A metal-organic framework gel with Cd2+ derived from only coordination bonds without intermolecular interactions and its catalytic ability

Article abstract of DOI:10.1039/c3nj00164d

A cyclohexane-based ligand (1) as a triconnected linker forms a supramolecular gel in the presence of transition metal ions, particularly Cd²⁺ and Zn²⁺. 1 can also be gelated with cadmium counter ions such as NO₃^-

Full text of DOI:10.1039/c3nj00164d

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A metal–organic framework gel with Cd2+ derived
from only coordination bonds without intermolecular
interactions and its catalytic ability†
Cite this: NewJ.Chem., 2013,
37, 2330
Hyo Hee Lee,z Sung Ho Jung,z Sunhong Park, Ki-Min Park and Jong Hwa Jung*
A cyclohexane-based ligand (1) as a triconnected linker forms a supramolecular gel in the presence of
transition metal ions, particularly Cd2+ and Zn2+. 1 can also be gelated with cadmium counter ions such
ꢀ
as NO3ꢀ, ClO4 and SO42ꢀ. The gels have been characterized by SEM, TEM, rheometry and single crystal
Received (in Montpellier, France)
11th February 2013,
X-ray crystallography. The Cd2+–cyclohexane-based ligand gel shows a fibrillar structure with diameters
ranging from 20–50 nm. X-ray analysis revealed that 1 forms a 1D coordination polymer structure with
the Cd2+ ion, with an octahedral structure. The rheological properties of the Cd2+–cyclohexane-based
ligand gel were strongly dependent on the concentration of Cd2+. Furthermore, the xerogel 1 with Cd2+
acted as a base catalyst in the Knoevenagel condensation reaction.
Accepted 2nd May 2013
DOI: 10.1039/c3nj00164d
www.rsc.org/njc
sites.33 The amide group is a versatile functional group because it
can participate in two types of intermolecular hydrogen-bonding
Introduction
Organogels and hydrogels composed of small molecules or ‘low
molecular weight gelators’ (LMWGs) which are linked into
fibers by supramolecular interactions are current topics of great
interest. Supramolecular gels have been studied as soft materials
for use in applications such as drug-delivery systems, tissue
engineering, sensing devices, separation and optoelectronic
devices.1–22 The incorporation of labile metals or anions within
these supramolecular gels is of particular interest because
metallogels and coordination polymer gels with metal–organic
framework structures are applicable in various fields such as
redox responsiveness,23,24 catalytic action,25,26 absorption,27,28
emission,29–32 magnetism28 and electron emission.26 In addition,
the gels composed of a metal–organic framework (MOF) structure
can be rapidly, efficiently and easily prepared under mild conditions
in comparison to a typical crystalline MOF, which would display
similar properties. In general, the organic ligands form the gel in the
presence of metal ions by the intermolecular interactions such as
H-bonds, p–p stacking and van der Waals interactions.23–32 Metal–
organic framework gels (MOFGs) with amide moieties are candidate
compounds that have functional organic sites as guest interaction
interactions: the –NH moiety acts as an electron acceptor and the
–CQO group acts as an electron donor.34 These multifunctional
moieties of amide groups tend to form hydrogen bonds among
themselves and interact negligibly with guest molecules after
formation of MOFGs. If MOFGs could be prepared without forming
amide–amide interactions, these groups would constitute attractive
interaction sites for selective sorption, reaction and/or catalysis on
the surface of the gel. To retain amide groups as guest-interaction
functional organic sites in the network, we employed a three-
connector ligand, 1,3,5-cyclohexane tricarboxylic acid tris[N-(4-
pyridyl)amide], containing three pyridyl groups as coordination
sites and three amide groups as guest interaction sites. Herein,
we describe an approach for the preparation of a MOFG with
Cd2+ using base-type functional organic sites and demonstrate
its base catalytic properties in a heterogeneous reaction. The
coordination structure of a MOFG with Cd2+ was proven by
single X-ray crystallography. Furthermore, the rheology of a
MOFG with Cd2+ exhibited the behavior of a typical organogel.
Results and discussion
Department of Chemistry and Research Institute of Natural Science, Gyeongsang
National University, Jinju 660-701, Korea. E-mail: jonghwa@gnu.ac.kr
† Electronic supplementary information (ESI) available: Details of the experi-
mental section, SEM and TEM images and measurement data. CCDC 922449. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3nj00164d
Ligand 1 was conveniently prepared in a one pot synthesis
according to methods described previously (Scheme 1). 1,3,5-
Cyclohexane tricarboxytrichloride was reacted with 4-amino-
pyridine in the presence of triethylamine in THF to yield 1, as
1
confirmed by H, 13C NMR, mass spectroscopy and elemental
‡ These authors contributed equally.
analysis.
c
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Scheme 1 Synthetic route of ligand 1.
