Immobilized palladium on surface-modified Fe3O 4/SiO2 nanoparticles: As a magnetically separable and stable recyclable high-performance catalyst for Suzuki and Heck cross-coupling reactions

Article abstract of DOI:10.1016/j.tet.2012.03.008

A palladium-based catalyst (Fe₃O₄/SiO ₂/HPG-OPPh₂-PNP) supported on chlorodiphenylphosphine- functionalized magnetic nanoparticles was successfully prepared from Fe ₃O₄/SiO₂ with sequential attachment of glycerol and chlorodiphenylphosphine, followed by treatment of an ethanolic solution of palladium chloride with hydrazine. The as-prepared catalyst was characterized by ICP-AES, FTIR, XRD, SEM, and TEM. The Fe₃O₄/SiO ₂/HPG-OPPh₂-PNP was found as a magnetically separable and highly active catalyst for Suzuki coupling reactions of aryl iodides, bromides, and chlorides as well as Heck reactions of aryl iodides and bromides. Under appropriate conditions, all reactions afforded the desired products in moderate to excellent yields. Moreover, this catalyst can be easily recovered by using a magnetic field and directly reused for at least six cycles without significant loss of its activity.

Full text of DOI:10.1016/j.tet.2012.03.008

Tetrahedron 68 (2012) 3577e3584
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Immobilized palladium on surface-modified Fe3O4/SiO2 nanoparticles:
as a magnetically separable and stable recyclable high-performance catalyst
for Suzuki and Heck cross-coupling reactions
*
Qingwei Du, Wei Zhang, Hao Ma, Jia Zheng, Bo Zhou, Yiqun Li
Department of Chemistry, Jinan University, 510632 Guangzhou, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A palladium-based catalyst (Fe3O4/SiO2/HPGeOPPh2ePNP) supported on chlorodiphenylphosphine-
functionalized magnetic nanoparticles was successfully prepared from Fe3O4/SiO2 with sequential at-
tachment of glycerol and chlorodiphenylphosphine, followed by treatment of an ethanolic solution of
palladium chloride with hydrazine. The as-prepared catalyst was characterized by ICP-AES, FTIR, XRD,
SEM, and TEM. The Fe3O4/SiO2/HPGeOPPh2ePNP was found as a magnetically separable and highly
active catalyst for Suzuki coupling reactions of aryl iodides, bromides, and chlorides as well as Heck
reactions of aryl iodides and bromides. Under appropriate conditions, all reactions afforded the desired
products in moderate to excellent yields. Moreover, this catalyst can be easily recovered by using
a magnetic field and directly reused for at least six cycles without significant loss of its activity.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 30 December 2011
Received in revised form 26 February 2012
Accepted 2 March 2012
Available online 8 March 2012
Keywords:
Magnetic nanoparticle support
Palladium
Heterogeneous catalysts
Suzuki reaction
Heck reaction
1. Introduction
Nanoparticles (NPs) have recently emerged as viable and ap-
pealing alternatives to these conventional materials due to its robust,
It is well-known that catalytic Suzuki and Heck cross-coupling
reactions are very important and powerful chemical methods for
the formation of CeC bonds.1 It as everyone knows that homoge-
neous catalysts show higher catalytic activities than their hetero-
geneous counterparts because they can dissolve in reaction media
making all catalytic sites accessible to the substrate.2 However,
homogeneous catalysis suffers from the problematic separation of
the expensive catalyst from the product for reuse.3 Moreover, the
homogeneous palladium catalysts tend to lose their catalytic ac-
tivity because of palladium metal aggregation and precipitation.4 In
addition, many transition metal catalysts used in homogeneous
catalytic systems are quite expensive. Industrial processes based on
these new inventions are less attractive unless a new and practical
strategy for recycling active catalysts is found. In this regard, het-
erogeneous Pd-based catalysts will be a promising alternatives,
owing to their air-stability, recoverability, reusability, and non-
residual properties.5 In the last few years, numerous solid sup-
ports, such as microporous polymers,6 mesoporous silica,7 metal
oxides,8 clays,9 carbon nanofibers,10 and molecular sieves,11 have
been adopted for Pd-based heterogeneous catalysts, in which Pd
complexes or nanoparticles are involved as active species.
readily available, high surface area properties.12 NPs-supported cat-
alysts mimic their homogeneous counterparts and can be considered
as ‘soluble’ analogues since they are readily dispersed and exhibit an
intrinsically high surface area, attribute that allow excellent accessi-
bility of substrates to the surface bound active catalytic sites. Mag-
netic nanoparticles (MNPs) as catalyst supports have been attracting
more and more attention because they can be easily recovered from
the reaction mixture simply by using an external magnet, thus
eliminating the necessity of tedious centrifugation, filtration, or
membrane separation steps.13 Recently, numerous functionalized
MNPs have been employed in a range of organic transformations,
showing excellent catalytic activities in CeC coupling,14 hydrogena-
tion,15 oxidation,16 amination,17 and Ritter reactions.18
Considering the Fe3O4 particles are reactive especially in an acid
environment and thereby to lose their magnetic properties, silica as
a protecting shell was usually utilized to coat the Fe3O4 particles to
form a core-shell (Fe3O4/SiO2) structure. Meanwhile, the silica shell
can prevent the aggregation of the Fe3O4 particles and provide
numerous surface SieOH groups for further modification.19 Herein,
we report the use of modified MNPs as catalyst vehicles to support
palladium nanoparticles and its application in Suzuki cross-
coupling and Heck coupling reactions. The new catalyst system
can be well-dispersed in the reaction medium, conveniently sepa-
rated from the reaction mixture by using a magnet, and reused for
several times without significant loss of its activity.
