Palladium nanoparticles immobilized on amphiphilic and hyperbranched polymer-functionalized magnetic nanoparticles: An efficient semi-heterogeneous catalyst for Heck reaction

Article abstract of DOI:10.1002/aoc.3707

To address the obstacles facing the use of palladium-based homogeneous and heterogeneous catalysts in C─C cross-coupling reactions, a novel semi-heterogeneous support was developed based on hyperbranched poly(ethylene glycol)-block-poly(citric acid)-functionalized Fe₃O₄ magnetic nanoparticles (Fe₃O₄@PCA-b-PEG). Because of the surface modification of the Fe₃O₄ nanoparticles with amphiphilic and hyperbranched polymers (PCA-b-PEG), these hybrid materials are not only soluble in a wide range of solvents (e.g. water, ethanol and dimethylformamide) but also are able to trap Pd²⁺ ions via complex formation of free carboxyl groups of the PCA dendrimer with metal ions. The reduction of trapped palladium ions in the dendritic shell of Fe₃O₄@PCA-b-PEG leads to immobilized palladium nanoparticles. The morphology and structural features of the catalyst were characterized using various microscopic and spectroscopic techniques. The catalyst was effectively used in the palladium-catalysed Mizoroki–Heck coupling reaction in water as a green solvent. In addition, the catalyst can be easily recovered from the reaction mixture by applying an external magnetic field and reused for more than ten consecutive cycles without much loss in activity, exhibiting an example of a sustainable and green methodology.

Full text of DOI:10.1002/aoc.3707

Received: 10 October 2016
DOI 10.1002/aoc.3707
Revised: 13 November 2016
Accepted: 15 November 2016
F U L L PA P E R
Palladium nanoparticles immobilized on amphiphilic and
hyperbranched polymer‐functionalized magnetic nanoparticles:
An efficient semi‐heterogeneous catalyst for Heck reaction
Seyed Jamal Tabatabaei Rezaei | Azin Shamseddin | Ali Ramazani |
Asemeh Mashhadi Malekzadeh | Pegah Azimzadeh Asiabi
Department of Chemistry, Faculty of Science,
To address the obstacles facing the use of palladium‐based homogeneous and het-
erogeneous catalysts in C─C cross‐coupling reactions, a novel semi‐heterogeneous
support was developed based on hyperbranched poly(ethylene glycol)‐block‐
poly(citric acid)‐functionalized Fe3O4 magnetic nanoparticles (Fe3O4@PCA‐
University of Zanjan, PO Box 45195‐313, Zanjan,
Iran
Correspondence
S. J. Tabatabaei Rezaei, Department of Chemistry,
Faculty of Science, University of Zanjan, PO Box
b‐PEG). Because of the surface modification of the Fe3O4 nanoparticles with
45195‐313, Zanjan, Iran.
Email: sjt.rezaei@znu.ac.ir
amphiphilic and hyperbranched polymers (PCA‐b‐PEG), these hybrid materials are
not only soluble in a wide range of solvents (e.g. water, ethanol and
dimethylformamide) but also are able to trap Pd2+ ions via complex formation of free
carboxyl groups of the PCA dendrimer with metal ions. The reduction of trapped
palladium ions in the dendritic shell of Fe3O4@PCA‐b‐PEG leads to immobilized
palladium nanoparticles. The morphology and structural features of the catalyst were
characterized using various microscopic and spectroscopic techniques. The catalyst
was effectively used in the palladium‐catalysed Mizoroki–Heck coupling reaction
in water as a green solvent. In addition, the catalyst can be easily recovered from
the reaction mixture by applying an external magnetic field and reused for more than
ten consecutive cycles without much loss in activity, exhibiting an example of a
sustainable and green methodology.
KEYWORDS
green methodology, magnetic nanoparticles, Mizoroki–Heck reaction, palladium,
semi‐heterogeneous
1
|
INTRODUCTION
but also to contamination of the final product by leached
metal, which makes the homogeneous catalysts inapplicable
The palladium‐catalysed Heck cross‐coupling reaction
between aryl halides and alkenes is a versatile method for
carbon–carbon bond formation in synthetic and medicinal
chemistry.[1–7] In this context, various palladium‐based com-
pounds, usually complexes with phosphane ligands, have been
utilized for this transformation under homogeneous condi-
tions.[8–14] Although these catalytic systems exhibit high
catalytic activity and selectivity, their separation from the
reaction mixture, recovery and reusability are challenging.
