Pd nanoparticles immobilized on the magnetic silica–chitosan nanocomposite (NiFe2O4@SiO2@CS-Pd NPs) promoted the biaryl synthesis

Article abstract of DOI:10.1007/s13738-019-01667-6

NiFe₂O₄ nanoparticles have been prepared by co-precipitation procedure and coated with SiO₂ following the St?ber process. Then, chitosan was covalently connected with silica to provide an excellent stabilizer and biosupport for the immobilization of Pd nanoparticles. This magnetic bionanocatalyst was characterized by Fourier transform infrared spectroscopy, field-emission scanning electron microscope, transmission electron microscope, energy-dispersive X-ray, X-ray powder detection, vibrating sample magnetometer, inductively coupled plasma atomic emission spectroscopy techniques and CHN analysis. The NiFe₂O₄@SiO₂@CS-Pd NPs exhibited high activity for the Suzuki coupling reactions of various aryl halides with arylboronic acids. Moreover, this catalyst could be recovered magnetically and recycled for nine times without significant loss of its activity.

Full text of DOI:10.1007/s13738-019-01667-6

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-019-01667-6
ORIGINAL PAPER
Pd nanoparticles immobilized on the magnetic silica–chitosan
nanocomposite (NiFe2O4@SiO2@CS‑Pd NPs) promoted the biaryl
synthesis
Fatemeh Rafee1 · S. Azam Hosseini1
Received: 31 October 2018 / Accepted: 5 April 2019
© Iranian Chemical Society 2019
Abstract
NiFe2O4 nanoparticles have been prepared by co-precipitation procedure and coated with SiO2 following the Stöber process.
Then, chitosan was covalently connected with silica to provide an excellent stabilizer and biosupport for the immobilization
of Pd nanoparticles. This magnetic bionanocatalyst was characterized by Fourier transform infrared spectroscopy, feld-
emission scanning electron microscope, transmission electron microscope, energy-dispersive X-ray, X-ray powder detection,
vibrating sample magnetometer, inductively coupled plasma atomic emission spectroscopy techniques and CHN analysis.
The NiFe2O4@SiO2@CS-Pd NPs exhibited high activity for the Suzuki coupling reactions of various aryl halides with
arylboronic acids. Moreover, this catalyst could be recovered magnetically and recycled for nine times without signifcant
loss of its activity.
Keywords Biomaterial supported catalysts · Chitosan · Magnetic nanoparticle support · Heterogeneous catalysts · Magnetic
separation · Biphenyls
Introduction
and are difcult to recover from reactions due to their small
size. Chitosan is able to reduce metal ions to a zero valent
oxidation state while controlling nanoparticle growth, and
also it can act as a useful stabilizing polymer for metal nano-
particles such as Ag [18], Au [19], Pd and Pt [20]. Due to
the strong afnity of this biopolymer to interact with the
metal nanoparticles leaching does not occur as with tradi-
tional inorganic supports such as silica, alumina and other
metal oxides [21].
The natural biopolymers such as cellulose, chitin, chitosan
and starch are widely applied as a biosupport due to the
availability, non-toxicity, biodegradability, renewability and
biocompatibility properties [1–3]. Among them, chitosan
was considered to be an ideal support for the immobilization
of catalytic metals [4–9]. The presence of available amine
and hydroxyl chelating functional groups in this biopolymer
led to the stabilization of metal anions specially palladium
and platinum. The highly active immobilized metal cata-
lysts supported on chitosan were used in the hydrogenation
and acetalization [10], oxidation [11], cycloaddition [12],
oxidative carboxylation of olefns [13] and cross-coupling
reactions [14–17].
The chemical instability, lower surface area, dissolution
and lower disperse ability of chitosan are the limiting fac-
tors in the use of this biopolymer [5, 22]. The restrictions
mentioned were improved in the magnetic functionalized
chitosan catalysts. These catalysts have attracted a great
research attention especially in biomedical applications.