* Corresponding author. E-mail address: tlyq@jnu.edu.cn (Y. Li).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2012.03.008

3578
Q. Du et al. / Tetrahedron 68 (2012) 3577e3584
2. Results and discussion
Fig. 1 showed the FTIR spectrum for the samples of Fe3O4 NPs,
Fe3O4/SiO2 microspheres, Fe3O4/SiO2/HPGeOPPh, and Fe3O4/SiO2/
HPGeOPPh2ePNP catalysts. In Fig. 1, the bands at 586 cmꢀ1 were
assigned to the vibration of the FeeO bonds, which demonstrate the
existence of Fe3O4 components. In addition, there was an obvious
peak at 3416 cmꢀ1 attributed tothe eOH groups. Compared with the
curve A, the bands at 960 and 1095 cmꢀ1 were assigned to the
2.1. Preparation and characterization of Fe3O4/SiO2/
HPGeOPPh2ePNP
The magnetic nanoparticle-supported Pd0 catalyst (Fe3O4/SiO2/
HPGeOPPh2ePNP), used for promoting Suzuki cross-coupling and
Heck coupling reactions, was synthesized via multiple steps from
commercial sourced ferric chloride hexahydrate and ferrous chlo-
ride tetrahydrate (Scheme 1). The first step was to prepare Fe3O4
particles according to the literature.20 Second, the obtained MNPs
were subsequently coated with silica (Fe3O4/SiO2) through the
140
Fe-O
120
Si-O
A
well-known Stober method.21 The Fe3O4/SiO2 core/shell structures
€
100
are then sequential treated by glycerol and chlorodiphenylph
osphine to form Fe3O4/SiO2/HPGeOPPh2.22 Finally, Pd0 nano-
particles (PNP) were immobilized on Fe3O4/SiO2/HPGeOPPh2 from
Fe3O4/SiO2/HPGeOPPh2ePNP through the reduction of PdCl2 by
hydrazine hydrate in ethanol. The carbon contents of Fe3O4/SiO2/
HPG are founded to be 3.62 wt % by elemental analysis. Based on
the results of the carbon contents, it is estimated that the glycerol
loading on Fe3O4/SiO2/HPG is 1.01 mmol/g. The loading amount of P
and Pd on Fe3O4/SiO2/HPGeOPPh2ePNP were found to be about
0.40 mmol/g and 0.38 mmol/g by Inductive coupled plasma-atomic
emission spectrometry (ICP-AES), respectively. The as-prepared
Fe3O4/SiO2/HPGeOPPh2ePNP catalyst was also characterized by
Fourier transform infrared spectroscopy (FTIR), X-ray diffraction
(XRD), scanning electron microscopy (SEM), and transmission
electron microscopy (TEM).
80
B
60
40
C
D
C-H
20
0
-OH
E
-20
-40
4000
3500
3000
2500
2000
1500
-1
1000
500
Wavenumber
(
cm
)
Fig. 1. FTIR spectra of Fe3O4 (A), Fe3O4/SiO2 (B), Fe3O4/SiO2/HPG (C), Fe3O4/SiO2/
HPGeOPPh2 (D), and Fe3O4/SiO2/HPGeOPPh2ePNP (E).
Scheme 1. Preparation of magnetic nanoparticle-supported nanopalladium catalyst.

Q. Du et al. / Tetrahedron 68 (2012) 3577e3584
3579
vibration of the SieO bonds, demonstrating the existence of SiO2
2.2. Room temperature Suzuki coupling of iodo- and
bromoarenes with arylboronic acids
components. After the polymerization, two obvious bands at 2860
and 2927 cmꢀ1 were observed, which were assigned to CeH
stretching, confirming the structure of Fe3O4/SiO2/HPG. Though the
IR spectrum of the Fe3O4/SiO2/HPGeOPPh2ePNP catalyst demon-
strates that almost no change occurs after immobilization of palla-
dium on the Fe3O4/SiO2/HPGeOPPh2 surface, but it was clearly
observed that the PNP was dispersed on the surface of Fe3O4/SiO2/
HPGeOPPh2 from the TEM image of the Fe3O4/SiO2/HPGeOPPh2e
PNP.
The catalytic property of the Fe3O4/SiO2/HPGeOPPh2ePNP cat-
alysts was initially tested in the Suzuki cross-coupling reaction. In
an effort to develop a better catalytic system, the influences of
solvent, base, and amount of catalyst on Suzuki cross coupling were
carefully examined at room temperature using the reaction of 4-
iodoanisole and phenylboronic acid for the model reaction. The
results are summarized in Tables 1e3, respectively.
The high angle XRD patterns of the Fe3O4 NPs, Fe3O4/SiO2 core-
shell nanospheres, Fe3O4/SiO2/HPG, and Fe3O4/SiO2/HPGeOPPh2
ePNP catalysts were shown in Fig. 2. It could be seen that the strong
As it is known, solvent plays a crucial role in the rate and the
product distribution of Suzuki coupling reactions. We screened
various solvents in the model reaction. We found that using
aqueous DMF (DMF/H2O¼1:2, v/v) as the solvent gave the highest
yield in 91% (Table 1, entries 1e8). Then, a series of bases were
taken into consideration for the coupling of 4-iodoanisole and
phenylboronic acid. K2CO3 and Cs2CO3 were found to be more ef-
fective (Table 1, entries 1 and 5). We finally chose K2CO3 as a base, as
it is much cheaper than Cs2CO3. As it can be seen from the Table 3,
with increasing the amount of catalysts (Table 3, entries 5e7) from
0.76 mol %, the yield of desired product does not further increase,
but reducing the amount of the catalyst (Table 3, entries 1e3), the
yields decreased apparently. Therefore, 0.76 mol % of the palladium
catalyst was enough to push the reaction to completion (Table 3,
entry 4).
On the basis of the optimized reaction conditions, Suzuki re-
actions of a wide range of substrates, such as aryl iodides and aryl
bromides were explored. The results were summarized in Table 4.
