Metallogels derived from silver coordination polymers of C3-symmetric tris(pyridylamide) tripodal ligands: Synthesis of ag nanoparticles and catalysis

Article abstract of DOI:10.1002/chem.201404959

By applying a recently developed crystal engineering rationale, four C3 symmetric tris(pyridylamide) ligands namely 1,3,5-tris(nicotinamidomethyl)-2,4,6-triethylbenzene, 1,3,5-tris(isonicotinamidomethyl)-2,4,6-triethylbenzene, 1,3,5-tris(nicotinamidomethyl)-2,4,6-trimethylbenzene, and 1,3,5-tris(isonicotinamidomethyl)-2,4,6-trimethylbenzene, which contain potential hydrogen-bonding sites, were designed and synthesized for generating AgI coordination polymers and coordination-polymer-based gels. The coordination polymers thus obtained were characterized by single-crystal X-ray diffraction. The silver metallogels were characterized by transmission electron microscopy (TEM) and dynamic rheology. Upon exposure to visible light, these silver metallogels produced silver nanoparticles (AgNPs), which were characterized by TEM, powder X-ray diffraction, energy dispersive X-ray and X-ray photoelectron spectroscopy. These NPs were found to be effectively catalyzed the reduction of 4-nitrophenolate to 4-aminophenolate without the use of any exogenous reducing agent.

Full text of DOI:10.1002/chem.201404959

DOI: 10.1002/chem.201404959
Full Paper
&
Metallogels
Metallogels Derived from Silver Coordination Polymers of C3-
Symmetric Tris(pyridylamide) Tripodal Ligands: Synthesis of Ag
Nanoparticles and Catalysis
Mithun Paul, Koushik Sarkar, and Parthasarathi Dastidar*[a]
Abstract: By applying a recently developed crystal engineer-
ing rationale, four C3 symmetric tris(pyridylamide) ligands
namely 1,3,5-tris(nicotinamidomethyl)-2,4,6-triethylbenzene,
1,3,5-tris(isonicotinamidomethyl)-2,4,6-triethylbenzene, 1,3,5-
tris(nicotinamidomethyl)-2,4,6-trimethylbenzene, and 1,3,5-
X-ray diffraction. The silver metallogels were characterized
by transmission electron microscopy (TEM) and dynamic
rheology. Upon exposure to visible light, these silver
metallogels produced silver nanoparticles (AgNPs), which
were characterized by TEM, powder X-ray diffraction, energy
dispersive X-ray and X-ray photoelectron spectroscopy.
These NPs were found to be effectively catalyzed the reduc-
tion of 4-nitrophenolate to 4-aminophenolate without the
use of any exogenous reducing agent.
tris(isonicotinamidomethyl)-2,4,6-trimethylbenzene,
which
contain potential hydrogen-bonding sites, were designed
and synthesized for generating AgI coordination polymers
and coordination-polymer-based gels. The coordination
polymers thus obtained were characterized by single-crystal
polymer[13] is known to produce metallogels. Metal-ligand coor-
dination plays a crucial role to form SAFiNs in such metallo-
gels.[14] Supramolecular gels that contain metal nanoparticles
(MNPs)[15] is another class of metallogel that offers opportuni-
ties to integrate the properties of MNPs and that of the
supramolecular gel matrix; such metallogels are important in
developing potential applications in catalysis,[16] sensing,[17]
photophysics,[18] synthesis of nanomaterials,[19] magnetic
materials,[20] and so forth.
Introduction
A wide spread interest has grown in designing supramolecular
gelators[1] considering their promising applications in sensing,[2]
catalysis,[3] drug delivery,[4] tissue engineering,[5] and so forth.
When a solution containing a small amount of gelator mole-
cule (often known as low-molecular-weight gelators or LMWGs
owing to their molecular weight less than 3000) is subjected
to various external stimuli like heating and cooling to room
temperature,[6] sound energy,[7] pH adjustment,[8] and so forth,
the resulting solid-like mass that can withstand its own weight
against gravity is a supramolecular gel. It is believed that 1D
self-assembly of the gelator molecules sustained by various
noncovalent interactions (such as hydrogen bonding, p–p
stacking, CꢀH···p interactions, van der Waals interactions, halo-
gen bonding, etc.) promotes the formation of 1D fibrous ag-
gregates; under suitable conditions, such 1D fibers entangle to
form self-assembled fibrillar networks (SAFiNs)[9] within which
the solvent molecules are immobilized resulting in gel. When
metal atoms are incorporated in the fibrous network as a part
of the gelator molecule, a metallogel[10] is formed. The self-as-
sembly of a discrete metal coordination complex,[11] a well-de-
fined coordination polymer,[12] or a cross-linked coordination
Designing molecules that may serve as a metalogelator is
a daunting task. Nevertheless, several attempts have been
made to design metalogelators that include 1) installing a well-
known gel-forming moiety, such as an aromatic-linker-steroid
(ALS), on a metallocene,[21] 2) introducing hydrophobic groups,
such as long alkyl chains on coordination complexes,[22] 3) uti-
lizing surface modifications with mixed ligands of different
lengths,[23] 4) slow formation of coordination polymers with
flexible ligands,[24] and 5) exploiting the well-studied supra-
molecular synthon.[25] Examples from our group[26] and else-
where[27] suggested that solvent occluded molecular solids are
good candidates for hydro or aqueous gelation, because of
the structural analogy that exists between solvent-occluded
molecular solids and gels—in both the cases, a large number
of solvent molecules are occluded. Based on this hypothesis,
we have recently demonstrated that coordination-polymer-
based metallogelators could indeed be designed.[28]
[a] M. Paul, K. Sarkar, Prof. P. Dastidar
Department of Organic Chemistry
Indian Association for the Cultivation of Science (IACS)
2A and 2B, Raja S. C. Mullick Road
Jadavpur, Kolkata, 700032, West Bengal (India)
Fax: (+91)33-2473-2805
In this report, we explored C3-symmetric bowl-shaped
tris(pyridylamide) ligands that contain potential hydrogen-
bonding sites for designing metallogels, since such ligands
were expected to form either metal–organic molecular
capsules[29] or coordination polymers,[30] wherein the possibility
of occluding solvent molecules existed in both the cases there-
E-mail: ocpd@iacs.res.in
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201404959.
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Table 1. Results of the reactions of the ligands with various AgI-salts
displaying optical images of the single crystals of the corresponding
coordination polymers.
AgNO3
AgBF4
AgPF6
AgCF3CO2
crystalline
L1 CPL1·NO3
crystalline crystalline
aggregate aggregate
aggregate
[{Ag(m-L1)3-
(CH3CN)}·NO3]
1
L2 CPL2·NO3
crystalline CPL2·PF6
CPL2·CF3CO3
aggregate
[{Ag3(m-L2)3(CH3CN)-
[{Ag(m-L2)3-
[{Ag2(m-L2)3(H2O)2}-
(CH3CH2OH)(NO3)}·
(CH3CN)}·PF6]
·2DMSO·7H2O·
2CF3CO2]
1
1
Scheme 1. Various tripodal ligands used in the present study.
