Bifunctional design of stable metal-organic framework bearing triazole–carboxylate mixed ligand: Highly efficient heterogeneous catalyst for knoevenagel condensation reaction under mild conditions

Article abstract of DOI:10.1016/j.catcom.2020.106032

A highly water stable zinc metal–organic framework (ZnMOF), {[Zn(HL)₂]}_n, was synthesized using a triazole–carboxylate-based mixed ligand (L = 5-(4H-1,2,4-triazol-4-yl)isophthalic acid). A 2D MOF was formed by hydrothermal synthesis, and extended to a 3D supramolecular network through strong hydrogen bonding. This MOF was fully characterized by Fourier-transformation infrared spectroscopy, thermogravimetric analysis, single-crystal X-ray diffraction (XRD), powder XRD and elemental analysis. Owing to the d¹⁰ configuration of this ZnMOF, its luminescent properties were suitable for the sensing of the CN^? ions over other anions, as inferred from the florescence result. However, regarding the catalytic mechanism, this ZnMOF showed a strong ability to react with CN^?, which might be due to the hydrogen bonding between the COOH groups without coordination. This interaction behavior with CN^? ions makes the ZnMOF a promising heterogeneous catalyst for Knoevenagel condensations using malononitrile and aldehyde derivatives as reactants under mild conditions. All reactions were conducted in water as a green solvent.

Full text of DOI:10.1016/j.catcom.2020.106032

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Bifunctional design of stable metal-organic framework bearing
triazole–carboxylate
mixed
ligand:
Highly
efficient
heterogeneous catalyst for knoevenagel condensation reaction
under mild conditions
Feixiong Liu, Sandeep Kumar, Shuangshuang Li, Hengzhi You,
Peng Ren, Limin Zhao
PII:
S1566-7367(20)30108-4
DOI:
https://doi.org/10.1016/j.catcom.2020.106032
CATCOM 106032
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
4 March 2020
16 April 2020
29 April 2020
Please cite this article as: F. Liu, S. Kumar, S. Li, et al., Bifunctional design of stable
metal-organic framework bearing triazole–carboxylate mixed ligand: Highly efficient
heterogeneous catalyst for knoevenagel condensation reaction under mild conditions,
Catalysis Communications (2019), https://doi.org/10.1016/j.catcom.2020.106032
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Bifunctional Design of Stable Metal-Organic Framework Bearing
Triazole–Carboxylate
Mixed
Ligand:
Highly
Efficient
Heterogeneous Catalyst for Knoevenagel Condensation Reaction
Under Mild Conditions†
Feixiong Liu,#a,b Sandeep Kumar, #b Shuangshuang Li,b Hengzhi You,b Peng Ren*b and Limin Zhao*a,c
a School of Chemistry and Chemical Engineering, Guangdong PharmaceuticalUniversity, Guangzhou
510000, China
b School of Science, Harbin Institute of Technology (Shenzhen), Shenzhen 518055, China
c Guangdong Cosmetics & Engineering Technology Research Center, Guangzhou 510000, China
# both the authors contributed equally in this work
Abstract
A highly water stable zinc metal–organic framework (ZnMOF), {[Zn(HL)2]}n, was
synthesized using a triazole–carboxylate-based mixed ligand (L= 5-(4H-1,2,4-triazol-4-
yl)isophthalic acid). A 2D MOF was formed by hydrothermal synthesis, and extended to a 3D
supramolecular network through strong hydrogen bonding between two 2D sheets. This MOF
was fully characterized by Fourier-transformation infrared spectroscopy, thermogravimetric
analysis, single-crystal X-ray diffraction (XRD), powder XRD and elemental analysis. Owing
to the d10 configuration of this ZnMOF, its luminescent properties were suitable for the
sensing of the CN ions over other anions, as inferred from the florescence result. However,
regarding the catalytic mechanism, this ZnMOF showed a strong ability to react with CN,
which might be due to the hydrogen bonding between the COOH groups without
coordination. This interaction behavior with CN ions makes the ZnMOF a promising
heterogeneous catalyst for Knoevenagel condensations using malononitrile and aldehyde
derivatives as reactants under mild conditions. All reactions were conducted in water as a
green solvent.
Keywords: MOF, Mixed ligand, Catalysis, Knoevenagel condensation, Luminescence
Corresponding authors: renpeng@hit.edu.cn, zhaolimin@gdpu.edu.cn

