Bifunctional 2D Cd(II)-Based Metal-Organic Framework as Efficient Heterogeneous Catalyst for the Formation of C-C Bond

Article abstract of DOI:10.1021/acs.cgd.7b01728

A porous two-dimensional (2D) metal-organic framework (MOF), namely, [Cd(PBA)(DMF)]·DMF (Cd-PBA), has been solvothermally synthesized by the reaction of 5-(4-pyridin-3-yl-benzoylamino)-isophthalic acid ligand (H₂PBA) and Cd(II) ions. Structural

Full text of DOI:10.1021/acs.cgd.7b01728

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A Bifunctional 2D Cd(II)-Based Metal-Organic Framework as
Efficient Heterogeneous Catalyst for the Formation of C-C Bond
Lei Hu, Gui-Xia Hao, Hai-Dong Luo, Chun-Xian Ke, Guang Shi,
Jia Lin, Xiao-Ming Lin, Umair Yaqub Qazi, and Yue-Peng Cai
Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 26 Mar 2018
Downloaded from http://pubs.acs.org on March 26, 2018
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Lei Hu,† Gui-Xia Hao,‡ Hai-Dong Luo,† Chun-Xian Ke,† Guang Shi,† Jia Lin,† Xiao-Ming Lin,*,†, §
Umair Yaqub Qazi,‖ and Yue-Peng Cai*,†
†School of Chemistry and Environment, South China Normal University, Guangzhou Key Laboratory of Materials for Energy
Conversion and Storage, 510006, P.R. China
‡
College of Chemistry and Environmental Engineering, Hanshan Normal University, Chaozhou,Guangdong, 521041, P.R.
China
§ State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of
Sciences, Fuzhou, Fujian, 350002, P.R. China
‖Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of
Science and Technology of China, Hefei, 230026, China
ABSTRACT: A porous two-dimensional (2D) metal–organic framework (MOF), namely, [Cd(PBA)(DMF)]·DMF (Cd-PBA), has
been solvothermally synthesized by the reaction of 5-(4-pyridin-3-yl-benzoylamino)-isophthalic acid ligand (H2PBA) and Cd(II)
ions. Structural analysis shows that Cd-PBA possesses 2D (3,6)-connected kgd net topology with the Schläfli symbol of
(43)2(46.66.83) and exhibits two distinct types of 1D opening channels along a- and c-axis. The incorporation of Cd metal centers
and -NH groups of amides endows Cd-PBA rich open metal sites and Lewis basic sites, which are applied as an efficient catalyst
for the important Knoevenagel condensation and cyanosilylation of aldehydes reactions. This Cd-PBA presents highly efficient
catalytic activity and recyclability for both Knoevenagel condensation reaction and cyanosilylation of various aldehydes with
trimethylsilyl cyanide (TMSCN). The excellent catalytic activity can be maintained at least four cycles without loss of obvious
catalytic activity. These results indicate Cd-PBA can serve as a promising heterogeneous catalyst toward C−C bond formation due
to the stability and high catalytic activity.
1. INTRODUCTION
metal-organic framework, [Eu2(PDC)3] (H2PDC=pyridine-3,5-
dicarboxylic acid), which could effectively accelerate the
cyanosilylation reaction.25 By utilizing of a versatile CuII/CuI
metal-organic framework as catalyst, three-component
coupling reactions of diisopropylamine, tosylazide, and
aromatic alkynes can be promoted as well as the oxidation of
benzylic compounds.26 As part of our continuous work, herein,
we reported the synthesis and characterization of a 2D (3,6)-
connected framework with kgd topology, namely,
Nowadays, porous metal−organic frameworks (MOFs), as
the particular field of materials science, is being extensively
studied due to their intriguing topologies and the
multifunctional properties, such as separation and gas storage,
catalysis science, magnetism, luminescence, etc.1-9
.
Particularly interesting are those related to the application of
heterogeneous catalysis. It is feasible and fundamental to
predict the possible catalytic properties and activity through
reasonable design and fine characterization of these
materials.10-14 For instance, the selection of ligand and metal
center can direct the structural diversity with variable
topological structures, and accessibility to the active center.15
Generally speaking, one effective strategy is to create
coordinatively unsaturated metal sites or introduce nanosized
metallic clusters/organometallic complexes into the MOFs
through post-modification method.16-19 Another effective
strategy is to fabricate functionalized ligands to synthesize
desired MOFs.20,21
[Cd(PBA)(DMF)]·DMF,
(H2PBA=5-(4-pyridin-3-yl-
benzoylamino)-isophthalic acid). Moreover, the catalytic
properties were also studied.
