Green synthesis of Pd/Fe3O4 nanoparticles using Euphorbia condylocarpa M. bieb root extract and their catalytic applications as magnetically recoverable and stable recyclable catalysts for the phosphine-free Sonogashira and Suzuki co

Article abstract of DOI:10.1016/j.molcata.2014.09.029

This work reports on the green synthesis of Pd/Fe₃O₄ nanoparticles using Euphorbia condylocarpa M. biebroot extract as reducing agents and stabilizers and their catalytic applications in ligand- and copper-free Sonogashira and Suzuki

Full text of DOI:10.1016/j.molcata.2014.09.029

Accepted Manuscript
Title: Green synthesis of Pd/Fe3O4 nanoparticles using
Euphorbia condylocarpa M. bieb root extract and their
catalytic applications as magnetically recoverable and stable
recyclable catalysts for the phosphine-free Sonogashira and
Suzuki coupling reactions
Author: Mahmoud Nasrollahzadeh S. Mohammad Sajadi
Akbar Rostami-Vartooni Mehdi Khalaj
PII:
S1381-1169(14)00432-4
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2014.09.029
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To appear in:
Journal of Molecular Catalysis A: Chemical
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Accepted date:
5-7-2014
10-9-2014
21-9-2014
Please cite this article as: M. Nasrollahzadeh, S.M. Sajadi, A. Rostami-
Vartooni, M. Khalaj, Green synthesis of Pd/Fe3O4 nanoparticles using Euphorbia
condylocarpa M. bieb root extract and their catalytic applications as magnetically
recoverable and stable recyclable catalysts for the phosphine-free Sonogashira and
Suzuki coupling reactions, Journal of Molecular Catalysis A: Chemical (2014),
http://dx.doi.org/10.1016/j.molcata.2014.09.029
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Green synthesis of Pd/Fe3O4 nanoparticles using Euphorbia condylocarpa M.
bieb root extract and their catalytic applications as magnetically recoverable and
stable recyclable catalysts for the phosphine-free Sonogashira and Suzuki
coupling reactions
Mahmoud Nasrollahzadeh,*,a S. Mohammad Sajadi,b Akbar Rostami-Vartoonia and Mehdi Khalajc
aDepartment of Chemistry, Faculty of Science, University of Qom, Qom 3716146611, Iran
bDepartment of Petroleum Geoscience, Faculty of Science, Soran University, PO Box 624, Soran, Kurdistan Regional Government, Iraq
cDepartment of Chemistry, Buinzahra Branch, Islamic Azad University, Buinzahra, Qazvin, Iran
ABSTRACT
This work reports on the green synthesis of Pd/Fe3O4 nanoparticles using Euphorbia condylocarpa M. bieb root
extract as reducing agents and stabilizers and their catalytic applications in ligand- and copper-free Sonogashira
and Suzuki coupling reactions. This method has such advantages as high yields, simple methodology and easy
work up. In addition, the catalyst can be recovered by using a magnet and reused several times without
significant loss of its catalytic activity. The catalyst was characterized using UV-vis, powder XRD, SEM and
EDS techniques.
Keywords: Pd/Fe3O4; Bio-reduction; Coupling reaction; Copper-free; Ligand-free
1. Introduction
The Sonogashira reaction is probably one of the most important, powerful and straightforward methods for
the synthesis of arylated internal alkyne compounds, C(sp2)-C(sp) bond, and has been widely applied to diverse
areas such as natural product synthesis, pharmaceuticals and material science [1-3].
The Sonogashira reaction is generally carried out in an organic solvent such as an amine, benzene, THF, or
DMF with a complex palladium catalyst in conjunction with CuX (X = Cl, Br, I) as a co-catalyst and in the
presence of toxic phosphine ligands and a stoichiometric amount of base under inert conditions, which make it
economically and environmentally malignant [4,5].
*Corresponding author. Tel.: +98 25 32103595; Fax: +98 25 32850953.
E-mail address: mahmoudnasr81@gmail.com (M. Nasrollahzadeh).
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However, there are several disadvantages in the literature methods such as the use of expensive and toxic
ligands, low yields, long reaction times, harsh reaction conditions, environmental pollution due to the side
products formed and tedious work-up [4,5]. In Sonogashira coupling reaction, the presence of copper salt as the
co-catalyst can result in the formation of diyne side products via Glaser-type oxidative dimerization of in situ
generated copper acetylide and can also decrease the efficiency to cross-coupling products [4,5].
