Magnetically palladium catalyst stabilized by diaminoglyoxime-functionalized magnetic Fe3O4 nanoparticles as active and reusable catalyst for Suzuki coupling reactions

Article abstract of DOI:10.1016/j.molcata.2014.10.012

A Pd supported on diaminoglyoxime (DAG)-functionalized Fe₃O₄ (Fe₃O₄/DAG/Pd) hybrid materials was fabricated for the first time. In this fabrication, DAG played an important role as a capping agent. The immobilized palladium catalyst was an efficient catalyst without added phosphine ligands for the Suzuki cross-coupling reaction of several aryl halides with phenylboronic acid in aqueous phase at room temperature. The yields of the products were in the range from 70% to 98%. The prepared heterogeneous nanocatalyst was characterized by XRD, XPS, EDS, FT-IR, ICP, FESEM, VSM and TEM. Interestingly, the novel catalyst could be recovered in a facile manner from the reaction mixture by applying an external magnet device and recycled eight times without any significant loss in activity.

Full text of DOI:10.1016/j.molcata.2014.10.012

Accepted Manuscript
Title: Magnetically palladium catalyst stabilized by
diaminoglyoxime-functionalized magnetic Fe3O4
nanoparticles as active and reusable catalyst for Suzuki
coupling reactions
Author: Hojat Veisi Javad Gholami Hiroshi Ueda Pourya
Mohammadi Mohammad Noroozi
PII:
S1381-1169(14)00456-7
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2014.10.012
MOLCAA 9314
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
4-9-2014
4-10-2014
9-10-2014
Please cite this article as: H. Veisi, J. Gholami, H. Ueda, P.M. </sup>Mohammad
Noroozi, Magnetically palladium catalyst stabilized by diaminoglyoxime-
functionalized magnetic Fe3O4 nanoparticles as active and reusable catalyst for
Suzuki coupling reactions, Journal of Molecular Catalysis A: Chemical (2014),
http://dx.doi.org/10.1016/j.molcata.2014.10.012
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Magnetically palladium catalyst stabilized by diaminoglyoxime-
functionalized magnetic Fe3O4 nanoparticles as active and reusable catalyst
for Suzuki coupling reactions
Hojat Veisi,a,* Javad Gholami,b Hiroshi Ueda,c Pourya Mohammadia Mohammad
Noroozia
aDepartment of Chemistry, Payame Noor University, 19395-4697 Tehran, Iran
bFaculty of Nano Material, Tarbiat Modares University, Tehran, Iran
cChemical Resources Laboratory, Tokyo Institute of Technology, 4259-R1-18 Nagatsuta-cho, Midori-ku,
Yokohama 226-8503, Japan
cYoung Researchers and Elite Club, Pharmaceutical Sciences Branch, Islamic Azad University, Tehran, Iran
E-mail: hojatveisi@yahoo.com
Highlights
A Pd supported on diaminoglyoxime (DAG)-functionalized Fe3O4 was reported.
Fe3O4/DAG/Pd show superior activity in the Suzuki reactions.
The products were produced in excellent yields under mild conditions.
Low palladium loading was used in the reaction.
The catalyst can be reused at least 8 consecutive cycles.
Abstract.
A
Pd supported on diaminoglyoxime (DAG)-functionalized Fe3O4
(Fe3O4/DAG/Pd) hybrid materials was fabricated for the first time. In this fabrication, DAG
played an important role as a capping agent. The immobilized palladium catalyst was an
efficient catalyst without added phosphine ligands for the Suzuki cross-coupling reaction of
several aryl halides with phenylboronic acid in aqueous phase at room temperature. The
yields of the products were in the range from 70% to 98%. The prepared heterogeneous
nanocatalyst was characterized by XRD, XPS, EDS, FT-IR, ICP, FESEM, VSM and TEM.
Interestingly, the novel catalyst could be recovered in a facile manner from the reaction
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mixture by applying an external magnet device and recycled eight times without any
significant loss in activity.
Keywords: Fe3O4/DAG/Pd, Magnetic nano composite, Catalysis, Recyclable, Suzuki reaction.