This leads not only to loss of expensive metal and ligands,
especially for large‐scale production. For this reason, many
attempts have been made to immobilize metal complexes on
a wide variety of solid supports, such as silica‐based materials,
magnetic nanoparticles, self‐assembled monolayers, carbon
nanomaterials, dendrimers and functional polymers.[15–23]
However, the reactivity and selectivity of the heterogenized
catalysts are significantly lower than those of their homoge-
neous analogues.[24,25]
The field of semi‐heterogeneous catalysis is at the frontier
between homogeneous and heterogeneous catalysis. These
Appl Organometal Chem. 2017;e3707.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.3707

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TABATABAEI REZAEI ET AL.
semi‐heterogeneous catalytic systems not only show high
activity and selectivity but also possess simplicity of catalyst
separation, recovery and recycling.[26–30] Magnetic nanopar-
ticles, with their intrinsic high surface‐to‐volume ratio, high
stability and reactivity, facile preparation and easy and com-
plete recovery by means of an external magnet, have emerged
as robust semi‐heterogeneous catalyst supports. However,
bare magnetic nanoparticles tend to agglomerate within a
short period of time. To overcome this inherent drawback
and to induce constant semi‐heterogeneous ability, the sur-
face of the magnetic nanoparticles has to protect by a suitable
coating materials.[31–35]
To take advantage of these characteristics and also as a
part of our ongoing research programme on the design of
new catalysts for the development of useful and green syn-
thetic methodologies,[36–40] in the present paper, we report a
novel and easy approach for immobilizing Pd nanoparticles
on amphiphilic and hyperbranched poly(ethylene glycol)‐
block‐poly(citric acid) (PCA‐b‐PEG)‐functionalized Fe3O4
magnetic nanoparticles (Fe3O4@PCA/Pd(0)‐b‐PEG). The
catalyst is designed with the aim of combining the superior
supporting property of hyperbranched PCA‐b‐PEG to effec-
tively immobilize and stabilize Pd nanoparticles in organic
and aqueous media with the magnetic property of Fe3O4
nanoparticles for simple catalyst separation and therefore to
improve catalyst reusability (Scheme 1). The synthesized
Fe3O4@PCA/Pd(0)‐b‐PEG nanocomposite remains soluble
in various polar organic and aqueous solvents, but can be
simply separated from reaction solutions by applying an
external magnetic field. The Fe3O4@PCA/Pd(0)‐b‐PEG cata-
lyst shows high efficiency for the Heck cross‐coupling reac-
tion in aqueous solution without the need of any ligand
under mild conditions and can be efficiently reused without
much loss in activity.
2
|
EXPERIMENTAL
2.1 | Apparatus and reagents
FeCl3, 4‐dimethylaminopyridine (DMAP), N,N′‐dicyclo-
hexylcarbodiimide (DCC), citric acid monohydrate,
dichloromethane (DCM), 25% ammonia solution, acetone,
poly(ethylene glycol) (PEG) with average molecular weight
of 2000 g mol−1, PdCl2 and molecular sieve were purchased
from Merck Chemical Co. FeSO4⋅7H2O was obtained from
Sigma‐Aldrich. All chemicals were of analytical grade and
used without purification, except for PEG which was purified
by azeotropic distillation with dry toluene and DCM which
was dried over P2O5 and distilled. Dimethylsulfoxide (DMSO)
was dried over activated molecular sieve. Toluene was dried
by refluxing over sodium and distilled just prior to use.
Scanning electron microscopy (SEM) was performed
with a field emission scanning electron microscope (Philips
XL‐30) operating at 17 kV. Transmission electron micros-
copy (TEM) analyses were performed with a LEO 912AB
electron microscope. The content of Pd in the composite
was determined using inductively coupled plasma mass spec-
trometry (ICP‐MS; Agilent ICP‐MS 7500 series) and thermo-
gravimetric analysis (TGA; STA 1500 instrument at a heating
rate of 10 °C min−1 in air). Fourier transform infrared (FT‐
IR) spectra were recorded with a Bomem MB‐Series FT‐IR
spectrophotometer. Reaction products were analysed using a
Varian 3900 GC (GC conversions obtained using n‐decane
as an internal standard and based on the amount of aryl halide
employed in relation to authentic standard product). Powder
X‐ray diffraction (XRD) data were collected with an XD‐
3A diffractometer using Cu Kα radiation. The magnetic prop-
erties of samples were determined at room temperature using
vibrating sample magnetometry (VSM; Meghnatis Kavir
Kashan Co., Kashan, Iran). An ultrasonic bath
(EUROSONIC® 4D ultrasound cleaner with a frequency of
50 kHz and an output power of 350 W) was used to disperse
materials in solvents.
