Facile and surfactant-free synthesis of Pd nanoparticles by the extract of the fruits of Piper longum and their catalytic performance for the Sonogashira coupling reaction in water under ligand- and copper-free conditions

Article abstract of DOI:10.1039/c4ra12875c

This work reports a facile synthesis of palladium nanoparticles (Pd NPs) by the extract of the fruits of Piper longum without any stabilizer or surfactant. The Pd NPs were found to be effective catalysts for the ligand-, amine- and copper-free Sonogashira

Full text of DOI:10.1039/c4ra12875c

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ARTICLE TYPE
Facile and surfactant-free synthesis of Pd nanoparticles by extract of
fruits of piper longum and their catalytic performance for Sonogashira
coupling reaction in water under ligand- and copper-free conditions
5
Mahmoud Nasrollahzadeh,a* S. Mohammad Sajadi,b Mehdi Mahamc and Ali Ehsania
aDepartment of Chemistry, Faculty of Science, University of Qom, PO Box 37185-359, Qom, Iran. E-mail: mahmoudnasr81@gmail.com; Fax: +98 25
32103595; Tel: +98 25 32850953
bDepartment of Petroleum Geoscience, Faculty of Science, Soran University, PO Box 624, Soran, Kurdistan Regional Government, Iraq
10 cDepartment of Chemistry, Aliabad Katoul Branch, Islamic Azad University, Aliabad Katoul, Iran
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
First published on the web Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
15
This work reports a facile synthesis of palladium nanoparticles
(Pd NPs) by extract of fruits of piper longum without any
stabilizer or surfactant. The Pd NPs were found to be effective
catalysts for ligandꢀ, amineꢀ and copperꢀfree Songashira coupling
20 reaction under aerobic conditions. The Pd NPs were characterized
with UVꢀvis, FTꢀIR and TEM methods. This method has the
advantages of high yields, simple methodology, and elimination
of ligand, copper, organic solvent and homogeneous catalysts and
easy work up. Water is used as the only solvent for the coupling
25 reaction. More importantly, the catalyst was recovered and
recycled by a simple decantation of the reaction solution and used
for five consecutive trials without significant loss of its reactivity.
Pdꢀcatalyzed Sonogashira reactions in aqueous medium have
been reported; however, many of these reactions are carried out in
60 an aqueousꢀorganic medium and in some cases, specialized
phosphine ligands and copper salts are required in order to
achieve high reaction efficiency.6ꢀ8 However, most of the above
reactions were carried out homogeneously and, thus, had intrinsic
problems such as difficulties in recovery, separation and
65 recycling of the expensive, toxic catalysts and contamination of
the ligand residue into the final product.9 Therefore, development
of mild, highly efficient and environmentally benign method for
the ligandꢀ, copperꢀ and amineꢀfree Sonogashira coupling
reaction under heterogeneous conditions has been a major
70 challenge in organic synthesis.
Introduction
The Sonogashira crossꢀcoupling reaction has become one of
30 the most important, powerful, and versatile tools in the formation
of C(sp)ꢀC(sp2) bonds over the past three decades, which plays
an important role in the synthesis of a variety of compounds,
including heterocycles, natural products, and pharmaceuticals.1ꢀ3
Generally, the Sonogashira reaction proceeds in the presence
35 of toxic phosphine ligands with the combination of palladium and
copper salts using a large excess 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
However, each of these methods suffer from different
40 drawbacks such as employing expensive phosphine ligands, use
of hazardous and expensive organic solvents, harsh reaction
conditions, tedious workꢀups, long reaction times, low yields,
inert atmosphere conditions, environmental pollution caused by
formation of side products.4,5 In Sonogashira coupling reaction,
45 Glaserꢀtype oxidative dimerization of the terminal alkynes cannot
be avoided in copperꢀmediated reaction, in which side products
(diyne) are generally difficult to separate from the desired
In recent years, there has been great interest in using
nanoparticles (NPs) as catalysts in organic reactions.10 Because of
their easy preparation and being relatively stable in air, the
nanoparticles of coinage metals were widely reported, and there
75 are many excellent examples of their applications as catalysts.11
Due to a higher available catalytic surface, heterogeneous
catalysts are more and more used in the form of nanoparticles.12
Additionally, nanocatalysis can make the products easily
removable from the reaction mixtures and make the catalysts
80 recyclable. Thus nanocatalysis is a promising alternative to the
homogeneous catalysis to afford the same products with high
reaction rates and high yields. However, these smallꢀsized
products require more complex production methods which often
imply the use of toxic and expensive chemicals. Therefore,
85 environmentally benign production methods of Pd nanocatalysts
are very desirable.
In the recent time, several researchers have achieved success
in the synthesis of metal nanoparticles with specific shape and
size using various plants extracts, bacteria and fungus.13,14
90 Greener synthesis helps to replace the hazardous chemicals that