The results indicated that the Fe3O4/SiO2/HPGeOPPh2ePNP was an
effective catalyst for the Suzuki reaction at room temperature. All of
these reactions proceed rapidly and completed in 15 min. Aryl
halides with electron-donating groups are less active than those
with electron-withdrawing groups in the para-positions. In order to
expand the scope of this methodology, a variety of substituted
arylboronic acids were employed in this system and found them
effective in such coupling (Table 4). However, compared with aryl
halides, the substituent on arylboronic acids had not obviously
influenced on the reaction time and the product yields.
characteristic diffraction peaks at 2
q
of 30.09ꢁ, 35.44ꢁ, 43.07ꢁ,
53.43ꢁ, 56.96ꢁ, and 62.55ꢁ corresponding to the diffraction of (220),
(311), (400), (422), (511), and (440) of the Fe3O4 (JCPDS 89-3854)
were found in all samples. This result means that the core/shell
Fe3O4/SiO2 microspheres have been synthesized successfully
without damaging the crystal structure of Fe3O4 core. Compared
with the curve A, the broad peak at 2
amorphous silica shell on the surface of the MNPs. Moreover, apart
from the original peaks, the appearance of the new peaks at 2 of
q
of 23ꢁ is assigned to the
q
40.01ꢁ, 46.54ꢁ, and 67.93ꢁ, which corresponding to (111), (200), and
(220) crystalline planes of Pd was observed in the spectrum
(Fig. 2D), indicating that Pd element exists in the form of Pd0, not
Pd2þ. The broad nature of the diffraction peak clearly indicates the
Pd composition is composed of small nanocrystals, which agrees
well with the TEM analysis.
2.3. Catalytic Suzuki coupling of chloroarenes with
phenylboronic acid
Impressed with the results from iodo- and bromoarenes, we
further tried to extend the scope of aryl halides. The cross-coupling
reaction of aryl chlorides with phenylboronic acids in the presence
of 0.020 g (0.76 mol % of Pd) of catalyst was carried out at 130 ꢁC,
the results are summarized in Table 5. When using DMF as a sol-
vent, K2CO3 as a base, and 1-chloro-4-nitrobenzne as the substrate,
the corresponding 4-nitrobiphenyl was produced in good yield
(Table 5, entries 1e7). The catalytic performance was excellent for
the substrate with electron-withdrawing groups (entries 7 and 8).
This may be due to the electron-withdrawing moiety facilitating
the oxidative addition step in the catalytic cycle. However, the ef-
ficiencies were lower as using the substrates with electron-
donating groups (entries 10 and 11).
Fig. 2. XRD patterns of Fe3O4 (A), Fe3O4/SiO2 (B), Fe3O4/SiO2/HPG (C), and Fe3O4/SiO2/
HPGeOPPh2ePNP (D).
Representative SEM images of some of the materials described
in Scheme 1 were shown in Fig. 3. No apparent change in the
morphology was observed after modifying with glycerol, chlor-
odiphenylphosphine, and anchoring palladium on the surface of
the MNPs. The TEM images of the Fe3O4 NPs, Fe3O4/SiO2 core-shell
nanospheres, and Fe3O4/SiO2/HPGeOPPh2ePNP catalysts were
shown in Fig. 4. As can be seen from the Fig. 4a, the Fe3O4 NPs had
a consistent particle size range of 10e20 nm. Subsequently, silica
was coated on the Fe3O4 to form Fe3O4/SiO2 core-shell particles
(Fig. 4b) through hydrolysis of tetraethyl orthosilicate (TEOS)
2.4. Performance of catalyst for the Heck reaction
Having demonstrated that the Fe3O4/SiO2/HPGeOPPh2ePNP
catalyst was highly effective for the Suzuki reaction, its activity was
also investigated in the Heck reaction of aryl halides and olefins. In
order to find the optimal reaction conditions, a series of experi-
ments were performed by using 4-iodoanisole and styrene as
model compounds. The model reactions were conducted by using
different bases, such as Na2CO3, K2CO3, Et3N, and Bu3N and a series
of solvents, such as EtOH, DMF, DMSO, CH3CN, and THF at the
according to the Stober method.21 Moreover, after anchoring of Pd,
€
the Pd nanoparticles are clearly distinguishable with the difference
in their contrast. Two sizes of particle became clearly visible: the
smaller particles (4e8 nm) are attached to the larger particles
(10e15) nm.

3580
Q. Du et al. / Tetrahedron 68 (2012) 3577e3584
Fig. 3. SEM images of (a) Fe3O4; (b) Fe3O4/SiO2; (c) Fe3O4/SiO2/HPG; (d) Fe3O4/SiO2/HPGeOPPh2; (e) Fe3O4/SiO2/HPGeOPPh2ePNP catalysts.
Fig. 4. TEM images of (a) Fe3O4; (b) Fe3O4/SiO2; (c) Fe3O4/SiO2/HPGeOPPh2ePNP catalysts.
specific temperatures. Furthermore, the effect of the amount of
catalyst on Heck coupling was also carefully examined. The optimal
conditions of the Heck reaction were given in Tables 6 and 7,
respectively.
As shown in Table 6, the base had obviously effect on the cou-
pling. Among the screened bases, K2CO3 had been proven to the
best base. Et3N and Bu3N gave moderate yields. Na2CO3 afforded
poor yields of the product (Table 6, entries 1e4). Among the
screened solvents, DMF was founded to be the best suitable media
for this catalyst (Table 6, entry 4). Other solvents, such as EtOH,
DMSO, CH3CN, and THF, were not satisfactory (Table 6, entries 4e8).
Table 1
Effect of solvent on the Suzuki cross-couplinga
Table 2
Effect of base on the Suzuki cross-couplinga
Entry
Solvent (v/v)
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
8
95% EtOH
EtOH
H2O
DMF
DMF/H2O¼1:1
DMF/H2O¼2:1
DMF/H2O¼3:1
DMF/H2O¼4:1
DMF/H2O¼1:2
DMF/H2O¼1:3
DMF/H2O¼1:4
DMF/H2O¼1:5
5 h
5.5 h
1.5 h
24 h
30
4 h
5 h
12 h
10
83
81
52
N.R.
