2D network structure of zinc(II) complex: A new easily accessible and efficient catalyst for the synthesis of pyrazoles

Article abstract of DOI:10.1002/aoc.6379

A Zn(II) coordination polymer {[Zn₃(paz)(btc)₂(H₂O)₈]·4H₂O}_n (noted as ZnPB), where paz = pyrazine, H₃btc = trimesic acid, was synthesized and characterized by single-crystal X-r

Full text of DOI:10.1002/aoc.6379

Received: 19 May 2021
DOI: 10.1002/aoc.6379
Revised: 1 July 2021
Accepted: 7 July 2021
R E S E A R C H A R T I C L E
2D network structure of zinc(II) complex: A new easily
accessible and efficient catalyst for the synthesis of
pyrazoles
Gang-Ming Cao
Xiao-Ling Lin
| Guo-Dong Zeng
| Guo-Ping Yang
| Ke Li
| Yu-Feng Liu
|
Jiangxi Key Laboratory for Mass
Spectrometry and Instrumentation,
Jiangxi Province Key Laboratory of
Synthetic Chemistry, East China
University of Technology, Nanchang,
China
Abstract
A Zn(II) coordination polymer {[Zn3(paz)(btc)2(H2O)8]ꢀ4H2O}n (noted as
ZnPB), where paz = pyrazine, H3btc = trimesic acid, was synthesized and
characterized by single-crystal X-ray diffraction, elemental analysis, infrared
spectroscopy and powder X-ray diffraction analyses. ZnPB shows a (3,3)-
connected 2D network with a point symbol of (63)2. Hydrogen bonds and πꢀꢀꢀπ
interactions connect adjacent 2D network to form a 3D packing structure. This
compound exhibits excellent catalytic activity in the reaction of condensation
and cyclization of sulfonyl hydrazides with 1,3-diketones for the synthesis of
pyrazoles. This solvent-free and halogen-free catalytic system represents an
effective economic and environmentally friendly method for the construction
of pyrazoles.
Correspondence
Guo-Ping Yang, Jiangxi Key Laboratory
for Mass Spectrometry and
Instrumentation, Jiangxi Province Key
Laboratory of Synthetic Chemistry, East
China University of Technology,
Nanchang, Jiangxi 330013, China.
Email: erick@ecut.edu.cn
Funding information
Research Found of East China University
of Technology, Grant/Award Numbers:
DHBK2019264, DHBK2019265,
DHBK2019267; Open Fund of the Jiangxi
Province Key Laboratory of Synthetic
Chemistry, Grant/Award Number:
JXSC202008; National Natural Science
Foundation of China, Grant/Award
Number: 22001034
K E Y W O R D S
2D network, condensation reaction, hydrazine, pyrazoles, zinc(II) complex
1 | INTRODUCTION
importance, various synthetic approaches have been
developed for the synthesis of pyrazoles over the past
Nitrogen-containing heterocycles are key core structures
that form the basis of many natural products, pharma-
ceutical and agrochemical.[1–7] Among them, pyrazole
moiety is synthetic unit of utmost importance in the
pharmaceutical industry,[8,9] which is known to have
abundant of strong biological activity such as
antipyretic,[10] antibacterial,[11] and pesticidal.[12] Nowa-
days, as privileged structure, pyrazole core skeleton are
found in many drugs, such as celecoxib (A),[13] lonazolac
(B),[14] and penthiopyrad (C),[14] which are already
booming in the market (Figure 1). Given its extreme
decades. It can be synthesized via 1.3-dipolar cycloaddi-
tion of diazo compounds,[15] reaction of acetylenic
ketones,[16] N-sulfonylhydrazones,[17] or chalcones[18]
and hydrazines.[19] But the direct condensation of hydra-
zines with 1,3-diketones is a more efficient method to
successfully obtain pyrazole derivatives, it represents the
most straightforward and effective method owing to sig-
nificantly high overall reaction efficiencies and improved
atom economy.[20] In this regards, various catalysts have
been explored to catalyze this reaction with water as sole
by-product. For instant, H2SO4,[10] Sc(OTf)3,[21]
Appl Organomet Chem. 2021;e6379.
wileyonlinelibrary.com/journal/aoc
© 2021 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.6379