2NO3·H2O·CH3CN]
1
L3 CPL3·NO3
crystalline crystalline
aggregate aggregate
CPL3·CF3CO3
by facilitating the metallogel formation. Thus, a new series of
C3-symmetric tripodal ligands were synthesized (L1–L4,
Scheme 1) and reacted with various AgI salts; AgI was
deliberately chosen as the metal center because of its flexible
coordination geometry that might promote the formation of
capsules[31] and metallogels under suitable conditions. The free
ligands and the corresponding AgI coordination compounds
were characterized by various physicochemical techniques, in-
cluding single-crystal X-ray diffraction (SXRD). Most of the coor-
dination compounds studied herein were indeed solvent-
occluded molecular solids as envisaged. Interestingly, reaction
of the ligands and AgI salts produced metallogels. Two of the
four ligands were also gelators. AgNPs generated from these
metallogels were exploited in catalyzing the conversion of
nitrophenol to aminophenol.[18a,22]
[{Ag3(m-L3)3Cl}·
2NO3]
[{Ag(m-L3)3}·CH3CN·
H2O·CF3CO2]
1
1
L4 crystalline
crystalline crystalline
aggregate aggregate
crystalline
aggregate
aggregate
conformation, F is understandably approximately 08 whereas
for syn-syn-anti conformation F must be much greater than
08. In the cases of L1 and L2, the value of F was 2.08 and 1.58,
respectively, whereas that for L3 was 25.98. In all the cases, oc-
cluded water molecules were present and stabilized through
various hydrogen-bonding interactions involving the amide
moiety and pyridyl N atoms. In the crystal structure of L1,
a cluster of six hydrogen-bonded water molecules were encap-
sulated by four ligand molecules through further hydrogen
bonding involving the amide moiety and the pyridyl N atoms
thereby leading to 1D hydrogen-bonded network. In the case
of L2, two hydrogen-bonded lattice occluded water molecules
were encapsulated by three ligand molecules sustained by fur-
ther hydrogen bonding from the amide and the pyridyl N
atoms giving rise to 3D hydrogen-bonded network. In L3, two
occluded water molecules were found to be hydrogen bonded
with the amide NH and pyridyl N of the ligand leading to a 3D
hydrogen bonded network. Thus, as envisaged, two of the li-
gands adopted the expected bowl-shaped conformation and
in all the cases, water molecules as solvates were occluded
within the crystal lattice sustained by various hydrogen-
bonding interactions (Figure 1).
Results and Discussion
In order to probe the hypothesis of metallogelation described
above, it was of utmost importance to study the single-crystal
structures of the ligands as well as the corresponding AgI
coordination compounds (Table 1).
Syntheses and single-crystal structures of the ligands
All the tripodal ligands were synthesized following literature
procedure (for details, see Experimental Section). Out of the
four lignads, L1, L2, and L3 could be crystallized from DMF/
MeOH/water mixture by slow evaporation at room tempera-
ture. Despite our best efforts, L4 could not be crystallized.
SXRD studies revealed that both L1 and L2 adopted a syn-
syn-syn bowl-shaped conformation, whereas L3 displayed
a syn-syn-anti conformation, which can be quantified from the
value of the dihedral angle (F) involving the central aromatic
plane and an imaginary plane passing through all the three N
atoms of the amide functionality. In the case of syn-syn-syn
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Figure 1. Crystal structure illustration of the ligands L1, L2, and L3; solvate
water (orange ball) encapsulation in a) L1, b) L2 and c) L3; in insets, the con-
formation of the ligand molecules as observed in these crystals structures.
Figure 2. Crystal structure illustration of CPL2·NO3; a) interdigited dimer of
the capsular assemblies sustained by argentophilic interactions; b) 1D
coordination polymer formed via amide (C=O)ꢀAg coordination.
Synthesis and crystal structures of the AgI coordination
compounds
tween two adjacent 1D polymeric chains sustained by OꢀH···O
hydrogen bonding involving the amide moieties. One mole-
cule of acetonitrile was also present in the crystal structure
sustained by CꢀH···N interactions involving the N of nitrile
moiety and aromatic CꢀH adjacent to the 4-pyridyl N atom.
Two of the three nitrate counterions were found to be hydro-
gen bonded with the amide NꢀH and the other one was coor-
dinated to one of the AgI metal centers. The presence of unac-
counted electron density indicated the existence of disordered
solvent molecules; SQUEEZE[33] calculations and thermogravi-
metric analysis(TGA) data suggested that there were additional
0.5 water molecules (wt loss=9.9% (obsd), 8.6% (calcd)) as
lattice-occluded solvents.
The ligands were treated with various AgI salts (nitrate, tetra-
fluoroborate, hexafluorophosphate, and trifluoroacetate) in
DMSO/acetonitrile at room temperature; the molar ratio of the
reacting components were kept 2:3 (ligand/metal) in order to
obtain molecular capsule like assembly (see Experimental
Section). Except for L4, all the other ligands produced X-ray
quality single crystals from AgNO3. On the other hand, L2 also
gave single crystals with AgPF6 and AgCF3CO2, whereas L3
gave crystals good for SXRD from AgCF3CO2. the ligand L4 did
not produce any single crystals with any of the AgI-salt used
herein.
The fact that the position of the pyridyl N atom seemed to
be important in controlling the supramolecular architecture of
the coordination polymers was evident from the structure of
CPL1·NO3, which was completely different from that of
CPL2·NO3; the ligand L1 differed from L2 only by the position
of the pyridyl N atom. In CPL1·NO3, the metal center adopted
a distorted tetrahedral geometry. For this we calculated the an-
gular structural parameter for tetrahedral geometry (t4) that
represented the degree of distortion from ideal tetrahedral ge-
ometry.[34] The t4 value is 1.0 for a perfect tetrahedral geome-
try. In this case, it ranged from 0.7 to 0.8 indicating significant
distortion from tetrahedral geometry. In the case of CPL2·NO3,
while one of the metal centers displayed distorted tetrahedral
geometry (t4 =0.7), the other two showed distorted trigonal
planar coordination (]N-Ag-N=99.4(3)–156.3(3)8). Careful
analyses of the structure of CPL1·NO3 revealed that it was
a 1D array of L1 coordinated to the adjacent AgI metal centers
through all the three 3-pyridyl N atoms. In such array, the 3-
pyridyl tripodal ligands indeed approached the metal center
SXRD data revealed that these crystals belonged to various
crystal systems that included triclinic, monoclinic, hexagonal,
and cubic (see Tables 4 and 5, in the Experimental Section).
Remarkably, in all the coordination compounds, the ligands
adopted syn-syn-syn conformation (F=0–2.98). However, true
capsular assembly was obtained only in the case of CPL2·NO3.
Herein, two bowl-shaped ligands coordinated with three AgI
metal centers through 4-pyridyl NꢀAg coordination. Two such
capsular assemblies were found to form interdigited dimers
sustained by argentophilic interactions,[32] owing to the closed
shell interactions involving Ag atoms (d10–d10). The interactive
distances of AgꢀAg in this structure were found to be 3.13
and 3.21 ꢁ, which was less than the summation of the van der
Waals radii of Ag atoms (3.44 ꢁ), confirming argentophilic in-
teractions. The interdigited dimers were further coordinated to
the adjacent dimers through >C=OꢀAg coordination resulting
in a 1D coordination polymeric chain (Figure 2). A lattice-oc-
cluded water molecule was found to be acting as a bridge be-
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from opposite sides in a significantly offset fashion unlike in
the capsular assembly as observed in CPL2·NO3, presumably
because of the angular ligating topology of L1 due to its 3-pyr-
idyl N atom. The nitrate counterion was found to be occluded
within such array sustained by NꢀH···O interactions involving
the nitrate O and amide NꢀH. Such 1D arrays were further
packed in parallel fashion stabilized by amide···amide hydro-
gen-bonding interactions leading to a 2D hydrogen bonded
network (Figure 3).
Figure 4. Crystal structure illustration; a) 3D coordination network of
CPL2·PF6, PF6ꢀ counterions shown in the space-filling model; b) parallel
packing of the 2D coordination network in CPL2·CF3CO2, the counter anion
trifluoroacetates (green), solvate DMSO (yellow) and water (red) molecules
were stabilized within the crystal lattice by hydrogen-bonding interactions.
(Figure 4). Thus, various counterions appeared to be
influencing the resulting supramolecular architecture of these
coordination compounds.
