Palladium supported on triazolyl-functionalized hypercrosslinked polymers as a recyclable catalyst for Suzuki-Miyaura coupling reactions

Article abstract of DOI:10.1039/d0ra01190h

A novel hypercrosslinked polymers-palladium (HCPs-Pd) catalyst was successfully preparedviathe external cross-linking reactions of substituted 1,2,3-triazoles with benzene and formaldehyde dimethyl acetal. The preparation of HCPs-Pd has the advantages of low cost, mild conditions, simple procedure, easy separation and high yield. The catalyst structure and composition were characterized by N₂sorption, TGA, FT-IR, SEM, EDX, TEM, XPS and ICP-AES. The HCPs were found to possess high specific surface area, large micropore volume, chemical and thermal stability, low skeletal bone density and good dispersion for palladium chloride. The catalytic performance of HCPs-Pd was evaluated in Suzuki-Miyaura coupling reactions. The results show that HCPs-Pd is a highly active catalyst for the Suzuki-Miyaura coupling reaction in H₂O/EtOH solvent with TON numbers up to 1.66 × 10⁴. The yield of biaryls reached 99%. In this reaction, the catalyst was easily recovered and reused six times without a significant decrease in activity.

Full text of DOI:10.1039/d0ra01190h

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Palladium supported on triazolyl-functionalized
hypercrosslinked polymers as a recyclable catalyst
for Suzuki–Miyaura coupling reactions†
Cite this: RSC Adv., 2020, 10, 17123
*
*
Cijie Liu,‡ Lijuan Zheng,‡ Dexuan Xiang,
You Shu, Yuejun Ouyang and Hongwei Lin
Shasha Liu, Wei Xu, Qionglin Luo,
A novel hypercrosslinked polymers-palladium (HCPs-Pd) catalyst was successfully prepared via the external
cross-linking reactions of substituted 1,2,3-triazoles with benzene and formaldehyde dimethyl acetal. The
preparation of HCPs-Pd has the advantages of low cost, mild conditions, simple procedure, easy separation
and high yield. The catalyst structure and composition were characterized by N2 sorption, TGA, FT-IR, SEM,
EDX, TEM, XPS and ICP-AES. The HCPs were found to possess high specific surface area, large micropore
volume, chemical and thermal stability, low skeletal bone density and good dispersion for palladium
chloride. The catalytic performance of HCPs-Pd was evaluated in Suzuki–Miyaura coupling reactions.
The results show that HCPs-Pd is a highly active catalyst for the Suzuki–Miyaura coupling reaction in
H2O/EtOH solvent with TON numbers up to 1.66 ꢀ 104. The yield of biaryls reached 99%. In this
reaction, the catalyst was easily recovered and reused six times without a significant decrease in activity.
Received 27th March 2020
Accepted 17th April 2020
DOI: 10.1039/d0ra01190h
rsc.li/rsc-advances
Heterogeneous catalysts are an attractive and versatile tool in metal-based catalysts such as Pd, Pt and Ru, the preparation of
industrial and academic research laboratories because of their HCPs is mainly based on acid-catalysed Friedel–Cras alkyl-
unique properties,1 which include high reactivity, stability, easy ation reactions,8 which provide quick cross-linking to form
separation, purication and good recyclability.2 Currently, strong linkages. These linkages lead to highly crosslinked
heterogeneous catalysis represents an important eld of networks with predominant porosity,9 resulting in extremely
research in green chemistry3 that receives frequent attention rigid networks that are difficult to collapse.10 Regarding the
and witnessed constant development in recent years. However, synthetic methodology, HCPs are mainly prepared by three
heterogeneous catalysts normally have inferior catalytic effi- methods.6a The rst method involves post-crosslinked polymer
ciency compared to homogeneous systems because of their long precursors.11 The Davankov resins, which are the rst examples
diffusion pathways to catalytic sites and the difference in elec- of HCPs, were prepared by Davankov in the 1970s via post-
tron density on active sites.4 In addition, most recovered crosslinking with external crosslinking agents.12 The second
heterogeneous catalysts suffer from the sintering and leaching method is the one-step polycondensation of functional mono-
of metals, the loss of surface area and degradation aer several mers.13 Recently, the design and synthesis of hypercrosslinked
reactions under typical reaction conditions.5 These drawbacks polystyrene and their applications have undergone rapid
could be overcome by using appropriate catalyst supports that development. For example, Yang designed microporous poly-
