Pd salen complex@CPGO as a convenient, effective heterogeneous catalyst for Suzuki–Miyaura and Heck–Mizoroki cross-coupling reactions

Article abstract of DOI:10.1007/s11164-018-3455-4

A Pd(II) Schiff base complex supported on graphene oxide nanosheets (Pd(II) salen@CPGO) has been synthesized and characterized by FT-IR, ICP-AES, XRD, SEM/EDX and TEM. The synthesized nanocatalyst has been found to be an efficient heterogeneous catalyst for Suzuki–Miyaura and Heck–Mizoroki coupling reactions. Pd(II) salen@CPGO could be separated and recovered easily from the reaction mixture and recycled several times without a discernible decrease in its catalytic activity. The construction of a solid sheet-supported Pd catalyst would be expected to be a promising system to perform heterogeneous catalytic reactions.

Full text of DOI:10.1007/s11164-018-3455-4

Res Chem Intermed
https://doi.org/10.1007/s11164-018-3455-4
Pd salen complex@CPGO as a convenient, effective
heterogeneous catalyst for Suzuki–Miyaura and Heck–
Mizoroki cross-coupling reactions
Mahdieh Ghabdian1
Mohammad Ali Nasseri1 Ali Allahresani1
•
•
•
Alireza Motavallizadehkakhky2
Received: 21 February 2018 / Accepted: 24 April 2018
Ó Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract A Pd(II) Schiff base complex supported on graphene oxide nanosheets
(Pd(II) salen@CPGO) has been synthesized and characterized by FT-IR, ICP-AES,
XRD, SEM/EDX and TEM. The synthesized nanocatalyst has been found to be an
efficient heterogeneous catalyst for Suzuki–Miyaura and Heck–Mizoroki coupling
reactions. Pd(II) salen@CPGO could be separated and recovered easily from the
reaction mixture and recycled several times without a discernible decrease in its
catalytic activity. The construction of a solid sheet-supported Pd catalyst would be
expected to be a promising system to perform heterogeneous catalytic reactions.
Keywords Suzuki–Miyaura reaction Á Heck–Mizoroki reaction Á Pd(II)
salen@CPGO nano catalyst Á Heterogeneous catalyst Á Recyclable
catalyst
Introduction
Suzuki–Miyaura and Heck–Mizoroki coupling reactions are two of most widely
used methods for carbon–carbon bond-forming reactions [1–6]. They have been
shown to be an effective method for the construction of C–C bonds and play a basic
role in the synthesis of plenty of natural products, biologically active compounds,
herbicides, pharmaceuticals, many organic building blocks and therapeutic drugs
and their intermediates. Regarding the significance of these coupling reactions, a
&
Mahdieh Ghabdian
ghabdianm@birjand.ac.ir
1
2
Department of Chemistry, Faculty of Science, University of Birjand,
P. O. Box 97175-615, Birjand, Iran
Department of Chemistry, School of Sciences, Islamic Azad University, Neyshabur Branch,
P. O. Box 9318813-639, Neyshabur, Iran
123

M. Ghabdian et al.
great number of homogeneous palladium catalysts such as [Pd(PPh3)4], [Pd(OAc)2]
and [PdCl2(PPh3)2] [7–9] have been reported. Although these catalysts exhibit
excellent activity, there remain some noticeable problems, such as tedious
separation, poor recyclability, high-cost ligands and sensitivity to air oxidation.
