Pd nanoparticles supported on Fe3O4@SiO2-Schiff base as an efficient magnetically recoverable nanocatalyst for Suzuki–Miyaura coupling reaction

Article abstract of DOI:10.1007/s11164-020-04223-7

Abstract: In this work, Pd nanoparticles (Pd-NPs) were decorated on modified magnetic nanoparticles (MNPs) and used as an efficient and recyclable catalyst for the Suzuki cross-coupling reaction of aryl halides with phenylboronic acid (PhB(OH)₂) in a green solvent. The prepared nanocomposite was characterized by field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, Fourier transforms infrared spectroscopy, X-ray powder diffraction, thermogravimetric analysis/differential thermal analysis, and vibrating sample magnetometry. All analyses confirmed the successful modification of MNPs and immobilization of Pd on modified MNPs. This catalyst exhibited superior catalytic activity and stability in the suzuki cross-coupling reaction of PhB(OH)₂ and aryl halide derivatives. This protocol includes some advantages, such as magnetically reusability of the catalyst, mild experimental conditions, green solvent, excellent yields of the product (52–98percent), and short reaction times (4–33?min). The catalyst could be reused for six successive runs without any significant loss of its efficiency. Graphic abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s11164-020-04223-7

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-020-04223-7
Pd nanoparticles supported on Fe3O4@SiO2‑Schif base
as an efcient magnetically recoverable nanocatalyst
for Suzuki–Miyaura coupling reaction
Najmeh Amirmahani1,2 · Nosrat O. Mahmoodi3 · Mohammad Malakootian2 ·
Abbas Pardakhty4 · Neda Seyedi5
Received: 19 May 2020 / Accepted: 21 July 2020
© Springer Nature B.V. 2020
Abstract
In this work, Pd nanoparticles (Pd-NPs) were decorated on modifed magnetic nan-
oparticles (MNPs) and used as an efcient and recyclable catalyst for the Suzuki
cross-coupling reaction of aryl halides with phenylboronic acid (PhB(OH)2) in
a green solvent. The prepared nanocomposite was characterized by feld emission
scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission
electron microscopy, Fourier transforms infrared spectroscopy, X-ray powder dif-
fraction, thermogravimetric analysis/diferential thermal analysis, and vibrating
sample magnetometry. All analyses confrmed the successful modifcation of MNPs
and immobilization of Pd on modifed MNPs. This catalyst exhibited superior cata-
lytic activity and stability in the suzuki cross-coupling reaction of PhB(OH)2 and
aryl halide derivatives. This protocol includes some advantages, such as magneti-
cally reusability of the catalyst, mild experimental conditions, green solvent, excel-
lent yields of the product (52–98%), and short reaction times (4–33 min). The
catalyst could be reused for six successive runs without any signifcant loss of its
efciency.
*
Nosrat O. Mahmoodi
mahmoodi@guilan.ac.ir; nosmahmoodi@gmail.com
1
2
3
4
5
Department of Chemistry, University Campus 2, University of Guilan, PO Box 41335-1914,
Rasht, Iran
Environmental Health Engineering Research Center, Kerman University of Medical Sciences,
PO Box 76175-531, Kerman, Iran
Department of Organic Chemistry, Faculty of Sciences, University of Guilan, PO
Box 41335-1914, Rasht, Iran
Pharmaceutics Research Center, Neuropharmacology Institute, Kerman University of Medical
Sciences, PO Box 76175-531, Kerman, Iran
Department of Chemistry, Faculty of Science, University of Jiroft, Jiroft, Iran
Vol.:(0123456789)
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N. Amirmahani et al.
Graphic abstract
HO OH
B
X
K2CO3
H2O:EtOH
75 oC
R
O
O
SiO2
Si
O
N
(CH2)3
(CH2)3
O
O
O
Pd
Fe3O4
O
O
N
R
R = H, 4-OMe, 4-NO2, 2-OMe,-NO2, 3NO2, 3-Me, 4-Me,
X = Br, I
Keywords Magnetite nanoparticles · Nanocomposites · Suzuki–Miyaura coupling
reaction · C–C coupling
Introduction
The formation of Ar–Ar bonds has major importance in various felds of organic and
medicinal chemistry. Biaryl compounds are useful frameworks for various polymers,
agrochemicals, pharmaceuticals, and natural products including favonoids, terpe-
nes, alkaloids, tannins, peptides, and glycopeptides [1, 2]. The classical methods for
the synthesis of biaryls are the Ullmann reaction [3, 4], Gomberg-Bachmann-Hey
reaction [5], Pschorr cyclization [6], and Gatterman synthesis [7].
