Elusive nanoscale metal-organic-particle-supported metallogel formation using a nonconventional chelating pyridine-pyrazole-based bis-amide ligand

Article abstract of DOI:10.1002/chem.201204242

Support group: A rare case of metallogel formation (see figure) supported by nanoscale metal-organic particles by using a nonconventional ditopic chelating ligand is reported.

Full text of DOI:10.1002/chem.201204242

COMMUNICATION
DOI: 10.1002/chem.201204242
Elusive Nanoscale Metal–Organic-Particle-Supported Metallogel Formation
Using a Nonconventional Chelating Pyridine–Pyrazole-Based Bis-Amide
Ligand
Satirtha Sengupta* and Raju Mondal*[a]
Dedicated to Professor Judith A. K. Howard, CBE, FRS
Nanoscale metal–organic particles (NMOPs) have recent-
ly gained immense importance as hybrid materials because
they show promising applications in various fields including
drug delivery, chemo- and biosensing, molecular electronics,
and as contrast agents in magnetic resonance imaging.[1]
One of the most interesting and useful features of NMOPs
is their ability to entrap guest molecules that range from
drug molecules[1b,e] to greenhouse gases and organic dyes.[2]
Notwithstanding these successes, entrapment of solvent mol-
ecules by using NMOPs for metallogel formation is ex-
tremely rare.[3] It is worth mentioning here that the self-as-
sembly process in the run up to the gel formation usually
takes place unidimensionally to generate a fibrous morphol-
ogy. Metallogels, a fast-expanding branch of supramolecular
gels, are formed when a discrete metal complex self-assem-
bles by means of noncovalent interactions to give rise to a
highly cross-linked, intertwined, three-dimensional network
while immobilizing a large amount of solvent molecules
within the framework. With the proper choice of the gela-
tors and transition metals, metallogels can be successfully
tuned to impart physicochemical properties such as magnet-
ic, spectroscopic, and catalytic properties.[4] Introduction of
functional groups such as urea, amide, and aromatic rings,
which are capable of forming various weak interactions like
hydrogen bonding or p–p stacking, have been found to be
instrumental in immobilizing solvent molecules. On the
other hand, ligands with multiple coordinating sites were
found to promote the requisite cross-linking network.[5]
However, even a cursory inspection would reveal that
most of the strategies adopted for generating metallogels
are more or less “one-dimensional”, that is, they use a diver-
gent exo-ditopic ligand with a single coordinating site, with
an overwhelming majority of urea- and amide-based bipyr-
idyl molecules.[5,6] In principle, as far as gel formation is con-
cerned, one can argue that there is a serious shortcoming in
pyridine-based bis-urea or bis-amide complexes. These types
of divergent bipyridine-based ligands have a natural tenden-
cy to form solid polymeric coordination polymers or MOFs,
which goes against the basic principle of gel formation. Con-
sidering the fact that gel formation represents a tradeoff be-
tween the dynamic solution phase and solid aggregate, any
judicious design of gelator molecule should aim to restrict
coordination polymer formation. Therefore, a convergent
chelating ligand should always have an edge over a diver-
gent ligand, provided the former type is still capable of im-
mobilizing large amount of solvents. The sheer dominance
of bent 3-pyridyl-based ligands over the linear 4-pyridyl-
based ligands in gel formation can be envisaged as corrobo-
rating evidence for this theory.
Inspired by the above-mentioned considerations, we have
successfully evaluated the usefulness of chelating ligands in
metallogel formation by using a simple pyridine–pyrazole-
based ligand, N1,N3-bis[5-(pyridin-2-yl)-1H-pyrazole-3-yl]-
AHCTUNGTREGiNNUN sophthalamide (BPPIPA). To the best of our knowledge,
[a] S. Sengupta, Dr. R. Mondal
Department of Inorganic Chemistry
Indian Association for the Cultivation of Science
2A & 2B Raja S.C. Mullick Road
Kolkata 32, W.B. (India)
this is the first example in which a bis-amide-based chelating
ligand has been used for metallogel formation. We chose a
pyridine–pyrazole moiety for two reasons: 1) pyrazole-based
compounds are biologically important molecules owing to
their antibacterial, antifungal, and antimicrobial proper-
ties;[7] and 2) the pyrazole group can play a dual role in
metal coordination as well as hydrogen-bond formation with
Fax : (+91)33-2473-2805
E-mail: icrm@iacs.res.in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201204242. It contains a picture
of the metallogels, elemental analysis, plots of the elastic modulus of
copper bromide gel, SAED diffraction patterns, FTIR spectra,
EDAX spectra, powder diffraction patterns of the xerogels, and the
local structure of the gel.
