (7200); 380 (4400)
350 (9200)
DQ5U·Hg2+
DQ5U·Cu2+
<0.1
<0.1
counterions for the cationic species listed above. As regards concentration. Since all images were taken with the same RGB
the metal ions, all the compounds can complex Cu2+ and Hg2+ camera and the same set of filters, the difference in color
apart from PQ8U, which does not show affinity toward Hg2+. corresponds to the different spectral properties of these
PQ6U is the only one able to complex also Ag+ in ethanolic fluorescent gels, as also proved by the fluorescence spectra
solution (see Table 2) with a Ka ≈ 103 M−1. In all of the cases, shown in Fig. 13.
the addition of increasing amounts of these metal ions caused
In Fig. 12b and c the cyan-emitting fine texture is clearly
changes in the absorption spectra, in particular the decrease recognizable, which at the highest concentration is extremely
of the n–π* band and the appearance of a new bathochromi- homogeneous along the whole sample (millimeter-to-centi-
cally shifted band centred between 340 and 397 nm (see, for meter scale) (Fig. 12c). Such a texture is not present at the
example, the absorption spectrum of PQ6U with Hg2+, lowest concentration at which no gelification is observed
Fig. 11a). It is worth noting that this new absorption band (Fig. 12a, only traces of random adhesion on the glass surface
allows for selective excitation of the complexed species in a are visible). The texture is in some cases strongly oriented,
fluorescence imaging setup, as discussed in the next section. probably due to the local mechanical shear-stress experienced
The complexation of Cu2+, Hg2+, Ag+ caused the quenching of by the gel during sample preparation.
the luminescence in the emission spectra (Fig. 11b). The
Fig. 12e and f show the presence of large fibrils which are
values of log Ka toward Hg2+ are similar for all the ligands responsible for the gelation of DQ5U in the presence of
(4.34 < log Ka < 4.74), and an analogous behavior was found AgNO3. Differently from what is observed for PQ5U, the length
for Cu2+ (3.47 < log Ka < 3.96), except for PQ8U exhibiting a of the fibrils for this ligand is several hundred microns, the
higher value (Table 2). No changes either in the absorption or width is within few microns and fibrillar interspace is larger
−
in emission spectra were observed for the addition of NO3
,
than in PQ5U gels. This microstructure strongly determines
F−, Cl−, Br−, H2PO4−, CH3COO−, except for PQ8U, which exhi- the macroscopic properties of the gels. At the microscopic
bits a low association constant (<102) toward F− with a slight scale we indeed observed an increased mobility of the liquid
enhancement in the emission spectrum.
phase within the large spaces visible among the fibrils, and in
Finally, we observed that the presence of water caused a some cases even the fibrils easily moved across each other, fol-
quenching of the luminescence for all the compounds, prob- lowing a general reorganization. DQ5U forms, as a result,
ably due to the formation of hydrogen bonds stronger than much weaker gels than PQ5U at all concentrations, as proved
those formed by ethanol molecules.
by the gel breaking upon little mechanical stresses.
The photophysical properties of the two gels were explored
with fluorescence spectroscopy. When DQ5U makes gel in the
presence of Ag+, it shows a broadened, red-shifted emission
band, which is responsible for the green color observed in the
fluorescence images. Such green emission can be selectively
excited, and thus observed separately from the blue native
ligand emission by using excitation wavelength >370 nm (as in
our microscope setup) or <300 nm. This observation is consist-
ent with the changes in the DQ5U absorption spectrum,
which, upon complexation with metals in diluted solutions,
Photophysics and fluorescence imaging of DQ5U and PQ5U
gels
Fluorescence microscopy was used to examine the microstruc-
ture of the gels. The fluorescence of the ligands can indeed be
used to visualize the fibril aggregates responsible for the gela-
tion process. Fig. 12a–c show the fluorescence of samples
at increasing concentration of ligands PQ5U and AgNO3
in ethanol; Fig. 12d–f are fluorescence images of DQ5U
and AgNO3 in the ethanol : DMF ratio 1 : 2 at increasing
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Fig. 11 Absorption (a) and emission (b) spectra of PQ6U (1 × 10−5 M) in EtOH at room temperature upon addition of increasing amounts of Hg2+ ions (from 0 to
15 equivalents).