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Introduction
Metal–organic frameworks (MOFs) are a class of hybrid porous materials formed by the
combination of metal ions/clusters and organic ligands/linkers.1,2 In the last few decades,
MOFs have received research interest owing to their anticipated applications in different
fields, including drug delivery,3 magnetism, catalysis,4,5 gas storage,6 separation,7 ion
exchange,8 and sensing.9,10 MOFs can be constructed from metals and ligands through self-
assembly to generate various topologies.11 The ligands shapes and connectivity with the
metal center play important roles in determining the dimensionality of the MOFs.12 New
developments in MOFs structure have led to the promising discovery of new materials for
catalysis.13 Importantly, MOFs can be constructed by changing the metal or ligand to obtain
the required functionality, which aids catalysis. Regarding catalytic activity, introducing
functionality into the organic ligand that can interact with the incoming reactant and generate
an unsaturated site at the metal center is very important.14 The incoming reactant can interact
with the framework through weak interactions such as hydrogen bonding,15 – interactions,
and van der Waals forces. These weak interactions help reactants interact with the active site,
where product formation occurs.16 Functional MOFs can be constructed using various
approaches, such as using carboxylate ligands, which provide rigidity and organic pillars, to
introduce functional sites through a three-component strategy.17 Mixed ligands can be
designed using both pillars and carboxylate ligands.
Herein, a mixed ligand has been synthesized by combining triazole and carboxylic groups.18
The triazole nitrogen acts as a Brønsted base, while the carboxylic group interacts with the
reactant through hydrogen bonding. This hydrogen bonding provides extra support for
catalysis. The Knoevenagel condensation is the most significant for C-C bond formation
reaction,19 and occurs between compounds bearing activated methylene groups, including
malononitrile and carbonyl compounds such as aldehydes.20 This reaction can be accelerated
by Brønsted basic and Lewis acidic sites.21 Solid bases (such as metal oxides), amines,
microwave, ionic liquids, and biotechnology-based approaches under homogeneous
conditions have been applied to this reaction, which require high catalyst loadings.
Furthermore, catalyst reuse in further transformations is difficult. Although there are
numerous reports on this reaction, further exploration is still needed. In this study, we have
constructed a 2D bifunctional zinc MOF (ZnMOF) through combination with a mixed ligand
(5-(4H-1,2,4-triazol-4-yl)isophthalic acid). This MOF was constructed with the production of

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both Brønsted basic and Lewis acidic functionalities in mind, allowing it to serve as an
efficient heterogeneous catalyst for the Knoevenagel condensation.22 The luminescent nature
of this MOF provided additional support to understanding the mechanism of this
condensation reaction. The detection of CN ions over other anions using this MOF
motivated us to perform the Knoevenagel condensation using malononitrile. This occurred
through weak hydrogen bonding between the CN ions and free carboxylic acid groups
present in the MOF, which accelerated the reaction.
Results and discussion
Herein, a new ZnMOF was prepared using a mixed triazole–carboxylate ligand (H2L).
Furthermore, this ZnMOF was employed as a sensor for CN ions and a heterogeneous
catalyst for C-C bond formation. Crystals of this ZnMOF were grown using a hydrothermal
reaction and used for single-crystal X-ray analysis.
Single-crystal structural analysis
Crystals of the ZnMOF suitable for single-crystal analysis were grown using the
hydrothermal technique and collected from the mother liquor.
Fig. 1. Coordination environment around the metal center (left) and the formation of a 2D
coordination network in the ZnMOF (right).
A 2D coordination network was formed in the ZnMOF, which crystallized in the
monoclinic C2/c space group. The asymmetric unit consisted of one metal center (Zn(II)) and
two ligands (HL1). The geometry around the Zn(II) metal center was a distorted tetrahedron
(butterfly shaped) surrounded by two nitrogen atoms of the triazole ring (Zn–Ntriazole distances
of 2.025 and 2.048 Å) and two oxygen atoms of the carboxylate group (distances of 1.948
and 1.945 Å) to form an N2O2 environment around the metal center (Figure 1). Interestingly,