provide a large reaction surface with a proper porous structure.6 triphenylamine networks via the oxidative polymerization of
Hypercrosslinked polymers (HCPs), a type of microporous triphenylamine and DCX crosslinkers.14 The third method is
organic polymer (MOP), are good support materials for noble external cross-linking reaction.15 Tan developed the knitting
metals to form heterogeneous catalysts.6a,7 HCPs have a large strategy using formaldehyde dimethyl acetal as an external
number of permanent pores formed by extensive chemical crosslinker to combine with rigid aromatic building blocks
crosslinking. Compared to other MOPs prepared by noble through the Friedel–Cras reaction.16 Recently, our group
synthesized three HCPs based on pyridine-functionalized N-
heterocyclic carbene via an external cross-linking reaction and
applied them in Suzuki–Miyaura coupling reactions.17 We then
prepared HCPs using the readily available raw material, phe-
Hunan Engineering Laboratory for Preparation Technology of Polyvinyl Alcohol (PVA)
Fiber Material, Key Laboratory of Research and Utilization of Ethnomedicinal Plant
Resources of Hunan Province, Huaihua University, Huaihua 418000, China. E-mail:
nanthroline, and used it to catalyse the Heck reaction with good
dexuanxiang@126.com; linhongwei1968@163.com
effect.18 HCPs have been widely used in catalysis, gas storage
† Electronic supplementary information (ESI) available: Experimental details,
and separation, and drug release because of the following
merits:19 (i) large surface areas and pores; (ii) superior chemical
spectral characterization data and supplementary gures. See DOI:
10.1039/d0ra01190h
‡ Cijie Liu and Lijuan Zheng contributed equally to this work.
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Meanwhile, a slight hysteresis loop indicates the presence of
some mesopores, and a sharp rise at high pressure (P/P0 ¼ 0.9–
1.0) implies a spot of macropores in these materials. The
apparent Brunauer–Emmett–Teller surface area (SBET) of the
HCPs (794 m2 gꢁ1) is larger than that of HCPs-Pd (731 m2 gꢁ1),
which can be attributed to the partial lling of the pores in
HCPs-Pd with metal (Table 1). These materials were further
analyzed by pore-size distribution analysis (Fig. 2). The similar
pore sizes of the HCPs and HCPs-Pd implies that the immobi-
lization of palladium on the ESI† did not change the pore-size
distribution. The presence of some mesopores and macro-
pores in HCPs-Pd is also essential because they could enable the
heterogeneous catalysts upon soaking in a certain solvent. As
a result, the catalytically active sites would better contact the
substrates. The content of Pd in HCPs-Pd was detected by ICP
analysis (Table 1), indicating a Pd content of 1.58%.
HCPs-Pd was then subjected to characterization by X-ray
photoelectron spectroscopy (XPS) to investigate the coordina-
tion states of palladium species with the ligand triazoles. As
shown in Fig. 3, the Pd 3d XPS spectrum of HCPs-Pd indicates
that only one state of Pd (the +2 oxidation state rather than the
metallic state) is present in the catalyst. This state corresponds
to the binding energies of 337.5 and 342.7 eV, which are
assigned to Pd2+ 3d5/2 and 3d3/2, respectively. Compared to the
homogeneous counterpart PdCl2 (337.9 and 343.1 eV), the Pd2+
Fig. 1 The structures of HCPs and HCPs-Pd.
and thermal stability; and (iii) low cost and simple and versatile
synthetic approach.
1,2,3-Triazoles are one of the most important classes of N-
heterocyclic compounds; they have been widely applied in
pharmaceutical drugs,20 bioconjugation,21 catalysts,22 mate-
rials23 and synthetic organic chemistry.23 Due to the large dipole
moment in the 1,2,3-triazole unit, many reports have found that
1,2,3-triazoles can coordinate with metals.24 This property gives
1,2,3-triazoles an important role in the eld of catalysis, espe-
cially heterogeneous catalysis.25
In this paper, we report the preparation of hypercrosslinked
polymers-palladium (HCPs-Pd) catalysts based on substituted
1,2,3-triazoles via external cross-linking reaction (Fig. 1) along
with the utilization of the HCPs-Pd in Suzuki–Miyaura coupling
reactions as a recyclable catalyst with high TON number. HCPs-
Pd is simply prepared with low cost, mild conditions and easy
separation. The catalyst possesses recyclability, high activity
and excellent yield, making it useful for Suzuki–Miyaura
coupling reactions.