Their most important benefts include the enhancement
of the stability in solution, improvement of the adsorption
capacity and disperse ability of due to high surface area and
surface functional groups and facile magnetic separation
[23–25].
Nanoparticles can efectively improve the loading and the
catalytic efciency of immobilized catalysts due to their high
surface to volume ratios. However, they tend to aggregate
* Fatemeh Rafee
f.rafee@alzahra.ac.ir
Biaryls present a signifcant interest for the synthesis of
biologically active compounds [26], natural products [27],
receptor macrocyclic molecules [28], liquid crystal mate-
rials [29] and asymmetric catalysts [30]. In view of the
1
Department of Chemistry, Faculty of Physics-Chemistry,
Alzahra University, Vanak, Tehran, Iran
Vol.:(0123456789)
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Journal of the Iranian Chemical Society
importance of biaryls, a number of efective catalytic meth-
ods have been developed for forming these compounds via
palladium catalyzed Suzuki–Miyaura cross-coupling reac-
tion of arylboronic acid, boronic ester and arylborane rea-
gents with aryl halides or trifates [31].
a magnetic stirring bar at 70 °C for 2 h. After complete
precipitation, the residue was washed with double distilled
water (3×25 ml) and dried in an oven at 90 °C over night;
then, it was calcinated at 600 °C for 4 h [38].
The immobilized Pd NPs on ligand-free chitosan [32], or
modifed chitosan for example thiourea-modifed chitosan
[14], silica–chitosan [15] and chitosan-g-mPEG hybrid
[33], chitosan/cellulose-Pd(II) catalyst [16], functionalized
chitosan–pd(II) complexes such as Schif base [34, 35] and
amino acid derivatives [36] and magnetic functionalized
chitosan-supported palladium catalyst [37] have been used
as efcient catalyst source for the Suzuki cross-coupling.
Silica coating process
Coating of these nanoparticles with silica was carried out
by ultrasonic dispersion of 0.5 g of NiFe2O4 nanoparticles
in 40 ml ethanol and 4 ml of deionized water. Then, 5 ml
ammonium hydroxide (25%) was added to the mixture. After
stirring for around 15 min, 2 ml of tetraethyl orthosilicate
(TEOS) was added drop by drop to the mixture. The reaction
was then allowed to proceed for 30 min in an ultrasonic bath
and then continuously stirred at room temperature for 12 h.
The silica-coated nanoparticles were collected by an exter-
nal magnet, followed by washing three times with ethanol
and deionized water and then dried in the oven at 80 °C for
12 h [39].
Experimentals
Materials and methods
All reagents and solvents used in this study were commer-
cially available and were purchased from commercial suppli-
ers (Acros, Merck and Aldrich) and used without any addi-
tional purifcation. Chitosan was purchased from Exir, and
its molecular weight was 100,000–300,000. FT-IR spectra
were recorded using a Bruker Tensor 27 spectrometer within
Chitosan coating process
For the synthesis of chitosan-coated magnetite nanoparti-
cles, amount of 0.5 g of the SiO2-coated NiFe2O4 nanoparti-
cles were dispersed in chitosan solution (contain 1 g chitosan
that dissolved in 40 ml of a 2% acetic acid solution) using
an ultrasonic bath for 30 min at 60 °C. The reaction was
then allowed to proceed at room temperature under continu-
ous stirring for 12 h. Then, the chitosan–coated NiFe2O4@
SiO2 nanoparticles were separated by an external magnet
and washed several times with ethanol and dried in the oven
at 80 °C overnight. This step was done according to the
reported process for chitosan coating of magnetite nanopar-
ticles [40].