Among these reported methods for the C–C coupling in organic reaction, tran-
sition-metal-catalyzed reaction, particularly, Pd-catalyzed, has been developed as
an atom- and step-economical tools to construct many organic molecules in recent
years. The Suzuki cross-coupling reactions are well-known, very important, and
powerful methodologies for C–C coupling formation in organic synthesis [8, 9].
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Pd nanoparticles supported on Fe3O4@SiO2‑Schif…
The Pd-complexes of electron-rich, sterically demanding phosphines or NHC
ligands are efective catalysts for sterically demanding Suzuki couplings, and those
ligands are believed to stabilize a putative monoligated Pd-complex [10].
These reactions have been traditionally carried out using many diferent Pd cata-
lysts such as Pd(PPh3)4, Pd(OAc)2, and Pd2(dba)3 C6H6 [9–13]. In this regard, Pd
nanocomposites, e.g., Pd‐imine@MNPs/Cs, PVP-Pd nanoparticles, Pd/IL-NH2/
SiO2/Fe3O4 and dendrimer-stabilized Pd nanoparticles (PdNPs) have attracted
much attention as a major alternative to traditional heterogeneous catalysts [14–28].
Recently, the magnetic nanoparticles (MNPs) due to their unique properties includ-
ing easy synthesis, low toxicity, cost-efective, and most importantly easy separation
from the reaction mixture have developed as a core magnetic support [29]. Among
the MNPs, iron oxide (Fe3O4) has attracted technological importance due to its cata-
lytic, magnetic, and electrical properties. Nano size Fe3O4 has large interfacial area,
fame retardancy, oxidative stability, and thermal resistance [30–32]. Many research
was completed for Suzuki coupling on the basis of Pd supported on Fe3O4@SiO2
such as Fe3O4@SiO2‐EDTA‐Pd NPs [33], Fe3O4@SiO2@3‐glycidoxypropyl tri-
methoxysilanetriazole@Pd [34], Fe3O4@SiO2/Schif base/Pd (II) [35], Fe3O4@
SiO2-Pd [36], Fe3O4@SiO2@mSiO2-Pd(II) [37], Fe3O4‐G4‐PAMAM dendrimers‐
Pd (0) [38], and Fe3O4@SiO2@l‐arginine@Pd(0) [39].
Here, in contribution to our interest in the synthesis of new nanocomposite
materials (NCM) [40–43], we report the synthesis and characterization of Fe3O4@
SiO2-APTMS-SAL-Pd NC immobilized onto MNPs core. Subsequent, the Fe3O4@
SiO2-APTMS-SAL-Pd NC as a heterogeneous nanocatalyst was applied for Suzuki
reaction.
The results indicated that the reactivity of this heterogeneous catalyst is similar to
a homogeneous catalyst. In addition, this catalyst was easily separated from reaction
mixture without signifcant loss of activity.
Experimental
Materials and characterization
All of the materials used were of analytical grade. The solvents were distilled
before use. The FT-IR spectra were recorded as KBr pellets, resolution 4 cm−1, and
number of scans 10 (FT-IR 6300 Brucker). The FESEM analyses was conducted
(MIRA3TESCANXMU). The TEM analysis was conducted (Philips CM30). The
EDS analysis was conducted (TESCAN mira2). The XRD pattern was obtained
with a Philips X‐Pert 1710 difractometer using Co Kα (λ=1.78897 Å) at a volt-
age of 40 kV and current of 40 mA. The data were collected from 10° to 80° (2θ)
with a scan speed of 0.02 s−1. The magnetic measurement of the catalyst was con-
ducted using a vibrating magnetometer/alternating gradient force magnetometer
(Lake Shore Cryotronics-7404). TGA/DTA was carried out with a thermal analyzer
under Ar fow at a heating rate of 10 °C min−1 in the range of 25–600 °C (PC Luxx
409-NETZSCH). The ICP analysis was performed by an ICP‐OES simultaneous
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N. Amirmahani et al.
model Arcos EOP spectrometer. 1H NMR spectra of products were recorded with a
Bruker INOVA 400 and 300 (MHz).