These drawbacks can be overcome by the use of heterogeneous catalysts. As a
suitable solution, covalent binding of Pd complexes onto the solid support through a
ligand spacer is the most efficient and widely used strategy. Therefore, a wide range
of matrixes have been developed for metal complexes covalent attachment, like
polymers [10], zeolites [11], silica spheres [12], magnetic particles [13] and carbon
nanotubes [14]. However, these immobilized catalysts’ potential and utility are
largely related to the support’s structural nature. For example, Pd nanomaterials
which were supported on functionalized silica were gradually deactivated after three
runs in Suzuki coupling reactions [15]. Recently, graphene and graphene oxide
(GO) with fascinating two-dimensional structures and exclusive characteristics have
emerged as ideal supports for various catalytic species. In addition to the availability
of surface functionalization, their two-dimensional structures allow excellent
dispersion of the catalytic species. They also facilitate mass transfers in the
reaction processes [16]. Reactive species can readily have access to the active sites
with restricted mass transfer resistance. Graphene oxide (GO) is a lamellar flexible
material having a great number of functional groups such as epoxy (C–O–C),
hydroxyl (OH) on basal planes and carboxyl (COOH) on the edges. Extra hydroxyl
groups on its surface provide the capability of reaction with different linkers and
ligands which could be available for immobilization of metal ions. The nontoxicity,
availability, thermal and mechanical stability of a graphene oxide nanosheet makes
it an excellent support, especially in heterogeneous catalysis. With these consid-
erations in mind, and to solve the problem of regeneration and reuse of the
expensive catalyst, the use of a Pd(II) salen@CPGO nanocatalyst is thus both
elegant and efficient. Additionally, GO is inexpensive in comparison with zeolites
such as MCM-41 and SBA-15 allowing GO to be the ideal option for catalyst
supports [17]. We here describe the synthesis of a Pd(II) salen@CPGO nanocatalyst
and demonstrate its application in Suzuki–Miyaura and Heck–Mizoroki coupling
reactions. The results show that the new catalyst, Pd(II) salen@CPGO, retains the
reactivity characteristic of a homogeneous catalyst but at the same time is easy to
separate off and reuse.
Experimental
Materials and methods
Highly pure chemical materials were purchased from Merck. Melting points were
determined in open capillaries using an Electrothermal 9100 apparatus. The infrared
(IR) spectra (cm-1) were recorded on a VERTEX 70 spectrometer (Bruker) in the
transmission mode in spectroscopic grade KBr pellets for all the powders. The size
and morphology of the magnetic nanoparticles were observed by using a Philips
CM10 transmission electron microscope operating at 100 kV. NMR spectra were
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Pd salen complex@CPGO as a convenient, effective…
recorded in CDCl3 on a Bruker Advance DRX-400 MHz instrument spectrometer
using TMS as internal standard. The purity determination of the products and
reaction monitoring were accomplished by TLC on silica gel polygram SILG/UV
254 plates. The Pd content of the catalysts was determined using ICP-AES of the
model Thermo Electron IRIS INTREPID II XSP DUO. The yields were determined
by a Shimadzu GC-17A gas chromatograph (GC) equipped with a flame ionization
detector with a capillary column (CBP-1, 30 m 9 0.32 mm 9 0.25 mm), using
helium as carrier gas.
Synthesis of 3-chloropropyltrimethoxysilane functionalized GO (CPGO, 1)
GO was prepared as described in the literature [18]. Brown powder of GO (1 g) was
dispersed in 70 mL of dry toluene for 20 min followed by dropwise addition of
3-chloropropyltrimethoxysilane (3 mL) diluted in 20 mL dry toluene [19]. The
resulting mixture was heated to reflux with continual stirring for 24 h under a
nitrogen atmosphere. After cooling the mixture, the black powdery sample was
filtered, washed with toluene four times and then washed with EtOH. The solid was
dried at 70 °C under vacuum for 6 h.
Preparation of salen
To a stirred solution of 2-hydroxybenzaldehyde (2.0 mmol) in 5 mL of ethanol,
diethylentriamine (1.0 mmol) was added dropwise and the reaction mixture was
stirred under reflux condition for 4 h. N,N-Bis(salicylidene)-ethylenetriamine was
obtained by being washed three times with water and methanol, as well as ethanol,
and dried at 80 °C in an oven under vacuum. In order to achieve the pure product,
the crude product was recrystallized from ethanol.
Synthesis of Pd(II) salen complex (2)
Salen ligand (1.0 mmol) was dissolved in MeOH (20 mL) and then a solution of
PdCl2 (1 mmol) in absolute MeOH (3 mL) was added dropwise. The resulting
mixture was stirred for 3–5 h at room temperature. After evaporating the solvent
from the filtrate, a solid mass was obtained which was washed with cold methanol
and dried under vacuum to obtain the Pd(II) salen complex 2.
General procedure for the preparation of Pd(II) salen@CPGO
For the preparation of the nanocatalyst, 1 g of CPGO(1) was dispersed in 75 mL of
dry toluene for 20 min. Later, 0.5 g of homogeneous Pd(II) salen complex (2) and
1.5 mmol triethylamine were added to the suspension. The mixture was refluxed for
20 h and the obtained immobilized catalyst was then washed successively with dry
toluene, EtOH and dichloromethane. The solid was dried at 70 °C under vacuum
condition for 6 h. The immobilized Pd(II) salen complex on CPGO was
characterized by XRD, TEM, FT-IR, SEM–EDX and ICP-AES (Scheme 1).