The ligand L3, which showed syn-syn-anti conformation in
its free ligand structure, adopted syn-syn-syn conformation in
the corresponding coordination compounds namely
CPL3·CF3CO2 and CPL3·NO3. Interestingly, in CPL3·CF3CO2, the
metal center displayed significantly distorted trigonal planar
geometry (]N-Ag-N=107.0(9)–129.0(9)8) and the structure
may be best described as 1D coordination polymer; the coun-
Figure 3. Crystal structure illustration of CPL1·NO3; a) 1D polymeric array;
b) the overall 2D network sustained by hydrogen-bonding interactions dis-
playing uncoordinated nitrates as counterion in the crystal lattice.
The shape and chemical structure of the counter anions ap-
peared to be influencing the resultant supramolecular struc-
tures as evident from the single-crystal structures of CPL2·PF6
and CPL2·CF3CO2; both the compounds differed from
ꢀ
terion CF3CO2 was uncoordinated and remained within the
1D array sustained by hydrogen bonding involving the amide
NꢀH and carboxylate O of the anion. The solvate water mole-
cule was found to be hydrogen bonded with the amide NꢀH,
whereas the lattice occluded acetonitrile was stabilized by
means of dispersion forces. In the case of CPL3·NO3, however,
an unexpected metal-coordinated chloride anion was observed
which presumably had come from the crystallizing solvent. In
this case, the metal center adopted a significantly distorted
tetrahedral geometry (t4 =0.7) that led to the formation of 3D
coordination network displaying trigonal symmetry (Figure 5).
Within the lattice, there were 1.5 water molecules as occluded
solvents revealed by SQUEEZE and TGA (wt loss=2.9% (obsd),
1.9% (calcd)).
ꢀ
CPL2·NO3 only by the counterions. While NO3 is trigonal
ꢀ
ꢀ
planar, PF6 and CF3CO2 are octahedral and tetrahedral, re-
spectively. The metal centers in both the cases (CPL2·PF6 and
CPL2·CF3CO2) adopted significantly distorted to near perfect
tetrahedral coordination geometry (t4 =0.5–1.0), whereas in
CPL2·NO3, the metal centers displayed distorted tetrahedral as
well as trigonal planar coordination geometry (vide supra).
While CPL2·PF6 displayed a cubic 3D coordination network,
CPL2·CF3CO2 was found to be a 2D coordination polymer. In
both the cases, the counterions were found to be uncoordinat-
ꢀ
ed; as expected the PF6 ion was stabilized in the crystal struc-
ꢀ
ture by means of dispersion forces, whereas the CF3CO2 ion,
which is capable of forming hydrogen bonds, was found to be
stabilized by NꢀH···O hydrogen bonding involving the amide
NꢀH and carboxylate O of the anion. In CPL2·CF3CO2, lattice
occluded solvent molecules (DMSO and water) were observed
in addition to three solvate water molecules as evident from
SQUEEZE calculations and TGA (wt loss=20.7% (obsd), 19.1%
(calcd)), whereas no such solvents were present in CPL2·PF6
The lattice occluded solvent content in the single crystal
structures of the free ligands as well as the corresponding AgI
coordination compounds was further verified by TGA (see
Experimental Section).
The crystalline phase purity of the ligands L2 and L3
and the coordination polymers CPL2·PF6, CPL2·CF3CO2,
CPL3·CF3CO2, and CPL3·NO3 were established by X-ray powder
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with the increase in [gelator], indicating the role of various
noncovalent interactions in gel network formation (Figure 6).
HR-TEM images of the dried gels of L1 and L3 revealed in-
teresting features of the morphology of the gel network
(Figure 7). Both DMSO/water and nitrobenzene gels of L1 dis-
played several micrometer long tapes (width=0.24 to 1.0 mm).
On the other hand, highly entangled fibers (width=0.08 to
0.12 mm) were observed in the DMSO/water gel of L3. Interest-
ingly, the nitrobenzene gel of L3 displayed a few twisted rib-
bons (width=0.55 mm and pitch=1.2 mm) of left-handed chir-
ality along with the several micrometer long flat tapes
(width=0.14–0.23 mm). Appearance of chiral morphology (heli-
ces, twisted tapes etc.) is known in the self-assembly of chiral
molecules.[35] Although L3 was molecularly achiral, the appear-
ance of the chiral twisted tape in the xerogel could be because
of its supramolecular chirality as evident from its chiral space
group (P21212; see Table 4 in the Experimental Section). The
Figure 5. Crystal structure illustration; a) 1D polymeric array of CPL3·CF3CO2;
b) 3D polymeric network in CPL3·NO3; metal-coordinated chloride shown in
green.
diffraction (PXRD) analyses (Figure S10–S18 in the Supporting
Information); in the case of L1and CPL1·NO3, the PXRD pat-
terns of the bulk solids were found to be poorly diffracting
and showed reasonable match below 208 in 2q with the
corresponding simulated PXRD patterns. In case of CPL2·NO3,
the simulated pattern did not match with the bulk crystals.
These may be attributed to the crystal instability due to loss
of solvent molecules.
Figure 6. Tgel versus [gelator] plots of L1 and L2 gels in DMSO/water (2:3 v/
v).
Gelation studies
Gelation of the free ligands
The main goal of the present study was to develop metal–or-
ganic systems that were capable of occluding solvent mole-
cules thereby producing metallogels under suitable conditions.
The foregoing discussion on the SXRD of the AgI coordination
compounds clearly established that out of the six coordination
compounds, four of them were lattice-occluded crystalline
solids. Therefore, we undertook gelation studies with the hope
of generating metallogels. Interestingly, C3-symmetric tris(pyri-
dylamide) ligands were also known to promote gelation.[26a]
Thus, all the ligands were scanned for gelation with various
solvents that included both polar and nonpolar solvents. It
was observed that only 3-pyridyl derivatives that is, L1 and L3,
were capable of gelling DMSO/water (2:3 v/v), DMF/water (2:3
v/v) and nitrobenzene, indicating the crucial role being played
by the position of the pyridyl N atoms on gelation (Table S1 in
the Supporting Information). The minimum gelator concentra-
tion (MGC) for these gels was in the range of 1–4 wt% and the
gel–sol dissociation temperature (Tgel) was within the range of
110–1368C. The Tgel versus [gelator] plot for DMSO/water (2:3
v/v) gels of L1 and L3 revealed that the Tgel values increased
Figure 7. HR-TEM images of the xerogels; for L1: a) DMSO/water (2:3 v/v);
b) nitrobenzene; for L3: c) DMSO/water (2:3 v/v); d) nitrobenzene. Inset: op-
tical images of the gels; TEM images were taken on freshly prepared gels
after smearing them on 300 mesh carbon coated copper grid followed
drying under vacuum.
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supramolecular chirality was also evident in the solid-state CD
of the crystalline particles of L3 (Figure S32 in the Supporting
Information). Achiral molecules displaying chiral morphologies
have also been reported in the literature.[36] It may be men-
tioned here that the coordination polymers displaying chiral
space groups (CPL2·NO3 and CPL3·NO3) were also CD active
(Figure S31 and S33 in the Supporting Information).
Table 2. Optical images of all the Ag metallogels (DMSO/water (2:3) v/v)
along with their MGC; all the optical images were taken soon after the
gels were formed.
AgNO3
AgBF4
AgPF6
L1
Metallogelation
Since the molar ratio for metal/ligand was 1:1 in the majority
of the crystal structures of the coordination polymers studied
herein, we deliberately kept the same metal/ligand ratio in
metallogelation studies in all the cases, except for CPL2·NO3
for which the ratio was kept at 3:2 (metal/ligand) as guided by
its crystal structure (vide supra). In a typical metallogelation ex-
periment, a DMSO (0.4 mL) solution of the ligand was added
to an aqueous solution (0.6 mL) of the silver salt; the resulting
mixture, on brief sonication, formed a homogeneous opaque
gel that was able to withstand its own weight against gravity
and was stable for more than a week at room temperature in
the dark.