Synthesis
Synthesis of Fe3O4 NPs
Fe3O4 NPs were prepared using simple chemical co-precipitation. In a 250-
mL three-necked bottom fask was added FeCl2·4H2O (5 mmol, 0.995 g) and
FeCl3·6H2O (10 mmol, 2.7 g) in 15 mL of deionized (DI) H2O and the resulting
mixture was degassed with Ar. When the temperature arrived at 80 °C, 30 mL of
NH4OH, 25% w/w was added into the solution until the pH adjusted to 11. Then,
the reaction mixture was stirred at r.t. for 1 h. The resulting mixture was refuxed
at 100 °C for 1 h. After this time, the obtained black solid was separated with a
magnet, and the collected Fe3O4 NPs (1 g) was washed with DI-H2O and EtOH for
several times and dried under vacuum at 80 °C for 5 h.
Coating Fe3O4 NPs with SiO2 (Fe3O4@SiO2)
Synthesis of Fe3O4@SiO2: Fe3O4 NPs (1.0 g) was frst diluted with DI-H2O (50 mL)
and stirred for 10 min. Then, tetraethyl orthosilicate TEOS (1 mL) diluted in EtOH
(20 mL) was added dropwise to this solution, and the reaction mixture was contin-
ued to react for 24 h at 80 °C. The obtained product was collected by magnetic sepa-
ration and washed with EtOH, DI-H2O and dried under vacuum.
Synthesis of Fe3O4@SiO2‑APTMS
Typically, Fe3O4@SiO2 (1 g) was added to 50 mL of EtOH and stirred for 10 min
under Ar atmosphere. APTMS (1 mL, 2 mmol) was added to the above mixture and
heated at 80 °C for 8 h; after completion of the reaction, the product was collected
by magnetic separation and washed with EtOH, DI-H2O and dried under vacuum.
Synthesis of Fe3O4@SiO2‑APTMS‑SAL‑Pd Nc
Fe3O4@SiO2-APTMS (1 g) and 2-hydroxybenzaldehyde (SAL) (0.63 g, 6 mmol)
in 50 mL of EtOH was heated at 60 °C for 24 h. The nanocomposites were mag-
netically separated and washed several times with EtOH, and dried under vacuum
at 80 °C for 6 h. Then, for loading Pd onto Fe3O4@SiO2-APTMS-SAL, Pd(OAc)2
(20 mg) and Fe3O4@SiO2-APTMS (500 mg) in acetone (25 mL) were stirred for
24 h at r.t. The obtained product was collected by magnetic separation and washed
with EtOH, DI-H2O and dried under vacuum. The Pd content in Fe3O4@SiO2-
APTMS-SAL-Pd NS was determined to be 3 w% (0.28 mmol g−1) based on ICP.
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Pd nanoparticles supported on Fe3O4@SiO2‑Schif…
General procedure for Suzuki–Miyaura cross‑coupling of aryl halides with PhB(OH)2
in the presence of Fe3O4@SiO2‑APTMS‑SAL‑Pd NC
The mixture of 0.01 g of catalyst, aryl halide (1 mmol), PhB(OH)2 (1 mmol, 0.12 g),
and K2CO3 (2 mmol, 0.28 g) in EtOH/H2O (1:1, 7 mL) was heated and stirred at
75 °C for appropriate time. The progress of the reaction was monitored by TLC.
After completion of the reaction, the NPs were separated from the reaction mixture
with an external magnet and washed with CH2Cl2 (2×25 ml) repeatedly and dried
over Na2SO4. The solvent was removed under reduced pressure to give the crude
product.
