A mononuclear Cu(II)-based metal-organic framework as an efficient heterogeneous catalyst for chemical transformation of CO2 and Knoevenagel condensation reaction

Article abstract of DOI:10.1016/j.inoche.2019.01.022

A novel 2D Cu(II) metal-organic framework, [(Cu(L)₂·(H₂O)₂·(NO₃)₂]_n (namely 1), can be synthesized by the self-assemble of 4-(5-methyl-3-pyridine)-1,2,4-triazole (namely L) and Cu(II) ion.

Full text of DOI:10.1016/j.inoche.2019.01.022

Accepted Manuscript
A mononuclear Cu(II)-based metal-organic framework as an
efficient heterogeneous catalyst for chemical transformation of
CO2 and Knoevenagel condensation reaction
Feng Guo
PII:
S1387-7003(18)31120-1
DOI:
https://doi.org/10.1016/j.inoche.2019.01.022
INOCHE 7238
Reference:
To appear in:
Inorganic Chemistry Communications
Received date:
Revised date:
Accepted date:
19 December 2018
9 January 2019
16 January 2019
Please cite this article as: Feng Guo , A mononuclear Cu(II)-based metal-organic
framework as an efficient heterogeneous catalyst for chemical transformation of CO2
and Knoevenagel condensation reaction. Inoche (2019), https://doi.org/10.1016/
j.inoche.2019.01.022
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ACCEPTED MANUSCRIPT
1
A mononuclear Cu(II)-based metal-organic framework as an
efficient heterogeneous catalyst for chemical transformation
of CO and Knoevenagel condensation reaction
2
Feng Guo,a*
a
School of Chemistry and Chemical Engineering, Yangtze Normal University,
Chongqing 408100, China
Corresponding Authors: Prof. Feng Guo
E-mail address: guofeng1510@yeah.net

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Abstract
A novel 2D Cu(II) metal-organic framework, [(Cu(L)
2
(H
2
O)
2
(NO
3
)
2
]
n
(namely 1),
can be synthesized by the self-assemble of 4-(5-methyl-3-pyridine)-1,2,4-triazole
namely L) and Cu(II) ion. The as-synthesized MOF 1 has mononuclear {CuN O }
(
4
2
and only one coordination mode of this L. The inorganic parts and organic ligands are
connected by each other to construct a 2D layer framework. The obtained MOF 1 has
been characterized and analysed in detail by single crystal X-ray diffraction, powder
X-ray diffraction (PXRD), elemental analysis, Fourier-transform infrared spectra
(
FT-IR), and thermogravimetric analysis (TGA). Furthermore, MOF 1 has open Cu
metal sites and lots of N atoms as Lewis basic sites, resulting in that it can be
employed as a heterogeneous catalysis for chemical transformation of CO and
2
epoxides into targeted cyclic carbonate and Knoevenagel condensation reaction. It
also can be reused four times at least without significantly reduced activity for these
catalytic reactions.
Keywords: metal-organic framework; catalysis; carbon dioxide; epoxide;
Knoevenagel condensation reaction.
Up to now, super large amounts of metal-organic frameworks (MOFs),[1-3] as a
novel class of inorganic-organic hybrid crystalline materials, have really attracted
surge of interesting, not only thanks to their fascinating structures,[4-6] but also their
potential practical applications in small gas adsorption,[7-12] enzyme immobilization,[13,
1
4]
luminescence sensor,[15-27] optical device material,[28-30] and catalysis[31-35]. Among
allthe related works, design and preparation of novel MOFs with interesting
frameworks is still a very interesting research field. Various distinctive constructions
and topologies have been successfully designed and generated by rationally using the
synthetic strategy, including organic linker, metal cation, temperature, pH value,
pressure, and solvent system.[36-39] Innumerous influencing factors, it is almost no
doubt that the organic linkers really show a very important role to adjust and control
the skeleton structures. Multifarious methods have been tried to design and prepare