The surface area and pore structure of the HCPs and HCPs-
Pd were investigated by nitrogen adsorption analyses at 77.3 K.
The nitrogen adsorption and desorption isotherms of the HCPs
exhibit type I adsorption–desorption isotherms (Fig. 2), similar
to the isotherms of HCPs-Pd. The isotherms show steep
nitrogen gas adsorption at low relative pressure (P/P0 < 0.05),
which reects abundant micropores in the polymers.
binding energy is negatively shied by 0.4 eV for 3d5/2 and 3d3/2
,
which can be attributed to the coordination with triazoles
immobilized on the hypercrosslinked polymers. The results
show that Pd2+ was successfully immobilized successfully on
the HCPs by coordination rather than by the physical adsorp-
tion of Pd2+ on the HCP surface.
The thermal stability was assessed by TGA in the tempera-
ꢁ1
ꢂ
ꢂ
ture range of 40–700 C with a heating rate of 20 C min in
nitrogen. The TGA traces of the HCPs and HCPs-Pd are shown
Fig. 2 N2 adsorption–desorption isotherms and the corresponding
pore size distributions of HCPs and HCPs-Pd.
Fig. 3 XPS spectra of HCPs-Pd.
Table 1 Physical properties of HCPs and HCPs-Pd
SBETa [m2 gꢁ1
]
SMicrob [m2 gꢁ1
]
VMicro [m3 gꢁ1
]
[Pd]d [wt%]
c
Sample
HCPs
HCPs-Pd
794
731
0
547
0.280
0.292
—
1.58
a
Surface area calculated from the nitrogen adsorption isotherm using the BET method. b Micropore volume derived using a t-plot method based on
the Halsey thickness equation. c Total pore volume at P/P0 ¼ 0.99. d Data were obtained by ICP-AES.
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Fig. 4 TGA curves of HCPs and HCPs-Pd.
in Fig. 4 along with the corresponding data analysis. Both
materials exhibited good thermal stability up to at least 300 ꢂC.
The T5% and T10% values of HCPs-Pd were clearly lower than
those of the HCPs.
Fig. 6 TEM images of HCPs and HCPs-Pd.
To investigate the surface morphologies of the HCPs and
HCPs-Pd, these materials were subjected to scanning electron
microscopy (SEM). As clearly shown in Fig. 5, numerous pores
were distributed on the surfaces of the HCPs, and the surface
morphology did not change remarkably aer loading with
palladium. The compositions of the HCPs and HCPs-Pd were
then investigated by energy-dispersive X-ray spectroscopy (EDX;
see ESI, Section III†). The results indicate that C, N, Pd and Cl
were the major elements in HCPs-Pd. Meanwhile, Pd was
distributed in the networks with a high degree of dispersion.
Transmission electron microscopy (TEM) was also employed to
study the hypercrosslinked polymers, and Pd nanoparticles
were found to be uniformly distributed in HCPs-Pd with good
dispersion (Fig. 6).
some of the results are summarized in Table 2, entries 12–17.
Eventually, the optimal results were obtained when the reaction
of 1a with 2a was carried out in the presence of NaOH with
HCPs-Pd in EtOH/H2O (4/1, v/v) at 60 ꢂC for 1.0 h to afford 3a in
99% yield (entry 8).