Fig. 2 Photographs of 1 with; (a) CdSO4, (b) Cd(NO3)2, (c) Cd(ClO4)2, (d) CdBr2,
Table 1 Gelation test of ligand 1 with metal ions (3.0 equiv.) in organic solvents
(e) CdCl2 and (f) CdI2 in H2O.
and water
Entry
Solvent
ZnSO4
CdSO4
1
2
3
4
3
6
7
8
MeOH
DCM
TKF
Toluene
AN
EA
CHC13
Ethanol
H2O
DMF
DMSO
H2O: DMSO
P
P
P
P
P
P
P
P
G
S
P
P
P
P
P
P
P
P
G
S
9
10
11
12
Fig. 3 (a) SEM and (b) TEM image of Cd2+ coordination polymer gel 1.
S
P
S
P
Note: P: precipitate, S: solution, G: stable gel.
micrometers in length (Fig. 3 and Fig. S1, ESI†). The SEM image
of Cd2+ coordination polymer gel 1 using different anions
showed a similar fiber structure with diameters of 10–20 nm.
The gelation abilities of 1 were tested with other cations
such as Zn2+, Co2+ and Cu2+ under the same conditions. Upon
the addition of Zn2+ the gel formation was induced (Fig. S2,
ESI†), whereas upon addition of Co2+ and Cu2+ no gel formation
occurred. It is known that Cd2+ and Zn2+ ions have octahedral
structures with ligands. Therefore, the octahedral coordination
polymer structure of 1 with metal ions could be gelatized in a
solvent. These findings suggest that the gel formation of 1 is
influenced by the coordination structure of the cations. We
examined the absorption and fluorescence emission properties
of sol 1 and coordination polymer gel 1 with metal ions. The
UV-vis absorption bands of Cd2+ coordination polymer gel 1
and sol 1 appeared at 262 nm (Fig. S3, ESI†), indicating that the
gelator 1 in the gel state did not undergo the p–p* transition.
The coordination polymer gel 1 with Cd2+ exhibited no emission
(Fig. S3, ESI†). Therefore, the formation of the coordination
polymer gel 1 originated from the coordination bond between
the gelator 1 and Cd2+, but was not due to intermolecular
interactions such as p–p stacking, H-bonds, and van der Waals
forces.
A coordination polymer gel based on 1 was prepared by
dissolving 1% (by weight) of 1 in water. To this solution a small
volume of metal ions in water was added in concentrations
varying from 1.0–4.0 equivalents with respect to the ligand
concentration. The samples were then left to stand for a week.
Table 1 and Table S1 in ESI† (abbreviated as S), summarize the
results of a gelation test of 1 in the presence of transition metal
ions (2.0 equivalents) with water. Opaque gel 1 was obtained
with Cd2+ and Zn2+ ions in water (Fig. 1). In addition, 1 can also
be gelated with a variety of cadmium counter ions such as
NO3ꢀ,ClO4ꢀ and SO42ꢀ, but 1 could not gelate when the counter
ion was Clꢀ, Brꢀ and Iꢀ (Fig. 2), because the Hofmeister effects
2ꢀ
of NO3ꢀ, ClO4ꢀ and SO4 are much weaker than those of Clꢀ,
Brꢀ and Iꢀ. The weak Hofmeister effect of anions did not
induce gelation, due to high solubility in water.
The morphology of xerogels obtained with different cations
and anions was investigated by scanning electron microscopy
(SEM) and transmission electron microscopy (TEM). The SEM
images of Cd2+ coordination polymer gel 1 clearly displayed a
fibrillar structure with diameters of 10–20 nm and several
To better understand the molecular structure of gel 1 with
Cd2+, we prepared a single crystal of 1 with Cd2+ by reacting 1
with cadmium sulfate in a dichloromethane–methanol mixture
in the presence of a small amount of HCl, which yielded a
colorless precipitate. Vapor diffusion of diethyl ether into a
DMF solution of this complex gave crystalline 1a (Tables S2 and
S3, ESI†). X-Ray analysis revealed that 1a is a 1D coordination
polymer with the formula {[Cd(H1)2(SO4)2]ꢁDMFꢁ4H2O}n as
shown in Fig. 4. The Cd(II) center is octahedrally coordinated
to the pyridyl nitrogen atoms from different bridging (1) and two
Fig. 1 Photographs of 1 with (a) CdSO4, (b) ZnSO4, (c) CuSO4, (d) CoSO4,
(e) NiSO4 and (f) MnSO4 in H2O.
