A palladium complex confined in a thiadiazole-functionalized porous conjugated polymer for the Suzuki-Miyaura coupling reaction

Article abstract of DOI:10.1039/c9ra06709d

Porous organic polymers (POPs) with well-distributed and tunable functional groups acting as ligands for specific reactions are promising supports for confining useful novel metals such as Pd, Au, and Pd. Herein, a thiadiazole-containing POP has been successfully synthesized and used for immobilizing Pd species. Pd immobilized inside the micropores (2.3 nm) of the POP material is easily prepared owing to a large amount of the strong anchoring group, thiadiazole, which is intrinsically distributed in the as-prepared POP. The rigid thiadiazole-containing polymer can stabilize the central metal rather than poisoning it. The as-prepared catalyst shows excellent catalytic activity in Suzuki-Miyaura coupling reactions under mild reaction conditions and low catalyst loading. Importantly, the intrinsically distributed thiadiazole ligands can stabilize the Pd moiety, preventing aggregation and leaching, and afford excellent catalytic lifetimes. Consequently, the catalyst can be reused 10 times without a significant loss of its catalytic activity.

Full text of DOI:10.1039/c9ra06709d

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A palladium complex confined in a thiadiazole-
functionalized porous conjugated polymer for the
Suzuki–Miyaura coupling reaction
Cite this: RSC Adv., 2019, 9, 33563
Yingjie Qian, Sang Yung Jeong, Sung-Hyeon Baeck,
and Sang Eun Shim
Myung-Jong Jin
*
Porous organic polymers (POPs) with well-distributed and tunable functional groups acting as ligands for
specific reactions are promising supports for confining useful novel metals such as Pd, Au, and Pd.
Herein, a thiadiazole-containing POP has been successfully synthesized and used for immobilizing Pd
species. Pd immobilized inside the micropores (2.3 nm) of the POP material is easily prepared owing to
a large amount of the strong anchoring group, thiadiazole, which is intrinsically distributed in the as-
prepared POP. The rigid thiadiazole-containing polymer can stabilize the central metal rather than
poisoning it. The as-prepared catalyst shows excellent catalytic activity in Suzuki–Miyaura coupling
reactions under mild reaction conditions and low catalyst loading. Importantly, the intrinsically
distributed thiadiazole ligands can stabilize the Pd moiety, preventing aggregation and leaching, and
afford excellent catalytic lifetimes. Consequently, the catalyst can be reused 10 times without
a significant loss of its catalytic activity.
Received 26th August 2019
Accepted 8th October 2019
DOI: 10.1039/c9ra06709d
rsc.li/rsc-advances
4
polymers (POPs) have been developed. Among them, POP-
based heterogeneous catalysts have attracted signicant
Introduction
Transition metal-catalyzed Suzuki–Miyaura coupling reac- interest owing to their potential applications in energy and
5
tions have been ubiquitously employed in organic chemistry gas storage. Recently, these have also emerged as prom-
6
because of their applications in the synthesis of pharma- ising materials for anchoring Pd. Because of their control-
1
ceuticals, natural products, and polymers. In general, the lable intrinsic functional groups and nanoporous structures
majority of the coupling reactions in the case of pharma- with high stability in organic solvents, POPs are suitable
1
a
1b
ceuticals and natural products rely on the use of homo- supports in catalysis. Wang et al. reported an efficient
geneous Pd complexes, which require an extensive heterogeneous catalytic system by anchoring Pd moiety onto
separation and recycling of toxic and expensive catalysts imine-based conjugated POPs and demonstrated the possi-
from products. Additionally, increasing environmental bility of POP applicability in Suzuki–Miyaura coupling
6
exposure with the emission of Pd from organic trans- reaction. Signicantly high turnover frequency value was
7
formations requires an effective control of the metal achieved by Zhu et al. using imine-tagged POP. Both
2
contamination of desired product and eco-system. There- representative POP-based immobilized catalysts exhibited
fore, many approaches have been presented in previous excellent catalytic activities in coupling reactions. However,
studies focusing on the development of new strategies and the limited reuse of heterogeneous catalysts still limits the
3
supports to adequately immobilize the catalytic species.