79
62
61
38
91
79
76
69
Entry
Base
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
8
K2CO3
Na2CO3
K3PO4$3H2O
Na3PO4$12H2O
Cs2CO3
NaOH
CH3ONa
CH3OK
10
25
55
50
10
180
40
30
91
63
88
78
90
38
71
81
9
10
11
12
15
25
25
a
a
Reaction conditions: 4-iodoanisole (1.0 mmol), phenylboronic acid (1.2 mmol),
K2CO3 (2.0 mmol), solvent (5.0 mL), and catalyst (0.015 g, 0.57 mol % of Pd) at room
temperature under air.
Reaction conditions: 4-iodoanisole (1.0 mmol), phenylboronic acid (1.2 mmol),
base (2.0 mmol), DMF/H2O (5.0 mL, v/v¼1:2), and catalyst (0.015 g, 0.57 mol % of Pd)
at room temperature under air.
b
b
Isolated yield based on 4-iodoanisole.
Isolated yield based on 4-iodoanisole.

Q. Du et al. / Tetrahedron 68 (2012) 3577e3584
3581
Table 3
Table 5
Effect of the amount of catalyst on the Suzuki cross-couplinga
Fe3O4/SiO2/HPGeOPPh2ePNP catalyzed Suzuki coupling of aryl chlorides with
phenylboronic acida
Entry
Amount of catalyst (mol % of Pd)
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
0.19
0.38
0.57
0.76
0.95
1.14
1.33
20
20
15
10
10
10
10
73
78
91
93
92
93
93
Entry Aryl chlorides
Base
Solvent
DMSO
PEG-400 58
DMF
DMF
Yieldsb (%) Product
1
2
3
4
5
6
7
8
4-NO2eC6H4Cl
4-NO2eC6H4Cl
4-NO2eC6H4Cl
4-NO2eC6H4Cl
4-NO2eC6H4Cl
4-NO2eC6H4Cl
4-NO2eC6H4Cl
4-NCeC6H4Cl
C6H5Cl
Cs2CO3
Cs2CO3
Cs2CO3
K3PO4$3H2O
45
1a
1a
1a
1a
1a
1a
1a
1m
1c
1d
1e
90
71
69
38
91
89
70
56
31
Na3PO4$12H2O DMF
CH3ONa
K2CO3
K2CO3
K2CO3
K2CO3
DMF
DMF
DMF
DMF
DMF
DMF
a
Reaction conditions: 4-iodoanisole (1.0 mmol), phenylboronic acid (1.2 mmol),
K2CO3 (2.0 mmol), DMF/H2O (5.0 mL, v/v¼1:2), and catalyst at room temperature
under air.
9
b
Isolated yield based on 4-iodoanisole.
10
11
4-MeeC6H4Cl
4-MeOeC6H4Cl K2CO3
a
Reaction conditions: aryl chloride (1.0 mmol), phenylboronic acid (1.5 mmol),
base (2.0 mmol), solvent (5.0 mL), and catalyst (0.020 g, 0.76 mol % of Pd) at 130 ꢁC
for 36 h under air.
Table 4
Fe3O4/SiO2/HPGeOPPh2ePNP catalyzed Suzuki cross-coupling reaction of aryl ha-
lides and arylboronic acid under optimized conditionsa
b
Isolated yield based on aryl chlorides.
Table 6
Effect of bases and solvents on the Heck couplinga
Entry
AreX
R2
Time (min)
Yieldsb (%)
Product
Entry
Base
Solvent
Temperature (ꢁC)
Time (h)
Yieldsb (%)
1
2
3
4
5
6
7
8
4-NO2eC6H4I
4-CleC6H4I
C6H5I
H
H
H
H
10
10
10
10
10
10
10
10
10
10
10
10
10
15
15
15
95
94
90
85
93
94
93
95
92
93
93
92
87
83
86
92
1a
1b
1c
1d
1e
1f
1g
1h
1i
1
2
3
4
5
6
7
8
Et3N
Bu3N
DMF
DMF
DMF
DMF
EtOH
DMSO
CH3CN
THF
100
100
100
100
78
100
80
67
1
1
1
1
5
12
12
12
74
83
61
90
67
79
Trace
47
4-MeeC6H4I
4-MeOeC6H4I
4-NO2eC6H4I
4-NO2eC6H4I
4-NO2eC6H4I
4-NO2eC6H4I
4-NO2eC6H4I
4-NO2eC6H4Br
3-NO2eC6H4Br
C6H5Br
Na2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
H
4-MeO
4-Me
4-Cl
4-F
4-CHO
H
H
H
H
H
9
10
11
12
13
14
15
16
1j
a
Reaction conditions: 4-iodoanisole (1.0 mmol), styrene (1.5 mmol), base
(2.0 mmol), solvent (5.0 mL), and catalyst (0.020 g, 0.76 mol % of Pd).
1a
1k
1c
1d
1e
1l
b
Isolated yield based on 4-iodoanisole.
4-MeeC6H4Br
4-MeOeC6H4Br
4-MeO2CeC6H4Br
Table 7
The effect of the amount of catalyst on Heck couplinga
H
a
Reaction conditions: aryl halides (1.0 mmol), arylboronic acid (1.2 mmol), K2CO3
(2.0 mmol), DMF/H2O (5.0 mL, v/v¼1:2), and catalyst (0.020 g, 0.76 mol % of Pd) at
room temperature under air.
b
Isolated yield based on aryl halide.
The results in Table 7 showed that excellent yields were obtained in
1 h in the presence of 0.95 mol % palladium was employed (Table 7,
entry 4). A moderate yield (51%) was obtained even at palladium
loading as low as 0.38 mol %. Further elevating the amount of
palladium to 1.14, 1.33, and 1.52 mol % could not apparently pro-
mote the reaction yields.
Under the optimized reaction conditions, nine of aryl halides
bearing both electron-donating and electron-withdrawing groups
reacted with styrene and acrylic acid, respectively, affording the cou-
pled products in excellent yields. The results were given inTable 8. The
results clearly demonstrated that the Fe3O4/SiO2/HPGeOPPh2ePNP
catalyst was effective for the Heck reaction. The catalytic activity of
Entry
Amount of catalyst (mol % of Pd)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
0.38
0.57
0.76
0.95
1.14
1.33
1.52
2
2
1.2
1
1
51
79
87
92
93
92
93
1
1
a
Reaction conditions: 4-iodoanisole (1.0 mmol), styrene (1.5 mmol), K2CO3
(2.0 mmol), DMF (5.0 mL), and catalyst at 100 ꢁC.
b
Isolated yield based on 4-iodoanisole.