Results and discussion
Catalyst preparation
The Fe3O4@SiO2-APTMS-SAL-Pd NC as a magnetic catalyst was prepared based
on the following procedure. Initially, the Fe3O4 NPs were prepared via co-precipi-
tation of Fe2+ and Fe3+ in the NH4OH solution [44]. Then, in order to protect mag-
netic NPs against oxidation, agglomeration and also improve their chemical stability,
TEOS as a source of silica core-shell was applied for coating MNPs [45]. Further
modifcation was done by 3-aminopropyltriethoxysilane (APTES). Since APTES
is an aliphatic aldehyde and easily oxidized, the terminal amine groups of APTES
reacted with the aldehyde group of SAL and formed an imine group. Subsequently,
Pd was loaded to the surface of Fe3O4@SiO2-APTMS-SAL via OH and imine of
Schif base in acetone for 24 h (Scheme 1).
CHO
SiO2
SiO2
MeO
MeO
Si
OMe
NH2
O
2
OH
SAL
2
Si
O
i
O
i
O
NH2
Fe3O4
O
i
O
i
Fe3O4
S
S
S
S
O
SiO2
SiO2
Fe3O4@SiO2-APTES
Fe3O4@SiO2
H
O
O
H
N
S
i
O
O
O
O
Si
SiO2
N
Pd(OAc)2
Si
OH
Pd
O
Fe3O4
O
Fe3O4
SiO2
O
O
O
O
N
i
S
O
O
O
i
S
i
S
N
O
OH
H
H
Fe3O4@SiO2-APTES-SAL
Scheme 1 Synthesis of Fe3O4@SiO2-APTMS-SAL-Pd NC
Fe3O4@SiO2-APTES-SAL-Pd NC
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N. Amirmahani et al.
Catalyst characterization
The FT-IR spectra of Fe3O4@SiO2 in Fig. 1a showed the absorption band at
3416 cm−1 that attributed to the O–H bonds attached to the surface of silica and also
the absorption bands at 1103 cm−1 and 475 cm−1 corresponding to the Si–O and
Fe–O bonds, respectively. After amino-functionalized Fe3O4@SiO2 by APTES, sev-
eral additional peaks appeared (Fig. 1b). The presented peak at 2870 and 2934 cm−1
is attributed to the –CH2– groups of aminopropyl from APTMS. In addition, the
adsoption bands at 3438 cm−1 is related to the amino-functional groups of ami-
nopropyl from APTMS [46]. The next Fe3O4@SiO2-NH2 reacted with SAL and
formed an imine group, and the new peak at 1642 cm−1 corresponds to the vibration
of C=N group shown in Fig. 1c [47, 48]. These results exhibited that the APTMS
and SAL were successfully immobilized on Fe3O4@SiO2.
The FT-IR spectrum of Fe3O4@SiO2-APTMS-SAL-Pd demonstrated that no
obvious changes occur after loading of Pd on the Fe3O4@SiO2-APTMS-SAL, which
was in agreement with the reported results [35].
The phase and purity of Fe3O4@SiO2-APTMS-SAL-Pd were examined by
XRD. Figure 2 shows the XRD powder pattern of Fe3O4@SiO2-APTMS-SAL-Pd.
Seven difraction peaks (2θ=30.4, 35.5, 43.2, 44.5, 53.4, 57.1 and 62.8) related to
their corresponding indices ((111) (2 2 0), (311), (4 0 0), (4 2 2), (511) and (4 4
0)) were observed in the case of Fe3O4 NPs (JCPDS 75-0033) (Fig. 1a). Also the
XRD pattern of Fe3O4@SiO2 nanospheres exhibited almost the same feature as
pure Fe3O4 (Fig. 1b), except that a broad peak centered in between 20 and 30 of 2θ
Fig. 1 FT-IR spectra of a Fe3O4@SiO2, b Fe3O4@SiO2-APTMS, c Fe3O4@SiO2-APTMS-SAL, and d
Fe3O4@SiO2-APTMS-SAL-Pd NC
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Pd nanoparticles supported on Fe3O4@SiO2‑Schif…
Fig. 2 The XRD pattern of
Fe3O4@SiO2-APTMS-SAL-Pd
NC
corresponding to SiO2 was observed, indicating that the coated SiO2 shell is crys-
talline. In the case of Fe3O4@SiO2-APTMS-SAL-Pd catalyst (Fig. 1c), besides the
characteristic difraction of Pd NPs, the obvious difraction peaks were observed at
2θ=40.1° and 46.7° that related to the difraction planes of Pd with miller index
(111) and (200), respectively, (JCPDS 05-0681). These results confrmed that the
Fe3O4@SiO2-APTMS-SAL-Pd NP was successfully prepared.