Two parallel reactions were conducted under the same
conditions as in Table 2 (entry 8) to determine whether the real
catalyst was the heterogeneous catalyst or the homogeneous Pd
species dissolved in solvent. The two reactions were quenched
aer the reaction took place for 20 min, and the heterogeneous
catalyst was separated from the reaction mixture via hot ltra-
tion. One of the reactions was separated by column chroma-
tography, affording biphenyl in 75% yield, while the other
The catalytic activity of the HCPs-Pd catalyst was evaluated in
the Suzuki–Miyaura coupling reaction with iodobenzene 1a and
phenylboric acid 2a as a model substrate. Initially, we screened
solvents including EtOH, DMSO, DMF, toluene, THF, CH3CN
and H2O (Table 2, entries 1–7). When alcohol and DMF were
used, the reaction proceeded with moderate yield, while using
H2O as solvent only provided a yield of 18%. In our previous
work, we found that the mixed solvent of water and ethanol is
more favorable for this reaction; other studies have also indi-
cated that the addition of some water to the solvent can speed
up the conversion and enhance the yield.26 Next, a series of
Table 2 Optimization of Suzuki–Miyaura reactiona
Entry
HCP-Pd
Solvent
Base
Yieldb (%)
1
2
3
4
5
6
7
8
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
EtOH
DMSO
DMF
Toluene
THF
CH3CN
H2O
EtOH/H2O (4 : 1)
EtOH/H2O (2 : 1)
EtOH/H2O (1 : 1)
EtOH/H2O (1 : 4)
EtOH/H2O (4 : 1)
EtOH/H2O (4 : 1)
EtOH/H2O (4 : 1)
EtOH/H2O (4 : 1)
EtOH/H2O (4 : 1)
EtOH/H2O (4 : 1)
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
K2CO3
K3PO4
Cs2CO3
Na2CO3
Et3N
84
55
79
39
reactions with the feed volume ratio of EtOH to H2O (VEtOH
-
: VH O) ranging from 4/1 to 1/4 was carried out (entries 8–11).
2
The best yield was obtained at VEtOH : VH O ¼ 4/1. Aer
2
NRc
Trace
18
screening the reaction solvent, a series of experiments was
carried out to investigate the reaction conditions, including the
base, reaction time, temperature, and the amount of catalyst;
99
92
89
51
96
98
95
69
9
10
11
12
13
14
15
16
17
Trace
76
NaOH
a
Reaction conditions: 1a (2.5 mmol), 2a (3.5 mmol), HCPs-Pd (1.0 mg,
1.5 ꢀ 10ꢁ4 mmol), 60 ꢂC, 1.0 h. Isolated yields. c No reaction.
b
Fig. 5 SEM images of HCPs and HCPs-Pd.
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Table 3 Comparison of HCP-Pd with other catalysts for Suzuki–Miyaura reaction
Entry
Catalyst
React condition
Yield
TONa (ꢀ103)
1
2
3
4
5
6
7
KAPs(Ph-PPh3)-Pd
Poly-NHC-2–Pd
Pd/SMP-PhPh3
HCP-Pd–I
MOPs–Pd–I
HCPs-bipy-Pd–I
HCPs-Pd
K3PO4$3H2O, H2O/EtOH, 80 ꢂC
K3PO4$3H2O, H2O/EtOH, 80 ꢂC
K3PO4$3H2O, H2O/EtOH, 80 ꢂC
98%
99%
99%
95%
97%
97%
99%
0.141 (ref. 7a)
1.737 (ref. 8d)
2.04 (ref. 27)
0.123 (ref. 17)
0.206 (ref. 18)
0.425 (ref. 28)
16.6 [this work]
K3PO4, H2O, 80 ꢂ
C
K3PO4, H2O/EtOH, 80 ꢂC
K2CO3, EtOH, 80 ꢂ
NaOH, EtOH, 60 ꢂC
C
a
TON number was calculated from the reported data.
reaction was allowed to react for another 40 min at the same external cross-linking reactions are good platforms to support
temperature, affording biphenyl in 77% yield. The similar yields palladium species as catalysts for Suzuki–Miyaura reaction.
of biphenyl for the two reactions suggest that the real catalyst This may be due to the large number of micropores in the
was the heterogeneous catalyst rather than the homogeneous microporous polymer, which can help improve the dispersion
Pd species leached from HCPs-Pd.
of palladium nanoparticles. However, the reported catalyst TON
The results obtained in this work were compared with values range from 0.123 to 2.04 ꢀ 103 (Table 3, entries 1–6); the
previous catalysts synthesized via external cross-linking reac- TON value in this work reached 16.6 ꢀ 103, indicating prom-
tions (Table 3). Various parameters such as base, solvent, ising application potential.
temperature, yield and TON number were compared. As shown
Having established the optimal conditions for the reaction,
in Table 3, the yield of the product varied from 95% to 99%, we investigated the reaction scope. Thus, a series of aryl boronic
indicating that microporous organic polymers synthesized by acids 2 were subjected to substrate 1; some of the results are
summarized in Table 4. All the reactions proceeded smoothly to
give the corresponding biphenyls 3b–l with excellent yields and
TON values. Chlorobenzene was also applied in this reaction,
Table 4 Suzuki–Miyaura reaction catalysed by HCPs-Pd-Ia
but the result was not satisfactory. The TON values and isolated
yields of the products demonstrate the practicality and effi-
ciency of HCPs-Pd in the Suzuki–Miyaura reaction.