3582
Q. Du et al. / Tetrahedron 68 (2012) 3577e3584
Table 8
and drying. The results are shown in Table 9. The results revealed
that the recovered catalyst had not significantly lost its activity after
The coupling reactions of aryl halides with olefinsa
six cycles.
Table 9
Reusabilities of the Fe3O4/SiO2/HPGeOPPh2ePNP catalyst
Entry
AreX
R2
Time (h)
Yieldsb (%)
Product
1
2
3
4
5
6
7
8
4-NO2eC6H4I
4-NO2eC6H4I
4-CleC6H4I
4-CleC6H4I
C6H5I
Ph
COOH
Ph
COOH
Ph
COOH
Ph
COOH
Ph
COOH
Ph
COOH
Ph
COOH
Ph
1
1
1
1
1
1
1
1
1
1
1
1
1.5
1.5
2
2
2
95
93
91
92
87
89
88
86
91
92
92
93
85
84
74
61
83
78
2a
2b
2c
2d
2e
2f
2g
2h
2i
C6H5I
4-MeeC6H4I
4-MeeC6H4I
4-MeOeC6H4I
4-MeOeC6H4I
4-NO2eC6H4Br
4-NO2eC6H4Br
C6H5Br
Run
Yield 1a
(Suzuki reaction)
Yield 2b
(Heck reaction)
Magnetic
catalyst
ICP analysis
(mmol/g of Pd)
9
10
11
12
13
14
15
16
17
18
2j
First
Second
Third
93
93
92
91
92
90
Fresh
0.380
2a
2b
2e
2f
2g
2h
2i
Reused
0.371
catalyst 1c
C6H5Br
Fourth
Fifth
93
91
91
89
4-MeeC6H4Br
4-MeeC6H4Br
4-MeOeC6H4Br
4-MeOeC6H4Br
Reused
0.363
COOH
Ph
COOH
catalyst 2d
Sixth
92
91
2
2j
a
Reaction conditions: 4-iodoanisole (1.0 mmol), phenylboronic acid (1.2 mmol),
K2CO3 (2.0 mmol), DMF/H2O (5.0 mL, v/v¼1:2), and catalyst (0.020 g, 0.76 mol % of
Pd) at room temperature under air.
a
Reaction conditions: aryl halides (1.0 mmol), olefins (1.5 mmol), K2CO3
(2.0 mmol), DMF (5.0 mL), and catalyst (0.025 g, 0.95 mol % of Pd) at 100 ꢁC.
b
Isolated yield based on aryl halide.
b
Reaction conditions: 4-iodoanisole (1.0 mmol), styrene (1.5 mmol), K2CO3
(2.0 mmol), DMF (5.0 mL), and catalyst (0.025 g, 0.95 mol % of Pd) at 100 ꢁC under air.
c
Fe3O4/SiO2/HPGeOPPh2ePNP depended on the type of halides. All
aryl iodides were rapidly converted to the corresponding products in
excellent yields (Table 8, entries 1e10). Both electron-rich and
electron-deficient aryl bromides also proceeded smoothly to afford
the desired products with excellent yields (Table 8, entries 11e18). The
reactivity of aryl bromides with electron-withdrawing substituent
(Table 8, entries 11 and 12) was higher than those with electron-
donating substituent (Table 8, entries 15e18).
After six cycles of reusability for Suzuki reaction.
After six cycles of reusability for Heck reaction.
d
Metal leaching of the catalyst was further investigated by ICP-
AES. The results indicated that only a trace amount of the Pd
metal is leached out from the catalyst surface after six cycles (for
both Suzuki and Heck reactions) (Table 9). Furthermore, the filtered
solution exhibited no reactivity to both Suzuki and Heck reactions.
The TEM images of the catalyst showed that the morphology and
size of the catalyst after six recycling had no apparently agglom-
eration and changes (Fig. 5b and c).
2.5. Recycling of the Fe3O4/SiO2/HPGeOPPh2ePNP catalyst for
the Suzuki and Heck reactions
For practical applications of heterogeneous catalyst, the re-
usability of catalyst is a very important factor. To clarify this issue,
catalytic recycling experiments were carried out using a Suzuki
reaction of 4-iodoanisole and phenylboronic acid, and a Heck re-
action of 4-iodoanisole and styrene as model reactions. After the
completion of reaction, the magnetic catalyst could be conveniently
and efficiently recovered from the reaction mixture with an external
magnet, it can be used in the next run after washing with ethanol
3. Conclusions
In conclusion, a nanosized Fe3O4/SiO2/HPGeOPPh2ePNP cata-
lyst with a high magnetic and excellent dispersibility was prepared
from Fe3O4/SiO2 with sequential attachment of glycerol and chlor-
odiphenylphosphine, followed by treatment with hydrazine in an
ethanolic solution of palladium chloride. The catalytic activities of
this novel magnetic palladium catalyst were investigated in Suzuki
Fig. 5. (a) Photographs showing the magnetic separation of the catalyst; (b) TEM images of the catalyst after six cycles of Suzuki reactions; (c) TEM images of the catalyst after six
cycles of Heck reactions.