Among various catalysts for the coupling reactions, homogeneous palladium catalysts have been widely
investigated [6-9] while less expensive heterogeneous palladium catalysts have received scanter attention. The
problem with homogeneous catalysis is the difficulty of separation of the catalyst from the reaction mixture and
the incapability of its application in consecutive reactions. Thus, the use of ligand- and copper-free
heterogeneous palladium catalysts is often desirable from the perspective of process development due to their
easy handling, simple recovery, and recycling.
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. Therefore, heterogeneous catalysts are mostly
used in the form of nanoparticles. Among various catalysts, iron oxide magnetic nanoparticles (Fe3O4-MNPs)
have been paid attention to in the past few years due to their unusual magnetic, physical and surface chemical
and catalytic properties, high stability and ease of recovery with an external magnet [10]. The potential
advantage of Fe3O4-supported metal catalysts is that it can be separated from the reaction medium by an external
permanent magnet without using any filtration or centrifugation. We have recently reported simple synthesis of
Pd/Fe3O4 nanoparticles and their applications in recyclable Sonogashira and Stille coupling reactions and N-
arylureas preparation [11,12].
There are several methods for the synthesis of Pd/Fe3O4 nanoparticles using toxic and expensive chemicals.
Thus, environmentally benign production methods of Pd/Fe3O4 nanoparticles without the use of harsh, toxic
reducing agents (eg. hydrazine hydrate, sodium borohydride, dimethylformamide, ethylene glycol, and so on),
and 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 [13,14]. Green synthesis of NPs has several advantages over conventional synthetic methods, such
as simplicity, very mild reaction conditions, and use of non-toxic solvents such as water, elimination of toxic
and dangerous materials and cost effectiveness as well as compatibility for biomedical and pharmaceutical
applications [15-19]. In addition, in this method, there is no need to use high pressure, energy, temperature and
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toxic chemicals. Despite the availability of methods for the synthesis of Pd/Fe3O4 NPs by various toxic and
expensive chemicals, there is no report on the biosynthesis of Pd/Fe3O4 NPs by utilizing the plants extract.
The plants of the Euphorbiaceae contain acrid, milky, or colorless juice. Chemical data are available for
several genera, especially Eubhorbia, in which more than 120 species have been investigated. The family
Euphorbiaceae is rich in flavonoids, particularly flavones and flavonols, which have been identified from
several genera. Furthermore, the previous studies in 1970 on Euphorbia condylocarpa demonstrated the
presence of phytochemicals such as flavonoids, tetracyclic triterpenoids, and trifolin in different parts of the
plant [20-22].
In continuation of our efforts to develop environmentally friendly synthetic methodologies [23-29], we now
wish to report a new and rapid protocol for the preparation of Pd/Fe3O4 NPs by using Euphorbia condylocarpa
M. bieb extract (Figure 1) as a reducing and stabilizing agent and the application of these NPs as novel and
stable heterogeneous catalysts in the copper- and phosphine-free Sonogashira and Suzuki coupling reactions.
Figure 1. Image of Euphorbia condylocarpa M. bieb.
2. Experimental
2.1. Instruments and reagents
All reagents were purchased from the Merck and Aldrich Chemical Companies and used without further
purification. Products were characterized by comparison of their physical and spectral data with those of
authentic samples. The NMR spectra were recorded in DMSO as the solvent. 1H NMR spectra were recorded on
a Bruker Avance DRX 300 MHz instrument. The chemical shifts (δ) are reported in ppm relative to TMS as the
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internal standard and J values are given in Hz. 13C 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. X-ray diffraction measurements were performed with a Philips powder diffractometer type
PW 1373 goniometer, equipped with a graphite monochromator crystal. The X-ray wavelength was 1.5405 A˚
and the diffraction patterns were recorded in the 2θ range (10-60) with scanning speed of 2º/min. Morphology
and particle dispersion was investigated by scanning electron microscopy (SEM) (Cam scan MV2300). The
chemical compositions of the synthesized nanostructures were measured by EDS performed in SEM.
