Pillar[5]arene-Based Organometallic Cross-Linked Polymer: Synthesis, Structure Characterization, and Catalytic Activity in the Suzuki-Miyaura Coupling Reaction

Article abstract of DOI:10.1021/acs.macromol.7b02701

Heterogeneous catalysts, as an emerging research hotspot, have showed excellent advantages on stability, recyclability, and separation from reactant compared with homogeneous catalysts in recent years. Herein, pillar[5]arene-based organometallic cross-linked polymer (P[5]-OCP) was constructed via the complexation between imidazolium-derivatived pillar[5]arene (P[5]) and Pd(OAc)₂. The chemical compositions and structure of the P[5]-OCP were confirmed by means of multiple characterization methods. The catalytic activity and process of P[5]-OCP were emphatically investigated in the Suzuki-Miyaura coupling reaction. The P[5]-OCP showed outstanding catalytic activity in mild reaction conditions. Meanwhile, the catalyst P[5]-OCP possessed excellent stability and recyclability, which could be reused at least five times without significant loss of activity. Furthermore, the P[5]-OCP is easily synthesized and cost-effective, which make it a candidate for applications in fine chemical engineering. [Figure Presented]

Full text of DOI:10.1021/acs.macromol.7b02701

Article
Cite This: Macromolecules XXXX, XXX, XXX−XXX
Pillar[5]arene-Based Organometallic Cross-Linked Polymer:
Synthesis, Structure Characterization, and Catalytic Activity in the
Suzuki−Miyaura Coupling Reaction
Yuezhou Liu, Baohui Lou, Liqing Shangguan, Jingsong Cai, Huangtianzhi Zhu, and Bingbing Shi*
Department of Chemistry, Zhejiang University, Hangzhou 310027, P. R. China
S
* Supporting Information
ABSTRACT: Heterogeneous catalysts, as an emerging
research hotspot, have showed excellent advantages on
stability, recyclability, and separation from reactant compared
with homogeneous catalysts in recent years. Herein, pillar[5]-
arene-based organometallic cross-linked polymer (P[5]-OCP)
was constructed via the complexation between imidazolium-
derivatived pillar[5]arene (P[5]) and Pd(OAc)2. The chemical
compositions and structure of the P[5]-OCP were confirmed
by means of multiple characterization methods. The catalytic
activity and process of P[5]-OCP were emphatically
investigated in the Suzuki−Miyaura coupling reaction. The
P[5]-OCP showed outstanding catalytic activity in mild
reaction conditions. Meanwhile, the catalyst P[5]-OCP possessed excellent stability and recyclability, which could be reused
at least five times without significant loss of activity. Furthermore, the P[5]-OCP is easily synthesized and cost-effective, which
make it a candidate for applications in fine chemical engineering.
Pillar[n]arenes are new symmetrical calixarenes analogues,
which were first discovered by Ogoshi and co-workers in
2008.13 Pillar[n]arenes, as an emerging macrocyclic host, are
composed of n hydroquinone units held together by methylene
bridges linking the para positions in cyclic arrays, and the
unique structure endows pillar[n]arenes with easy functional-
ization properties in supramolecular chemistry and material
chemistry.14−28 The synthesis process of pillar[n]arenes is
efficient and low-cost, which makes pillar[n]arenes more
suitable to construct functional materials, such as polymeric
materials,29,30 self-assembly materials,31,32 metal−organic
frameworks (MOFs),33 supramolecular organic frameworks
(SOFs),34 and hybrid materials.35 Nevertheless, employing
pillararene-based polymer materials in heterogeneous catalysis
has rarely been explored. Therefore, pillararene-based polymer
materials are the emerging candidates for challenging problems
in efficient heterogeneous catalytic field.