1
the 400–4000 cm−1 range. H NMR and 13C NMR spec-
tra were recorded with a Brucker avance 400 and 100 MHz
machine, respectively. XRD pattern of the catalyst was
recorded with a PW 3710 X-ray difractometer (Philips) at
room temperature using monochromatic Cu Kα radiation
with a wavelength of λ = 0.15418 nm. The peak position
and intensity were obtained between 5° and 80° with a rate
of 0.04° s−1. The morphology of the catalyst was observed
using a FE-SEM instrument (HITACHI (S4160) Japan) and
TEM (PHILIPS CM30). EDX pattern was recorded using a
TESCAN, VEGA 3 LMU instrument. Magnetic susceptibil-
ity measurements were carried out using a vibrating sample
magnetometer (VSM, MDK Co. Ltd, Iran) with the magnetic
feld range of between + 10,000 and − 10,000 Oe at room
temperature. Elemental analysis was performed on Eager
300 for EA1112 CHNS instrument.
Preparation of NiFe2O4@SiO2@CS‑Pd catalyst
1 g of NiFe2O4@SiO2@CS, 0.02 g (0.12 mmol) of PdCl2
and 10 ml of ethanol were placed in a round bottom fask
equipped with magnetic stirrer and a refux condenser. The
mixture was stirred and refuxed for 8 h. Then, NiFe2O4@
SiO2@CS-Pd nanocomposite was separated by a perma-
nent magnet and washed fve times with deionized water
and dried in the oven at 80 °C for 10 h.
Synthesis of nickel ferrite (NiFe2O4) nanoparticles
To prepare NiFe2O4 nanoparticles by co-precipitation tech-
nique, the solutions of metallic salts (FeCl3·6H2O, 100 ml,
0.2 M and NiCl2·6H2O, 100 ml, 0.1 M) were prepared sepa-
rately and mixed together. The solution was stirred for an
hour. Then, in order for pH to reach 13, a solution of NaOH
(~18 ml, 3 M) was added slowly to the fask. Finally, oleic
acid (3 drops) was added to the solution as surfactant to
prevent the aggregation and agglomeration of the nanopar-
ticles. Then, the suspension was vigorously stirred using
General procedure for the Suzuki–Miyaura reaction
of aryl halides with arylboronic acids
To a round-bottomed fask containing a mixture of aryl
halide (1 mmol), arylboronic acid (1.1 mmol) and K2CO3
(2 mmol) in 2 ml of EtOH, NiFe2O4@SiO2@CS-Pd catalyst
(0.002 g, 8×10−5 mol% Pd) was added and stirred at 75 °C
for the time specifed in Table 2. After completion of the
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Journal of the Iranian Chemical Society
reaction (monitored by TLC (n-hexane/EtOAc, 9: 1) or GC),
the reaction mixture was cooled to room temperature and the
catalyst was separated by applying an external magnet. Then,
the reaction mixture was diluted with water and the resultant
mixture extracted with n-hexane to isolate the products. The
combined organic layers were dried over MgSO4, and the
solvent was evaporated under reduced pressure.
oxidation processes. The organic-silica based polymeric
materials having metal chelating ability anchored to the
silica surface and ofer advantages such as combination of
the hardness of silica and polymer functional properties and
also enhancement of catalytic centers [15, 45, 46].
Characterization of NiFe2O4@SiO2@CS‑Pd
Figure 1 shows the FT-IR spectrum of NiFe2O4, NiFe2O4@
SiO2, pure chitosan, NiFe2O4@SiO2@CS and palladium
supported on NiFe2O4@SiO2-CS catalyst. In the spectrum
of pure chitosan (Fig. 1a), the characteristic absorption
bands appear at 3422 (O–H and N–H stretching vibra-
tions), 2922 (C–H stretching vibrations), 1653 (N–H bend-
ing vibration), 1423 (C–N stretching vibrations), 1383
(CH3 symmetrical angular deformation) 1150–1100 cm−1
(C–O–C and C–O stretching vibrations. All spinel ferrites
have two signifcant absorption bands in the low-frequency
region (1000–400 cm−1), the highest and lowest ones. This
spectrum (Fig. 1b) demonstrates that the highest band at
651 cm−1 corresponds to the intrinsic stretching vibration
of metal–oxygen at the tetrahedral site and the lowest one
corresponds to the octahedral metal–oxygen stretching
at about 467 cm−1. The absorption broad bands at 3424
and 1634 cm−1 assigned to OH stretching and bending
modes of water. As shown in Fig. 1c, the strong absorption
band at 1091 cm−1 is related to Si–O–Si antisymmetric
stretching vibrations; the absorption peaks at 796 cm−1
and 473 cm−1 are due to the Si–O–Si symmetric stretch-
ing and Si–O–Si or O–Si–O bending mode, respectively.