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CAO ET AL.
FIGURE 1 Typical pharmaceuticals containing a
pyrazole moiety
SCHEME 1 ZnPB catalyzed the
condensation cyclization
FIGURE 2 Coordinating environment of (a) ZnPB, (b) btc ligand, and (c) paz ligand
P2O5ꢀSiO2,[22]
nano-SSA,[3]
PSSA,[23]
[a-Zr
and
As part of ongoing research on green catalysis
chemistry,[26–31] we focus on the coordination polymers
(CPs) constructed by transition-metal ions and organic
ligands, which had been studied widely in the past
decades due to their multifaceted chemical and physical
properties as well as the potential application in gas/ion
adsorption, catalysis, conductivity, luminescence, sensor,
and so on.[32] CPs constructed by Zn(II) ions or clusters
show advantages in heterogeneous catalysis due to the
d10 electron configuration and conjugative effects. For
instance, Tran et al. reported the synthesis of
2-benzylidenemalononitrile catalyzed by ZIF-8.[33] Arai
et al. reported a novel trinuclear Zn(II) complex and cata-
lyzed asymmetric iodolactonization in up to 99.9% ee.[34]
Markad et al. synthesized two new Zn(II) CPs for the cat-
alyze of Michael addition of malononitrile to
2-enoylpyridines with 99% conversion.[35] All above men-
tioned indicated that Zn complexes have a good role in
catalytic organic synthesis.
(CH3PO3)1.2(O3PC6H4SO3H)0.8],[24]
(C2H8N)12Na2[H10{Ce(H2O)5}2(Te2W37O132)]ꢀ39H2O[25]
have been developed for the preparation of pyrazoles via
condensation of hydrazines with 1,3-diketones. Although
these reported procedures have their own merits, they
also have suffered from various drawbacks, such as harsh
reaction conditions, inconvenient operation, large load-
ing, and difficult preparations of the catalyst. And the
reaction was usually carried out in homogeneous solu-
tion thus far. Thus, product–catalyst separation remains
a big challenge, as homogeneous catalysts are difficult to
be separated from the product mixtures for recycling,
which strongly hamper their extensive application. None-
theless, despite the significance of these reports, it is
highly desirable to develop a facile, efficient approach to
synthesize pyrazoles using an inexpensive heterogeneous
catalyst, with excellent activity, in an operationally sim-
ple and environmentally friendly manner.

CAO ET AL.
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Hence, we reported
a
Zn(II) CP {[Zn3(paz)
system represents an effective economic and environ-
mentally friendly method for the construction of
pyrazoles (Scheme 1).
(btc)2(H2O)8]ꢀ4H2O}n (ZnPB), which was constructed by
cheap pyrazine and trimesic acid in conventional aque-
ous solution. Single-crystal X-ray analysis reveals that
ZnPB shows a (3,3)-connected 2D network with a point
symbol of (63)2; hydrogen bonds and πꢀꢀꢀπ interactions
connect adjacent 2D network to form a 3D packing struc-
ture. This compound exhibits excellent catalytic activity
in the reaction of condensation and cyclization of
sulfonyl hydrazides with 1,3-diketones for the synthesis
of pyrazoles. This solvent-free and halogen-free catalytic
2 | RESULTS AND DISCUSSION
2.1 | Crystal structure
Single-crystal analysis shows that ZnPB has a (3,3)-
connected 2D network with P-1 space group. The
FIGURE 3 (a) The 2D network of ZnPB and (b) schematic depiction of the (3,3)-connected 2D network
FIGURE 4 Schematic depiction of the (a) hydrogen bonds and (b) πꢀꢀꢀπ interactions in 1