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the ligand contained two carboxylic groups, only one of which was deprotonated and bound
with the metal center in a monodentate fashion, while the other remained in the carboxylic
acid form and showed no binding with the metal center.
The ligand contained four binding sites, namely, two nitrogen atoms in the triazole ring
and two carboxylic groups. However, only one triazole nitrogen and one carboxylic group
participated in coordination bonds with the metal. In this case, the ligand acted as a bidentate
ligand because only two atoms participated in coordination bond formation, while the Zn(II)
metal was tetracoordinated and connected with the four other metal atoms, making it a four
connected node. After combining these two nodes, a 4-c; uninodal net topology was formed
(Figure S5, Supporting information). All crystallographic parameters, bond angles and
distances are shown in Tables S1-S3 (Supporting information).
As discussed above, two types of carboxylic groups were present, namely, one bound with
the metal (monodentate) and another that remained in the carboxylic acid form. Owing to the
nature of this ligand, two different types of hydrogen bonding were observed (Table S4,
Supporting information). The first type was between two carboxylic acid groups of two
different ligands situated in different 2D sheets, while the second type was between the
second carboxylic acid group and the oxygen of the bonded carboxylate group. Therefore, the
asymmetric unit consisted of a metal with two ligands (HL) in which both ligands behaved
differently. The carboxylic acid group of one ligand generated a dimer with the other
carboxylic acid group of another molecule, while the other carboxylic acid did not show this
type of dimeric behavior, but did show hydrogen bonding with the bonded carboxylate group,
as shown in Figure S6 (Supporting information). Owing to this hydrogen bonding, a three
dimensional supramolecular network was formed from this 2D MOF. A 3D porous network
was formed when packed three dimensionally (see Figure S7, Supporting information).
Luminescence properties of the ZnMOF
To study the photoluminescence properties of the ZnMOF, fluorescence spectra were
recorded in suspension using a finely ground powder sample, exhibiting a strong and broad
absorption with a λmax of 310 nm corresponding to −* and n−* transitions (Figure S9,
Supporting information).23 Based on the absorption data, of the ZnMOF emission spectra
exhibited a strong emission band at 410 nm (excited at 310 nm), as shown in Figure S9,
(Supporting information).

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Fig. 2. (a) Change in luminescence intensity with respect to different anions and (b) bar
diagram of the ZnMOF.
This emission was attributed to the strong ligand–metal interaction, because the ligand
alone showed no fluorescence behavior.24 The of ZnMOF emission was further studied in
different solvents, including water, methanol, ethanol, acetonitrile, dimethylformamide, and
dimethylacetamide, which were found to influence the fluorescence intensity. The
maximum ZnMOF emission intensity was observed in water, which was used as the solvent
for further experiments accordingly (Figure S10, Supporting information). As water is also a
green solvent, its use made this work environmentally friendly.
Detection of CN ions in water
For anion sensing, the ZnMOF (1 mg) was suspended in distilled water (2 mL) and
fluorescence spectra were recorded by adding anion solution (40 mL, 510-3 M) prepared in
distilled water. For anion sensing, different anions, such as fluoride (F), chloride (Cl),


bromide (Br), acetate (CH3COO), nitrate (NO3 ), dihydrogen phosphate (H2PO4 ),

hydrogen sulfate (HSO4 ) and cyanide (CN), were used. The results obtained illustrated that
all anions enhanced the fluorescence intensity, but the maximum enhancement was observed
with cyanide ions (Figure 2). This intensity enhancement might be due to the strong
interaction of CN with the carboxylic acid groups. Accordingly, the transfer of energy
between the metal center and ligand is increased, and enhanced fluorescence intensity is
observed. As the MOF showed a good interaction with CN ions it was applied to C-C bond
formation reaction.
Catalytic activity of ZnMOF for C-C bond formation reaction

