Green synthesis of water-dispersable palladium nanoparticles and their catalytic application in the ligand- and copper-free Sonogashira coupling reaction under aerobic conditions

Article abstract of DOI:10.1016/j.molcata.2014.04.004

Pd nanoparticles were prepared from PdCl₂ by green method and were characterized with UV-vis, XRD and TEM methods. Catalytic studies indicated that Pd nanoparticles exhibited high activity for ligand-, amine- and copper-free Sonogashira couplin

Full text of DOI:10.1016/j.molcata.2014.04.004

Accepted Manuscript
Title: Green synthesis of water-dispersable palladium
nanoparticles and their catalytic application in the ligand- and
copper-free Sonogashira coupling reaction under aerobic
conditions
Author: Mahmoud Nasrollahzadeh Mehdi Maham
Mohammad Mostafa Tohidi
PII:
S1381-1169(14)00142-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2014.04.004
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To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
26-2-2014
3-4-2014
4-4-2014
Please cite this article as: M. Nasrollahzadeh, M. Maham, M.M. Tohidi,
Green synthesis of water-dispersable palladium nanoparticles and their catalytic
application in the ligand- and copper-free Sonogashira coupling reaction
under aerobic conditions, Journal of Molecular Catalysis A: Chemical (2014),
http://dx.doi.org/10.1016/j.molcata.2014.04.004
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Green synthesis of water-dispersable palladium nanoparticles and their catalytic
application in the ligand- and copper-free Sonogashira coupling reaction under
aerobic conditions
*,a
b
a
Mahmoud Nasrollahzadeh, Mehdi Maham and Mohammad Mostafa Tohidi
a
Department of Chemistry, Faculty of Science, University of Qom, Qom 3716146611, Iran
b
Department of Chemistry, Aliabad Katoul Branch, Islamic Azad University, Aliabad Katoul, Iran
ABSTRACT
Pd nanoparticles were prepared from PdCl
2
by green method and were characterized with UV-Vis, XRD and
TEM methods. Catalytic studies indicated that Pd nanoparticles exhibited high activity for ligand-, amine- and
copper-free Sonogashira coupling between aryl iodides and terminal alkynes under mild and aerobic conditions.
This method has the advantages of high yields, simple methodology and easy work up. More importantly, the
catalyst exhibits high catalytic activity, superior cycling stability and excellent substrate applicability.
Keywords: Palladium; Sonogashira; Copper-free; Ligand-free; Water
1
. Introduction
The Sonogashira coupling reaction of terminal alkynes with aryl or vinyl halides provides a powerful and
2
versatile method for C(sp)-C(sp ) bond formation, which has been widely applied to diverse areas such as
heterocycles natural product synthesis, pharmaceuticals, biologically active molecules and material science [1-
3
].
The Sonogashira reaction usually proceeds in the presence of a homogeneous palladium catalyst, a copper
salt as a co-catalyst, toxic phosphine ligands and a stoichiometric amount of base in an organic solvent such as
an amine, benzene, THF, or DMF under inert conditions, which were economically and environmentally
malignant [4,5].
*
Corresponding author. Tel.: +98 25 32103595; Fax: +98 25 32850953.
E-mail address: mahmoudnasr81@gmail.com (M. Nasrollahzadeh).
1
Page 1 of 22

