A 2D-layered Cd(II) MOF as an efficient heterogeneous catalyst for the Knoevenagel reaction

Article abstract of DOI:10.1016/j.jssc.2020.121846

A Cd(II) coordination polymer based on a polytopic compartmental ligand was synthesized, and used as an efficient heterogeneous catalyst for the Knoevenagel reaction between benzaldehyde and malononitrile under mild reaction conditions. The solid catalyst

Full text of DOI:10.1016/j.jssc.2020.121846

Journal of Solid State Chemistry xxx (xxxx) xxx
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
A 2D-layered Cd(II) MOF as an efficient heterogeneous catalyst for the
Knoevenagel reaction
,
*
Lincy Tom a, M.R.P. Kurup b
a Department of Applied Chemistry, Cochin University of Science and Technology, Kochi, 682 022, Kerala, India
b Department of Chemistry, School of Physical Sciences, Central University of Kerala, Tejaswini Hills, Periye, Kasaragod, 671 320, Kerala, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
2D MOF
Polytopic compartmental ligand
Catalysis
Heterogeneous
Knoevenagel reaction
A Cd(II) coordination polymer based on a polytopic compartmental ligand was synthesized, and used as an
efficient heterogeneous catalyst for the Knoevenagel reaction between benzaldehyde and malononitrile under
mild reaction conditions. The solid catalyst was characterized using single crystal XRD, X-ray powder diffraction,
SEM, TGA, UV diffuse reflectance, infrared spectroscopy and elemental analysis. The compound is a two-
dimensional (2D) MOF with a grid structure. Topological analysis of the framework revealed that it is a 2,4-con-
nected binodal net. The catalytic activity was tested between various benzaldehydes containing different sub-
stituents with malononitrile. The effect of reaction parameters such as solvent, time, reactant ratio and catalyst
amount was investigated. Furthermore, the catalyst stability was examined through reusability experiments and it
is observed that the catalyst can be recycled at least five times without significant drop in its activity.
1. Introduction
enhancing the catalytic activities [11]. Compared to ditopic linkers based
MOFs the polytopic compartmental linkers-based MOFs can more func-
Porous coordination polymers or metal organic frameworks (MOFs)
are built from multi-functionalized organic molecules that are bound
together by inorganic units. The advantageous features of these materials
has caught increasing attention in recent years and turns out to be one of
the fastest growing areas in synthetic chemistry and materials science.
Compared to conventionally used porous inorganic materials, these
metal-organic structures exhibit the potential for more flexible rational
design, by controlling the size and the functionalization of the organic
linkers and nodes [1,2]. Because of their unparalleled structural di-
versities, highly-ordered porous structure, high specific surface area and
great structural designability, they have wide range of applications in the
growing fields of molecular storage, drug delivery, sensing and catalysis
[3–9].
The design and synthesis of MOFs with desired properties are of
immense interest in heterogeneous catalysis [10]. As is known, the
architectural diversities and pore surface properties of MOFs greatly
depend on reactivity and flexibility of linkers and coordination modes of
metal nodes, the designing of ligands with suitable functional groups is a
key strategy for the construction of MOFs with required catalytic prop-
erties. The presence of multiple catalytic sites with appropriate coordi-
nation environments for metal ions also play a significant role in
tionalize as heterogeneous catalysts due to the presence of multiple
catalytic centers. Double hydrazone ligands, due to their specific geom-
etry, including the different relative orientation of N-donors and the
zigzag conformation of the spacer moiety (–CR¼N–N¼CR–) between the
two terminal coordination groups, favors polymerization with novel
network patterns not achievable by other rigid linking ligands, such as
bipyridine and dicarboxylate ligands. All the known MOF linkers bind
metal ion in a monodentate or bidentate coordination mode and a tet-
radentate spacer is never involved in metal coordination environment.
The terminal nitrogens of these polytopic linkers have been shown to
bind exodentate to metal nodes and the compartmental atoms lock by
endodentate binding (Fig. 1).
As one of the most promising inorganic-organic hybrid materials for
catalysis, MOFs function as solid catalysts or catalyst supports for several
organic transformations such as Knoevenagel reaction, aldol condensa-
tion, oxidation, hydrogenation, Paal–Knorr reactions Suzuki cross-
coupling, transesterification reaction, Friedel-Crafts alkylation, and
epoxide ring-opening reaction [12–24]. Knoevenagel reaction is one of
the most versatile reaction in organic chemistry and is widely employed
for the condensation of aldehydes with active methylene compounds
with numerous applications in the synthesis of useful chemicals as well as
* Corresponding author.
E-mail address: mrp@cukerala.ac.in (M.R.P. Kurup).
https://doi.org/10.1016/j.jssc.2020.121846
Received 30 September 2020; Received in revised form 3 November 2020; Accepted 3 November 2020
Available online xxxx
0022-4596/© 2020 Elsevier Inc. All rights reserved.