Inspired by the unique pillar-shaped and rigid structure of
pillar[5]arene, here we cross-linked imidazolium-derivatived
pillar[5]arene (P[5]) with Pd(OAc)2 to construct an organo-
metallic cross-linked polymer by using the “bottom-up”
approach.36 The metal ions (Pd2+) were successfully immobi-
lized and dispersed in the cross-linked polymer by the
complexation between the imidazolium groups on P[5] and
palladium ions. Then, we investigated the catalytic activity of
INTRODUCTION
■
In 2010, the Nobel Prize in Chemistry was awarded collectively
to Richard F. Heck, Ei-ichi Negishi, and Akira Suzuki for their
great contributions to palladium-catalyzed cross-couplings in
organic synthesis. Palladium-catalyzed carbon−carbon coupling
reactions between aryl halides and arylboronic acids, which are
very crucial methods to construct biaryl units in organic
synthesis,1 are the most powerful and general tools for the
synthesis of herbicides, pharmaceuticals, natural products,
liquid crystalline materials, and conducting polymers.2,3 In the
past decade, significant progress has been achieved in the
constructing biaryl units by using palladium-based catalysts,
especially using palladium-based homogeneous catalysts with
high activities and broad substrate scopes.4,5 However, some
disadvantages of palladium-based homogeneous catalysts, such
as complicated synthesis, hard separation, and intricate
recycling, definitely hindered the development of palladium-
based homogeneous catalysts in industry.6,7 Compared with
palladium-based homogeneous catalysts, palladium-based het-
erogeneous catalysts which are generally prepared by
immobilizing catalytic activity sites on solid supports, porous
materials, and polymers have shown remarkable advantages in
stability, recyclability, and separation from the reactant.8−10
The key characteristics of palladium-based heterogeneous
catalysts are the intrinsic robustness and recyclability that
satisfy wide applications in both academia and industry.
Therefore, finding convenient methods and building blocks to
prepare efficient palladium-based heterogeneous catalysts has
consequently become a research hotspot in recent years.11,12
Received: December 21, 2017
Revised: January 28, 2018
© XXXX American Chemical Society
A
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Scheme 1. P[5]-OCP Derived from the Complexation between P[5] and Pd(OAc)2 and Cartoon Representation of the Cross-
Linked Structure of the P[5]-OCP
MHz, DMSO-d6, room temperature) δ (ppm): 9.23 (s, 10H), 8.14 (s,
P[5]-OCP in the Suzuki−Miyaura coupling reaction. In this
10H), 7.49 (s, 10H), 6.72 (s, 10H), 4.76 (s, 20H), 4.47 (s, 20H), 3.75
technique, water was used as solvent in the mild conditions,
(s, 30H), 3.50 (s, 10H). 13C NMR (100 MHz, DMSO-d6, room
and the catalyst showed excellent catalytic capacity. Further-
more, P[5]-OCP possessed excellent stability and recyclability,
which could be reused at least five times without significant loss
of activity. The easy and cost-effective construction of this
heterogeneous catalyst (P[5]-OCP) makes it candidate for
applications in fine chemical engineering.
temperature) δ (ppm): 148.6, 136.9, 127.8, 123.3, 122.7, 66.4, 49.0,
40.1, 35.8. The solid-state 13C CPMAS NMR δ (ppm): 148.0, 137.1,
123.0, 113.5, 65.8, 49.2, 36.5, 27.8.
Synthesis of P[5]-OCP. To a solution of compound P[5] (1.00 g,
0.40 mmol), palladium acetate (0.45 g, 2.00 mmol) in DMF (20 mL)
was stirred vigorously at 110 °C under argon for 48 h. The grayish-
yellow product was obtained by filtration and washed with DMF,
methylbenzene, and CH2Cl2. Solid-state 13C CPMAS NMR δ (ppm):
177.5, 148.8, 122.2, 113.6, 66.0, 49.3, 37.1, 31.6, 28.0.
General Procedure for the Suzuki−Miyaura Coupling
Reaction. Aryl halide (1.0 mmol), arylboronic acid (1.5 mmol),
K2CO3 (2.0 mmol), and P[5]-OCP (0.5 mol % Pd) were added to 5
mL of distilled water at 80 °C. After the reaction was completed
(monitored by TLC), the mixture was extracted with CH2Cl2 and
evaluated by GC.
