Encapsulation of Pd(II) into superparamagnetic nanoparticles grafted with EDTA and their catalytic activity towards reduction of nitroarenes and Suzuki-Miyaura coupling

Article abstract of DOI:10.1002/aoc.3258

A robust, safe and magnetically recoverable palladium catalyst was synthesized by anchoring Pd(II) onto ethylenediaminetetraacetic acid-coated Fe₃O₄ (Fe₃O₄@EDTA) magnetic nanoparticles. The Fe₃O₄ magnetic nanoparticle-supported Pd(II)-EDTA complex catalyst thus obtained was characterized using scanning and transmission electron microscopies, thermogravimetric analysis, vibrating sample magnetometry, X-ray diffraction, and inductively coupled plasma atomic emission and Fourier transform infrared spectroscopies. Fe₃O₄@EDTA-Pd(II) was screened for the Suzuki reaction and reduction of nitro compounds in water. The Pd content of the catalyst was measured to be 0.28 mmol Pd g^-1. In addition, the Fe₃O₄@EDTA-Pd catalyst can be easily separated and recovered with an external permanent magnet. The anchored solid catalyst can be recycled efficiently and reused five times with only a very slight loss of catalytic activity.

Full text of DOI:10.1002/aoc.3258

Full Paper
Received: 17 October 2014
Revised: 12 November 2014
Accepted: 14 November 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3258
Encapsulation of Pd(II) into superparamagnetic
nanoparticles grafted with EDTA and their
catalytic activity towards reduction of
nitroarenes and Suzuki–Miyaura coupling†
Kobra Azizi, Ehsan Ghonchepour, Meghdad Karimi and Akbar Heydari*
A robust, safe and magnetically recoverable palladium catalyst was synthesized by anchoring Pd(II) onto ethylenediaminetetra-
acetic acid-coated Fe3O4 (Fe3O4@EDTA) magnetic nanoparticles. The Fe3O4 magnetic nanoparticle-supported Pd(II)–EDTA com-
plex catalyst thus obtained was characterized using scanning and transmission electron microscopies, thermogravimetric analysis,
vibrating sample magnetometry, X-ray diffraction, and inductively coupled plasma atomic emission and Fourier transform
infrared spectroscopies. Fe3O4@EDTA–Pd(II) was screened for the Suzuki reaction and reduction of nitro compounds in water.
The Pd content of the catalyst was measured to be 0.28 mmol Pd gꢀ1. In addition, the Fe3O4@EDTA–Pd catalyst can be easily
separated and recovered with an external permanent magnet. The anchored solid catalyst can be recycled efficiently and reused
five times with only a very slight loss of catalytic activity. Copyright © 2015 John Wiley & Sons, Ltd.
Keywords: magnetic nanoparticles; reduction; Suzuki; heterogeneous catalysis
advantages of easy separation of magnetic nanoparticle-supported
catalysts by assembling a novel magnetic supported Pd catalyst.
Introduction
Magnetic nanoparticles have attracted worldwide attention as
catalyst supports, because of their response to an applied magnetic
field.[1] The development of functionalization strategies for un-
coated and coated magnetic nanomaterials allows the combination
of an enormous number of possibilities for the immobilization of
catalytic active species such as organocatalysts, metal complexes,
metal nanoparticles and enzymes.[2] Conventional catalyst supports
include polymers, carbon, silica, alumina and other metal oxides.[3]
Magnetic nanoparticle-supported catalysts can be simply and
efficiently separated from the reaction system using an external per-
manent magnet, thus avoiding complicated operating processes.[4]
Tailoring the surface properties of magnetic nanoparticles can be
carried out using various approaches: (a) the non-covalent approach
with surfactants or polymers; (b) coating with oxides, carbon,
polymer or metallic layers; and (c) the covalent approach between
hydroxyl groups on the nanoparticle surface and anchoring agents
such as carboxylic acid, phosphonic acid and dopamine derivatives.[5]
The selection of the correct immobilization method is of primary
importance in the synthesis of high-quality magnetic catalysts.
