Synthesis, crystal structure and catalytic property of a highly stable 3D Cu(II)-organic framework

Article abstract of DOI:10.1016/j.molstruc.2020.127923

A three-dimensional (3D) Cu(II)-based MOF, [CuL·Cl]_n (namely complex 1), was successfully generated through the in-situ reaction of CuCl₂, NaN₃, and 4-(4-cyanostyryl)pyridine. Notably, –CN and N₃ ^? ca

Full text of DOI:10.1016/j.molstruc.2020.127923

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Synthesis, crystal structure and catalytic property of a highly stable 3D Cu(II)-organic
framework
Chunyan Chen
PII:
S0022-2860(20)30247-7
DOI:
https://doi.org/10.1016/j.molstruc.2020.127923
MOLSTR 127923
Reference:
To appear in: Journal of Molecular Structure
Received Date: 19 January 2020
Revised Date: 16 February 2020
Accepted Date: 16 February 2020
Please cite this article as: C. Chen, Synthesis, crystal structure and catalytic property of a highly stable
3D Cu(II)-organic framework, Journal of Molecular Structure (2020), doi: https://doi.org/10.1016/
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Graphical Abstract
A 3D Cu(II)-based MOF was successfully synthesized and used as a heterogeneous
catalyst for the CO2 cycloaddition reaction and Knoevenagel condensation reaction.

1
Synthesis, crystal structure and catalytic property of a
highly stable 3D Cu(II)-organic framework
Chunyan Chen,a*
a
School of Materials & Food Engineering, Zhongshan Institute, University of
Electronic Science & Technology of China, China.
Corresponding Author: Chunyan Chen
E-mail address: yannanfei800@126.com

2
Abstract
A three-dimensional (3D) Cu(II)-based MOF, [CuL·Cl]n (namely complex 1), was
successfully generated through the in-situ reaction of CuCl2, NaN3, and
–
4-(4-cyanostyryl)pyridine. Notably, –CN and N3 can directly react to form a
tetrazolate group during the crystal growth process. This reaction was further proved
by single crystal X-ray diffraction data. Thanks to the excellent stability, various
N-donors as Lewis basic sites and Cu(II) centres as Lewis acid sites, the as-made
complex 1 has a potential application as a bifunctional heterogeneous catalyst for the
CO2 cycloaddition reaction and Knoevenagel condensation reaction. In addition,
complex 1 has the excellent recyclability for both reactions at least six cycles.
Keywords: In situ reaction; heterogeneous catalyst; CO2 cycloaddition reaction;
Knoevenagel condensation reaction; recyclability.
1. Introduction
Recently, metal–organic frameworks (MOFs) [1,2] are organic-inorganic hybrid
crystalline materials, which have been developed rapidly thanks to their various
structures and greatly potential applications in small gas sorption [3-5], bio-carrier
[6,7], luminescent detector [8-10], optical device [11,12], and catalysis [13,14].
Compared with other solid materials, MOFs have lots of unique features, including
structural diversity, designability, multifunctional site, and adjustability [15-17].
Taking into account of some previous reports, diversiform MOFs can be tuned and
constructed by rationally choosing metal centres and organic building blocks with
various coordination sites to achieve excellent properties [18-20]. Among all
properties, MOFs exhibit greatly potential applications as heterogeneous catalysts for
many catalytic reactions because of their catalytic centres, including Lewis acid sites
(open metal site) [21-23], Brønsted acidic sites (–COOH and –OH) [24-26], and
Lewis basic sites (–NH2) [27-29]. However, the poor stability of MOFs greatly limits
the development of these materials as heterogeneous catalysts in practical applications.
Up to now, some useful approaches have been developed to enhance the stability of

3
MOFs to expand their real usability, such as surface hydrophobic coating, soft and
hard acid base theory, multi-wall and high connected multicore structure [30-32].
Bridging organic linkers have significantly important influences for the structures and
properties of MOFs. Most attentions have been still focused on carboxylic acid
organic ligands due to their excellent coordination abilities and easy preparation of
MOFs [33,34]. On the other hand, another sort of linkers focuses on the heterocyclic
nitrogen rings (pyridine, imidazole, triazole, and tetrazole) in the assemble process of
MOFs with transition metals [35-38]. Compared with the carboxylic acid linkers, such
linkers are always difficult to prepare MOFs, but the as-synthesized MOFs always
have the excellent stability. In addition, the free N-donors and metal centers in MOFs
can be served as Lewis basic sites and Lewis acid sites for different catalytic reactions
[39-41]. Hence, it is a significant change to construct highly stable MOFs based on
organic linkers with various N-donors to investigate their catalytic applications.
Based on the above discussion, we chosen an organic linker
4-(4-cyanostyryl)pyridine with pyridine and cyano groups as a bridging linker to
–
assemble with Cu(II) and NaN3. The cyano group can react with N3 to generate
tetrazole groups in suit, which was further assembled with Cu(II) to construct MOFs.
As we expect, a novel three-dimensional (3D) Cu(II)-organic framework, [CuL·Cl]n
(namely complex 1), has been successfully prepared via the solvothermal reaction;
meanwhile, the tetrazole groups are found in the structure of complex 1. The
as-prepared sample was detailly characterized and analysed by lots of testing methods.
More importantly, complex 1 exhibits the excellent catalytic performance for the CO2
cycloaddition reaction and Knoevenagel condensation reaction with the outstanding
recyclability.
2. Experimental
2.1. Materials and general methods
All chemicals and solvents were purchased and used directly. Powder X-ray
diffraction (PXRD) patterns were carried out on a Bruker D8 GADDS from 5° to 40°.