2.2. Preparation of Euphorbia condylocarpa M. bieb root extract
50 g of dried powder of E. condylocarpa M. bieb was extracted by boiling in 300 mL double distilled water
for 20 min and the aqueous extract was centrifuged at 7000 rpm to obtain the supernatant as extract (Figure 2).
Figure 2. Photograph of Euphorbia condylocarpa M. bieb root extract.
2.3. Preparation of Pd/Fe3O4 nanoparticles
For green synthesis of Pd supported on magnetite nanocatalyst, 0.5 g FeCl3·6H2O and 0.1 g PdCl2 were
dissolved in 30 mL aqueous extract of the E. condylocarpa at 60 ºC under vigorous stirring. Then, a solution of
1.0 M Na2CO3 was added dropwise to the mixture to obtain alkaline pH while changing the color to dark brown
and monitoring the product formation using UV-vis at times raining 5 to 35 min. After being stirred again for 5
hours at the same temperature, a suspension was formed which gave precipitate of Pd/magnetite nanoparticles
on centrifugation at 7000 rpm and the obtained nanoparticles were washed with ethanol and distilled water,
respectively, and then dried.
2.4. General procedure for Sonogashira coupling reaction
To a stirred solution of 1.0 mmol of aryl halide in 4 mL of DMF, 1.0 mmol of terminal alkyne, 1.5 mmol of
piperidine and 0.2 mol% of Pd/Fe3O4 NPs were added and the mixture was heated on an oil bath at 110 °C for
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24 h under inert atmosphere (N2). After completion of the reaction (as monitored by TLC), the reaction mixture
was cooled to room temperature and the catalyst was separated using a magnetic separator. The solvent was
evaporated at reduced pressure, and the residue was subjected to gel permeation chromatography to afford pure
products. All the products are known compounds and the spectral data and melting points were identical to those
reported in the literature [30-33].
2.5. General procedure for Suzuki coupling reaction
To the stirred solution of arylboronic acid (0.6 mmol), aryl halide (0.5 mmol), and K3PO4 (1.5 mmol) in 5
mL of MeOH at room temperature was added Pd/Fe3O4 NPs (0.1 mol%). The reaction mixture was stirred at 60
ºC for the appropriate times under a dry nitrogen atmosphere. After completion of the reaction (as monitored by
TLC), the reaction mixture was cooled to room temperature and the catalyst was separated using a magnetic
separator. The solvent was evaporated at reduced pressure to obtain the product in crude form which was
1
purified afterwards by column chromatography. The purity of the compounds was checked by H NMR and
yields are based on aryl halide. All the products are known and the spectroscopic data (FT-IR and NMR) and
melting points were consistent with those reported in the literature [34-36].
3. Results and discussion
3.1. Reduction of metal ions
Reduction kinetics play a key role in controlling the nucleation and growth of nanoparticles. In this work, we
have employed Euphorbia condylocarpa M. bieb root extract as a reducing and stabilizing agent for the
synthesis of Pd/Fe3O4 NPs. As shown in Scheme 1, metal ions were reduced to nano zero valent (NZV) metallic
particles using flavonoid (FlOH) contents of the plant as green (without any hazardous impact on the
environment) chemicals (reducing agents) as an alternative to the commonly used hydride reducing agent,
sodium borohydride.
nFlO (radical) + nM0
nFlOX + nM0 (Nucleation)
nFlOH + M+n
nFlO (Radical) + M+n
nM0 + M+n
Mn+n (Growth)
+n
Mn+n + Mn
+2n
M2n
(M2n+2n)n + (FlOH)n
metalic NZV
Scheme 1. Reducingability of antioxidantphenolicsto produce metalic NPs
where FlOH and NZV are flavonoid and nano zero valent,respectively.
3.2. Characterization of catalyst
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The stable Pd/Fe3O4 NPs obtained were fully characterized by UV-vis, SEM, EDS and XRD and their
behavior as catalyst was tested in the Sonogashira and Suzuki coupling reactions.
3.2.1. UV-vis and FT-IR analysis
Figure 3 shows the UV-vis spectra of Euphorbia condylocarpa M. bieb root extract. The UV spectrum with
nm
bands at λmax 387 (band 1) which is due to the transition localized within the B ring of cinnamoyl system;
whereas the one depicted at 283 (band 2) is consistent with absorbance of ring A of benzoyl system (Scheme 2).