1. Introduction
The biaryl moiety is an important structural motif in a great number of biologically active
compounds and functional molecules [1]. Such molecules include the natural product
biphenomycin [2], the pharmaceuticals valsartan, telmisartan, felbinac, losartan, and imatinib
[3], the agrochemical boscalid [4], and liquid crystals for LCD screens [5]. Biaryls can be
synthesized by using Pd-catalyzed Suzuki [6], Negishi [7], Kumada [8], Hiyama [9], and
Stille [10] reactions. The Suzuki reaction has become part of the standard toolbox of organic
chemists. The importance of the reaction is also reflected by the fact that one of the inventors,
Akira Suzuki, was awarded with the Nobel Prize in Chemistry in 2010.
Development of this reaction is greatly dependent on the reactivity of the palladium
catalyst. Generally, catalysts are divided into two groups: Most examples are homogeneous
systems and the others are heterogeneous systems. Despite the observed beneficial effects
with homogeneous catalyst, problems associated with the separation and recovery of the
expensive active catalyst, limit their use in industrial and synthetic applications. In light of
this, continuous efforts are now being made to carry out the reactions with heterogeneous
system to aid recovery, recyclability and re-use of the catalyst. Immobilization of
homogeneous catalysts on various insoluble supports (especially porous materials with high
surface areas) can lead to simplify catalyst recycling via filtration or centrifugation. But, a
substantial decrease in the activity of the immobilized catalyst is frequently observed due to
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the loss of the catalyst in the separation processes and/or diffusion factors. In the attempt to
resolve such problems, nanoparticles (NPs) have used as alternative soluble matrixes for
supporting homogeneous catalysts. When the size of the support materials is decreased to the
nanometer scale, the surface area of NPs will increase dramatically. As a consequence, NPs
could have higher catalyst loading capacity and higher dispersion than many conventional
support matrixes, leading to the improved catalytic activity of the supported catalysts.
However, conventional separation methods may become inefficient for support particle sizes
below 100 nm. The incorporation of magnetic NPs (MNPs) into supports offers a solution to
this problem. The renewed interest in synthesis of magnetic nanoparticles (MNPs) is due to
their technological applications such as data storage, biological imaging, and biomedicine
[11-15]. One of the attractive features of MNPs is the possibility of fast and cost-efficient
separation by applying an external magnetic field, which makes them as the ideal candidates
for practical use in catalysis process. This new direction in catalysis galvanized the academic
research and lead to the development of a great number of magnetic-based catalysts,
especially magnetite (Fe3O4) that found application in the various reactions [16-25]. Several
accounts about synthesis, characterization and application of Fe3O4 MNPs utilized in the
coupling reaction have been reported in the literature [26-37].
It should be noted that magnetostatic interactions between particles, makes Fe3O4 particles
susceptible for agglomeration. On the other hand, the oxidation of Fe2+ causes depletion of
magnetism of Fe3O4. Thus, in order to decrease the agglomeration degree and increase the
oxidation resistance, effective coating of magnetite particles is essential. Several molecules
have been used to coat on the surface of MNPs. In recent years, modified magnetic
nanoparticles have received a lot of attention as support for preparation of noble metal
nanoparticles. However, most of these techniques require many reaction steps to introduce
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functional groups to the magnetic surface and they use organosilica precursors as organic
shell in order to prepare a suitable support for trapping metal nanoparticles [38-43]. The
organosilane precursors not only involve complicated synthesis and purification method, but
also are very expensive and toxic. Therefore, from both environmental and economic points
of view, preparation of the modified magnetic nanoparticles via a simple method and without
using organoalkoxysilane compounds is highly desirable. In this study, diaminoglyoxime
(DAG), which is inexpensive and nontoxic materials, was used as good candidate for
incorporation of palladium nanoparticles.
In continuing our efforts towards the development of efficient and environmentally benign
heterogeneous catalysts [44-47], herein, we will report a simple preparation of palladium
nanoparticles incorporated into Fe3O4/diaminoglyoxime nanocomposite as a new
magnetically recoverable heterogeneous catalyst. The synthesized Fe3O4/DAG/Pd
nanocomposites exhibited efficient catalytic activities for various Suzuki cross-coupling
reactions. The Fe3O4 nanocrystal was used as a support of Pd nanoparticles, as well as
enhancing the dispersion of catalysis active sites and improving its catalytic activity. More
importantly, the synthesized Fe3O4/DAG/Pd nanocomposite presented good magnetic
property, and could be easily separated from the reaction mixture by using a magnet. By
utilizing this property, it could be reused for 7 cycles without losing its catalytic activity,
indicative of a potential application in industry.