c
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To garner an insight into the thermally promoted stability of
Cd2+ coordination polymer gel 1, the transition temperature
(Tsol–gel) of coordination polymer gel 1 was measured using
differential scanning calorimetry (DSC) (Fig. S4, ESI†). Cd2+
coordination polymer gel 1 showed a sharp phase transition at
115 1C, undergoing an endothermic reaction. This endothermic
thermogram is due to the transition of the Cd2+ coordination
polymer gel 1 into a solution phase (Fig. S4, ESI†).
Rheological information is an indicator of the behavior of
the gels when they are exposed to mechanical stress, especially
the ‘‘storage’’ (or ‘‘elastic’’) modulus G0, which represents the
ability of the deformed material to ‘‘snap back’’ to its original
geometry, and the ‘‘loss’’ (or ‘‘viscous’’) modulus G00, which
represents the tendency of a material to flow under stress. Two
rheological criteria required for a gel are: (i) the independence
of the dynamic elastic modulus, G0, with respect to the oscillatory
frequency, and (ii) G0 must exceed the loss modulus G00 by about 1
order of magnitude. We first used dynamic strain sweep to
determine the proper conditions for the dynamic frequency sweep
of the gel at different concentrations of Cd2+.
Fig. 4 Single crystal structure of 1a ({[Cd(H1)2(SO4)2]ꢁDMFꢁ4H2O}n).
As shown in Fig. 6a, the values of the storage modulus (G0)
and the loss modulus (G00) exhibited a weak dependence from
0.1 to 1.0% of strain (with G0 dominating G0 0), indicating that
the sample is a gel. The values of both G0 and G00 of the gel in
the presence of 3.0 equivalents of Cd2+ dramatically increased
in comparison to the gels in the presence of only 2.0 equivalent
of Cd2+. These results reflect that the gel was stabilized with an
almost complete coordination polymer structure in the
Fig. 5 Powder X-ray diffraction patterns of (a) crystal 1 with CdSO4 and xerogel
1 with (b) CdSO4 and (c) Cd(NO3)2.
oxygen atoms from sulfate anions by twofold rotation symmetry presence of 3.0 equivalents of Cd2+
.
(C2 axis) through Cd(II). In addition, an acidic proton protonates
N4 in the presence of a small amount of HCl as solvent.
We used dynamic frequency sweep to study the gel after
setting the strain amplitude at 0.08% (within the linear
Wide-angle X-ray diffraction (WAXD) was used to probe the response region of the strain amplitude). G0 and G00 were
molecular packing of gel 1 with the cadmium ion. Powder XRD almost constant with the increase of frequency from 0.1 to
patterns of ligand 1 with CdSO4 in the bulk crystal had the 100 rad sꢀ1 (Fig. 6b). The value of G0 was about 10 times larger
same diffraction pattern as that of the xerogel 1 with CdSO4. than that of G0 0 over the whole range (0.1–100 rad sꢀ1),
Furthermore, the strong similarities between the powder XRD suggesting that the gel is fairly tolerant to external force. As
patterns of the xerogel 1 wꢀith CdSO4 and those with other observed by changes in dynamic strain sweep, the values of
counter ions such as ClO4 and NO3 2ꢀsuggested that the both G0 and G00 of the gel at values above 3.0 equivalents of Cd2+
ꢀ
molecular packing of the gel 1 with SO4 as the counter ion concentration were larger than that of the gel in the presence of
was the same as the molecular packing of the gel 1 with other only 2.0 equivalent of Cd2+. Furthermore, time-dependent
counter ions (Fig. 5).
oscillation measurements were used to monitor the gelation
Fig. 6 (a) Strain sweep at a frequency of 1 rad sꢀ1 of the Cd2+ coordination polymer gel 1. (b) Frequency sweep of G0 and G0 0 for the Cd2+ coordination polymer gel 1
at a strain of 0.01%. (c) Time sweep of G0 and G0 0 for the Cd2+ coordination polymer gel 1 at a strain of 0.01% and a frequency of 1 rad sꢀ1 (2 equiv.: G0:
3 equiv.: G0:E, G0 0: K).