These data confrm the successful coating of silica layers
on NiFe2O4.
Result and discussion
Preparation of NiFe2O4@SiO2@CS‑Pd catalyst
Nickel ferrite (NiFe2O4) is one of the most important mem-
bers of the spinel ferrites, with a particular ferromagnetism
quality that have high saturation magnetization, high Curie
temperature, chemical stability and relatively high perme-
ability [41–43]. We have already prepared NiFe2O4 nanopar-
ticles and investigated its catalytic activity in the synthesis
of 5-substituted 1H-tetrazoles [44].
The magnetic nanoparticle-supported Pd catalyst
(NiFe2O4@SiO2@CS-Pd NPs) was synthesized via multi-
ple steps The frst step was to prepare NiFe2O4 nanoparticles
from commercial sourced ferric chloride (FeCl3·6H2O) and
nickel chloride (NiCl2·6H2O) in proper pH and the presence
of oleic acid as a surfactant. Second, the obtained MNPs
were subsequently coated with silica (NiFe2O4@SiO2)
through the well-known Stöber method with TEOS. The
NiFe2O4@SiO2 core/shell structures are then sequentially
treated by chitosan to form NiFe2O4@SiO2@CS. Finally,
Pd nanoparticles were immobilized on this support through
the reaction with PdCl2 in ethanol that chitosan acted as a
stabilizer and reducing agent (Scheme 1).
The comparison of the spectra of NiFe2O4@SiO2@CS
with those of pure Chitosan and NiFe2O4@SiO2 indicates
successful immobilization of chitosan on the surface of
NiFe2O4@SiO2 nanoparticles (Fig. 1d).
The surface oxidation and agglomeration of magnetic
nanoparticles result in a signifcant decrease in their mag-
netic properties, surface area and catalytic activity. Due to
the strong surface afnity of the magnetic nanoparticles
toward silica as a chemically inert material with high sur-
face area and good stability, it often employed as coating
layer material over the surface. This surface modifcation
can weaken the particle–particle magnetic bipolar interac-
tions and protects the nanoparticles from aggregation and
Scheme 1 Procedure for preparation of NiFe2O4@SiO2@CS-Pd cat-
Fig. 1 FT-IR spectra of chitosan (a), NiFe2O4 (b), NiFe2O4@SiO2 (c),
alyst
NiFe2O4@SiO2@CS (d), NiFe2O4@SiO2@CS-Pd (e)
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Journal of the Iranian Chemical Society
In the powder XRD pattern of the NiFe2O4@SiO2@
CS-Pd nanocomposite (Fig. 2), the diffraction peaks
observed at 2θ = 30.71°, 36.18°, 37.93°, 43.99°, 54.35°,
57.89,° 63.44° and 74.80° correspond to crystalline planes
(220), (311), (222), (400), (422), (511), (440) and (533) of
NiFe2O4 nanoparticles (with the cubic spinal structure),
respectively. No difraction peaks of impurities such as
α-Fe2O3 or NiO and other secondary phases are detected.
It is clearly observed that the refection peaks are sharp and
narrow, indicating a high degree of crystallinity and phase
purity of the product. Also the XRD pattern of this nano-
composite contains the new characteristic peaks at 40.09°,
46.68°, 67.93° which are indexed, respectively, to the (111),
(200) and (220) crystalline planes of Pd nanocrystal suggest-
ing the formation of metallic Pd NPs (Pd0). The broad peaks
appeared from 18° to 24° cover the amorphous silica phase
around the magnetic core.