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CAO ET AL.
SCHEME 2 Gram-scale synthesis of pyrazoles
TABLE 1 Crystallographic data of ZnPB
Complex
CCDC No.
Formula
Fw
ZnPB
2052269
C22H34N2O24Zn3
906.62
T/K
293.15
Crystal system
Space group
a (Å)
Triclinic
P-1
7.6645(11)
10.1903(14)
10.7263(15)
87.218(2)
89.250(2)
82.928(2)
830.4(2)
462
b (Å)
c (Å)
α (ꢁ)
β (ꢁ)
γ (ꢁ)
V (Å3)
F(000)
Z
FIGURE 5 Recyclability of ZnPB in direct condensation
reaction of 1a and 2a under the optimized reaction conditions
1
ρcalcd (g cmꢂ3
μ (mmꢂ1
)
1.813
asymmetric unit of ZnPB consists of two Zn(II) atoms,
half paz ligand, one btc ligand, four coordinating water
molecules, and two solvent water molecules. Zn1 is coor-
dinated with three carboxylate oxygen atoms from two
btc ligands (O1/O3#1/O4#1), one nitrogen atom from
one paz ligand (N1), and two coordinating water mole-
cules (O7/O8) with a distorted octahedral geometry. Zn2
is located in the center of a slight distorted octahedral
geometry, coordinated with two carboxylate oxygen
atoms (O5/O5#2) from two btc ligands and four water
molecules (O9/O10/O9#2/O10#2) (Figure 2a). The btc
ligands in ZnPB show monodentate mode in two carbox-
ylate group (O1O2/O5/O6) and monochelating mode for
O3O4. Thus, one btc ligand coordinates with three
Zn(II) atoms (Zn1/Zn2/Zn1#4) (Figure 2b). The paz
ligand shows a C2 symmetry and coordinates with two
Zn1 atoms (Zn1/Zn1#3) (Figure 2c).
)
2.249
Reflections collected
Unique reflections
Parameter
6615
3541 (Rint = 0.0144)
248
GOF
1.053
R1a (I ≥ 2σ(I)]
wR2b (all data)
0.0300
0.0921
aR1 = Σ(jjF0j ꢂ jFcjj)/ΣjF0j.
2
bwR2 = Σ[w(F0 ꢂ F02)2]/Σ[w(F02)2]1/2
.
be simplified as a three-connected node. Both Zn2 and
paz ligand are simplified as two-connected nodes because
they only connect two btc ligands or Zn1 atoms. Thus,
the three-connected Zn1 atoms and btc ligands are con-
nected by two-connected Zn2 and paz ligands to form a
2D network (Figure 3a). The 2D structure of ZnPB can
be simplified as a (3,3)-connected network with a point
symbol of (63)2 (Figure 3b).
Topologically, each Zn1 is coordinated with two btc
ligand and one paz ligand, which can be simplified as a
three-connected node. The btc ligands in ZnPB can also

CAO ET AL.
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TABLE 2 Evaluation of reaction
parameters
Entry
Catalyst (mol%)
—
Temperature (ꢁC)
Time (min)
Yielda (%)
1
2b
r.t.
r.t.
r.t.
40
60
80
80
80
80
15
15
15
15
15
15
30
60
60
29
41
63
69
75
82
93
99
92
ZnSO4
ZnPB
3
4
ZnPB
5
ZnPB
6
ZnPB
7
ZnPB
8
9c
ZnPB
ZnPB
Note: Reaction conditions: naphthalene-2-sulfonohydrazide 1a (1 mmol), 1,3-diketone 2a (1 mmol), ZnPB
5 mol%, EtOH (0.1 ml).
aThe yields were determined by GC with biphenyl as the internal standard.
bThe loading of ZnSO4 based on Zn2+
cThe loading of ZnPB 3 mol%.
.
Four coordinated water molecules and two solvent
water molecules in ZnPB connect adjacent 2D network
with hydrogen bonds and πꢀꢀꢀπ interactions to form a 3D
packing structure (Figure 4a,b). The distances of donator
and acceptor of hydrogen bonds are 2.763(4) Å for
was only 41% (Table 2, Entry 2). When the ZnPB (5 mol
%) was employed as catalyst under the identical condi-
tions, the desired product 3a was obtained in 63% yield
(Table 2, Entry 3). Afterward, the reaction temperature,
the reaction time, and the loading of ZnPB were
explored (Table 2, Entries 4–9). The results clarified that
both the increase of reaction temperature and the exten-
sion of reaction time could make the reaction proceed
smoothly. Using 5 mol% ZnPB at 80ꢁC, 60 min
produced the best outcome (Table 2, Entry 8).
The substrate scope was explored under optimized
conditions by using 5 mol% ZnPB as the catalyst. Table 3
shows the results of ZnPB-catalyzed condensation
cyclization of sulfonyl hydrazides with 1,3-diketones.
Naphthalene-2-sulfonohydrazide 1a reacted with
1,3-diketone 2a under the optimized reaction conditions,
giving 99% yield of the desired pyrazole product 3a
(Table 3, Entry 1). Benzenesulfonyl hydrazide derivatives
with functional groups are suitable for this reaction with
a slight adjustment of reaction temperature and reaction
time. Both electron-withdrawing and electron-donating
substituted groups in benzene ring of benzenesulfonyl
hydrazide reacted with acetylacetone, producing the
desired products in excellent yields (Table 3, Entries 2–8).
To our delight, benzenesulfonyl hydrazide bearing halo
substituted groups such as Cl and Br could also converted
to the desired pyrazoles (Table 2, 3e–3h), which provides
the potential for further functionalization via cross
O7ꢀꢀꢀO1#1, 2.692(3)
Å
for O8ꢀꢀꢀO6#3, 2.731(3)
Å
for O9ꢀꢀꢀO4#4, and 2.786(3) Å for O10ꢀꢀꢀO2#5. And the
solvent water O12 also connects adjacent 2D network
with hydrogen bonds with the distances of 2.788(4) Å for
O12ꢀꢀꢀO2#6 and 2.795(4) Å for O12ꢀꢀꢀO5#7. The benzene
rings of btc ligands from adjacent 2D network have πꢀꢀꢀπ
interactions with the distance of 3.924 and 4.134 Å for
the centers of benzene rings. The FT-IR spectrum and
PXRD patterns of ZnPB is shown in Figures S1 and S2.
2.2 | Catalytic activity
To the best of our knowledge, Zn CPs has excellent Lewis
acid catalytic activity. In order to explore the catalytic
performance of ZnPB, in the preliminary experiments,
we performed condensation cyclization reaction with
naphthalene-2-sulfonohydrazide 1a and 1,3-diketone 2a
as the substrates and screened a series of reaction condi-
tions. First, the control experiment indicated the reaction
could be occurred without a catalyst (Table 2, Entry 1).
The catalytic performance of ZnSO4 as the raw material
for the synthesis of ZnPB was evaluated, and the yield