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M. Ghabdian et al.
Dry Toluene
a
(CH3O)3Si(CH2)2Cl
GO
CPGO (1)
O
OH
N
N
N
H
HO
OH
EtOH, Reflux
+
b
M
e
O
H
H2N
NH2
R
,
t
P
.
dC
N
H
l
2
H
N
N
2+Pd
N
2+Pd
OH
HO
Dry Toluene, Et3N
Reflux, 20 h
Pd(II) salen@CPGO
Scheme 1 Schematic for the preparation of the catalyst
Pd(II) salen complex (2)
General procedure for catalytic Suzuki–Miyaura coupling
In a typical experiment, known quantities of aryl halides (1.0 mmol) and
arylboronic acid (1.2 mmol), were dissolved in DMF (3.0 mL) in a 50-mL round-
bottomed flask and stirred for 10–15 min. K2CO3 (2.0 mmol) and Pd salen@CPGO
nanocatalyst (0.7 mol%) were added to the reaction flask. The reaction mixture was
heated at 80 °C and stirred. The progress of the reaction was monitored by TLC at
regular intervals. After completion, the reaction mixture was cooled to room
temperature and the mixture was filtered and then extracted with ethyl acetate
(3 9 10 mL). The combined organic layers were treated with saturated brine
solution and dried over anhydrous sodium sulfate. The removal of the solvent yields
the crude product, which after purification by column chromatography afforded the
desired products.
123

Pd salen complex@CPGO as a convenient, effective…
General procedure for Heck–Mizoroki coupling reaction
A 50-mL round-bottomed flask was charged with aryl haides (1.0 mmol), methyl
acrylate/styrene (1.2 mmol), Et3N (2.0 mmol), and Pd(II) salen@CPGO nanocat-
alyst (1 mol%) in DMSO (3.0 mL). The reaction mixture was heated to 110 °C for
an appropriate time. At the end of the reaction, the mixture was cooled to room
temperature. The reaction mixture was diluted with water and extracted with ethyl
acetate (3 9 10 mL). The combined organic layers were dried over anhydrous
Na2SO4, and the removal of solvent yielded the crude product, which after
purification by column chromatography afforded the desired products.
General procedure for recycle reaction
After the reaction, the catalyst was obtained by centrifugation separation. The
catalyst could be directly reused for several runs after being washed three times with
ethanol and was dried under vacuum (Fig. 1).
Results and discussion
Characterization of the heterogeneous catalyst
As shown in Scheme 1, the Pd (II) salen complex was immobilized on a CPGO
support through an in situ strategy. Briefly, in order to produce a Schiff base, salicyl
aldehyde was condensated with the diethylentriamine, then PdCl2 was added to
prepare the Pd salen complex. Eventually, the synthesized Pd salen complex
anchored onto the CPGO. The Pd(II) Salen@CPGO catalyst was characterized by
FT-IR, TEM, SEM/EDX, XRD and ICP-AES. 3-Chloropropylterimethoxysilane,
which plays the role of coupling agent, provided a large surface to anchor the active
complex and increased the efficiency of the heterogeneous catalyst which was used
for anchoring the homogeneous Cu(II) catalyst onto the GO.
The FT-IR spectrum of GO (Fig. 2e) shows a broad peak appearing at
3399 cm-1, assigned to the stretching mode of an O–H bond, revealing the
Fig. 1 Reuse performance of the catalyst
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M. Ghabdian et al.