2. EXPERIMENTAL SECTION
2.1. General Information. Organic ligand was synthesized
based on the procedures described previously.27-29 All reagents
and solvents employed were purchased from commercial
sources and used without further purification. Infrared spectra
were collected from KBr pellets on a Nicolet/Nexus-670 FT-
IR spectrometer. Perkin-Elmer 240 elemental analyzer was
used to carry out the elemental analyses. Thermogravimetric
analyses (TGA) were performed on a Netzsch Thermo
Microbalance TG 209 F3 Tarsus from room temperature to
We have been studying on the design of functionalized
MOFs and application for heterogeneous catalysis.22-24 As
reported in our previous work, we obtained a polyhedral
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Crystal Growth & Design
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900 oC under flowing nitrogen. X-ray powder diffraction
patterns were characterized on a Bruker D8 Advance
diffractometer with Cu target tube. A Varian 500 MHz
spectrometer was used to record 1H NMR spectra.
Quantachrome Autosorb-iQ-MP gas sorption analyzer was
applied to measure sorption isotherms. The size and shape
were characterized by using field emission scanning electron
microscopy (FESEM, TESCAN Maia 3, Czech) operating at 5
kV and transmission electron microscopy (TEM, JEM-
2100HR, Japan) coupled with an acceleration voltage of 200
kV.
3. RESULTS AND DISCUSSION
3.1. Structure and Characterization of Cd-PBA MOF.
Structural analysis reveals that Cd-PBA crystallizes in the
triclinic Pī space group. As depicted in Figure 1a, the Cd ion is
six-coordinated and exhibits an octahedral coordination
surrounded by one N atom, four carboxylate oxygen atoms of
three different PBA2- ligands, and one oxygen atom from a
coordinated DMF molecule. The neighboring Cd(II) ions are
connected together through the carboxylate groups of PBA2-
ligands to form a 1D infinite chain (Figure 1b), in which the
carboxylate groups adopt the bidentate chelating and bidentate
bridging coordination modes. These chains are further linked
by PBA2- ligands using the N atoms to give rise to a 2D layer
network (Figure 1c). A better insight into the nature of this
structure can be achieved by the application of a topology
approach, each PBA2- ligand acts as organic trinodal building
block and the binuclear cadmium atoms serve as hexatopic
node geometry (Figure S1). Thus, the whole framework can be
ascribed as a binodal (3,6)-connected kagomé dual (kgd) net
topology with the Schläfli symbol of (43)2(46.66.83) analyzed
by Topos 4.0 program (Figure 1d).31 Although the adjacent
layers are packed in an ABCABC arrangement (Figure S2),
the stacking still leads to two different types of one-
dimensional (1D) opening channels with effective size of 8.9 ×
8.0 Å2 and 9.3 × 7.8 Å2 along a- and c- axis, respectively
(Figure 1e and 1f), in which the void space is filled with DMF
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2.2 Preparation of [Cd(PBA)(DMF)]·DMF. A mixture
containing H2PBA (72.4 mg, 0.2 mmol), Cd(NO3)2·4H2O (60
mg, 0.2 mmol), and dimethylformamide (DMF, 6 mL) was
sealed in a glass reaction bottle. The system was heated at 90
ºC for 3 d and then cooled to room temperature. Block crystals
were obtained by filtration and washed with DMF. FT-IR
(KBr, cm-1): 3366 (s), 1646 (vs), 1558 (w), 1425 (w), 1394 (w),
1307 (w), 782 (w), 717 (w). Anal. calcd for C26H26N4O7Cd
(618.91): C 50.46, H 4.23, N 9.05 %; found: C 50.42, H 4.27,
N 9.03 %.