Fig. 12 Fluorescence microscopy images of PQ5U (a–c) and DQ5U (d–f) equimolar with Ag+ at various concentrations. Note the presence of fibrillar aggregates.
exhibits a decrease in the n–π* band gap and the appearance polarization is maintained, possibly due to the high ligand
of a new red-shifted band centred at 380 nm (see Fig. 11a). In orientation in the fibrils.
contrast, the gels formed by PQ5U do not show any significant
Ag-complexes obtained with the ligand PQ6U: a solvate and
two polymorphs of the unsolvated form
shift in the emission band compared to the native ligand. The
lifetimes measured for gel emissions were comparable to that
observed in dilute solutions: 1.3 ns for DQ5U and 1.4 ns for
PQ5U.
When gelation tests were performed in the presence of the
ligand PQ6U, three different crystalline materials were recov-
ered from the solutions, which were characterized via X-ray
single diffraction as an acetonitrile solvate, [Ag(PQ6U)2]-
NO3·CH3CN, and (in the case of EtOH or MeOH solutions) two
polymorphic modifications of the unsolvated complex,
[Ag(PQ6U)2]NO3. No crystalline material suitable for diffraction
Interestingly, the gel emissions show a rather high fluores-
cence anisotropy (r ∼ 0.3 for the DQ5U gel and r ∼ 0.2 for the
PQ5U gel), indicating that the emitters do not have much
rotational freedom when packed in the fibrillar structure, and
that in the case of migration of excitation energy the
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Fig. 13 Fluorescence spectra of DQ5U/Ag+ gel and PQ5U/Ag+ gel at different
excitation wavelength. Solid line: DQ5U/Ag+ gel excited at 380 nm. Dotted line:
DQ5U/Ag+ gel excited at 350 nm. Dashed line: PQ5U/Ag+ gel excited at 330 nm
(this emission spectrum is invariant with excitation wavelength). Note that the
two emissions of DQ5U/Ag+ gel attributable to the free ligand and to the fibrils
can be separated by selecting the suitable excitation wavelength (350 and
380 nm respectively, see also the emission map in Fig. ESI-9†).
experiments were obtained when DQ5U and PiQ5U were
treated with AgNO3.
The solvate [Ag(PQ6U)2]NO3·CH3CN. The acetonitrile
solvate [Ag(PQ6U)2]NO3·CH3CN is characterized by the pres-
ence of two complex molecules in the asymmetric unit, which
Fig. 15 The 1-D ribbon (a) involving one of the two conformational isomer
complexes in crystalline [Ag(PQ6U)2]NO3·CH3CN, with each PQ6U ligand brid-
ging two silver cations; the urea fragments interact via hydrogen bonds with the
differ for the relative conformation of the urea groups with nitrate anions on both sides of the ribbon (b). The second conformational
isomer bridges two such ribbons, via HB interactions with the nitrate anions.
respect to the quinoline-Ag-quinoline fragment (see Fig. 14).
Solvent molecules (c) interact via the N atom with the quinoline moieties. HCH
The two complexes can therefore be considered conformation-
atoms omitted for clarity.
al isomers: the effect on the Ag+ coordination geometry is pro-
found, and two distinct patterns can be easily recognized in
the crystal. In the first complex isomer the Ag+ ion is linearly
are in turn connected, via hydrogen bonds involving the same
nitrate groups, with the second conformational isomer (central
coordinated by two nitrogen atoms from the quinoline units
[Ag+⋯N = 2.197(5) Å; N–Ag+–N angle = 180.0(2)°, see Fig. 15a],
part of Fig. 15b). The solvent molecules direct the N-extremity
but two oxygen atoms belonging to two adjacent ligands also
towards the silver cation, and interact via N⋯HC interactions
with the quinoline moieties, as can be seen in Fig. 15c.