2.2 | Synthesis of Fe3O4–(COOH)n nanoparticles
Initially, FeSO4⋅7H2O and FeCl3 were dissolved in water in a
1:2 molar ratio under nitrogen protection. The resulting dark
orange solution was stirred for 30 min at 80 °C. An aqueous
NH3 solution (1.5 M) was then added dropwise to the above
hot solution with stirring over a period of 15 min. Instant col-
our change from dark orange to black was found to occur
with particle formation. Stirring was continued for a further
30 min followed by cooling to room temperature. Then
4 ml of an aqueous solution of citric acid (0.5 g ml−1) was
added to the reaction mixture and reaction temperature was
slowly raised to 90 °C under reflux and reacted for 60 min
with continuous stirring. The solvent was removed by mag-
netic decantation. Washing of the particles was done several
times with water and then ethanol to make the iron dispersion
SCHEME 1 Synthetic route employed for preparation of Fe3O4@PCA‐b‐PEG
magnetic nanoparticles

TABATABAEI REZAEI ET AL.
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free of any residual salts used during the co‐precipitation. The
final supernatant was decanted magnetically to afford the as‐
prepared superparamagnetic Fe3O4–(COOH)n nanoparticles.
in an oil bath. The reaction was monitored by TLC (or GC
if necessary). After completion of the reaction, the catalyst
was magnetically separated, washed several times with abso-
lute ethanol, air‐dried and used directly for the next reaction
run without further purification. After separation of the cata-
lyst, the collected solution was poured into water and
extracted with CH2Cl2. The combined organic phases were
dried over Na2SO4, filtered and evaporated in vacuum. The
mixture was then purified by column chromatography over
silica gel (eluent: ethyl acetate–petroleum ether, 1:4) or
recrystallization to afford a product with high purity. All of
the products were characterized by comparison of NMR
spectral data with those of authentic samples.
2.3 | Synthesis of Fe3O4@PCA
A total of 0.1 g of Fe3O4–(COOH)n was added to a polymer-
ization ampoule equipped with a magnetic stirrer and vacuum
inlet. Citric acid monohydrate (1 g) was added to the ampoule
and it was sealed under vacuum. The mixture was heated to
120 °C and was stirred at this temperature for 30 min. After
removing water via the vacuum inlet, the reaction tempera-
ture was raised to 140 °C and stirred at this temperature for
1 h. Again produced water was removed via the vacuum inlet
and the reaction temperature was raised to 160 °C. The poly-
merization was continued at this temperature under dynamic
vacuum (open vacuum inlet) for 1.5 h. Coated nanoparticles
were separated using a permanent magnet and washed with
tetrahydrofuran (THF) and then ethyl acetate and dried for
characterization.
3
| RESULTS AND DISCUSSION
3.1 | Synthesis and characterization of supported
catalyst
Our approach was guided by five imperatives: (i) the catalyst
should be isolated simply by applying an external magnetic
field; (ii) the designed system should be hydrophilic to have
good solubility in water as reaction solvent; (iii) the support
should encapsulate Pd nanoparticles to control their size and
uniform distribution; (iv) the catalyst (Pd) and substrates
should concentrate in nanosize sites; and (v) developing an
efficient, inexpensive, environmentally friendly and benign
synthetic process for the facile conversion of the Heck reaction.
The chemical synthesis of Fe3O4@PCA‐b‐PEG nano-
composite and subsequent loading of Pd are outlined in
Schemes 1 and 2. Firstly, Fe3O4 magnetic nanoparticles were
obtained by the classical chemical co‐precipitation method in
basic conditions.[39] Then the Fe3O4 magnetic nanoparticle
surfaces were functionalized with COOH groups using citric
acid at 90 °C. Both citric acid and functionalized magnetic
nanoparticles contain alcoholic and acidic hydroxyl func-
tional groups; hence citric acid can be polymerized onto the
functionalized magnetic nanoparticles through polyconden-
sation reaction.[41] In this step, poly(citric acid) dendrimer
was grafted onto magnetic nanoparticles (Scheme 1). In order
to increase the water solubility of the magnetic nanocompos-
ite, the surface of Fe3O4@PCA nanoparticles was further
2.4 | Synthesis of Fe3O4@PCA‐b‐PEG
PEG (Mn = 2000 g mol−1, 4.5 mmol, 8.1 g) and DMAP
(5.25 mmol, 0.641 g) were dissolved in dry DCM (30 ml)
at 0 °C, followed by addition of Fe3O4@PCA (1 g, 1.5 mmol
COOH groups calculated by titration). Then DCC (4.5 mmol,
0.928 g) and dry DCM (20 ml) were added to the mixture
dropwise in a period of 30 min with stirring. The reaction
mixture was stirred at 0 °C for another 1 h and then at room
temperature for 48 h. Modified Fe3O4@PCA‐b‐PEG nano-
particles were separated using a permanent magnet and
washed with DCM and then ethanol and dried for
characterization.