Ethylenediaminetetraacetic acid (EDTA) is an aminopoly-
carboxylic acid and water-soluble solid. Its carboxylate groups bind
to an iron oxide surface and the hydrophobic tails provide the steric
hindrance that stabilizes nanoparticles in solution. EDTA has the
ability to ‘sequester’ metal ions such as Pd2+ and Cu2+.
Herein, we report the preparation of a magnetically separable Pd
catalyst and its catalytic properties in Suzuki reaction and reduction
of nitro compounds in water. The purpose of this investigation
was (a) the evaluation of the encapsulation of Pd2+ into
superparamagnetic nanoparticles grafted with EDTA; (b) the
measurement of the catalytic activity in Suzuki reaction and
reduction of nitro compounds in water; and (c) the investigation
of the stability of the catalyst as well as its recycling characteristics.
Experimental
Chemicals, Instrumentation and Analysis
All solvents and chemicals purchased were of analytical grade and
used without further purification. FT-IR spectra were obtained in
the region 400–4000 cmꢀ1 using a Nicolet IR100 instrument with
spectroscopic grade KBr. Powder X-ray diffraction (XRD) spectra
were recorded at room temperature with a Philips X-Pert 1710
diffractometer using Co Kα (λ =1.78897 Å) at a voltage of 40 kV
and current of 40 mA and data were collected from 10° to 90°
(2θ) with a scan speed of 0.02° sꢀ1. The morphology of the
catalyst was studied using scanning electron microscopy (SEM;
In addition, from the viewpoint of green chemistry, it is desirable
to use water to replace toxic organic solvents as the reaction
medium.[6]
As part of our on-going research towards the development of
eco-friendly and green organic transformations,[7] we attempted
to join the high catalytic activity of supported catalysts with the
*
Correspondence to: Akbar Heydari, Chemistry Department, Tarbiat Modares
University, PO Box 14155-4838, Tehran, Iran. E-mail: heydar_a@modares.ac.ir
†
This article is dedicated to Gen. Qassem Soleimani.
Chemistry Department, Tarbiat Modares University, PO Box 14155-4838, Tehran,
Iran
Appl. Organometal. Chem. (2015)
Copyright © 2015 John Wiley & Sons, Ltd.

K. Azizi et al.
Philips XL 30 and S-4160) with coated gold equipped with disper-
sive X-ray spectroscopy capability. The magnetic properties of
Fe3O4@EDTA–PdCl2 nanoparticles were measured with a vibrating
magnetometer/alternating gradient force magnetometer (MDK
Co., Iran, www.mdk-magnetic.com). Thermogravimetric analysis
(TGA) and differential thermal analysis (DTA) were performed using
a thermal analyser with a heating rate of 20°C minꢀ1 over a temper-
ature range of 25–1100°C under flowing compressed nitrogen.
Surface Modification of Fe3O4
The magnetic catalyst (Fe3O4@EDTA–PdCl2) was prepared by dis-
solving a mixture of 10 mmol FeCl3ꢁ6H2O and 5 mmol FeCl2ꢁ4H2O
salts in 100 ml of deionized water under vigorous stirring (800 rpm).
An aqueous ammonia solution (25% w/w, 30 ml) was then added to
the stirring mixture to maintain the reaction pH about 11. Simulta-
neously, disodium salt dihydrate solution (1.1 g, 2 mmol in 20 ml of
water) was added to this black suspension.
The resulting black dispersion was continuously stirred for 1 h at
room temperature and then refluxed for 1 h. Fe3O4 nanoparticles
were separated from the aqueous solution using magnetic
decantation, and were washed with water several times. After that
2 mmol of PdCl2 and 4 mmol of Na2CO3 were added with stirring
at 40°C for 6 h. The suspension was refluxed for 2 h. Finally, nano-
particles were separated from the aqueous solution by magnetic
decantation, washed several times with water, ethanol and diethyl
ether before being dried in an oven overnight. The synthesis route
is depicted in Scheme 1.
