The study and application of three highly porous hyper-crosslinked catalysts possessing similar catalytic centers

Article abstract of DOI:10.1016/j.polymer.2019.01.009

Three new catalysts, in which salen ligands were successfully locked in knitting network polymers, were synthesized through the Friedel-Crafts reaction. As a result, the catalysts possessed high specific surface area as high as 512 m²/g with micro-/mesoporous structures, and showed layered morphologies. Owing to the high surface area, the abundant active centers and pores, the Poly-salen-a-Pd(II) displayed superior performance in the Suzuki-Miyaura coupling reaction of various substrates. In particular, the effect from similar catalytic centers for the catalytic properties and stability was investigated by a series of experiments and analyses such as thermogravimetric analysis (TGA), scanning and transmission electron microscopies (SEM and TEM), X-ray photoelectron spectroscopy (XPS), atomic absorption spectroscopy (AAS). Notably, three catalysts showed high activity in the reaction, and displayed excellent performance in the hot filtration experiments. This strategy is very cost-effective and convenient, which was highly suggestive to diverse professional audiences in a wide range of heterogeneous catalyst design and application.

Full text of DOI:10.1016/j.polymer.2019.01.009

Accepted Manuscript
The study and application of three highly porous hyper-crosslinked catalysts
possessing similar catalytic centers
Jiajun Bi, Yahao Dong, Di Meng, Dajian Zhu, Tao Li
PII:
S0032-3861(19)30018-7
DOI:
https://doi.org/10.1016/j.polymer.2019.01.009
JPOL 21158
Reference:
To appear in: Polymer
Received Date: 24 October 2018
Revised Date: 29 December 2018
Accepted Date: 4 January 2019
Please cite this article as: Bi J, Dong Y, Meng D, Zhu D, Li T, The study and application of three highly
porous hyper-crosslinked catalysts possessing similar catalytic centers, Polymer (2019), doi: https://
doi.org/10.1016/j.polymer.2019.01.009.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT
Graphical Abstract:
Three similar salen ligands were successfully locked into network via the
Friedel-Crafts reaction. As a result, the obtained catalysts possessed the advantages of
abundant pores, high surface areas, facile synthesis approach and low cost of
synthesizing. Meanwhile, Poly-salen-a-Pd displayed excellent performance in the
Suzuki-Miyaura coupling reaction of various substrates, and the superior catalytic
capacity of three catalysts was further confirmed by hot filtration experiments and
comparison with other catalysts. On the other hand, it is found that the stability and
catalytic capacity of three heterogeneous catalysts were influenced by the pore size
distribution, surface area, pore shape, thermal stability of polymers and structure of
monomers.

ACCEPTED MANUSCRIPT
1
2
The study and application of three highly porous hyper-crosslinked
catalysts possessing similar catalytic centers
3
4
5
Jiajun Bi,a Yahao Dong,a Di Meng,a Dajian Zhu,a,b * Tao Lia,*
a Hubei Key laboratory of Material Chemistry and Service Failure; Key laboratory of Material
Chemistry for Energy Conversion and Storage, Ministry of Education; School of Chemistry and
Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
b Shenzhen Research Institute of Huazhong University of Science and Technology, 9 Yuexing Third
Avenue, Shenzhen 518057, China
6
7
8
9
10
11
*Corresponding author:
12
13
14
E-mail for Tao Li: taoli@hust.edu.cn
E-mail for Dajian Zhu: zhudj@hust.edu.cn
1

ACCEPTED MANUSCRIPT
15 Abstract
16
17
18
19
20
21
22
23
24
25
26
27
28
29
Three new catalysts, in which salen ligands were successfully locked in knitting
network polymers, were synthesized through the Friedel-Crafts reaction. As a result,
the catalysts possessed high specific surface area as high as 512 m2/g with
micro-/mesoporous structures, and showed layered morphologies. Owing to the high
surface area, the abundant active centers and pores, the Poly-salen-a-Pd(II) displayed
superior performance in the Suzuki-Miyaura coupling reaction of various substrates.
In particular, the effect from similar catalytic centers for the catalytic properties and
stability was investigated by a series of experiments and analyses such as
thermogravimetric analysis (TGA), scanning and transmission electron microscopies
(SEM and TEM), X-ray photoelectron spectroscopy (XPS), atomic absorption
spectroscopy (AAS). Notably, three catalysts showed high activity in the reaction, and
displayed excellent performance in the hot filtration experiments. This strategy is very
cost-effective and convenient, which was highly suggestive to diverse professional
audiences in a wide range of heterogeneous catalyst design and application.
30
Keywords: Suzuki reaction, Friedel-Crafts reaction, heterogeneous catalyst
31 1. Introduction
32
33
34
35
36
The heterogenization of homogeneous catalysts is extensively utilized in
industrial processes due to their compliance with the principles of green
chemistry, including enhancing the economy of the process and decreasing
metal pollution of desired products [1-3]. In the last decade, a variety of
microporous materials supporting Pd as catalysts have been reported for the
2