They are related to the π → π transitions and these absorbent bands demonstrate the presence of flavon nuclei.
Figure 3. UV-vis spectrum of Euphorbia condylocarpa M. bieb root extract.
B
O
A
C
O
Scheme 2.
Flavon nuclei
The FT-IR analysis was carried out to identify the possible biomolecules responsible for the reduction of
Au/Pd nanoparticles and capping of the bioreduced Au and Pd nanoparticles synthesized by the root extract
(Figure 4). The FT-IR spectrum depicted some peaks at 3436 to 3000, 1643, 1561, 1413 and 1277 cm-1, which
represent free OH in molecule and OH group forming hydrogen bonds, carbonyl group (C=O), stretching C=C
aromatic ring and C-OH stretching vibrations, respectively. These peaks suggested the presence of flavonoid
and phenolics acids in the Euphorbia condylocarpa M. bieb root extract. The presence of flavonoid and
phenolics acids in the extract could be responsible for the reduction of metal ions and formation of the
corresponding metal nanoparticles. Flavonoid and phenolics acids could be adsorbed on the surface of metal
nanoparticles, possibly by interaction through π-electrons in the absence of other strong ligating agents.
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Figure 4. FT-IR spectrum of Euphorbia condylocarpa M. bieb root extract.
The UV-vis spectra of Pd/Fe3O4 bimetallic nanoparticles synthesized using the aqueous extract of Euphorbia
condylocarpa at different times have been shown in Figure 5. The surface plasmon resonance peak is centered at
380 nm and developed with increasing the time ranging 5 min to 35 min in which the appearance of the maxima
in 35 min indicates the end of the reaction.
Figure 5. UV-vis spectra of Pd/Fe3O4 bimetallic NPs.
3.2.2. X-ray powder diffraction analysis
The crystalline structure of Pd/Fe3O4 NPs was confirmed with powder XRD (X-ray Powder Diffraction)
measurements. The presence of palladium and iron was confirmed with powder XRD measurements (Figure 6).
The X-ray diffraction pattern revealed that iron nanoparticles are very air-sensitive and easily oxidized to Fe3O4.
The results showed that the products consisted of a face-centered cubic (fcc) structure.
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Figure 6. XRD pattern of Pd/Fe3O4 NPs.
3.2.3. Scanning electron microscopy (SEM) of Pd/Fe3O4 nanoparticles
The shape and size of the Pd/Fe3O4 NPs were determined by scanning electron microscopy (SEM). Figure 7
shows FE SEM images of the Pd/Fe3O4 NPs. It is clearly observed that the Pd grain pervaded on Fe3O4 surface,
which displays a good combination between Fe3O4 and Pd NPs. Hence, Pd nanoparticles appear as bright dots
over the surface of Fe3O4 with average sizes of less than 39 nm.
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Figure 7. FE SEM images of Pd/Fe3O4 NPs.
3.2.4. The EDS analysis
The elemental composition of the Pd/Fe3O4 NPs was also analyzed by the Energy Dispersive X-ray
Spectroscopy (EDS). This technique further confirmed that Pd/Fe3O4 NPs was composed of palladium, iron and
oxygen (Figure 8). The EDS results show that Fe, Pd and oxygen concentrations are about 22.7, 12.2 and
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62.7%, respectively. The excess oxygen is due to physical absorption of oxygen from the environment during
sample preparation.
Figure 8. EDS spectrum of Pd/Fe3O4 NPs.
3.3. Evaluation of the catalytic activity of Pd/Fe3O4 NPs through the Sonogashira coupling reaction
In most transition metal catalyzed reactions, ligands play a key role. Various ligands such as phosphine and
nonphosphine used in palladium catalyzed coupling reactions are described in the literature. Most of these
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/Fe3O4 NPs were used as heterogeneous
catalysts in the copper- and ligand-free Sonogashira coupling reaction.