2. Experimental
2.1. Preparation of the magnetic Fe3O4 nanoparticles (MNPs)
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Naked Fe3O4 nanoparticles were prepared by chemical coprecipitation of Fe3+ and Fe2+
ions with a molar ratio of 2:1. Typically, FeCl3·6H2O (5.838 g, 0.0216 mol) and FeCl2·4H2O
(2.147 g, 0.0108 mol) were dissolved in 100 mL deionized water at 85 ºC under N2
atmosphere and vigorous mechanical stirring (500 rpm). Then, 10 mL of 25% NH4OH was
quickly injected into the reaction mixture in one portion. The addition of the base to the
Fe2+/Fe3+ salt solution resulted in the formation of the black precipitate of MNPs immediately.
The reaction continued for another 25 min and the mixture was cooled to room temperature.
Subsequently, the resultant ultrafine magnetic particles were treated by magnetic separation
and washed several times by DI water.
2.2. Preparation of the Fe3O4/DAG
The obtained MNPs powder (500 mg) was dispersed in 50 mL ethanol/water (volume
ratio, 1:1) solution by sonication for 20 min, and then DAG (250 mg) was added to the
mixture. After mechanical agitation at 40 ºC for 5 h, the suspended substance was separated
with centrifugation. The settled product was re-dispersed in ethanol by sonication and then
was isolated with magnetic decantation for 5 times. The precipitated product (Fe3O4/DAG)
was dried at room temperature under vacuum.
2.3. Preparation of the Fe3O4/DAG/Pd(0)
The Fe3O4/DAG (500 mg) were dispersed in CH3CN (30 mL) by ultrasonic bath for 30
min. Subsequently, a yellow solution of PdCl2 (30 mg) in 30 mL acetonitrile was added to
dispersion of Fe3O4/DAG and the mixture was stirred for 10 hours at room temperature. Then,
the Fe3O4/DAG/Pd(II) was separated by magnetic decantation and washed by CH3CN, H2O
and acetone respectively to remove the unattached substrates.
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The reduction of Fe3O4/DAG/Pd(II) by hydrazine hydrate was performed as follows: 50 mg
of Fe3O4/DAG/Pd(II) was dispersed in 60 mL of water, and then 100 μL of hydrazine hydrate
(80%) was added. The pH of the mixture was adjusted to 10 with 25% ammonium hydroxide
and the reaction was carried out at 95 °C for 2 h. The final product Fe3O4/DAG/Pd(0) was
washed with water and dried in vacuum at 40 °C. Scheme 1 depicted the synthetic procedure
of Fe3O4/DAG/Pd(II) and Fe3O4/DAG/Pd(0). The concentration of palladium in
Fe3O4/DAG/Pd(0) was 2.83 wt% (0.28 mmol/g), which was determined by ICP-AES.
2.4. Suzuki-Miyaura coupling reaction
In a typical reaction, 7 mg of the Fe3O4/DAG/Pd(0) (7 mg = 0.0021 mmol Pd) was
placed in a 25 mL Schlenk tube, 1 mmol of the aryl halide in 5 mL of water/ethanol (1:1)
was added 0.134 g (1.1 mmol) of phenyl boronic acid, 0.276 mg of K2CO3 (2 mmol). The
mixture was then stirred for the desired time at room temperature. The reaction was
monitored by thin layer chromatography (TLC). After completion of the reaction, 5 mL
ethanol was added, and the catalyst was removed by external magnet. Further purification
was achieved by column chromatography.