Thermal gravimetric analysis on a polycrystalline sample of
[Ag(PQ6U)2]NO3·CH3CN shows a weight loss of ca. 5% in the
temperature range 100–175 °C, corresponding to the loss of
one mole of acetonitrile per mole of complex (see ESI†).
interact with the silver cation, [O⋯N = 2.941(7)–2.981(7) Å] (see
Fig. 15a). This results in the formation of a 1-D ribbon, with
each PQ6U ligand bridging two silver cations and the urea frag-
ments interacting via hydrogen bonds with the nitrate anions
on both sides of the ribbon (see Fig. 15b). Two such ribbons
Form I of the unsolvated complex [Ag(PQ6U)2]NO3. Unlike
the previously described acetonitrile solvated complex, in crys-
talline [Ag(PQ6U)2]NO3, Form I, the coordination geometry of
the Ag+ ion is distorted trigonal, with two quinoline units
forming a bent backbone [Ag+⋯N = 2.146(3)–2.151(3) Å] and
the nitrate anion entering the first coordination sphere (see
Fig. 16a) [Ag+⋯O = 2.743(3)–2.821(3) Å]. The urea groups on
−
the ligands interact via hydrogen bonding with two NO3
anions [O⋯N = 2.922(5)–2.929(4) Å]. The complexes are
organized in the solid in a criss-cross pattern (Fig. 16b), with
the nitrate anions acting as a “glue” via hydrogen bonding
interactions, as can be seen in Fig. 16c, which shows how a
single anion simultaneously interacts with three cationic
complexes.
Fig. 14 The two conformational isomers present in the asymmetric unit of
[Ag(PQ6U)2]NO3·CH3CN. HCH atoms omitted for clarity.
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coordinating the silver cation are oriented so as to form a
“tweezer” that traps the nitrate anion, as can be seen from
Fig. 18. The neutral unit is then repeated in a head-to-tail
fashion, thus forming rows (Fig. 19a) and layers, which are
then covered by a second layer shifted so as to fill the cavities
below (Fig. 19b).
A fifth polymorphic modification of [Ag(PQ5U)2](NO3). In
the course of our gelling tests a fifth polymorph of the supra-
molecular complex [Ag(PQ5U)2](NO3), for which four poly-
morphs had been characterized by us in a previous work,9 was
crystallized from MeOH. For the sake of completion we report
here its structure, although the data quality for this complex
are not at the level of those reported for polymorphs I–IV.9
Fig. 20 shows a comparison of all polymorphic modifi-
cations for crystalline [Ag(PQ5U)2](NO3): it can be seen that the
conformation of the supramolecular complex in Form V
closely resembles that of Form IV. Two independent molecules
are present in the asymmetric unit; they are connected via
hydrogen bonds between the urea groups [N(H)urea⋯OCO
=
2.891(2)–2.941(2) Å), thus forming a large supramolecular ring,
Fig. 16 (a) Coordination sphere around the Ag+ ion in crystalline [Ag(PQ6U)2]-
NO3 Form I, and (b) the criss-cross arrangement of the complexes; the nitrate
anions act as a hydrogen bonding “glue” by simultaneously interacting with
three neighbouring cationic complexes. HCH atoms omitted for clarity.
A variable temperature X-ray diffraction measurement on a
crystalline powder sample of [Ag(PQ6U)2]NO3·CH3CN shows
that, upon heating the sample to 155 °C, a desolvation process
takes place, yielding [Ag(PQ6U)2]NO3 Form I (Fig. 17).
Fig. 18 The “tweezer-like” complex trapping the nitrate anion in crystalline
[Ag(PQ6U)2]NO3 Form II.
Form II of the unsolvated complex [Ag(PQ6U)2]NO3. In crys-
talline [Ag(PQ6U)2]NO3, Form II, the two quinoline ligands
Fig. 19 Rows (top) and layers (bottom) of tweezers in crystalline [Ag(PQ6U)2]-
NO3 Form II. A second layer (evidenced in light-orange) filling the underlying
cavities is partly shown on top of the first layer.
Fig. 17 Variable temperature powder diffraction experiment showing the con-
version [Ag(PQ6U)2]NO3·CH3CN → [Ag(PQ6U)2]NO3 upon heating.
16956 | Dalton Trans., 2013, 42, 16949–16960
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Paper
This study confirms that structure–property relationships
are difficult to predict when dealing with gel formation. Even
compounds with close structural relationships show very
different gelling behaviors or no formation of gels at all.
Clearly, prediction of gelling behaviour from a knowledge of
the molecular structure of the complexes and of the nature of
the solvents is still a chimaera, and the investigator can only
tackle the problem by trial-and-error.
Experimental
Synthesis
Fig. 20 Comparison of the [Ag(PQ5U)2]+ units observed in Forms I to IV9 (copy-
right RSC) and in Form V. H atoms omitted for clarity.