L3
also capable of producing metal nanoparticles (MNPs).[39] There
are examples in which supramolecular gelators are able to
reduce metal ions to produce MNPs within the gel bed. Steed
et al. first reported the in situ synthesis of AgNPs from a silver
metallogel.[19a] The MNPs have been considered as promising
materials for catalysis owing to their unique large surface-to-
volume ratios and quantum-size effects.[18a,22] In the present
study, it was observed that the white opaque metallogels
turned light brown upon exposure to visible light; such a colo-
ration was indicative of possible AgNP formation (Figure 8).
UV/Vis spectra of the isolated NPs gave an absorption band
at 435 nm in EtOH at room temperature; the absorption band
was attributed to the surface plasmon band of the spherical
Remarkably, as in the cases of the free ligands, the 3-pyridyl
derivatives that is, L1 and L3, gave gels in presence of the Ag-
salts, whereas the 4-pyridyl derivative resulted in precipitates.
The fact that these gels were metallogels was confirmed by
their thermo-irreversibility, which indicated the metal–organic
coordination polymeric nature of the gel networks (Table 2).
Both the ligands L1 and L3 gave metallogels with all the
Ag-salts except AgCF3CO2, suggesting the crucial role of the
counter anion in gelation.[37]
Various PXRD patterns (experimental bulk solid, xerogel, and
simulated) of MGL1·NO3 and MGL3·NO3 did not show a signifi-
cant match because of the poor
diffraction of the xerogel; this
was attributed to the solvent
loss as well as the formation of
the AgNPs during the experi-
ments. Thus, it was not possible
to decipher the supramolecular
structure of the metallogel net-
works. On the other hand, in the
cases of L1and L3, the PXRD
patterns of the xerogels reasona-
bly matched with the corre-
sponding simulated and bulk
patterns, thereby establishing
the structure of the gel network
in the xerogels (Figure S10–S18
in the Supporting Information).
It is worth mentioning that
AgI can easily be reduced to
metallic Ag0 even in presence of
weak reducing agents, such as
ascorbic acid.[38] Various small or-
ganic compounds containing
Figure 8. Optical images of metallogels containing AgNPs; all the optical images of the metallogels were taken
thiol/amide/urea moieties were after 21 days of their formation.
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AgNPs.[40] The intensity of the absorption band increased with
the increase in concentration of the Ag salt used and with the
aging time. This enhancement might be attributed to the
amount of AgNPs being formed. Keeping the metallogels in
the dark significantly retarded the coloration, which was attrib-
uted to the slow rate of AgNP formation in the absence of
light. Existence of AgNPs in the metallogels was further con-
firmed by HRTEM. It was clearly seen that AgNPs adhered to
the fibers of the gel network. The size distribution of the
AgNPs generated from the metallogels of L1 (MGL1·NO3,
MGL1·BF4 and MGL1·PF6) were within the range of 7–11 nm,
whereas for those of L3 (MGL3·NO3, MGL3·BF4 and MGL3·PF6),
the range was 9–13 nm (Figure 9).
Understandably, the amide moiety, as well as the light
energy, reduced the AgI to AgNPs, which in turn were adhered
to the gel network. Since AgI was part of the coordination
polymeric gel network, such reduction of metal ion must lead
to the degradation of the gel network. Indeed, the metallogels
collapsed upon keeping the gels for about 2–3 weeks in pres-
ence of visible light, thereby supporting such hypothesis. Thus,
AgI was reduced to AgNPs without any exogenous reducing
agent. The color intensity was remarkably enhanced with the
increasing silver metal concentration and time (Figure 9).
The growth rate of the AgNPs increased in presence of UV
light under which, within a day, the NPs were formed. The
effect of the concentration of Ag salts on the size distribution
of the NPs was also studied. For this purpose, we chose to
work with the metallogel derived from L1 and AgNO3. It was
observed that as the concentration of Ag salt increased, the
size of the NPs gradually increased; in some cases, agglomera-
tion of AgNPs away from the gel fiber was also noted (Fig-
ure S27 in the Supporting Information). No significant effect of
anions on the size distribution of the NPs was observed. The
crystalline nature of the AgNPs was confirmed by PXRD on the
NPs derived from L1 and AgNO3. The diffraction peaks ap-
peared at 38, 44, 65 and 788 for 2q. These diffraction peaks
matched perfectly well with that of the face-centered cubic
(fcc) lattice of crystalline silver.[41] The results obtained from
PXRD studies were further supported by the selected area elec-
tron diffraction (SAED) analyses of the AgNPs in TEM. The first
five electron diffraction rings were indexed as (111), (200),
(220), (311), and (400), which matched well with that of the
face-centered cubic (fcc) silver. The existence of the AgNPs was
further confirmed by energy dispersive X-ray (EDX). The exis-
tence of Ag0 in the AgNPs isolated from the metallogels was
confirmed by X-ray photoelectron spectroscopy (XPS; Fig-
ure 10).The peaks observed at 368 and 374 eV were attributed
to Ag3d5/2 and Ag3d3/2, respectively.[42]
Figure 9. Charaterization of silver NPs derived from Ag metallogels of L1;
effect of a) the concentration of silver salt and b) aging time on the charac-
teristic surface plasmon peak in the UV/Vis spectrum of silver NPs isolated
from MGL1·NO3; TEM micrographs of silver NPs adhered on the gel fibers
obtained from c) MGL1·NO3; d) MGL1·BF4; e) MGL1·PF6 and their corre-
sponding size distribution plots; all the TEM samples were prepared from
the corresponding gel containing nanoparticles aged for 7 days.
UV light and characterized by its typical surface plasmon peak
at ca. 435 nm in the UV/Vis spectrum), the reaction did not go
through indicating the possible role of the ligand in catalyzing
the same reaction in metallogels. The metallogel-catalyzed
conversion could be observed visually; the characteristic
yellow color of 4-nitrophenolate turned to colorless within
a few seconds, when a solution of 4-nitrophenolate was placed
over the metallogel bed at room temperate. The characteristic
absorption band of 4-nitrophenolate (400 nm) disappeared
and that of 4-aminophenolate (316 nm) appeared in the UV/
Vis spectra (Figure 11).
Having successfully prepared and characterized AgNPs in
the metallogels, we then explored its catalytic behavior; for
this purpose, we considered the conversion of 4-nitrophenol
to 4-aminophenol as a model reaction. It was observed that
the AgNPs present within the metallogels were capable of cat-
alyzing the reaction without the use of any other exogenous
reducing agent like NaBH4. Interestingly, when 4-nitrophenol
was treated with the separately prepared AgNPs (prepared by
exposing a dilute solution of AgNO3 containing NaBH4 under
Since two of the ligands namely L1 and L3 were gelators,
we also explored the possibility of preparing AuNPs on the
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ly prepared gels between parallel plates on a rheometer. For
these experiments, gels of L1 and L3 were prepared keeping
the concentration at 3 and 5 wt%, respectively, in DMSO/water
(2:3 v/v) and the same concentrations were maintained for
their corresponding silver metallogels (DMSO/water (2:3 v/v)).
First we performed oscillatory amplitude sweep on the gel
samples in order to determine the strain at which the dynamic
frequency sweep needed to be performed (Figure S34 in the
supporting information). Thus, the elastic modulus (G’) and vis-
cous modulus (G’’) of the gels were measured as a function of
strain amplitude at a fixed frequency (w=10 radsꢀ1). From
these experiments a strain of 0.01% was determined for the
frequency sweep measurements. The plot of G’ and G’’ against
the angular frequency w at a constant strain of 0.01% exhibit-
ed that G’ was frequency invariant over the entire frequency
range and the values of G’ were much greater than those of
G’’, indicating the viscoelastic (gel) nature of the materials
(Figure 12). It was interesting to note that the values of G’ for
the metgallogels were much larger (2–22 times) than those of
the organogels derived from the free ligands indicating the
presence of AgNPs in the gels.[15a]
Figure 10. a) PXRD pattern of the xerogel of MGL1·NO3 after formation of
silver NPs predominantly displaying diffraction peaks due to crystalline
AgNPs; b) selected area electron diffraction of AgNPs; c) EDX spectrum of
AgNPs adhered on gel (MGL1·NO3) fiber; d) XPS of AgNPs isolated from the
metallogelMGL1·NO3; both PXRD and XPS were recorded after 21 days of
the gel formation, whereas EDX and SAED were recorded after 7 days of the
gel formation.