Fig. 3 Room temperature magnetization curve of NiFe2O4@SiO2@
CS-Pd catalyst
A magnetic measurement of magnetic chitosan nano-
composite was investigated with a vibrating sample mag-
netometer at room temperature in the applied magnetic feld
sweeping from −10 to 10 kOe (Fig. 3). It can be seen that
this catalyst shows ferromagnetic behavior. The saturation
magnetization (Ms) value for the NiFe2O4@SiO2@CS-Pd
catalyst (11.7 emu g−1) is determined to be signifcantly
lower than that of bare NiFe2O4 that was reported in the
literature (~40 emu g−1). The low magnetic susceptibility of
chitosan-coated magnetic nanoparticles arises from a double
coating of magnetite core by diamagnetic silica shell and
polymeric layer. The magnetic properties of the silica- and
polymer-coated samples depend on the NiFe2O4. In this
sample, there is less magnetic material per gram, and the
magnetization readings are divided by substantial mass of
silica and chitosan. The other is the non-magnetic coating
layer at the surface, afecting the uniformity or magnitude
of magnetization due to quenching of surface moments [47].
Even with this reduction in the saturation magnetization,
the catalyst can still be separated from the reaction mixture
using an external magnetic feld.
The morphology and size of the nanoparticles were
obtained from FE-SEM and TEM analysis. The FE-SEM
photograph (Fig. 4) of the nanocomposite demonstrates that
the prepared magnetic nanoparticles are spherical in shape
with slight agglomeration. TEM images of the catalyst at dif-
ferent magnifcations show that Pd nanoparticles are spheri-
cal, narrowly distributed and well dispersed in the polymer
matrix with average size <10 nm in diameter.
In the EDX spectrum of the nanocomposite (Fig. 5), the
presence of Ni, Fe, Si and O signals indicates that the nickel
ferrite nanoparticles are loaded into silica, and the higher
intensity of the Si peak compared with the Ni and Fe peaks
indicates that the magnetic nanoparticles were trapped by
Fig. 2 XRD pattern of NiFe2O4@SiO2@CS-Pd nanocomposite
Fig. 4 FE-SEM and TEM images of NiFe2O4@SiO2@CS-Pd
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Journal of the Iranian Chemical Society
SiO2. Figure 5 also reveals the presence of palladium on the
surface of chitosan matrix. The exact amount of palladium
in the catalyst as determined by inductively coupled plasma
atomic emission spectroscopy (ICP-AES) was 4.2 wt%
(0.39 mmol/g). Also the content of carbon, nitrogen and
hydrogen is found by CHN analysis to be 10.42, 2.21 and
10.70%, respectively.
The investigation of catalytic activity
of the NiFe2O4@SiO2@CS‑Pd in C–C coupling
In 2015, Jadhav et al. [15] reported that in silica–chitosan
hybrid material, chitosan (as ligand and support to stabilize
palladium) covalently connected with silica. This biopoly-
mer provides a platform to Pd and prevents the removal of
palladium from the silica surface.
Fig. 5 EDX spectra of NiFe2O4@SiO2@CS-Pd
Table 1 Optimization of reaction conditions for the Suzuki cross-coupling reactiona
Entry
Solvent
Base
Catalyst (g)
Temp. (°C)
Conversion
(%)b
1
2
3
4
5
6
7
8
H2O
H2O
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
NaHCO3
NaOAc
Na2CO3
KOH
KHCO3
K2CO3
K2CO3
K2CO3
K2CO3
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
–
rt
100
rt
–
–
90
100
–
100
–
50
80
–
70
90
85
–
80
100
100
EtOH
EtOH
DMSO
DMF
Me–Ph
MeCN
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
75
Refux
Refux
110
80
75
75
75
75
75
75
9
10
11
12
13
14
15
16
17
0.001
0.002
0.003
75
75
75
aReactions conditions: 1-bromo-4chlorobenzene (1 mmol), phenylboronic acid (1 mmol), base (2 mmol), NiFe2O4@SiO2@CS-Pd nanocatalyst
(g), solvent (3.0 ml), 1 h
bDetermined by GC and TLC
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Journal of the Iranian Chemical Society
Table 2 Suzuki cross-coupling
reaction using NiFe2O4@SiO2–
CS-Pd nanocatalyst
Entry Ar-X
ArB(OH)2
Time (min) Yield (%)a m.p. (°C) Refs.