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novel linkers to synthesize interesting topologies and attractive frameworks. The
neutral ligands based on pyridine rings without any carboxylic acid groups have been
widely used to construct cationic MOFs,[40-44] which can be used as host materials for
exchanging or capturing pollutions. Inspired by many similar investigations on design
such MOFs, it is also very interesting research area to design and synthesize an
organic ligand to construct these cationic MOFs in the crystallographic point, which
not only can form lots of fascinating architectures but also excellent applications. As
we know, CO
affect people’s life and health. The most promising method to solve this problem is
CO capture and transformation. CO as C1 source can react with epoxides to
2
is a common greenhouse gas to result in a series of serious problems to
2
2
generate cyclic carbonates, which is a useful raw and processed material in numerous
industrial fields.[45] In addition, the Knoevenagel condensation reaction is also a
common organic reaction and widely used in many application fields. Hence, it is a
significant important job to design and prepare high-efficient heterogeneous catalysts
for these reactions.
In this communication, we selected a neutral organic ligand without carboxylic
acid groups 4-(5-methyl-3-pyridine)-1,2,4-triazole (namely L).[46] As we expected,
this ligand can be successfully assembled with Cu(NO
3
2
) to synthesize a novel
two-dimensional (2D) cationic MOF material, [(Cu(L)
2
(H
2
O)
2
(NO
3
)
2
]
n
(namely 1).
2.5H O
The blue block crystals can be generated via mixing this ligand and Cu(NO
3
)
2
2
in a 20 mL capped vessel containing water and EtOH mixed solution and evaporation
at room temperature for one week.[47] Meanwhile, the resultant 1 can be considered as
a high-efficient heterogeneous catalysis not only for chemical conversion of CO and
2
epoxides into cyclic carbonate without any solvent due to the open metal Cu(II) sites
as Lewis acid sites, but also can catalyze the Knoevenagel condensation reaction
because of their Lewis basic sites.
The single crystal X-ray diffraction data showed that the obtained 2D MOF 1
crystallizes in the trigonal crystal system and R–3c space group.[48] In the

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corresponding asymmetric unit, it contains a half Cu(II), one L organic ligand, a half
NO – –, and one terminal coordinated H
, one third NO O molecule (Fig. S1). As seen
3
3
2
in Fig. 1a, the L ligand has only one coordination mode, which connects two
crystallographic independence Cu(II) cations by the priding N atom and one atom in
the 1,3,4-triazole group. On the other hand, each Cu(II) has only one coordinated
mode and octahedral configurationas shown in Figs. 1b and 1c, which is linked with
four nitrogen atoms (Cu-N = 2.0227-2.0314 Å) form three different ligands and two
oxygen atoms (Cu-O = 2.4296 Å) coming from two coordinated water molecules. As
illustrated in Figs. 1d and 1e, the terminal framework shows a 2D structure and packs
as a hexagon structure, which also contains NO –
in all structures as counter ions. The
3
results obviously demonstrate that 1 not only has lots of open metal Cu(II) sites but
also contains lots of free nitrogen atoms as Lewis basic sitessimultaneously, resulting
in its great potential applications, especially in the catalytic field.
Fig. 1.(a) The coordination mode of the L ligand; (b and c) the coordination environment
octahedral configurationof Cu(II); (d and e) view of a single 2D network and the 2D networks
along different axises (the hydrogen atoms are omitted for clarity and C, black; N, blue; O, red; Cu,
green); (f) the triangle in the 2D layer.
As obviously displayedin Fig. 2a, the TGA result of the fresh as-synthesized 1
demonstrated a slow weight loss of 28.76% before 300 °C prevailingly due to the loss