ACCEPTED MANUSCRIPT
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
model Suzuki-Miyaura coupling reaction, such as metal-organic frameworks
(MOFs) [4,5], hyper-crosslinked polymer (HCPs) [6,7], covalent-organic
frameworks (COFs) [8-10] and activated carbons (ACs) [11,12]. In particular,
hyper-crosslinked polymers (HCPs), as an important class of microporous
organic polymers (MOPs), are attracting widespread attention due to their
outstanding properties, including facile preparation, tuneable pore sizes, high
surface areas and porous frameworks. In addition, the extensive chemical
cross-linkage could effectively prevent the collapse of frameworks. In 2012,
our group and co-workers introduced a procedure by “knitting”
triphenylphosphine ligand with benzene to form functionalized microporous
network polymer as support to load Pd, the final catalyst displayed high surface
area and afforded rapid conversion for the Suzuki-Miyaura coupling reaction of
chlorobenzene [13]. On the basis of the above study, our group and co-workers
also further knitted N-heterocyclic carbenes (NHCs) into frameworks as Pd
catalysts for the Suzuki-Miyaura coupling reaction in 2015 [7]. Following the
“knitting” strategy, porous polymers could be produced with the use of a series
of ligands. The porous property ensures the high dispersion of metal and the
diffusion of organic reactant molecules, thereby enhancing the conversion rate
of substrates and the stability of catalysts. Therefore, the “knitting” strategy is
powerful to make some desirable ligands be knitted into the skeletal framework
without additional chemical modification [14].
The salen ligands have been proven to be excellent directional ligands, which are
3

ACCEPTED MANUSCRIPT
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
tended to create a N2O2 co-ordination pocket to coordinate different metal ions
[15,16], thereby highly strengthening the N2O2-metal bonds. Thus, salen-type
complexes have attracted much attention in the fields of electrochemistry [17-18] and
catalysis [15]. For example, Leng’s group synthesized Co-salen-TBB-Py for fixation
of carbon dioxide (CO2) under mild and solvent-cocatalyst-free conditions [19]. A
non-symmetrical salen-type palladium (II) catalyst, which is readily immobilized onto
Merrifield resin and functionalized silica for Suzuki-Miyaura coupling reaction, was
reported by Styring et al [20]. The integration of salen ligands into the backbone of
HCPs will enhance the metal binding capacity of the catalyst. In addition, catalysts
will possess diversity with the use of extensive salen building blocks. Xiao and his
co-workers synthesized POL-salen with high surface area (490 m2g-1) [21]. Further,
Xiao’s groups also prepared Co3+/POL-salen with high concentration of Co3+/salen,
which afforded excellent performance in epoxide hydration [22]. Based on the above
considerations, it is thus of perfect strategy to develop efficient catalysts using
different salen ligands as the building blocks.
The biaryl units are fundamental components of herbicides, natural products,
pharmaceuticals, conducting polymers, liquid crystalline materials and ligands for
catalysts [4,20]. Therefore, there is an increasing demand to obtain biaryl by a simple,
fast, green and cost-effective method. The Suzuki-Miyaura coupling reaction in the
present of Pd catalysts has become one of the most powerful methods to form biaryl
and demonstrated high selectivity, air- and moisture-stable and relatively low toxicity.
For example, Jie’s group synthesized the catalyst of UiO-67-3-PI-Pd, which was
4

ACCEPTED MANUSCRIPT
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
efficient for the Suzuki-Miyaura coupling reaction with a very low Pd loading (0.054
mol%) [23]. The catalyst of MONFs-PPh3@Pd was prepared by Huang’s group via
hyper-crosslinking core-shell multicomponent bottlebrush copolymers, and displayed
excellent performance for the Suzuki-Miyaura coupling reaction [6]. To date, the
Suzuki-Miyaura coupling reactions have achieved extensive investigation.
Nonetheless, a small fraction of catalysts showed low catalytic efficiency for some
substrates, because the low or negligible surface area of catalysts limited the
interaction between active centers and substrates. In this context, it is attractive to
synthesize highly efficient heterogeneous catalysts with both micro-/mesoporous and
a controllably high surface area. In this sense, salen HCPs is a promising material as
support for Pd catalyst to catalyze extensive substrates.
Thus, we herein report the strategy of knitting different salen ligands into the
network framework and the subsequent application of these high surface area
polymers supporting Pd as versatile catalysts in Suzuki-Miyaura coupling
reaction (in Scheme 1). The catalytic properties and stability of the catalysts
possessing different active centers will also be examined to determine their
effect on the cross-coupling reaction. Furthermore, this work demonstrates that
Poly-salen-Pd(II) exhibited excellent catalytic activity for the Suzuki-Miyaura
reaction in aqueous media comparing with other Pd catalysts, and the
subsequent gram-scale experiments and hot filtration test further confirmed the
excellence of Poly-salen-Pd(II) catalysts.
101
102
5

ACCEPTED MANUSCRIPT
103
104
Scheme 1. The synthesis of monomers and catalysts.
105 2. Experimental section
106
107
2.1 Chemicals
All the chemicals were commercially available and used as received without further
108
109
110
111
112
113
114
115
purification. Benzene, salicylaldehyde, o-phenylenediamine, ethylenediamine and
FeCl3 (anhydrous) were obtained from Sinopharm Chemical Reagent Co., Ltd.
Cyclohexanediamine was purchased from Adamas Reagent Co., Ltd. Formaldehyde
dimethyl acetal (FDA, 99.0%) came from Aladdin. The solvents, such as methanol,
1,2-dichloroethane (DCE) and dichloromethane (DCM) were purchased from
Sinopharm Chemical Reagent Co., Ltd and used as received.
2.2 Characterization equipment
Nuclear Magnetic Resonance (NMR) spectra were recorded in CDCl3 or
6