ꢀ
solvents using the built-in hydrogen-bonding site (N H).
BPPIPA can be prepared in high yields by the reaction of
3{5}-amino-5{3}-(pyrid-2-yl)-1H-pyrazole[8] with isophthalolyl
chloride in the presence of triethylamine. BPPIPA itself is
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not a gelator, but in the presence of copper chloride and
bromide it gives rise to a green transparent gel instantane-
ously. The ligand is soluble in a wide variety of solvents
such as hot THF, 1,4-dioxane, DMF, and DMSO. It can gel
copper halides in all the aforementioned solvents in the
presence of water as cosolvent, but gel formed in DMF/
water (3:2) was found to be the strongest amongst all and
hence is discussed in detail in this Communication. Addition
of copper halide (0.1 to 0.4 equiv with respect to the ligand
concentration) to a 1 wt% ligand solution (critical gel con-
centration) in a DMF/water mixture gave partial gels, but
the addition of copper salt (0.5 equiv) instantly gelled the
total mixture. Complete gels were obtained up to 1.0 equiv
copper halide concentration, after which partial gels were
formed (see Figure S1 in the Supporting Information). The
ligand is highly selective towards copper halides because no
gels formed with other copper analogues and other first-row
transition metals under similar conditions. A sample of
freshly prepared gel without sonication exhibited thixotropic
behavior by transforming into a viscous sol upon shaking,
which re-formed into gel upon standing. Moreover, the gel
that re-formed was found to be stronger than the gel ob-
tained initially. However, stronger gels can also be obtained
by brief sonication of the freshly formed gel. Polar solvents,
especially water, were found to be imperative towards gelat-
ion as no gels were formed in nonpolar solvents . The sol–
gel transition temperature, Tgel, as determined by the drop-
ping-ball method was found to be (65ꢁ3)8C for 0.5 equiv
copper chloride, which increased to (80ꢁ3)8C for 0.6 equiv
copper chloride, above which the gel persisted beyond the
boiling point of water suggesting high thermal stability of
the gels. However, the Tgel value for copper bromide gel was
found to be slightly higher than its chloride counterpart with
(70ꢁ3)8C for 0.5 equiv copper bromide, which increased to
(83ꢁ3)8C for 0.6 equiv copper bromide. Wherever ob-
served, the gels were thermoreversible and stable under am-
bient conditions on a bench top over several weeks.
tively, in the ligand shifted to 1467 and 1570 cmꢀ1 in the xe-
rogel, thereby suggesting metal coordination to both the
pyridine and pyrazole nitrogen. Copper bromide gel exhibit-
ꢀ
ed similar behavior with the N H stretching, C=O stretch-
ꢀ
ing, and N H bending of the amide functionality shifting to
3411, 1604, and 1544 cmꢀ1, respectively, in the xerogel,
whereas the C=N stretching of pyrazole and pyridine was
observed at 1469 and 1571 cmꢀ1, respectively.
To gain insight into the absorption properties of the
ligand and its metal complexes, UV/Vis titration (Figure 1)
Elemental analysis (see Table 1 in the Supporting Infor-
mation) performed with the xerogels revealed a 1:1 stoichi-
ometry of ligand/metal irrespective of the copper concentra-
tions.
To probe the participation of the amide groups in the self-
assembly process through hydrogen-bonding interactions,
infrared spectroscopy was performed with the free ligand
and its corresponding xerogel (see Figure S6 in the Support-
ing Information). The ligand displayed strong absorption
Figure 1. UV/Vis absorption spectra of ligand titrated with a) aliquots of
copper chloride (runs of 0.1 up to 2 equiv, 0.2 up to 4 equiv), and b) ali-
quots of copper bromide (runs of 0.1 up to 2 equiv, 0.2 up to 4 equiv).
Inset images show the corresponding d–d transitions.
bands at 3454, 1674, and 1541 cmꢀ1 that correspond to N H
ꢀ
ꢀ
stretching, C=O stretching vibrations, and N H bending vi-
brations of the amide groups, respectively, which shifted to
3413, 1608, and 1546 cmꢀ1 in the copper chloride xerogel.