2.5 | Preparation of Fe3O4@PCA/Pd(0)‐b‐PEG
nanocomposite
Aqueous solutions of PdCl2 (0.25 g in 3 ml) and
Fe3O4@PCA‐b‐PEG (0.25 g in 10 ml) were mixed and
placed in an ultrasonic bath for 10 min to well disperse
Pd2+ ions in the dendritic (PCA) shell of the Fe3O4@PCA‐
b‐PEG nanocomposite. The mixture was stirred at room tem-
perature for 8 h and reduction was carried out by the addition
0.08 ml of an aqueous solution of NaBH4 (0.01 M) to the
mixture and stirring at room temperature for 1 h. The
Fe3O4@PCA/Pd(0)‐b‐PEG nanocomposite was magnetically
separated, washed well with water and ethanol (2 × 50 ml)
and dried under vacuum at 50 °C for 4 h.
2.6 | General procedure for catalytic tests
A mixture of an aryl halide (1.0 mmol), an alkene (methyl
acrylate or acrylic acid; 1.2 mmol), K2CO3 (1.2 mmol) and
Fe3O4@PCA/Pd(0)‐b‐PEG (92 mg, 0.05 mol% Pd with
respect to aryl halide) in water (3 ml) was heated to 90 °C
SCHEME 2 Synthetic route employed for preparation of Fe3O4@PCA/
Pd(0)‐b‐PEG magnetic nanoparticles

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TABATABAEI REZAEI ET AL.
modified using PEG (Scheme 1). PEG is amphiphilic and
soluble in water as well as in many organic solvents (e.g.
methylene chloride, ethanol, toluene, acetonitrile and chloro-
form).[42–48] In the final step, Pd2+ ions were introduced into
the hyperbranched PCA‐b‐PEG polymer on the surface of
Fe3O4 and they were subsequently reduced by sodium boro-
hydride, resulting to the formation of the Fe3O4@PCA/
Pd(0)‐b‐PEG nanocomposite (Scheme 2).
Figure 1 shows photographs of water, ethanol and
dimethylformamide (DMF) dispersions of as‐prepared Fe3O4,
Fe3O4@PCA‐b‐PEG and Fe3O4@PCA/Pd(0)‐b‐PEG mag-
netic nanoparticles after a week of static placement. During
the preparation process of Fe3O4, a black precipitate appeared
instantly as aqueous NH3 solution was added. Even after soni-
cation for 30 min, evident precipitation still occurred on stand-
ing for several minutes (Figure 1). By comparison, the dark
brown aqueous dispersions of Fe3O4@PCA‐b‐PEG and
Fe3O4@PCA/Pd(0)‐b‐PEG magnetic nanoparticles exhibit
high stability, and no obvious precipitate appears after static
placement of one week or even longer. Similar results are
observed for ethanol and DMF solvents (Figure 1). Therefore,
the surface modification of Fe3O4 nanoparticles with amphi-
philic and hyperbranched PCA‐b‐PEG has a great effect on
the stability of aqueous and organic solvent dispersions.[42–48]
Dynamic light scattering (DLS) is a common technology
used to measure the size of nanoparticles, emulsions and
molecules that have been dispersed or dissolved in a liquid.
The size of Fe3O4@PCA/Pd(0)‐b‐PEG was determined using
DLS measurements. As shown in Figure 2(A), the size distri-
bution diagram of the particles in aqueous solution indicates
a uniform distribution with an average hydrodynamic diame-
ter of 49.5 nm. Figure 2(B, C) shows SEM and TEM images
of
the
Fe3O4@PCA‐b‐PEG
nanocomposite.
The
Fe3O4@PCA‐b‐PEG nanospheres are clearly evident in both
SEM and TEM images with an average diameter of ca 15 nm.
TEM observations indicate that Pd nanoparticles are dis-
persed into the hyperbranched polymers on the surface of
Fe3O4 (Figure 2D). This is due to the presence of a large
number of oxygen heteroatoms, as anchoring sites, on the
surface of Fe3O4@PCA‐b‐PEG nanocomposite (Scheme 2).
The difference in nanoparticle size measured by TEM (or
SEM) and DLS should be attributed to the fact that the latter
is the hydrodynamic diameter of nanoparticles in water,
whereas the former reveals the morphological size of the
nanoparticles in the solid state.[42–48]
To further investigate the structural features of as‐pre-
pared Fe3O4, Fe3O4@PCA‐b‐PEG and Fe3O4@PCA/Pd(0)‐
b‐PEG magnetic nanoparticles, the materials were further
characterized using FT‐IR spectroscopy, XRD, TGA, X‐ray
photoelectron spectroscopy (XPS) and VSM. Figure 3 shows
the FT‐IR spectra of Fe3O4, Fe3O4@PCA and Fe3O4@PCA‐
b‐PEG magnetic nanoparticles. In all three spectra, the char-
acteristic absorption band of Fe3O4 appears in the range
565–585 cm−1 [39]
In Figure 3(B, C), the peaks in the range
.