However, most of traditional Pd-catalyzed reactions suffer from different drawbacks such as harsh reaction
conditions, tedious work-ups, long reaction times, inert atmosphere conditions, use of toxic polar solvents and
expensive ligands, environmental pollution caused by formation of side products and low yields of the products
that restrict their usage in practical applications [4,5]. In Sonogashira coupling reaction, copper salts usually
play an important role in assisting tansmetalation by the in situ generation of copper acetylide. However, it can
also induce a Glaser-type oxidative homocoupling of the terminal acetylene to yield a diyne as a side reaction
[
4,5]. In addition, amines, such as triethylamine and piperidine, required in most Sonogashira reactions, have a
bad smell and add to the environmental burden.
The use of aqueous media in metal-catalyzed reactions have received considerable attention because water-
based synthetic processes are inherently safer (water is non-toxic and non-flammable) as well as being
inexpensive. Moreover, the products can be isolated easily by extraction. Several examples of Pd-catalyzed
Sonogashira reactions in aqueous media have been reported; however, many of these reactions are carried out in
an aqueous-organic and in some case, special phosphine ligands and copper salts are required in order to reach
high reaction efficiency [6-8]. In addition, among various catalysts for the Sonogashira coupling reactions,
homogeneous palladium catalysts have been widely investigated, while less expensive heterogeneous palladium
catalysts received scanter attention [9]. Thus, the use of ligand-free heterogeneous palladium catalysts is often
desirable from the perspective of process development due to their easy handling, simple recovery, and
recycling. Therefore, it is desirable to develop a mild, highly efficient and environmentally benign method for
the ligand-, copper- and amine-free Sonogashira coupling reaction under heterogeneous conditions.
Palladium (Pd) has widely been paid attention during the past few years due to its application potential as the
homogeneous or heterogeneous catalyst for the carbon-carbon coupling reactions [10-14]. The problem with
homogeneous catalysis is the difficulty to separate the catalyst from the reaction mixture and the impossibility to
reuse it in consecutive reactions. However, heterogeneous catalysts allow a more easy separation from the
reaction mixture. Since catalysis takes place on metal surface, nanoparticles (NPs) are much more reactive than
the particulate metal counterpart due to their small sizes and large surface areas. So, heterogeneous catalysts are
more and more used in the form of nanoparticles.
There are several methods for the synthesis of palladium nanoparticles using toxic and expensive chemicals.
Thus, environmentally benign production methods of Pd nanocatalysts without the use of harsh, toxic reducing
agents (eg, hydrazine hydrate, sodium borohydride, dimethylformamide, ethylene glycol, and so on), and
2
Page 2 of 22

expensive chemicals are very desirable. Recently, extensive research effort has been made in utilizing various
biological systems such as bacteria, fungus and plant extracts for the green synthesis of metal nanoparticles
[
15,16]. Green synthesis of NPs has several advantages over chemical synthesis, such as simplicity, very mild
reaction conditions, and use of nontoxic solvents such as water, elimination of toxic and dangerous materials
and cost effectiveness as well as compatibility for biomedical and pharmaceutical applications [17-21]. Also, in
this method there is no need to use high pressure, energy, temperature and toxic chemicals. Despite the
availability of methods for the green synthesis of Pd NPs by various plants the potential of gums as biological
materials for the synthesis of nanoparticles is yet to be fully explored [22,23].
Gums are naturally occurring polysaccharide components in plants, which are fundamentally economical and
easily available. They have assorted applications as thickeners, food emulsifiers, viscosifiers, sweeteners, and so
on in confectionery, and as binders and drug release modifiers in pharmaceutical dosage forms [24].
In continuation of our efforts to develop environmentally friendly synthetic methodologies [25-33], we now
wish to report a new and rapid protocol for the preparation of Pd NPs by using Sour cherry tree gum (Figure 1)
as a reducing and stabilizing agent and their application as novel and stable heterogeneous catalysts for the
copper- and phosphine-free Sonogashira coupling reaction in water under aerobic conditions (Scheme 1).
Figure 1. Image of Sour Cherry tree Gum.
3
Page 3 of 22

Pd NPs
K CO , H O, 60 C
ArI
+
R
Ar
R
o
2
3
2
Scheme 1. Sonogashira coupling in water
2
. Experimental
.1. Instruments and reagents
All reagents were purchased from the Merck and Aldrich chemical companies and used without further
2
purification. Products were characterized by comparison of their physical and spectral data with authentic
samples. Sour Cherry tree gum was obtained from the Bande Pay area, (city of Babol, province of Mazandaran,
1
Iran). The NMR spectra were recorded in DMSO. H NMR spectra were recorded on a Bruker Avance DRX
3
00 MHz instruments. The chemical shifts (δ) are reported in ppm relative to the TMS as an internal standard
1
3
and J values are given in Hz. C NMR spectra were recorded at 75 Hz. FT-IR (KBr) spectra were recorded on a
Perkin-Elmer 781 spectrophotometer. Melting points were taken in open capillary tubes with a BUCHI 510
melting point apparatus and were uncorrected. TLC was performed on silica gel polygram SIL G/UV 254 plates.
Transmission electron microscopy (TEM) was recorded on a JEOL JSM 100CX. XRD was recorded on a
Bruker D8 XRD instrument SWAX.
2
.2. Preparation of palladium nanoparticles
Sour Cherry tree Gum was powdered in a Prestige high-speed mechanical blender. Then, 1.0% (w/v) of
homogenous gum stock solution was prepared by adding this powder to reagent bottle containing ultra-pure
water and stirring overnight at room temperature. Then this solution was centrifuged to remove the insoluble
materials and the supernatant was used for all the experiments. In a typical synthesis of Pd NPs, 10 mL of the
2
gum solution prepared above was added dropwise to 30 mL of 0.001 M aqueous solution of PdCl with constant
II
o
stirring at 80 °C. Reduction of palladium ions (Pd ) to palladium (Pd ) was completed around 30 min (as
monitored by UV-Vis and FT-IR spectra of the solution). The color of the reaction mixtures gradually changed
from transparent yellow to dark brown in 30 min at 80 °C indicating the formation of palladium nanoparticles.
Then the colored solution of palladium nanoparticles was centrifuged at 6000 rpm for 30 min to completely
dispersing.
2
.3. General procedure for Sonogashira coupling reaction
4
Page 4 of 22