Figure 1. XRD patterns of Fe3O4@EDTA and Fe3O4@EDTA–PdCl2.
of aryl halide, 1.1 mmol of phenylboronic acid, 4 ml of water and
2 mmol of K2CO3 were added to the round-bottom flask and
heated at 80°C in an oil bath. In the case of water-insoluble aryl
halides, 1 mol% of n-Bu4NBr was also added to the reaction
mixture. After stirring at this temperature for 3–5 h, the hetero-
geneous mixture was cooled to room temperature and the
nanoparticles were collected with a permanent magnet from
the reaction mixture and washed with distilled water and
CH2Cl2 repeatedly. Water (20 ml) was added to the reaction
mixture and extracted with EtOAc (2 ×20 ml) and dried over
Na2SO4. The solvent was removed under reduced pressure to
give the crude product. The solid was recrystallized from a
CH2Cl2–n-hexane solvent mixture to afford the desired product.
Catalytic Reduction of Nitroarenes in the Presence of
Fe3O4@EDTA–PdCl2
To a solution of nitro compound (1 mmol) in water (4 ml), NaBH4
(5 mmol) was added along with an appropriate amount of
Fe3O4@EDTA–Pd (20 mg). In the case of water-insoluble nitro
compounds, 1 ml of EtOH was also added to the reaction mixture.
The reaction was heated at 45°C in an oil bath for 2–30 min. The
progress of the reaction was monitored by TLC. After completion
of the reaction, the nanoparticles were collected with a permanent
magnet from the reaction mixture and washed with distilled water
and CH2Cl2 repeatedly. Water (50 ml) was added to the reaction
mixture and extracted with EtOAc (2 ×20 ml) and dried over
Na2SO 4. The solvent was removed under reduced pressure to give
the crude product. The residue was subjected to silica-gel plate
chromatography using n-hexane–ethyl acetate as an eluent to
afford pure product.
Results and Discussion
XRD Patterns
As can be seen in Fig. 1, six characteristic peaks of Fe3O4 (2θ =30.1°,
35.4°, 43.0°, 53.5°, 57.0° and 62.5°) are observed. These six peaks
correspond to the six crystal faces (220), (311), (400), (422), (511)
and (440), respectively. As is shown in Fig. 1, the XRD peaks match
well with peaks of Fe3O4, which is in agreement with work done by
Wei et al.[8] This indicates that the crystalline structure of Fe3O4
magnetic nanoparticles remains after surface modification with
EDTA (XRD pattern of Fe3O4@EDTA). The results indicate that the
resultant particles are Fe3O4 with a spinel cubic structure. All
diffraction peaks and positions for Fe3O4 match well with those
from JCPDS card no. 76-1849. Also, the six peaks of Fe3O4 can be
seen in the XRD pattern of the synthesized Fe3O4@EDTA–PdCl2.
This shows that the prepared Fe3O4 nanoparticles in this stage
are pure with a spinel structure and the supporting of the sur-
face of Fe3O4 with Pd (II)–EDTA does not result in a phase
change of the magnetic nanoparticles.
Catalytic Suzuki Cross-Coupling Reaction in the Presence of
Fe3O4@EDTA–PdCl2
A round-bottom flask equipped with a magnetic stirrer bar was
charged with Fe3O4@EDTA–PdCl 2 (20 mg). Amounts of 1 mmol
FT-IR Spectra
The organic moieties anchored on the surface of Fe3O4 were inves-
tigated using FT-IR spectroscopy. The FTIR spectra of Fe3O4,
Fe3O4@EDTA and Fe3O4@EDTA–PdCl2 are depicted in Fig. 2(a)–(c).