Fig. 2 The FT-IR spectra of (a) Schiff base, (b) Pd salen complex 2, (c) Pd salen@CPGO heterogeneous
catalyst, (d) CPGO, (e) GO
abundance of hydroxyl groups in GO. The strong bands at 1735, 1619, 1220 and
1059 cm-1 revealed the presence of C=O, C=C, C–OH and epoxy functional
groups, respectively, in the GO. Figure 2d which is assigned to CPGO shows an
obvious decrease in the intensities of peaks such as 3399 and 1728 cm-1 which
correspond to the OH and C=O stretching vibrations, respectively, and we can see a
clear increase in the intensities of the peaks at 1049 and 1080 which provide direct
evidence for the strong stretching vibrations of Si–O in CPGO. The FTIR spectrum
of the Schiff base shows an enhanced sharp band near 1636 cm-1 which is assigned
to stretching vibration of the C=N group (Fig. 2a). In the spectrum of Pd(II) salen 2
(Fig. 2b), the band related to the C=N functional group shifts to a lower position
(* 20 cm-1) compared to the Schiff base. This red shift to a relatively lower
frequency might be attributed to the coordination of Pd through C=N. In the
spectrum of Pd(II) salen@CPGO (Fig. 2c), the new stretching vibration bands at
1620 and 1080 cm-1 and a significant decrease in the intensities of peaks such as
3399 and 1737 cm-1 which correspond to the OH and C=O stretching vibrations,
respectively, suggesting the grafting of the homogeneous Pd(II) salen ligand
1
complex on the CPGO surface. The H and 13C NMR spectra of 2 are consistent
with the structure of the complex. In the 1H NMR spectrum, a proton due to –CH=N
was observed at 8.3 ppm, which upon coordination to the Pd(II) ion showed a
downfield shift and occurred at 8.5 ppm for 2. Also, the 13C NMR signals for imine-
carbon of salen ligand appeared at 166.84 and this value undergoes a downfield shift
(169.44 ppm) upon coordination to the Pd(II) ion.
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Pd salen complex@CPGO as a convenient, effective…
Fig. 3 The XRD patterns of (a) GO, (b) CPGO, and (c) heterogeneous catalyst
Figure 3 shows the XRD patterns of GO and CPGO and the heterogeneous
catalyst. Upon oxidation of the graphite, the basal reflection (002) peak appearing at
2h = 26.6° (d spacing = 0.335 nm) shifts to a lower angle (2h = 11.79°, d spac-
ing = 0.79 nm) (Fig. 3a). The increase in d spacing is due to the intercalation of
water molecules and the formation of oxygen-containing functional groups between
the layers of the graphite. By immobilization of 3-chloropropyltrimethoxysilane on
the GO, a broad peak appears at 2h = 15–27° ,depending on amorphous silica
(Fig. 3b). The decrease in the intensity of the 002 reflection peak of GO and the
appearance of a broad peak at 2h = 15–27° in the CPGO indicated the
immobilization of 3-chloropropyltrimethoxysilane on the GO. By immobilization
of the homogeneous Pd(II) salen complex on CPGO, the 002 reflections peak of GO
completely disappeared (Fig. 3c).
The morphological properties of Pd(II) salen@CPGO were determined by means
of TEM and SEM. The TEM of Pd salen@CPGO (Fig. 4a, b) revealed the
nanoscopic features of CPGO. The quantitative energy dispersive spectroscopy
(EDX) mapping of Pd salen @CPGO demonstrates that the elements N, O, Pd and
Si are homogeneously distributed on the whole surface of this compound, which
indicates that the functional groups are anchored uniformly on the GO, whereas the
EDX of CPGO only recognizes the elements C, O, Cl and Si (Fig. 4c, d). These
results proved that the Pd salen complex (2) was successfully immobilized on the
CPGO (1) surface. The crumpling characteristic of Pd(II) salen @CPGO along with
the homogeneous distribution of the active sties should be of benefit for its catalytic
performance. Analysis by ICP-AES demonstrated that 0.66 mmol g-1 based on the
Pd element was the loading of the homogeneous Pd (II) complex (2) in the
heterogeneous catalyst.
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M. Ghabdian et al.
Fig. 4 a, b The TEM of heterogeneous catalyst Pd(II) salen@CPGO, c the SEM images and
corresponding EDX analysis of CPGO, d the Pd(II) salen@CPGO heterogeneous catalyst
Catalytic performance of the Pd(II) salen@CPGO for the Suzuki–Miyaura
cross-coupling reaction
The catalytic feature of Pd(II) salen@CPGO was studied in C–C bond formation by
means of Suzuki–Miyaura cross-coupling of iodoobenzene with phenylboronic acid
as a model reaction. The critical factors for the reaction outcomes, like the catalyst
amount, solvent, base and temperature, were compared for the purpose of attaining
the optimal conditions. The conclusion of the optimization of the conditions is
shown in Table 1. The reaction was carried out when various amounts of the
heterogeneous Pd(II) salen@CPGO (0.5–1 mol% based on the Pd element) were
present at 80 °C. By using 0.7 mol% of the nanocatalyst, the highest yield of the
desired product in a short reaction time was obtained. By applying 0.5 mol% of the
catalyst, a significant decrease in the reaction yield was observed, while additional
amounts of catalyst (1 mol%) did not importantly increase the efficiency of the yield
nor the rate of the reaction (Table 1, entries 1–3). In order to compare the
homogeneous Pd (II) complex (2) was also investigated under the same reaction
conditions (Table 1, entry 4).