According to the EDS analysis prepared for an elemental investigation of the
Fe3O4@SiO2-APTMS-SAL-Pd NC (Fig. 3). The results showed W% to be 42.05%
O, 26.17% Si, 15.15% Fe, 7.45% N, 7.12% C, and 2.05% Pd in accordance with
the expected values for confrming the chemical structure of Fe3O4@SiO2-APTMS-
SAL-Pd (Fig. 3).
The typical morphology and size of the synthetic catalyst were recorded by
FESEM and TEM, as shown in Fig. 4. The FESEM image shows the formation of
uniform and single-dispersed nanoparticles (Fig. 4a, b). The TEM image also con-
frmed the successful coating of amorphous silica on Fe3O4 MNs, which is seen as
a bright area surrounding the dark Fe3O4 core. A slight aggregation and stacking
texture could be observed from the magnifed image, which was due to the mag-
netic attraction between the particles structure of the catalyst (Fig. 4c, d); the aver-
age diameter of the particle size is about 11–16 nm.
TGA/DTA analysis of Fe3O4@SiO2-APTMS-SAL-Pd NC shows a weight loss
due to the elimination of physisorb and chemisorbed solvent and surface OH groups
on the support below 200 °C [49]. Thermal degradation of the catalyst happened
following 200 °C which shows the high thermo-stability of the catalyst. The second
and third weight loss of 9.63% in the range of 200–600 °C due to the degrading of
organic remains (Fig. 5).
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N. Amirmahani et al.
Fig. 3 EDS analysis of Fe3O4@SiO2-APTMS-SAL-Pd NC
The magnetic properties of Fe3O4@SiO2-APTMS-SAL-Pd NC were estimated by
VSM (Fig. 6). The magnetization curve of NCs proved a ferromagnetic character.
Additionally, the specifc magnetization (M) versus the applied magnetic feld (H)
curve indicated the coercive force (Hc), saturation magnetization (Ms), and rem-
nant magnetization (Mr) values for NCs as 0.01 OE, 14.23 emu/g, and 0.389 emu/g,
respectively. These values confrm the sufcient magnetic strength of NCs for sim-
ple separation [50].
Fe3O4@SiO2-APTMS-SAL-Pd was investigated as a catalyst in the synthesis of
biaryls via the Suzuki cross-coupling reaction. In this work, the cross-coupling of
iodobenzene with PhB(OH)2 was chosen as the model reaction. To optimize the
reaction conditions, the typical reaction was performed in the presence of difer-
ent amounts of Fe3O4@SiO2-APTMS-SAL-Pd catalyst, base, and temperature. As
shown in Table 1 in the absence of a catalyst, the reaction did not proceed even after
a prolonged reaction time (Table 1, entry 1). In the presence of 0.15 mol% cata-
lyst, the reaction produced the desired product in low yield (Table 1, entries 2). In
another attempt, it was found that 0.28 mol% Pd amount of the catalyst with the
appropriate amount of aryl halide and PhB(OH)2 is the optimized performance in
the Suzuki crossover reaction (entry 3, Table 1). By increasing T to 75° C, the reac-
tion was performed with high yield and short reaction time (15 min) (Table 1, Input
3, 10–12). Reaction optimization was performed in the presence of various bases
including K2CO3, KOH, Na2CO3, and CaCO3 (Table 1, entries 3, 7–9). Further
experiments showed that the use of K2CO3 as base in 15 min resulted in a 98% yield
of target biphenyl product (Table 1, entry 3). Finally, a typical reaction was per-
formed in the presence of various solvents, e.g., EtOH–H2O (1:1), EtOH, and H2O
(Table 1, entries 3, 5, 6). The results showed that the use of EtOH–H2O (1:1) as a
solvent produced the biphenyl, after 15 min, in 98% yield (Table 1, entry 3).