Typical band ascribed to the Fe–O stretching vibration appears at
561 cmꢀ1 for the bare magnetic nanoparticles. The C–H stretching
Scheme 1. Preparation of Fe3O4@EDTA–PdCl2.
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Encapsulation of Pd(II) into superparamagnetic nanoparticles
Figure 2. FT-IR spectra of (a) Fe3O4, (b) Fe3O4@EDTA, (c) Fe3O4@EDTA–
PdCl2 (fresh) and (d) Fe3O4@EDTA–PdCl2 (after fifth run).
band of Fe3O4@EDTA (2800–2950 cmꢀ1) is stronger than that of
Fe3O4@EDTA–PdCl2 demonstrating that Pd interacts with the EDTA
and weakens the C–H vibration. A broad band with a shoulder at 566
cmꢀ1 is assigned to the formation of COO–Pd complex. Comparing
C¼O vibration in the FT-IR spectrum of Fe3O4@EDTA at 1629 cmꢀ1
with that of Fe3O4@EDTA–PdCl2 at 1615 cmꢀ1 proves formation of
the complex.
FT-IR measurements reveal that EDTA is chemisorbed onto the
surface of Fe3O4 nanoparticles as a carboxylate. The separation of
the asymmetric and symmetric stretch of the carboxylate positions,
which is defined as Δν (1629 – 1532 =97 cmꢀ1), is ascribed to
bidentate chelation, and the interaction between COOꢀ group
and Fe atom is covalent.
Although no significant change in the catalytic activity of the
catalyst is observed, we performed an analysis of the catalyst after
the fifth run in order to determine any change in the catalyst
structure. The nature of the recovered catalyst was traced using
FT-IR spectroscopy. The results indicate that the catalyst shows no
change in its FT-IR spectrum after the fifth run (Fig. 2(d)).
Figure 3. SEM images of (a) Fe3O4@EDTA, (b) Fe3O4@EDTA–PdCl2 and (c)
Fe3O4@EDTA–PdCl2 after the fifth run in the reduction of nitro benzene.
properties. Saturation magnetization decreases as the particle size
is reduced due to enhancement of surface spin effects, and as
particle size decreases the coercivity vanishes and becomes a
superparamagnetic state. Fe3O4@EDTA–Pd(II) dispersed in solvent
SEM Analysis
The morphology of the synthesized catalyst was confirmed using
SEM. Regarding the incorporation of Pd(II), the images obtained
(Fig. 3(b)) do not reveal significant changes in the Fe3O4@EDTA
morphology (Fig. 3(a)) nor in the reused Fe3O4@EDTA–Pd(II), even
after the fifth run (Fig. 3(c)).
Transmission electron microscopy confirms the formation of
single-phase Fe3O4 nanoparticles, with spherical morphology
(Fig. 4).
Vibrating Sample Magnetometry
The magnetic properties of Fe3O4@EDTA–Pd(II) were studied
using vibrating sample magnetometry. Magnetization (emu gꢀ1
)
as a function of applied field (Oe) is depicted in Fig. 5 with the
confined field from ꢀ10 000 to 10 000 Oe. Bare nanocrystals
of Fe3O4 have high saturation magnetization of 73.7 emu gꢀ1
at room temperature,[9] which decreases to 48.0 emu gꢀ1 for
Fe 3O 4@EDTA–Pd(II).
Zero remanence and coercivity of the magnetization curve
demonstrate that these nanoparticles have superparamagnetic
Figure 4. Transmission electron micrograph of Fe3O4@EDTA–PdCl2.
Appl. Organometal. Chem. (2015)
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K. Azizi et al.
is present in this temperature range,[11] and the weight loss pattern
of Fe3O4@EDTA–PdCl2 up to about 600°C corresponds to the for-
mation of the most stable oxides, PdO and FeO.[12] The mass loss
is about 5.5%.