The G’ values as determined from frequency sweep experi-
ments were found to gradually increase with the increase in
AgNO3 concentration in the case of the metallogel derived
from L1 and AgNO3, clearly indicating the crucial role of excess
Ag salt on the mechanical strength of the metallogel network
(Figure 13).
The possible effect of the concentration of the metal salt on
the gel network strength could be due to their possible adher-
ence to the gel network through electrostatic and hydrogen-
bonding (involving nitrate and amide moieties) interactions.
The yield stress of the metallogels as determined from the am-
plitude sweep experiments revealed interesting observations;
there appeared to be effects of counterions on the yield stress
(Table 3). In both the cases, the yield stress of the metatallogel
ꢀ
containing NO3 ion was relatively larger than that of the met-
allogels containing the other anions. This could be because of
ꢀ
the possible hydrogen-bonding interactions of the NO3 with
the amide backbone of the ligands in the gel network.
Table 3. The yield stress values of different metallogels at the same
concentration determined from amplitude sweep experiment.
Figure 11. Reduction of 4-nitrophenolate to 4-aminophenolate by AgNPs
present within the metallogel MGL1·NO3.
Yield stress [Pa]
AgNO3
AgBF4
AgPF6
L1
L3
37
33
24
29
21
26
corresponding gel beds. Indeed, both the ligands were able to
generate AuNPs on the corresponding DMSO/water gel bed
and the gels containing AuNPs were able to catalyze the same
reaction considered above (Figure S29 and S30 in the Support-
ing Information).
Conclusions
Attempts have been made to design metallogels based on a re-
cently demonstrated crystal-engineering strategy.[27,28] Thus, we
have designed and synthesized a new series of C3-symmetric
tripodal ligands equipped with hydrogen-bonding functionali-
ties and pyridyl coordinating sites that indeed produced Ag
Rheology
To evaluate the viscoelastic response of the gels and the met-
allogels we performed dynamic rheology[19a,43] by placing fresh-
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Figure 12. Frequency sweep experiments of the gels and metallogels.
metallogels when treated with various Ag salts under suitable
conditions. Reaction of these ligands with varoius Ag salts re-
sulted in six coordination polymers that were characterized by
SXRD. Under suitable conditions the ligands L1 and L3 gave
Ag metallogels, which were characterized by TEM and rheolo-
gy. Interestingly, these metallogels produced AgNPs at room
temperature upon exposure to light. The AgNPs were charac-
terized by TEM, SAED, PXRD, EDX, and XPS. The results re-
vealed that AgI was reduced to Ag0 presumably by the ligands
thereby producing AgNPs without the use of any exogenous
reducing agent like NaBH4. Interestingly, an important organic
conversion namely reduction of 4-nitrophenolate to 4-amino-
phenolate was catalyzed by these AgNPs.
Experimental Section
Materials and method
All the chemicals were commercially available and used without
further purification. 1,3,5-Tris(nicotinamidomethyl)-2,4,6-triethylben-
zene (L1), 1,3,5-tris(isonicotinamidomethyl)-2,4,6-triethylbenzene
(L2), 1,3,5-tris(nicotinamidomethyl)-2,4,6-trimethylbenzene (L3),
and 1,3,5-tris(isonicotinamidomethyl)-2,4,6-trimethylbenzene (L4)
were synthesized by coupling acid chloride (nicotinoyl/isonicotino-
yl) with 1,3,5-trisamine-2,4,6-triethylbenzene or 1,3,5-trisamine-
2,4,6-trimethylbenzene. The central scaffold 1,3,5-trisamine-2,4,6-tri-
methylbenzene and 1,3,5-trisamine-2,4,6-triethylbenzene were syn-
thesized following a known procedure.[44] Elemental analyses ware
carried out using a PerkinElmer 2400 Series-II CHN analyzer. FT-IR
spectra were recorded using PerkinElmer Spectrum GX, and TGA
analyses were performed on a SDT Q Series 600 Universal VA.2E TA
instrument. UV/Vis spectra were recorded with a VARIAN CARY 50
Bio UV-visible spectrophotometer. The solid-state CD spectra were
recorded at room temperature with Jasco J-815 spectropolarimeter.
The X-ray photo electron spectroscopic measurements were per-
formed in SPECS (Germany) source-MgKa (1253.6 eV) by placing
the sample containing gel-embedded NPs with carbon black on
Figure 13. a) Frequency sweep of MGL1·NO3 with varying AgNO3 concen-
tration; b) G’ versus concentration of silver salt.
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a copper plate. The values of elemental ratios were calculated
using Casa Software. The mass spectrum was recorded on QTOF
Micro YA263. NMR spectra (1H and 13C) were recorded using
500 MHz spectrometer (Bruker Ultrasheild Plus-500).
545.01 [M+Na]+; FT-IR (KBr): n˜ =3444, 3419, 3288, 3242, 3066,
3002, 1635 (s), 1593 (s), 1531 (s), 1471 (m), 1421 (m), 1359, 1296
(m), 1159, 1027 (s), 709 cmꢀ1(s). Thermogravimetric data analyses
(Figure S3 in the Supporting Information) revealed a weight loss of
7.86% that occurred within a temperature range of 51–868C; this
was attributed to the weight loss of two solvated water molecules
(calculated weight loss 6.44%); this observation was in good
agreement with the single crystal structure of L3.
Snythesis and characterization of the ligands
Synthetic procedure for the ligands L1, L2, L3 and L4: For L1
and L2, dichloromethane (100 mL), triethylamine (700 ml, 5 mmol)
and acid chloride (nicotinoyl or isonicotinoyl) (890 mg, 5 mmol)
were placed in a 250 mL 2-neck round bottom flask. The reaction
mixture was then stirred at room temperature under nitrogen at-
Characterization data of L4: Yield: 520 mg (57.5%). m.p: 1688C; ele-
mental analysis calcd (%) for C30H30N6O3: C 68.95, H 5.79, N 16.08;
1
found: C 68.44, H 5.54, N 15.71; H NMR (500 MHz, [D6]DMSO): d=
8.67 (d, J=2.0 Hz, N-H, 3H), 8.66 (d, J=1.5 Hz, 6H), 7.72 (q, J=
2.0 Hz, 6H), 4.56 (d, J=6.0 Hz, 6H), 2.40 ppm (s, 9H); 13C NMR
(500 MHz, [D6]DMSO): d=164.71, 150.04, 141.35, 137.11, 132.30,
121.46, 16.08 ppm; ESI-MS: m/z calcd for C30H30N6O3: 522.6 [M]+;
found: 545.34 [M+Na]+; FT-IR (KBr): n˜ =3323, 3307, 3024, 1637 (s),
1596, 1550, 1541 (s), 1508 (s), 1481, 1407 (s), 1298 (s), 844 (m),
mosphere.
A
solution of 1,3,5-trisamine-2,4,6-triethylbenzene
(399 mg, 1.6 mmol) in THF (50 mL) was then added slowly. Then
the mixture was allowed to react at room temperature followed by
reflux at 608C. A precipitate was formed that was then filtered and
washed properly with THF and dichloromethane. The ligands were
used without further purification. Similarly the ligands L3 and L4
were synthesized by using 1,3,5-trisamine-2,4,6-trimethylbenzene
instead of 1,3,5-trisamine-2,4,6-triethylbenzene.