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Pd nanoparticles supported on Fe3O4@SiO2‑Schif…
Fig. 4 FESEM image of Fe3O4@SiO2-APTMS-SAL-Pd NC (a, b) and TEM image of Fe3O4@SiO2-
APTMS-SAL-Pd NC (c, d)
After optimizing the reaction conditions, the generality of this method was inves-
tigated by reacting to a wide range of aryl iodides and aryl bromides with PhB(OH)2
in the presence of Fe3O4@SiO2-APTMS-SAL-Pd at 75 °C.
Both aryl iodides and aryl bromides delivered the products with high yields
(Table 2, 3, entries 1–7). The coupling of aryl iodides was superior and aforded
the desired product in excellent yields. The results showed that the presence of
electron-donor groups in the ortho and para position of aryl halide increased the
reactivity in comparison to the meta position. However, the presence of electron-
withdrawing groups in the ortho and para position of aryl halide decreased the
reactivity in comparison to the meta position (Table 2). The catalyst turnover
number (TON) and turnover frequency (TOF) are two crucial parameters that are
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N. Amirmahani et al.
Fig. 5 TGA/DTA diagram of Fe3O4@SiO2-APTMS-SAL-Pd NC
Fig. 6 VSM of Fe3O4@SiO2-APTMS-SAL-Pd NC
used for comparing catalyst efciency in homogenous and heterogeneous media
[51]. In heterogeneous catalytic reactions, the TON (=mol of product/mol of cat-
alyst) and TOF (= TON/time [h]) can be determined on the basis of the amount
of product biaryls which was formed. The TON and TOF values of Fe3O4@SiO2-
APTMS-SAL-Pd are given in Table 2.
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Pd nanoparticles supported on Fe3O4@SiO2‑Schif…
Table 1 Cross-coupling reactions between phenylboronic acid and iodobenzenes under various reaction
conditions
Entry
Catalyst (mol%)
Base
Solvent
Temp. (°C)
Yielda (%)
1
2
3
4
5
6
7
8
0
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
CaCO3
KOH
EtOH–H2O (1:1)
EtOH–H2O (1:1)
EtOH–H2O (1:1)
EtOH–H2O (1:1)
EtOH
25
75
75
75
75
75
75
75
75
60
50
80
0
70
98
98
80
60
95
20
70
50
35
98
0.15
0.28
0.45
0.28
0.28
0.28
0.28
0.28
0.28
0.28
0.28
H2O
EtOH–H2O (1:1)
EtOH–H2O (1:1)
EtOH–H2O (1:1)
EtOH–H2O (1:1)
EtOH–H2O (1:1)
EtOH–H2O (1:1)
9
10
11
12
K2CO3
K2CO3
K2CO3
Table 2 Suzuki–Miyaura cross-
coupling of aryl halides with
PhB(OH)2 catalyzed by Fe3O4@
SiO2-APTMS-SAL-Pd NC
Entry
Aryl halidea
Yieldb (%)
TONc
TOFd
1
2
3
4
5
6
7
8
C6H5I
C6H5Br
98
95
89
88
85
75
87
89
82
77
76
79
350
140
339.3
317.8
314.3
303.6
267.9
310.7
317.9
292.9
275
135.7
127.14
125.7
121.43
107.14
124.28
127.14
117.14
110
4-OCH3–C6H5I
4-OCH3–C6H5Br
3-OCH3–C6H5I
3-OCH3–C6H5Br
2-OCH3–C6H5I
3-NO2–C6H5I
3-NO2–C6H5Br
4-NO2–C6H5I
4-NO2–C6H5Br
2-NO2–C6H5I
9
10
11
12
271.4
282.1
108.57
112.86
aReaction conditions: Arylhalide (1.0 mmol), phenylboronic acid
(1 mmol), Fe3O4@SiO2-APTMS-SAL-Pd (0.28 mol%), and K2CO3
(2 mmol) were stirred in H2O/EtOH 1:1 (7 mL) at 75 °C for 15 min
bIsolated yield
cTON turnover number
dTOF turnover frequency
The catalytic performance of Fe3O4@SiO2-APTMS-SAL-Pd NC during the
Suzuki coupling reaction was compared with other literature reported Pd catalysts
(Table 3).