Initially, we employed p-iodoanisole and phenylacetylene as model substrates for the development of
optimized conditions. Control experiments show that there is no reaction in the absence of catalyst (Table 1,
entry 10). However, addition of the catalyst to the mixture has rapidly increased the formation of product in high
yields. The reactivity of the catalyst in the presence of different bases was also investigated. The results
indicated that base had a demonstrative effect on the yield of product. Among the tested bases (DMAP,
NaHCO3, Et3N, piperidine, NaOH, KOH, pyridine), piperidine was found to be superior since it gave the highest
yield of product (Table 1, entry 7). A decrease in the catalyst loading from 0.2 to 0.08 mol% afforded the
product in lower yield (Table 1, entry 8). No significant improvement on the yield was observed using higher
amounts of the catalyst (Table 1, entry 9) and 0.2 mol% of the catalyst was found to be optimum. The best result
was obtained with 1.0 mmol of p-iodoanisole, 1.0 mmol of phenylacetylene, 0.2 mol% of Pd/Fe3O4 NPs and 1.5
mmol of piperidine at 110 ºC, which gave the product in an excellent yield. No surfactant or ligand was
10
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required. Since no copper salt was used, the undesired formation of Glaser-type oxidative homocoupling
product, which is a diyne, was also avoided.
Table 1.
Copper-free Sonogashira reaction of phenylacetylene with p-iodoanisole in the presence of different solvents
and basesa
Entry
1
2
3
4
5
6
7
8
Pd/Fe3O4 NPs (mol %)
Solvent
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
Base
DMAP
Pyridine
NaHCO3
NaOH
KOH
NEt3
piperidine
piperidine
piperidine
piperidine
Yieldb (%)
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.08
0.5
0.0
0
0
39
Trace
Trace
36
85
40
86
0
9
10
a Reaction conditions: p-iodoanisole (1.0 mmol), phenylacetylene (1.0 mmol), Pd/Fe3O4 (0.2 mol%), base (1.5
mmol), DMF (4.0 mL), 110 °C, 24 h.
Having optimized the conditions, we explored the general applicability of Pd/Fe3O4 NPs as catalysts for
copper-free coupling of different alkynes with aryl iodides and bromides containing electron-withdrawing or
donating substituents. As indicated in Table 2, it is evident that our method is reasonably general and can be
applied to several types of aryl halides. In all cases, the reaction gave the corresponding products in good to
excellent yields. In addition, steric hindrance of the substituent did not influence the product yield in the copper-
free Sonogashira reaction of deactivated aryl halides using Pd/Fe3O4 NPs (Table 2, entries 4 and 9). Good yield
is also found with chlorobenzene (Table 2, entry 19).
Table 2.
Sonogashira coupling reaction of different aryl halides with terminal alkynesa
Pd/Fe3O4 NPs, DMF
Piperidine, 110 ºC
X + RC CH
C
CR
R
R
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Entry
1
Aryl halide
Alkyne
Yield b (%)
88
I
2
3
4
85
83
84
MeO
I
Me
I
Me
I
5
6
7
8
9
80
83
87
90
85
O2N
F3C
I
I
OMe
OMe
OMe
I
MeO
I
Me
I
10
11
12
13
14
15
16
17
18
19
85
86
81
80
74
73
71
81
83
70
F
I
Br
I
Br
MeO
Br
Me
Br
Br
O2N
NC
Br
Br
Cl
OMe
OMe
MeO
Br
a Reaction conditions: Aryl halide (1.0 mmol), alkyne (1.0 mmol), Pd/Fe3O4 (0.2 mol%), piperidine (1.5 mmol),
DMF (4.0 mL), 110 °C.
b Yields after work-up.
To show the advantages of proposed method in comparison with other reported methods, we summarized
some of results for Sonogashira coupling reaction between chlorobenzene with phenylacetylene in Table 3,
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which shows that Pd/Fe3O4 NPs is an equally or more efficient catalyst with respect to reaction time and yield
than previously reported ones.
Table 3.
Comparison present methodology with other reported methods in the Sonogashira coupling reaction between
chlorobenzene with phenylacetylene
Entry
Catalyst
Polystyrene resin supported palladium(II) complex
Polystyrene anchored Pd(II) azo complex
Pd(0)-MCM-41
Time (h) Temperature (oC) Yielda (%) [Ref.]
1
2
3
4
5
6
3
r.t.
80
35 [37]
6 [38]
12
24
24
24
24
100
90
29 [39]
47 [40]
Pd(II)-MCM-41
Pd@agarose-Fe3O4
130
110
53 [41]
70 [This work]
Pd/Fe3O4 NPs
a Yields after work-up.