To clarify the specific catalytic activity of Pd(II) salen@CPGO in the Suzuki–
Miyaura cross-coupling reaction, the model reaction was performed in the presence
of GO and when no catalyst was present. The application of just GO afforded a low
yield of the corresponding cross-coupling product and, when the heterogeneous
catalyst was absent, we did not observe any product (Table 1, entries 5, 6).
In addition, the different solvent effects on the model reaction were investigated
in depth to achieve the best optimal catalyst conditions (Table 1, entries 2, 7–12).
When DMF was applied, the best performance of the catalytic system was obtained
(Table 1, entry 2). The results of Table 1 proved that a basic environment was also
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Pd salen complex@CPGO as a convenient, effective…
Table 1 Optimization of reaction conditions for Suzuki–Miyaura coupling reaction
Entry Catalyst (mol%)
Solvent
Base
Temp (°C) Time (min) Yield (%)a
1
Pd salen@CPGO (0. 5) DMF
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
NaHCO3
Et3N
80
80
80
80
80
80
80
80
80
80
60
60
80
80
80
80
80
80
r.t
60
35
75
2
Pd salen@CPGO (0.7)
Pd salen@CPGO (1)
Pd salen complex (0.7)
GO (0.7)
DMF
97
3
DMF
35
98
4
DMF
35
97
5
DMF
160
160
120
60
Trace
n.r
68
6
No catalyst
DMF
7
Pd salen@CPGO (0.7)
Pd salen@CPGO (0.7)
Pd salen@CPGO (0.7)
Pd salen@CPGO (0.7)
Pd salen@CPGO (0.7)
Pd salen@CPGO (0.7)
Pd salen@CPGO (0.7)
Pd salen@CPGO (0.7)
Pd salen@CPGO (0.7)
Pd salen@CPGO (0.7)
Pd salen@CPGO (0.7)
Pd salen@CPGO (0.7)
Pd salen@CPGO (0.7)
Pd salen@CPGO (0.7)
Pd salen@CPGO (0.7)
EtOH/H2O
DMF/H2O
DMSO
H2O
8
75
9
35
85
10
11
12
13
14
15
16
17
18
19
20
21
120
120
120
120
120
50
45
EtOH
THF
60
52
DMF
78
DMF
74
DMF
Na2CO3
Cs2CO3
KOH
85
DMF
60
80
DMF
120
160
160
160
35
63
DMF
No base
K2CO3
K2CO3
K2CO3
n.r
Trace
75
DMF
DMF
60
110
DMF
98
Reaction conditions: Iodobenzene (1 mmol), phenylbronic acid (1.2 mmol), Pd(II) salen@CPGO, base
(2 mmol), solvent (3 mL), at different temperature
aDetermined by GC with a CBP1 column (Shimadzu 30 m 9 0.32 mm 9 0.25 mm)
essential for the Suzuki–Miyaura cross-coupling reaction (Table 1, entry 18).
Therefore, in order to investigate its impact, several inorganic and organic bases
were tested in the Suzuki–Miyaura reaction of iodobenzene (1 mmol) and
phenylboronic acid (1.2 mmol) with the Pd(II) salen@CPGO catalyst (0.7 mol%)
in dimethylformamide (3 mL). The common organic bases such as Et3N generally
used in the Suzuki–Miyaura reaction were found to be less reactive in our catalytic
system (Table 1, entry 14). The same behavior was also observed with hydroxide
ion (Table 1, entry 17) and sodium bicarbonate base (Table 1, entry 13), as in both
cases the reaction did not complete even after 120 min. In contrast, carbonate bases,
i.e., Na2CO3, Cs2CO3 and K2CO3, provided excellent yields of the cross-coupling
product (Table 1, entries 2, 15, 16). Thus, we have considered K2CO3 for further
optimization.
