Magnetite nanoparticles immobilized Salen Pd (II) as a green catalyst for Suzuki reaction

Article abstract of DOI:10.1016/j.catcom.2012.05.026

In this work, we described a methodology to immobilize Salen Pd (II) complex on the surface of magnetite nanoparticles. The palladium catalyst exhibited efficient catalytic activity in coupling reactions with aryl iodides or bromides and phenylboronic aci

Full text of DOI:10.1016/j.catcom.2012.05.026

Catalysis Communications 26 (2012) 199–203
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Magnetite nanoparticles immobilized Salen Pd (II) as a green catalyst for
Suzuki reaction
a,
Xiaodong Jin a, Kunyu Zhang b, Jian Sun a, Jia Wang a, Zhengping Dong a, , Rong Li
⁎
⁎
a
The Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000,
PR China
b
School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 May 2012
Received in revised form 30 May 2012
Accepted 31 May 2012
Available online 8 June 2012
In this work, we described a methodology to immobilize Salen Pd (II) complex on the surface of magnetite
nanoparticles. The palladium catalyst exhibited efficient catalytic activity in coupling reactions with aryl
iodides or bromides and phenylboronic acid at 100 °C for 1–3 h under DMF/H2O. Furthermore, the catalyst
could be magnetically isolated with a permanent magnet and the yields achieved above 85% after reused at
least 5 times.
© 2012 Elsevier B.V. All rights reserved.
Keywords:
Palladium
Magnetite
Recycling
Suzuki coupling reaction
1. Introduction
nanoparticles are easily prepared and surface functionalized and they
can be recycled from the solution by external magnetic field. Hence,
The palladium-catalyzed Suzuki and Heck reactions have been
shown as an efficient method for the construction of C\C bonds
and play an important role in pharmaceutical industry and organic
synthesis [1,2]. High activity for coupling reaction can be achieved
using homogeneous palladium catalysts. Therefore, it is not surprising
that much efforts have been devoted to the development of homoge-
neous systems for this reaction, such as phosphine ligands [3], N-
heterocyclic carbenes [4], bidentate ligands [5], pyridines [6], and
simple amines [7]. Schiff bases are an important class of ligand and
have been studied extensively. A few reports investigating the
viability of Schiff-type ligands for coupling reactions exist, the Salen
Pd (II) complex were found to act as efficient catalyst in coupling
reactions [8–10].
the catalyst supported on Fe3O4 nanoparticles can be easily separated
from the reaction system and reused [18,19,22,24]. In addition, the
reported coupling reactions were mostly carried out in organic solvents,
to begin with environmental benign, the development of highly efficient
heterogeneous catalysts to facilitate coupling reaction in benign medium
is highly desirable [16,20].
In this work, our interest in this area led us to explore the Salen Pd
(II) complex immobilized onto the surface of magnetic nanoparticles,
which can be sufficiently applied even for Suzuki reaction under envi-
ronmentally friendly solvent, and then can be easily separated from
the reaction mixture to reuse. Herein, we wish to describe the synthe-
sis of reusable magnetite nanoparticles supported Salen Pd (II) com-
plex and its catalytic activity in the Suzuki reaction.
However, many noble metals used in homogeneous system are quite
expensive and it is difficult to separate the soluble catalyst from the
reaction system. To address these problems, heterogenization of the ho-
mogeneous catalyst is a promising option. Thus, efforts have been made
to immobilize them on diverse soluble and insoluble supports, such as
inorganic solids [11–15], polymers [16,17], and nanoparticles [18–24].
Nowadays, magnetite nanoparticles, especially Fe3O4 nanoparticles
have attracted increasing interest because of their unique physical prop-
erties including the high surface area, superparamagnetism, low toxicity
and their potential applications in various fields [25–27]. The Fe3O4
2. Experimental
2.1. Characterization methods
Pd content of the catalyst was measured by inductively coupled plas-
ma (ICP) on IRIS Advantage analyzer. FT-IR spectra were recorded on a
Nicolet NEXUS 670 FT‐IR spectrometer with a DTGS detector, and sam-
ples were measured with KBr pellets. The size and morphology of the
magnetic nanoparticles were observed by a Hitachi-600 electron micro-
scope, with an accelerating voltage of 100 kV. XRD measurements were
performed on a Rigaku D/max-2400 diffractometer using Cu-Kα radia-
tion as the X-ray source in the 2θ range of 10–80°. Magnetic measure-
ments of samples were investigated with a Quantum Design vibrating
⁎
Corresponding authors. Tel.: +86 931 891 3597; fax: +86 931 891 2582.
E-mail addresses: dongzhp@lzu.edu.cn (Z. Dong), liyirong@lzu.edu.cn (R. Li).
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2012.05.026