The freshly prepared solution of palladium in water (2 mol%) was added to the aryl iodide (1 mmol), alkyne
1.2 mmol), and K CO (2 mmol) in a 25 mL flask and the mixture was vigorously stirred at 60 ºC for the
appropriate times under aerobic conditions. After completion (as monitored by TLC), the reaction mixture was
cooled and the organic layer was extracted with EtOAc, washed with water, dried over MgSO , filtered and
(
2
3
4
evaporated under reduced pressure. The residue was purified by column chromatography. The purity of the
1
compounds was checked by H NMR and yields are based on aryl iodide. All the products are known and the
spectroscopic data (FT-IR and NMR) and melting points were consistent with those reported in the literature [6-
9
3
3
].
. Results and discussion
II
o
.1. Reduction of Pd to Pd
Reduction kinetics plays a key role in controlling the nucleation and growth of nanoparticles. In this work,
we have employed Sour cherry tree gum as a reducing agent. To date, the reduction mechanism of palladium
with Sour cherry tree gum has not been thoroughly investigated. Gums from plants may act both as reducing and
II
capping agents in nanoparticle synthesis. Pd ions were reduced to their zero-valent states in water solution at
7
0 ºC, and under aerobic conditions.
3
.2. Characterization of catalyst
The stable Pd NPs obtained were fully characterized by UV-Vis, TEM and XRD, and their behavior as
catalyst was tested for the Sonogashira coupling reaction in water.
3
.2.1. UV-Vis analysis
The progression of the reaction, formation and stability of palladium nanoparticles were controlled by UV-
II
Vis spectroscopy (Figure 2). The yellow color of the Pd solution (λmax 415 nm) immediately changed into dark
II
o
brown indicating reduction of Pd to Pd and formation of Pd NPs as characterized by UV-Vis spectrum [34].
The synthesized palladium nanoparticles by the this method are quite stable and no obvious variance in the
shape, position and symmetry of the absorption peak is observed even after two months which indicates that the
as-prepared palladium nanoparticles can remain stable. The results suggested that Sour cherry tree gum plays an
important role in the reduction and stabilization of palladium to palladium nanoparticles.
5
Page 5 of 22

Figure 2. UV-Vis absorbance spectra of Pd nanoparticles.
3
.2.2. X-Ray powder diffraction analysis
Phase investigation of the crystallized product was performed by XRD and the powder diffraction pattern of
Pd NPs is presented in Figure 3. The presence of palladium was confirmed with powder XRD measurements.
The XRD analysis of NPs dried powders indicated a crystalline structure with peaks at 20.0˚, 28.8˚, 40.0˚, 46.0˚,
5
0.5˚ and 66.7˚, values found also by other authors in Pd NPs synthesized in the presence of a variety of
stabilizing agents [35]. The particles size can be found by applying Sherrer’s equation and the average particles
size is found to be 11 nm.
Figure 3. XRD pattern of Pd nanoparticles.
6
Page 6 of 22

3
.2.3. Transmission electron microscopy (TEM) of Pd NPs
The size and shape of the products were examined by transmission electron microscopy (TEM). Figure 4
shows TEM images of the as-produced palladium nanosphere. The products are of spherical morphology and
have very narrow diameter distributions. Figure 5 shows the size distribution histogram of the nanoparticles.
The particle sizes distributed in the range of 2.5-15 nm, with an average particle size of 5 ± 2 nm synthesized.
Figure 4. TEM images of Pd nanoparticles.
Figure 5. Particle size distribution histogram.
3
.3. Evaluation of the catalytic activity of Pd nanoparticles through the Sonogashira coupling reaction
In most of transition metal catalyzed reactions, ligands play a key role. Various ligands such as phosphine
and nonphosphine for palladium catalyzed coupling reactions are described in the literature. Most of these
7
Page 7 of 22