Please cite this article as: L. Tom, M.R.P. Kurup, A 2D-layered Cd(II) MOF as an efficient heterogeneous catalyst for the Knoevenagel reaction, Journal
of Solid State Chemistry, https://doi.org/10.1016/j.jssc.2020.121846

L. Tom, M.R.P. Kurup
Journal of Solid State Chemistry xxx (xxxx) xxx
Fig. 1. Schematic representation of the design and construction of CP-Cd. (a) Structural formula of H2DDIH and binding sites, (b) pictorial representation of the ligand and (c)
self-assembly formation of CP-Cd.
heterocyclic compounds of biological significance [25,26]. The reaction
is generally catalyzed by weak soluble bases in super-stoichiometric
amounts. To address the problems of recyclability and high loading
common with homogeneous base catalysts, efficient and recyclable solid
base catalyst systems such as amine-functionalized mesoporous zirconia,
cation-exchanged zeolites, chitosan hydrogel, amine-functionalized
superparamagnetic nanoparticles, etc. have been utilized for these reac-
tion [27–30]. However, most of these currently used methods suffer from
one disadvantage or another which include, high-power microwaves,
tedious preparation procedures, high reaction temperature as well as the
use of toxic metals. Therefore, further development of new types of
catalysts based on cheap and environmentally tolerable metal complexes
that could be easily recyclable and could show high efficiency under mild
conditions is desirable.
Calcd for H2DDIH: C, 69.63; H, 4.49; N, 18.74%. Found: C, 69.92; H,
4.68; N, 18.93%.
2.3. Synthesis of [Cd(DDIH)2H2O]n (CP–Cd)
A solid mixture of cadmium acetate dihydrate, Cd(OAc)2⋅2H2O,
(0.266 g, 1 mmol) and, 12-diphenylethane-1,2-dione bisisonicotinylhy-
drazone (H2DDIH) (0.896, 2 mmol) was dissolved in 20 mL of DMF/THF
(3:1 v/v) mixture. The solution was then heated to 80 ꢁC for 45 min and
cooled to room temperature. The mother liquor was kept for slow
evaporation. After 5 days orange crystals were collected with a yield of
26% (0.156 g). Anal. Calcd. for C26H20CdN6O3: C, 54.13; H, 3.49; N,
14.57%. Found: C, 53.91; H, 3.62; N, 14.27%. IR spectrum (cmꢀ1): 3442
s, 1625 m, 1607 m, 1562 m, 1382 s, 1233 w, 1045 w, 710 m.
In the present paper, we wish to report the application of a Cd(II)
MOF from a flexible dihydrazone linker as an efficient heterogeneous
catalyst for the room temperature liquid phase Knoevenagel reaction.
Detailed characterization of the MOF using various analytical methods,
crystal structure, TG, PXRD, SEM etc. has also been performed. The
catalyst stability was also ascertained by performing leaching and reus-
ability experiments and a plausible mechanism has been proposed.
2.4. X-ray crystallography
Crystallographic data were collected with a Bruker SMART APEX II
diffractometer with graphite monochromated Mo-K
α (λ ¼ 0.71073 Å) X-
ray source. The unit cell dimensions were measured and the data col-
lections were performed at 296 K. Bruker SMART software was used for
data acquisition and Bruker SAINT Software for data integration [31].
Absorption corrections were carried out using SADABS based on Laue
symmetry using equivalent reflections [32]. The structure was solved by
direct methods and refined by full-matrix least-squares on F [2],
including all reflections with the SHELXL-2018/3 software package [33].
All non-hydrogen atoms were refined with anisotropic displacement
parameters and positions of hydrogen atoms were located in the differ-
ence Fourier maps and were placed in calculated positions with isotropic
displacement parameters set to 1.2Ueq of the attached atom (1.5 for Me).
The studied crystal was refined as a two-component twin and the twin
data refinement was subsequently carried out with a scale factor of BASF
¼ 0.2927. The site-occupancy factors [0.501 (6)/0.499 (6)] for the
disordered atoms were refined using one common parameter. Their
displacement parameters were refined using SIMU and DELU restraints
and EADP instruction was used for the dummy atoms constrains. The
possibility of spurious bonds was eliminated by PART instructions. All
the drawings were made using, Diamond 3.2 k and Mercury 3.10.2 [34,
35]. The crystallographic data, refinement parameters, bond lengths and
angles are summarized in Tables S1 and S2.
2. Experimental
2.1. Materials and methods
All reagents and solvents were purchased from commercial sources
and were used without further purification. CHN analyses were done
using an Elementar Vario EL III elemental analyzer. IR spectra were
recorded using a KBr pellet method on a JASCO FT-IR-5300 Spectrometer
in the 4000-400 cmꢀ1 region. TGA analysis was carried out using a
PerkinElmer STA6000/8000 under flowing nitrogen from 35 to 400 ꢁC at
a heating rate of 10 ꢁC/min. Powder X-ray diffraction (PXRD) data were
collected using a Bruker D8 Advance diffractometer system with Cu-K
α
radiation. BET experiment was carried out using a Belsorp-mini II in-
strument (BEL Sorp II, Japan). Before the sorption experiment, sample
was activated at 100 ꢁC overnight. Solid state UV–Vis spectra were
recorded using an Ocean Optics USB-4000 spectrometer and BaSO4 as a
reference. Scanning Electron Microscopy (SEM) micrographs were
recorded using a JEOL Model JSM-6390LV instrument. Gas chromatog-
raphy was conducted using an Agilent 7890-GC instrument using a flame
ionization detector.