All reactants and reagents were purchased from Sigma-Aldrich
and used without further purification. Reagent grade solvents
were used.
Synthesis of 1-phenyl-3-(quinolin-5-yl)urea (PQ5U) and
1-phenyl-3-(quinolin-8-yl)urea (PQ8U). The ligands PQ5U and
PQ8U were synthesized according to the literature procedure.10
5-Aminoquinoline or 8-aminoquinoline (0.36 g, 2.5 mmol) was
dissolved in dry dichloromethane (ca. 10 mL); a solution of
phenylisocyanate (0.27 mL, 2.5 mmol) in dichloromethane was
then added dropwise, and the mixture was stirred at room
temperature for 3 h. The products formed were filtered and
recrystallized from methanol. PQ5U and PQ8U were identified
by X-ray powder diffraction. See Fig. ESI-0.†
Fig. 21 The supramolecular ring formed by the two independent complexes in
crystalline [Ag(PQ5U)2](NO3) Form V. HCH omitted for clarity.
to which two nitrate anions are hydrogen bonded [O⋯N =
2.891(2)–3.171(3) Å], see Fig. 21.
Synthesis of 1-phenyl-3-(quinolin-6-yl)urea (PQ6U). 6-Amino-
quinoline (72.3 mg, 5 mmol) was dissolved in 15 mL of dry
dichloromethane, then
a
solution of phenylisocyanate
(0.54 mL, 5 mmol) in dichloromethane was added dropwise
over two hours; the resulting mixture was stirred at RT for an
additional two hours. The product formed (white powder) was
Conclusions
Starting from our findings on the behaviour of compound recovered by filtration and washed with cold dichloromethane
1-phenyl-3-(quinolin-5-yl)urea (PQ5U)9 in the presence of (5 × 2 mL). Recrystallization from alcohol (methanol or
AgNO3 as a supergelator of the alcohols MeOH, EtOH, n-PrOH, ethanol) or acetonitrile afforded only a polycrystalline powder.
i-PrOH and n-BuOH as well as of acetonitrile10 we have Yield: 85%.
explored a whole class of new, strictly related, compounds. We
1H-NMR (DMSO-d6, 400 MHz, ppm): 9.05 (1H, s), 8.8 (1H,
have synthesised and structurally characterized by single s), 8.7 (1H, m), 8.2 (1H, d, J = 8.4 Hz), 8.1 (1H, d, J = 2.4 Hz),
crystal and powder diffraction the derivatives 1,3-di(quinolin- 7.9 (1H, d, J = 9.2 Hz), 7.7 (1H, m), 7.4 (3H, m), 7.3 (2H, t, J =
5-yl)urea (DQ5U), 1-phenyl-3-(quinolin-6-yl)urea (PQ6U), 1-(3,5- 7.2 Hz), 7.0 (1H, t, J = 7.2 Hz). See Fig. ESI-1.†
bis(pyrid-2-yl)-1,2,4-triazol-4-yl)-3-phenylurea (PPT4U) and 1-(iso-
Synthesis of 1-(isoquinolin-5-yl)-3-phenylurea (PiQ5U).
quinolin-5-yl)-3-phenylurea (PiQ5U) and examined their gelling 5-Aminoisoquinoline (72.1 mg, 5 mmol) was dissolved in
behaviors when treated under the same conditions as their 15 mL of dry acetone, then a solution of phenylisocyanate
parent 1-phenyl-3-(quinolin-5-yl)urea (PQ5U) in the formation of (0.54 mL, 5 mmol) in acetone was added dropwise over two
Ag-complexes. Of all compounds, only DQ5U proved capable of hours; the resulting mixture was stirred at RT for an additional
gelating a mixture of solvents EtOH–DMF 1 : 2 (v/v) when mixed four hours. The product formed (brownish powder) was recov-
with 1 equivalent of AgNO3. As evidenced by the “tube-inversion” ered by filtration and washed with cold acetone (5 × 2 mL).
test, the critical gelation concentration1 (CGC, i.e. the lowest con- Colourless, needle-shaped crystals suitable for X-ray diffraction
centration at which gelation occurs) at room temperature was were obtained by vapor diffusion of water into a dimethylsulf-
found to be 0.17% (w/v) for the gel DQ5U-AgNO3. Compounds oxide solution of the freshly synthesized ligand. Yield: 75%.