4
Thermogravimetric analyses (TGA) data were measured on a TGA Q500 under N2
from room temperature to 800 °C. The catalytic yield was obtained on a Thermo
Fisher Trace ISQ GC/MS. Elemental analysis of C, H and N was performed on a
CE-440 analyzer (Leeman Laboratories).
2.2. Synthesis of complex 1
CuCl2 (0.1 mmol), NaN3 (0.1 mmol), and 4-(4-cyanostyryl)pyridine (0.1 mmol) were
all added in a mixture solution of ethanol (1.2 mL) and H2O (2.8 mL) and carefully
sealed in a Teflon-lined steel. The mixture was putted and heated at 165 °C for 4 days.
Then, it was cooled slowly to room temperature at a decreasing rate of 5 °C h–1.
Finally, blue crystals with high quality were collected and washed with ethanol. These
crystals were dried in air with the yield of 61% based on the linker. Anal. Calc. (%)
for C14H10CuN5: C, 53.85; H, 3.21; N, 22.44. Found: C, 54.01; H, 3.26; N, 22.39.
2.3. Single crystal X-ray diffraction of complex 1
Single crystal X-ray diffraction of complex 1 was measured on a Bruker SMART
CCD diffractometer with Mo Kα (λ = 0.71073 Å) radiation. The structure of complex
1 can be well solved by using SHELXL-2015 [42,43] in the OLEX2 program [44]. All
non-H atoms were added by the anisotropic parameters. The crystal data and structure
refinement of complex 1 are summarized in Table 1.
2.4. The Knoevenagel condensation reaction
In a typical catalytic reaction, benzaldehyde with different substitutional groups (1.00
mmol) and malononitrile (1.05 mmol) were both dissolved in toluene (4 mL) in a
reaction bottle with complex 1 (100 mg). The reaction mixture was heated in an oil
bath at 100 °C for 4 hours under slowly stirring. The catalytic yields can be calculated
by using the GC method with biphenyl as an internal standard substance.
2.5. The CO2 cycloaddition reaction

5
In a typical reaction, epoxides (10 mmol), n-Bu4NBr (0.35 mmol), and complex 1
(100 mg) were all added in a high-pressure reaction equipment with CO2 (1 MPa) at
100 °C for 8 hours. The yields of cyclic carbonates can be obtained by the GC
approach with n-dodecane as an internal standard.
Table 1. Crystal data and structure refinement.
Sum formula
Formula weight
Crystal system
Space group
a (Å)
C14H10CuN5
311.82
monoclinic
P21/c
13.4232(12)
9.1569(9)
10.1126(9)
90
b (Å)
c (Å)
α (º)
β (º)
99.916(2)
90
γ (º)
Volume (Å3)
1224.4(2)
4
Z
Dx (g cm-3)
Mu (mm-1)
Nref
1.692
1.779
2621
R(int)
0.0336
Goodness-of-fit on F2 1.045
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
0.0885, 0.0875
0.0467, 0.0915
3. Results and discussion
3.1. Structural description for complex 1
The as-synthesized crystals can be generated by mixing 4-(4-cyanostyryl)pyridine,