2.5. Catalytic studies
2.2. Synthesis of H2DDIH (1,2-diphenylethane-1,2-dione
bisisonicotinylhydrazone)
The condensation reaction of benzaldehyde derivatives and malono-
nitrile was performed in a 10 mL beaker equipped with a magnetic
stirrer. For each reaction, a benzaldehyde derivative (1.0 mmol), malo-
nonitrile (1.1 mmol), MeOH (5 mL), and a MOF catalyst (3 mol%) were
placed in the 10 mL beaker. Then, the reaction mixture was stirred with a
magnetic stirring bar at 400 rpm at room temperature. The reaction was
monitored periodically by analyzing the sample by GC until completion
of the reaction. The percentage conversion and selectivity were
Isonicotinic acid hydrazide (0.274 g, 2 mmol) was added to 20 mL
DMF solution of 1,2-diphenylethane-1,2-dione (0.420 g, 1 mmol). The
mixture was heated under stirring for 2 h at 60 ꢁC. The white crystalline
product obtained after slow evaporation of the solution was washed and
dried in vacuum. C26H20N6O2 (H2DDIH); Yield ¼ 54% (0.246 g). Anal.
2

L. Tom, M.R.P. Kurup
Journal of Solid State Chemistry xxx (xxxx) xxx
3.2. Characterization and stability study
Experimental PXRD pattern of CP-Cd corroborated well with the
simulated single crystal X-ray (SXRD) data (Fig. 6a). The sharp peak
below 10ꢁ (with 2θ of 8.38) indicates a highly crystalline material that
clearly disclosed the phase purity of the bulk sample synthesized. The
binding sites of the linker involved in coordination with the cadmium
ions have been examined by the comparison of the infrared spectra of
H2DDIH and the complex (Fig. 6b). FTIR data for CP-Cd showed a sharp
peak at 3442 cmꢀ1 corresponding to the O–H stretching vibration of
coordinated water. The spectra of H2DDIH exhibited bands at 3456 and
1658 cmꢀ1 due to the
ν
(N–H) and
ν
(C O) vibrations respectively. The
–
–
absence of these bands in the spectra of CP-Cd is consistent with the
iminolization of the amide functionality and subsequent deprotonation of
iminolic group and coordination of H2DDIH to Cd(II) ion through imi-
nolate oxygen atom. The band appearing at about 1382 cmꢀ1 is attrib-
–
uted to the ν(C–O) (iminolic) mode. The ν(C N) band of the H DDIH
–
2
(1594 cmꢀ1) has shifted to the higher wavenumber (1625 cmꢀ1) upon
complexation which suggests the coordination of the azomethine nitro-
gen to cadmium. The compound CP-Cd showed band in the 1562 cmꢀ1
Fig. 2. View (at the 50% probability level) of the asymmetric unit of CP-Cd with
the atom numbering scheme.
–
region, assigned to newly formed C N due to iminolization of the linker
–
during complexation [37].
Thermal degradation investigations are necessary as many applica-
tions depend on their thermal stability [38]. As depicted in Fig. 6c, TGA
analysis under dinitrogen atmosphere revealed that CP-Cd showed
thermal stability up to ~300 ꢁC followed by degradation of the organic
moiety. Weight loss of about 3.1% between 60 and 100 ꢁC suggests the
escape of coordinated water molecule present in the MOF framework. It
was also found that a sharp weight-loss step of over 55% was observed as
the temperature increased from 300 to 400 ꢁC, indicating the thermal
decomposition of the CP-Cd in that temperature range. This TGA result
ensured the applicability of the CP-Cd across a wide temperature range.
The morphology of CP-Cd was observed by SEM. The SEM image
determined by GC analysis. The product was confirmed by GC-MS
analysis. Prior to the experiments, the sample was activated at 110 ꢁC
overnight.
To evaluate a potential influence of leaching of active species from the
heterogeneous catalysts, the Knoevenagel condensation reaction was
started as mentioned in the experimental procedure. Then a part of the
reaction mixture was filtered to remove the catalyst at the reaction
temperature and the obtained liquid phase was re-evaluated using the
same starting material under the optimal catalytic conditions. In order to
perform the catalyst recycling experiments, the used catalyst was sepa-
rated by centrifugation, washed with acetone to remove any physisorbed
reagents and dried in air. It is then reused for the next run. The above
procedure was repeated four times.
exhibited a hexagonal shape with an average particle size of 15
(Fig. 7).
μM
3. Results and discussion
3.3. Specific surface area and optical band gap
3.1. Structure and topology
The gas adsorption-desorption study of CP-Cd was conducted to
measure the porosity, but the sample did not respond (Fig. S1). The BET
surface area is determined to be 4.85 m2gꢀ1 and the CO2 adsorption
isotherm shown adsorption of carbon dioxide gas up to 7.8 cm3gꢀ1
(Fig. S2). The data revealed very low surface area and porosity. Despite
the stable framework and an adequate effective pore size, to our surprise
no nitrogen or carbon dioxide diffusion into the pores was observed. It is
probable that the carbon dioxide and nitrogen adsorbates interact very
strongly with the pore windows, which block other molecules from
passing into the pore, as the framework has no other additional open
channel along the ‘a’ and ‘b’ axes. Otherwise, as observed in SXRD data
the offset packing of the 2D network perhaps blocks the through channels
supporting the nonporous nature [39,40].
The optical band gap is closely related to its photocatalytic ability. In
order to explore the photocatalytic ability of the compound, the mea-
surements of diffuse reflectivity were performed to obtain the band gap
(Eg), which was determined as the intersection point between the energy
axis and the tangential line extrapolated from the linear portion of the
adsorption edge in a plot of Kubelka-Munk function against energy Eg.