3. Results and discussion
3.1. Catalyst characterization
The Fe3O4 particles were synthesized according to the literature using ferric chloride
hexahydrate and ferrous chloride tetrahydrate upon addition of NH4OH. The Fe3O4/DAG
particles were prepared by coating diaminoglyoxime (DAG) on the surface of Fe3O4
particles. For the preparation of palladium loaded materials, Fe3O4/DAG particles were
treated with a PdCl2, and then a hydrazine solution was added. Finally, the mixture was
collected by an external magnet, followed by drying in vacuum (Scheme 1). The Pd content
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in Fe3O4/DAG/Pd catalyst was determined 2.83 wt.% (0.28 mmol/g) by ICP-AES. The
prepared catalyst was characterized by FTIR, XRD, FESEM, EDX, TEM, ICP and VSM.
N
NH2
NH2
NH2
O
OH
HO
HO
N
O
O
O
HO
OH
FeCl3.6H2O
FeCl2.4H2O
N
NH2
NH4OH, N2
N
Fe3O4 NP
O
Fe3O4 NP OH
O
HO
HO
DAG
20 min
85 C
25 min
O
OH
OH
O
Fe3O4 MNPs
PdCl2
Fe3O4/DAG
MeCN
Cl
H2
N
H2
N
O
Pd
O
N
Pd(0)
N
O
O
O
Cl
NH2NH2
O
O
NH2
NH2
N
Fe3O4 NP
O
N
O
Fe3O4 NP
O
O
O
O
O
O
O
Fe3O4/DAG/Pd(II)
Fe3O4/DAG/Pd(0)
Scheme 1. Preparation of Fe3O4/DAG/Pd nanocatalyst.
The FT-IR spectrum for the MNPs alone (Fig. 1a) shows a stretching vibration at 3415
cm-1 which incorporates the contributions from both symmetrical and asymmetrical modes of
the O-H bonds which are attached to the surface iron atoms. The bands at low wavenumbers
(≤700 cm-1) come from vibrations of Fe-O bonds of iron oxide, in which for the bulk Fe3O4
samples appear at 570 and 375 cm-1 but for Fe3O4 nanoparticles at 623 and 577 cm-1 as a blue
shift, due to the size reduction [48, 49]. From the Fig. 1b, it is clear that the broad band at
3250 and 3498 cm-1 are referred to the O-H and N-H stretching vibration and the band at
1447 cm-1 is referred to the C=N stretching vibration [50]. The peaks at 1653 and 1447 cm-1
are to confirm the presence of DAG groups in the structure of Fe3O4/DAG catalyst.
Successful immobilization of Pd nanoparticles on the surface of Fe3O4/DAG nanoparticles
(Fig. 1c) could be confirmed by absence of NH stretching (3498 cm-1) and decreasing the
band at 1653 cm-1 (in the Fig. 1b) to 1645 cm-1 at the next spectra. Also, Fig. 1c shows that
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coordination mode of diaminoglyoxime to Fe3O4 does not change after immobilization of
palladium.
Fig. 1. FT-IR spectra of (a) Fe3O4, (b) Fe3O4/DAG and (c) Fe3O4/DAG/Pd.
Unambiguous evidence of palladium particles on Fe3O4/DAG is provided via X-ray
powder diffraction analysis (XRD) (Fig. 2). The material had similar diffraction peaks at 2θ =
30.2º, 35.6º, 42.3º, 53.6º, 57.1º and 62.6º, corresponding to the (1 1 1), (2 2 0), (3 1 1), (4 0
0), (4 4 0) and (5 1 1) are readily recognized from the XRD pattern; which could be indexed
to a cubic-phase of Fe3O4 [37]. This result indicated that the crystal structure of Fe3O4 MNPs
was not changed after modification with diaminoglyoxime and palladium nanoparticles. Also,
the appearance of the new peaks at 2θ = 39, 46.5 and 68.0 are attributed to the Pd species,
that corresponding to the (1 1 1), (2 0 0) and (2 2 0) are readily recognized from the XRD
pattern; which could be indexed to a face-centered cubic (fcc) palladium. (JCPDS No. 89–
4897). The results from XRD imply that the Pd nanoparticles have been successfully
immobilized onto the surface of the magnetic nanoparticles. The result shows that the
structure of Fe3O4 has been not damaged in the of catalyst preparation process.
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Fig. 2. XRD pattern of Fe3O4/DAG/Pd.