200
X. Jin et al. / Catalysis Communications 26 (2012) 199–203
Scheme 1. Preparation of palladium catalyst.
sample magnetometer (VSM) at room temperature in an applied mag-
netic field sweeping from −15 to 15 kOe.
The mixture was stirred at 100 °C in an air atmosphere and for re-
quired time. After reaction completion, the catalyst was recovered
magnetically by placing a magnet, and the solvent was evaporated
to remove water. The products were collected and analyzed by GC
(P.E. AutoSystem XL) or GC–MS (Agilent 6890 N/5973 N). The sepa-
rated catalyst washed with absolute ethanol could be reused by addi-
tion of new portions substrate after drying at 50 °C under vacuum.
2.2. Catalyst preparation
All chemicals were of analytical grade and used as received with-
out further purification. The magnetite nanoparticles (MNPs) were
synthesized according to the method reported previously [25]. The
silica coated magnetite nanoparticles (SMNPs) were prepared
through a reverse microemulsion method [26].
3. Results and discussion
3.1. Catalyst preparation and characterization
2.2.1. Preparation of Salen complex modified Fe3O4@SiO2 (SMNPs-Salen)
At first, the Salen complex was synthesized by the following
procedure [28]. 5.0 mmol (610.6 mg) of salicylaldehyde was added in
30 ml of absolute methanol, to which 3-aminopropyltrimethoxysilane
(5.0 mmol, 1107.0 mg) was then added. After 3 h, 0.4 g of as-prepared
Fe3O4@SiO2 microspheres was then added and stirred for 6 h, the
Fe3O4@SiO2 immobilized Salen complex was obtained.
The immobilization of the Salen Pd (II) complex onto the surface
of SMNPs consists of the following steps as shown in Scheme 1. Palla-
dium contents (analyzed by ICP-AES): 3.85 wt.%. Fig. 1 shows FT-IR
spectra of SMNPs, SMNPs-Salen and SMNPs-Salen Pd (II). The obser-
vation of two broad bonds at around 460–590 cm−1 indicates the
presence of magnetite nanoparticles. The silica coated magnetite
nanoparticles are confirmed by observation of a broad band about
1000–1100 cm−1. In Fig. 1(c), the subtractive spectra show bands at
1628 cm−1 and 1447 cm−1 due to C_N and C\O stretching vibra-
tions. Some weak bands at 1400–1500 cm−1 assign to stretching vi-
brations of aromatic rings of Salen complex are observed, and that
are not present in parent SMNPs. The bands at 2937 cm−1 and
1638 cm−1 upon reaction of Salen complex with Pd shifted to lower
frequency (2928 cm−1, 1628 cm−1) indicate to form Pd complex.
Also, weak bands at 755 cm−1 and 1193 cm−1 in Fig. 1(c) are charac-
teristic of the presence of Pd and formation of SMNPs-Salen Pd (II)
nanomaterials.
2.2.2. Preparation of palladium catalyst (SMNPs-Salen Pd (II))
For preparation of Fe3O4@SiO2 nanoparticles immobilized Salen
Pd (II), a solution of PdCl2 (0.5 mmol, 88.5 mg) in absolute methanol
(30 ml) was dropwise added to the above obtained solution of
Fe3O4@SiO2 immobilized Salen complex, and the mixture was stirred
overnight. The resulting product was separated magnetically, washed
with absolute ethanol 3 times and dried at 50 °C under vacuum.
2.3. Catalytic Suzuki reactions
In a typical procedure, aryl halide (1.0 mmol), phenylboronic acid
(1.5 mmol), K2CO3 (2.0 mmol), DMF/H2O as solvent (5.0 ml, v/
v=1:1) and the catalyst were taken in a 10 ml round bottom flask.
The TEM image in Fig. 2 shows that the SMNPs (Fig. 2(a)) and
SMNPs-Salen Pd (II) (Fig. 2(b)) are of uniform spherical shape. How-
ever, aggregation gives rise to increasing the size of observed
nanoparticles as seen in the TEM image (Fig. 2(b)).
The XRD pattern of palladium catalyst confirms the presence of
crystalline Fe3O4 by the corresponding reflections of (220), (311),
(400), (422), (511) and (440) crystal planes as indicated in Fig. 3.
Comparing with XRD pattern of SMNPs (Fig. 3(a)) and SMNPs-Salen
Pd (II) (Fig. 3(b)), immobilization palladium complex on the surface
of Fe3O4 nanoparticles did not significantly affect the structure of
nanoparticles.
Fig. 4 shows the magnetization curves of the samples. It can be
seen that the saturation of the obtained catalyst decreases from 25
to 16 emu/g for the SMNPs. The superparamagnetic properties of
the prepared SMNPs-Salen Pd (II) are attractive for their application.
3.2. Catalytic Suzuki reaction
In this study, we first tested SMNPs-Salen Pd (II) catalyst in the
Suzuki reaction of iodobenzene with phenylboronic acid in DMF as
a model reaction. The coupling reaction in the presence of 1.0 mol%
Fig. 1. FT-IR spectra of (a) SMNPs, (b) SMNPs-Salen and (c) SMNPs-Salen Pd (II).