The Kubelka–Munk function, F(R)¼(1ꢀR) [2]/2R, was obtained from the
UV–Vis DRS data, where R is the reflectance of an infinitely thick layer at
a given wavelength. As shown in Fig. 8, the compound CP-Cd showed a
strong UV absorption around 325 nm. It suggests that the compound
could efficiently utilize ultraviolet light and generate more electron
under ultraviolet light irradiation [41]. The corresponding band gap
energy (Eg) estimated from the steep absorption edge is 2.55 eV. The
results clearly indicate that the compound is a potential wide gap
The compound CP-Cd was isolated as orange air stable crystals by
combination of H2DDIH and Cd(OAc)2 in mixed solvent system. It is
worthwhile to point out that, in this reaction, the products do not depend
on the ligand-to-metal ratio. The single-crystal X-ray structure of the
compound was solved in an orthorhombic space group Iba2. The asym-
metric unit is composed of one independent Cd(II) ion, deprotonated 1,2-
diphenylethane-1,2-dione bisisonicotinylhydrazone ligand, DDIH and
one coordinated water molecule (Fig. 2). As shown in Fig. 3, Cd1 is seven-
coordinated by four nitrogen atoms and two oxygen atoms from DDIH;
oxygen atom from water molecule, revealing a distorted pentagonal
bipyramidal geometry. The oxygen atoms and azomethine nitrogens in
the equatorial plane around the metal center adopts a chelating coordi-
nation mode to generate a five membered Cd(II) containing ring. The
terminal nitrogens which are axial to the equatorial plane are further
linked to other Cd(II) centers to form a non-interpenetrating two-
dimensional network. The topology of the resulting structure can be
defined as
a (2,4) connected uninodal net with Schlafli symbol
{33⋅44⋅55⋅62⋅7} (Fig. 4). The hexagonal channels are running down the
crystallographic ‘c’ axis (15.13 ꢂ 4.3 Å). Packing fraction and total po-
tential solvent area volume were calculated using PLATON program [36].
The packing index value of 43.7% and solvent area volume of 3389.93
Å3, showed the complex to be potentially porous with wide
solvent-accessible channels (Fig. 5). The metal connected infinite 2D
grids are stacked parallel to each other along the ‘a’ axis with an … ABAB
… arrangement.
semi-conductive material and probably
a good candidate as a
3

L. Tom, M.R.P. Kurup
Journal of Solid State Chemistry xxx (xxxx) xxx
Fig. 3. (a) 2D capped stick view of the framework CP-Cd. (b) the Cd-centers are shown as purple coordination polyhedra (c) the distorted pentagonal bipyramidal
geometry around Cd center. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 4. (a) 2D frameworks stacked parallel to each other along the ‘a’ axis (b) The topological network formed by CP-Cd. (c) Packing of layers with … ABAB … arrangement.
Fig. 5. (a) Perspective image showing the 3D structure of CP-Cd along ‘c’ axis (b) solvent accessible voids modelled using the program Mercury.
photocatalyst.
factors, such as time, catalysts, temperature and solvents have been
tested in order to reveal the most suitable parameters for the Knoeve-
nagel condensation reaction. Consequently, the optimization of the re-
action conditions (solvent, reactant ratio, reaction time, and the amount
of catalyst) was carried out in a model malononitrile-benzaldehyde sys-
tem. Substrate stoichiometry for the reaction has been chosen as 1:1.5 in
the model catalytic reaction. As inspired by green chemistry principles, it
was decided to carry out the reaction under room temperature condi-
tions. Scheme 1 shows the representative Knoevenagel condensation
reaction and details of the catalytic reaction procedure is provided in the
3.4. Catalytic study
Thermal and chemical stability of the framework, presence of labile
H2O ligand and infinite sheet structure of CP-Cd could offer a platform to
accomplish heterogeneous catalytic reactions. Therefore, we have
assessed the MOF, CP-Cd for its activity in the Knoevenagel reaction by
studying the condensation of benzaldehyde with malononitrile to form
benzylidene malononitrile as the principal product. Several reaction
4

L. Tom, M.R.P. Kurup
Journal of Solid State Chemistry xxx (xxxx) xxx
Fig. 6. (a) X-ray powder diffractogram of CP-Cd (b) comparison of FT-IR spectra of CP-Cd and H2DDIH (c) TGA analysis of CP-Cd.
Scheme 1. The Knoevenagel condensation reaction of benzaldehyde and
malononitrile.
substrate ratio. Benzaldehyde versus malononitrile at molar ratios of 1:1,
1:1.5, 2:1, and 1:2 have been tested. The reaction rate at 1:1 ratio was
low. Using the reagent molar ratio of 1:1.5, the reaction gave 96% con-
version after 15 min, and then it could go to completion after 30 min with
no trace amount of benzaldehyde being detected by GC. Further increase
of the malononitrile nucleophile did not significantly accelerate the re-
action rate. As a result, the 1:1.5 ratio of benzaldehyde and malononitrile
was chosen as the optimal substrate ratio. Moreover, increasing the
benzaldehyde molar ratio resulted in a significant drop in the reaction
rate. It was observed that only 70% conversion was achieved for the
reaction using 1 equiv of malononitrile and 1.5 equiv of benzaldehyde
under same condition.
Fig. 7. SEM micrographs of CP-Cd.
Experimental section.
To gauge the catalytic efficiency, experiments were conducted in
different solvents such as methanol, ethanol, ethyl acetate, acetonitrile
and tetrahydrofuran. From Table 1, it can be deduced that methanol is
the most suitable reaction solvent for further catalytic studies. Ethanol
also gave satisfying yield. Tetrahydrofuran and acetonitrile gave com-
parable, but still lower yields of the desired product. Ethyl acetate gave
very poor yield. The effect of reaction time in the Knoevenagel conden-
sation shows that 15 min is the best reaction time.