The TEM image of the Fe3O4/DAG/Pd catalyst revealed that the Pd nanoparticles with
nearly spherical morphology were formed on the surface of the modified Fe3O4 nanoparticles.
In the transmission electron microscopy images, iron oxide nanoparticles of 10-15 nm in
diameter and palladium nanoparticles of ∼2 nm entrapped in iron oxide are observed. Also,
the TEM image of the Fe3O4/DAG/Pd catalyst shows that the functionalized magnetic
nanoparticles possess almost spherical morphology with nearly monodispersity.
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Fig. 3. TEM image of Fe3O4/DAG/Pd and the histogram illustrating the Pd nanoparticle size
distribution.
Presence of Pd atoms in Fe3O4/DAG/Pd were confirmed using energy dispersive X-ray
spectroscopy (EDS), which was recorded at random points on the surface for qualitative
analysis (Fig. 4). The EDS spectrum also showed the signals of iron and oxygen atoms,
which also indicated their presence in the composite.
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Fig. 4. Energy-dispersive X-ray spectroscopy (EDS) of the catalyst.
Fig. 5 shows the FESEM image of the synthesized Fe3O4/DAG/Pd loaded magnetite
nanoparticles. It was confirmed that the catalyst was made up of uniform nanometer-sized
particles.
Fig. 5. FESEM images of Fe3O4/DAG/Pd.
X-ray photoelectron spectroscopy (XPS) is a powerful tool to investigate the electron
properties of the species formed on the surface, such as the electron environment, oxidation
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state, and the binding energy of the core electron of the metal. Fig. 6 shows the the XPS
spectrum of Fe3O4/DAG/Pd (recorded by Shimadzu ESCA-3400 electron spectrometer). The
XPS spectrum of Fe 2p contains two peaks. There are two strong peaks located at 712.6 and
726.2 eV, which are the typical Fe 2p1/2 and Fe 2p3/2 XPS signals of magnetite [51].
Moreover, the investigation of the Fe3O4/DAG/Pd at the Pd 3p level shows peaks at 531.8
and 553.4 eV for Pd 3p3/2 and Pd 3p1/2, respectively, which clearly indicates that the Pd
nanoparticles are stable as metallic state in the nanocomposite structure (Fig. 6). In
comparison to the standard binding energy of Pd0, with Pd 3p3/2 of about 532.4 eV and Pd
3p1/2 of about 560.2 eV [52], it can be concluded that the Pd peaks in the Fe3O4/DAG/Pd
shifted to lower binding energy than Pd0 standard binding energy. It has been reported that
the position of Pd 3p peak is usually influenced by the local chemical/physical environment
around Pd species besides the formal oxidation state, and shifts to lower binding energy when
the charge density around it increases [53]. Therefore, the peaks at 553.4 and 531.8 could be
due to Pd0 species bound directly to amino groups in the Fe3O4/DAG, which is in agreement
with IR results. The peaks corresponding to oxygen (O 2s), carbon (C 1s), and nitrogen (N
1s) are also clearly observed in XPS elemental survey of the catalyst.
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Fig. 6. XPS spectra of Pd/Fe3O4 catalyst for Pd 3p, Fe 2p, C 1s, N 1s and O 2s. .
The magnetic properties of Fe3O4 and Fe3O4/DAG and Fe3O4/DAG/Pd were characterized
by a vibrating sample magnetometer (VSM). The saturation magnetization of Fe3O4 NPs at
room temperature by measuring the magnetization curve (Fig. 7) was 61.6 emu/g; however,
its minor decrease to 61.4 emu/g further suggested a DAG coating formation on the surface
of Fe3O4 NPs. The Ms value of Fe3O4/DAG/Pd is major decreased due to the Pd coating and
the immobilized of the Fe3O4/DAG (55.3 emu g-1). There is no hysteresis, suggesting that
Fe3O4/DAG/Pd MNPs are superparamagnetic. Even with this reduction in the saturation
magnetization, the catalyst can still be efficiently separated easily from the solution with the
help of an external magnetic force.
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Fig. 7. Room temperature magnetization curves of (a) Fe3O4, (b) Fe3O4/DAG and
(c) Fe3O4/DAG/Pd.