ꢀ
The decrease in the N H and C=O stretching frequency in
the xerogel coupled with an increase in the N H bending vi-
was carried out by taking a 1ꢁ10ꢀ5 m concentration (in 3:2
DMF/water mixture) of the ligand and sequentially adding
measured aliquots of metal salt solution into it. A strong p–
p* absorption band appeared at 285 nm in the free ligand,
which redshifted to 292 nm upon sequential addition of
metal salt solution up to 2 equiv of copper, with the peak
ꢀ
bration band supports the fact that significant intermolecu-
lar hydrogen bonding between the carbonyl oxygen atom
and the amide hydrogen atom has taken place in the gel
state. Moreover, the stretching vibrations of C=N bonds in
pyrazole and pyridine groups at 1495 and 1579 cmꢀ1, respec-
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Chem. Eur. J. 2013, 19, 5537 – 5541

Metallogel Formation
COMMUNICATION
gradually decreasing in intensity. A weak, broad band at
around 320 nm in the free ligand spectrum, which might be
attributed to intraACHTUNGTRENNUNGliACHTUNGTRENNUNGgand charge transfer (ILCT), showed a
gradual increase in energy without any significant shifting of
the peak. However, a further increase in the copper salt
concentration from 2 to 4 equiv resulted in the generation
of separate absorption peaks at around 300 nm without any
significant change or shift, which suggests complete com-
plexation of the metal with the ligand at 2 equiv of copper,
above which the spectra of the copper–ligand complex was
observed. Moreover, increasing the concentration of the
ligand to 10ꢀ3 m and subsequently titrating with aliquots of
the corresponding copper salts gave a broad peak (due to
ligand field and Jahn–Teller effects) centered around 600–
650 nm (Figure 1, inset), which corresponded to a d–d
2
(2Eg! T2g) transition, thus indicating octahedral geometry
around the metal ion.
Furthermore, EPR spectroscopy was used as a tool to de-
termine the environment around the copper ion, which
showed a similar spectrum both for the copper chloride and
its bromide analogue at all copper concentrations. The X-
band ESR spectrum (see Figure S5 in the Supporting Infor-
mation) recorded at 77 K revealed anisotropic character
that contained four poorly resolved super hyperfine absorp-
tion-like peaks that correspond to gj j, thus indicating the in-
teraction of the copper(II) odd electron with four similar
atoms, which in the present case is nitrogen. Moreover, the
Figure 2. Plot of elastic modulus (G’, black squares) and viscous modulus
(G’’, red circles) for copper bromide gel, and elastic modulus (G’, magen-
ta squares) and viscous modulus (G’’, green circles) for copper chloride
gel as a function of applied stress [Pa] for 1 wt% gel prepared by using
0.8 equivalents of copper halide.
be nearly constant up to about 6% strain, which denotes the
linear viscoelastic region (LVR), after which a steep drop in
the values of both the moduli was observed, thereby sug-
gesting complete disruption of the gel network. The strength
of copper bromide gel was found to be slightly greater than
that of its chloride analogue, which in turn was strongly de-
pendent on the concentration of copper added, with the
strength gradually increasing up to 0.8 equiv of copper hal-
ides, after which it decreased. However, even the strongest
gel at 0.8 equiv of copper displayed an elastic modulus of
about 1200 Pa with a yield stress of about 85 Pa, which sug-
gests the formation of a moderately strong metallogel.
Furthermore, to study the morphology of the metallogel,
field-emission scanning electron microscopy (FESEM) and
transmission electron microscopy (TEM) were performed
on the xerogels. A freshly prepared sample of the xerogel
revealed an aggregation of numerous spherical particles
(NMOPs) of an approximate diameter that ranged from 50
to 120 nm (Figure 3a,b), which was also visible even in the
sol state (Figure 3c), but with a considerable decrease in
size. The gel that re-formed from the sol state as well as the
gel that formed after sonication was found to contain slight-
ly larger NMOPs with an average diameter of 200 nm (Fig-
ure 3d). Since these NMOPs are smaller in size they are sup-
posed to provide a large surface area that could support
many interparticle cross-linking sites in the network, which
can effectively interact with and entrap the solvent mole-
cules that can give rise to the increased stability of the gel in
a polar aqueous solvent medium.[3] However, over a course
of several weeks, these particles underwent a morphological
transformation to a fibrillar structure (Figure 3e), even
though very small NMOPs were still visible in the TEM mi-
crograph. Moreover, the formation of these NMOPs was
further confirmed by using atomic force microscopy (AFM)
g-tensor values, gkACHTUNGTRENNUNG(2.34)>g?ACHTUNGTERN(NUGN 2.04)>2.0023, indicate that
the ground state of copper is predominantly dx2ꢀy2 and has a
distorted-octahedral geometry with axial symmetry. The ex-
change interaction parameter G value (calculated as G=(gk
ꢀ2)/(g?ꢀ2)) was found to be 8.5 (that is, G>4), which indi-
G
cates that the exchange interaction between copper centers
in the gel state is negligible. Moreover, the absence of any
half-field signal around 1600 G due to a DMs = ꢁ2 transition
also rules out any Cu–Cu interaction, which suggests that
the complex is mononuclear. Thus, on the basis of the above
spectral studies and elemental analysis taken together, we
can propose that the copper atom forms a distorted-octahe-
dral geometry with four nitrogen atoms of two chelating
BPPIPA molecules occupying the equatorial position,
whereas the axial positions are occupied by chlorine atoms.