671 cmꢀ1
.
Characterization of coordination polymers
Characterization data of L1: Yield: 580 mg (64%); m.p: 2378C after
crystallization from DMF/MeOH/H2O (1: 2: 2 v/v); elemental analysis
calcd (%) for C33H42N6O6: C 64.06, H 6.84, N 13.58; found: C 64.01,
[{Ag(m-L1)3(CH3CN)}·NO3] (CPL1·NO3): This polymer was synthe-
/
sized by carefully adding a solution of AgNO3 (22.3 mg, 0.13 mmol)
in acetonitrile (10 mL) to a solution of L1 (50 mg, 0.08 mmol)
DMSO/acetonitrile (1:4; 10 mL). The resulting solution was kept in
the dark for slow evaporation at room temperature. After two
weeks well-formed block-shaped colorless crystals were obtained.
The crystals were washed in acetonitrile and characterized by ele-
mental analysis, PXRD, and FT-IR. Yield: 41.02% (28 mg,
0.036 mmol); elemental analysis calcd (%) for C35H39AgN8O6: C
54.20, H 5.07, N 14.45; found: C 54.36, H 4.93, N 14.06; FT-IR (KBr
pellet): n˜ =3277, 3061,1656 (s), 1595 (s), 1532 (s), 1478, 1366 (s),
1295 (m), 1153 (m), 1052 (m), 1029 (s), 953, 825, 700 cmꢀ1 (m).
Thermogravimetric data analyses (Figure S4 in the Supporting In-
formation) revealed a weight loss of 5.29% that occurred within
a temperature range of 50–808C; this was attributed to the weight
loss of one coordinated acetonitrile molecule (calculated weight
loss 6.76%); this observation was in good agreement with the
single crystal structure of CPL1·NO3.
1
H 6.24, N 13.29; H NMR (500 MHz, [D6]DMSO): d=8.99 (s, 3H), 8.67
(s, 3H), 8.66 (d, J=2.5 Hz, 3H), 8.20 (d, J=7 Hz, 3H), 7.47 (q, J=
8 Hz, 3H), 4.59 (d, J=7 Hz, 6H), 2.84 (q, J=11 Hz, 6H), 1.127 ppm
(t, J=12 Hz, 9H); 13C NMR (500 MHz, [D6]DMSO): d=15.90, 23.28,
36.01, 123.28, 129.02, 135.18, 143.87, 145.60, 148.62, 151.70,
164.73 ppm; ESI-MS: m/z calcd for C33H36N6O3: 564.68 [M]+; found:
587.17 [M+Na]+; FT-IR (KBr): n˜ =3282, 2966, 2871, 1629 (s), 1591
(m), 1529 (s), 1473 (m), 1417 (m), 1355, 1288 (m), 1199, 1027, 892
(s), 858 (m), 705 cmꢀ1. Thermogravimetric data analyses (Figure S1
in the Supporting Information) revealed a weight loss of 10.3%
that occurred within a temperature range of 34–1778C; this was
attributed to the weight loss of three solvated water molecules
(calculated weight loss 8.72%); this observation was in good
agreement with the single crystal structure of L1.
Characterization data of L2: Yield: 520 mg (57.5%); m.p: 1708C
after crystallization from DMF/MeOH/H2O (1: 2: 2 v/v); elemental
analysis calcd (%) for C33H40N6O5: C 65.98, H 6.71, N 13.99; found: C
65.44, H 6.54, N 13.71; 1H NMR (500 MHz, [D6]DMSO): d=8.72 (d,
J=4 Hz, 3H), 8.66 (d, J=5.5 Hz, 3H), 7.74 (d, J=6 Hz, 3H), 4.55 (d,
J=4.5 Hz, 6H), 2.81 (d, J=7.5 Hz, 6H), 1.09 ppm (t, J=7 Hz, 9H);
13C NMR (500 MHz, [D6]DMSO): d=16.74, 23.34, 38.55, 122.08,
132.32, 141.87, 144.60, 150.69, 165.23 ppm; ESI-MS: m/z calcd for
C33H36N3O6: 564.68 [M]+; found: 587.38 [M+Na]+; FT-IR (KBr): n˜ =
3475, 3332, 3045, 2970, 1666 (s), 1635 (s), 1604, 1544 (s), 1527 (s),
1488 (m), 1411, 1361, 1313, 1220, 1068 (m), 1043, 1002, 854 (s), 757
(s), 709 (s), 684 cmꢀ1 (m). Thermogravimetric data analyses (Fig-
ure S2 in the Supporting Information) revealed a weight loss of
4.86% that occurred within a temperature range of 25–1008C; this
was attributed to the weight loss of two solvated water molecules
(calculated weight loss 5.99%); this observation was in good
agreement with the single crystal structure of L2.
[{Ag3(m-L2)3(CH3CN)(CH3CH2OH)(NO3)}·2NO3·H2O·CH3CN]
/
(CPL2·NO3): This polymer was synthesized by carefully adding a so-
lution of AgNO3 (22.3 mg, 0.13 mmol) in acetonitrile (10 mL) to a so-
lution of L2 (50 mg, 0.08 mmol) DMSO/acetonitrile (1:4; 10 mL).
The resulting solution was kept in the dark for slow evaporation at
room temperature. After two weeks well-formed block-shaped col-
orless crystals were obtained. The crystals were washed in acetoni-
trile and characterized by elemental analysis, PXRD and FT-IR.
Yield: 61.1% (32 mg, 0.017 mmol); elemental analysis calcd (%) for
C72H86Ag3N17O17: C 48.44, H 4.86, N 13.34; found: C 46.24, H 4.18, N
11.69; FT-IR (KBr pellet): n˜ =3275, 3046, 2968, 1651 (s), 1639 (s),
1362 (s), 1223, 1065, 1016, 850, 760 cmꢀ1. Thermogravimetric data
analyses (Figure S5, Supporting Information) revealed a weight loss
of 8.64% that occurred within a temperature range of 50–1298C;
this was attributed to the weight loss of one acetonitrile molecule,
one coordinated and 0.5 uncoordinated water moleucles (accord-
ing to SQUEEZE calculation) and one ethanol molecule (calculated
weight loss 9.98%); this observation was in good agreement with
the single crystal structure of CPL2·NO3.
Characterization data of L3: Yield: 599 mg (76.4%); m.p: 2808C
after crystallization from DMF/MeOH/H2O (1: 2: 2 v/v); elemental
analysis calcd (%) for C30H34N6O5: C 64.5, H 6.13, N 15.04; found: C
1
64.21, H 5.84, N 14.96; H NMR (500 MHz, [D6]DMSO): d=8.96 (s, N-
H, 3H), 8.67 (dd, J=3.5 Hz, 3H), 8.62 (s, 3H), 8.17 (dd, J=8.0 Hz,
3H), 7.46 (q, J=5.0 Hz, 3H), 4.58 (d, J=4.5 Hz, 6H), 2.43 ppm (s,
9H); 13C NMR (500 MHz, [D6]DMSO): d=164.48, 151.66, 148.55,
143.88, 137.03, 135.13, 131.79, 129.85, 123.27, 16.11 ppm; ESI-MS:
m/z calcd for C30H30N6O3: 522.60 [M]+; found: 523.04 [M+H]+,
[{Ag(m-L2)3(CH3CN)}·PF6] (CPL2·PF6): This polymer was synthe-
/
sized by carefully adding a solution of AgPF6 (32.8 mg, 0.13 mmol)
in acetonitrile (10 mL) to a solution of L2 (50 mg, 0.08 mmol) in
DMSO/acetonitrile (1:4; 10 mL). The resulting solution was kept in
the dark for slow evaporation at room temperature. After two
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weeks well-formed block-shaped colorless crystals were obtained.