ACCEPTED MANUSCRIPT
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
DMSO-d6 on a Bruker AV400 MHz spectrometer with the internal reference of
tetramethylsilane (TMS) at room temperature. The FT-IR spectra were conducted in
the wave number 4000-400 cm-1 using a Bruker VERTEX 70 FT-IR
Spectrometer. Pd content of samples was obtained by atomic absorption
spectroscopy (AAS). Transmission electron microscopy (TEM) was recorded
on a Tecnai G20 microscope (FEI Corp. USA) instrument operated at an
accelerating voltage of 200kV. Polymer surface areas, N2 adsorption isotherm
(77.3 K) and pore size distributions were analyzed using V-Sorb 2800P surface
area and porosity analyzer (10-5 bar). The yields were measured in a Fuli
GC9790 equipping with a FFAP (30 m×0.25 mm×0.25 µm) capillary column
and a flame ionization detector (FID). X-Ray photoelectron emission spectra
(XPS) were performed on a Kratos AXIS ULTRA DLD-600 W system with the
passing energy flow of 100 eV. Thermogravimetric analysis (TGA) was
recorded on a Perkin-Elmer Instruments Pyris1 TGA. The “Pd (mmol%)” is the
amount of Pd relative to the substrate in the Suzuki-Miyaura coupling reaction.
UV-Vis absorption spectra were recorded on UV-2600. Emission spectra were
13
obtained using RF-5301PC. C MAS NMR measurement was performed on
Bruker AVANCE III HD 400MHZ.
2.3 General procedure for synthesis of salen ligands:
Synthesis of salen-a
7

ACCEPTED MANUSCRIPT
1
136
137
138
139
140
141
142
143
144
145
146
147
148
149
Salen-a was synthesized following reported procedures [24,25]. H NMR (400
MHz, CDCl3) δ 13.20 (d, J = 5.3 Hz, 2H), 8.81 (s, 2H), 7.42 (s, 6H), 7.26 (s,
2H), 7.00 (s, 2H), 6.92 (t, J = 7.4 Hz, 2H).
Synthesis of salen-b
1
Salen-b was synthesized following reported procedures [25,26]. H NMR (400 MHz,
DMSO-d6) δ 13.40 (s, 2H), 8.59 (s, 2H), 7.66-7.09 (m, 4H), 6.88 (dd, J = 11.8, 7.9 Hz,
4H), 3.92 (s, 4H).
Synthesis of salen-c
1
Salen-c was synthesized following reported procedures [25,26]. H NMR (400
MHz, CDCl3) δ 13.43 (d, J = 61.7 Hz, 2H), 8.30 (d, J = 38.1 Hz, 2H), 7.39-7.04
(m, 4H), 7.04-6.56 (m, 4H), 3.67-3.15 (m, 2H), 2.06-1.83 (m, 4H), 1.82-1.52
(m, 4H).
2.4 General procedure for synthesis of salen functionalized polymer networks
(Poly-salen):
150
151
152
153
154
155
156
Three Poly-salen functionalized polymer precursors were synthesized by the
external cross-linking of benzene and salen (a, b and c). Poly-salen-a,
Poly-salen-b and Poly-salen-c were synthesized following reported procedures
[7,13].
Poly-salen-a: salen-a (1.10 g, 3 mmol), benzene (1.71 g, 20 mmol), FeCl3 (9.77
g), 1,2-dichloroethane (10 mL) and dimethoxymethane (4.67 g) were used in
this polymerization.
8

ACCEPTED MANUSCRIPT
Poly-salen-b: salen-b (0.98 g, 4 mmol), benzene (1.75 g, 20 mmol), FeCl3
(9.50 g), 1,2-dichloroethane (10 mL) and dimethoxymethane (4.68 g) were used
in this polymerization.
157
158
159
160
161
162
163
164
Poly-salen-c: salen-c (1.17 g, 4 mmol), benzene (1.75 g, 20 mmol), FeCl3 (9.47
g), 1,2-dichloroethane (10 mL) and dimethoxymethane (4.66 g) were used in
this polymerization.
2.5 General procedure for synthesis of Pd(II)-containing polymer networks
(Poly-salen-Pd(II)).
165
166
167
168
169
170
Poly-salen-a-Pd(II): Poly-salen-a and Pd(OAc)2 were used in catalyst
preparation (see supporting information).
Poly-salen-b-Pd(II): Poly-salen-b and Pd(OAc)2 were used in catalyst
preparation (see supporting information).
Poly-salen-c-Pd(II): Poly-salen-c and Pd(OAc)2 were used in catalyst
preparation (see supporting information).
171 3. Results and discussion
172
173
174
175
176
177
178
3.1 Characterization of the catalysts
Three ligands were analyzed by the UV-Vis spectra (see Figure S1). The
spectra (salen-a, salen-b and salen-c) exhibited absorption bands at 269 nm, 267
nm and 263 nm, respectively, which may be assigned to the phenyls. The bands
of three salen ligands at 310-335 nm are attributed to azomethine CH=N bonds
[27,28]. The property of three salen ligands was also investigated by the
Fluorescence spectroscopy (see Figure S1). The emission peak of salen-a with
9

ACCEPTED MANUSCRIPT
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
π-conjugation of 1,2-phenylenediamine bridge is at λF,max=511 nm, and
emission peaks of salen-b and salen-c without π-conjugation of
1,2-ethylenediamine bridge are at λF,max=437 and 434 nm, respectively [26].
The monomers were also studied by the FT-IR spectra, and the results were
displayed in the Figure 1. The spectra of salen-a, salen-b and salen-c showed
the bands at 1614 cm-1, 1635 cm-1 and 1629 cm-1, which corresponded to the
C=N stretching vibrations. The spectra of three monomers also displayed a
series of bands in the range of 1600-1450 cm-1 and 900-650 cm-1, which were
attributed to aromatic skeleton stretching and C-H bending vibrations of
1
benzene ring, respectively [14,29-32]. Together with the H NMR, the spectra
of FT-IR disclosed that the synthesized monomers are consistent with the
expected salen structure.
Three polymers were investigated by the FT-IR spectra (see Figure 1), and the
spectra of Poly-salen-a, Poly-salen-b and Poly-salen-c displayed the bands
around 1600-1660 cm-1, which could be assigned to the C=N stretching
vibrations. The peaks around 1250-950 cm-1 corresponded to C-H in-plane
bending of the benzene skeleton [14, 29-32]. The results indicated that the salen
functional groups were introduced successfully into the network.
10