EXPERIMENTAL SECTION
■
Materials and Instrumentation. All reagents were commercially
available and used as supplied without further purification. H NMR
1
and 13C NMR spectra were recorded with a Bruker Avance DMX 400
spectrophotometer with use of the deuterated solvent as the lock and
the residual solvent or TMS as the internal reference. Scanning
electron microscopy investigations were carried out on a JEOL
6390LV instrument. SEM samples were prepared by dispersing P[5]-
OCP in DMSO via the vacuum freeze-drying methodology. Fourier
transform infrared spectroscopy spectra were recorded on a Thermo
Nicolet iS10 spectrometer. Transmission electron microscopic
investigations were carried out on a Hitachi HT-7700 instrument.
Solid-state NMR spectra were recorded on a Bruker 400WB AVANCE
III spectrometer. Elemental analyses were carried out on a EA1112
instrument. Thermogravimetric analysis was carried out on a
DSCQ1000 thermal gravimetric analyzer. Diffuse reflectance Fourier
transform infrared (DRIFT) spectra of adsorbed CO were obtained on
a Tensor 27 spectrometer (Bruker) equipped with a liquid-nitrogen-
cooled mercury−cadmium−tellurium detector, a diffuse reflectance
accessory, and a high-temperature reaction chamber. GC measure-
ments were carried out using an Agilent 7890B instrument configured
with an FID detector and a DB-624 column.
RESULTS AND DISCUSSION
■
Synthesis and Structure Characterization. The syn-
thetic route of P[5]-OCP is depicted in Scheme 1. P[5], which
was prepared based on our previous work,37 was selected as the
rigid scaffold for the preparation of the cross-linked polymer.
And then, P[5] was treated with Pd(OAc)2 in N,N-dimethyl-
formamide (DMF) at 110 °C for 48 h. The solid was formed
on the bottom of the glassware and washed with DMF,
methylbenzene, and dichloromethane to get the polymer P[5]-
OCP as a grayish-yellow solid.
To get the information on the chemical components of the
P[5]-OCP, elemental analysis (EA) by combustion was
conducted (Figure S6). The observed contents of carbon
(35.73 wt %), nitrogen (9.03 wt %), and hydrogen (4.14 wt %)
in the P[5]-OCP matched well with the calculated values of for
[(C19H22N4O2PdBr2)n] (C 37.56, N 9.22, H 4.15 wt %), which
clearly supported the formation of the cross-linked polymer.
On the other hand, the result of inductively coupled plasma
atomic emission spectrum (ICP) showed that the content of
palladium in P[5]-OCP is 17.12 wt %, corresponding to the
theoretical value of 17.60 wt %.
Synthesis of 2. The synthetic route of P[5] is shown in the
Supporting Information (Scheme S1). A solution of 1 (6.74 g, 23.0
mmol) and paraformaldehyde (0.70 g, 23.0 mmol) in 1,2-dichloro-
ethane (150 mL) was stirred vigorously at room temperature. Then
boron trifluoride diethyl etherate (6.52 g, 46.0 mmol) was added to
the solution, and the mixture was stirred at room temperature for 1 h.
Methyl alcohol (150 mL) was added to quench the reaction. The
mixture was evaporated to afford the crude product, which was isolated
by flash column chromatography using ethyl acetate/petroleum ether
1
(1:100) to give 2 as a white solid (3.7 g, 48%). H NMR (400 MHz,
In order to further confirm the structure of P[5]-OCP, 13C
cross-polarization at magic angle spin (CPMAS) spectra and
solid Fourier transform infrared spectroscopy (FT-IR) spectra
experiments were carried out. As shown in Figure 1, in the
solid-state 13C CPMAS NMR spectra of P[5]-OCP, all the
signals of the peaks can be clearly assignable to the
corresponding carbon atoms in the expected structure. In
particular, the signals of the peaks at δ = 137.1 and 177.5 ppm
chloroform-d, room temperature) δ (ppm): 6.92 (s, 10H), 4.24 (t, J =
5.7 Hz, 20H), 3.85 (s, 10H), 3.64 (t, J = 5.6 Hz, 20H). 13C NMR (100
MHz, chloroform-d, room temperature) δ (ppm): 148.6, 128.0, 115.0,
67.9, 29.7, 28.4.