The TGA and DTA results reveal that the thermal stability of syn-
thesized Fe3O4@EDTA–PdCl2 catalyst is maintained at higher
temperature. The amount of Pd (3%) of Fe3O4@EDTA–PdCl2
(0.28 mmol Pd gꢀ1) was determined using inductively coupled
plasma atomic emission spectroscopy (ICP-AES). The TGA curves
and ICP-AES results also confirm the successful supporting of
the Pd(II)–EDTA complex onto the magnetic surface.
Catalytic Activity
Reduction of nitro compounds is widely used in various industries
such as the agrochemical, dye and pharmaceutical industries. This
reaction was chosen to evaluate catalytic activity of the recyclable
complexes supported on magnetic nanoparticles. Many reducing
agents have been utilized for their nitro compound reduction, the
most classic being metallic reagents in the presence of an acid.
However, this methodology is messy and environmentally
hazardous.[13] Thus, methods involving cleaner and cheaper alter-
natives such as NaBH4 in water are necessary.
Figure 5. Magnetization curve of Fe3O4@EDTA–PdCl2.
and subjected to an external magnetic field can immediately
gather nanoparticles, then with slight shaking can be promptly
re-dispersed. These results indicate that the particles display
good magnetic characteristics that could be useful when they
are applied as a magnetic catalyst.
Our preliminary experiments show that Fe3O4@EDTA–Pd2+ is a
more efficient promoter for rapid and convenient reduction of
nitro compounds with NaBH4 in aqueous medium. The yields of
aniline from the reduction of nitrobenzene with 1 molar equivalent
TGA/DTA
The amount of Pd(II)–EDTA complex supported on the magnetic
nanoparticles was estimated using TGA (Fig. 6). The Pd complex
exhibits thermal stability up to 200°C, which confirms that this
complex is free from any types of water molecules.[10]
of NaBH4 and catalytic amounts (20 mg) of Fe3O4@EDTA–Cu2+
,
Fe3O4@EDTA–Fe2+, Fe3O4@ EDTA–Ni2+
, ,
Fe3O4@EDTA–Mn2+
Fe3O4@EDTA–Co2+ and Fe 3O4@EDTA–Pd2+ within 1 h at room
temperature are summarized in Table 1 (entries 1–6).
The second step within the temperature range 200–360°C is
associated with an exothermic DTA peak in this range. The broad
exotherm is due to the presence of a complex EDTA moiety in
the matrix. There are two peaks accompanied by mass loss of
2.5% in the temperature range 200–360°C in the DTA curve. The
first peak is due to the partial decomposition of the ligand. The final
decomposition step appears above 320°C corresponding to the
complete thermal decomposition of the complexes and the loss
of their organic portion. The mass loss is about 2.5%, which corre-
sponds to the EDTA molecules that bind directly with Fe3O4 nano-
particles. The TGA curve of magnetic nanoparticle combined with
EDTA remains unchanged above 360°C, implying that iron oxide
Keeping other conditions the same, the reaction was also
tested using other mild reducing agents, namely hydrazine and
isopropanol (Table 1, entries 7 and 8). Among the reductants used
(entries 6–8), NaBH4 is demonstrated as appropriate (entry 6) and
superior to N2H4 and isopropanol. A little NaBH4 is always
decomposed in the reaction medium during the course of reaction.
So an excess of NaBH4 was always employed. The reaction shows a
strong dependence on molar ratio of reductant. The reaction was
carried out in the presence of 1–5 mmol of NaBH4 under the same
conditions, separately, which result in a serious improvement of
yield (entries 6 and 9–12).
Finally, we investigated the reaction temperature and amount of
catalyst. The reaction proceeds smoothly at room temperature. An
increase in temperature speeds up the reaction (entry 12). More by-
products are obtained when the reaction is conducted at high tem-
perature (entries 13 and 14). Also, increasing the amount of catalyst
does not improve the yield (entry 16).