3.4. Evaluation of the catalytic activity of Pd/Fe3O4 NPs through the Suzuki coupling reaction
The Suzuki coupling reaction catalyzed by palladium (Pd) catalysts in the presence of phosphine ligands is
one of the most powerful tools for selective C-C bond formation for the construction of biaryl skeletons, which
are often considered as partial structures in pharmaceuticals, natural products, polymers and material science
[34-36]. However, Pd-catalyzed Suzuki coupling under ligand-free conditions is a topic of considerable interest
because of both economic and environmental reasons. The efficiency and stability of Pd/Fe3O4 NPs prompted us
to use the catalyst for the commercially important Suzuki coupling reactions.
For optimization of the reaction conditions, we chose the reaction of 0.5 mmol of iodobenzene with 0.6
mmol of phenylboronic acid in the presence of 0.1 mol% of Pd/Fe3O4 nanoparticles and 1.5 mmol of base at 60
°C in MeOH (5 mL) as the model reaction and the effects of the various bases (K3PO4, Et3N, NaHCO3, NaOAc)
on the reaction were examined. The reaction was significantly influenced by the base employed and the best
result was obtained in the case of K3PO4. In the absence of base, the reactions did not proceed after a long
period of time. Increasing the amount of catalyst showed no substantial improvement in the yield. The best
result was obtained with iodobenzene (0.5 mmol), phenylboronic acid (0.6 mmol), catalyst (0.1 mol%), K3PO4
(1.5 mmol) and MeOH (5 mL), which gave the product in a good yield (93%).
Next, using the optimized procedure, a variety of arylboronic acids and aryl halides possessing both electron-
releasing and electron-withdrawing groups were employed. As shown in Table 4, reactions of aryl iodides were
complete at 60 ºC after 12 h, while the aryl bromides and aryl chlorides required longer reaction times.
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Table 4.
Suzuki coupling reaction of different aryl halides with arylboronic acidsa
Pd/Fe3O4 NPs, MeOH
X
B(OH)2
+
K3PO4, 60 oC
R'
Yield b (%)
R
R'
R
Entry
1
Aryl halide
Arylboronic acid
Time (h)
12
95
I
B(OH)2
B(OH)2
B(OH)2
B(OH)2
2
3
4
12
12
12
91
MeO
I
95, 92c
92
Me
I
O2N
I
MeO
5
6
12
18
91
95
I
B(OH)2
Br
B(OH)2
B(OH)2
B(OH)2
B(OH)2
B(OH)2
B(OH)2
B(OH)2
B(OH)2
7
8
18
18
18
24
24
24
24
91
92
95
95
78
83
95
MeO
Br
Me
Br
Br
9
O2N
10
11
12
13
Cl
MeO
Cl
Me
Cl
Cl
O2N
a
Reaction conditions: Aryl halide (0.5 mmol), arylboronic acid (0.6 mmol), Pd/Fe3O4 (0.1 mol%), K3PO4 (1.5
mmol), MeOH (4.0 mL), 60 °C.
b Yields after work-up.
c Yields after the fifth cycle.
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The present method offers several notable features, compared with the other literature works [42-49] on the
Suzuki coupling reaction, i.e., (1) The use of plant extract as an economic and effective alternative represents an
interesting, fast and clean synthetic route for the large scale synthesis of Pd/Fe3O4 NPs, (2) avoidance of the
toxic ligands and homogeneous catalysts, (3) wide substrate scope and generality, (4) higher yields, (5) higher
turnover number (TON) (6) in our method, because of the magnetic nature of the Pd/Fe3O4 NPs catalyst, it can
be recovered by simply using external magnets, with high efficiency and up to 99% recovery of catalyst. Also,
According to the UV-vis results, the synthesized Pd/Fe3O4 NPs by this method are quite stable and can be kept
under inert atmosphere for several months.
3.5. Catalyst recyclability
The reusability of the catalysts is one of their most important advantages, which makes them useful for
commercial applications. We studied the reusability of these heterogeneous Pd/Fe3O4 NPs catalysts in
Sonogashira coupling reaction (Figure 9). After completion of the reaction, the catalyst was recovered by the
application of an external magnet, then washed with ethyl acetate, and dried in a hot air oven at 100 °C for 2 h.