practical use of POP-based heterogeneous catalysts in
However, the application of heterogenized catalytic systems industrial sector.
has been limited by leaching and deactivation over time due
Prior studies suggest that introducing foreign elements
to the insufficient affinity between the central metal and (N, O, S) to the POP is crucial for enhancing the recycling
8
support. Thus, it is essential to introduce substantial performance. Among the electronegative foreign elements,
anchoring groups into a heterogeneous catalyst to enhance S, which is a widely known Pd poison owing to the strong
the long-term stability of the catalyst.
interaction between S and Pd, is of considerable interest.
In the past few decades, an impressive number of solid However, only a few studies have examined the coupling
supports, including silica matrices, graphene, metal– transformation of sulfur-containing heterogeneous cata-
8
b,9
organic frameworks, metal oxides, and porous organic lysts.
Zhang et al. reported that the encapsulation of
metal inside the cavities of POP was crucial and benecial in
the Suzuki–Miyaura coupling reactions in the presence of
Department of Chemistry & Chemical Engineering, Inha University, 100 Inha-ro,
Nam-gu, Incheon 22212, South Korea. E-mail: seshim@inha.ac.kr
8
b
strong anchoring thiol groups.
Additionally, a rigid,
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Scheme 1 (a) Synthesis of fully conjugated DTE and (b) schematic representation of the synthesis of DTE supported Pd@DTE.
dangling thiol ligand, where the thiol groups are far apart to were purchased from TCI. 1,3,5-Triethynylbenzene, cop-
entirely bond the Pd species, was developed and it exhibited per(I)iodide (CuI), triethylamine (TEA), and all other chem-
excellent reusability owing to the rigid framework of the icals were obtained from Sigma Aldrich. X-ray photoelectron
9
b
sulfur-containing ligand. Inspired by the previous inves- spectroscopy (XPS) data were collected by using an ESCA
tigations, it was envisioned that the presence of a rigid 2000 photoelectron spectrometer (Thermo Scientic) with
sulfur-containing anchoring groups that are evenly distrib- monochromated Al Ka radiation (hn ¼ 1486.6 eV). Fourier
uted inside the POP could facilitate the binding of the metal transform-infrared vacuum spectrometer (FT-IR) spectra
and ligand, thus enhancing the long-term stability of the were obtained by employing a Bruker VERTEX 80V. Solid-
catalyst. Herein, the synthesis of a thiadiazole-containing state NMR spectra were collected using a Bruker Digital
POP and its application in Suzuki–Miyaura coupling reac- AVANCE III (400 MHz) spectrometer. Field-emission trans-
tions is described. The synthesized catalyst exhibits the mission electron microscopy (FE-TEM) images were ob-
highest catalytic activity in comparison with the previously tained by employing a JEOL JEM-2100F microscope. The Pd
reported sulfur-containing heterogeneous catalysts. The content was obtained through inductively coupled plasma-
catalyst also shows high stability over multiple catalytic atomic emission spectroscopy (ICP-AES) using OPTIMA
cycles and easy recyclability.
4300 DV (Perkin-Elmer). The specic surface area and pore
size distribution of the catalyst were investigated by
measuring the nitrogen adsorption and desorption
isotherms at 77 K on a BELSORP-MAX system (BEL, Japan).
GC/GC-MS analyses were performed using an Agilent 6890N
GC instrument coupled to an Agilent 5975 Network Mass
Selective Detector.
Experimental
Materials and characterization
All chemicals were commercially available and used as-received
without any further purication. 4,7-dibromobenzo[c][1,2,5]
thiadiazole and tetrakis(triphenylphosphine)-palladium(0)
13
Fig. 1 (a) FT-IR spectra of 1,3,5-triethylnylbenzene and DTE and (b) solid-state C NMR spectrum of DTE.
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Fig. 2 (a) C 1s. (b) N 1s. (c) S 2p. (d) Pd 3d XPS spectra of Pd@DTE.