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CAO ET AL.
TABLE 3 Substrate scope of ZnPB-catalyzed condensation cyclization reaction
Entry
Hydrazide
1,3-Diketone
Conditions
Product
3a
Yielda (%)
1
80ꢁC, 60 min
99
2
3
4
5
6
7
8
80ꢁC, 30 min
80ꢁC, 45 min
80ꢁC, 60 min
100ꢁC, 90 min
100ꢁC, 90 min
100ꢁC, 90 min
100ꢁC, 90 min
3b
3c
3d
3e
3f
99
99
99
97
95
92
95
3g
3h

CAO ET AL.
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TABLE 3 (Continued)
Entry
Hydrazide
1,3-Diketone
Conditions
Product
3i
Yielda (%)
9
80ꢁC, 30 min
99
96
98
96
10
11
12
100ꢁC, 90 min
80ꢁC, 45 min
80ꢁC, 60 min
3j
3k
3l
Note: Reaction conditions: hydrazide 1 (1 mmol), 1,3-diketone (1 mmol), ZnPB (5 mol%), EtOH (0.1 ml).
aIsolated yield.
coupling. Furthermore, the difference in yields of 3e–3g
indicated the same substituents at different positions had
slight influence on the efficiency of this transformation
(1e > 1f > 1g). Subsequently, the scope of the reaction
was further explored using a variety of 1,3-diketones.
3-Methylpentane-2,4-dione 2b and 3-chloropentane-2,
4-dione 2c could also react well with various
benzenesulfonyl hydrazide, delivering an excellent yields
of 96% to 99% (Table 3, Entries 9–12).
were carried out by using ZnPB as the catalyst under the
optimal reaction conditions. The results shown in Figure 5
indicated that the reaction did not exhibit any significant
change in yield after six cycles, the catalyst was stable
under the reaction conditions, and ZnPB could be used at
least for six times without significant loss of activity.
3 | CONCLUSIONS
Moreover, the gram-scale synthesis was investigated
under identical conditions (Scheme 2). To our delight,
the present protocol was found to be scalable and practi-
cal. The 97% and 93% yields of corresponding products
were obtained, respectively, when the reaction was con-
ducted on 5 mmol scale.
In addition, the facile separation of the product and
the recyclability of the catalyst are important features of
the designed catalyst system. The recycling experiments of
naphthalene-2-sulfonohydrazide 1a and 1,3-diketone 2a
In summary, a new (3,3)-connected Zn(II) CP ZnPB has
been synthesized and fully characterized. ZnPB can be
used as Lewis acid catalyst for the condensation cycliza-
tion to synthesize pyrazoles in high yields with low cata-
lyst loadings and short reaction time. Importantly, this
reaction could be scaled up, and the heterogeneous cata-
lyst was used at least for six times. Investigations on the
synthesis and potential catalytic performance of
Zn(II) CPs are in progress.