2.3. X-Ray Structure Determination. X-ray reflection
intensities were performed with
a Bruker APEX II
diffractometer at 296 K using graphite monochromatic Mo-K
radiation (λ=0.71073 Å). The empirical absorption corrections
were used to correct the reflections. The structure was solved
by the direct method and refined by full-matrix least-squares
on F2 using SHELXL programs (SHELXTL-2014).30 The non-
hydrogen atoms were refined with anisotropic displacement
parameters at the final cycles. Isotropic displacement
parameter was used to place the organic hydrogen atoms in
calculated positions. The details of the crystal parameters, data
collection, and refinement are summarized in Table S1, and
the selected bond lengths are listed in Table S2. CCDC
1580065 contains the crystallographic data.
molecules. PLATON32 calculation suggests
a
solvent
accessible volume of 507.1 Å3 (39.6 % of unit cell) by
excluding the guest solvent molecules.
2.4. Sample Activation. Prior to the catalytic test, the as-
prepared Cd-PBA samples were soaked in CH3OH for three
days. Further, the samples were filtrated and dried at 120 °C
for 5 h under vacuum to remove the physical and chemical
guest molecules, generating the dehydrated phase Cd-PBA.
2.5. Typical Procedure for Knoevenagel condensation
reaction. In a 10 mL Pyrex-glass screw-cap vial were placed
successively benzaldehyde (100 L, 1 mmol), malononitrile
(111 L, 2 mmol), ethanol (5 mL), and activated Cd-PBA (9.5
mg, 0.02 mmol, 2 %). Then the mixture was stirred with a
Teflon-coated magnetic stir at room temperature for 2 h. After
that, the supernatant was filtered and concentrated under
reduced pressure. The reaction crudes were further purified by
flash column chromatography and the yields of the reaction
1
products were determined by H NMR spectroscopy by using
dibromomethane as an internal standard.
2.6. Typical Procedure for Cyanosilylation Reaction. A
mixture of benzaldehyde (26.5 mg, 0.25 mmol), trimethylsilyl
cyanide (TMSCN, 49.6 mg, 0.5 mmol), and n-hexane (5 mL)
was placed into a Pyrex-glass screw-cap vial (10 mL), and Cd-
PBA catalyst (2 %) was added and the resulting reaction
mixture was stirred with a Teflon-coated magnetic stir at room
Figure 1. Crystal structure of Cd-PBA. (a) Coordination environment
of Cd(II). (b) 1D chain connected through the carboxylate groups in
Cd-PBA. (c) Polyhedral view of 2D network. (d) View of the (3,6)-
connected kgd net topology with the Schläfli symbol of (43)2(46.66.83).
A space-filling view of the porous network showing two different
types of 1D channels along the crystallographic (e) a-axis, and (f) c-
axis, in which the solvent molecules are omitted for clarity. All
hydrogen atoms are omitted for clarity.
1
temperature for 8 h. The yields were determined by H NMR
analysis using dibromomethane as an internal standard.
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Field emission scanning electron microscopy (FESEM) and
accelerated in comparison with other organic solvents.
However, due to the toxicity of benzene solvent, we used the
ethanol as catalytic solvent instead in this case.
transmission electron microscopy (TEM) were firstly applied
to observe the morphology of the as-prepared Cd-PBA
samples. As presented in Figure S3, the average size of MOF
particle is approximate to be 3 μm. To confirm the phase
purity of Cd-PBA, PXRD pattern for the bulk samples was
measured at room temperature. Figure S4 shows that all the
diffraction peaks perfectly match well with the simulated
pattern results, confirming the phase purity of the obtained
crystal samples. The intensity differences may be due to the
preferred orientation of the crystalline samples.
Herein, we employed a molar ratio of 1:2 to select aromatic
benzaldehyde and malononitrile compounds in ethanol at
room temperature for 2 h with 2 % catalyst loading, which
generated the corresponding product yield of 91 % for 2 h
(Table 1, entry 1). The yield is slightly less than the powerful
Lewis base catalyst [Cd3(tipp)(bpdc)2] with a 93 % conversion
after 1 h with 0.6 % catalyst loading.36 However, the
experimental condition was conducted at the temperature of
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Meanwhile, thermogravimetric analysis (TGA) for crystal
samples was also performed under nitrogen atmosphere. As
shown in Figure S5, the TG curve in the range of 100-150 °C
shows a first loss of 12.52 %, corresponding to the loss of one
solvent DMF molecule (calcd. 12.61 %). The second weight
loss of 11.71 % ranges from 150 to 400 °C due to the loss of
one coordinated DMF molecule (calcd 11.81 %). Above
500 °C , an abrupt weight loss appears, which is ascribed to the
decomposition of the framework. The final residual after
thermal decomposition was presumed to be CdO (found
21.1 %, cacld 20.7 %).