PQ5U and PiQ5U, though strictly related, do not show any
1H-NMR (DMSO-d6, 400 MHz, ppm): 9.3 (1H, s), 9.0 (1H, s),
gelling behaviour. On the other hand molecule PQ6U forms the 8.8 (1H, s), 8.5 (1H, d, J = 6 Hz), 8.2 (1H, d, J = 7.6 Hz), 7.9 (1H,
stable complexes [Ag(PQ6U)2]NO3·CH3CN, and [Ag(PQ6U)2]NO3 d, J = 6.4 Hz), 7.8 (1H, d, J = 8.4 Hz), 7.6 (1H, t, J = 6.4 Hz), 7.5
when reacted with AgNO3, depending on the solvent. Compound (2H, m), 7.3 (2H, m), 7.0 (1H, t, J = 7.6 Hz). See Fig. ESI-2.†
PiQ5U : AgNO3, in contrast, does not yield single crystals or poly-
Synthesis of 1,3-di(quinolin-5-yl)urea (DQ5U). Tri-ethyl-
crystalline powders suitable for powder diffraction studies.
amine (0.2 mL) and 67.3 mg (0.226 mmol) of triphosgene were
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added to a solution of 5-aminoquinoline (98.4 mg, 0.68 mmol)
Synthesis of [Ag(PQ6U)2]NO3. PQ6U (0.061 mg, 0.23 mmol)
in 10 mL of dry dichloromethane; the resulting mixture was and AgNO3 (20 mg, 0.11 mmol) were suspended in ca. 7 mL of
stirred at RT for 1 hour. The off-white precipitate formed was ether ethanol and heated up until a clear solution was
recovered through filtration and washed with dichloromethane obtained, which was allowed to slowly cool to room tempera-
(5 × 2 mL). Slow evaporation of a dimethyl sulfoxide solution ture. Colourless needle-shaped crystals of Form I suitable for
yielded small block-shaped, colourless crystals. Yield: 20%.
X-ray diffraction grew overnight. Form I was also obtained
1H-NMR (DMSO-d6, 400 MHz, ppm): 9.3 (2H, s), 8.945 (2H, upon desolvation of [Ag(PQ6U)2]NO3·CH3CN. When the syn-
m), 8.6 (2H, d, J = 8.8 Hz), 8.1 (2H, m), 7.7 (4H, m), 7.6 (2H, thesis was repeated after one month, only crystals of a second
m). See Fig. ESI-3.†
polymorph, Form II, were obtained (see below). Since then, we
Synthesis of 1-phenyl-3-(3,5-bis(pyrid-2-yl)-1,2,4-triazol-4-yl)- have not been able to reproduce Form I from any solvent: even
urea (PPT4U). A solution of phenyl isocyanate (0.27 mL, if CH3CN was used, only form II was directly obtained.
2.5 mmol) in 8 mL of dichloromethane was added dropwise to
1H-NMR spectroscopy
a stirring solution of 4-amino-3,5-di-pyridyl-4H-1,2,4-triazole
(0.6 g, 2.5 mmol) in 25 mL of dichloromethane over a period
1H-NMR spectra were recorded on a Varian Mercury400,
of 2 hours. The formed precipitate was collected through fil-
1
chemical shifts of H-NMR signals were expressed in parts per
tration and washed with dichloromethane, yielding 0.88 g of a
million (δH) using internal standard TMS (δH = 0.00). DMSO-d6
was bought from the Cambridge Isotope Lab.
white solid in the form of colourless needles (Form I). If Form
I was left in the CH2Cl2 solution for 48 hours, plate-like crys-
tals were obtained (Form II). The same behaviour was observed
if the reaction was conducted in alcohols. Yield = 98%, m.p. =
Structural characterization
decomposes above 200 °C. 1H-NMR (400 MHz, DMSO-d6,
TMS) δ: 9.832 (s, 1H), 9.450 (s, 1H), 8.697–8.686 (d, 2H, CH),
8.118–8.098 (d, 2H), 8.034–7.991 (t, 2H), 7.544–7.551 (m, 2H).
See Fig. ESI-4.† Form II could also be obtained directly via crys-
tallization from acetone, or via kneading of Form I with a drop
of methanol or ethanol.