Moreover, temperature plays a significant role in the reaction yield. We
performed the model reaction at various temperatures (25, 60, 80 and 110 °C) in
DMF in the presence of the heterogeneous catalyst and K2CO3. There was a
remarkable fall in the yield at 25 °C (Table 1, entry 19). The reaction proceeded
123

M. Ghabdian et al.
faster at 60 °C in comparison with 25 °C. However, when the temperature was
higher than 60 °C (80 and 110 °C), a clear rise in intensity was observed in the yield
of the coupling reaction (Table 1, entries 2, 21). Thus, we concluded that the
optimum temperature is 80 °C.
A variety of structurally divergent aryl iodides, bromides, and chlorides reacted
with phenylboronic acid to generate the desired coupling products under the
optimized reaction conditions (Table 2). The coupling reactions of aryl iodides and
aryl bromides occurred in a short time with good to high yields. Even though aryl
bromides required a longer time to afford the same result in comparison with aryl
iodides (compare entries 1–10 with 11–15), they gave good yields compared with
their chloride counterparts. This might be due to the leaving group ability of
halogens (I [ Br [ Cl [ F). In the present catalytic system, the electronic as well
as steric effects on the yields and reaction rates were also studied. A good efficiency
with a range of both electron-rich and electron-poor substituted aryl halides (such as
4-NO2C6H4, 4-OMeC6H4, 4-MeC6H4, 4-CNC6H4, 4-CHOC6H4 and 4-NH2C6H4)
was shown by this catalytic system. It is noteworthy that aryl iodides containing
electron-withdrawing groups, such as –COCH3, –CN and –NO2, were discovered to
be more active than those including electron-donating groups, such as –CH3, –
OCH3 and –NH2 (compare entries 2, 3 with 4, 7, 8, 9, 10). It is also true for aryl
bromides and aryl chlorides (compare entries 12, 13 with 14, 15 and 17 with 18, 19).
Catalytic performance of the Pd(II) salen@CPGO for the Heck–Mizoroki
cross-coupling reaction
Encouraged by the excellent results obtained from the Suzuki–Miyaura cross-
coupling reactions, we proceeded to investigate the catalytic activity of the Pd(II)
salen@CPGO catalyst in Mizoroki–Heck cross-coupling reactions.
The cross-coupling reaction between iodobenzene (1 mmol) and methyl acrylate/
styrene (1.2 mmol) was taken as the model reaction. The reaction conditions were
optimized by following a series of reactions as listed in Table 3. Reactions were
carried out by varying different parameters such as the amounts of catalyst, solvent,
base and temperature to determine the optimum conditions. Among the bases used
for the reaction, Et3N was found to be superior. Regarding the solvents, DMSO was
found to be promising at a temperature of 110 °C, followed by the reaction time of
55 min with 1 mol% of palladium catalyst. For comparison, the homogeneous Pd
(II) complex (2) was also investigated under the same reaction conditions (Table 3,
entry 5). To demonstrate the efficiency of the catalytic activity of Pd(II)
salen@CPGO in the Heck–Mizoroki cross-coupling reaction, the model reaction
was performed without using a catalyst. After a long period of time, a poor yield of
product was observed such as happened in the presence of GO (Table 3, entries 6,
7).
Subsequently, the optimized reaction conditions were applied for the reaction of
a vast range of aryl halides involving electron-donating and electron-withdrawing
groups on the aromatic ring to investigate the scope and generality of the present
method in the Heck–Mizoroki cross-coupling reaction. The results are summarized
in Table 4 from which if can be seen that all reactions of different substituted aryl
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Pd salen complex@CPGO as a convenient, effective…
Table 2 Suzuki–Miyaura cross-coupling reaction of different aryl halides catalyzed by Pd(II) salen@
CPGO
Pd(II) salen@CPGO
B(OH)2
+
X
K2CO3, DMF
Entry
1
Ar-X
Time (min)a
35
Yield (%)b
97
I
2
3
60
65
84
80
I
MeO
Cl
I
4
35
92
I
Me
45
85
90
80
O2N
I
5
6
S
I
7
45
45
87
91
93
94
93
87
90
95
89
65
63
85
90
OHC
HOOC
H3COC
NC
I
8
I
9
45
I
10
11
12
13
14
15
16
17
18
19
35
I
50
Br
3 h
3 h
50
HO
H2N
O2N
OHC
Br
Br
Br
90
Br
3 h
4 h
4 h
4 h
Cl
HO
NC
Cl
Cl
Cl
O2N
aReaction conditions: Iodobenzene (1 mmol), phenylbronic acid (1.2 mmol), Pd(II) salen@CPGO
(0.7 mol%), base (2 mmol), Solvent (3 mL), at 80 °C
b
Determined by GC with a CBP1 column (Shimadzu 30 m 9 0.32 mm 9 0.25 mm)
123