With such results in hand, we then decided to investigate the effect of
catalyst concentration on reaction conversion. The catalyst concentra-
tion, with respect to the cadmium content in the CP-Cd, was studied in
The reaction has been further optimized to determine the best
Fig. 8. (a) Kubelka-Munk diffuse reflectance spectra and (b) Tauc plot for CP-Cd.
5

L. Tom, M.R.P. Kurup
Journal of Solid State Chemistry xxx (xxxx) xxx
Table 1
Table 2
Optimization of the parameters for Knoevenagel reaction between benzaldehyde and
Knoevenagel condensation reaction of various aldehydes with malononitrile with
catalyst CP-Cd.
malononitrile with CP-Cd as the catalysta.
Entry
Amount of catalyst (mol
%)
Solvent
Time
(min)
Conversion
(%)
Sr. No
1
Substrate
Product
Yield (%)
96
1
2
3
1.5 mol% CP-Cd
1.5 mol% CP-Cd
1.5 mol% CP-Cd
Methanol
Ethanol
Ethyl
acetate
THF
Acetonitrile
Methanol
Methanol
Methanol
Methanol
Methanol
15
15
15
85
83
32
2
3
4
80
84
93
4
5
6
7
8
9
10
1.5 mol% CP-Cd
1.5 mol% CP-Cd
1.5 mol% CP-Cd
2 mol% CP-Cd
2.5 mol% CP-Cd
3 mol% CP-Cd
2 mol% CP-Cd (B:M ¼
1:1)
15
15
20
15
15
15
15
60
74
87
96
97
97
82
11
12
2 mol% CP-Cd (B:M ¼
Methanol
Methanol
15
15
70
97
1.5:1)
2 mol% CP-Cd (B:M ¼
1:2)
5
6
88
69
13
14
15
Blank
H2DDIH
Cd(OAc)2⋅H2O
Methanol
Methanol
Methanol
15
15
15
5
10
28
a
All reactions except entries 10,11,12 were carried out at reactant ratio B:M ¼
1:1.5.
the range of 1.5–3 mol%. It was observed that quantitative conversion of
benzaldehyde was achieved within 15 min at 2 mol% catalyst. Further
increase in the former had no significant effect on reaction conversion. As
expected, a lower reaction rate was observed for the reaction using 1.5
mol% catalyst. However, the reaction could afford a conversion of 93%
after 30 min, and complete conversion of benzaldehyde was still obtained
after 1 h. Decreasing the catalyst concentration to 1 mol% led to a sig-
nificant drop in the reaction rate. The results indicated that the CP-Cd
catalyst was quite active in the Knoevenagel reaction and best results
were obtained in methanol as the solvent medium with 2 mol% catalyst
along with substrates (benzaldehyde and malononitrile) in 1:1.5 stoi-
chiometry at room temperature leading to 96% conversion within 15 min
reaction time. The reaction between benzaldehyde and malononitrile
was also monitored in blank, as well as using free ligand precursor
H2DDIH and the metal salt Cd(OAc)2⋅2H2O, instead of the catalysts. In
the absence of the catalyst, the reaction showed trace yield at room
temperature. Only 10–30% of product yield was obtained using
Cd(OAc)2⋅2H2O or H2DDIH as catalyst.
In order to determine the general applicability and efficiency of CP-
Cd, we have also investigated the catalytic activity with different types
of substituted aromatic aldehydes in the reaction with malononitrile
applying the optimized reaction conditions based on the model reaction.
The results summarized in Table 2 indicate that the MOF CP-Cd could
convert all benzaldehyde derivatives studied to the corresponding
condensation product effectively. The highest yield was obtained by
using p-nitrobenzaldehyde (93%), whilst the lowest one was for p-
methoxybenzaldehyde (56%). All other substituted benzaldehydes under
the optimized reaction conditions gave 69–88%. The presence of a sub-
stituent on benzaldehyde such as an electron withdrawing or electron
donating group has a profound effect on the product yield in the Knoe-
venagel condensation reaction. Electron-withdrawing substituent pro-
motes the reactivity, relative to an electron-donor moiety. The results
clearly demonstrate that CP-Cd possess the capacity for catalyzing the
Knoevenagel reaction with a variety of aldehydes under ambient reaction
conditions. Although the relationship between the structure and catalytic
activity is not clearly understood, the high activity of CP-Cd may be due
to the presence of labile H2O ligands and the easier accessibility of the
Cd(II) center to the substrate. As revealed from N2 and CO2 adsorption
data, CP-Cd showed very low surface area and porosity. Hence, the
condensation reaction by CP-Cd occurs on the surface of the catalyst.
Leaching is a crucial issue observed in the catalytic systems, leading to
7
56
loss of activity due to the migration of active sites from solid support to
the liquid phase. In order to examine the possibility of leaching and the
heterogeneity of our catalytic system, an experiment was performed
following the procedure described by Sheldon et al. [42] After the re-
action mixture was stirred for 8 min (only ca. 50% conversion of benz-
aldehyde was then observed) the solid catalyst was removed by filtration.
The removal of the solid catalyst ceased the condensation reaction and
negligible conversion was observed even upon an extended time, sup-
porting the heterogeneity of the catalyst with no leaching of the active
species (Fig. 9a).