X. Jin et al. / Catalysis Communications 26 (2012) 199–203
201
Fig. 2. TEM images of (a) SMNPs, (b) SMNPs-Salen Pd (II) and (c) SMNPs-Salen Pd (II) after the five cycles.
catalyst and K2CO3 as a base at 100 °C for 2 h afforded the corresponding
product in excellent yield (entry 1). We next investigated the reaction in
DMF/H2O, as shown in Table 1, after 2 h heating, the best yields were
obtained in the presence of 1.0–0.5 mol% catalyst at 100 °C (entries
2–3), when the loading of catalyst was decreased to 0.1 mol%, the yield
was influenced (entry 4). Upon changing the temperature and time, the
reactions were coupled well with only 0.5 mol% of catalyst at 100 °C for
1 h (entries 5–7). And then, we obtained the similar results under the
same conditions in EtOH (entries 8–9). It is remarkable that 85% of yield
was obtained in the presence of 0.5 mol% catalyst at 130 °C for 2 h in
water (entry 11). The results were summarized in Table 1, the best cata-
lytic activity of the catalyst was found in DMF/H2O solvent together with
K2CO3 for required time, the optimized condition was thus used for all fur-
ther studies.
Further, we investigated the general applicability of SMNPs-Salen Pd
(II) in the coupling of various aryl halides under the optimized condition.
As summarized in Table 2, the coupling of deactivated aryl iodides
proceeded smoothly at 100 °C for 1 h and the corresponding products
obtained high yields (entries 1–6). We further explored the catalytic ac-
tivity for the coupling of aryl bromides, lengthen reaction time to 3 h,
aryl bromides and their derivatives efficiently reacted with phenylboronic
acids, to yield Suzuki products in good to excellent yields (entries 7–12). It
is worth to mention that aryl iodides and bromides bearing an electron-
withdrawing group obtained excellent yields (entries 4, 10–11, and 14).
Meanwhile, for aryl iodides and bromides with electron-donating
groups such as –CH3, –OCH3 and –OH, good to highly yields were
achieved (entries 2–3, 5–6, 8–9, and 13), however, the reaction of
4-bromoanilines was observed a moderate yield (85%) (entry
12). Besides, the protocol tolerated the presence of functional groups
in the -ortho and -meta position of the aryl halides (entries 6 and
13–14). Then, in order to test the feasibility of this protocol for challeng-
ing substrates, we conducted the coupling of several aryl chlorides with
phenylboronic acid, however, poor yields were obtained in the reactions
with 0.5 mol% or 1.0 mol% under 100–130 °C for 6 h (entries 15–19). In-
terestingly, it is worth noting that the coupling of 4-chloroacetophenone
possessing an electron-withdrawing group with phenylboronic acid
gave the product in 43% yield in the presence of 0.5 mol% catalyst at
130 °C for 6 h (entry 19).
Fig. 3. XRD pattern of (a) SMNPs and (b) SMNPs-Salen Pd (II).
Table 1
The Suzuki reaction of iodobenzene with phenylboronic acid using Pd catalyst.a
Entry Solvent
Catalyst (mol%) Temp (°C) Time (h) Yield (%)b
1
2
3
4
5
6
7
DMF
1.0
100
100
100
100
100
100
80
2
2
2
2
1
0.5
1
100
100
100
64
100
73
DMF/H2O (v/v=1:1) 1.0
0.5
0.1
0.5
0.5
0.5
46
8
9
10
11
12
EtOH
H2O
1.0
0.5
0.5
0.5
0.5
100
100
100
130
130
2
1
2
2
100
96
41
85
62
1
a
Reaction condition: iodobenzene (1.0 mmol), phenylboronic (1.5 mmol), K2CO3
Fig. 4. Room temperature magnetization curve of (a) SMNPs and (b) SMNPs-Salen Pd
(II).
(2.0 mmol), solvent (5.0 ml), Pd catalyst (0.1–1.0 mol%), and 100–130 °C.
b
Yield was determined by GC analysis.