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CAO ET AL.
4 | EXPERIMENTAL SECTION
4.1 | General methods and materials
at room temperature. Colorless block crystals of ZnPB
were collected after one month. Anal. calc. for
C22H34N2O24Zn3 (1): C, 29.15; H, 3.78; N, 3.09%;
found: C, 28.97; H, 4.01; N, 2.89%. IR (cmꢂ1): 3129 (m),
1606 (m), 1518 (m), 1470 (w), 1430 (s), 1355 (vs), 1109
(w), 1059 (w), 980 (w), 928 (w), 814 (w), 752 (s),
710 (vs), 654 (s), 607 (s), 561 (s).
All reagents were purchased and used without further
purification. FT-IR data were obtained on a Bruker
VERTEX 70 FT-IR spectrophotometer in the 4000- to
500-cmꢂ1 region. Powder X-ray diffraction (PXRD) was
performed on a D/MAX-3C diffractometer with Cu-Kα
radiation (λ = 1.5406 Å) at room temperature. The prod-
ucts were isolated by column chromatography on silica
gel (200–300 mesh) using petroleum ether (60ꢁC to 90ꢁC)
and ethyl acetate. All compounds were characterized by
1H NMR, 13C NMR and mass spectroscopy, which were
consistent with those reported in related literatures.
NMR spectra were determined on Bruker Ascend 500 in
CDCl3. 1H NMR chemical shifts were referenced to resid-
ual solvent as determined relative to CDCl3 (7.26 ppm).
The 13C NMR chemical shifts were reported in ppm rela-
tive to the carbon resonance of CDCl3 (central peak is
4.4 | Typical procedure of ZnPB-
catalyzed condensation reaction
To a 4-ml reaction vial, benzenesulfonyl hydrazide
(1 mmol), 1,3-diketone (1 mmol), and ZnPB (5 mol%)
were added. Then, the reaction was carried out in screw-
cap vials with a Teflon seal at 80ꢁC for the desired time.
After the reaction, the mixture was purified by column
chromatography (petroleum ether/EtOAc) to afford the
desired products. However, some products could also be
obtained by recrystallization. In addition, the reacted cat-
alyst can be washed three times with EtOAc and EtOH,
respectively, and can be recycled after being dried under
vacuum at 50ꢁC for 3 h.
1
77.0 ppm). H NMR peaks were labeled as singlet (s),
doublet (d), triplet (t), and multiplet (m). The coupling
constants, J, are reported in Hertz (Hz). GC analyses
were performed on a Shimadzu GC-2014C equipped with
a capillary column (HP-5, 30 m ꢃ 0.25 μm) using a flame
ionization detector.
ACKNOWLEDGMENTS
We thank the National Natural Science Foundation of
China (22001034), the Open Fund of the Jiangxi Province
Key Laboratory of Synthetic Chemistry (JXSC202008),
and the Research Found of East China University of
Technology (DHBK2019265, DHBK2019267, and
DHBK2019264) for financial support.
4.2 | X-ray crystallography
The single-crystal X-ray diffraction data were collected
on Bruker D8 Smart Apex II diffractometer with graphite
monochromated Mo Kα radiation (λ = 0.71073 Å). Inten-
sities were collected by ω-scan and reduced on APEX II
(Bruker AXS Inc., 2009) and a multiscan absorption cor-
rection was applied.[36] The positions of hydrogen atoms
were determined with theoretical calculation. The struc-
tures were solved and refined on Olex2 using SHELXTL
package.[37–39] Parameters of the crystal data collection
and refinement are given in Table 1. Selected bond
lengths and bond angles are given in Tables S1 and S2.
Crystallographic files can be obtained via http://www.
ccdc.cam.ac.uk/data_request/cif with the CCDC number
of 2052269.
AUTHOR CONTRIBUTIONS
Gang-Ming Cao: Data curation; investigation; methodol-
ogy. Guo-Dong Zeng: Data curation; investigation. Ke
Li: Formal analysis; funding acquisition; validation. Yu-
Feng Liu: Funding acquisition; methodology. Xiao-Ling
Lin: Data curation; methodology. Guo-Ping Yang:
Funding
acquisition;
methodology;
project
administration.
DATA AVAILABILITY STATEMENT
The data that support the findings of this study
are available in the supplementary material of this
article.
4.3 | Synthesis of ZnPB
ORCID
Gang-Ming Cao https://orcid.org/0000-0001-8990-6463
Guo-Dong Zeng https://orcid.org/0000-0002-2649-4382
Ke Li https://orcid.org/0000-0002-0365-8612
Yu-Feng Liu https://orcid.org/0000-0002-0015-5278
Xiao-Ling Lin https://orcid.org/0000-0002-0099-0310
Guo-Ping Yang https://orcid.org/0000-0002-4891-2290
ZnSO4ꢀ7H2O (0.12 mmol, 0.0345 g), H3btc (0.06 mmol,
0.0126 g), and paz (0.12 mmol, 0.01 g) were dissolved in
20-ml H2O, and the mixture was adjusted to pH = 7
with dilute NaOH solution. The mixture was stirred for
10 min and then filtered. The filtrate keeps stand still

CAO ET AL.
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How to cite this article: G.-M. Cao, G.-D. Zeng,
K. Li, Y.-F. Liu, X.-L. Lin, G.-P. Yang, Appl
Organomet Chem 2021, e6379. https://doi.org/10.
1002/aoc.6379