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The Knoevenagel condensation reaction proceeds owing to the Lewis acidic and Brønsted
basic nature of the catalyst,25,26 and occurs between derivatives of benzaldehyde and
malononitrile.27 Brønsted basic site helps in activate the methylene group of malononitrile,
while the Lewis acidic site enhances the reactivity of benzaldehyde. Therefore, to determine
the potential of using both ZnMOF sites, the Knoevenagel condensation reaction was
performed in the presence of this MOF.
To successfully perform this reaction, benzaldehyde (0.3 mmol) and malononitrile (0.33
mmol) were stirred with the catalyst (5 mol%) in solvent for 2 h. Dodecane (30 L, internal
standard) was then added to the reaction mixture, followed by quenching with water (10 mL)
and extraction of the product using dichloromethane (10 mL). The organic layer was dried
over anhydrous sodium sulfate and afforded the desired product. The conversion/yield and
purity of the product were confirmed by GCMS analysis.
Table 1. Knoevenagel reaction between benzaldehyde and malononitrile with ZnMOF
at room temperature.
Yielda (% )
Entry
Time (h)
Solvent
Temp. (°C)
Catalyst
(mol% )
5
1
2
2
2
MeOH
EtOH
25
25
66
5
55
3
4
5
2
2
2
DCM
CH3CN
H2O
25
25
25
25
25
25
25
25
25
5
5
5
17
15
100
42
66
91
67
30
31
6
7
8
0.5
1
2
H2O
H2O
H2O
5
5
3
1
NA
9
10
11b
2
2
2
H2O
H2O
H2O
Zn(ClO4)2
aYield was calculated by GC using dodecane as standard, b5mol% catalyst was used.
The reaction conditions were optimized with respect to time and catalyst amount using the
malononitrile and benzaldehyde system with the ZnMOF as catalyst (Table 1). As compared
to no catalyst (30%) and Zn(ClO4)2 salt (31%), yield was improved significantly up to 100%
with this ZnMOF (Entry 5, Table 1). From solvent screening, the maximum conversion was
obtained in water (100%) compared with other common solvents, such as DCM (17%),

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MeOH (66%), EtOH (55%), and acetonitrile (15%) when using the catalyst (5 mol%) at room
temperature.
Table 2. Knoevenagel reaction between benzaldehyde derivatives and malononitrile
with ZnMOF at room temperature.

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Yielda (%)
Entry
-R
Product
1
H
100
4-F
100
2
4-NO2
4-OMe
4-Cl
100
70
3
4
5
100
Pent.
100
6
Hex.
50
58
7
8
4-CH3
24
74*
Nepth.
Anth.
9
17
13*
10
0
20*
Phen.
11
aYield was calculated by GC using dodecane as standard,*in MeOH.
Furthermore, different catalyst amounts 1 mol% (67%), 3 mol% (91%) were tested, but a
catalyst loading of 5 mol% gave the best conversion (100%) at room temperature in water.
Other than catalyst amount, the reaction time was screened, with only 43% yield observed
after 30 min, while 66% and 100% yield were achieved after 1 and 2h, respectively.
Furthermore, we extended the substrate scope of the ZnMOF as heterogeneous catalyst using
different aldehyde derivatives (Table 2), including both aliphatic and aromatic aldehydes. As
a 2D MOF was employed for catalysis, aldehydes of different sizes behaved differently.