M. Ghabdian et al.
Table 3 Optimization of reaction conditions for the Heck–Mizoroki coupling reaction
Entry Catalyst (mol%)
Solvent
Base
Temp (°C) Time (min) Yield (%)a
1
Pd salen@CPGO (0. 25) DMSO
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
110
110
110
110
110
110
110
90
80
55
70
2
Pd salen@CPGO (0.5)
Pd salen@CPGO (1)
Pd salen@CPGO (1.5)
Pd salen complex (1)
GO (1)
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
EtOH/H2O
88
3
55
95
4
55
95
5
55
95
6
160
160
140
80
Trace
n.r
65
7
No catalyst
8
Pd salen@CPGO (1)
Pd salen@CPGO (1)
Pd salen@CPGO (1)
Pd salen@CPGO (1)
Pd salen@CPGO (1)
Pd salen@CPGO (1)
Pd salen@CPGO (1)
Pd salen@CPGO (1)
Pd salen@CPGO (1)
Pd salen@CPGO (1)
Pd salen@CPGO (1)
Pd salen@CPGO (1)
Pd salen@CPGO (1)
Pd salen@CPGO (1)
Pd salen@CPGO (1)
9
DMSO/H2O Et3N
90
75
10
11
12
13
14
15
16
17
18
19
20
21
22
DMF
Et3N
Et3N
Et3N
Et3N
110
90
80
83
H2O
120
120
100
120
110
110
110
120
160
160
160
55
40
EtOH
60
48
THF
60
58
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
NaHCO3 110
69
K2CO3
Na2CO3
Cs2CO3
KOH
110
110
110
110
110
r.t
65
70
68
52
No base
Et3N
n.r
Trace
62
Et3N
80
Et3N
160
95
Reaction conditions: Iodobenzene (1 mmol), olefin (1.2 mmol), Pd(II) salen@CPGO, base (2 mmol),
solvent (3 mL), in different temperature
aDetermined by GC with a CBP1 column (Shimadzu 30 m 9 0.32 mm 9 0.25 mm)
iodides with methyl acrylate or styrene generated products with high yields in short
reaction times (Table 4, entries 1–14). Aryl bromides also reacted efficiently with
olefins under optimized conditions, resulting in good yields (Table 4, entries
15–20). However, aryl chlorides gave lower yields when compared to their iodide
and bromide counterparts (Table 4, entries 21–25). Notably, aryl halides with
electron-withdrawing substituents react with olefins more rapidly than the ones with
electron-donating substituents. It was observed that the ortho-substituted aryl
halides afforded the corresponding products with proper yields in longer reaction
times than those attained with para-substituted ones showing steric effects (Table 4,
compare entries 6, 17 with 7,18). From these results, it can be concluded that the
Pd(II) salen@CPGO catalyst can efficiently catalyze Mizoroki–Heck cross-coupling
reactions.
In order to understand the distinctiveness of the Pd(II) salen@CPGO catalyst, we
compared the results of the Suzuki–Miyaura and Mizoroki–Heck cross-coupling
reactions of the catalyst with other supported heterogeneous catalysts [20–29], as
123

Pd salen complex@CPGO as a convenient, effective…
Table 4 Heck–Mizoroki cross-coupling reactions of different aryl halides catalyzed by Pd(II) salen@
CPGO
Pd(II) salen@CPGO
+
X
or
O
or
O
Et3N, DMSO
O
O
Entry
Ar-X
Olefin
Time (min)a
55
Yield (%)b
95
COOMe
I
I
1
2
55
65
96
83
COOMe
COOMe
3
4
I
75
80
80
80
MeO
MeO
I
I
5
6
COOMe
COOMe
120
3 h
90
40
H2N
I
NH2
7
8
I
COOMe
45
89
Cl
I
COOMe
COOMe
COOMe
9
70
65
65
70
82
87
92
88
OHC
I
10
11
12
HOOC
H3COC
H3COC
I
I
I
COOMe
COOMe
13
60
90
92
70
NC
I
S
I
14
15
COOMe
100
82
Br
COOMe
COOMe
COOMe
16
17
3 h
3 h
5 h
78
75
45
HO
Br
Br
H2N
NH2
18
Br
123