1
2
Ph–Br
p-CHO–PhBr
PhB(OH)2
PhB(OH)2
5
30
98
96
98
94
97
98
95
96
98
93
96
95
94
98
98
94
97
90
68–70
56–58
118–120
58–60
[48]
[49]
[48]
[50]
[51]
[48]
[52]
[51]
[51]
[50]
[51]
[53]
[51]
[54]
[54]
[55]
[53]
[48]
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
p-MeOC–Ph–Br PhB(OH)2
45
m-O2N–Ph–I
p-NC–Ph–Br
p-Cl–Ph–Br
p-CHO–PhBr
p-NC–Ph–Br
p-Cl–Ph–Br
m-O2N–Ph–I
p-NC–Ph–Br
p-Me–Ph–I
p-MeO–Ph–I
p-MeOC–Ph–Br 3,4,5-tri-fouro-PhB(OH)2
p-CHO–Ph–Br 3,4,5-tri-fouro–PhB(OH)2
p-MeOC–Ph–Br m-MeO–PhB(OH)2
p-MeOC–Ph–Br p-MeOC–PhB(OH)2
Ph–Cl
PhB(OH)2
PhB(OH)2
PhB(OH)2
p-Me–PhB(OH)2
p-Me–PhB(OH)2
p-Me–PhB(OH)2
p-Me–PhB(OH)2
p-Me–PhB(OH)2
p-Me–PhB(OH)2
p-Me–PhB(OH)2
30
45
84–86
15
30
30
15
74–76
104–106
109–111
125–127
76–78
30
30
30
30
180
180
120
120
180
109–111
118–120
108–110
119–122
109–111
48–50
189–190
68–70
PhB(OH)2
Reactions conditions: aryl halide (1 mmol), arylboronic acid (1 mmol), K2CO3 (2 mmol), NiFe2O4@
SiO2@CS-Pd nanocatalyst (8×10−5 mol%), ethanol (3.0 ml), 75 °C
aIsolated yields
scope and generality of the process. The results are summa-
rized in Table 2. Both electron-rich and electron-defcient
aryl halides and arylboronic acids were superior and deliv-
ered products with good to excellent yields. Prolonged reac-
tion time was required for the less active aryl chlorides to
obtain a moderate yield. A wide range of functional groups,
such as nitryl (NO2), nitrile (CN), methyl, methoxy, and car-
bonyl (COCH3 and CHO), were tolerated in the reactions.
To study the recyclability of NiFe2O4@SiO2@CS-Pd, we
carried out a set of experiments using the recycled catalyst
for the Suzuki–Miyaura coupling reaction of 1-bromo-4-chlo-
robenzene with phenyl boronic acid under optimized condi-
tions. After the completion of the frst reaction, the catalyst
was recovered magnetically, washed with ethanol and fnally
dried for the next run. Then, a new reaction was carried out
Fig. 6 Reusability of the NiFe2O4@SiO2@CS-Pd catalyst
with fresh reactants and solvent under the same conditions.
Base on the results, this catalyst could be used at least nine
times without any signifcant loss of its catalytic activity
(Fig. 6).
In order to show the catalytic activity of synthesized Pd
nanocatalyst in the Suzuki–Miyaura coupling reaction, we
employed the coupling reaction of 4-bromoanisole with phe-
nylboronic acid as a model reaction to optimize the reaction
conditions. The best results were obtained in ethanol as the
solvent and K2CO3 as base by employing 0.002 g of nano-
catalyst (8 × 10−5 mol% palladium) at 75 °C. As this catalyst
is not sensitive to oxygen, the reactions were carried out
under air atmosphere (Table 1).