Another point of great concern for most of solid catalysts is the
recoverability as well as the reusability of the catalyst. MOF based cat-
alysts are heterogeneous in nature and they can be easily separated from
the catalytic system and show good catalytic performance, product yield
and reusability. In order to evaluate the reusability and stability of CP-Cd
during the Knoevenagel condensation, we have performed recycling tests
in five consecutive experiments and its catalytic activity was monitored
(Fig. 9b). It was observed that more than 90% conversion was still ach-
ieved at the fifth run. After the completion of five cycles, the catalyst was
recovered by filtration and washed with acetone and dried. Furthermore,
FT-IR and powder X-ray diffraction of CP-Cd taken before and after the
reaction showed that the structure of the solid remained intact (Fig. 10).
The efficiency of our catalyst in comparison with various MOF based
catalysts for the Knoevenagel condensation reaction with our investiga-
tion is presented in Table 3. The obtained yield in our system is par/
better than that of the reported ones. Furthermore, our present study is
conducted at room temperature with 15 min reaction time to achieve
96% conversion which makes the reaction energy efficient and eco-
friendly. Table 3 reports Knoevenagel reaction investigated with longer
reaction time, higher temperature and catalyst loading. In comparison
with other reported catalysts, ours have the advantages of being easy-to-
prepare, rather cheap, highly active and are recyclable without appre-
ciable loss of catalytic activity.
A mechanism for the Knoevenagel reaction has been proposed. The
labile water coordinated Cd(II) center acts as an active Lewis acidic
6

L. Tom, M.R.P. Kurup
Journal of Solid State Chemistry xxx (xxxx) xxx
Fig. 9. (a) Plots of yield vs. time for the Knoevenagel condensation reaction of benzaldehyde and malononitrile in the presence of CP-Cd (black curve) and after
filtering off the CP-Cd (red curve) (b) Catalyst recycling studies. Yield of five consecutive cycles. (For interpretation of the references to colour in this figure legend, the
reader is referred to the Web version of this article.)
Fig. 10. (a) Powder X-ray diffraction pattern and (b) FTIR spectra of CP-Cd before and after reaction.
Table 3
A comparison of catalytic activity of various MOFs in the Knoevenagel condensation reaction of benzaldehyde and malononitrile.
MOF Catalyst
Catalyst mol%
Solvent
Time (h)
Temp (Cꢁ)
Conversion (%)
Ref
1
2
3
4
5
6
7
8
{[Cd(4-btapa)2(NO3)2]⋅6H2O⋅2DMF}n
{[Gd2(L)3(dmf)4]⋅4DMF⋅3H2O}n
[PbL2]⋅2DMF⋅6H2O
ZIF-8
[Zn4(TBCB)⋅(H2O)6]⋅5DMAc
TMU-5
UPC-30
ZIF-9
Zn-MOF-NH2
CAU-1-NH2
4
10
3
Benzene
DCM
12
20 min
24
6
4
0.5
5
6
4.5
7
6
1
1
5
24
6
RT
RT
RT
RT
90
RT
RT
RT
80
98
96
100
97
99
68
94
99
98
98
78
99
94
94
77
78
99
95
[43]
[44]
[45]
[46]
[47]
[48]
[49]
[50]
[51]
[52]
[53]
[52]
[54]
[55]
[56]
[57]
CH3CN
Toluene
1,4 dioxane
Methanol
DCM
Toluene
DMF
Ethanol
p-Xylene
H2O
Ethanol
THF
Toluene
p-Xylene
CH3CN
Methanol
5
2
10
5
9
10
11
12
13
14
15
16
17
18
40
{[Ni4(
μ
6-MTB)2(
μ
2-H2O)4(H2O)4]⋅10DMF⋅11H2O}n
130
RT
40
50
70
130
60
RT
{[Zn(ADA)(L)]⋅2H2O}n
CAU-1-NH2
[Zn(κN-H3L)(H2O)3]⋅3H2O
[Zn2dobdc]n
3
1
4
4
3
{[Ni4(
μ
6-MTB)2(
μ
2-H2O)4(H2O)4]⋅10DMF⋅11H2O}n
[Tb(BTATB)(DMF)2(H2O)]n
[Cd(DDIH)2(H2O)]n
24
15 min
[58]
present study
catalytic site. On the basis of crystal structure and previous reports [39,
58,59] a plausible mechanism of the Knoevenagel reaction on the Lewis
acidic site is illustrated in Fig. 11. The reaction initiates from the attack of
the polarized carbonyl oxygen from the aldehyde with the metal center,
favoring the attack of malononitrile. Simultaneously, detachment of an
acidic proton from the methylene group of malononitrile, give rise to
carbonium anion intermediate. In the second step, the newly formed
carbanion reacts with the carbonyl group of aromatic aldehyde via
nucleophilic attack to give an intermediate. Lastly, the intermediate
undergoes rearrangement and quickly converted to the benzylidenema-
lononitrile product after the absorption of one proton and loss of water.
4. Conclusion
In summary, a Cd(II) MOF has been synthesized with the reaction of
cadmium acetate dihydrate and double hydrazone linker 1,2-diphenyl-
ethane-1,2-dione bisisonicotinylhydrazone. The linker is capable of
coordinating Cd(II) center through endodentate and exodentate coordi-
nation and of generating novel coordination polymer. The MOF was
characterized using a variety of different techniques and studied prop-
erties such as thermal and chemical stability, phase purity etc. The crystal
structure of MOF revealed interesting two dimensional grid structure
with uninodal topology. The semiconducting property of these MOF has
also been reported. The compound acts as efficient heterogeneous
7

L. Tom, M.R.P. Kurup
Journal of Solid State Chemistry xxx (xxxx) xxx
[5] P. George, N.R. Dhabarde, P. Chowdhury, Mater. Lett. 186 (2017) 151–154.