M. Ghabdian et al.
Table 4 continued
Entry
Ar-X
Olefin
Time (min)a
Yield (%)b
COOMe
19
20
80
2 h
3 h
90
80
84
O2N
Br
Br
COOMe
COOMe
OHC
O2N
21
22
Cl
COOMe
COOMe
COOMe
3 h
5 h
2 h
3 h
54
48
50
45
Cl
23
24
25
HO
NC
NC
Cl
Cl
Cl
aReaction conditions: Iodobenzene (1 mmol), olefin (1.2 mmol), Pd(II) salen@CPGO (1 mol%), base
(2 mmol), Solvent (3 mL), at 110 °C
bDetermined by GC with a CBP1 column (Shimadzu 30 m 9 0.32 mm 9 0.25 mm)
Table 5 Comparison of the catalytic efficiency of Pd(II) salen@CPGO with various catalysts for the
Suzuki–Miyaura reaction
Entry
Catalyst
Experimental conditions
Yield (%)
Halide
References
1
2
3
4
5
6
7
PdNP@PPh2–SiO2
Pd/SBA-15
60 °C, 3 h
85 °C, 5 h
90 °C, 5 h
80 °C, 1 h
80 °C, 19 h
80 °C, 1 h
80 °C, 35 min
94
88
92
90
97
96
97
Br
Br
Br
Br
Br
I
[20]
[21]
MCM-48-Py-Pd
Pd-CS@SiO2
[22]
[23]
Pd/CM
[24]
GO-NHC-Pd (II)
Pd(II) salen@CPGO
[25]
I
This work
Reaction conditions: aryl halide, phenylbronic acid (1.2 mmol), base, solvent, in different temperature
and time
Table 6 Comparison of the catalytic efficiency of Pd(II) salen@CPGO with various catalysts for the
Heck–Mizoroki reaction
Entry
Catalyst
Experimental conditions
Yield (%)
Halide
References
1
2
3
4
5
6
Pd/CM
80 °C, 3 h
91
85
96
80
99
95
I
[24]
SiO2-SHÁPd
SBA-SHÁPd
HMS-OPPh2-Pd
AL(OH)3/Pd
Pd(II) salen@CPGO
100 °C, 12 h
120 °C, 15 h
120 °C, 3 h
130 °C, 19 h
110 °C, 55 min
I
[26]
I
[27]
I
[28]
Br
I
[29]
This work
Reaction conditions: aryl halide, olefin, base, solvent, in different temperature and time
123

Pd salen complex@CPGO as a convenient, effective…
shown in Tables 5 and 6, respectively. These tables illustrate clearly that the Pd(II)
salen@CPGO shows a better activity in less time and milder reaction conditions for
both Suzuki–Miyaura and Mizoroki–Heck cross-coupling reactions. Facile recovery
of the Pd(II) salen@CPGO catalyst, mild reaction conditions, and better activity are
the key advantages of the Pd(II) salen@CPGO catalyst over other reported catalysts.
Conclusions
We have successfully synthesized a new heterogeneous Pd (II) salen complex
tethered onto CPGO and evidenced by FT-IR, ICP-AES, XRD, SEM/EDX and
TEM. The catalyst exhibited high catalytic activity towards the C–C coupling for
the Suzuki–Miyaura and Heck–Mizoroki cross-coupling reactions with various
electronically diverse substrates. Catalytic tests showed that the heterogeneous
Pd(II) salen@CPGO is the most active in DMF and DMSO at 80 and 110 °C using a
base for the Suzuki–Miyaura and Heck–Mizoroki couplings, respectively. Further-
more, the heterogeneous catalyst showed good recoverability and relatively high
stability against leaching of active species and can be recycled many times without
significant loss of catalytic activity.
Acknowledgements The authors are grateful to the University of Birjand for financial support.
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