Hot fltration tests of fresh catalyst in the frst use and also
recycled catalyst showed that coupling conversion was only
40% and no improvement of the product yield observed even in
longer times. ICP-AES analysis data showed that the amount
of palladium leaching from the surface into the fltrate after
catalyst recycling was <1%.
The chemical structure of the recycled catalyst after the 3
run was investigated by using of FT-IR spectroscopy (Fig. 7)
and XRD analysis (Fig. 8). The same difraction peaks and
We then explored the synthesis of various biphenyl deriv-
atives under the optimized conditions, in order to probe the
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Journal of the Iranian Chemical Society
Fig. 7 FT-IR spectra of recycled
NiFe2O4@SiO2@CS-Pd cata-
lyst after the 3 run
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber cm-1
absorption bands of fresh catalyst were observed in XRD pat-
tern and FT-IR spectrum of recycled catalyst, respectively.
Therefore, results confrm that the catalyst retains its main
structure during Suzuki coupling reactions.
The magnetic properties investigation of the recycled cata-
lyst showed that the magnetic intensity does not change after
the 3 run (Fig. 9). These results indicate that the amount of
palladium leaching is very low. Metal leaching will increase
the amount of magnetic material per gram and so the magnetic
intensity will increase.
To show the merit of the present work, we compared the
results of NiFe2O4@SiO2–CS-Pd with the previously reported
catalyst systems in the synthesis of biphenyl via cross-cou-
pling of bromobenzene with phenylboronic acid (Table 3).
The results obtained with this catalytic system are superior to
others, so that it gave a better yield in shorter time with lower
loading of palladium catalyst under mild reaction conditions.
The mechanism of the Suzuki coupling reaction is pre-
sented in Scheme 2. Oxidative addition, transmetalation and
reductive elimination are the three main steps in this coupling
reaction [63]. The catalytic cycle begins with the oxidative
addition of the aryl halide to Pd(0) and aryl palladium (II)
intermediate formation (Ar-Pd(II)-X). In transmetalation step,
arylboronic acid which is activated by base reacts with aryl
palladium (II) complex. Finally, reductive elimination of Ar-
Pd(II)-Ar intermediate produces the desired coupling products.
Fig. 8 XRD pattern of recycled NiFe2O4@SiO2@CS-Pd catalyst after
the 3 run (red) and fresh catalyst (blue)
Fig. 9 Room temperature magnetization curve of recycled NiFe2O4@
SiO2@CS-Pd catalyst
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Table 3 Comparison results
Entry Catalyst
(mol%)
1
Solvent
Time
24 h
Yield (%) Refs.
of NiFe
2O4@SiO2–CS-Pd
with other catalysts for the
coupling of bromobenzene with
phenylboronic acid
1
Pd–Fe3O4 heterodimer
nanocrystal catalyst
Refux DMF/H2O
81
[56]
2
3
Pd0/mesoporous NiFe2O4-300 0.08
NMP/H2O, 80 °C
DMF/H2O, 90 °C
12 h
1 h
97
69
[57]
[58]
Pd/NiFe2O4
0.1
1
30 min 98
4
5
6
7
8
9
10
Pd-CS@SiO2
1
EtOH 95%, 80 °C
TBAB, 90 °C
H2O, 100 °C
TBAB, H2O, 100 °C 6 h
H2O/DMF, 80 °C
EtOH 50%, 50 °C
1 h
5 h
2 h
90
[15]
[59]
[33]
[60]
[61]
[62]
Our
Pd NPs@chitosan
CS-g-mPEG350 Pd (0)
Cell-Pd(0)
Pd@Al2O3–CELL
Cell-AMP-Pd
0.1
0.5
0.3
1
97
79
92
95
91
98
1.5 h
3.5 h
5 min
0.75
NiFe2O4@SiO2-CS-Pd
8 × 10−5 EtOH 95%, 75 °C
Acknowledgements We gratefully acknowledge the partial fnancial
support received from the research council of Alzahra University.
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