[6] A. Legrand, A. Pastushenko, V. Lysenko, A. Geloen, E. Quadrelli, J. Canivet,
D. Farrusseng, Chem. Nano Mater. 2 (2016) 866–872.
[7] A.R. Chowdhuri, D. Bhattacharya, S.K. Sahu, Dalton Trans. 45 (2016) 2963–2973.
[8] E. Rahmani, M. Rahmani, Microporous Mesoporous Mater. 249 (2017) 118–127.
[9] A. Karmakar, H.M. Titi, I. Goldberg, Cryst. Growth Des. 11 (2011) 2621–2636.
[10] M. Ksibi, S. Rossignol, J.M. Tatibouet, C. Trapalis, Mater. Lett. 62 (2008)
4204–4206.
[11] D.M. Alonso, S.G. Wettstein, J.A. Dumesic, Chem. Soc. Rev. 41 (2012) 8075–8098.
[12] A. Karmakar, M.M.A. Soliman, G. Rúbio, M. Guedes da Silva, A.J.L. Pombeiro,
Dalton Trans. 49 (2020) 8075–8085.
[13] J. Gascon, U. Aktay, M.D. Hernandez-Alonso, G.P.M. Klink, F.J. Kapteijn, Catal 261
(2009) 75–87.
[14] T. Dewa, T. Saiki, Y.J. Aoyama, J. Am. Chem. Soc. 123 (2001) 502–503.
[15] F.X.L. Xamena, O. Casanova, R.G. Tailleur, H.J. Garcia, Catal 255 (2008) 220–227.
[16] H. Liu, Y. Liu, Y. Li, Z. Tang, H.J. Jiang, Phys. Chem. C 114 (2010) 13362–13369.
[17] S. Opelt, S. Turk, E. Dietzsch, A. Henschel, S. Kaskel, E. Klemm, Catal. Commun. 9
(2008) 1286–1290.
[18] F.X.L. Xamena, A. Abad, A. Corma, H.J. Garcia, Catal 250 (2007) 294–298.
[19] Y. Zhou, J. Song, S. Liang, S. Hu, H. Liu, T. Jiang, B.J. Han, Mol. Catal. A. 308
(2009) 68–75.
[20] N.T.S. Phan, K.K.A. Le, T.D. Phan, Appl. Catal., A 382 (2010) 246–253.
[21] U. Ravon, M. Savonnet, S. Aguado, M.E. Domine, E. Janneau, D. Farrusseng,
Microporous Mesoporous Mater. 129 (2010) 319–329.
Fig. 11. Proposed catalytic cycle for the Knoevenagel condensation reaction
catalyzed by CP-Cd (M ¼ Cd(II) center; X ¼ O of coordinated H2O).
[22] K.K. Tanabe, S.M. Cohen, Inorg. Chem. 49 (2010) 6766–6774.
[23] A. Corma, H. García, F.X.L. Xamena, Chem. Rev. 110 (2010) 4606–4655.
[24] A. Helal, K.E. Cordova, Md E. Arafat, M. Usman, Z.H. Yamani, Inorg. Chem. Front.
79 (2020) 3571–3577.
[25] F. Freeman, Chem. Rev. 80 (1980) 329–350.
[26] L.F. Tietze, Chem. Rev. 96 (1996) 115–136.
[27] K.M. Parida, S. Mallick, P.C. Sahoo, S.K. Rana, Appl. Catal., A 381 (2010) 226–232.
[28] L. Martins, K.M. Vieira, L.M. Rios, D. Cardoso, Catal. Today 133 (2008) 706–710.
[29] K.R. Reddy, K. Rajgopal, C.U. Maheswari, M.L. Kantam, New J. Chem. 30 (2006)
1549–1552.
[30] N.T.S. Phan, C.W. Jones, J. Mol. Catal. Chem. 253 (2006) 123–131.
[31] Smart and Saint, Area Detector Software Package and SAX Area Detector
Integration Program, Bruker Analytical X-ray, Madison, WI, USA, 1997.
[32] Sadabs, Area Detector Absorption Correction Program, Bruker Analytical X-ray,
Madison, WI, 1997.
catalyst for the Knoevenagel condensation under mild reaction condi-
tions. It effectively catalyzes the reaction of various aldehydes and
malononitrile producing the corresponding benzylidenemalononitriles in
high yields. Moreover, the catalyst could be recovered and reused up to
five times without a significant degradation in catalytic activity. PXRD
and FTIR measurements of the fresh and reused catalyst showed no
structural changes and leaching experiment indicated no contribution of
active Cd2þ species in the reaction solution during the course of the re-
action, thus proving the catalyst stability under the present experimental
conditions. It is apparent that the CP-Cd catalyst can be an alternative to
other solid catalysts for the Knoevenagel reaction. Further explorations
into the uses of this catalyst in other organic transformations, as well as
mechanistic investigations, are ongoing.
[33] G.M. Sheldrick, Acta Crystallogr. C71 (2015) 3–8.
[34] K. Brandenburg, Diamond Version 3.2g, Crystal Impact GbR, Bonn, Germany, 2010.
[35] C.F. Macrae, P.R. Edgington, P. McCabe, E. Pidcock, G.P. Shields, R. Taylor,
M. Towler, J. van de Streek, J. Appl. Crystallogr. 39 (2006) 453–457.
[36] A.L. Spek, Acta Crystallogr. D65 (2009) 148–155.