Also, the reaction in the presence of Fe3O4@EDTA–Pd2+ without
NaBH4 and in the presence of NaBH4 without Fe3O4@EDTA–Pd2+
was investigated. The observations indicate that cooperation of
Fe3O4@EDTA–Pd2+ and NaBH4 is essential for the reaction (entries
17 and 18).
The reduction of nitrobenzene was studied in detail in other sol-
vents. In CH3CN the reduction time is 20 min, higher in comparison
to the time observed for aqueous medium (2 min). Nitrobenzene
ended up in the final product (perfect atom economy), as the E fac-
tor would be minimized. In the past fifteen years green chemistry
and sustainability, supported by the underlying concepts of E fac-
tors and atom economy (atom utilization), have become a powerful
impetus.[14] With the optimized conditions in hand, a series of nitro
Figure 6. TDA and DTA of Fe3O4@EDTA–PdCl2.
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Encapsulation of Pd(II) into superparamagnetic nanoparticles
Table 1. Optimization of the reduction of nitrobenzenea
Entry
Catalyst
Catalyst amount (mg)
Reductant (mmol)
Temperature (°C)
Yield (%)
1
Fe3O4@EDTA–Cu2+
Fe3O4@EDTA–Fe2+
Fe3O4@EDTA–Ni2+
Fe3O4@EDTA–Mn2+
Fe3O4@EDTA–Co2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
20
20
20
20
20
20
20
20
20
20
20
20
20
20
10
30
—
20
NaBH 4 (1)
NaBH 4 (1)
NaBH 4 (1)
NaBH 4 (1)
NaBH 4 (1)
NaBH 4 (1)
N2H4 (1)
25
25
25
25
25
25
25
25
25
25
25
45
70
100
45
45
45
45
25
ND
ND
ND
ND
41
2
3
4
5
6
7
<10
<5
45
8
Isopropanol (1)
NaBH 4 (2)
NaBH 4 (3)
NaBH 4 (4)
NaBH4 (5)
NaBH 4 (5)
NaBH 4 (5)
NaBH 4 (5)
NaBH 4 (5)
NaBH 4 (5)
—
9
10
11
12
13
14
15
16
17
18
66
73
94
80
78
40
90
ND
ND
aReaction conditions: nitrobenzene (1 mmol), reductant, H2O (4 ml), in the presence of catalyst, 1 h.
reduction of nitrobenzene to aniline under the optimized condi-
tions were compared with the best ones published for this reaction
using other catalysts, the data being listed in Table 2. The advan-
tages of the catalyst under investigation in the present case are ev-
ident regarding the time, conditions of the reactions, ease of
separation and reusability.
The Suzuki reaction of aryl halides with phenylboronic acid is a
powerful synthetic method for the generation of biaryl inorganic
chemistry. We explored the catalytic activity of Fe3O4@EDTA–PdCl2
for the Suzuki reaction. The study was initiated with phenylboronic
acid and iodobenzene as model substrates for the reaction (Table 3).
No desired product is obtained in the presence of Fe3O4@EDTA–
FeCl2 and Fe3O4@EDTA–NiCl2 nanoparticles. A high temperature is
also required for the coupling reactions of aryl iodide in water
without an organic co-solvent. Notably, the yield of biphenyl is up
to 69% from 21% when the reaction temperature is increased to
80°C from 25°C. It is found that 80°C gives the best result (Table 3, en-
try 7). Raising the reaction temperature does not improve the yield,
so temperatures higher than 80°C disfavour the reaction. Water is
generally accepted to be a green solvent, as it causes no environmen-
tal concerns. Surfactants such as cetyltrimethylammonium bromide
(CTAB) and tetrabutylammonium bromide (TBAB), which are usually
used as phase-transfer agents to promote organic transformations
Scheme 2. Reduction of nitroarenes.
compounds were chosen to establish the scope and generality of
the method (Scheme 2).