Q. Du et al. / Tetrahedron 68 (2012) 3577e3584
3583
cross-coupling reaction and Heck cross-coupling. Compared to
other magnetic Pd-loaded catalysts reported previously in litera-
ture, Fe3O4/SiO2/HPGeOPPh2ePNP showed greatly high catalytic
activities for both Suzuki and Heck reactions in the presence of low
Pd loading (0.76 mol % for Suzuki coupling and 0.95 mol % for Heck
coupling, respectively). Moreover, the catalyst can be conveniently
separated and recovered from the reaction system by a magnet, and
can be reused for at least six times without noticeable lose of its
activity. In addition, the new magnetic catalyst is more economic
and environmentally friendly because of its low Pd leaching after six
cycles. We expect that this novel heterogeneous magnetic nano-
catalyst has more wide applications in Pd-catalyzed reactions.
washing and separation steps, the resulting solid was dried over-
night in avacuum to give the Fe3O4/SiO2/HPG. Then, Fe3O4/SiO2/HPG
(1.0 g), Et3N (10 mL), and toluene (50 mL) were placed in a dry flask
and irradiated by ultrasonic for 15 min. After that, diphenyl-
chlorophosphine (5 mL) was added dropwisewhilst stirring, and the
solutionwas stirred additionally for 16 h. The product was separated
by a magnet and washed with ethanol repeatedly and dried under
vacuum at 80 ꢁC to give Fe3O4/SiO2/HPGeOPPh2.
4.4. Loading and characterizing of Pd NPs on Fe3O4/SiO2/
HPGeOPPh2 NPs
The as-synthesized Fe3O4/SiO2/HPGeOPPh2 (1.0 g) and PdCl2
(0.1 g) were first dispersed in an ethanol solution (30 mL) under
ultrasonication for 0.5 h. Then, hydrazine hydrate solution (0.3 mL)
was slowly dropped into the above mixture under reflux and the
solution was stirred further for 12 h. The Fe3O4/SiO2/
HPGeOPPh2ePNP were collected by an external magnet and
washed with excess of deionized water for 4e5 times and finally
with ethanol and dried under vacuum. The as-prepared catalyst was
characterized by ICP-AES, FTIR, XRD, SEM, and TEM. The high angle
XRD patterns and TEM image of the Fe3O4/SiO2/HPGeOPPh2ePNP
catalyst clearly showed that the PNP was dispersed on the surface of
Fe3O4/SiO2/HPGeOPPh2. The Pd loading was determined to be
0.38 mmol/g operated by the Analytical and Testing Center of Jinan
University using an inductively coupled plasma-atomic emission
spectrometry (ICP-AES). In a typical procedure, a Fe3O4/SiO2/
HPGeOPPh2ePNP catalyst (15 mg) was dissolved in 30 mL of freshly
prepared aqua regia (30 mL) by stirring and the volume was brought
to 100 mL with distilled water. Subsequently, the Pd content was
determined with the aid of the ICP-AES technique based on stan-
dard method (JY/T 015e1996).
4. Experimental section
4.1. General
Melting points were measured on an Electrothermal X6 mi-
croscopic digital melting point apparatus. IR spectra were recorded
on a Nicolet 6700 spectrometer using KBr pellets. 1H NMR spectra
were recorded on a 300 MHz or 500 MHz Bruker Avance in-
strument using CDCl3 or DMSO as solvent and TMS as internal
standard. Elemental analyses were performed on a PerkineElmer
EA2400II elemental analyzer. The elemental palladium content of
the catalysts was determined by PerkineElmer Optima 2000DV
inductively coupled plasma (ICP) spectroscopy. Scanning electron
microscopy (SEM) was performed on a Philips XL 30ESEM in-
strument. Transmission electron microscopy (TEM) was performed
with a Philips Tecnai instrument operating at 40e100 kV. The
chemicals were obtained from commercial sources and used di-
rectly as received.
4.2. Synthesis of silica-coated magnetite nanoparticles
4.5. General procedure for the Suzuki coupling of bromo- and
First, Fe3O4 MNPs were prepared by the coprecipitation
method.20 FeCl2$4H2O (9.94 g, 0.05 mmol) and FeCl3$6H2O (16.2 g,
0.06 mmol) were mixed in an aqueous solution (200 mL). Ammonia
solution (25 wt %) was added into the mixture to adjust the pH
value to 9 while vigorous stirring. The resulted mixture was kept at
room temperature for 12 h. MNPs thus synthesized were separated
from the solution with a permanent magnet and washed several
times with water until the pH value was 7. Then, the MNPs were
dried in a vacuum oven at 80 ꢁC for 24 h. Next, the obtained MNPs
iodoarenes using Fe3O4/SiO2/HPGeOPPh2ePNP as catalyst
Aryl halide (1.0 mmol), arylboronic acid (1.2 mmol), K2CO3
(2.0 mmol), and Pd-containing catalyst (0.76 mol % Pd) were added
to a 25 mL round-bottom flask equipped with a magnetic stirring
bar. DMF/H2O (5.0 mL, v/v¼1:2) were added, and the reaction
mixture was stirred at room temperature. The completion of re-
action was judged by TLC, the catalyst was separated with a magnet
and washed with diethyl ether (2ꢂ10 mL), deionized water
(2ꢂ10 mL), and then dried under vacuum for the next run. The
aqueous phase was extracted with diethyl ether (2ꢂ15 mL) and the
combined organic phases were concentrated. These crude products
were further purified by recrystallization.
were subsequently coated with silica through the well-known
21
€
Stober method. Fe3O4 MNPs (1.0 g) was dispersed in a mixture
of isopropanol (250 mL) and water (10 mL) by ultrasonic irradia-
tion. After addition of 40 mL of ammonia solution (25 wt %),
a mixture of TEOS (3.0 mL) and isopropanol (50 mL) was added
dropwise to the reaction solution with mechanical stirring at room
temperature. The suspension was mechanically stirred vigorously
for 18 h at room temperature. The resulting Fe3O4/SiO2 particles
were washed by deionized water and ethanol and then collected by
an external magnet. Finally, the Fe3O4/SiO2 particles were dried in
a vacuum oven at 80 ꢁC for 24 h.
4.6. General procedure for the Suzuki coupling of aryl
chlorides and phenylboronic acid
A mixture of aryl chloride (1 mmol), phenylboronic acid
(1.2 mmol), K2CO3 (2.0 mmol), DMF (5.0 mL), and the solid catalyst
was charged in a 25 mL round-bottom flask and stirred at 130 ꢁC.
The end of reaction was tracked by TLC, the catalyst was separated
with a magnet and washed with diethyl ether (2ꢂ10 mL), deionized
water (2ꢂ10 mL), and then dried under vacuum for the next run.