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Larger aldehydes showed lower conversions, while smaller aldehydes gave excellent
conversions under the optimized conditions in water. From the evidence, we have proposed a
mechanism for the Knoevenagel condensation reaction catalyzed by the ZnMOF, as shown
in Scheme 1.
Scheme 1. Proposed mechanism of the Knoevenagel condensation reaction using ZnMOF as
catalyst.
The florescence spectra showed that CN anions had a stronger interaction with the
framework compared with those of other ions. Therefore, in the proposed mechanism, the
cyanide group of malonitrile interacts with the deprotonated carboxylic acid group, which
moves malononitrile closer to the Brønsted base (triazole nitrogen) and, removes hydrogen
from the activated carbon center (methylenic group of malononitrile) as shown by
intermediate A (Scheme 1). After deprotonation, the malononitrile acts as a nucleophile.
In the second step (intermediate B), the aldehyde group interacts with the Lewis acidic site
present at the metal center (unsaturated metal) and becomes polarized, increasing the
electrophilicity of the aldehyde. This polarization enhances nucleophilic attack by

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malononitrile at the polarized carbonyl group of the aldehyde to form a C−C bond. After the
C-C bond formation, dehydration occurs. The third step (intermediate C) then involves
charge neutralization while, the final step (intermediate D), shows the removal of water as a
side product and formation of the final product.
Conclusions
In summary, we have constructed a 2D Metal-Organic Framework (MOF) with bi-
functional catalytic sites. Although this MOF is highly stable in water, all catalytic reactions
were conducted in water as a green solvent. Furthermore, the use of both functionalities
(Lewis acid and Brønsted base) has been demonstrated in C-C bond formation reaction.
Interestingly, of the two carboxylic groups, only one is deprotonated and binds with the metal
center, while the other is not deprotonated and remains in the carboxylic acid form. This
deprotonated carboxylic acid group expand the 2D MOF sheet to a 3D supramolecular
network through strong hydrogen bonding. Owing to its florescent nature, the ZnMOF was
used to screen anions, with CN ions shown to have a high affinity for interaction with the
framework through hydrogen bonding with the carboxylic acid group.
Additionally, the high affinity of the CN ions for interacting with the framework allowed the
Knoevenagel condensation, a C-C bond formation reaction between benzaldehyde and
malononitrile, to be performed. As this reaction used both the Lewis acid and Brønsted base
natures of the catalyst, the ZnMOF showed high efficiency toward this conversion.
Furthermore, hydrogen bonding interactions with the cyanide group of malononitrile, which
brought malononitrile close to the catalytic site, enhanced the conversion. This research is
particularly relevant to the design of 2D metal organic framework bearing bi-functional sites,
which act as heterogeneous catalysts for the C-C bond formation reactions.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (21702038),
Shenzhen Fundamental Research Project (JCYJ20170811161344569), Special Funds of Key
Disciplines Construction from Guangdong and Zhongshan Cooperating, and a startup grant
from the Harbin Institute of Technology, Shenzhen. We acknowledge Prof. Yingchun Li at
HITSZ for providing photoluminescence spectroscopy facility.
† Electronic supplementary information (ESI) available: Details of the synthesis and
characterization; experimental section; crystallographic information table; packing digram of

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1
ZnMOF; crystallographic information files (CIF); H NMR, 13C NMR, GC, GCMS and PL
spectra. Crystallographic data: CCDC-1985716 for ZnMOF. For ESI and crystallographic
data in CIF or other electronic format see DOI: xxx.
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FeixiongLiu:Experiments conductions;SandeepKumar:Experiments conductions, Writing;
Shuangshuang Li: Experiments conductions;Hengzhi You:Supervision; PengRen:Supervision,
Writing- Reviewingand Editing; LiminZhao: Supervision, Project Design

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Declaration of interests
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.

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Graphical abstract
A bi-functional MOF formed by the combination of Zn(II) and mixted triazole-carboxylate
based ligands for the detection of CN- over other ions and an efficient heterogenous catalyst
for the Knovengal Condensation Reaction

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Highlights


Design of bifunctional zincMOF, bearingtriazole-carboxylatemixed ligand
Heterogeneous catalyst for C-C bond forming reactions such as Knoevenagel
condensation



Highly efficientcatalysis undermild conditions, wateras solvent at roomtemperature
Selectivesensingof CN ion overotheranions by ZnMOF, usingflorescence technique
Detection of CN ion supports the proposed mechanismof Knoevenagel condensation