3050–3630 cm−1 are attributed to hydroxyl functional groups
of grafted PCA. In these spectra the absorption band of car-
bonyl groups of poly(citric acid) appears in the range 1502–
1722 cm−1 and the peaks in the range 1255–1443 cm−1 are
due to C─O.[41] Also, overlapped peaks in the range 1075–
1150 cm−1 for C─O─C appear, consistent with the addition
of PEG ether units.[42] The asymmetric absorption band of
carboxyl group is negatively shifted in the spectra of
Fe3O4@PCA/Pd(II)‐b‐PEG (Figure 3D) and Fe3O4@PCA/
Pd(0)‐b‐PEG (Figure 3E), implying coordination of carboxyl
groups with Pd(II) and Pd(0), respectively.[49]
Figure 4 presents the XRD patterns of as‐prepared Fe3O4,
Fe3O4@PCA‐b‐PEG and Fe3O4@PCA/Pd(0)‐b‐PEG mag-
netic nanoparticles. The three XRD patterns show all the
major peaks corresponding to Fe3O4, due to their similar
compositions. The peaks at 2θ = 30.1°, 35.6°, 43.1°, 53.2°,
57.2° and 62.9° can be assigned to the (220), (311), (400),
(422), (511) and (440) planes, respectively.[39] The other
three peaks of Fe3O4@PCA/Pd(0)‐b‐PEG (Figure 4C) are
FIGURE 1 Digital photos of water, ethanol and DMF dispersions of (a)
Fe3O4, (b) Fe3O4@PCA‐b‐PEG and (c) Fe3O4@PCA/Pd(0)‐b‐PEG magnetic
nanoparticles after a week of static placement

TABATABAEI REZAEI ET AL.
5 of 10
FIGURE 3 FT‐IR spectra of (a) Fe3O4, (b) Fe3O4@PCA, (c) Fe3O4@PCA‐
b‐PEG, (d) Fe3O4@PCA/Pd(II)‐b‐PEG and (e) Fe3O4@PCA/Pd(0)‐b‐PEG
magnetic nanoparticles
FIGURE 4 XRD patterns of (a) Fe3O4, (b) Fe3O4@PCA‐b‐PEG and (c)
Fe3O4@PCA/Pd(0)‐b‐PEG magnetic nanoparticles
characteristic of face‐centred cubic crystalline Pd, corre-
sponding to the planes (111), (200) and (220) at 2θ values
of about 40.02°, 46.59° and 68.03°, respectively.[39]
The surface composition and chemical oxidation states of
the prepared Fe3O4@PCA/Pd(0)‐b‐PEG catalyst were char-
acterized using XPS. The XPS survey scan (Figure 5A)
shows the presence of Pd 3d signal together with Fe 2p,
O 1 s and C 1 s signals derived from Fe3O4@PCA‐b‐PEG.
Thus, the XPS data also confirm the presence of Pd in the
prepared Fe3O4@PCA/Pd(0)‐b‐PEG catalyst. The high‐reso-
lution Pd 3d XPS scan of Fe3O4@PCA/Pd(0)‐b‐PEG is illus-
trated in Figure 5(B) which reveals the presence of Pd 3d5/2
and 3d3/2 peaks at binding energies of 335.3 and 340.75 eV,
respectively. These results are consistent with those reported
for metallic Pd(0) oxidation state.[40]
FIGURE 2 (a) DLS plot of Fe3O4@PCA/Pd(0)‐b‐PEG in water. (b) SEM
image of Fe3O4@PCA‐b‐PEG. TEM images of (c, c) Fe3O4@PCA‐b‐PEG and
(d, d) Fe3O4@PCA/Pd(0)‐b‐PEG at different magnifications
The ICP‐MS technique was used to calculate the weight
percent of Pd in the catalyst. The data show that the amount

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TABATABAEI REZAEI ET AL.
FIGURE 6 TGA curves of (a) Fe3O4, (b) Fe3O4@PCA‐b‐PEG and (c)
Fe3O4@PCA/Pd(0)‐b‐PEG
FIGURE 5 (a) XPS survey spectrum of Fe3O4@PCA/Pd(0)‐b‐PEG, and
(b) high‐resolution Pd 3d XPS spectrum of Fe3O4@PCA/Pd(0)‐b‐PEG
of Pd immobilized on Fe3O4@PCA/Pd(0)‐b‐PEG is about
5.6 wt%. The actual weight loss corresponding to the com-
bustion of supported hybrid material was determined using
TGA as a weight loss between 100 and 800 °C, i.e. excluding
the weight loss due to desorption of water (Figure 6). Com-
paring the curves demonstrates that the amount of Pd in the
synthesized catalyst is about 5.55 wt%, which is consistent
with the results obtained using the ICP‐MS technique.