The aqueous phase was extracted with diethyl ether (2ꢂ15 mL) and
the combined organic phases were concentrated. These crude
products were further purified by recrystallization.
4.3. Synthesis of glycerol and chlorodiphenylphosphine-
functionalized magnetite nanoparticles
Fe3O4/SiO2 nanoparticles (1.0 g) were mixed with CH3OK
(2.345 mmol) and anhydrous THF (25 mL) in a flask. The mixturewas
stirred for 1 h, and then anhydrous dioxane (100 mL) was added, and
the flask was kept in an oil bath at 95 ꢁC. Glycidol (67.675 mmol) was
added dropwise over a period of 15 h. After completion of the
monomer addition, the mixture was stirred for an additional 1 h. The
mixture was quenched and dispersed in ethanol and subsequently
separated by a magnet and washed with ethanol. After repeated
4.7. General procedure for the Heck coupling of styrene with
aryl halides
To the Heck coupling reactions of styrene with aryl halides, the
aryl halides (1.0 mmol), styrene (1.5 mmol), K2CO3 (2.0 mmol), and

3584
Q. Du et al. / Tetrahedron 68 (2012) 3577e3584
the catalytic amount of catalyst (0.95 mol % Pd) were added into
DMF (5.0 mL). The reaction mixture was stirred at 100 ꢁC. The end
of reaction was monitored by TLC. After completion of the reaction,
the mixture was cooled to room temperature, and the catalysts
were separated with a magnet and washed with diethyl ether
(2ꢂ10 mL), deionized water (2ꢂ10 mL), and dried under vacuum for
the next run. The resultant residual mixture diluted with H2O
(20 mL), followed by extraction twice (2ꢂ15 mL) with diethyl ether.
The organic fraction was dried over MgSO4, and crude product was
obtained after removing the ethereal solution by a rotary evapo-
rator. The desired pure product was further purified by re-
crystallization with ethanol.
References and notes
1. (a) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457e2483; (b) Heck, R. F. Acc.
Chem. Res. 1979, 12, 146e151; (c) Dounay, A. B.; Overman, L. E. Chem. Rev. 2003,
103, 2945e2963.
2. (a) Corma, A.; Garcia, H. Adv. Synth. Catal. 2006, 348, 1391e1412; (b) Widegren,
J. A.; Finke, R. G. J. Mol. Catal. A: Chem. 2003, 198, 317e341.
3. Garrett, C. E.; Prasad, K. Adv. Synth. Catal. 2004, 346, 889e900.
4. (a) Reetz, M. T.; de Vries, J. G. Chem. Commun. 2004, 1559e1563; (b) Astruc, D.
Inorg. Chem. 2007, 46, 1884e1894.
5. Yin, L. X.; Liebscher, J. Chem. Rev. 2007, 107, 133e173.
6. Ogasawara, S.; Kato, S. J. Am. Chem. Soc. 2010, 132, 4608e4613.
7. (a) Karimi, B.; Abedi, S.; Clark, J. H.; Budarin, V. Angew. Chem., Int. Ed. 2006, 45,
4776e4779; (b) Polshettiwar, V.; Hesemann, P.; Moreau, J. J. E. Tetrahedron
2007, 63, 6784e6790; (c) Nezhad, A. K.; Panahi, F. Green Chem. 2011, 13,
2408e2415.
€
€
8. Prockl, S. S.; Kleist, W.; Gruber, M. A.; Kohler, K. Angew. Chem., Int. Ed. 2004, 43,
1881e1882.
4.8. General procedure for the Heck coupling of acrylic acid
with aryl halides
ꢀ
9. Poyatos, M.; Marquez, F.; Peris, E.; Claver, C.; Fernandez, E. New J. Chem. 2003,
27, 425e431.
10. Zhu, J.; Zhou, J. H.; Zhao, T. J.; Zhou, X. G.; Chen, D.; Yuan, W. K. Appl. Catal., A
2009, 352, 243e250.
In a typical experiment, a mixture of aryl halide (1.0 mmol),
acrylic acid (1.5 mmol), K2CO3 (3.0 mmol), DMF (5.0 mL), and Fe3O4/
SiO2/HPGeOPPh2ePNP catalyst (0.025 g, 0.95 mol % Pd) was
charged in a 25 mL round-bottom flask and stirred at 100 ꢁC. The
end of reaction was monitored by TLC. After completion of the re-
action, the mixture was cooled down to room temperature and the
catalyst was separated with a magnet and washed with diethyl
ether (2ꢂ10 mL), deionized water (2ꢂ10 mL), and dried under
vacuum for the next run. The resultant residual mixture diluted
with H2O (20 mL) and aqueous layer was acidified using diluted
aqueous HCl. The organic material was then extracted twice
(2ꢂ15 mL) with diethyl ether. The organic fraction was dried over
MgSO4, and removing the solvent leaving the crude product. The
pure product was obtained by recrystallization from ethanol.
11. Venkatesan, C.; Singh, A. P. J. Catal. 2004, 227, 148e163.
€
12. (a) Wittmann, S.; Schatz, A.; Grass, R. N.; Stark, W. J.; Reiser, O. Angew. Chem.,
Int. Ed. 2010, 49, 1867e1870; (b) Coker, V. S.; Bennett, J. A.; Telling, N. D.;
Henkel, T.; Charnock, J.; Laan, G. v.; Pattrick, R. A. D.; Pearce, C. I.; Cutting, R. S.;
Shannon, I. J.; Wood, J.; Arenholz, E.; Lyon, I. C.; Lloyd, J. R. ACS Nano 2010, 4,
2577e2584; (c) Islama, R. U.; Witcomb, M. J.; Scurrell, M. S.; Otterlo, W. V.;
€
Mallick, K. Catal. Commun. 2010, 12, 116e121; (d) Shylesh, S.; Schunemann, V.;
Thiel, W. R. Angew. Chem., Int. Ed. 2010, 49, 3428e3459; (e) Hutchings, G. J.