60 °C . Meanwhile, our catalyst presents a significant
improvement compared to the other reported MOFs
catalysts.37-41 For comparison, a series of control experiments
were conducted. Obviously, when the reaction was carried out
in the absence of catalyst, only 6 % conversion of target
products were achieved, demonstrating the excellent catalytic
performance of Cd-PBA for the formation of C-C bond.
Additionally, to verify the active sites, we used
Cd(NO3)2·4H2O as homogeneous catalyst instead of Cd-PBA,
however, a negligible conversion of 3 % was observed. By
utilizing the H2PBA ligand as catalyst, which can catalyze the
reaciton with a yield of 39 % under the similar condition.
Furthermore, when Cd(NO3)·4H2O was used as a catalyst,
H2PBA ligand was utilized as additive, a moderate yield of
31 % was observed. These results indicate that –NH groups of
amides have influence on promoting the catalytic reaction.
When the unactivated Cd-PBA samples were used instead of
the activated ones, the catalytic results are inferior to that of
dehydrated Cd-PBA under the similar conditions (Yield:
54 %), which indicates that the catalytic active sites might be
easily blocked by the solvent molecule, and the substrates
have to compete with the solvent molecules during the
catalytic process.42
Nitrogen sorption experiments were carried out to evaluate
the pore characteristics. To activate the sample, the fresh
samples were immersed in CH3OH solvent for three days, then
was heated under vacuum at 120 °C for 5 h to remove all the
solvent molecules, generating the dehydrated phase Cd-PBA.
No obvious weight loss of the dehydrated phase was observed
at the starting temperature (Figure S6), suggesting the
complete removal of all the uncoordinated and coordinated
solvent molecules. Moreover, PXRD pattern further confirmed
that the framework remained intact and the preservation of its
crystallinity (Figure S4). Based on the nitrogen
adsorption/desorption isotherms in Figure S7, the porous Cd-
PBA exhibited typical type-I curve with a Brunauer-Emmett-
Teller surface area of 768 m2 g-1 and a total pore volume of
0.47 cm3 g-1. The pore-size distribution calculated by Barrett-
Horvath–Kawazoe (HK) method reveals the pore diameter is
about 8.5 Å, which is agreement with the crystal structure
analysis (Figure S8). This porous and robust framework
property could provide a platform to carry out heterogeneous
catalytic reaction.
Table 1. Reaction of benzaldehyde with malononitrile in the presence
of Cd-PBA: Effect of solvents.
Yield [a]
(%)
Isolated yield [b]
(%)
Entry
Solvent
Time (h)
3.2. Catalytic Performances. It is interesting to note that
there exits the –NH groups of amides in the framework,
suggesting that desolvated Cd-PBA might serve as an active
catalyst for Lewis base promoted reactions. Therefore, the
Lewis base-catalyzed Knoevenagel condensation reactions of
carbonyl compounds with malononitrile in the presence of Cd-
PBA as catalyst were carried out. As we know, the
Knoevenagel reaction is a weakly basic condensation reaction,
as an important member of the C-C bond formation reaction
family, which is employed in the synthesis of fine chemicals
and pharmaceutical products.33-35 Because Cd-PBA is insoluble
in most organic solvents, we used ethanol, cyclohexane, n-
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Ethanol
Cyclohexane
n-Hexane
2
6
91
51
57
72
92
6
86
40
42
65
85
-
6
Acetonitrile
Benzene
4
2
Dichloromethane
Tetrahydrofurane
DMF
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6
75
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58
-
10
a
Reaction conditions: catalyst (0.02 mmol), benzaldehyde (1 mmol),
malononitrile (2 mmol), solvent (5 mL). Yield determined by 1H
hexane,
acetonitrile,
benzene,
dichloromethane,
b
tetrahydrofurane, and DMF as solvents. As shown in Table 1,
the reaction yields in ethanol and benzene solvents were more
NMR analysis using CH2Br2 as an internal standard. Isolated yield
after column chromatography.
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