The recovered catalyst was reused under similar conditions for the next run, and the catalytic behavior of the
Pd/Fe3O4 NPs was found to be unaltered over five consecutive cycles. Thus, the magnetic nanocatalyst is stable
during the Sonogashira coupling reaction. The reusability of the catalyst was also studied for the Suzuki
coupling reaction under the present reaction conditions (Table 4, entry 3).
I
Yield %
+ C6H5C CH
First Run
88 %
88 %
87 %
First Recycle
Second Recycle
Pd/Fe3O4 NPs
Third Recycle
Fourth Recycle
Fifth Recycle
85 %
84 %
83 %
C
C6H5C
Figure 9. Reusabilityof Pd/Fe3O4 NPs for Sonogashira couplingreaction.
4. Conclusions
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In conclusion, we have established that the Pd/Fe3O4 NPs generated by Euphorbia condylocarpa M. bieb
extract used as a reducing agent and stabilizer is a highly efficient, magnetically recoverable and recyclable
catalyst for the phosphine-free Sonogashira and Suzuki coupling reactions. The use of a green, renewable
reagent such as Euphorbia condylocarpa M. bieb provides a great advantage in terms of safety, economy, and
sustainability. The present method has the advantages of readily available starting materials, straightforward and
easy work-up procedures, elimination of dangerous and toxic ligands and homogeneous catalysts, high yields,
low catalyst loading, and tolerance for a wide variety of functionalities. The catalyst is eco-friendly because it
produces little waste and can be recovered by a magnet and successively reused without significant loss of
activity.
Acknowledgments
We gratefully acknowledge the Iranian Nano Council and the University of Qom for the support of this work.
References
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Scheme captions
Scheme 1. Reducing ability of antioxidant phenolics to produce metalic NPs where FlOH and NZV are
flavonoid and nano zero valent, respectively.
Scheme 2. Flavon nuclei.
nFlO (radical) + nM0
nFlOX + nM0 (Nucleation)
nFlOH + M+n
nFlO (Radical) + M+n
nM0 + M+n
Mn+n (Growth)
+n
Mn+n + Mn
+2n
M2n
(M2n+2n)n + (FlOH)n
metalic NZV
Scheme 1. Reducingability of antioxidantphenolicsto produce metalic NPs
where FlOH and NZV are flavonoid and nano zero valent,respectively.
B
O
A
C
O
Scheme 2.
Flavon nuclei
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Figure captions
Figure 1. Image of Euphorbia condylocarpa M. bieb.
Figure 2. Photograph of Euphorbia condylocarpa M. bieb root extract.
Figure 3. UV-vis spectrum of Euphorbia condylocarpa M. bieb root extract.
Figure 4. FT-IR spectrum of Euphorbia condylocarpa M. bieb root extract.
Figure 5. UV-vis spectra of Pd/Fe3O4 bimetallic NPs.
Figure 6. XRD pattern of Pd/Fe3O4 NPs.
Figure 7. FE SEM images of Pd/Fe3O4 NPs.
Figure 8. EDS spectrum of Pd/Fe3O4 NPs.
Figure 9. Reusability of Pd/Fe3O4 NPs for Sonogashira coupling reaction.
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Figure 1. Image of Euphorbia condylocarpa M. bieb.
Figure 2. Photograph of Euphorbia condylocarpa M. bieb root extract.
Figure 3. UV-vis spectrum of Euphorbia condylocarpa M. bieb root extract.
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Figure 4. FT-IR spectrum of Euphorbia condylocarpa M. bieb root extract.
Figure 5. UV-vis spectra of Pd/Fe3O4 bimetallic NPs.
Figure 6. XRD pattern of Pd/Fe3O4 NPs.
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Figure 7. FE SEM images of Pd/Fe3O4 NPs.
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Figure 8. EDS spectrum of Pd/Fe3O4 NPs.
I
Yield %
88 %
+ C6H5C CH
First Run
First Recycle
88 %
Second Recycle
87 %
Pd/Fe3O4 NPs
Third Recycle
Fourth Recycle
Fifth Recycle
85 %
84 %
83 %
C
C6H5C
Figure 9. Reusabilityof Pd/Fe3O4 NPs for Sonogashira couplingreaction.
Table captions
24
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Table 1. Copper-free Sonogashira reaction of phenylacetylene with p-iodoanisole in the presence of different
solvents and bases.