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of guest and coordinated molecules (calculated 28.88%). When the heating
temperature further increased in air, the structure of MOF 1 began to decompose,
resulting in the last remnant CuO (expt: 14.83%; calcd: 14.71%). The PXRD profile
of the as-synthesized solid sample was carried outat ambient temperature. The
characteristic diffraction lines of prepared 1 are similar with the simulated from the
single crystal data of 1 (Fig. 2b). The PXRD profiles exhibited that the final phase
purity of the obtained crystalline materials.
Fig. 2. (a) The TGA data of the as-synthesized 1; (b) the PXRD patterns of the simulated (black)
and the as-synthesized (red) sample.
In virtue ofopen Cu(II) metal sites in 1, MOF 1 was applied to as a Lewis acid
catalysis to investigateits catalytic performancefor the solvent-free conversion of CO
2
and epoxides into cyclic carbonate (Table 1).[49-56] The as-synthesized 1 was collected
by filtering and drying in air. In a typical trial, a mixture of epoxide substrate
(
20mmol), catalyst (1, 100 mg), and tetrabutylammonium bromide (TBABr, 100mg)
as a co-catalyst was performed the catalytic reaction with 2.0 MPa CO at 85°C for 4
hours under the free-solvent condition. Table 1 contained the results of the CO
cycloaddition reaction with different epoxides, which was easily calculated by using
the GC method. As seen in entry 1, the propylene carbonate yield of CO and
-(chloromethyl)oxirane reached up to 99% after 4 hours. Nevertheless, the final
2
2
2
2
propylene carbonate efficiencies of only MOF 1 or TBABr were only 8% and 11%
under the same reaction besides the catalytic as control experiments to illustrate the
necessary of MOF 1 (entries 2 and 3). The different epoxides were further expanded

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to investigate the influence of different functional groups for this CO
2
reaction using
MOF 1 as catalytic. The measurement results illustrated that the yields were
decreased from 99% for propylene oxide to 91% for styrene oxide, 83% for tert-butyl
glycidyl ether, and 67% of styrene oxide, respectively (entries 4-6). Based on the
other similar reports, this performance can be predominantly ascribed to large-sized
epoxides and different electronic effects of these groups in substrates to influence the
catalytic results.[49-56] The catalytic mechanism may be attributed to the common
process as the other reports. Firstly, this reaction can be initiated by the interaction
between the open Cu(II) Lewis site and the oxygen atom in the epoxide ring. Then,
-
the epoxide rings are further opened by the Br ion from TBABr to attack the
less-hindered carbon atoms from the coordinated epoxides. Subsequently, the oxygen
anions of the opened epoxy rings can rapidly interact with the surrounding CO
2
molecules to form alkycarbonate anions. Finally, the corresponding cyclic carbonates
can be achieved through the cyclization process.[49-56]
Table 1. The catalytic results of cyclic carbonates from CO
2
and different epoxides.
O
1, TBABr
O
+
CO2
O
O
80℃, 2MPa
R
R
Entry
Substrate
Product
Yield (%)
O
O
1
Cl
99
O
O
Cl
O
O
2
3
Cl
8
O
O
Cl
O
O
Cl
11
O
O
Cl

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7
O
O
O
4
5
6
91
83
67
O
O
O
O
O
O
O
O
O
O
O
O
O
Meanwhile, the basic catalytic ability of 1 for the Knoevenagel condensation
reaction was further studied in detail due to the various Lewis basic sites in the whole
structure like the other previous reports.[57-65] The typical experiment was performed
as the following condition in Table 2: substrate (1 mmol), malononitrile (1.1 mmol),
catalyst (1, 100 mg), and toluene (5 mL) in a 20 mL glass flask at 80 °C under
continuously stirring. The corresponding yields of targeted products were successfully
monitored by applying the GC approach. Table 2 summed up the results based on
different aldehyde substrates and malononitrile in the presence of 1. As found in entry
1
of Table 2, the final yield of 2-benzylidenemalononitrile was about 96% after
reacting 3 hours. To prove the necessary of MOF 1 for this reaction, the catalytic
reaction almost didn’t happen in the absent of 1 as a control experiment (entry 2).
Various different substituted aldehyde reactants have been applied to study the effects
of different substituents with the catalysis 1. The catalytic results furthermore
demonstrated that the superior conversions (>99%) can be detected during existing
the withdrawing groups (–F and –NO
the electron donor groups on the aldehyde for one –OMe group in entry 5 and two
OMe groups in entry 6 were reduced to 89 and 74, respectively. The catalytic
2
, entries 3 and 4), while the catalytic yields of
–
performance is also similar with the previous reports.[57-65] The catalytic mechanism
for this reaction may be ascribed to lots of N atoms in MOF 1, which can served as
common Lewis basic sites to activate this reaction like other similar reports.[57-63]