Synthesis of P[5]. A mixture of 2 (3.36 g, 2.00 mmol) and N-
methylimidazole (3.28 g, 40.0 mmol) in acetonitrile (50 mL) was
stirred in a 150 mL round-bottom flask at 85 °C for 36 h to form a
precipitate. After cooling, the mixture was filtrated and recrystallized
1
from acetonitrile to give a white solid (3.20 g, 69%). H NMR (400
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Figure 1. Solid-state 13C CPMAS NMR spectra of P[5] and P[5]-
OCP.
can be assigned to the carbon atom d on imidazole groups of
P[5] and carbene carbon atom l connected to the palladium
atom. The disappeared signal of carbon atom d and the
emerging signal of l provided the straightforward evidence for
the formation of the cross-linked polymer P[5]-OCP. In
addition, the structure of P[5]-OCP was further verified by
solid FT-IR spectra. As shown in Figure S7, the main peaks
related to P[5] and P[5]-OCP were marked and assigned.
Initially, compound P[5] showed strong absorbance bands
including O−C, Ar−C, and CC of the benzene rings and
NC stretching vibrations at 1057, 1201, 1391−1502, and
1634 cm−1, respectively. Compared with the peaks of
compound P[5], the adsorption peak of P[5]-OCP at 1661
cm−1 is due to the CC stretching vibrations, indicating the
cross-linking reaction between P[5] and Pd(OAc)2.
Figure 2. (a) SEM images of P[5]-OCP. (b) TEM images of P[5]-
OCP. (c) SEM images of P[5]-OCP recycled after five times of the
Suzuki−Miyaura coupling reaction.
Scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) were further utilized to investigate
the micromorphology of P[5]-OCP. The results of SEM and
TEM showed that the cross-linked polymer adopted an
irregular spherical shape with the size of 105 nm. The powder
X-ray diffraction spectrum (XRD) experiment is usually used to
analyze materials in a crystalline or noncrystalline state. In
order to have a deeper sight into P[5]-OCP, the XRD
experiment was also carried out at 298 K. As shown in Figure
S8, the broad peak at 40° indicated the noncrystalline structure
of P[5]-OCP. Furthermore, the thermogravimetric analysis
(TGA) of P[5]-OCP (Figure S9) showed that it is stable up to
309 °C under a nitrogen atmosphere, indicating the excellent
thermostability of the cross-linked polymer P[5]-OCP.
In order to gain the oxidation states of Pd species in P[5]-
OCP, X-ray photoelectron spectroscopy (XPS) measurements
of Pd(OAc)2 and P[5]-OCP were carried out. As shown in
Figure 3, the XPS spectra of P[5]-OCP contained two main
peaks with binding energies of 336.8 and 342.0 eV, assignable
to Pd 3d5/2 and Pd 3d3/2, indicating that no Pd(0) is present in
P[5]-OCP, and the Pd species is only in the +2 state. In
addition, the binding energy 336.8 eV in P[5]-OCP shifted
negatively by 1.7 eV in comparison with that of 338.5 eV for
Pd(OAc)2, which indicated the strong coordination between
P[5] and Pd2+.38
Figure 3. XPS analysis for Pd 3d spectra: (a) Pd(OAc)2; (b) P[5]-
OCP; (c) P[5]-OCP recycled after five times of the Suzuki−Miyaura
coupling reaction.