ACCEPTED MANUSCRIPT
197
198
Figure 1. FT-IR spectra of monomers (salen-a, salen-b and salen-c) and Poly-salen
199
(Poly-salen-a, Poly-salen-b and Poly-salen-c).
200
201
202
203
204
205
206
207
208
Then, the surface morphology of three catalysts was studied by the scanning
electron micrograph (SEM) and transmission electron microscopy (TEM). The
TEM images of three fresh catalysts (see Figure 2a-c) gave the direct evidence
that the catalysts contain abundant nanosheets. The SEM images (see Figure
S2) of three catalysts preliminarily disclosed that three catalysts were porous.
The results (see Figure 2a-c and Figure S2) showed that the morphology of
these catalysts was hierarchical and porous. The Pd species were present in the
+2 state in the fresh Poly-salen-Pd(II), because no black Pd nanoparticles were
observed in the TEM images (see Figure 2a-c).
11

ACCEPTED MANUSCRIPT
a
b
c
f
e
d
209
210
211
212
Figure 2. TEM images of Poly-salen-Pd(II). a: Poly-salen-a-Pd(II); b:
Poly-salen-b-Pd(II); c: Poly-salen-c-Pd(II); d: Poly-salen-a-Pd(II) after fourth recycle;
e: Poly-salen-b-Pd(II) after third recycle; f: Poly-salen-c-Pd(II) after second recycle.
213
214
215
216
217
218
219
220
221
TGA (thermogravimetric analysis) was conducted under inert atmosphere and
the results were shown in Figure 3. The representative TGA curves of samples
showed mass changes at different temperature ranges. The TGA curves of
Poly-salen-a, Poly-salen-b and Poly-salen-c showed the first negligible weight
°
losses at below 350 C, which revealed that these materials exhibit excellent
°
thermal stability up to 350 C [33-37]. Three polymers were also analyzed by
the XPS, The N spectra of Poly-salen-a, Poly-salen-b and Poly-salen-c were
divided into two peaks. The peaks of C=N, C-N of Poly-salen-a corresponded
401.1 eV and 399.1 eV (see Figure S3). The peaks of C=N of Poly-salen-b and
12

ACCEPTED MANUSCRIPT
222
223
224
225
226
227
228
Poly-salen-c corresponded 400.6 eV and 400.5 eV [38]. The N atomic
concentration of Poly-salen-a, Poly-salen-b and Poly-salen-c was analyzed by
the XPS, and the data were 0.91%, 1.09%, 1.05%, respectively. The solid-state
13C MAS NMR spectrum of Poly-salen-a was shown in the Figure S4. The peak
near 36 ppm was observed, which may be attributed to the carbon in the
methylene linker formed from the Friedel-Crafts reaction [7,13] (see Figure
S4).
229
230
Figure 3. TGA curves of Poly-salen (Poly-salen-a, Poly-salen-b and Poly-salen-c).
XPS measurements were performed to identify the elemental compositions of
Poly-salen-a-Pd(II) and coordination states of Pd species. The full spectrum of
XPS analysis indicated that the Poly-salen-a-Pd(II) contains C, N, O and Pd
elements (see Figure S4). The Pd 3d5/2 and 3d3/2 peaks were identified at 338.5
eV and 343.9 eV, respectively [7,39,40], proving that the Pd species were
present in the +2 state rather than in the metallic state (see Figure 4).
231
232
233
234
235
236
13

ACCEPTED MANUSCRIPT
237
238
Figure 4. XPS spectra for Pd on fresh Poly-salen-a-Pd(II) and Poly-salen-a-Pd(II)
239
after fourth recycle.
240
241
242
243
244
245
246
247
248
249
250
251
The porous properties of the samples were further measured by nitrogen
adsorption analysis. All materials of Poly-salen and Poly-salen-Pd(II) gave rise
to type I nitrogen gas sorption isotherm with hysteresis loops. The data in Table
1 disclosed that Poly-salen-Pd(II) displayed a dramatic decrease in surface area
after anchoring Pd(II), it is possible that metal loading in the partial pores and
simple increase in mass lead to decrease in surface area. The adsorption and
desorption isotherms of Poly-salen and Poly-salen-Pd(II) in Figure 5 showed
steep nitrogen gas uptake curves at low relative pressure (P/P0 < 0.05),
suggesting the existence of abundant micropores. The hysteresis loop and a
sharp rise (P/P0 > 0.9) were observed, indicating that the materials include a
number of mesoporous and some macropores. Compared with the
non-metallized parent networks, the surface areas of the Poly-salen-Pd(II)
14

ACCEPTED MANUSCRIPT
252
253
254
255
256
257
258
259
260
261
catalysts were smaller from Brunauer-Emmet-Teller (BET) analysis in Table 1.
However, the Poly-salen-Pd(II) catalysts still exhibited obvious microporous,
which was clearly observed in the curves of micropore-size distribution (see
Figure 5). The porous structure in the heterogeneous support is necessary to
maximize the accessibility of the catalyst active sites toward the organic
reactant molecules. Moreover, the porous structure improved the dispersion of
Pd active sites [7,13,14]. Meanwhile, the content of Pd(II) in the
Poly-salen-Pd(II) was detected by atomic absorption spectroscopy (AAS) and
summarized in Table 1.
262
263
264
265
266
Figure 5. N2 sorption isotherms and micropore-size distributions calculated using SF
methods.
(a):Poly-salen-a
(b):Poly-salen-a-Pd(II)
(c):Poly-salen-b
(d):Poly-salen-b-Pd(II) (e):Poly-salen-c (f):Poly-salen-c-Pd(II).
267
Table 1. The characterization of the polymers and catalysts.
15