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X. Jin et al. / Catalysis Communications 26 (2012) 199–203
Table 2
The Suzuki reaction of aryl halides with phenylboronic acid using Pd catalyst.a
Entry
1
Aryl halide
Product
Temp (°C)
100
Time (h)
1
Yield (%)b
100
2
100
100
100
100
100
100
100
100
100
100
100
100
100
100
110
130
130
130
1
1
1
1
1
3
3
3
3
3
3
3
3
6
6
6
6
6
99
97
3
4
100
94
5
6
97
7
100
96
8
9
94
10
11
12
13
14
15
16
17
18
99
96
85
94
93
11
16
31
40c
43
19
a
Reaction condition: aryl halide (1.0 mmol), phenylboronic (1.5 mmol), K2CO3 (2.0 mmol), DMF/H2O (5.0 ml, v/v=1:1), Pd catalyst (0.5 mol%), 100–130 °C.
Yield was determined by GC analysis.
The loading of Pd catalyst is 1.0% mol.
b
c
The recycling of the catalyst is an important issue in the heteroge-
studied by ICP-AES analysis. The Pd content was found to be
3.85 wt.% before reaction and 3.49 wt.% after five reaction cycles,
which confirmed negligible Pd leaching. In addition, the isolated solu-
tion did not exhibit any further reactivity. Fig. 2(c) presents the TEM
image of the catalyst after the five cycles. It can be seen that the mor-
phology of recycled SMNPs-Salen Pd (II) does not go any drastic
change, but aggregation is more obvious.
neous reaction for commercial application. After the separation from
reaction mixture by magnet, the catalyst has been reused in model re-
action for five reaction cycles. As shown in Table 3, the yields were
above 85% in all five reaction cycles and the decreased catalytic activ-
ity as recycling proceeded may be due to the significant loss of palla-
dium. Metal leaching of the catalyst before and after reaction was

X. Jin et al. / Catalysis Communications 26 (2012) 199–203
203
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Acknowledgment
This work was supported by the Fundamental Research Funds for
the Central Universities (lzujbky-2012-68).
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