In the case of water-insoluble nitro compounds, 1 ml of EtOH was
also added to the reaction mixture. The results obtained for the
Table 2. Various catalytic systems for the reduction of nitrobenzene
Entry
Catalyst
Catalyst amount (mmol)
Reductant (mmol)
Temperature (°C)
Time (min)
Yield (%)a
1
2
3
4
5
NiCl2ꢁ6H2O
BiCl3
0.2
4
8
5
4
5
Room temperature
Room temperature
Room temperature
50–60
20
120
10
92[15]
86[16]
94[17]
95[18]
92
1.5
CuSO4
2
Charcoal
Present work
0.4 g
0.02 g
120
5
45
aIsolated yield.
Appl. Organometal. Chem. (2015)
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K. Azizi et al.
Table 3. Optimization of the Suzuki coupling reaction of 4-iodobenzene and phenylboronic acida
Entry
Catalyst
Catalyst amount (mg)
Additive
Base
Temperature (°C)
Yield (%)
1
Fe3O4@EDTA–Fe2+
Fe3O4@EDTA–Ni2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
Fe3O4@EDTA–Pd2+
20
20
20
20
20
20
20
20
20
20
20
20
20
20
10
30
—
—
—
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
CaCO3
Et3N
100
100
100
25
40
60
80
80
80
80
80
80
80
80
80
80
80
ND
ND
68
21
37
47
69
94
36
61
85
56
60
ND
51
94
ND
2
3
—
4
—
5
—
6
—
7
—
8
TBAB
SDSb
CTAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
9
10
11
12
13
14
15
16
17
—
K2CO3
K2CO3
K2CO3
aReaction conditions: 4-iodobenzene (1 mmol), phenylboronic acid (1.1 mmol), base (2 mmol), H2O (4 ml), 3 h.
bSodium dodecylsulfate.
A series of available bases were screened. Excellent yields are
obtained with inorganic bases like K2CO3 and Na2CO3, among
which K2CO3 is the best choice (Table 3, entries 8 and 11). No
significant difference is observed when slightly increasing the
catalyst loading (entry 16). Also the reaction was investigated in
the absence of Fe3O4@EDTA–Pd2+. The results indicate that
Fe3O4@EDTA–Pd2+ is essential for the reaction (entry 17).
Encouraged by these results with iodobenzene, we further
expanded the reaction scope (Scheme 3). Under the optimized con-
ditions described above, chloroarenes are generally not converted.
Hence, 4-chlorobenzene does not react with phenylboronic acid,
with a modest 30% conversion into coupled product being observed
after 10 h. Investigations show that application of the mentioned
conditions for the Suzuki reaction of aryl iodides and bromides in wa-
ter at 80°C is inefficient for chloroarenes and leads to homocoupling
of the phenylboronic acid as major process. In order to show the ac-
cessibility of the present work in comparison with results reported in
the literature,[19–21] we summarize some of the results for the prepa-
ration of biphenyl in Table 4, which shows that Fe3O4@EDTA–PdCl2
is the most efficient catalyst with respect to the reaction time and
temperature and exhibits broad applicability in terms of yield. Also,
after completion of the reaction, the catalyst can be removed from
the reaction vessel with the assistance of an external magnet and
reused at least five times without significant loss of activity.
Scheme 3. Suzuki reaction of various aryl halides and phenylboronic acid.
in the aqueous phase, improve the yield of the reaction. Under the
described conditions at 80°C in neat water and using K2CO3 as base,
the best performance was obtained using TBAB (entry 8).
Table 4. Various catalytic systems for the Suzuki reaction of iodobenzene
Entry
Catalyst
Pd@SMPa
Solvent
Temperature (°C)
Time (h)
Yield (%)
1
2
3
4
Dioxane
80
15
2
96[19]
Pd(OAc)2/bisphosphinite
PdNPs/PSb
Methanol
DMF–H2O (1:1)
Water
Room temperature
93[20] (conversion)
100[21] (conversion)
94
110
80
12
3
Present work
aPalladium(II)–phosphine complexes supported on magnetic nanoparticles.
bPalladium nanoparticle catalyst based on styrene–divinylbenzene.