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Table 2. The yields of Knoevenagel condensation reaction.
CN
CN
O
1
+
NC
CN
8
0 °C, toluene
R
H
R
Entry
Substrate
Product
Yield (%)
1
96
5
2
3
4
5
6
>99
>99
89
74
Additionally, it is significantly important for heterogeneous catalyses to handily
recollect and reuse after reactions. For MOF 1, it can be recycled by centrifuging at
-1
8
000 rmin for 3 minutes and further re-catalyse these two different reactions. The
reactions of CO and 2-(chloromethyl)oxirane, and benzaldehyde and malononitrileas
2
the reaction models. As shown in Fig. 3, Figs. S2, and S3, the PXRD profiles of
reused 1 after using four times and the catalytic performance both clearly indicated
that the structure of 1 can retain very well for these catalytic reactions and the
catalytic effects. The above mentioned results obviously illustrated that 1 can be
implemented as a bifunctional heterogeneous catalysis for the solvent-free chemical
transformation of CO and epoxides into cyclic carbonate, and the Knoevenagel
2

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condensation reaction.
Fig. 3. The recyclability of MOF 1 for chemical transformation of CO (a) and
2
Knoevenagel condensation reaction (b).
In conclusion, a novel mononuclear Cu(II)-based metal-organic framework was
successfully generated, which can be considered as an outstanding heterogeneous
catalyst for the CO cycloaddition reaction and Knoevenagel condensation reaction.
2
More importantly, MOF 1 exhibited excellent recyclability for these reactions at least
four times. We hope that this work can offer a useful and meaningful method to use
such ligands to synthesize MOFs with targeted functions.
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[
47]Preparation of [(Cu(L)
Cu(NO •2.5H O (10 mg, 0.043 mmol), L (5 mg, 0.031 mmol), ethanol (EtOH,
.5 mL) and H O (1 mL) was putted into a 20 mL capped vessel and evaporated
2
(H
2
O)
2
(NO
3
)
2
]
n
(namely 1):
A
mixture of
3
)
2
2
0
2
at room temperature for one week. The polyhedron blue crystals of the
as-synthesized 1 were collected by filtration and dried in air (yield: 58% based on
L). EA (%) Calc. for C16
H
20
N
10
8
O Cu: C, 35.29; H, 2.94; N, 25.74; Found: C,
-1
3
3
5.21; H, 2.98; N, 25.79. The selected FT-IR result (KBr pellet cm , Fig. S4):
410 (br), 3122 (s), 3054 (s), 1612 (s), 1545 (s), 1351 (s), 1205 (s), 1154 (s), 1111

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15
(
s), 1055 (s), 967 (s), 876 (s), 697 (s), 642 (s).
48]Crystal data: C48 : F = 1569.87, trigonal, space group R-3c, a=
6.5216(12), b= 16.5216(12), c= 46.798(4)Å, α= 90, β= 90, γ=120,V =
[
H
60
O
21
N
29Cu
3
w
1
3
-3
-1
1
1062.6(18) Å , Z = 6, Dc= 1.414 g cm , μ = 1.745 mm , R(int) = 0.021, Nref =
457, R =0.0753, wR = 0.2336 [I > 2σ(I)], R =0.0789, wR = 0.2402 [all data],
GOF= 1.080. The single crystal data of 1wasmeasured on a Bruker SMART CCD
diffractometer with Cu K (λ = 0.71073 Å) radiation at 293(2) K. The final crystal
2
1
2
1
2
α
structure can be solved by the direct method and further refined by the
2
full-matrixleast-squares on F of the obtained crystal data by using the
SHELXL-2015.[66] Allnon-hydrogen atoms were also refined through anisotropic
displacement parameters. The selected bond lengths and angles of 1 are
bothsummarised in Table S1.
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2
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Graphical Abstract-Pictogram
A novel mononuclear Cu(II)-based MOF was successfully generated and used as an
heterogeneous catalyst for the CO
2
cycloaddition reaction and Knoevenagel
condensation reaction.

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Highlights
1
2
3
. A novel 2D MOF was fabricated by neutral ligand and Cu(II).
. It can be served as a bifunctional heterogeneous catalysis.
. It possesses excellent recyclability at least four times.