The crystals were washed in acetonitrile characterized by elemental
analysis, PXRD and FT-IR. Yield: 52.0% (36 mg, 0.022 mmol); ele-
mental analysis calcd (%) for C70H78AgF6N14O6P: C 53.48, H 5.00, N
12.47; found: C 53.28, H 4.97, N 12.86; FT-IR (KBr pellet): n˜ =3285
(s), 3052, 2970, 2256, 1656 (s), 1606, 1527 (s), 1488, 1412, 1370,
1284, 1225, 1066 (m), 1029 (m), 860 (m), 765 (m), 704 (m),
473 cmꢀ1. Thermogravimetric data analyses (Figure S6 in the Sup-
porting Information) revealed a weight loss of 5.94% that occurred
within a temperature range of 30–1008C; this was attributed to
the weight loss of one coordinated acetonitrile molecule (calculat-
ed weight loss 4.78%); this observation was in good agreement
with the single crystal structure of CPL2·PF6.
porting Information) revealed a weight loss of 5.73% that occurred
within a temperature range of 34–708C; this was attributed to the
weight loss of one CH3CN and one water molecules (calculated
weight loss 4.45%); this observation was in good agreement with
the single crystal structure of CPL3·CF3CO2.
Transmission electron microscopy
The TEM were recorded in Jeol instrument using carbon coated
300 mesh Cu grid at 200 KV. Samples for TEM were prepared by
smearing a small amount of gel on a 300 mesh carbon coated
copper grid followed by drying under vacuum. The xerogel thus
coated on the TEM grid was observed under the transmission elec-
tron microscope.
[{Ag2(m-L2)3(H2O)2}·2DMSO·7H2O·2CF3CO2]
(CPL2·CF3CO2): This
/
polymer was synthesized by carefully adding solution of
a
AgCF3CO2 (28.7 mg, 0.13 mmol) in acetonitrile (10 mL) to a solution
of L2 (50 mg, 0.08 mmol) in DMSO/acetonitrile (1:4; 10 mL). The re-
sulting solution was kept in the dark for slow evaporation at room
temperature. After two weeks well-formed block-shaped colorless
crystals were obtained. The crystals were washed in acetonitrile
and characterized by elemental analysis, PXRD and FT-IR. Yield:
50.5% (42 mg, 0.022 mmol); elemental analysis calcd (%) for
C74H102Ag2F6N12O21S2: C 47.04, H 5.44, N 8.90; found: C 46.63, H
5.14, N 8.73. FT-IR (KBr pellet): n˜ =3305 (s), 2960, 2923, 2871, 2256,
1645 (s), 1632 (s), 1535 (s), 1489, 1412, 1382, 1370, 1305, 1206 (m),
1186, 1063, 1065 (m), 845 (m), 802 (m), 671 cmꢀ1. Thermogravimet-
ric data analyses (Figure S7 in the Supporting Information) revealed
a weight loss of 20.69% that occurred within a temperature range
of 40–1158C; this was attributed to the weight loss of two coordi-
nated, seven solvated, three uncoordinated water molecules (ac-
cording to SQUEEZE calculation) and two solvated DMSO mole-
cules (calculated weight loss 19.15%); this observation was in
good agreement with the single crystal structure of CPL2·CF3CO2.
Catalysis
When an aqueous solution (2.0 mL) of sodium 4-nitrophenolate
(0.0125 mm) was placed over a MG bed (1.0 mL, 2–4 wt% in
DMSO/water (2:3 v/v)) containing Ag nanoparticles, 4-nitropheno-
late was reduced to 4-aminophenolate within a few seconds as
could be observed by the yellow solution turning colorless. The
completion of the reaction was monitored by UV/Vis spectroscopy.
In case of the organogel containing Au nanoparticles, an aqueous
solution (2.0 mL) of sodium 4-nitrophenolate (0.0125 mm) was
placed over the gel bed (1.0 mL, 2 wt%, DMSO/water (2:3 v/v))
and the reduction of the nitro derivative took place within a few
seconds as observed by the color change.
Table 4. Crystallographic data for ligands L1–L3.
L1
L2
L3
[{Ag3(m-L3)3·Cl}·2NO3] (CPL3·NO3): This polymer was synthesized
/
formula
Mr
C33H42N6O6
618.73
C33H36N6O5
596.68
C30H32N6O5
556.62
by carefully adding a solution of AgNO3 (19.2 mg, 0.11 mmol) in
acetonitrile (10 mL) to a solution of L3 (40 mg, 0.07 mmol) DMSO/
acetonitrile (1:4; 10 mL). The resulting solution was kept in the
dark for slow evaporation at room temperature. After two weeks
well-formed block-shaped yellow crystals were obtained. The crys-
tals were washed in acetonitrile and characterized by elemental
analysis, PXRD and FT-IR. Yield: 42% (40 mg, 0.032 mmol); elemen-
tal analysis calcd (%) for C60H60Ag3ClN14O12: C 47.15, H 3.96, N
12.83; found: C 48.47, H 4.45, N 11.75; FT-IR (KBr pellet): n˜ =1650
(s), 1637, 1556, 1541 (s), 1523, 1508, 1475 (m), 1419 (m), 1363,
1338, 1294, 702 cmꢀ1. Thermogravimetric data analyses (Figure S8
in the Supporting Information) revealed a weight loss of 2.91%
that occurred within a temperature range of 39–648C; this was at-
tributed to the weight loss of 1.5 lattice occluded water molecules
(calculated weight loss 1.91%); this observation was in good
agreement with the single crystal structure of CPL3·NO3.
crystal size [mm] 0.91ꢂ0.67ꢂ0.05 0.75ꢂ0.38ꢂ0.20 0.50ꢂ0.18ꢂ0.08
crystal system
space group
a [ꢁ]
b [ꢁ]
c [ꢁ]
monoclinic
P21/n
8.902(4)
25.729(12)
14.747(7)
90.00
99.051(16)
90.00
3336(3)
4
triclinic
¯
P1
orthorhombic
P212121
23.4135(7)
24.1817(7)
5.0004(2)
90.00
90.00
90.00
2831.12(16)
4
1.306
12.192(9)
12.408(10)
12.569(11)
115.714(11)
96.623(13)
100.736(16)
1641(2)
2
1.207
632
0.083
100(2)
a [8]
b [8]
g [8]
V [ꢁ3]
Z
1calcd [gcmꢀ3
]
1.232
1320
0.086
100(2)
F(000)
1176
0.091
100(2)
0.0335
m MoKa [mmꢀ1
T [K]
]
Rint
0.0526
0.0757
range of hkl
ꢀ7/7,ꢀ21/21,
ꢀ12/12
1.58/17.09
ꢀ11/11,ꢀ12/12, ꢀ28/28,ꢀ27/29,
[{Ag(m-L3)3}·CH3CN·H2O·CF3CO2]
(CPL3·CF3CO2): This polymer
/
ꢀ11/12
ꢀ6/6
was synthesized by carefully adding a solution of AgCF3CO2
(31 mg, 0.14 mmol) in acetonitrile (10 mL) to a solution of L3
(50 mg, 0.09 mmol) DMSO/acetonitrile (1:4; 10 mL). The resulting
solution was kept in the dark for slow evaporation at room tem-
perature. After two weeks well-formed block-shaped colorless crys-
tals were obtained. The crystals were washed in acetonitrile and
characterized by elemental analysis, PXRD and FT-IR. Yield: 45.9%
(35 mg, 0.043 mmol); elemental analysis calcd (%) for
C34H35AgF3N7O6: C 50.88, H 4.40, N 12.22; found: C 50.06, H 4.35, N
11.82; FT-IR (KBr pellet): n˜ =3838–3274, 3064, 1697 (s), 1683 (s),
1635 (s), 1595, 1521, 1473 (m), 1419,1296 (m), 1203,1174,
1126 cmꢀ1. Thermogravimetric data analyses (Figure S9 in the Sup-
min/max q [8]
1.74/20.34
1.21/25.58
reflns collected/ 13265/1973/1569 13015/1312/1745 43237/3102/2574
unique/observed
[I>2s(I)]
data/restraints/ 1973/0/417
parameters
goodness
of fit on F2
final R indices
[I>2s(I)]
R indices
(all data)
3112/0/400
1.091
3102/0/389
1.048
1.407
R1 =0.0569
wR2 =0.1653
R1 =0.0715
wR2 =0.1842
R1 =0.0561
wR2 =0.1414
R1 =0.1264
wR2 =0.1991
R1 =0.0431
wR2 =0.1254
R1 =0.0587
wR2 =0.1254
Chem. Eur. J. 2014, 20, 1 – 15
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11
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
Table 5. Crystallographic data for coordination polymers.