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Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2015)

Encapsulation of Pd(II) into superparamagnetic nanoparticles
Figure 7. Visualization of the Fe3O4@EDTA–PdCl2 collection process using
an external magnet: (A) during stirring with magnetic bar; (B) nanoparticles
adsorbed on the magnetic stirring bar and a magnet attracts the magnetic
stirring bar and Fe3O4@EDTA–PdCl2 nanoparticles after completion of
reaction.
Figure 10. Time course of Suzuki–Miyaura reaction between
phenylboronic acid and 4-iodobenzene (standard run) and after hot
filtration of the catalyst.
complete by separating it from the reaction mixture with an exter-
nal permanent magnet (Fig. 7). Also, there is no change in the struc-
ture of the catalyst after the reaction: all of the observed XRD peaks
are the same as the recorded pattern of primary catalyst (Fig. 8).
The nanoparticles were then washed with ethyl acetate, air-dried
and used directly for the next round of reactions without further
purification. It is found that the Fe3O4 nanoparticle catalyst can
be recovered and reused five times without significant loss of cata-
lytic activity. A series of five consecutive reductions of nitrobenzene
with NaBH4 were carried out (Fig. 9).
Figure 8. Powder XRD powder pattern of Fe3O4@EDTA–PdCl2 after
The results suggest that although a little loss of activity is
observed gradually, the Fe3O4@EDTA–PdCl2 catalyst still retains a
relatively high activity and always good selectivity for the desired
product, and 80% yield can be achieved in the fifth run. Subse-
quently, the leaching of palladium was analysed using ICP-AES in
the Suzuki cross coupling reaction, and no absorption is observed.
This result indicates that the Fe3O4@EDTA–PdCl2 catalyst is stable
under the experimental conditions. Also, a hot filtration test was
performed to assess the impact of palladium leaching (Fig. 10):
the magnetic catalyst was removed from the reaction system. After
30 min from the beginning of the reaction and removal of catalyst
using a magnet, we observe that the reaction does not complete
even after 24 h at elevated reaction temperature (100°C) (in order
to avoid possible re-coordination or precipitation of soluble palla-
dium upon cooling). This clearly confirms that active species do
not float in the reaction medium or on the surface. From these
analyses, we conclude that effect of palladium leaching is unimpor-
tant for the present catalyst system.
reaction.
Conclusions
Figure 9. Activity lost as a function of the number of times of reuse
of Fe 3O 4@EDTA–PdCl 2 in the reduction of nitrobenzene (reaction
times: 2–5 min).
We have demonstrated for the first time the use of magnetic EDTA
as a support for immobilizing metal catalyst, i.e. Pd2+, Ni2+ and Fe2+
.
The catalytic activity of the Fe3O4@EDTA–PdCl2 nanoparticles was
studied for the reduction of nitro group and the Suzuki–Miyaura
cross-coupling reaction. The main advantage of the developed cat-
alyst is that the magnetic nature of the catalyst allows its facile re-
covery using an external magnetic effect. The catalysis was truly
heterogeneous and Fe3O4@EDTA–Pd could be reused without an
The reusability of the Fe3O4@EDTA–PdCl 2 catalyst was evalu-
ated using the reduction of nitrobenzene as a model reaction.
The procedure is simple: the Fe3O4@EDTA–PdCl2 catalyst is firstly
applied to the reaction, and then recovered after the reaction is
Appl. Organometal. Chem. (2015)
Copyright © 2015 John Wiley & Sons, Ltd.
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Acknowledgements
We acknowledge Tarbiat Modares University for partial support of
this work.
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