ACCEPTED MANUSCRIPT
Pd2+
SBET
(m2g-1)b
Vmicro
(cm3g-1)c
Vtotal
(cm3g-1) d
Yield
(%)e
AAS
(wt%)a
Samples
Poly-salen-a
Poly-salen-a-Pd(II)
Poly-salen-b
0
699.7
512.1
484.3
290.7
798.3
325.0
0.21
0.15
0.15
0.08
0.24
0.09
0.88
0.65
0.59
0.50
0.48
0.23
0
99
0
4.0
0
Poly-salen-b-Pd(II)
Poly-salen-c
4.7
0
99
0
Poly-salen-c-Pd(II)
4.2
99
268
269
270
271
272
273
a: Data were obtained by atomic absorption spectroscopy (AAS); b: Langmuir surface
area; c: SF micropore volume; d: Single point adsorption total pore volume at P/P0 =
0.99; e: bromobenzene (1.0 mmol), phenylboronic acid (1.5 mmol), K2CO3 (1.5
°
mmol), 3 mL of solvent (EtOH:H2O=3:2, v/v), 80 C, 0.38 mmol% Pd(II), t=10 min,
determined by GC.
3.2 Application of Poly-salen-a-Pd(II) in Suzuki-Miyaura coupling reaction
274
275
276
277
278
279
280
281
282
283
The Poly-slen-a-Pd(II) was chosen as model catalyst according to the results of
Table 1, and afforded excellent performance in the Suzuki-Miyaura coupling
reaction. Universally it is acknowledged that factors like base, solvent and
temperature largely affect the catalytic performance [40,41]. In order to
improve the catalytic capacity of the Poly-slen-a-Pd(II), several typical solvents
were studied. The yields of single solvents such as dioxane, acetonitrile, ethyl
acetate and anhydrous ethanol corresponded to 14%, 14%, 20% and 82%,
respectively (see Table S1). Water is not only green but also perfect for
dissolving inorganic base. Therefore, the solvents with different volume ratios
of EtOH/H2O were evaluated and the results were satisfactory. The mixed
16

ACCEPTED MANUSCRIPT
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
solvent (EtOH:H2O=3:2, v/v) is one of the best choices with higher yield than
99%, and the yields with the solvents of (EtOH:H2O=4:1, v/v) and
(EtOH:H2O=1:4, v/v) corresponded to 93% and 99%, respectively (see Table
S1). The ethanol could solve the organic substrates and products, and water
could dissolve the inorganic base, which would benefit to chemical interactions
of base and substrates in the mixed solvent.
Subsequently, we also evaluated the effect of bases for the Suzuki-Miyaura coupling
reaction in the optimal solvent (EtOH:H2O=3:2, v/v). The common bases were
investigated, including NaHCO3, NaOH, K2CO3, CH3COOK, HCOONa·2H2O and
KHCO3, and the yields of products were 60%, 95%, >99%, 16%, 12% and 64%,
respectively. Remarkably, K2CO3 was the best base among the above bases (see Table
S1). Thus, the optimum reaction conditions of Poly-salen-a-Pd(II) were observed in 3
mL of solvent (EtOH:H2O=3:2, v/v) using 0.38 mmol% Pd(II) and K2CO3 (1.5 mmol)
as base in the reaction of bromobenzene (1.0 mmol) and phenylboronic acid (1.5
mmol). Further, Poly-salen-a-Pd(II), Poly-salen-b-Pd(II) and Poly-salen-c-Pd(II) were
studied under the optimum reaction conditions, and the results were summarized in
Table 1. The results demonstrated that three catalysts showed excellent performance
in the Suzuki-Miyaura coupling reaction.
In order to expand the scope of substrates, the catalytic capacity of
Poly-salen-a-Pd(II) was examined in the Suzuki-Miyaura coupling reaction of
phenylboronic acid and various aryl halides (Table 2, entries 1-7). All of aryl
halides with electron-donating group (-CH3) coupling with phenylboronic acid
17

ACCEPTED MANUSCRIPT
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
can achieve >90% yields within 3.5 h. For example, 4-methyl bromobenzene
can achieved the yield of 99% with 3.5 h (Table 2, entry 3). The
Poly-salen-a-Pd(II) also could promote the conversion of 4-methyl
iodobenzene, and the yield was 91% in 3 hours (Table 2, entry 2). However, the
aryl bromide with electron-withdrawing group (-COCH3) need longer time to
achieve high yield, for example, 4-bromoacetophenone was converted to
4-phenylacetophenone with the yield of 97% in 6 h (Table 2, entry 7).
In addition to the derivatives of aryl halides, the catalytic capacity of
Poly-salen-a-Pd(II) for the derivatives of phenylboronic acid was also
investigated in the Suzuki-Miyaura coupling reaction. The yields were higher
than 95% when 4-methylphenylboronic acid was used as substrate to couple
with aryl halides within 4 hours (Table 2, entries 8-11). It is difficult for
2-methylphenylboronic acid to achieve high yield under the identical conditions
because of steric hindrance, so the catalyst with 1.12 mmol% Pd(II) was used
(Table 2, entries 12 and 14). Meanwhile, the reaction time of iodobenzene as
substrate was shorter than the case of bromobenzene at the same condition, this
may be explained by the different bond dissociation energies and the ionic radii
of the halogens (Table 2, entries 12 and 14). Furthermore, the yield of coupling
products was 99% in the presence of 4-methoxyphenylboronic acid coupling
with bromobenzene (Table 2, entry 13), and the yield (99%) of coupling
reaction between iodobenzene and 4-acetylphenylboronic acid was achieved
within 6 h (Table 2, entry 15).
18