CPL1·NO3
CPL2·NO3
CPL2·PF6
CPL2·CF3CO2
CPL3·NO3
CPL3·CF3CO2
formula
Mr
crystal size [mm]
crystal system
space group
a [ꢁ]
b [ꢁ]
c [ꢁ]
C35H39AgN8O6
775.61
0.80ꢂ0.20ꢂ0.05
monoclinic
P21
8.7897(9)
13.4841(14)
15.2555(15)
90.00
96.994(3)
90.00
C72H86Ag3N17O17
1785.19
0.67ꢂ0.11ꢂ0.10
triclinic
C70H78Ag2F6N14O6P
1572.17
0.80ꢂ0.20ꢂ0.11
cubic
C74H102Ag2F6N12O21S2 C60H60Ag3ClN14O12
C34H35AgF3N7O6
802.56
0.30ꢂ0.17ꢂ0.16
triclinic
1889.54
0.58ꢂ0.38ꢂ0.14
monoclinic
P21
1528.28
0.16ꢂ0.13ꢂ0.09
hexagonal
R3
¯
P1
¯
¯
P1
Pa3
14.3323(10)
16.4524(11)
18.8507(13)
106.596(2)
90.058(3)
110.645(2)
3960.6(5)
2
19.2258(3)
19.2258(3)
19.2258(3)
90.00
90.00
90.00
7106.46(19)
4
1.469
14.8529(4)
16.4270(4)
18.4606(5)
90.00
15.3847(6)
15.3847(6)
22.8201(9)
90.00
10.6355(9)
12.1360(10)
13.3031(11)
77.235(2)
85.710(2)
87.008(2)
1668.8(2)
2
a [8]
b [8]
95.5010(10)
90.00
90.00
120.00
g [8]
V [ꢁ3]
Z
1794.6(3)
2
1.435
4483.4(2)
2
4677.6(3)
3
1calcd [gcmꢀ3
]
1.497
1.400
1.628
1.597
F(000)
800
0.617
100(2)
0.0492
1828
0.810
100(2)
0.0161
3228
0.651
100(2)
0.1004
1956
0.568
100(2)
0.0540
2316
1.049
100(2)
0.0566
820
0.678
100(2)
0.0388
m MoKa [mmꢀ1
T [K]
]
Rint
range of hkl
ꢀ9/9,ꢀ14/14,ꢀ16/ ꢀ14/14,ꢀ17/17,ꢀ19/ ꢀ24/24,ꢀ24/24,ꢀ23/ ꢀ20/20,ꢀ22/22,ꢀ25/ ꢀ20/15,ꢀ18/18,ꢀ29/ ꢀ15/15,ꢀ17/14,ꢀ18/
16
19
23
24
29
19
min/max q [8]
2.02/22.72
1.13/21.84
1.83/27.45
94626/2709/1836
1.11/29.89
96795/23195/17047 21586/2409/1047
1.77/27.58
1.57/31.22
25285/9616/7752
reflns collected/unique/ 13849/4761/4092 40875/9451/6735
observed [I>2s(I)]
data/restraints/parame- 4761/1/455
ters
9451/0/973
2709/0/152
23195/1/1056
2409/1/273
9616/0/462
goodness of fit on F2
1.187
1.277
1.372
1.187
1.653
1.182
final R indices [I>2s(I)] R1 =0.0507
wR2 =0.1340
R1 =0.0673
wR2 =0.1838
R1 =0.0965
wR2 =0.1943
R1 =0.0501
wR2 =0.1582
R1 =0.0892
wR2 =0.2010
R1 =0.0633
wR2 =0.1690
R1 =0.0889
wR2 =0.1808
R1 =0.0828
wR2 =0.2128
R1 =0.0937
wR2 =0.2179
R1 =0.0566
wR2 =0.1533
R1 =0.0738
wR2 =0.1723
R indices (all data)
R1 =0.0610
wR2 =0.1444
Rheology
Powder X-ray diffraction
The rheological response of the freshly prepared gels and metallo-
gels were studied using a controlled stress rheometer (Anton Parr
MCR-102, Anton-Parr Instruments Ltd, New Castle, DE, Austria).
PXRD data were collected using Bruker AXS D8 Advance powder
(CuKa1 radiation, l=1.5406 ꢁ) diffractometer equipped with super
speed LYNXEYE detector. The sample was prepared by making
a thin film of finely powdered sample (~30 mg) over a glass slide.
The experiment was carried out with a scan speed of 0.3 sec/step
(step size=0.028) for the scan range of 5–358 2q.
Acknowledgements
Single-crystal X-ray crystallography
P.D. thanks Department of Science & Technology (DST), New
Delhi, India for financial support. M.P. thanks CSIR, New Delhi
(Grant No. 09/080(0693)/2010-EMR-I) for SRF. SXRD data were
collected at the DBT-funded X-ray Diffraction Facility under the
CEIB program in the Department of Organic Chemistry, IACS,
Kolkata.
Single-crystal X-ray diffraction data were collected using MoKa (l=
0.7107 ꢁ) radiation on a BRUKER APEX II diffractometer equipped
with CCD area detector. Data collection, data reduction, structure
solution/refinement were carried out using the software package
of APEX II. All the structures (L1, L2, L3, CPL1·NO3, CPL2·NO3,
CPL2PF6, CPL2·CF3CO2, CPL3·NO3 and CPL3·CF3CO2) were solved
by direct methods and refined in a routine manner. In all the cases,
nonhydrogen atoms were treated anisotropically. Whenever possi-
ble, the hydrogen atoms were located on a difference Fourier map
and refined. In other cases, the hydrogen atoms were geometrical-
ly fixed. Details of the crystallographic measurements are given in
Tables 4 and 5. CCDC 1016281 (L1), 1016282 (L2), 1016283 (L3),
1016284 (CPL1·NO3), 1016285 (CPL2·NO3), 1016286 (CPL2·PF6),
1016287 (CPL2·CF3CO2), 1016288 (CPL3·NO3), and 1016289
(CPL3·CF3CO2) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif (or from the Cambridge Crys-
tallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK;
fax: (+44) 1223–336–033; or deposit@ccdc.cam.ac.uk.
Keywords: coordination polymers
·
gels homogeneous
·
catalysis
·
nanoparticles
·
metallogels · supramolecular
chemistry
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Received: August 22, 2014
Published online on && &&, 0000
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Chem. Eur. J. 2014, 20, 1 – 15
www.chemeurj.org
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FULL PAPER
&
Metallogels
M. Paul, K. Sarkar, P. Dastidar*
&& – &&
Metallogels Derived from Silver
Coordination Polymers of C3-
Symmetric Tris(pyridylamide) Tripodal
Ligands: Synthesis of Ag
Gelling silver together: A series of co-
ordination-polymer-based metallogels
were synthesized following a crystal
engineering rationale. In situ generated
silver nanoparticles within the metallo-
gels were found to be catalytically
active.
Nanoparticles and Catalysis
Chem. Eur. J. 2014, 20, 1 – 15
www.chemeurj.org
15
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