Magnetic properties of bare Fe3O4 and Fe3O4@PCA/
Pd(0)‐b‐PEG nanoparticles were investigated using VSM.
The related magnetization curves are depicted in Figure 7.
This confirms that the obtained nanoparticles have
superparamagnetic behaviour at room temperature with neg-
ligible hysteresis loop. The saturation magnetization of
Fe3O4 and Fe3O4@PCA/Pd(0)‐b‐PEG nanoparticles is found
as 49.7 and 41.5 emu g−1, respectively, which means that the
catalyst can be quickly recycled from reaction solution using
an external magnetic field (inset of Figure 7). The slight
decrease in the saturation magnetization of the modified
FIGURE 7 Magnetization curves of (a) Fe3O4 and (b) Fe3O4@PCA/Pd(0)‐
b‐PEG. Inset shows a digital image of magnetic separation of catalyst after
reaction
Fe3O4@PCA/Pd(0)‐b‐PEG nanoparticles may be owing to
the effect of higher density of polymer on the surface of
Fe3O4 nanoparticles.[50,51]
3.2 | Catalytic Activity of Fe3O4@PCA/Pd(0)‐b‐PEG in
heck cross‐coupling reactions
In general, C─C cross‐coupling reactions are performed in a
polar organic solvent, such as N‐methyl‐2‐pyrrolidone
(NMP) and DMF, or in water.[1–7] Considering that the
Fe3O4@PCA/Pd(0)‐b‐PEG magnetic nanocatalyst is fully

TABATABAEI REZAEI ET AL.
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soluble in water, in this study, we initially investigated the
possibility of the Heck cross‐coupling reaction of
bromobenzene with methyl acrylate in pure water without
any ligand under aerobic conditions catalysed by
Fe3O4@PCA/Pd(0)‐b‐PEG. As evident from Table 1, of the
bases screened, K2CO3 shows the highest yield of 98%
(Table 1, entry 1). Since the effect of base was optimized,
the influence of the amount of catalyst on the reaction was
studied in the next step. We find that with increasing amount
of catalyst (Table 1, entry 8), the yield of the product does
not change, and decreasing the amount of catalyst (Table 1,
entries 9–12) leads to a decrease in the yield of product. The
effect of temperature on the reaction progress was also inves-
tigated, the highest yield being obtained at 90 °C (Table 1,
entries 13–15). We also screened the use of diverse solvents
such as water, DMF, NMP, toluene, THF and ethanol (
Table 1, entries 1 and 16–20) and excellent yield of the cou-
pling product is obtained when using water (Table 1, entry 1).
In order to investigate the catalytic applicability of
Fe3O4@PCA/Pd(0)‐b‐PEG, we studied the Heck coupling
reactions of acrylic acid and methyl acrylate with various aryl
halides under similar conditions (Table 2). The results show
that various aryl iodides or bromides bearing either elec-
tron‐withdrawing or electron‐donating groups (Table 2,
entries 1–28) can be efficiently converted to the correspond-
ing coupling products in high yields under optimum
conditions. The reaction is also facile with sterically hindered
substrates (Table 2, entries 21, 22, 25 and 26), although a lon-
ger reaction time (6 h) is required. Furthermore, a catalytic
amount of Fe3O4@PCA/Pd(0)‐b‐PEG can be used in the Heck
coupling of even less reactive aryl chloride derivatives in mod-
erate yields with an increase in the reaction time as the elec-
tron‐donating properties of substituents increase (Table 2,
entries 29–34). It should be also noted that aryl iodides and
bromides can be selectively coupled with olefins in the pres-
ence of aryl chlorides. When the substrate contains both
iodo/bromo and chloro substituents, methyl acrylate and
acrylic acid couple readily only with the iodo (or bromo) func-
tion (Table 2, entries 7, 8, 27 and 28).
To determine Pd(0) as the active site of Fe3O4@PCA/
Pd(0)‐b‐PEG, the Hg(0) poisoning test was conducted.[40]
To carry out the test, 0.1 ml (1.35 g) of mercury was added
to 3 ml of water along with 0.092 g (Pd(0) content of
0.05 mol%) of Fe3O4@PCA/Pd(0)‐b‐PEG, and the mixture
was stirred overnight at room temperature. To this, 1 mmol
of bromobenzene, 1.2 mmol of methyl acrylate and
1.2 mmol of K2CO3 were added. The mixture was then
heated to 90 °C in an oil bath. The progress of the reaction
was monitored using the method described in Section 1.