Table 2. Sonogashira coupling reaction of different aryl halides with terminal alkynes.
Table 3. Comparison present methodology with other reported methods in the Sonogashira coupling reaction
between chlorobenzene with phenylacetylene.
Table 4. Suzuki coupling reaction of different aryl halides with arylboronic acids.
Table 1.
Copper-free Sonogashira reaction of phenylacetylene with p-iodoanisole in the presence of different solvents
and basesa
Entry
1
2
3
4
5
6
7
8
Pd/Fe3O4 NPs (mol %)
Solvent
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
Base
DMAP
Pyridine
NaHCO3
NaOH
KOH
NEt3
piperidine
piperidine
piperidine
piperidine
Yieldb (%)
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.08
0.5
0.0
0
0
39
Trace
Trace
36
85
40
86
0
9
10
a Reaction conditions: p-iodoanisole (1.0 mmol), phenylacetylene (1.0 mmol), Pd/Fe3O4 (0.2 mol%), base (1.5
mmol), DMF (4.0 mL), 110 °C, 24 h.
Table 2.
Sonogashira coupling reaction of different aryl halides with terminal alkynesa
25
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Pd/Fe3O4 NPs, DMF
Piperidine, 110 ºC
X + RC CH
C
CR
R
R
Entry
1
Aryl halide
Alkyne
Yield b (%)
88
I
2
3
4
85
83
84
MeO
I
Me
I
Me
I
5
6
7
8
9
80
83
87
90
85
O2N
F3C
I
I
OMe
OMe
OMe
I
MeO
I
Me
I
10
11
12
13
14
15
16
17
18
19
85
86
81
80
74
73
71
81
83
70
F
I
I
Br
Br
MeO
Br
Me
Br
Br
O2N
NC
Br
Br
Cl
OMe
OMe
MeO
Br
a Reaction conditions: Aryl halide (1.0 mmol), alkyne (1.0 mmol), Pd/Fe3O4 (0.2 mol%), piperidine (1.5 mmol),
DMF (4.0 mL), 110 °C.
b Yields after work-up.
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Table 3.
Comparison present methodology with other reported methods in the Sonogashira coupling reaction between
chlorobenzene with phenylacetylene
Entry
Catalyst
Polystyrene resin supported palladium(II) complex
Polystyrene anchored Pd(II) azo complex
Pd(0)-MCM-41
Time (h) Temperature (oC) Yielda (%) [Ref.]
1
2
3
4
5
6
3
r.t.
80
35 [37]
6 [38]
12
24
24
24
24
100
90
29 [39]
47 [40]
Pd(II)-MCM-41
Pd@agarose-Fe3O4
130
110
53 [41]
70 [This work]
Pd/Fe3O4 NPs
a Yields after work-up.
Table 4.
Suzuki coupling reaction of different aryl halides with arylboronic acidsa
Pd/Fe3O4 NPs, MeOH
X
B(OH)2
+
K3PO4, 60 oC
R'
R
R'
R
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Entry
1
Aryl halide
Arylboronic acid
B(OH)2
Time (h)
12
Yield b (%)
94
I
2
3
4
12
12
12
91
95, 92c
92
MeO
I
B(OH)2
Me
I
B(OH)2
O2N
I
B(OH)2
MeO
5
6
12
18
91
94
I
B(OH)2
B(OH)2
B(OH)2
B(OH)2
B(OH)2
B(OH)2
B(OH)2
B(OH)2
B(OH)2
Br
7
8
18
18
18
24
24
24
24
91
92
95
95
78
83
95
MeO
Br
Me
Br
Br
9
O2N
10
11
12
13
Cl
MeO
Cl
Me
Cl
Cl
O2N
a
Reaction conditions: Aryl halide (0.5 mmol), arylboronic acid (0.6 mmol), Pd/Fe3O4 (0.1 mol%), K3PO4 (1.5
mmol), MeOH (4.0 mL), 60 °C.
b Yields after work-up.
c Yields after the fifth cycle.
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Abbreviations
XRD: X-ray Powder Diffraction
SEM: Scanning Electron Microscopy
EDS: Energy Dispersive X-ray Spectroscopy
VSM: Vibrating sample magnetometer
FT-IR spectroscopy: Fourier transform infrared spectroscopy
NMR: Nuclear Magnetic Resonance
29
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