Synthesis of [Ag(PQ6U)2]NO3·CH3CN. PQ6U (55 mg,
0.21 mmol) and AgNO3 (17 mg, 0.104 mmol) were suspended
in 6 mL of acetonitrile and heated up until a clear solution
was obtained, which was allowed to slowly cool to room temp-
erature. Colourless prisms suitable for X-ray diffraction grew
overnight.
Single-crystal data for all compounds (except PQ6U; see below)
were collected using an Oxford X’Calibur S CCD diffractometer
equipped with a graphite monochromator (Mo-Kα radiation,
λ = 0.71073 Å) and operating at room temperature. Crystal data
and details of measurement are listed in Tables 3 and 4. All
non-hydrogen atoms were refined anisotropically; HNH atoms
were either directly located or added in calculated positions;
HCH atoms for all compounds were added in calculated posi-
tions and refined riding on their respective carbon atoms.
SHELX9713a was used for structure solution and refinement on
F2; PLATON13b was used for hydrogen bonding analysis; Scha-
kal9913g and Mercury13e were used for molecular graphics.
Table 3 Crystal data and details of measurements for the ligands
PPT4U
Form I (needle)
PPT4U
Form II (plate)
PQ6Ua
PiQ5U
DQ5U
Formula
Fw (g mol−1
a (Å)
b (Å)
c (Å)
C16H13ON3
263.29
19.5009(3)
4.6309(7)
14.8429(9)
90
C16H13ON3
263.29
24.020(2)
4.5893(3)
11.828(1)
90
C18H14ON2
314.34
4.593(1)
11.696(2)
13.723(5)
90
C19H15N6O
343.37
11.128(1)
10.2052(9)
16.432(2)
90
C19H15N6O
343.37
6.1197(4)
8.0872(7)
17.676(1)
90
)
α (°)
β (°)
γ (°)
99.0647(7)
90
96.411(7)
90
90
90
105.40(1)
90
91.449(6)
90
Z
4
4
2
4
2
V (Å3)
1323.70(3)
—
—
Monoclinic
P21/n
—
—
—
1295.7(2)
1.350
0.087
Monoclinic
P21/c
4414
737.2(6)
1.416
0.092
Orthorhombic
P22121
2002
1799.0(3)
1.268
0.084
Monoclinic
P21/c
7850
874.50(11)
1.357
0.091
Monoclinic
P21
4122
2847
0.0560
0.1258
Dcalc (Mg m−3
)
μ (mm−1
)
Cryst. system
Space group
Collected reflns
Indep. reflns
R1 [on F02, I > 2σ(I)]
wR2 (all data)
Rwp (%)
2641
1386
4037
0.0651
0.01469
—
—
—
0.0503
0.0760
—
—
—
0.0554
0.1072
—
—
9.44
3.505
2.69
Rexp (%)
—
χ2
a Structural solution from powder data.
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Table 4 Crystal data and details of measurements for all complexes described
[Ag(PQ6U)2]NO3
Form I
[Ag(PQ6U)2]NO3
Form II
[Ag(PQ5U)2]NO3
Form V
[Ag(PQ6U)2]NO3·CH3CN
Formula
Fw (g mol−1
a (Å)
b (Å)
c (Å)
α (°)
β (°)
γ (°)
C
34H29O5N8Ag
C32H26O5N7Ag
696.47
12.9856(7)
16.975(1)
27.061(1)
90
90
90
8
C32H26O5N7Ag
696.47
C32H26O10N7Ag
696.47
21.981(2)
12.949(1)
22.251(3)
90
101.16(1)
90
8
)
737.53
7.3672(2)
9.0123(4)
25.5246(8)
85.865(3)
87.607(3)
69.359(3)
2
1581.56(1)
1.549
0.694
8.4802(6)
10.7958(10)
15.5706(9)
86.193(6)
85.546(6)
87.377(7)
2
1416.87(18)
1.632
0.768
Z
V (Å3)
5964.9(6)
1.551
0.730
6213.8(12)
1.489
0.701
Dcalc (Mg m−3
)
μ (mm−1
)
Cryst. system
Space group
Triclinic
P1
23 963
7541
0.0966
0.1728
Orthorhombic
Pbca
25 623
7150
0.0605
0.1083
Triclinic
P1
16 091
9317
0.0702
0.2429
Monoclinic
P21/c
47 123
12 967
0.1531
0.4592
ˉ
ˉ
Collected reflns
Indep. reflns
R1 [on F02, I > 2σ(I)]
wR2 (all data)
For phase identification purposes X-ray powder diffracto-
For structure solution and refinement purposes X-ray
grams in the 2θ range 5–40° (step size, 0.02°; time/step, 20 s; powder diffractograms in the 2θ range 5–60° (step size, 0.01°;
0.04 rad soller; VxA 40 × 40) were collected on a Panalytical time/step, 50 s; 0.02 rad soller; VxA 40 × 40) were collected on
X’Pert PRO automated diffractometer equipped with an X’Ce- a Panalytical X’Pert PRO automated diffractometer equipped
lerator detector (Bragg–Brentano geometry), using CuKα radi- with an X’Celerator detector.