ꢀ
Synthesis of DTE
and diethyl ether. Pd@DTE was dried at 75 C under
ꢁ1
vacuum. The Pd content of 0.18 mmol g was measured by
ICP-AES analysis.
4
1
,7-Dibromobenzo[c][1,2,5]thiadiazole (0.75 mmol) and
,3,5-triethynylbenzene (0.5 mmol) were dissolved in the
mixture of anhydrous DMF/TEA (15 ml/15 ml), named A,
followed by purging for 10 min. CuI (0.04 mmol) and Pd
catalyst (0.025 mmol) were dissolved in TEA (5 ml) and
anhydrous DMF (5 ml), respectively. Aer purging all solu-
tions for 10 min, the Pd and CuI solutions were simulta-
neously poured into solution A. The polymer DTE was
General procedure for the Suzuki coupling reaction
Aryl bromide or chloride (0.4 mmol), aryl boronic acid (0.48
mmol), TBAB (0.2 mmol), K
2
CO
3
(0.8 mmol), and Pd@DTE
O/
(2.0 mg, 0.1 mol%) were mixed in a specic amount of H
2
EtOH. The mixture was stirred at a specic temperature under
air. The reaction was monitored by GC. In the recycling exper-
iment, aer the reaction was completed, the catalyst was
recovered simply by ltration. The separated catalyst was
washed with water, ethanol, and diethyl ether. Thereaer, it was
used in the next run under the same reaction conditions. The
catalytic performance was conrmed by GC in the presence of n-
dodecane as an internal standard.
ꢀ
synthesized at 120 C for two days under an inert atmo-
sphere. Aer the completion of the reaction, the generated
polymer was puried by ltration, and washed with copious
amounts of THF, acetone, DMF, and MC. The obtained DTE
ꢀ
was dried at 75 C under vacuum.
Preparation of Pd-anchored DTE (Pd@DTE)
DTE (100 mg) was dispersed in anhydrous THF (30 ml) and
the mixture was ultrasonicated for 30 min. Aer sonication,
Results and discussion
Pd(II) acetate (5 mg, 0.022 mmol) was added to the mixture, As shown in Scheme 1, the catalyst can be prepared in two steps.
ꢀ
followed by stirring at 75 C for 2 h. Aer the anchoring, the First, the fabrication of polymer DTE is achieved using the
mixture was cooled down to room temperature and ltered, Sonogashira coupling reaction. Pd@DTE is obtained by
and thereaer washed with copious amounts of THF, MC, immobilizing Pd(II) acetate to DTE. The structural
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chemical states of elements and elemental composition of the
samples. The C spectrum is deconvoluted and split into three
2
peaks at 284.6, 285.7, and 287 eV, which are attributed to sp ,
sp, and imine C (C]N), respectively (Fig. 2a). The nitrogen
spectrum of Pd@DTE shows the binding energies of 399.45 and
400.38 eV, corresponding to the imine N (N]C) and N–S,
respectively (Fig. 2b). Notably, the ratio of N]C to N–S is close
to 1, which implies that the intrinsic structure is maintained
even aer the anchoring of Pd moiety to DTE. S and Pd spectra
are also deconvoluted and show three peaks centered at 165.8,
2
+
3
37.78, and 343.08 eV which correspond to S–N, Pd (Fig. 2c
11
and d).
The morphology of Pd@DTE was determined by TEM.
Fig. 3a–d shows the images of fresh Pd@DTE at different
magnications. Fig. 3e illustrates the EDS mapping of the
composition elements, nitrogen, sulfur, and palladium.
Based on the EDS mapping results, DTE is uniformly
anchored to Pd moiety. Moreover, the elemental composi-
tion of Pd@DTE is close to the theoretical values. Because of
the large size of DTE, it is difficult to visualize the distri-
bution of nano-sized Pd.
The nitrogen isotherm was collected at 77 K to illustrate the
effect of Pd@DTE fabrication process (Fig. 4). The results for
BET surface area (SBET) and total pore volumes (Vtotal) of DTE
and Pd@DTE are summarized in Table 1. Both isotherms with
type IV hysteresis show micro- and mesoporous characteristics.