From Table 3, it can be concluded that the prepared catalyst shows a shorter
reaction time as well as a higher yield in comparison to the other listed catalysts.
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N. Amirmahani et al.
Table 3 Comparison of catalytic performance of Fe3O4@SiO2-APTMS-SAL-Pd NC in the coupling
reaction of phenylboronic acid with iodobenzene
Entry Catalyst (mol%)
Conditions
Time (min) Yield (%) Refs.
1
2
3
NHC‐Pd(II) complex (0.2)
K3PO4·3H2O, H2O, TBAB,
40 °C
Et3N, DMF, 100 °C
300
30
98
96
78
[52]
[53]
[54]
γ-Fe2O3-acetamidine‐Pd
(0.12)
Bis(oxamato)palladate(II)
Et3N, n‐Bu4NBr, 120 °C
120
complex (5)
4
5
SiO2-pA-Cyan-Cys-Pd (0.5) K2CO3, H2O, 100 °C
300
30
95
95
[55]
[56]
Pd-isatin Schif base- γFe2O3 Et3N, Solvent‐free, 100 °C
(0.5)
6
7
Pd–BOX (2)
K2CO3, DMF, 70 °C
360
15
100
98
[57]
This work
Fe3O4@SiO2-APTMS-SAL- K2CO3, H2O/EtOH,75 °C
Pd (3)
Recycling of catalyst
The recycling of the catalyst further explored in the model reaction of phenyl
iodide with PhB(OH)2 in the presence of 10 mg of Fe3O4@SiO2-APTMS-SAL-Pd
NC. The catalyst was able to recycle simply via attaching an external magnet after
completion of the reaction and washed with H2O/EtOH, dried under vacuum, and
reused in a subsequent reaction. The results indicated that Fe3O4@SiO2-APTMS-
SAL-Pd NC showed notably catalytic activity in all the six cycles (Fig. 7). The cata-
lyst leaching test was completed; for this purpose, the ICP technique was applied,
the results of which showed an insignifcant amount of Pd loaded on Fe3O4 @
SiO2-APTMS-SAL-Pd NC after six periods of change. TEM images of the reused
catalyst, after six runs, showed that the catalyst structure was preserved and as well
the SiO2 shell is presented (Fig. 8). These results confrm the strong ability of SiO2
to protect Fe3O4 NPs and prevent them from leaching and accumulating during recy-
cling processes.
Fig. 7 Recycling of the catalyst
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Pd nanoparticles supported on Fe3O4@SiO2‑Schif…
Fig. 8 TEM images of the reusable catalyst remained unchanged after the sixth reaction of phenyl iodide
with PhB (OH)2
Conclusions
In summary, a simple and convenient process for the synthesis of Fe3O4@SiO2-
APTMS-SAL-Pd NC has been presented. Fabrication of this catalyst essentially
includes several steps, frstly, the Fe3O4 NPs were synthesized using simple chemi-
cal co-precipitation and coated with SiO2 (Fe3O4@SiO2). The surface of Fe3O4@
SiO2 NPs was modifed with several organic reagents such as APTMS and SAL
(Fe3O4@SiO2-APTMS-SAL). Finally, Pd NPs were decorated on the surface of sup-
port, Fe3O4@SiO2-APTMS-SAL-Pd NC. The synthesized nanocatalyst is character-
ized by simple methods and demonstrates excellent catalytic activity for the C–C
Suzuki–Miyaura cross-coupling reaction in an environmentally friendly solvent sys-
tem. The attractive features of this method are a clean reaction, mild experimental
conditions, kind of base, green solvent, simple procedure, high yield, as well as ease
of preparation of the catalyst. This catalyst was completely recovered from the reac-
tion mixture using an external magnet and was used several times without any loss
of activity in terms of yield and purity. The TEM images of the reusable catalyst
remained unchanged after the sixth reaction time.
Acknowledgements The authors express appreciation to University of Guilan, Faculty Research Com-
mittee for its support of this investigation.
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