[37] L. Tom, N. Aiswarya, S.S. Sreejith, M.R.P. Kurup, Inorg. Chim. Acta. 473 (2018)
223–235.
CRediT authorship contribution statement
Lincy Tom: Conceptualization, Methodology, Investigation, Soft-
ware, Writing - original draft, Funding acquisition. M.R.P. Kurup: Data
curation, Visualization, Supervision, Writing - review & editing.
[38] A. Kurt, E.J. Kaya, Appl. Polym. Sci. 115 (2010) 2359–2367.
[39] B. Parmar, P. Patel, V. Murali, Y. Rachuri, R.I. Kureshy, N.H. Khan, E. Suresh, Inorg.
Chem. Front. 5 (2018) 2630–2640.
[40] T.K. Maji, R. Matsuda, S. Kitagawa, Nature Mat 6 (2007) 142–148.
[41] X. Zhang, L. Cui, X. Zhang, F. Jin, Y. Fan, J. Mol. Struct. 1134 (2017) 742–750.
[42] R.A. Sheldon, M. Wallau, I.W.C. E Arends, U. Schuchardt, Acc. Chem. Res. 31
(1998) 485–493.
Declaration of competing interest
[43] S. Hasegawa, S. Horike, R. Matsuda, S. Furukawa, K. Mochizuki, Y. Kinoshita,
S. Kitagawa, J. Am. Chem. Soc. 129 (2007) 2607–2614.
[44] R.K. Das, A. Aijaz, M.K. Sharma, P. Lama, P.K. Bharadwaj, Chem. Eur J. 18 (2012)
6866–6872.
[45] X.M. Lin, T.T. Li, L.F. Chen, L. Zhang, C.Y. Su, Dalton Trans. 41 (2012)
10422–10429.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
[46] U.P.N. Tran, K.K.A. Le, N.T.S. Phan, ACS Catal. 1 (2011) 120–127.
[47] H. He, F. Sun, B. Aguila, J.A. Perman, S. Ma, G. Zhu, J. Mater. Chem. 4 (2016)
15240–15246.
[48] M.Y. Masoomi, S. Beheshti, A. Morsali, J. Mater. Chem. 2 (2014) 16863–16866.
[49] W. Fan, Y. Wang, Z. Xiao, L. Zhang, Y. Gong, F. Dai, R. Wang, D. Sun, Inorg. Chem.
56 (2017) 13634–13637.
Acknowledgements
Lincy Tom gratefully acknowledges the CSIR, New Delhi, India for
financial support in the form of Senior Research Fellowship (09/
239(0506)2014-EMR-l). The authors are indebted to SAIF, Cochin Uni-
versity of Science and Technology, Kochi, Kerala, India for elemental
analysis, SEM and X-ray diffraction studies. We also thank Poornaprajna
Research Center (PPISR), Bangalore for gas adsorption studies.
[50] L.T.L. Nguyen, K.K.A. Le, H.X. Truong, N.T.S. Phan, Catal. Sci. Technol. 2 (2012)
521–528.
ꢀꢁ
[51] M. Opanasenko, A. Dhakshinamoorthy, M. Shamzhy, P. Nachtigall, M. Horacek,
ꢁ
H. Garcia, J. Cejka, Catal. Sci. Technol. 3 (2013) 500–507.
[52] A. Dhakshinamoorthy, N. Heidenreich, D. Lenzen, N. Stock, CrystEngComm 19
(2017) 4187–4193.
ꢁ
Appendix A. Supplementary data
ꢀꢁ
ꢁꢀ
[53] M. Almasi, V. Zelenak, M. Opanasenko, J. Cejka, Dalton Trans. 43 (2014)
3730–3738.
[54] A. Karmakar, A. Paul, K.T. Mahmudov, M. Guedes da Silva, A.J.L. Pombeiro, New J.
Chem. 40 (2016) 1535–1546.
[55] P. Valvekens, M. Vandichel, M. Waroquier, V.V. Speybroeck, D. De Vos, J. Catal.
317 (2014) 1–10.
Supplementary data to this article can be found online at https://do
i.org/10.1016/j.jssc.2020.121846.
ꢀꢁ
ꢁꢀ
ꢀ
[56] M. Almasi, V. Zelenak, M. Opanasenko, J. Cejka, Dalton Trans. 43 (2014)
References
3730–3738.
[57] N. Wei, M.Y. Zhang, X.N. Zhang, G.M. Li, X.D. Zhang, Z.B. Han, Cryst. Growth Des.
[1] H. Li, M. Eddaoudi, M. O’Keeffe, O.M. Yaghi, Nature 402 (1999) 276–279.
[2] J.L.C. Rowsell, O.M. Yaghi, Microporous Mesoporous Mater. 73 (2004) 3–14.
[3] J. Jiang, H. Furukawa, Y.B. Zhang, O.M. Yaghi, J. Am. Chem. Soc. 138 (2016)
10244–10251.
14 (2014) 3002–3009.
[58] A. Karmakar, A. Paul, K.T. Mahmudov, M. Fatima C. Silva, A.J.L. Pombeiro, New J.
ꢀ
Chem. 40 (2016) 1535–1546.
[59] Z. Miao, Y. Luan, C. Qi, D. Ramella, Dalton Trans. 245 (2016) 13917–13924.
[4] A. Dhakshinamoorthy, H. Garcia, Chem. Soc. Rev. 43 (2014) 5750–5765.
8