Fig. 3 Characterizations of Pd@DTE: (a and b) low-magnification TEM
images. (c and d) High-magnification TEM images. (e) Energy disper-
sive X-ray spectroscopy (EDS) mapping of composition element, M, S,
and Pd, respectively.
Aer the anchoring of Pd moieties, S
from 309 ꢂ 7 to 244 ꢂ 5 m g . As some of the micropores are
of catalyst 4 decreases
BET
2
ꢁ1

lled with Pd moieties during the Pd treatment process, the
decrease in micropores can be predicted. Meanwhile, it
demonstrates that Pd species are successfully entered into the
pores.
2-Bromoanisole was initially selected as a model starting
material to optimize the reaction conditions (Table 2). First, the
reaction was carried out with 0.1 mol% of catalyst Pd@DTE and
ꢀ
2
.0 eq. K CO in different solvents at 50 C (entries 1–6). Based
2
3
on the solvent effect, EtOH was found to be the most effective
organic solvent. EtOH/H O was used as the reaction media
2
because this aqueous-organic media allowed the highest
transformation performance in most cases. Furthermore,
t
K
2
CO
3
, KOH, Na
2
CO
3
, KO Bu, and TEA were also tested and
K CO was determined to be the most effective base (entries 7–
2
3
11).
To maximize the catalytic performance of Pd@DTE, the ratio
of H O to EtOH was varied. Fig. 5 shows the yield as a function
of time for Suzuki–Miyaura coupling reaction of 2-bromoani-
sole with phenylboronic acid at different volume ratios of H
4
Nitrogen adsorption–desorption isotherms of DTE and to EtOH. Among the various combinations of the reaction
2
2
O
Fig.
Pd@DTE.
media, the highest yield of 97.2% is obtained at H
2
O : EtOH of
2. The difference in catalytic activity at different ratios of reac-
tion media allows the modication and enhancement of the
catalytic activity.
characteristics of the as-synthesized DTE were determined by
1
3
The Suzuki–Miyaura coupling reactions of aryl halides with
aryl boronic acid were carried out under the optimized reaction
conditions (Table 3). The sterically hindered 2,5-dibromoto-
luene and deactivated 4-bromophenol, 2-bromoanisole, and 4-
bromoanisole show good catalytic activities in 6 h (entries 1–5).
FT-IR and solid-state C NMR (Fig. 1). The disappearance of the
ꢁ
1
C–H stretch band at 3278 cm and corresponding peaks from
1
3
C NMR spectrum illustrate the successful synthesis of DTE
10
from building units. Next, XPS was employed to obtain the
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Table 1 BET surface area (SBET) and the total pore volumes (Vtotal) from N adsorption–desorption isotherms for DTE and Pd@DTE
Pore size
[nm]
2
ꢁ1
2
ꢁ1
2
ꢁ1
2
ꢁ1
Sample
S
BET [m g
]
V
total [m g
]
V
meso [m g
]
V
micro [m g
]
DTE
Pd@DTE
309 ꢂ 7
244 ꢂ 5
0.174
0.141
0.094
0.075
0.08
0.066
2.3
2.2
a
Table 2 Optimization for Suzuki–Miyaura coupling of 2-bromoanisole with phenylboronic acid
b
Entry
Base
Catalyst (mol%)
Solvent (total 2 ml)
Yield (%)
1
2
3
4
5
6
7
8
9
K
K
K
K
K
K
K
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
H
EtOH
Isopropanol
Toluene
Propylene glycol
DMF
EtOH/H
EtOH/H
EtOH/H
EtOH/H
EtOH/H
2
O
57.5
42.0
11.5
0.7
20.4
0.8
84.1
65.6
58.1
81.9
37.2
2
2
2
2
2
O (1 : 1)
O (1 : 1)
O (1 : 1)
O (1 : 1)
O (1 : 1)
KOH
KO Bu
Na
NEt
t
1
1
0
1
2
CO
3
3
a
Reaction conditions: aryl halide (0.4 mmol), boronic acid (0.48 mmol), Pd@DTE (0.1 mol%), base (0.8 mmol), TBAB (0.2 mmol), solvent (2 ml),
0 C. GC yield was determined using n-dodecane as an internal standard.
ꢀ
b
5
Activated compounds such as 4-suitable, 4-bromobenzene, 4- methyl acetophenone shows excellent results because of the
bromobenzonitrile, and 2-bromonitrobenzene exhibit excellent existence of an activating acetophenone moiety in the molec-
performances (entries 6–9). The sterically hindered 3-bromo-4- ular structure (entry 10).