ligands are air and moisture sensitive, difficult to prepare, and expensive. Thus, catalysis under ligand-free
conditions is an area of high importance. In this work, the prepared Pd NPs were used as a catalyst in the
copper- and ligand-free Sonogashira coupling reaction.
For optimization of the reaction conditions, we chose the reaction of 1.0 mmol of p-iodoanisole with 1.2
mmol of phenylacetylene in the presence of 2.0 mol% of Pd NPs and 4.0 mL of water as the model reaction, and
the effects of the bases were examined. As shown in Table 1, the reaction was influenced significantly by the
2 3
base employed and the best result obtained in the case of K CO (Table 1, entry 5). Increasing the amount of
catalyst showed no substantial improvement in the yield. No surfactant, ligand or organic co-solvent was
required. Since no copper salt used, the undesired formation of Glaser-type oxidative homocoupling product, a
diyne, was also avoided.
Table 1.
a
Copper-free Sonogashira reaction of phenylacetylene with p-iodoanisole in the presence of different bases
b
Entry
Base
KOH
Yield (%)
1
2
3
4
5
6
21
0
DMAP
NaOH
20
78
91
79
Na
2
CO
CO
PO
3
K
2
3
K
3
4
a
Reaction conditions: p-iodoanisole (1.0 mmol), phenylacetylene (1.2 mmol), Pd NPs (2.0 mol%), base (2.0
mmol), water (4.0 mL), 60 °C, 5 h.
b
Isolated Yield.
Next, using the optimized procedure, a variety of aryl iodides possessing both electron-releasing and
electron-withdrawing groups were employed. As indicated in Table 2, it is evident that our method is reasonably
general and can be applied to several kinds of aryl iodides. In all cases the reaction gives the corresponding
products in good to excellent yields under the reaction conditions.
8
Page 8 of 22

Table 2.
a
Sonogashira coupling reaction of different aryl iodides with terminal alkynes
b
Entry
1
Aryl iodide
Alkyne
Time (h)
Yield (%)
5
5
5
2
2
89
91
93
94
93
I
2
3
4
5
MeO
Me
I
I
O N
I
2
F3C
I
6
7
8
9
OH
OH
OH
3
3
2
5
88
87
91
91
I
MeO
I
O N
I
2
OH
Me
OH
Me
MeO
I
1
0
4
92
O N
I
2
a
2 3
Reaction conditions: aryliodide (1.0 mmol), phenylacetylene (1.2 mmol), Pd NPs (2.0 mol%), K CO (2.0
mmol), water (4.0 mL), 60 °C.
b
Yields are after work-up.
3
.4. Catalyst recyclability
The reusability of the catalysts is one of the most important benefits and makes them useful for commercial
applications. The recycling potential of Pd NPs catalyst was studied by model Sonogashira coupling reaction for
9
Page 9 of 22

p-iodoanisole with phenylacetylene in three consecutive cycles under similar reaction conditions. The fact that
water is an excellent solvent for the Pd NPs, but not for the reagents and products, allows an easy
extraction/separation procedure from the reaction mixture. Thus, after each run, the reaction mixture was
centrifuged at 10,000 rpm at room temperature for 15 min and filtered. Then the residue was washed with ethyl
acetate and reused. The catalytic activity did not decrease considerably after three catalytic cycles (Figure 6).
This test clearly demonstrates the stability of catalyst over multiple runs and supporting its stability over
extended duration.
I
Yield %
+
C H C CH
6 5
First Run
MeO
96 %
5 %
3 %
2 %
First Recycle
9
Pd NPs
Second Recycle
Third Recycle
9
9
OMe
C
C H C
6
5
Figure 6. Reusability of Pd NPs for Sonogashira coupling reaction.
4
. Conclusions
In conclusion, Sonogashira coupling reaction under moderate conditions can be achieved in water in the
presence of Pd NPs generated by Sour Cherry tree gum as reductant. This method proved to be highly efficient
and green for the following reasons: (i) the use of a green a renewable reagent such as Sour Cherry tree gum
provides a great advantage in terms of safety, economy, and sustainability, (ii) using water as the solvent to
conduct the Sonogashira reaction is very environmentally friendly and is an important component of green
chemistry, (iii) the convenient preparation, high efficiency and reusability of the catalyst, (iv) low catalyst
loading, high yields and a simple work-up procedure, and (v) elimination of dangerous and toxic reagents,
ligands and organic solvents. The synthesized Pd NPs by this method are quite stable and can be kept under
inert atmosphere for several months.
1
0
Page 10 of 22