This gives rise to a one-dimensional infinite network, which
can be considered the basic building unit of gel. Subsequent-
ly, higher dimensionality is achieved through hydrogen
bonding and p–p stacking among these one-dimensional
strands, as shown in Figure S8 in the Supporting Informa-
tion.
The strength of the copper halide gels of BPPIPA
(1 wt%) was further probed by using stress sweep rheome-
try (Figure 2) in which the viscous modulus and the elastic
modulus was measured as a function of increasing strain am-
plitude from 0.01 to 200% keeping the frequency constant
at 1 Hz and temperature at 298 K. In both cases, the viscous
modulus (G’’) was found to be less than the elastic modulus
(G’), which is suggestive of gel-like materials by one order
of magnitude. Both the values of G’ and G’’ were found to
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S. Sengupta and R. Mondal
could be obtained. Energy-dispersive X-ray spectroscopy
(EDAX) analysis performed on these NMOPs (Figure S4 in
the Supporting Information) was found to contain both the
ligand and the metal elements, which ruled out the possibili-
ty of generation of copper nanoparticles within the gel
matrix. Moreover, this microcrystallization is supported by
the powder XRD pattern of the xerogels (Figure S6 in the
Supporting Information), which revealed the slightly crystal-
line nature of the copper chloride gel, but its bromide ana-
logue was completely amorphous.
In conclusion, we have reported for the first time, to the
best of our knowledge, a novel pyridine–pyrazole-based bis-
amide molecule that is capable of chelating copper chloride
and bromide to form instantaneously green transparent gels.
Primary investigation revealed the gels to be made up of re-
cently discovered nanoscale metal–organic particles
(NMOPs), which find useful applications in the field of
medicine and nanotechnology. Fresh gels were found to be
mechanoresponsive to stress and displayed pronounced thix-
otropic behavior. However, sonication leads to the forma-
tion of stronger gels having high thermal stability. The
ligand was found to be highly selective toward copper hal-
ides only, thus making it a good candidate for both selective
cation and anion capture. Further studies on gelation with
the pyridyl–pyrazole amine in the generation of other aro-
matic and aliphatic amides are being pursued in our lab.
Acknowledgements
R.M. gratefully acknowledges financial assistance from the Council of
Scientific and Industrial Research (CSIR), India (01ACHTUNTRGNEU(GN 2327)/09/EMR II/
Figure 3. a) SEM image of NMOPs. TEM images of b) freshly prepared
copper chloride gel, c) sol after shaking, d) gel after sonication,
e) NMOPs transformed to fibers after about a month. f) AFM images of
NMOPs (note that the spherical particles are highly deformed, which
might be due to the expulsion of solvent molecules while drying).
CSIR). S.S.G. is thankful to CSIR, India for research fellowships. S.S.G.
also acknowledges Anindita Das, Polymer Science Unit, for helping in
rheology experiments and Debajyoti Pramanik, Department of Inorganic
Chemistry, for assisting in UV/Vis spectroscopy and electron-spin reso-
nance spectroscopy studies.
(Figure 3f). AFM performed on xerogels revealed that the
NMOPs had a base size that varied from 100 to 200 nm with
heights that ranged from 40 to 100 nm. Nevertheless, the
spheres were highly deformed, which might be due to the
exudation of solvent molecules while drying. To the best of
our knowledge, this is the first report of NMOPs that sup-
port metallogel formation by using a pyridine–pyrazole-
based bis-amide molecule.
Selected-area electron diffraction (SAED) performed on
these NMOPs revealed the individual particles to be crystal-
line in the case of copper chloride gel but amorphous in the
case of copper bromide gel (see Figure S3 in the Supporting
Information). This is attributed to the fact that crystals
begin to grow within the copper chloride gel matrix over a
period of a few weeks, the microcrystalline nature of which
prevented their crystal structure determination. However,
no such phenomenon was observed in its bromide analogue,
thereby accentuating the fact that the initial NMOPs that
constituted the copper chloride gel crystallized within the
gel matrix over a month, although no direct evidence of it
Keywords: chelates · copper · gels · halides · nanoparticles
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Received: November 28, 2012
Published online: March 18, 2013
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