Fig. 5 The yield versus time for Suzuki–Miyaura coupling reaction of 2-bromoanisole with phenylboronic acid at different volume ratio of H
and EtOH at 50 C.
2
O
ꢀ
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Encouraged by the results of these coupling reactions,
some hetero halides and severely sterically hindered aryl
halides were also investigated (Table 4). Coupling of the
heteroaryl structure is more difficult than aryl halides owing
to presence of elements such as nitrogen or sulfur, which
deactivate the catalyst. Therefore, the investigation of the
performance of heteroaryls is also important. 2-Bromo-
thiophene shows excellent yield in 20 h (entry 1). Addition-
ally, 2-bromo-3-methylpyridine, 2-bromopyridine, and 3-
bromopyridine can couple in good to excellent yields at
ꢀ
5
0 C in 20 h (entries 2–4). The excellent catalytic activity of
Pd@DTE is also supported by the coupling of 2,5-dibromo-
pyridine, 2,5-dibromothiophene, 2-bromo-3-aminopyridine,
and mesitylene bromide in good yields (entries 5–8).
The reusability of the catalyst is one of the most impor-
tant features of an attractive heterogeneous catalyst. The
recycling of Pd@DTE was examined in the Suzuki–Miyaura
coupling of 4-iodobenzene with phenylboronic acid (Table
5) The catalytic system exhibits excellent reusability even
Fig. 6 Hot filtration experiments. Suzuki–Miyaura coupling reaction of
ꢀ
2-bromoanisole with phenylboronic acid was carried out at 100 C.
a
Table 3 Suzuki–Miyaura coupling of aryl halides with arylboronic acids
b
Entry
Aryl halide
Product
Time (h)
Yield (%)
1
2
6
4
94.2
98.4
3
4
5
4
4
6
99.2
98.1
97.8
6
7
1
1
100
100
8
9
4
1
100
100
1
0
1
100
a
Reaction conditions: aryl halide (1 mmol), arylboronic acid (1.5 mmol), Pd@DTE (0.1 mol%), K
2
CO
3
(2.0 mmol), TBAB (0.5 mmol), H
2
O (2.0 ml),
ꢀ
b
EtOH (2.0 ml), 50 C. GC yield was determined using n-dodecane as an internal standard.
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a
Table 4 Suzuki–Miyaura coupling of aryl halides with arylboronic acids
b
Entry
1
Aryl halide
Product
Time (h)
20
Yield (%)
100
91
2
20
3
4
20
20
20
97.2
96.9
89.3
c
5
c
6
20
20
92
93
c
7
c
8
20
87
a
Reaction conditions: aryl halide (1 mmol), arylboronic acid (1.5 mmol), Pd@DTE (0.1 mol%), K
2
CO
3
(2.0 mmol), TBAB (0.5 mmol), H
2
O (2.0 ml),
ꢀ
b
c
ꢀ
EtOH (2.0 ml), 50 C. GC yield was determined using n-dodecane as an internal standard. The reaction was carried out at 75 C.
a
Table 5 Reusability of Pd@DTE in the Suzuki–Miyaura coupling
aer 10 cycles of reuse, which is an outstanding result
among the sulfur-containing heterogeneous catalysts (Table
6
). The high stability of the rigid sulfur-containing Pd@DTE
catalysts in the reaction media is owing to the robust
molecular structure with strong interaction between the Pd
moiety and thiadiazole-containing framework.