}
, G0 0: J,
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process of gel 1 with Cd2+ (Fig. 6c). The time sweep shows the monochromated Cu K radiation (40 kV, 30 mA) and an imaging
rapid increase of G0 and G0 0 in the initial stage of gelation, plate (R-AxisIV).
followed by a slower long term approach to a final pseudo-
Preparation for TEM and SEM observations. For transmission
equilibrium plateau. At the end of the experiment, the value of electron microscopy (TEM), a piece of the gel was placed on a
G0 was about an order of magnitude higher than G00. All values carbon-coated copper grid (400 mesh) and removed after one min,
of G0 and G0 0 in the coordination polymer gel are strongly leaving some small patches of the sample on the grid. The speci-
influenced by the concentration of 1 with Cd2+.
mens were examined using a JEOL JEM-2010 transmission electron
To characterize the base-type properties of xerogel 1, we microscope operating at 200 kV using an accelerating voltage of
performed a Knoevenagel condensation reaction catalyzed by 100 kV and a 16 mm working distance. Scanning electron micro-
xerogel 1. The Knoevenagel reaction is well known, not only as a graphs of the samples were taken with a field emission scanning
weak base-catalyzed model reaction but also as a reaction that electron microscope (FE-SEM, Philips XL30 S FEG). The accelerating
generates a C–C bond. Knoevenagel condensation reactions of voltage for SEM was 5–15 kV and the emission current was 10 mA.
benzaldehyde with each of the active methylene compounds
DSC measurement. Differential scanning calorimetry (DSC)
(malononitrile, ethyl cyanoacetate and cyano-acetic acid tert- was performed on a Seiko DSC6100 high-sensitivity differential
butyl ester) were catalyzed by xerogel 1. The conversion yields of scanning calorimeter equipped with a liquid nitrogen cooling unit.
the respective adducts were obtained as follows: 80% for Samples of gels were hermetically sealed in a silver pan and
malononitrile, 73% for ethyl cyanoacetate and 75% for cyano- measured against a pan containing alumina as the reference. The
acetic acid tert-butyl ester. The results indicate that the reaction thermograms were recorded at a heating rate of 0.5 1C minꢀ1
mainly occurred on the surface of xerogel 1. The heterogeneity
.
Rheological measurements. The rheological properties were
and recyclability of xerogel 1 in the Knoevenagel condensation observed on freshly prepared gels using a controlled stress
of benzaldehyde and malononitrile were also examined. The rheometer (AR-2000ex, TA Instruments Ltd, New Castle, DE,
catalyst ability of xerogel 1 showed good recyclability. In addition, USA). Cone type geometry of 40 mm diameter was employed
xerogel 1 is easily isolated from the reaction suspension by filtration throughout. Dynamic oscillatory work maintained a frequency
alone and can be reused without loss of activity.
of 1.0 rad sꢀ1. The following tests were performed: increasing
the amplitude of oscillation up to 100% apparent strain on
shear, time and frequency sweeps at 25 1C (60 min and from
0.1–100 rad sꢀ1, respectively), and a heating run up to 90 1C at a
Conclusions
We have reported the formation of a simple MOF-based gel scan rate of 1.0 1C minꢀ1. Unidirectional shear routines were
with Cd2+. A cyclohexane-based ligand as a triconnected linker performed at 25 1C covering a shear-rate regime between 10ꢀ1
efficiently produced the hydrogel by simple mixing with Cd2+ and and 103 sꢀ1. Mechanical spectroscopy routines were completed
Zn2+. The ligand 1 forms a coordination polymer structure based with transient measurements. In doing so, the desired stress
on the octahedral Cd2+ center. The rheological properties of the was applied instantaneously to the sample and the angular
Cd2+-cyclohexane-based ligand gel were dependent on the concen- displacement was monitored for 60 min (retardation curve).
tration of Cd2+. Furthermore, the amide groups of gel 1 acted as a
Photophysical studies. UV-Vis absorption and emission
base-catalyst in the Knovenagel condensation reaction. This spectra of the gels were observed at room temperature and in
research is particularly relevant in the context of MOF-type gel the range of 200–800 nm. The absorption properties of gel 1
chemistry in the generation of new materials with unusual proper- with Cd2+ were studied extensively. UV-vis absorption and
ties. The present results also suggested that the combination of fluorescence spectra of gel 1 (0.01 mM) were observed in the
metal ions and a designed ligand can provide a MOF-type gel with presence of Cd2+ (1.0–4.0 equiv.).
functional organic sites that show base-catalytic performance.