ation without a monochromator. The program Mercury13e was
used for the calculation of X-ray powder patterns on the basis
Thermogravimetric analysis (TGA)
of single crystal data. In all cases the identity between bulk
materials and single crystals was verified by comparing calcu-
lated and experimental powder diffraction patterns.
TGA analyses were performed using a Perkin-Elmer TGA-7.
Each sample, contained in a platinum crucible, was heated in
a nitrogen flow (20 cm3 min−1) at a rate of 5 °C min−1, up to
decomposition. Sample weights were in the range 5–10 mg.
Structure determination and refinement of PQ6U from powder Photophysical measurements
data
Absorption spectra were recorded using
a Perkin-Elmer
Powder diffraction data were analyzed with the software Lambda 45 spectrophotometer. For the fluorescence spectro-
expo2010,14a which is designed to analyze monochromatic and scopy measurements, uncorrected emission and corrected
non-monochromatic data. Peaks were automatically chosen in excitation spectra were obtained using a Perkin-Elmer LS 55
the 2θ range 5–40°, and a monoclinic cell was found (see spectrofluorimeter. Luminescence quantum yields (un-
Table 1), using the algorithm N-TREOR,14b with a volume of certainty
15%) were determined using quinine sulfate in
1282.65 Å3. The structure was then solved by simulated anneal- 0.05 M H2SO4 aqueous solution as a reference (Φ = 0.53). In
ing using a molecular model built with the Avogadro soft- order to allow comparison of emission intensities, corrections
ware,15 and refined by Rietveld analysis with the software were performed for instrumental response, inner filter effects,
TOPAS.16
and phototube sensitivity.17 Values of log Ka were obtained by
A shifted Chebyshev function with 13 parameters and a fitting spectrophotometric and spectrofluorimetric data with
Pseudo–Voigt function (TCHZ type) were used to fit back- SPECFIT/32, a global analysis software. Mg2+, Cu2+, Ag+, Hg2+
ground and peak shape, respectively. A spherical harmonics were delivered in the form of their trifluoromethanesulfonate
model was used to describe the preferred orientation. A rigid salts dissolved in EtOH, while a Cd(ClO4) solution in EtOH
−
body was applied on the whole molecule. Bond distances and was used as a source of Cd2+. NO3−, F−, Cl−, Br−, H2PO4 were
torsion angles of the urea group present in the PQ6U molecule delivered in the form of their tetrabutylammonium salts dis-
were refined in a limited range around an equilibrium value. solved in EtOH, while a CH3COONa·3H2O solution in EtOH
An overall thermal parameter for the C, N, O atoms of the was used as a source of CH3COO− ions.
PQ6U molecule was adopted. Refinement converged with χ2 =
Photophysical measurements on gels were performed using
2.69, Rwp = 9.44%, Rexp = 3.505%. Fig. ESI-4† shows experi- an Edinburgh FLS920 equipped with photomultiplier Hama-
mental, calculated and difference curves. Peaks at ca. 8° (101) matsu R928P and with Glann–Thompson polarizers for aniso-
and 18° (4 0 0) show a certain degree of preferred orientation tropy measurements. The same instrument connected to a
that could not be modeled properly.
PCS900 PC card was used for the TCSPC experiments.
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Fluorescence microscopy
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Argon lamp with a 490 18 nm excitation filter (MF390-18,
Thorlabs).
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to obtain a thin film which did not dry out during measurements.
Acknowledgements
We acknowledge financial support from the University of
Bologna. The help of Dr Stefano Grilli with solution NMR
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