ACCEPTED MANUSCRIPT
328
329
Table 2. Suzuki-Miyaura coupling reaction of various substrates catalyzed by
Poly-salen-a-Pd(II)a.
Poly-salen-a-Pd(II)
Br
B(OH)2
+
R2
R1
base
solvent
R1
R2
2
1
330
Entry
Substrate 1
Substrate 2
Time(h)
Yield(%)b
B(OH)2
Br
1
2
0.3
99
B(OH)2
B(OH)2
B(OH)2
I
3
91
99
90
3
Br
3.5
0.2
4c
I
5c
3
98
Br
B(OH)2
B(OH)2
B(OH)2
B(OH)2
B(OH)2
B(OH)2
B(OH)2
O
6
7
Br
6
6
99
97
99
99
96
97
O
Br
I
8
4
9
Br
4
Br
10
11
3.5
3.5
I
I
B(OH)2
12c
13
2
7
3
6
92
99
99
99
O
B(OH)2
Br
Br
I
B(OH)2
14c
15
O
B(OH)2
19

ACCEPTED MANUSCRIPT
331
332
a Reaction conditions: aryl halide (1.0 mmol), phenylboronic acid (1.5 mmol), 3 mL of
solvent (EtOH:H2O=3:2, v/v), 0.38 mmol% Pd(II), K2CO3 (1.5 mmol), 60 °C.
333
334
b Determined by GC.
c 1.12 mmol% Pd(II).
335
336
337
338
339
340
341
342
343
3.3 Scale-up experiment
The scale-up reactions were carried out with model reaction, and the results
were shown in Table 3. The catalysts played positive roles within 10 min for
0.785 g bromobenzene (263 equivalents of substrate with respect to the Pd
catalyst; Figure S5). In particular, the coupling reaction of bromobenzene was
performed on a scale of 1.57 g with 0.11 mmol% Pd(II) to give the product
with a yield of 98%. Under the similar condition, the Poly-salen-b-Pd(II) and
Poly-salen-c-Pd(II) also achieved the yields of 99% (see Table 3). Clearly, the
results illustrated that the Poly-salen-Pd(II) catalysts can be feasible in the
344
345
gram-scale experiment for the Suzuki-Miyaura coupling reaction.
Table 3. Scale-up experiment of three catalysts (Poly-salen-Pd (II)).
80 °C Cat.
K2CO3
+
B(OH)2
Br
EtOH/H2O=3:2
346
Entry
Substrate(g)
Pd(mmol%)
0.11
Yield(%)
Poly-salen-a-Pd(II)
Poly-salen-b-Pd(II)
Poly-salen-c-Pd(II)
1.57
1.57
1.57
98
99
99
0.13
0.12
347
348
Conditions: bromobenzene (1.57 g, 10 mmol), phenylboronic acid (15 mmol), 30 mL
of solvent (EtOH:H2O=3:2, v/v), K2CO3 (15 mmol), 15 min.
20

ACCEPTED MANUSCRIPT
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
3.4 Hot filtration experiment
In order to determine if the coordination between polymer supports and Pd(II)
are relatively stable, a hot filtration experiment was adopted. We stopped the
reaction immediately at 20 min and removed the solid catalyst from the reaction
mixture. The Pd content of the mother liquid from Poly-salen-a-Pd(II),
Poly-salen-b-Pd(II) and Poly-salen-c-Pd(II) was very low, accompanying
0.15%, 0.25% and 0.26%, respectively. The results disclosed that the catalysts
were stable without obvious Pd leaching from the polymer supports. Therefore,
it is fair to conclude that the catalysts are heterogeneous in nature [7,13,42,43].
3.5 The test of stability
On the basis of above studies, the stability of the Poly-salen-a-Pd(II),
Poly-salen-b-Pd(II) and Poly-salen-c-Pd(II) was further evaluated for
comparison under the similar condition (see supporting information), and the
results were summarized in Table 4. The results revealed that
Poly-salen-a-Pd(II) is more stable than the others (Poly-salen-b-Pd(II) and
Poly-salen-c-Pd(II)). Keep in view different electronic effect of monomers, the
salen-a is strong π-conjugated systems, and the electronic effect of salen-a is
stronger than others (salen-b and salen-c). Meanwhile, the Pd leaching of
Poly-salen-a-Pd(II) is lower than others (Poly-salen-b-Pd(II) and
Poly-salen-c-Pd(II)). Furthermore, the role of the pore size distributions in these
materials is also equally important [14]. A larger microporous volume and total
pores volume in the structure ensured transport of reactants/products
21