3.2. Catalytic activity
In continuation of our interest on environmentally benign chemical processes
[52], herein, after characterization of the prepared nanocatalyst, in the present work
we disclose that Fe3O4/DAG/Pd can be used as an active and stable magnetically
separable nanocatalyst in the Suzuki-Miyaura reaction (Scheme 2).
B(OH)2
Fe3O4/DAG/Pd(0)
R
K2CO3, EtOH-H2O, r.t.
+
R
H2
N
Pd
X
O
N
Pd
O
O
Pd
O
X = I, Br, Cl
Pd
NH2
Pd
N
Fe3O4 NP
O
O
Pd
Pd
O
Pd
O
Scheme 2. Fe3O4/DAG/Pd for Suzuki cross-coupling reaction.
Initially, we explored optimized the conditions for the coupling reaction between
bromobenzene and phenylboronic acid as a model reaction at room temperature.
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Table 1. Optimization of the conditions for the Suzuki-Miyaura reaction of bromobenzene
with phenylboronic acid.a
Entry
Solvent
DMF
toluene
EtOH
H2O
Pd (mol%)
0.2
Base
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
NaOAc
Et3N
Time (min) Yield (%)b
1
2
3
4
5
6
7
8
9
80
60
60
90
40
60
90
60
60
90
80
65
77
55
98
65
80
75
96
0.2
0.2
0.2
0.2
0.2
0.2
0.1
0.3
EtOH/ H2Oc
EtOH/ H2Oc
EtOH/ H2Oc
EtOH/ H2Oc
EtOH/ H2Oc
K2CO3
K2CO3
No base
10 EtOH/ H2Oc
0.2
Trace
aReaction conditions: bromobenzene (1 mmol), PhB(OH)2 (1.1 mmol), Fe3O4/DAG/Pd, solvent (3 ml).
bIsolated yield
cEtOH/ H2O= 1:1
The influence of reaction parameters such as the base (Et3N, NaOAc and K2CO3), the
solvent (nonpolar, protic and aprotic), and catalyst amount were tested (Table 1). Among the
bases evaluated, K2CO3 was found to be the most effective. H2O/EtOH (1:1) was the best
choice as solvent. The effect of catalyst loading was investigated employing different
quantities of the catalyst ranging from 0.1 mol% to 0.3 mol%. (Table 1, entries 5, 8,9). The
best result was obtained with 0.007 g (0.2 mol%) of the catalyst (Table 1, entry 5).
Table 2
Heterogeneous Suzuki-Miyaura reaction of aryl halides with phenylboronic acid catalyzed by
Fe3O4/DAG/Pd at room temperature.a
Entry RC6H4X
X
I
Br
Cl
I
Br
Cl
I
Br
I
Time (h) Yield (%)b
1
H
0.16
0.66
8
98
98
85
98
96
75
98
96
98
96
2
H
3
H
4
5
6
7
8
9
10
4-CH3
4-CH3
4-CH3
4-COCH3
4-COCH3
4-CH3O
4-CH3O
0.20
1
12
0.3
0.8
0.3
1.5
Br
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11
12
13
14
15
16
17
4-Cl
2-CHO
3-NO2
Br
Br
I
Br
I
1
5
0.5
3
2
96
70
96
88
96
96
90
3-NO2
1-Naphthyl
2-Thienyl
2-Thienyl
I
Br
1
5
aReactions were carried out under aerobic conditions in 3 mL of H2O/EtOH (1:1), 1.0 mmol arylhalide,1.1 mmol
phenylboronic acid and 2 mmol K2CO3 in the presence of catalyst (0.007 g, 0.2 mol% Pd).
bIsolated yield.
To investigate the generality of this cross-coupling, we studied the reaction of
phenylboronic acid with a range of aryl halides (I, Br, Cl) under the optimized conditions
(Table 2). Under the optimized conditions, phenyl iodides, bromides and chlorides all reacted
efficiently with phenylboronic acid (Table 2, entries 1-6). Aryl halides with electron-
withdrawing or releasing groups reacted with phenylboronic acid to afford the corresponding
products in high yields. It was found that the yield when using an ortho substituated aryl
bromide was lower (Table 2, entry 12) than those obtained with para- or meta-substituted
aryl bromides (Table 2, entries 4-11, 13,14). Notably, heteroaryl halides such as 2-
bromothiophene and 2-iodothiophene with phenylboronic acid gave the corresponding
coupled products in 96% and 90% yields, respectively (Table 2, entries 16 and 17).