6
CuCl2, and NaN3 in a mixture solution of water and ethanol. The large single crystal
of complex 1 with high-quality was used to collect the single crystal X-ray diffraction
data, illustrating that it crystalized in the P21/c space group of the monoclinic system
with the cell parameters of a = 3.4232(12) Å, b = 9.1569(9) Å, c = 10.1126(9) Å, α =
90°, β = 99.916(2)°, γ = 90°. The asymmetric structural unit of complex 1 has one Cu
atom and an organic linker, which evidently exhibits the formation of tetrazole in this
linker as the similar report [45] (Fig. 1a). Fig. 1b shows the coordination mode of the
bridging linker as µ4- : : : η
η1 η1 η1 1 to connect with four Cu atoms by one pyridine and
three N-donors from tetrazole rings. Each Cu atom links with four N-donors from
three different tetrazole rings and one pyridine. Interestingly, all Cu atoms and
tetrazole rings can generate a two-dimensional (2D) layer (Fig. 1c), which are further
connected with each other by pyridine rings from linkers to finally construct a 3D
structure. The 3D structure of complex 1 is shown in different directions (Figs. 1d-1f).
<Figure 1>
3.2. PXRD and thermal analysis
As displayed in Fig. 2a, the PXRD peaks of as-synthesized bulk samples were
measured at room temperature, which are consistent well with those of the simulated
pattern. The result clearly proves that the as-synthesized samples are purity as the
obtained single structure. The TGA data of as-synthesized complex 1 indicates that
the resultant sample has the outstanding stability before ~300 °C; meanwhile, the
skeleton of complex 1 shows a rapid weight loss with the increasing of heating
temperature (Fig. 2b). The TGA result shows that complex 1 has the excellent heating
stability.
<Figure 2>
3.3. Catalytic properties
Thanks to lots of Lewis base sites in complex 1, the as-synthesized sample may be
investigated and applied as a basic catalyst for the Knoevenagel condensation reaction

7
[46-51]. Prior these catalytic reactions, as-synthesized 1 was soaked in fresh toluene
for 1 hour, which was further filtered and dried in air. In the typical catalytic reaction,
the benzaldehyde containing various substitutional groups (1.00 mmol) and
malononitrile (1.05 mmol) were both dissolved in 4 mL toluene in a reaction bottle.
The addition of complex 1 (100 mg) in the reaction system is considered as a catalyst.
The reaction was heated in an oil bath at 100 °C for 4 hours under slowly stirring. The
yields can be obtained by the GC method with biphenyl as an internal standard
substance. All catalytic results in this work are summarized in Table 2. As seen in
entry 1, the product yield of 2-benzylidenemalononitrile is higher than 99% within 4
hours. This reaction was further selected as a reaction mode to investigate the kinetic
catalytic rate (Fig. 3). The catalytic result indicates that the catalytic reaction can be
almost completely reacted after 4 hours at 100 °C. To evaluate the necessity of
complex 1 for the reaction, the reaction will quickly stop once filtering complex 1
from the reaction system. In addition, the yield of this reaction is only 9% in the
absence of complex 1 (entry 2). The control experiments illustrate that complex 1 is
the necessary catalysis for the Knoevenagel condensation reaction. The catalytic
substrates were expanded with different sizes and functional groups under the same
reaction condition. It evidently exhibits that the catalytic results are also higher than
99% (entries 3 and 4), which is mainly attributed to the electron-withdrawing groups
of –F and –NO2 in aldehydes to facilitate the reaction. Contrastively, the aldehydes
with the electron-donor and large size group of –OMe show lower catalytic yields to
88% and 81% for one –OMe group (entry 5) and two –OMe groups (entry 6),
respectively. The catalytic performance is similar with some previous reports [46-51].
<Figure 3>
Table 2. All catalytic yields of different aldehyde reactants.
CN
CN
O
+
NC
CN
R
H
R

8
Entry
1
Substrate
Product
Yield (%)
>99
CN
CN
O
H
CN
CN
O
2
3
4
5
6
9
H
CN
CN
O
>99
>99
88
H
F
F
CN
O
CN
H
O2
N
O
N
2
O
CN
H
CN
O
O
CN
O
81
CN
H
O
O
O
O
The stability and recyclability of complex 1 are both important factors for
heterogeneous catalysts. After each reaction of benzaldehyde as the substrate,
complex 1 can be easily re-collected by centrifuging at 8000 r min-1 for 2 minutes.
The reused sample can be recovered after washing with toluene for 2 times. As seen
in Fig. 4a, the catalytic yields of reused samples can be well kept even after recycling
six times. The corresponding PXRD profile of complex 1 after reusing six times are
well preserved to prove the high stability of complex 1 for this reaction (Fig. 4b).
<Figure 4>
Because of the metal sites as Lewis acid catalytic centres, complex 1 can be
considered as a potential heterogeneous catalyst for the CO2 cycloaddition reaction
with epoxides to form cyclic carbonates [52-62]. The prepared samples after washing