Gelation test. Gels were prepared by dissolving one percent
by weight of 1 in water. To this solution a small volume of metal
ions in organic solvents was added in varying concentrations
from 1.0 to 4.0 equivalents with respect to the ligand concen-
tration. The volume of the metal salt solution was typically
Experimental
General
All reagents were purchased from Aldrich and Tokyo Kasei around 1–5 wt% of the total solution. The samples were then
Chemicals and used without further purification. 1H and 13C left to stand, typically for a week. The formation of the gel states
NMR spectra were measured using a Bruker ARX 300 apparatus. IR was found to depend on the concentration of metal salt added.
spectra were obtained for KBr pellets, in the range 400–4000 cmꢀ1
,
Knoevengael condensation reaction catalyzed by MOFG with
with a Shimadzu FT-IR 8400S instrument, fluorescence spectra Cd2+. A solution of benzaldehyde (0.21 mL, 2.1 mmol) and
were recorded on a RF-5301PC spectrophotometer and mass malononitrile (or ethylcyanoacetate or cyano-acetic acid tert-
spectra were obtained using a JEOL JMS-700 mass spectrometer. butyl ester) (0.132 g, 2.0 mmol) in CHCl3 (1 mL) was stirred for
The optical absorption spectra of the samples were obtained at 5 min. Then MOFG 1 (0.10 g, 0.08 mmol, 4 mol%) was added.
25 1C using a UV-vis spectrophotometer (Hitachi U-2900). In The suspension was stirred at room temperature for 12 h. The
1
addition, X-ray diffraction patterns of MOFG and crystal sample progress of the reaction was monitored by H NMR (CDCl3) or
were measured using a Rigaku diffractometer (Type4037) gas chromatography. The catalyst was removed by filtration,
using graded d space elliptical side-by-side multilayer optics, washed with benzene, and recovered.
c
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X-ray crystallography. Data were collected on a Bruker Smart
Paper
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APEX2 ULTRA diffractometer using a graphite monochromated
Mo Ka (l = 0.71 073 Å) radiation source at 173(2) K. The frame
data was processed to give structure factors using SAINT-plus.35
The structure was solved by direct methods and refined by full-
matrix least-squares methods on F2 for data using SHELXTL
software.36 The crystal of 1a appears to contain a certain
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crystal lattice in a severely disordered manner. The solvent
could not be reliably modeled and therefore the SQUEEZE
technique37 was used to subtract its contribution from the
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crystal lattice.
Preparation of ligand 1. A solution of 4-aminopyridine
(17.0 g, 18.0 mmol) and distilled triethylamine (24 mL,
18.4 mmol) in distilled THF (200 mL) was added dropwise to
a solution of 1,3,5-cyclohexane tricarboxytrichloride (10 g,
18 mmol) in THF (60 mL) at 0 1C. The reaction mixture was
stirred for 12 h. The temperature was allowed to step up to
room temperature. The brown crude product 1 was collected by
filtration and washed with THF. The product was recrystallized
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under vacuum for 1 day at room temperature (21.4 g, 30%). Mp:
327 1C. 1H NMR (300 MHz, MeOD-d4): 8.4–8.39 (s, CH), 7.69
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(s, CH), 2.66 (s, CH2), 2.2–2.15 (m, CH2), 1.9–1.83 (m, CH2); 13
C
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NMR (300 MHz, DMSO-d6): 174.4, 150.7, 146.15, 113.7, 44.0,
31.1 ppm; IR (KBr, cmꢀ1): 1669, 1591, 1506, 1413, 1381, 1328,
1302, 1250, 1210, 1169, 1001, 855, 821 cmꢀ1; MS (ESI) m/z
445.17 (M+H)+ calcd for C24H24N6O3: 444.19; anal. calcd for
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C
24H24N6O3: C 64.85, H 5.44, N 18.91%; found: C 64.12, H 5.58,
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This work was supported by a grant from the World Class
25 T. Tu, W. Assenmacher, H. Peterlik, R. Weisbarth, M. Nieger
Project (WCU) supported by Ministry of Education Science and
Technology (R32-2008-000-20003-0), and NRF (2012R1A4A1027750
and 2012-002547), and the Environmental-Fusion Project (191-091-
004), Korea. In addition, this work was partially supported by a grant
from the Next-Generation BioGreen 21 Program (SSAC, grant#:
PJ009041022012), Rural development Administration, Korea.
¨
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