Fourier transform infrared (DRIFT) spectra experiment for
P[5]-OCP was performed (Figure 4). The bands at 2152 and
2129 cm−1 are ascribed to CO, which is linearly adsorbed on
Pd. The band at 1889 cm−1 is ascribed to the bridge-bond CO
on two Pd atoms and CO adsorbed between Pd clusters. The
intensity of bridge-bound CO relative to linear-bound CO of
P[5]-OCP is determined as 0.19, suggesting the high dispersity
of palladium in the polymer, which contributed to the reaction
process as a catalyst.38
Catalytic Activities. Palladium-catalyzed carbon−carbon
coupling reactions between aryl halides and arylboronic acids
are very crucial methods to construct biaryl units in organic
synthesis. In order to investigate the catalytic activities of P[5]-
OCP, Suzuki−Miyaura coupling reaction between aryl halides
and arylboronic acids was evaluated in the presence of P[5]-
OCP and K2CO3 in H2O at 80 °C. Herein, we chose water as
In order to further study the dispersity of palladium in the
polymer P[5]-OCP, the CO-absorbed diffuse reflectance
C
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different substrates, including electron-donating and electron-
withdrawing compounds, were tested in the Suzuki−Miyaura
coupling reactions, giving the corresponding biaryls in excellent
yields. Moreover, in the previous work, aryl chlorides have low
reaction activity in the Suzuki−Miyaura reaction catalyzed by
most of the present heterogeneous palladium catalysts.12
Herein, the catalyst P[5]-OCP was also used in these highly
challenging substrates. Particularly, the less active aryl chlorides
also gave good yields (100, 80.0, and 91.4%) with longer
reaction time (12 h, entries 18−20), indicating the catalyst has
excellent catalytic activity and universality.
In order to demonstrate the importance of P[5] for the
reaction, an attempt to study the interaction between P[5] and
1
bromobenzene was carried out by H NMR characterization
1
(Figure S10). The resulting H NMR spectrum showed that
obvious upfield shifts in the signals of all the protons from
bromobenzene while the signals of protons of P[5] shifted
downfield, indicating that bromobenzene was encapsulated into
the cavity of P[5]. These facts suggested that the high yield was
attributable to the host−guest interaction between the P[5]
and aryl halide, which enhances the interaction between
reagents and palladium. As control experiments, 1-bromo-4-
tert-butylbenzene was chosen as reactant which cannot enter
into the cavity of P[5]. As expected, the yield decreased when
the substrates possessed bulky substituents (entry 23),
indicating that the substrates with bulky substituents have
negative effect on the reaction. On the other hand, adiponitrile
can be used as competitive guest due to the strong
complexation with pillararene. As shown in the table, the
yield of the reaction of bromobenzene and phenylboronic acid
was nearly 100%, while the yield dropped to 78.9% in the same
reaction condition except the excess adiponitrile, indicating the
importance of the host−guest interaction between the P[5] and
aryl halide.
To make sure whether the catalyst P[5]-OCP is practically
working in a heterogeneous pathway, or whether it is only a
precatalyst for more active soluble species, a series of different
control tests have been carried out.39 First, the hot-filtration
test was conducted by using P[5]-OCP as a catalyst. In the
absence of the reactant, the mixture of P[5]-OCP and K2CO3
in distilled water was heated for 1 h at 80 °C. Then the filtrate
from the mixture was used to catalyze the reaction of
bromobenzene and phenylboronic acid, but no coupling
product was observed. Next, when the reaction of bromo-
benzene and phenylboronic acid was proceeded for about 20
min in the presence of P[5]-OCP, the conversion of
bromobenzene reached about 60.79%. The reaction was
allowed to react another 100 min after removing the catalyst,
but no distinct progress (1.90% by GC) was observed.
However, it was not powerful evidence to prove that the
catalytic activity of P[5]-OCP was performed in a heteroge-
neous way because the leached and soluble Pd species could be
redepositing back on the polymer backbone during the hot
filtration. So, mercury poisoning test was also employed to
explore the catalytic process of bromobenzene and phenyl-
boronic acid. Interestingly, as one drop of Hg was added in the
reaction, the yield is up to 100%, which provided the proof that
P[5]-OCP is not a precatalyst for the formation of Pd
nanoparticles. In addition, poly(4-vinylpyridine) (P4VP), an
efficient poison for the Pd-catalyzed Suzuki coupling reaction,
was also used to ascertain the reaction process. Then, the
coupling reaction of bromobenzene and phenylboronic acid
was carried out in the presence of P4VP (P4VP/Pd 400:1). It is
Figure 4. DRIFT spectra of CO absorbed at 25 °C on P[5]-OCP.
solvent for the Suzuki−Miyaura coupling reaction because
water is a green solvent and nontoxic.