J. Mater. Chem. 2009, 19, 1222e1235.
13. (a) Hu, A.; Yee, G. T.; Lin, W. B. J. Am. Chem. Soc. 2005, 127, 12486e12487; (b)
Phan, N. T. S.; Gill, C. S.; Nguyen, J. V.; Zhang, Z. J.; Jones, C. W. Angew. Chem., Int.
Ed. 2006, 45, 2209e2212; (c) Abu-Reziq, R.; Alper, H.; Wang, D.; Post, M. L. J. Am.
ꢀ
ꢀ
Chem. Soc. 2006, 128, 5279e5282; (d) Dalaigh, C. o.; Corr, S. A.; Gun’ko, Y.;
Connon, S. J. Angew. Chem., Int. Ed. 2007, 46, 4329e4332; (e) Lu, A. H.; Salabas,
€
E. L.; Schuth, F. Angew. Chem., Int. Ed. 2007, 46, 1222e1244; (f) Shi, F.; Tse, M. K.;
€
€
€
Zhou, S. L.; Pohl, M. M.; Radnik, J.; Hubner, S.; Jahnisch, K.; Bruckner, A.; Beller,
M. J. Am. Chem. Soc. 2009, 131, 1775e1779; (g) Luque, R.; Baruwati, B.; Varma, R.
S. Green Chem. 2010, 12, 1540e1543; (h) Lee, J.; Lee, Y.; Youn, J. K.; Na, H. B.; Yu,
T.; Kim, H.; Lee, S. M.; Koo, Y. M.; Kwak, J. H.; Park, H. G.; Chang, H. N.; Hwang,
M.; Park, J. G.; Kim, J.; Hyeon, T. Small 2008, 4, 143e152.
4.9. Catalyst recycling
14. (a) Xuan, S. H.; Jiang, W. Q.; Gong, X. L. Dalton. Trans. 2011, 40, 7827e7830; (b)
Liu, J. M.; Peng, X. G.; Sun, W.; Zhao, Y. W.; Xia, C. G. Org. Lett. 2008, 10,
3933e3936; (c) Baruwati, B.; Guin, D.; Manorama, S. V. Org. Lett. 2007, 9,
5377e5380; (d) Wang, Z. F.; Xiao, P. F.; Shen, B.; He, N. Y. Colloid. Surface A 2006,
276, 116e121; (e) Stevens, P. D.; Li, G. F.; Fan, J. d.; Yen, M.; Gao, Y. Chem.
Commun. 2005, 4435e4437; (f) Zheng, Y.; Stevens, P. D.; Gao, Y. J. Org. Chem.
2006, 71, 537e542.
After completion of the reaction, the catalyst was separated by
magnet (Fig. 5a) and extensively rinsed with diethyl ether
(2ꢂ10 mL), deionized water (2ꢂ10 mL), and dried under vacuum for
the next run. The catalyst recycling procedure was the same
mentioned in Sections 4.5 and 4.7.
15. (a) Yi, D. K.; Lee, S. S.; Ying, J. Y. Chem. Mater. 2006, 18, 2459e2461; (b) Panella,
B.; Vargas, A.; Baiker, A. J. Catal. 2009, 261, 88e93; (c) Zhang, J. L.; Wang, Y.;
Ji, H.; Wei, Y. G.; Wu, N. Z.; Zuo, B. J.; Wang, Q. L. J. Catal. 2005, 229, 114e118.
16. Polshettiwar, V.; Varma, R. S. Org. Biomol. Chem. 2009, 7, 37e40.
17. Luo, S. Z.; Zheng, X. X.; Cheng, J. P. Chem. Commun. 2008, 5719e5721.
18. Ma’mani, L.; Heydari, A.; Sheykhan, M. Appl. Catal., A 2010, 384, 122e127.
19. (a) Leng, Y. H.; Sato, K.; Shi, Y. G.; Li, J. G.; Ishigaki, T.; Yoshida, T.; Kamiya, H.
J. Phys. Chem. C 2009, 113, 16681e16685; (b) Morel, A. L.; Nikitenko, S. I.;
Gionnet, K.; Wattiaux, A.; Lai-Kee-Him, J.; Labrugere, C.; Chevalier, B.; Deleris,
G.; Petibois, C.; Brisson, A.; Simonoff, M. ACS Nano 2008, 2, 847e856.
20. (a) Nishioa, K.; Ikedaa, M.; Gokonb, N.; Tsubouchi, S.; Narimatsua, H.; Mochi-
zuki, Y.; Sakamotoa, S.; Sandhuc, A.; Abe, M.; Handa, H. J. Magn. Magn. Mater.
2007, 310, 2408e2410; (b) Shao, D. D.; Xu, K. K.; Song, X. J.; Hu, J. H.; Yang, W. L.;
Wang, C. C. J. Colloid Interface Sci. 2009, 336, 526e532.
Acknowledgements
We are grateful to the National Natural Science Foundation of
China (Nos. 21072077 and 20672046), the Guangdong Natural
Science Foundation (Nos. 10151063201000051 and 81510632010
00016), the Guangzhou Science & Technology Project (2010Y1-
C511), and the Fundamental Research Funds for the Central Uni-
versities (No. 11611712) for financial support.
€
21. Stober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62e69.
22. (a) Khan, M.; Huck, W. T. S. Macromolecules 2003, 36, 5088e5093; (b) Zhou, L.;
Gao, C.; Xu, W. G. ACS Appl. Mater. Inter. 2010, 2, 1483e1491; (c) Zhou, L.; Gao,
C.; Xu, W. G.; Wang, X.; Xu, Y. H. Biomacromolecules 2009, 10, 1865e1874; (d)
Zhang, B. S.; Wang, W.; Shao, D. D.; Hao, X. Q.; Gong, J. F.; Song, M. P. Bio-
macromolecules 2010, 29, 2579e2587; (e) Zhang, B. S.; Wang, C.; Gong, J. F.;
Song, M. P. J. Organomet. Chem. 2009, 694, 2555e2561.
Supplementary data
Supplementary data related to this article can be found online at
doi:10.1016/j.tet.2012.03.008.