Acknowledgments
We gratefully acknowledge the Iranian Nano Council and University of Qom for the support of this work.
References
[
[
[
[
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1
2
Page 12 of 22

Scheme caption
Scheme 1. Sonogashira coupling in water.
Pd NPs
K CO , H O, 60 C
ArI
+
R
Ar
R
o
2
3
2
Scheme 1. Sonogashira coupling in water
1
3
Page 13 of 22

Figure captions
Figure 1. Image of Sour Cherry tree Gum.
Figure 2. UV-Vis absorbance spectra of Pd nanoparticles.
Figure 3. XRD pattern of Pd nanoparticles.
Figure 4. TEM images of Pd nanoparticles.
Figure 5. Particle size distribution histogram.
Figure 6. Reusability of Pd NPs for Sonogashira coupling reaction.
1
4
Page 14 of 22

Figure 1. Image of Sour Cherry tree Gum.
Figure 2. UV-Vis absorbance spectra of Pd nanoparticles.
1
5
Page 15 of 22

Figure 3. XRD pattern of Pd nanoparticles.
Figure 4. TEM images of Pd nanoparticles.
1
6
Page 16 of 22

Figure 5. Particle size distribution histogram.
I
Yield %
96 %
+
C H C CH
6 5
First Run
MeO
First Recycle
9
5 %
3 %
2 %
Pd NPs
Second Recycle
Third Recycle
9
9
OMe
C
C H C
6
5
Figure 6. Reusability of Pd NPs for Sonogashira coupling reaction.
1
7
Page 17 of 22

Table captions
Table 1. Copper-free Sonogashira reaction of phenylacetylene with p-iodoanisole in the presence of different
bases.
Table 2. Sonogashira coupling reaction of different aryl iodides with terminal alkynes.
Table 1.
a
Copper-free Sonogashira reaction of phenylacetylene with p-iodoanisole in the presence of different bases
b
Entry
Base
KOH
Yield (%)
1
2
3
4
5
6
21
0
DMAP
NaOH
20
78
91
79
Na
2
CO
CO
PO
3
K
2
3
K
3
4
a
Reaction conditions: p-iodoanisole (1.0 mmol), phenylacetylene (1.2 mmol), Pd NPs (2.0 mol%), base (2.0
mmol), water (4.0 mL), 60 °C, 5 h.
b
Isolated Yield.
1
8
Page 18 of 22

Table 2.
a
Sonogashira coupling reaction of different aryl iodides with terminal alkynes
b
Entry
1
Aryl iodide
Alkyne
Time (h)
Yield (%)
5
5
5
2
2
89
91
93
94
93
I
2
3
4
5
MeO
Me
I
I
O N
I
2
F3C
I
6
7
8
9
OH
OH
OH
3
3
2
5
88
87
91
91
I
MeO
I
O N
I
2
OH
Me
OH
Me
MeO
I
1
0
4
92
O N
I
2
a
2 3
Reaction conditions: aryliodide (1.0 mmol), phenylacetylene (1.2 mmol), Pd NPs (2.0 mol%), K CO (2.0
mmol), water (4.0 mL), 60 °C.
b
Yields are after work-up.
1
9
Page 19 of 22

Abbreviations
XRD: X-ray Powder Diffraction
TEM: Transmission electron microscopy
FT-IR spectroscopy: Fourier transform infrared spectroscopy
NMR: Nuclear Magnetic Resonance
2
0
Page 20 of 22

Graphical Abstract
Green synthesis of water-dispersable palladium nanoparticles and their catalytic
application in the ligand- and copper-free Sonogashira coupling reaction under
aerobic conditions
*
Mahmoud Nasrollahzadeh, Mehdi Maham and Mohammad Mostafa Tohidi
I
R'
+ RC
CH
Pd NPs
R'
C
RC
*
Corresponding author. Tel.: +98 25 32103595; Fax: +98 25 32850953.
E-mail address: mahmoudnasr81@gmail.com (M. Nasrollahzadeh).
2
1
Page 21 of 22

Highlights:
ꢀ
ꢀ
Green synthesis of Pd nanoparticles.
Sonogashira coupling reactions of different aryl iodides using Pd nanoparticles in
water.
ꢀ
ꢀ
Catalyst was characterized using the powder XRD, TEM and UV-Vis.
Catalyst can be easily recovered and reused.
2
2
Page 22 of 22