ACCEPTED MANUSCRIPT
371
372
373
374
375
376
377
378
379
380
inside/outside the porous materials, thereby reducing the possibility for
plugging the pores to enhance the stability. The Poly-salen-a-Pd(II) contains
not only micropores but also a fair amount of mesopores and macropores in the
structure. The data in Table 1 disclosed that the microporous volume of
Poly-salen-b-Pd(II)
and
Poly-salen-c-Pd(II)
was
smaller
than
Poly-salen-a-Pd(II), and the sequence of total pores volume is
Poly-salen-a-Pd(II) > Poly-salen-b-Pd(II) > Poly-salen-c-Pd(II). The results
indicated that the structure of monomers, pore shape, surface area and pore size
distribution may play important roles in the stability.
Table 4. Stability of Poly-salen-Pd(II) in the Suzuki-Miyaura coupling reaction.
80 °C Cat.
K2CO3
+
B(OH)2
Br
EtOH/H2O=3:2
381
Runs
Poly-salen-a-Pd(II)a (%) Poly-salen-b-Pd(II)a (%) Poly-salen-c-Pd(II)a (%)
1
2
3
4
99
90
90
80
99
60
58
---
99
20
---
---
382
383
384
385
386
387
388
389
Conditions: bromobenzene (1.0 mmol), phenylboronic acid (1.5 mmol), K2CO3 (1.5
mmol), 3 mL of solvent (EtOH:H2O=3:2, v/v), 80 °C, 0.38 mmol% Pd(II).
a determined by GC.
----: not determined.
Three catalysts after used were also characterized by TEM (see Figure 2d-f).
The images of TEM showed that the Pd(0) was observed on the catalysts, but
no significant change occurred in terms of the morphology of the
Poly-salen-Pd(II) after used. The slight aggregation of Pd nanoparticles was
22

ACCEPTED MANUSCRIPT
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
observed in the Poly-salen-a-Pd(II) after fourth recycle. The nanoparticles of
Poly-salen-b-Pd(II) after third recycle and Poly-salen-c-Pd(II) after second
recycle were heavily agglomerated. The aggregation of Pd species resulted in
the decrease of catalytic activity and the longer time was necessary for the
coupling reaction in the recycle experiment.
We also investigated the Pd species states of Poly-salen-a-Pd(II) after used by
XPS (see Figure 4). The results revealed that Pd species were present in the
metallic state, corresponding to the binding energy (B.E.) of 336.4 eV and
341.7 eV in the Pd 3d5/2 and 3d3/2 levels, respectively.
3.6 Study of activity
The catalytic capacity of three catalysts was further investigated. Three catalysts and
salen-Pd(II) both achieved 99% yield in the Suzuki-Miyaura coupling reaction under
specified condition (Table 5, entries 4-9). Meanwhile, three catalysts were compared
with other Pd catalysts. As shown in Table 5, three catalysts (Poly-salen-Pd(II))
exhibit extensive progress in reaction time and temperature. For example, the yield of
coupling reaction could achieve 97% with Tbpo-Pd(II) as catalyst in 30 min [44],
however, the reaction time of three catalysts (Table 5, entries 4, 6 and 8) was shorter
°
than that of TbPo-Pd(II). Furthermore, the low temperature (80 C) were used than
°
that of SMNPs-Salen Pd(II) (100 C) [45], and the reaction time (10 min) was shorter
than the case of MCMSalenPd (120 min) [46].
410
411
Table 5. Study of catalytic capacity of Poly-salen-Pd(II) for the Suzuki-Miyaura
coupling reaction.
23

ACCEPTED MANUSCRIPT
Cat.
+
B(OH)2
Br
412
Entry
Catalyst
Temperature ( °C ) Time(min) Yield (%)
Ref.
1
TbPo-Pd(II)
80
30
97
[44]
2
3
4
5
6
7
8
9
SMNPs-Salen Pd(II)
100
180
100
[45]
MCMSalenPd
130
80
120
10
>98a
99
[46]
Poly-sale-a-Pd(II)
this work
salen-a-Pd(II)
Poly-salen-b-Pd(II)
salen-b-Pd(II)
80
80
80
80
80
10
10
10
10
10
99
99
99
99
99
---
this work
---
Poly-salen-c-Pd(II)
salen-c-Pd(II)
this work
---
413
414
Conditions of entries 4-9: bromobenzene (1.0 mmol), phenylboronic acid (1.5 mmol),
K2CO3 (1.5 mmol), 3 mL of solvent (EtOH:H2O=3:2, v/v). a: Conversion
415
416 4. Conclusions
417
418
419
420
421
422
423
424
425
To summarize, we designed a strategy to synthesize a series of Poly-salen via
simple Friedel-crafts reaction and then immobilize Pd(II) on the supports. The
porous structure of the support improved the diffusion of organic reactant
molecules and ensured the homogeneous dispersion of Pd active sites, thereby
improving the catalytic performance. More specifically, compared with other
Pd catalysts, the porous heterogeneous catalysts, Poly-salen-Pd(II), also
displayed excellent catalytic performance under the mild conditions and
aqueous reaction media for the Suzuki-Miyaura coupling reaction. The catalytic
experiments of various substrates and gram-scale experiments further
24

ACCEPTED MANUSCRIPT
426
427
428
429
430
431
demonstrated the efficiency of above catalysts. Importantly, the results
indicated that the structure of monomers, pore shape, surface area, thermal
stability of polymers and pore size distribution play important roles in the
stability and catalytic capacity. The highlights of these materials may be also
useful and potential in other fields of research as well as in catalytic
applications.
432 Conflicts of interest
The authors declare no competing financial interests.
434 Acknowledgements
433
435
436
437
438
439
440
The authors acknowledge financial support from the National Natural Science
Foundation of China (21473064), Shenzhen Science and Technology Innovation
Program (JCYJ20150630155150201), Innovation Foundation of Huazhong
University of Science and Technology (2015TS151). The authors thank for the
analysis supplied by the Analytical and Testing Center, Huazhong University of
Science and Technology, Wuhan, China.
441
442 References
443
444
445
446
447
[1] I. L. C. Buurmans, B. M. Weckhuysen, Heterogeneities of individual catalyst
particles in space and time as monitored by spectroscopy, Nat. Chem., 4 (2012)
873-886.
[2] C. M. A. Parlett, K. Wilson, A. F. Lee, Hierarchical porous materials: catalytic
applications, Chem. Soc. Rev., 42 (2013) 3876-3893.
25