Yieldb
(%)
TON
Reusability test
Entry
R1
X
R2
3
(ꢀ104)
The reusability is a very important property of a supported
catalyst.29 Thus, we investigated the recycling performance of
HCPs-Pd in Suzuki–Miyaura reaction with 1a and 2a. First, the
reaction was carried out in the presence of NaOH with HCPs-Pd
as a catalyst under the optimized conditions. The catalyst was
then recovered by ltering, washing with water and methanol,
and drying in a vacuum oven for 3.0 h. The recovered HCPs-Pd
was reused in the reaction six times, affording biphenyl in 99–
95% yields with no remarkable decrease in yield (Fig. 7). The
recovered catalyst was analyzed by SEM and EDX (see ESI†). The
SEM images indicated that the recovered catalyst retained
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
H
I
I
I
I
I
I
I
I
I
I
I
I
H
3a
3b
3c
3d
3e
3f
3g
3h
3d
3i
99
96
96
99
94
95
92
94
99
97
97
93
93
94
92
93
95
92
93
91
1.66
1.61
1.61
1.66
1.58
1.60
1.55
1.58
1.66
1.63
1.63
1.56
1.56
1.60
1.55
1.56
1.60
1.55
1.56
1.53
2-Me
3-Me
4-Me
2-F
3-F
4-F
4-CN
H
H
H
H
H
4-Me
4-F
4-CN
H
H
H
9
4-Me
4-OMe
3,5-(Me)2
4-CN
H
H
H
H
4-Me
2-Me
3-Me
H
10
11c
12
13
14
15
16
17
18
19
20
3j
3h
3a
3d
3g
3h
3d
3b
3c
3k
Br
Br
Br
Br
Br
Br
Br
CH2Br
H
21
H
I
3l
92
1.55
a
Reaction conditions: 1a (2.5 mmol), 2a (3.3 mmol), NaOH (3.3 mmol),
b
HCPs-Pd (1.0 mg), EtOH/H2O (10 mL), 60 ꢂC, 1.0 h. Isolated yields.
c
The reaction time was 3.0 h.
Fig. 7 Recyclability test of HCPs-Pd in Suzuki–Miyaura reaction.
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numerous pores on its surface. EDX revealed the Pd was still
uniformly distributed on the HCPs without obvious agglomer-
ation. In the last cycle, the palladium concentration in the
ltrate was calculated by AAS to be 10 mg Lꢁ1. The very small
amount of Pd in solution demonstrates that little metal leach-
ing occurred in the reaction medium.
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Conclusion
In summary, we have prepared a novel heterogeneous catalyst
supported on hypercrosslinked polymers. The catalyst was
synthesized via the external cross-linking reaction of
substituted 1,2,3-triazoles with benzene and formaldehyde
dimethyl acetal. The structure and composition of the catalyst
were characterized by FT-IR, N2 sorption, TGA, SEM, EDX, TEM,
XPS and ICP-AES. The results show that HCPs-Pd exhibits high
specic surface area, large microporous volume, chemical and
thermal stability, and good dispersion of palladium chloride.
The catalytic performance of HCPs-Pd was evaluated in Suzuki–
Miyaura coupling reactions. The results show that HCPs-Pd is
a highly active catalyst for the Suzuki–Miyaura coupling reac-
tion, affording biaryl products with TON values reaching 1.66 ꢀ
104. In this reaction, the catalyst could be reused six times
without any obvious loss of catalytic activity.
Conflicts of interest
The authors declare that there are no conicts of interest.
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(b) K. Song, Z. Zou, D. Wang, B. Tan, J. Wang, J. Chen and
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Acknowledgements
¨
S. Farsadpour, L. T. Ghoochany, S. Shylesh, G. Dorr,
This research was nancially supported by the Natural Science
Foundation of Hunan Province (No. 2018JJ3409), Scientic
Research Program of Huaihua University (No. HHUY2019-05),
Scientic Research Fund of Hunan Provincial Education
Department (17A166), the Foundation of Huaihua University
Double First-rate Applied Characteristic Discipline Construc-
tion Projects of Materials Science and Engineering (No.
19CKA003), and the Foundation of Hunan Double First-rate
Discipline Construction Projects (No. YYZW2019-02).
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