To determine whether Pd@DTE is working in a hetero-
b
Entry
Time (h)
Yield (%) geneous manner, a hot-ltration test was carried out in the
Suzuki–Miyaura coupling reaction of 2-bromoanisole with
1
2
3
4
5
6
7
8
9
1
st batch
nd batch
rd batch
th batch
th batch
th batch
th batch
th batch
th batch
0th batch
1
1
1
1
1
1
1
1
1
1
100
100
99.1
98.5
99.2
98.2
98
98.7
98.4
97.6
ꢀ
phenylboronic acid at 100 C (Fig. 6). During the reaction,
the heterogeneous catalyst was separated by ltration aer
.1 h (conversion 75%); the reaction was carried out for
0
a further 1 h. The GC analysis showed that the supernatant
has no catalytic activity. ICP-OES analysis of the supernatant
conrms that Pd is absent in the solution. These results
suggested that no leaching of Pd species takes place during
the catalysis and demonstrated the effectiveness of the
Pd@DTE in conning Pd moiety originated from the
intrinsic features of DTE, including evenly distributed
a
Reaction conditions: aryl halide (1 mmol), arylboronic acid (1.5
CO (2.0 mmol), TBAB (0.5 mmol),
mmol), Pd@DTE (0.1 mol%), K
2
ꢀ
3
b
H
2
O (2.0 ml), EtOH (2.0 ml), 50 C. GC yield was determined using strong thiadiazole ligand and microporous structure.
n-dodecane as an internal standard.
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Table 6 Comparison of Pd@DTE with reported sulfur containing heterogeneous Pd catalysts
Yield
Number of cycles
& Substrate
ꢀ
Catalyst
Solvent
Temp. ( C) Loading (mol%) Time (h) (%)
Substrate
TOF References
250 This work
Pd@DTE
Water & ethanol 50
0.1
4
100
PdNPs@COF
P.SBA-15SHPd
ZrDMTD-Pd
Water & DMF
Ethanol
50
60
80
0.1
0.12
1
3
82.9
80
276 8b
222 9a
17.2 9b
3
Ethanol
5
86
Pd–P/polythiophene Water & ethanol RT
1
24
87
N/A
4
9c
Pd@CB-PN
Water
RT
5
2
100
10
9d
Conclusion
Notes and references
In summary, a rigid thiadiazole-containing porous polymer
DTE is fabricated, followed by the immobilization of Pd moiety
to DTE. First, the intrinsic sulfur ligand is retained during the
preparation process of DTE at elevated temperature. Second,
the Pd moiety is successfully anchored on DTE, which is
conrmed by XPS and TEM data. According to the elemental
analysis and TEM mapping results, we found that the elemental
composition of as prepared DTE and Pd@DTE is very close to
theoretical values. The catalytic activity was conrmed by
carrying out the Suzuki–Miyaura coupling reactions of aryl and
heteroaryl bromides. The catalyst showed excellent catalytic
activity in coupling activated aryl bromides with phenylboronic
acid in short reaction period. The highly sterically hindered
mesitylene bromide and 2,5-dibromotoluene was coupled in 20
and 6 h, respectively. Additionally, heteroaryl deactivated by
foreign elements was also exhibited good to excellent catalytic
activities in 20 h. From the catalytic results the robustness
structure of the DTE, and strong bond between rigid thiadiazole
ligand and Pd moiety allow the excellent activity of catalyst and
its reuse for at least 10 times without a signicant loss of its
catalytic activity.
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Conflicts of interest
4
(a) D. K. Shen, L. Chen, J. P. Yang, R. Y. Zhang, Y. Wei,
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There are no conicts to declare.
Acknowledgements
This work was supported by the Basic Science Research
Program through the National Research Foundation of Korea
250–254; (c) Q. H. Yang, Q. Xu and H.-L. Jiang, Chem. Soc.
Rev., 2017, 46, 4774–4808; (d) W.-N. Wang, W.-J. An,
B. Ramalingam, S. Mukherjee, D. M. Niedzwiedzki,
(NRF), funded by the Ministry of Education (grant number:
NRF-2015R1A4A1042434).
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