ACCEPTED MANUSCRIPT
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
[3] K. Wang, Z. Jia, X. Yang, L. Wang, Y. Gu, B. Tan, Acid and base coexisted
heterogeneous catalysts supported on hypercrosslinked polymers for one-pot
cascade reactions, J. Catal., 348 (2017) 168-176.
[4] F. Nouri, S. Rostamizadeh, M. Azad, Post-synthetic modification of IRMOF-3
with an iminopalladacycle complex and its application as an effective
heterogeneous catalyst in Suzuki-Miyaura cross-coupling reaction in
H2O/EtOH media at room temperature, Molecular Catalysis, 443 (2017)
286-293.
[5] A. K. Jana, R. Hota, S. Natarajan, Palladium nanoparticles encapsulated in
[M(C19H11N2O2)2·H2O] (M = Co and Mn) as a potential catalyst for the
homocoupling of aryl halides, Cryst. Growth Des., 16 (2016) 6992-6999.
[6] Y. Xu, T. Wang, Z. He, A. Zhong, W. Yu, B. Shi, K. Huang, Synthesis of
triphenylphosphine-based microporous organic nanotube framework supported
Pd catalysts with excellent catalytic activity, Polym. Chem., 7 (2016)
7408-7415.
[7] S. Xu, K. Song, T. Li, B. Tan, Palladium catalyst coordinated in knitting
N-heterocyclic carbene porous polymers for efficient Suzuki-Miyaura coupling
reactions, J. Mater. Chem. A, 3 (2015) 1272-1278.
[8] S. Lu, Y. Hu, S. Wan, R. McCaffrey, Y. Jin, H. Gu, W. Zhang, Synthesis of
ultrafine and highly dispersed metal nanoparticles confined in a
thioether-containing covalent organic framework and their catalytic
applications, J. Am. Chem. Soc., 139 (2017) 17082-17088.
26

ACCEPTED MANUSCRIPT
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
[9] R. S. B. Gonçalves, A. B. V. d. Oliveira, H. C. Sindra, B. S. Archanjo, M. E.
Mendoza, L. S. A. Carneiro, C. D. Buarque, P. M. Esteves, Heterogeneous
catalysis by covalent organic frameworks (COF): Pd(OAc)2@COF-300 in
cross-coupling reactions, ChemCatChem, 8 (2016) 743-750.
[10] S. Ding, J. Gao, Q. Wang, Y. Zhang, W. Song, C. Su, W. Wang,
Construction of covalent organic framework for catalysis: Pd/COF-LZU1 in
Suzuki-Miyaura coupling reaction, J. Am. Chem. Soc
19816-19822.
., 133 (2011)
[11] A. A. Kurokhtina, E. V. Larina, A. F. Schmidt, A. Malaika, B. Krzyżyńska,
P. Rechnia, M. Kozłowski, Mechanistic studies of the Suzuki-Miyaura reaction
with aryl bromides using Pd supported on micro- and mesoporous activated
carbons, J. Mol. Catal. A-Chem., 379 (2013) 327-332.
[12] E. Guillén, R. Rico, J. M. López-Romero, J. Bedia, J. M. Rosas, J.
Rodríguez-Mirasol, T. Cordero, Pd-activated carbon catalysts for
hydrogenation and Suzuki reactions, Appl. Catal. A-Gen, 368 (2009) 113-120.
[13] B. Li, Z. Guan, W. Wang, X. Yang, J. Hu, B. Tan, T. Li, Highly dispersed Pd
catalyst locked in knitting aryl network polymers for Suzuki-Miyaura coupling
reactions of aryl chlorides in aqueous media, Adv. Mater., 24 (2012)
3390-3395.
[14] Z. Jia, K. Wang, T. Li, B. Tan, Y. Gu, Functionalized hypercrosslinked
polymers with knitted N-heterocyclic carbine-copper complexes as efficient
27

ACCEPTED MANUSCRIPT
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
and recyclable catalysts for organic transformations, Catal. Sci. Technol., 6
(2016) 4345-4355.
[15] S. Bhunia, R. A. Molla, V. Kumari, S. M. Islam, A. Bhaumik, Zn(II) assisted
synthesis of porous salen as an efficient heterogeneous scaffold for capture and
conversion of CO2, Chem. Commun., 51 (2015) 15732-15735.
[16] R. M. Haak, A. Decortes, E. C. Escudero-Adán, M. M. Belmonte, E. Martin,
J. Benet-Buchholz, A. W. Kleij, Shape-Persistent octanuclear zinc salen
clusters: synthesis, characterization, and catalysis, Inorg. Chem., 50 (2011)
7934-7936.
[17] S. Realista, P. Ramgi, B. d. P. Cardoso, A. I. Melato, A. S. Viana, M. J.
Calhorda, P. N. Martinho, Heterodinuclear Ni(II) and Cu(II) Schiff base
complexes and their activity in oxygen reduction, Dalton Trans., 45 (2016)
14725-14733.
[18] C. S. Martin, T. R. Dadamos, M. F. Teixeira, Development of an
electrochemical sensor for determination of dissolved oxygen by nickel-salen
polymeric film modified electrode, Sensor. Actuat. B-Chem, 175 (2012)
111-117.
[19] C. Zhang, D. Lu, Y. Leng, P. Jiang, Metal-salen-bridged ionic networks as
efficient bifunctional solid catalysts for chemical fixation of CO2 into cyclic
carbonates, Molecular Catalysis, 439 (2017) 193-199.
28