The catalyst is found to be completely inactive towards
the Heck reaction under these conditions (Figure 8). This
result demonstrates that the metallic mercury amalgamates
TABLE 1 Optimization of reaction conditions for Heck reaction catalysed by Fe3O4@PCA/Pd(0)‐b‐PEGa
Entry
Solvent
Catalyst amount (mol% Pd)
Base
Temp. (°C)
Yield (%)b
1
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
DMF
NMP
Toluene
THF
EtOH
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.06
—
K2CO3
Na2CO3
Et3N
90
90
98
95
90
63
54
58
85
98
—
31
48
79
98
85
Trace
89
92
33
37
63
2
3
90
4
NaOAc
Cs2CO3
NaOH
90
5
90
6
90
7
DABCO
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
90
8
90
9
90
10
11
12
13
14
15
16
17
18
19
20
0.02
0.03
0.04
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
90
90
90
100
80
r.t.
90
90
90
Reflux
Reflux
aReaction conditions: 1 mmol of bromobenzene, 1.2 mmol of methyl acrylate, 1.2 mmol of base for 4 h.
bIsolated yield.

8 of 10
TABATABAEI REZAEI ET AL.
TABLE 2 Heck reaction of acrylic acid and methyl acrylate with various
aryl halidesa
Entry
R1ArX
R2
CO2Me
Time (h)
Yield (%)b
1
C6H5I
2
2
>99
98
97
95
>99
>99
96
94
97
93
98
95
94
91
93
90
91
88
87
85
85
82
90
88
87
85
92
90
52
47
44
39
40
35
2
C6H5I
CO2H
3
p‐CH3C6H4I
p‐CH3C6H4I
p‐FC6H4I
CO2Me
CO2H
4
4
4
5
CO2Me
CO2H
2
6
p‐FC6H4I
2
7
p‐ClC6H4I
CO2Me
CO2H
4
8
p‐ClC6H4I
4
9
p‐OHC6H4I
p‐OHC6H4I
C6H5Br
CO2Me
CO2H
4
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
4
CO2Me
CO2H
4
C6H5Br
4
FIGURE 8 Kinetic profiles for coupling of bromobenzene to methyl acrylate
under normal conditions and in presence of excess Hg(0)
p‐CH3C6H4Br
p‐CH3C6H4Br
p‐OHC6H4Br
p‐OHC6H4Br
m‐OHC6H4Br
m‐OHC6H4Br
o‐OHC6H4Br
o‐OHC6H4Br
m,p‐(OH)2C6H3Br
m,p‐(OH)2C6H3Br
m‐OCH3C6H4Br
m‐OCH3C6H4Br
m‐OCH3, p‐OHC6H3Br
m‐OCH3, p‐OHC6H3Br
m‐ClC6H4Br
m‐ClC6H4Br
C6H5Cl
CO2Me
CO2H
5
5
CO2Me
CO2H
5
reaction of the filtrate under similar conditions did not pro-
ceed significantly. The stability and reusability of the
Fe3O4@PCA/Pd(0)‐b‐PEG catalyst were studied for the
Heck cross‐coupling reaction of bromobenzene with methyl
acrylate in optimum conditions for up to ten cycles. After
each experimental run, the reaction products were extracted
into CH2Cl2 followed by a simple decantation and
5
CO2Me
CO2H
5
5
CO2Me
CO2H
5
5
CO2Me
CO2H
6
6
CO2Me
CO2H
5
5
CO2Me
CO2H
6
6
CO2Me
CO2H
5
5
CO2Me
CO2H
12
12
24
24
24
24
C6H5Cl
p‐CH3C6H4Cl
p‐CH3C6H4Cl
p‐OHC6H4Cl
p‐OHC6H4Cl
CO2Me
CO2H
CO2Me
CO2H
aReaction conditions: 1 mmol of R1ArX, 1.2 mmol of olefin,1.2 mmol of K2CO3,
3 ml of water, Fe3O4@PCA/Pd(0)‐b‐PEG (0.05 mol% Pd) at 90 °C.
bIsolated yield of pure product.
with Pd(0), thus quenching the activity of the Pd(0) nano-
particles that would otherwise catalyse the C─C cross‐cou-
pling reaction.
For practical applications of semi‐heterogeneous cata-
lysts, especially in industry, the heterogeneity and level of
reusability are very important. To address these issues, we
undertook a series of tests as described below. Our prelimi-
nary studies show that Fe3O4@PCA/Pd(0)‐b‐PEG is very sta-
ble to air and moisture. Furthermore, the catalyst was
magnetically separated from the reaction mixture after ca
50% conversion at the reaction temperature (90 °C). Further
FIGURE
9
(a) Effect of recycling on the catalytic efficiency of
Fe3O4@PCA/Pd(0)‐b‐PEG. (b) TEM image of catalyst after being reused
ten times

TABATABAEI REZAEI ET AL.
9 of 10
TABLE 3 Comparison of results for Mizoroki–Heck cross‐coupling of aryl bromides with acrylate with some other previously catalysts
Entry
Catalyst
Additive
Solvent
Base
Atmosphere
Temp. (°C) Time (h)
Yield (%)
Ref.
1
Fe3O4@PCA/Pd(0)‐b‐PEG
—
Water
K2CO3
—
90
2
>99
Present
work
[52]
2
3
PNP‐SSS
—
Water
NMP
K2CO3
K2CO3
—
Reflux
100
6
90
92
[53]
Palladium dichloro‐bis(aminophosphine)
complexes
TBAB
N2
14
[54]
[55]
[22]
[40]
[56]
4
5
6
7
8
PdCl2(DBPF)2
TBAC
—
DMAc
DMF
Cy2NMe
NaOAc
K2CO3
K2CO3
K2CO3
—
—
—
—
—
80
100
90
24
24
10
2
94
76
94
95
86
Pd(0)MCM‐41
Pd@MCOP
—
DMSO–water
NMP
PdNs‐PAMAM‐g‐MWCNTs
—
100
80
Imidazolium ionic liquid‐tagged
Pd complex
—
Water
5.5
[23]
[57]
[58]
[59]
[60]
9
Nano tetraimine Pd(0) complex
PdNs@H40‐PCL‐PEG
Fe3O4‐NH2–Pd
Eco‐Pd3
—
Water
K2CO3
K2CO3
K2CO3
NaOAc
K2CO3
—
—
—
—
—
90
r.t.
2.5
1.5
93
98
10
11
12
13
—
Water
—
NMP
130
140
100
10
>99
97
TBAB
TDTAT
NMP–water
Water
24
24
PdCl2
88
PNP = palladium nanoparticles; SSS = silica starch substrate; DBPF = di‐bisphosphenylferrocene; TDTAT = N2,N4,N6‐tridodecyl‐1,3,5‐triazine‐2,4,6‐triamine; TBAB =
tetrabutylammonium bromide; TBAC = tetrabutylammonium chloride; MCM = mesoporous silica matrix; MCOP = mesoporous covalent organic polymer; Eco = eco‐
catalysts.
subsequently the conversion of product was determined by
means of GC. Also, the magnetically separated catalyst was
washed several times with absolute ethanol, air‐dried and used
directly for the next reaction run. The results shown in Figure 9
(A) demonstrate that the recovered catalyst does not signifi-
cantly lose its activity after ten cycles, clearly illustrating the
high stability and excellent reusability of the catalyst. A
TEM image of the catalyst shows that the morphology and size
of the catalyst after recycling 10 times do not change signifi-
cantly (Figure 9B). Also, the amount of Pd species leached
into the reaction solution was measured using the ICP‐MS
technique. The amount of Pd leaching after the first and last
run is only 0.011 and 0.057%, respectively. Therefore, we
may conclude that the observed catalysis is truly heteroge-
neous in nature.
reactions of aryl halides with olefinic compounds in an
environmentally friendly solvent under ligand‐free ambient
conditions. The modification of Fe3O4 magnetic nanoparti-
cles with amphiphilic and hyperbranched polymers (PCA‐
b‐PEG) was carried out to enhance the dispersibility of
the magnetic nanoparticles in polar solvents and to stabi-
lize the immobilized Pd nanoparticles via coordination
with the free carboxyl groups of the PCA dendrimer. High
stability and reusability of catalyst, short reaction times,
easy purification and very low Pd leaching are other
advantages of this green catalytic process. Further efforts
to extend the application of this semi‐heterogeneous sys-
tem in other Pd transformations are currently in progress
in our laboratory.
Comparison of the results for various other catalysts
employed previously for the Mizoroki–Heck cross‐coupling
of aryl bromides with acrylate indicates that our synthesis
methodology (Table 3, entry 1) uses greener and milder reac-
tion conditions as well as showing better catalytic activity
compared to other reported catalysts. These systems require
external additives (entries 3, 4, 12 and 13), an inert atmo-
sphere (entry 3) and longer reaction times (entries 3–6 and
11–13), involve difficulties of catalyst separation from the
reaction mixture (entries 3, 4, 8–10 and 13), and require
higher temperature (entries 2, 11 and 12) and use of non‐
ecofriendly organic solvents (entries 3–7, 11 and 12).
ACKNOWLEDGEMENT
We are grateful to the University of Zanjan Research Council
for partial support of this work.
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