The heterogeneity of the catalyst was evaluated to study whether the reaction using solid
Pd catalysts occurred on the Fe3O4/DAG surface or was catalyzed by Pd species leached in
the liquid phase. To address this issue, two separate experiments were conducted with
bromobenzene and phenyl boronic acid. In the first experiment, the reaction was terminated
after 20 minutes; at this juncture, the catalyst was separated from the reaction mixture by an
external magnet and the reaction was continued with the filtrate for an additional 60 minutes.
In the second experiment, the reaction was terminated after 20 minutes. In both cases, the
desired product was obtained in the same yield (45%). Pd was not detected in the filtrate in
either experiment by ICP-AES. These studies demonstrate that only the Pd bound to
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Fe3O4/DAG during the reaction is active, and the reaction proceeds on the heterogeneous
surface. So, based on these evidences, a reaction mechanism for Suzuki coupling with the
prepared nanocatalyst was proposed (Scheme 3).
Pd0
Ar
X
Oxidative addition
Fe3O4 NP
Pd0
Reductive elimination
Pd0
Ar Ph
X
Ar
Ph
Ar
Pd2+
Pd2+
Suzuki Reaction
Fe3O4 NP
Pd0
Fe3O4 NP
Pd0
Pd0
Pd0
Ph B(OH)2
Transmetalation
XB(OH)2
Scheme 3. Possible mechanism of Suzuki coupling reaction using Fe3O4/DAG/Pd.
In the view of industrial purposes and green chemistry, reusability of the catalyst was
tested by carrying out repeated runs of the reaction on the same batch of the catalyst in the
case of the model reaction (Table 1). In order to regenerate the catalyst, after each cycle, it
was separated by a magnet, and washed several times with deionized water and ethanol.
Then, it was dried in oven at 50 °C and used in the next run. The results show that this
catalyst can be reused eight times without any significant loss in activity performance (Fig.
8). This high catalytic activity can be possibly related to the strong interaction of Pd
nanoparticles with active sites of diaminoglyoxime. This result also demonstrated that the
palladium leaching of the catalyst was low
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Fig. 8. The recycling of the Fe3O4/DAG/Pd for the Suzuki coupling reaction under similar
conditions.
4. Conclusion
In conclusion, Fe3O4 nanoparticles were coated with diaminoglyoxime (DAG) as organic
shell via a co-precipitation method. The Fe3O4/DAG nanocomposites were effectively
employed as support for immobilization of Pd nanoparticles. The immobilized palladium was
found to be effective in the Suzuki coupling of various aryl halides and especially less
reactive aryl chlorides with phenyl boronic acid. This new heterogeneous catalyst showed the
following advantages: (a) simple preparation; (b) easy separation after reaction by an external
magnet; and (c) reusability for several times without any significant loss in its catalytic
activity. It looks that diaminoglyoxime have played an important role in stabilization of
catalyst particles. These unique results open new perspectives for the application of these
types of magnetic catalysts in other organic reactions.
Acknowledgement
We are thankful to Payame Noor University (PNU) and Tokyo Institute of Technology
for partial support of this work.
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Graphical Abstract
Magnetically palladium catalyst stabilized by diaminoglyoxime-
functionalized magnetic Fe3O4 nanoparticles as active and reusable catalyst
for Suzuki coupling reactions
Hojat Veisi,* Javad Gholami, Hiroshi Ueda, Pourya Mohammadi, Mohammad Noroozi
B(OH)2
Fe3O4/DAG/Pd(0)
R
K2CO3, EtOH-H2O, r.t.
+
R
H2
N
Pd
X
O
N
Pd
O
O
Pd
O
X = I, Br, Cl
Pd
NH2
Pd
N
Fe3O4 NP
O
O
Pd
Pd
O
Pd
O
22
Page 22 of 22