9
with CH2Cl2 were directly dried in air before the catalytic reaction of CO2 and
epoxides. The reaction condition is set as 1 MPa CO2 at 100 °C for 8 hours by mixing
epoxides (10 mmol), n-Bu4NBr (0.35 mmol), and complex 1 (100 mg) in a
high-pressure reaction equipment under slowly stirring. The yields of targeted cyclic
carbonates can be calculated by the GC approach with n-dodecane as an internal
standard substance. All catalytic results are concluded in Table 3. Only complex 1 or
n-Bu4NBr as the catalyst for epichlorohydrin and CO2 has a very low catalytic yields
of complex 1 (31%, entry 1) or n-Bu4NBr (~9%, entry 2), displaying that they can’t
catalyse this CO2 reaction by independent one. However, complex 1 is able to
catalyse this reaction with a high yield of >99% with n-Bu4NBr simultaneously (entry
3). All results claim that complex 1 and n-Bu4NBr can catalyse this reaction by the
synergy catalysis process. Different epoxides are selected as substrates for this CO2
reaction. The yields gradually reduce to 91% (entry 4), 82% (entry 5), and 79% (entry
6) for 1,2-epoxy-3-allyloxypropane, benzyl phenylglycidyl ether, and tert-butyl
glycidyl ether, which may be mainly ascribed to the different groups with different
size and electronic effects [52-62]. The recyclability is also investigated by the
reaction and the corresponding PXRD pattern. As shown in entry 7 and Fig. 5a, the
catalytic performance of recollected samples can be well preserved for the chemical
transformation of epichlorohydrin and CO2 for six times. The PXRD pattern can well
preserve the main diffraction peaks to illustrate the high stability of recycled complex
1 after the successive using six times (Fig. 5b).
Table 3. The yields of this CO2 cycloaddition reaction with epoxides.
O
O
CO2
O
O
1 MPa
R
R
Entry
Catalyst
Epoxide
Product
Yield%

10
O
1
complex
1
31
O
O
Cl
Cl
O
O
2
3
4
n
-Bu4NBr
9
O
O
O
O
Cl
Cl
Cl
Cl
O
O
complex
1
+
>99
91
n
-Bu4NBr
O
O
O
complex
1 +
O
O
n
-Bu4NBr
O
O
O
O
5
complex
1
+
82
O
O
n
-Bu4NBr
O
O
O
O
6
7
complex
1
+
79
O
O
n
-Bu4NBr
O
O
O
Reused complex
-Bu4NBr
>99
O
O
1
+ n
Cl
Cl
O
<Figure 5>
The assumptive catalytic mechanism can be deduced from the previous reports for
this CO2 fixating reaction (Fig. 6) [52-62]. This reaction can be caused by the weak
interaction between O atoms in epoxide rings and Cu(II) sites in complex 1. Epoxide
rings will rapidly attacked by Br– from n-Bu4NBr to open the epoxide rings. Then,
oxygen anions can be reacted with CO2 to form the ionic alkycarbonate intermediate.
The cyclic carbonate product will generate by the cyclization process of the
alkycarbonate anion. However, the accurate mechanism is also significantly far
beyond our research capabilities.
<Figure 6>
4. Conclusion

11
In summary, a 3D Cu(II)-organic framework based on 2D Cu-tetrazole layers can
be constructed and served as a bifunctional heterogeneous catalyst for the
Knoevenagel condensation reaction and chemical fixation of CO2 with epoxides. Due
to the excellent stability of complex 1, it has the excellent stability and recyclability
for both catalytic reactions at least six cycles.
Appendix A. Supplementary data
The luminescent spectra and crystal table are listed in the supporting information.
The CCDC reference number is 1977907.
Acknowledgements
This research did not receive any specific funding.
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Fig. 1. (a) The asymmetrical unit; (b) the coordination mode of this organic linker; (c)
the 2D Cu(II)-based layer; and (d-f) the 3D structure of complex 1 (H atoms are
removed for clarity).
Fig. 2. (a) The PXRD pattern of simulated (bottom, black) and as-synthesized
complex 1 (up, red); and (b) the TGA curve of as-synthesized complex 1.
Fig. 3. The kinetic catalytic rates of benzaldehyde as the substrat in complex 1 (black),
after filtering complex 1 (blue), and without any catalyst (pink).
Fig. 4. Recyclability (a) and PXRD profiles (b) of complex 1.
Fig. 5. The recyclability (a) and the corresponding PXRD profiles (b) of complex 1.
Fig. 6. The assumptive catalytic mechanism for this CO2 fixating reaction.

1
Synthesis, crystal structure and catalytic property of a
highly stable 3D Cu(II)-organic framework
Chunyan Chen,a*
a
School of Materials & Food Engineering, Zhongshan Institute, University of
Electronic Science & Technology of China, China.
Corresponding Author: Chunyan Chen
E-mail address: yannanfei800@126.com

2
Figure 1
Figure 2

3
Figure 3
Figure 4
Figure 5

4
Figure 6

1. A Cu(II)-organic framework was successfully synthesized via the solvothermal
reaction.
2. It can catalyse the Knoevenagel condensation reaction and chemical fixation of
CO2 with epoxides.
3. The as-synthesized sample has the excellent stability and recyclability.

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.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: