Preparation of monometallic (Pd, Ag) and bimetallic (Pd/Ag, Pd/Ni, Pd/Cu) nanoparticles via reversed micelles and their use in the Heck reaction

Article abstract of DOI:10.1016/j.tet.2012.02.028

The metal nanoparticles (NPs) have been prepared using a water-in-oil microemulsion system of water/dioctyl sulfosuccinate sodium salt (aerosol-OT, AOT)/isooctane at 25 °C. Since the NPs produced in this system can endure forcing conditions (100 °C), this system has been used for the synthesis of nano-catalysts in the Heck reactions. FE-SEM, DLS, and UV/vis analyses have been used to characterize the surface morphology, size, and proof of the formation of all the prepared metal NPs, respectively. In addition, the effects of some reaction parameters (here, bases and solvents) were optimized. Differences in the catalytic properties of the synthesized NPs have also been investigated. Consequently, the Pd/Cu (4:1) bimetallic NP showed the highest activity in the C-C coupling reaction of the iodobenzene with the styrene, thus it is employed as the superior catalyst in this study. Therefore, the Pd/Cu (4:1) bimetallic NPs were further investigated using TEM and XRD analyses. This catalyst system is also reusable for six runs with very negligible reduction in the efficiency.

Full text of DOI:10.1016/j.tet.2012.02.028

Tetrahedron 68 (2012) 3001e3011
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Preparation of monometallic (Pd, Ag) and bimetallic (Pd/Ag, Pd/Ni, Pd/Cu)
nanoparticles via reversed micelles and their use in the Heck reaction
*
Felora Heshmatpour , Reza Abazari, Saeed Balalaie
Department of Chemistry, Faculty of Science, K.N. Toosi University of Technology, P.O. Box 16315-1618, Tehran 15418, Islamic Republic of Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 August 2011
Received in revised form 30 January 2012
Accepted 13 February 2012
Available online 20 February 2012
The metal nanoparticles (NPs) have been prepared using a water-in-oil microemulsion system of water/
dioctyl sulfosuccinate sodium salt (aerosol-OT, AOT)/isooctane at 25 ꢀC. Since the NPs produced in this
system can endure forcing conditions (100 ꢀC), this system has been used for the synthesis of nano-
catalysts in the Heck reactions. FE-SEM, DLS, and UV/vis analyses have been used to characterize the
surface morphology, size, and proof of the formation of all the prepared metal NPs, respectively. In
addition, the effects of some reaction parameters (here, bases and solvents) were optimized. Differences
in the catalytic properties of the synthesized NPs have also been investigated. Consequently, the Pd/Cu
(4:1) bimetallic NP showed the highest activity in the CeC coupling reaction of the iodobenzene with the
styrene, thus it is employed as the superior catalyst in this study. Therefore, the Pd/Cu (4:1) bimetallic
NPs were further investigated using TEM and XRD analyses. This catalyst system is also reusable for six
runs with very negligible reduction in the efficiency.
Keywords:
Reverse micelle
Bimetallic nanoparticles
Nano-catalyst
Heck reaction
CeC coupling reaction
Aryl chlorides
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
supported by the ligands, such as phosphine, phosphorus, amines,
carbene, thiolate etc. However, the ligands, especially phosphine
Due to great progress of nano-catalysts in nano-chemistry and
nano-technology, many researchers have recently realized their
importance in organic reactions.1 For example, vinylation of aryl
halides, which was discovered by Heck2 and Mizoroski3 (Scheme 1)
in the early 1970s (commonly called the HeckeMizoroki reaction),
is an important reaction for the synthesis of CeC bond.4e7 Recently,
the commercial and economic aspects to the industrialization of
these reactions have been focused on.8e10 Conventionally, these
reactions are carried out in the presence of the palladium catalysts
ligands are usually unrecoverable, sensitive to air, expensive, toxic,
and degradable at elevated temperatures.11,12 In addition, these li-
gands have adverse effects on biological systems.9 In order to avoid
these problems, some researchers have made use of the active
catalysts anchored to solid supports.13,14 In this context, a wide
range of organic and inorganic supports including mesoporous
silica, zeolites, active carbon, dendronized support have been
applied.15e18 These catalysts are reported to be highly stable and
non-destructive; however, some of these supports have a limitation
R"
R'
R"
catalyst
Base
R'
R'
Base-HX
X
R"
R' = Aryl, Vinyl
R" = Aryl, CN, COCH3
X = Cl, Br, I
Scheme 1. The HeckeMizoroki coupling reaction.
in reusability and thermal stability (which should be also consid-
ered as important advantages in the organic reaction).19e21
Metal NPs are of the highest importance due to their very unique
chemical and physical properties.22 In comparison to the bulk state,
* Corresponding author. Tel.: þ98 21 23064230; fax: þ98 21 22853650; e-mail
addresses: Heshmatpour@kntu.ac.ir, f.heshmatpour@yahoo.com (F. Heshmatpour),
Reza.nano2011@gmail.com (R. Abazari), Balalaie@kntu.ac.ir (S. Balalaie).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2012.02.028

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F. Heshmatpour et al. / Tetrahedron 68 (2012) 3001e3011
the metal NPs due to their small particle size and high surface-to-
volume ratio23 bring about novel thermal, electronic, optical, mag-
netic, and chemical properties,24 and can also cause specific catalytic
activities.25,26 As recorded in the literature, several methods have
been developed for metal NPs preparation, including sonochemical
methods, citrate reduction, alcohol reduction, laser oblation
methods, g-radiolysis technique, reverse micelle methods etc. Be-
cause particle morphology may vary depending on synthesis con-
ditions and since the particles in nano-scale have size-dependent
properties.27 Therefore, it is important to adopt a reliable method
for developing uniform and de-agglomerated particles.
The reverse micelle route provides excellent control over the
particle size and particle shape of the NPs.28 Exchanges between
the micelles in the solution result in uniform distribution of mi-
celles volume throughout the solution. This makes it possible to
control NPs size through controlling micelles diameter. Since all
micelles are equal in size, the prepared NPs are fairly homogeneous
and uniform.29,30 Another advantage of this system is that, metal
NPs can be synthesized at room temperature, and the surfactants
surrounding the particles in the solution can easily be removed.
Since the catalytic activity of transition metals is related to their
(d) orbital properties, selecting a suitable metal is the first step for
selectivity control and synthesis of the organic reactions.1 The im-
portance of applying palladium as supported systems of Pd/ligand
and Pd/C as a versatile tool in various CeC,31 CeN,32 and CeO33
coupling reactions, including Negishi, Kumada, Suzuki Miyaura,
Stille, aryl amination etc., is recognized. However, the yield of the
catalysts can change by addition of another element to the metal,
thus bimetallic NPs create new properties compared to the
monometallic NPs.34
Fig. 1. Digital photographs of microemulsions before (the upper) and after (the lower)
reduction with hydrazine hydrate. (a) Pd; (b) Ag; (c) Pd/Ag (1:1); (d) Pd/Ni (1:1); (e)
Pd/Cu (4:1).
For many years, only aryl bromides and iodides were used in
Heck reactions. However, the previously used Pd catalysts delayed
oxidationeaddition step in Pd(II) complexes in aryl chlorides due to
the strong CeCl bond.10,35 Therefore, efforts have been made to
achieve a better catalyst to make the CeCl bond active. Accordingly,
a small number of heterogeneous Pd catalysts have been reported
to activate aryl chlorides in difficult conditions.36e40 In this pro-
tocol, we have also developed a novel nano-catalyst anchored to
AOT, which can effectively activate CeCl bond in aryl chlorides.
So far, different methods have been adopted using monometallic
NPs to conduct the Heck reaction.31,41e43 On the other hand, appli-
cation of bimetallic NPs prepared bymicroemulsion has notyet been
properly investigated in most organic reactions. In this paper, our
purpose is to design an effective catalytic system in which the Heck
reaction is conducted by using bimetallic NPs. Then, the reaction
conditions are optimized and the results are evaluated.
DLS analysis was used to characterize the particle size distri-
bution. Fig. 3 shows the particle size distribution of Pd, Ag, and Pd/
M NPs, obtained from of DLS analysis, and Fig. 4 shows the UV/vis
absorption spectra of these mono- and bimetallic metal NPs. DLS
data have shown a narrow particle size distribution. Both the UV/
vis absorption spectra and DLS analysis indicate the formation of
metallic NPs.
UV/vis spectrometry was chosen to prove the formation of metal
NPs because of its reliability and feasibility. Reducing the metal ions
causes a decrease in the plasmon band intensity and a flattening of
the peak. As shown in Fig. 4, absorption peaks are flattened after
the addition of reducing agent indicating that the metal ions vanish.
In addition, blue shift after the addition of reducing agent supports
the formation of NPs.
For Pd NPs before and after reduction (synthesized by the same
method), similar spectra have already been reported in the litera-
ture.44 The absence of Pd plasmon band (lmax¼420 nm) in the Pd/M
(M¼Ag, Ni, and Cu) bimetallic NPs indicates that only bimetallic
NPs are formed. If bimetallic NPs were not formed, two adsorption
peaks would be observed in UV/vis spectra for each metal. It has
been proved in advance that physical mixtures of the two NPs have
two individual spectra.45 Appearance of only one absorption band
indicates the formation of bimetallic NPs. On the other hand, ex-
istence of Pd metal in bimetallic NPs usually suppresses the surface
absorption plasmon,46e49 which is more obvious for Pd/Ni and Pd/
Cu in our work. Also, it has been proved that bimetallic NPs bring
about some changes in the surface plasmon band.50,51 These ob-
servations are compatible with Mie theory,52 first reported in 1908.
According to this theory, when a metal is of nanometer dimensions
(in contrast with the bulk state), a size-dependent blue shift in the
photoelectron band (i.e., hypsochromic effect) occurs. Such a be-
havior of small particles is due to the size-dependent distribution of
the electron energy levels (quantum-size effect). In other words,
2. Results and discussion
2.1. Analysis of catalyst NPs
Metal NPs color change is very evident after the addition of the
reducing agent, which implies that the metal NPs are reduced. For
instance, the brown color of the Pd/Cu (4:1) changed immediately
into black after the addition of hydrazine hydrate. All the samples
changed into dark brown or black after reduction. These color
changes before and after reduction process are shown in Fig. 1.
The preparation conditions affect the structure, shape, and size
distribution of the metal NPs. Therefore, due to the said reasons (as
stated in the introduction); the reverse micellar method is one of
the most preferred candidates. The first question relating to the
metal NPs is to investigate the aggregation state, size, and mor-
phology. The surface morphology of all the synthesized samples
was investigated by FE-SEM, as shown in Fig. 2. The FE-SEM images
indicate that the synthesized NPs are approximately uniform and
spherical in shape.
normal position of plasmon band shifts toward
a shorter

F. Heshmatpour et al. / Tetrahedron 68 (2012) 3001e3011
3003
Fig. 2. FE-SEM images of (a) Pd, (b) Ag, (c) Pd/Ag (1:1), (d) Pd/Ni (1:1), (e) Pd/Cu (4:1).
wavelength with a decrease in the particle size. These effects can be
explained by the fact that absorption band is closely dependent on
the particle size. Also, with the more reduction of the metal ions,
the absorption band is broadened (due to the ded interband
transitions) and then vanished.
In Fig. 4(a), the absorption spectrum of the micelle solution
without metal salt is shown. According to this figure, the absorption
band (lmax¼234 nm) is related to the presence of the surfactant
molecules, which is broadened after reducing agent is added to the
solution. However, this is out of the scope of the present study and
will be discarded.
For further investigation of the Pd/Cu (4:1) bimetallic NPs (as
the superior catalyst in this study), the structure and size of the
considered NPs were characterized using XRD and TEM analyses.
Among the most frequently used techniques, TEM analysis is in-
dispensable for the metal NPs studies. For TEM analysis, the syn-
thesized sample was prepared by placing a drop of microemulsion
solution onto the carbon coated copper TEM grids and drying it in
air at room temperature. In Fig. 5, the TEM image of the Pd/Cu (4:1)
bimetallic NPs shows the existence of uniform, mono-disperse, and
spherical particles.
X-ray diffraction is an effective method for investigation of the
solid structure of NPs. Therefore, the particle structure of the Pd/Cu
(4:1) NPs was further characterized by the XRD analysis. In order to
prepare the catalyst sample for XRD analysis, methanol was first
added to the reverse micellar solutions for phase separation, then
the mixture was centrifuged, and the catalyst was washed with
methanol three times, and finally the obtained catalyst was dried at
room temperature. Fig. 6 shows the phase composition of Pd/Cu
(4:1) catalyst by the XRD analysis. The phase was identified by
comparison with the Joint Committee on Powder Diffraction
Standards (JCPDSs). Because of the different peak positions of Pd,
Cu, and Pd/Cu NPs in the XRD pattern, it is very effective to employ
the XRD analysis for the determination of the Pd/Cu (4:1) NPs
structure. The characteristic peaks for Pd (JCPDS number of card,
05-0681; 2
(JCPDS number of card, 85-1326; 2
marked by their indices ((111), (200), (220), (311), (222)),
q
¼40ꢀ, 46.7ꢀ, 68ꢀ, 82ꢀ, and 86.0ꢀ) and those for Cu
q
¼43.3ꢀ, 50.5ꢀ, 74.2ꢀ, and 89ꢀ),

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F. Heshmatpour et al. / Tetrahedron 68 (2012) 3001e3011
Fig. 3. Particle size distribution of mono- and bimetallic NPs by DLS analysis.
demonstrate the presence of the face-centered cubic (fcc) Pd and
Cu crystallite planes. The XRD pattern of the Pd/Cu (4:1) catalyst in
not detected, showing that the Cu NPs were very stable against
further oxidation.
Using the DebyeeScherrer equation, the average particle size (t)
of the Pd/Cu (4:1) NPs is calculated to be 15 nm as follows (Eq. 1).
Fig. 6 appears at 2
q
values of 40.32ꢀ, 47.18ꢀ, 68.28ꢀ, 82.34ꢀ, and
86.12ꢀ corresponding to the planes (111), (200), (220), (311), and
(222) with fcc structure. In this XRD pattern, the five characteristic
diffraction peaks are corresponding to the planes of Pd, but the
presence of copper could affect the diffraction angles of Pd (shift to
l
t ¼ k =
b
cos
q
(1)
Due to the small particle size of the NPs, the peaks observed in
the XRD pattern are relatively broad. The calculated size matches
approximately to the size observed from TEM image and DLS
analysis. In this equation, t is the average particle size, k is the so-
higher 2
particle or alloy formation. To give an example, despite the fact that
the 2 value of diffraction peak of Pd for the plane (111) was located
at 40ꢀ, the 2
value of diffraction peak of the Pd/Cu (4:1) catalyst in
the plane (111) in our XRD pattern is located at 40.32ꢀ, demon-
strating that the formation of single-phase alloy causes the 2 value
to shift upward by
0.32ꢀ. In other words, this issue indicates that
q values) as a result of copper incorporation into the Pd
q
q
called shape factor, which usually takes a value of about 0.9,
the wavelength of the incident radiation, is the full width in ra-
is the angle at
l is
b
q
dians at half-maximum intensity (FWHM), and
maximum diffraction curve intensity.
q
D
a portion of Cu has entered the Pd lattice, forming the Pd/Cu alloy
phase. Also, since the radius of the Cu metal is smaller than that of
the Pd, the 2q values of the diffraction peaks of the Pd/Cu alloy are
All the analyses have consistently shown fairly uniform NPs
with small size. Recent literature has shown that synthesizing
uniform NPs for organic reactions has attracted much attention.
larger than those of the Pd particles, which follows the Vegard’s
law.53 The XRD pattern of Pd/Cu (4:1) showed that no characteristic
peak of Cu existed, which can be attributed to the low content of
the Cu in the Pd/Cu (4:1) bimetallic NP. Also, no characteristic peaks
for cuprous oxides (Cu(OH)2, Cu2O or CuO) were observed. Despite
the presence of dissolved oxygen in the solvent, the diffraction
2.2. Optimization of the reaction conditions
2.2.1. Catalytic testing. Initially, the Heck reaction of iodobenzene
and styrene was chosen as a model reaction. Loading of the catalyst,
bases, and solvents were screened to optimize reaction conditions.
peaks of oxides phase (i.e., 2
q
¼34ꢀ, 36.5ꢀ, 42.58ꢀ, and 61.4ꢀ) were

F. Heshmatpour et al. / Tetrahedron 68 (2012) 3001e3011
3005
Fig. 4. Typical UV/vis spectra of microemulsions before (I) and after (II) reduction with hydrazine hydrate. (a) Pd; (b) Ag; (c) Pd/Ag (1:1); (d) Pd/Ni (1:1); (e) Pd/Cu (4:1).
Since most papers in the literature indicate that the use of the
metals, such as Pd, Cu, and Ni in different systems can activate the
CeX bond (X¼Cl, Br, and I) in the coupling reactions, we have in-
vestigated these metals as bimetallic NPs in our system. For prac-
tical application in the Heck reaction, the lifetime, thermal stability
and reusability of prepared catalysts are very important factors.
A number of catalyst sources were examined in optimization
experiments. The rate and activity of the prepared catalysts in the
Heck reaction are compared and listed in Table 1. It should be noted
the turn over numbers (TONs) and the turn over frequencies (TOFs),
which are defined as mmol product/mmol catalyst and mmol
product/mmol catalyst per hour, were calculated from the isolated
yield. The reaction with no catalyst yielded only a small amount of
product (entry 1). When metal salts were used, results were not
acceptable (entries 2 and 3) as palladium(II) sources did not show
significant catalytic activity. Application of silver NPs showed that
the silver can decrease the Pd NPs activity in this reaction (entries
4e6). When a ratio of 1:1 of the bimetallic NPs was applied (entries
6e8), Pd/Cu showed the highest product yield. Therefore, different
ratios of this catalyst were investigated (entries 9e11). The highest
catalytic activity was achieved by Pd/Cu (4:1) bimetallic NP (entry
10). When this ratio changed to 5:1, the reaction yield decreased
(entry 11). As a result, the catalytic efficiency decreases in the order
of Pd/Cu (4:1)>Pd>>Pd/Ni (1:1)>Pd/Ag (1:1)>Ag. (For the sake of
simplicity, only the 4:1 ratio is here mentioned for Pd/Cu as the best
catalyst in the said order). All of the achieved products were trans-
isomers, which were identified by GC.
In practical use, developing catalysts that keep their catalytic
activity for a prolonged time is a major problem. In the present
work, we have studied the potential recycling process of Pd (as
a common metal in the Heck reaction) and Pd/Cu (4:1) (as the
superior catalyst in this study) catalysts. The catalytic stability of
these two catalysts up to seven runs at 100 ꢀC (in the Heck reaction
of the iodobenzene with the styrene) is shown in Fig. 7. Before the
next run, the catalyst system was easily separabledby cen-
trifugationdfrom the reaction mixture by decantation of the liquid

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F. Heshmatpour et al. / Tetrahedron 68 (2012) 3001e3011
Fig. 5. TEM image of spherical Pd/Cu (4:1) bimetallic NPs formed in the microemulsion.
Fig. 6. X-ray diffraction (XRD) spectrum of Pd/Cu (4:1) bimetallic NPs synthesized in
water/AOT/isooctane reverse micelles.
Fig. 7. Comparison of yield and reusability of Pd and Pd/Cu (4:1) NPs as catalysts in the
Heck reaction of the iodobenzene with the styrene at 100 ꢀC.
Table 1
the sixth run the yield was 8% (TON¼514, TOF¼28.5 hꢁ1). These
results can be attributed to the decreasing amount of Pd catalyst
versus the number of cycles, which in turn show that the contri-
bution of Pd catalyst in the CeC coupling reaction of the iodo-
benzene with the styrene is important. On the other hand, the
feature of the Heck reaction with Pd/Cu (4:1) is different so that
almost no loss of catalytic activity was observed after six runs.
However, after the sixth run, more decrease in the efficiency was
observed, compared to the previous runs (TON¼4491,
TOF¼249.5 hꢁ1), i.e., a 22% decrease compared with the first run.
These results show that the amount of leached catalyst after each
run is low. In general, this catalyst (Pd/Cu (4:1)) is easily separable
and recyclable in six successive runs with little reduction in cata-
lytic efficiency.
The effect of different catalysts on the reaction of iodobenzene and styrenea
Entry
Catalyst
Yieldb (%)
TONc
TOFc (hꢁ1
)
1
2
3
4
5
6
7
8
None
PdCl2
Trace
11
d
d
713
40
Pd (OAC)2
Pd
Ag
Pd/Ag (1:1)
Pd/Ni (1:1)
Pd/Cu (1:1)
Pd/Cu (2:1)
Pd/Cu (4:1)
Pd/Cu (5:1)
14
907
4806
d
50
267
d
74d
Trace
7
455
25
79
177
202
329
296
22
1424
3187
3642
5921
5335
49d
56d
91d
82d
9
10
11
a
Reaction condition: iodobenzene (2 mmol), styrene (3 mmol), NEt3 (4 mmol),
catalyst (4.5ꢂ10ꢁ4 mmol), MeOH (4 mL), 100 ꢀC, 18 h.
b
Yield was determined by GC, which was then compared to an internal standard.
Calculated from the isolated yields.
Yields are an average of two runs.
c
d
2.2.2. Effect of the base. Bases perform an essential role in the Heck
reaction since they can accelerate and regenerate catalysts. In order
to regenerate catalyst for Heck reaction, base is required to neu-
tralize and remove HX from HeCateX intermediate. In the process
of coupling reaction, it is important to select a base and solvent, so
these parameters were optimized and the experimental results are
presented in Tables 2 and 3, respectively. The reaction without
catalyst or base afforded no product (entry 1).
phase, and then the catalyst was washed with diethyl ether, dried at
60 ꢀC, and was used for a new run. Each reaction was run for 18 h,
which was the necessary time for completing the reaction. In case
of Pd catalyst, when this treatment was performed, a significant
decrease in the catalytic activity of Pd NPs was observed after each
run, so that by the second run, the activity decreased with a yield of
74e29%. Then, the catalytic activity decreased gradually so that in
Based on the type of catalytic system applied, bases show
different effects on Heck reaction.54 In view of that, previous

F. Heshmatpour et al. / Tetrahedron 68 (2012) 3001e3011
3007
Table 2
On the contrary, low yields were achieved when NaOH was used
(entry 6).
The effect of different bases on the reaction of iodobenzene and styrenea
Entry
Base
Yieldb (%)
TONc
TOFc (hꢁ1
)
NEt3 has previously been reported as a base with low product
yield;56e58 this may be due in part to blocking of free coordination
sites on the palladium center by ligands, dendrimers, organic and
inorganic supports. However, in our study, the applied NEt3 has
given a desired effect, which is thought to be due to the existence of
free coordination site at the catalyst center. Because of the great
importance attached to the bases, it is required that they should be
further investigated in the future.
1
2
3
4
5
6
7
8
None
NEt3
Trace
91d
75d
19
35
11
d
d
5921
4879
1235
2274
713
329
271
69
126
40
K2CO3
KOAC
Na2CO3
NaOH
CsCO3
NaPO4
31
40
2012
2605
112
145
a
Reaction condition: iodobenzene (2 mmol), styrene (3 mmol), base (4 mmol),
Pd/Cu (4:1) (4.5ꢂ10ꢁ4 mmol), MeOH (4 mL), 100 ꢀC, 18 h.
2.2.3. Effect of the solvent. Several organic solvents, such as DMF,
CH3CN, THF, n-hexane, DMSO, MeOH, and toluene were examined.
The experimental results are summarized in Table 3. Similar to
bases, the solvents can also cause different effects on the Heck re-
action, depending on the used catalyst system.59,60 In our system,
DMF was found to be more effective than other solvents (entry 1),
which was consistent with some of the previous reports. Solvents
like CH3CN and DMSO did not produce a good yield possibly due to
their high coordination ability (entries 2 and 5). The lowest effi-
ciency resulted when non-polar n-hexane solvent was applied
(entry 4).
b
Yield was determined by GC, which was then compared to an internal standard.
Calculated from the isolated yields.
Yields are an average of two runs.
c
d
Table 3
The effect of different solvents on the reaction of iodobenzene and styrenea
Entry
Solvent
Yieldb (%)
TONc
TOFc (hꢁ1
)
1
2
3
4
5
6
7
DMF
CH3CN
THF
n-Hexane
DMSO
MeOH
Toluene
100
13
27
6510
842
1754
d
362
47
97
d
Both our study and previous reports have indicated that aprotic
polar solvents give a good performance. Therefore, the best system
for this reaction was DMF in combination with NEt3, and Pd/Cu
(4:1) bimetallic NP as catalyst.
N.R
17
1106
5921
2733
61
329
152
91d
42
a
Reaction condition: iodobenzene (2 mmol), styrene (3 mmol), NEt3 (4 mmol),
Pd/Cu (4:1) (4.5ꢂ10ꢁ4 mmol), solvent (4 mL), 100 ꢀC, 18 h.
b
Yield was determined by GC, which was then compared to an internal standard.
Calculated from the isolated yields.
Yields are an average of two runs.
2.3. Heck cross-coupling reactions
c
d
Using the optimized reaction conditions, a broad sampling of
functionalized substrates was employed with good to excellent
yields. A range of substituents including OMe, CHO, COCH3, NO2,
NH2, COOH, OH, CN, I, Br, and Cl were compatible with this pro-
cedure. The results are listed in Table 4, entries 1e18. We found that
the yields of the CeC couplings were in the range of 66e100%,
reports have mentioned NaHCO3,9 K3PO4,43 and KOAc55 as the best
bases. In Table 2, entries 2e8, effects of different bases on the Heck
reaction are compared. The highest yield and selectivity were
observed in the presence of K2CO3 and NEt3 (entries 2 and 3).
Table 4
Heck reaction of aryl halides with olefins catalyzed by Pd/Cu (4:1)a
Entry
1
Aryl halide
Olefin
Product
Yieldb (%)
94
TONc
6291
TOFc (hꢁ1
)
349.5
I
I
O
O
2
100
5919
329
CH3
CH3
CN
3
4
93
88
5185
6401
288
I
CN
O
O
O
O
355.5
Br
CH3
CH3
CH3
5e
66d
5867
245.5
Cl
CH3
O
O
MeO
OHC
O
6
7
77
96
5859
7497
325.5
416.5
MeO
OHC
Br
Br
CH3
CH3
CH3
O
(continued on next page)
CH3

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F. Heshmatpour et al. / Tetrahedron 68 (2012) 3001e3011
Table 4 (continued )
Entry
Aryl halide
Olefin
Product
Yieldb (%)
TONc
6906
TOFc (hꢁ1
)
Br
O
O
8
91
383.5
CH3
CH3
O
O
O
O
H3COC
O2N
9
94
97
74
7293
7570
5445
405
Br
H3COC
O2N
CH3
CH3
CH3
CH3
O
10
11
420.5
302.5
Br
CH3
O
N
N
CH3
Br
O
Br
O
O
O
O
O
12
90
77d
73d
78d
85d
7077
6978
6717
7064
7834
393
291
280
294
326.5
CH3
CH3
CH3
CH3
CH3
CH3
O
HO
13e
14e
15e
16e
17e
HO
Cl
Cl
Cl
Cl
Cl
CH3
O
MeO
H2N
OHC
MeO
H2N
CH3
O
CH3
O
OHC
O2N
CH3
O
O
O
O2N
79d
84d
7165
7822
298.5
326
CH3
CH3
CH3
O
HOOC
18e
Cl
HOOC
CH3
a
Reaction condition: aryl halides (2 mmol), olefin (3 mmol), NEt3 (4 mmol), Pd/Cu (4:1) (4.5ꢂ10ꢁ4 mmol), DMF (4 mL), 100 ꢀC, 18 h.
b
c
All compounds are characterized by comparison of GC analysis.
Calculated from the isolated yields.
Yields are an average of two runs.
d
e
At 120 ꢀC for 24 h.
corresponding to the TONs from 5867 to 5919. This catalyst system
corresponding coupling products in excellent yields after 18 h
(entries 1e3). It is usually difficult to activate aryl halides because
the CeX bond (X¼I, Br, and Cl) has a relatively high bond energy.
When the CeC coupling reaction changed from iodine to chlorine,
the yield decreased because of the reaction sensitivity to the nature
of halogen (entries 2, 4, and 5). It is clear that the reaction of
chlorobenzene with olefins took about 24 h for completion. As
has relatively higher TONs and TOFs than some other systems. The
structures of the products were deduced according to the physical
properties and spectroscopic data.
The nature of the olefin exerts a significant effect on the yield
and selectivity of the reaction. The Heck reaction of various olefins
with iodobenzene proceeded easily at 100 ꢀC resulting in

F. Heshmatpour et al. / Tetrahedron 68 (2012) 3001e3011
3009
indicated in the table, the coupling reactions of aryl chlorides with
4.2. Characterization
methyl vinyl ketone required higher temperatures and extended
reaction times because the oxidative-addition of CeCl bond to
catalyst species is usually difficult. The reaction of aryl halides with
electron-withdrawing in the para and meta positions proceeded
smoothly (entries 7e10 and 16e18), while aryl halides with
electron-donating substituents were less reactive (entries 6 and
13e15). It would seem that presence of strong electron-
withdrawing groups on the aryl halide makes it more susceptible
to further oxidative-addition to the catalyst species. The trans-
isomer was formed as the dominant coupling product in all cases.
As a good advantage of the presented catalyst by this method,
product was obtained with excellent yield and complete conver-
sion (100%).
The size, phase composition, shape or morphology of the Pd/Cu
(4:1) bimetallic NPs were characterized by dynamic light scattering
(DLS), field emission scanning electron microscope (FE-SEM), X-ray
diffraction (XRD), transmission electron microscopy (TEM) analy-
ses, and UV/vis spectra. Also, other synthesized mono- and bi-
metallic NPs (Pd, Ag, Pd/Ag (1:1), Pd/Ni (1:1)) were characterized
just by the FE-SEM, DLS analyses, and UV/vis spectra. FE-SEM im-
ages were obtained on a Hitachi S-1460 field emission scanning
electron microscope using accelerating voltage of 15 kV. The par-
ticle size distribution was measured using Brookhaven 90 Plus, DLS
spectrometer. The UV/vis spectra of the reverse micelle solutions
containing various NPs were measured in a quartz cell using
a Camspec M330 UV/vis spectrometer (200e800 nm). TEM mea-
surement for Pd/Cu (4:1) NPs was performed on a Philips model EM
208S instrument operated at an accelerating voltage of 100 kV. XRD
analysis of the Pd/Cu (4:1) NPs was carried out using a Philips
We suggest that the presence of copper in Pd/Cu bimetallic NPs
results in the facilitation of electron transfer in the cycle. In general,
according to the recent studies, the addition of the second metal to
the catalytic system can improve the activity, selectivity, and sta-
bility of catalysts in the organic reactions.
diffractometer (Model TW 1800). Nickel filtered Cu K
a
radiation
ꢀ
source was used to produce X-ray (
l¼1.542 A), and scattered ra-
diation was measured with a proportional counter detector at
a scan rate of 4ꢀ/min. The scanning angle was from 20ꢀ to 90ꢀ,
operating at a voltage of 40 kV applying potential current of 30 mA.
Gas Chromatography (GC) data was recorded on a GC Shimadzu 14B
3. Conclusion
In conclusion, a simple method for preparing spherical metal
NPs (Pd, Ag, Pd/Ag, Pd/Ni, and Pd/Cu) has been adopted based on
the reverse micelle technique. The FE-SEM, UV/vis, and DLS anal-
yses all have indicated the formation of NPs. In the CeC coupling
process, the catalytic activities of these mono- and bimetallic NPs
have been compared. The catalytic efficiency in the Heck reaction
has decreased in the order of Pd/Cu (4:1)>Pd>>Pd/Ni (1:1)>Pd/Ag
(1:1)>Ag. Bases and solvents have been screened to optimize the
reaction conditions. The Pd/Cu (4:1) bimetallic NP proves to be the
superior catalyst with high activity and excellent selectivity for the
Heck reaction of aryl halides with alkenes. Thus, in order to de-
termine the phase composition, structure, and more accurate size
of Pd/Cu (4:1) bimetallic NPs as the best catalyst have been further
investigated using TEM and XRD analysesdin addition to FE-SEM,
UV/vis, and DLS analyses. This catalyst was simply recovered and
recycled for further reactions up to six runs with a negligible loss of
efficiency. In our experiments, the aryl chloride has been success-
fully activated using the presented catalyst. This catalyst is non-
toxic, reusable, thermally stable, and makes it possible for the
reactions to occur in air. The trans-isomer was formed as the
dominant coupling product in all cases. The present study is the
first report in which the Pd/Cu (4:1) bimetallic NP, developed in the
reverse micelle system, is exploited as an excellent catalyst in the
Heck reaction.
with a CBP5 column (25 mꢂ0.25 mmꢂ0.25
mm).
4.3. Preparation of the catalyst
4.3.1. Preparation of the Pd and Ag monometallic NPs. In two sep-
arate 25 mL beakers, two microemulsions with different aqueous
phases, one with 0.036 mmol Pd(OAc)2 (or 0.03 mmol AgNO3) and
the other one with 0.8 mmol N2H4$H2O were prepared (note that
each microemulsion was formed with 0.1 M AOT concentration and
a given amount of isooctane as oil phase). Then, the microemulsion
containing an aqueous solution of hydrazine hydride was added to
the microemulsion containing an aqueous solution of Pd(OAc)2 or
AgNO3. Obviously, the solution became dark brown immediately
after mixing. The corresponding reduction reactions can be
expressed as (Eqs. 2 and 3):
2Pd2þ þ N2Hþ þ 5OHꢁ/2Pd0 þ N2 þ 5H2O
(2)
5
4Agþ þ N2H5þ þ 5OHꢁ/4Ag0 þ N2 þ 5H2O
(3)
The reaction was allowed to proceed with stirring for 40 min.
Then, methanol was added to the beaker to make phase separation
and to wash the solution. The final mixture was centrifuged to get
samples. Finally, the synthesized NPs were characterized by FE-
SEM, DLS analyses, and UV/vis spectra.
4. Experimental
4.1. Materials
4.3.2. Preparation of the Pd/M (M¼Ag, Ni, and Cu) bimetallic
NPs. Fig. 8 illustrates the general sketch for the preparation process
of bimetallic NPs highly dispersed using the reverse micelle
method.61 This general sketch has been applied in our study to
synthesize the Pd/Cu (1:1) bimetallic NP as follows. Three micro-
The following chemicals were used in the synthesis pro-
cedure: palladium(II) acetate (Pd(OAc)2 ꢃ99%, Merck), nickel(II)
acetate tetrahydrate (Ni(OAc)2$4H2O 98%, Aldrich), copper(II)
acetate monohydrate (Cu(OAc)2$H2O ꢃ99%, Merck), palladium(II)
chloride (PdCl2 ꢃ59%, Merck), silver nitrate (AgNO3 99.8%,
Merck) as precursor salts; dioctyl sulfosuccinate sodium salt
(aerosol-OT, AOT 98%, Aldrich) as a surfactant; hydrazine hydrate
(N2H4$H2O ꢃ99%, Merck) as a reducing agent; isooctane (ꢃ99%,
Merck) as the oil phase; methanol (ꢃ99.9%, Merck) as a washing
agent and solvent; and water used in the experiments was
deionized (DI) and doubly distilled prior to use as the aqueous
phase in reverse micelle system. Substrates were purchased from
Merck Company, and were used without further purification or
drying.
emulsions
with
different
aqueous
phases
containing
0.018 mmol Pd(OAc)2 (as microemulsion A), 0.018 mmol
Cu(OAc)2$H2O (as microemulsion B), and 0.8 mmol N2H4$H2O (as
microemulsion C) were prepared. Then, microemulsion A was
mixed to microemulsion B; hereafter the resultant microemulsion
is called system I. After mechanical agitation for about 15 min,
microemulsion C was added to system I. The corresponding re-
duction reactions can be expressed as (Eq. 4):
2Pd2þ þ N2Hþ þ 5OHꢁ/2Pd0 þ N2 þ 5H2O
(4)
5

3010
F. Heshmatpour et al. / Tetrahedron 68 (2012) 3001e3011
and washed with water to remove base and salt. The remaining
solution was then washed with acetone to remove adsorbed or-
ganic substrate. Finally, the prepared samples were analyzed
by GC.
Acknowledgements
We would like to thank K.N. Toosi University of Technology
research council for financial support.
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