Table 1 summarizes the catalytic activity of P[5]-OCP
toward a variety of aryl halides and arylboronic acids. The
Table 1. Suzuki−Miyaura Coupling Reactions Catalyzed by
P[5]-OCP
ab
,
entry
X
R1
R2
T (h)
yield (%)
1
I
H
H
1
1
100
100
100
100
74.3
81.1
100
82.6
100
95.3
88.0
96.3
86.6
73.9
91.5
90.9
91.6
100
80.0
91.4
2
I
p-OCH3
p-NO2
H
H
3
I
H
2
4
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Br
Br
Br
Br
H
2
5
H
CH3
OCH3
H
6
6
H
6
7
p-OCH3
p-OCH3
p-OCH3
p-CN
p-CN
p-CN
p-CH3
p-NO2
p-CHO
m-CHO
o-CHO
H
6
8
CH3
OCH3
H
6
9
6
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
6
CH3
OCH3
H
6
6
6
H
8
H
6
H
6
H
6
H
12
12
12
2
p-OCH3
m-CHO
H
H
H
c
H
100
d
H
H
2
80.6
p-C(CH3)3
H
H
6
67.4
e
H
2
78.9
a
Reaction conditions: aryl halide (1.0 mmol), arylboronic acid (1.5
mmol), K2CO3 (2.0 mmol), P[5]-OCP (0.5 mol % Pd), H2O (5 mL),
b
c
80 °C. Yield were determined by GC/MS. The reaction was
performed in the presence of Hg. The reaction was performed in the
presence of P4VP. The reaction was performed in the presence of
d
e
adiponitrile.
D
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noteworthy that this new control experiment give biphenyl in
80.6% conversion after 2 h at 60 °C, leading to a gradual
decrease in catalytic activity of P[5]-OCP. Thus, the current
measurements clearly indicated that catalyst P[5]-OCP is partly
operating in an incompletely heterogeneous pathway.
The stability and reusability are very important factors of
catalysts. The excellent catalytic activity of P[5]-OCP
encouraged us to investigate its recyclability. The recycled use
of P[5]-OCP was examined in the reaction of bromobenzene
and phenylboronic acid. After the end of the reaction, P[5]-
OCP was isolated through simple extraction, dried in vacuo,
and then employed for the next run. It was found that the
recycling can be successfully achieved in five reaction runs and
no detectable loss in performence (Figure 5). In agreement
and recycling potential. Furthermore, this work can provide a
new perspective to introduce pillararenes into functional
materials and enrich the field of material science and
supramolecular chemistry.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.7b02701.
Additional NMR and FT-IR spectra, thermogravimetric
analyses, XRD and GC/MS spectra (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: bingbingshi@zju.edu.cn (B.S.).
■
ORCID
Bingbing Shi: 0000-0001-9523-5758
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Jie Fan from Zhejiang University for the support of
DRIFT spectra experiment for P[5]-OCP. This work was
supported by the Fundamental Research Funds for the Central
Universities.
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Figure 5. Reusability of the P[5]-OCP catalyst in the Suzuki−Miyaura
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Miyaura coupling reaction. Compared with P[5]-OCP freshly
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CONCLUSION
■
Organometallic cross-linked polymer P[5]-OCP was prepared
based on the complexation between P[5] and Pd(OAc)2. The
cross-linked structure of the polymer P[5]-OCP was analyzed
by solid-state 13C CPMAS NMR, FT-IR, electron microscopy,
EA, ICP, and XPS. The result of DRIFT spectra experiment
indicated the high dispersity of palladium in P[5]-OCP. The
P[5]-OCP demonstrated excellent activity in a serious of
Suzuki−Miyaura coupling reactions. The catalyst can be reused
at least five times without significant loss of activity, indicating
the outstanding stability and reusability. This cross-linked
polymer can extend the development of heterogeneous
catalysts because of its cost-effectiveness, high catalytic activity,
E
DOI: 10.1021/acs.macromol.7b02701
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