cause toxicity, minimizes harmful pollution to the environment
when debris such as surfactants/dispersants released by the large
scale industries and lead to an ecoꢀfriendly environment.15ꢀ19
Piper species, widely distributed in the tropical and subtropical
95 regions of the world are used medicinally for presence of various
phytochemicals. Piper longum or long piper belonging to the
family Piperaceae is a slender aromatic climber with red fruits
(Figure 1). These fruits turn grayish black with pungent smell
products and copper acetylide is
a potentially explosive
reagent.4,5 In addition, amines, such as triethylamine and
50 piperidine, required in most Sonogashira reactions, have a bad
smell and add to the environmental burden. Therefore, the
development of convenient methods for Sonogashira reactions is
an important objective.
The use of water, the most abundant and nonꢀtoxic solvent for
55 reactions is reclaiming its importance due to pressing
environmental, economical, and safety concerns. Moreover, the
products can be isolated easily by extraction. Several examples of
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when dried. The primary active constituents namely piperine,
piplartine, and piperlongumine are found to be present in the
plant. Also piper Longum consists of polyphenolics such as
glycoside flavonoids, alkaloids, lignans, steroids and organic
acids.20ꢀ23
5
Figure 2. HPLC chromatogram of alcoholic extract of fruits of Piper
longum; The characterized compounds according the peak numbers are 1-
45 Pipataline, 2- Pellitorine, 3- Sesamine, 4- Brachystamide B, 5-
Guineensine, 6- 5ꢀHydroxyꢀ7,3',4'ꢀTrimethoxyflavone, 7- 5ꢀHydroxyꢀ
7,4'ꢀdimethoxyflavone,
8-5,4'ꢀDihydroxyꢀ7ꢀmethoxyflavone,
9-
Diaeudesmin, 10- Piperine, 11- Dihydropiperlonguminine, respectively.
Figure 1. Image of Piper longum leaves and fruits.
50
Moreover, The UV spectrum of extract (Figure 3) shows
bonds at λmax 320 nm (bond Ι) due to the transition localized
within the ring of cinnamoyl system; whereas the one centred at
241 nm (bond ΙΙ) is for ring related to the benzoyl system. They
are related to the π → π* transitions and these absorbent bonds
Based on our continuous investigations on copper, palladium,
10 iron and silver nanoparticles catalyzed reactions,24 we wish to
report herein an ecofriendly, clean, nonꢀtoxic, facile chemically
preparative method for the synthesis of palladium NPs using
extract of fruits of piper longum without any stabilizer or
surfactant. The Pd NPs were characterized by TEM, FTꢀIR and
15 UVꢀvisible techniques. Also, the catalytic activity of Pd NPs for
the ligandꢀ and copperꢀfree Sonogashira coupling reactions in
water was studied under aerobic conditions (Scheme 1). To the
best of our knowledge, no report has been issued on the
biosynthesis of Pd NPs using extract of fruits of piper longum.
55 demonstrate the presence of polyphenolics.
20
Pd NPs
Ar
R
ArX +
R
K2CO3, H2O, 80 oC
X = I, Br
R = Ph, CH2OH
Scheme 1. Sonogashira coupling reaction of aromatic aryl halides and
terminal alkynes catalyzed by Pd NPs in water.
Figure 3. UVꢀvis spectrum of alcoholic extract of fruits of piper longum.
Result and Discussion
The FTꢀIR analysis was carried out to identify the possible
biomolecules responsible for the reduction of PdCl2 and capping
60 of the bioreduced Pd NPs synthesized by extract of fruits of piper
longum. The FTꢀIR spectrum of the crude extract, (Figure 4)
depicted some peaks at 3500 to 3000, 1700 and 1650, 1522, 1396
and 1122 cmꢀ1 which represent free OH in molecule and OH
group forming hydrogen bonds, carbonyl group (C=O), stretching
65 C=C aromatic ring and CꢀOH stretching vibrations, respectively.
These peaks suggested the presence of flavonoid and other
phenolics in the plant leaves extract. The presence of flavonoid
and other phenolics in the extract could be responsible for the
reduction of metal ions and formation of the corresponding metal
70 nanoparticles.
Plant extracts may act both as reducing agent and stabilizing
25 agents in the synthesis of nanoparticles. This study was
performed, to predict the plant based reducing agents that are
involved in the reaction between extract of the fruits of the piper
longum and PdCl2. In this study we used the fruits of piper
longum as a major part of the plant for presence of antioxidant
30 glycoside flavonoids as potent reducing agents to green
synthesize of palladium nano particles.
Characterization of extract of fruits of piper longum
To confirm the presence of reducing agents inside the extract
and its ability to green synthesis of metallic nanoparticles, HPLC
35 fingerprinting analysis and spectroscopic methods were used. The
HPLC chromatogram demonstrates the presence of 11
constituents as Pipataline, Pellitorine, Sesamine, Brachystamide
B, Guineensine, 5ꢀHydroxyꢀ7,3',4'ꢀTrimethoxyflavone, 5ꢀ
Hydroxyꢀ7,4'ꢀdimethoxyflavone,
40 methoxyflavone, Diaeudesmin,
Dihydropiperlonguminine, respectively (Figure 2).
5,4'ꢀDihydroxyꢀ7ꢀ
Piperine and
2
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spectrum. The synthesized palladium nanoparticles by this
method are quite stable and no obvious variance in the shape,
position and symmetry of the absorption peak is observed even
30 after one month indicating the stability of Pd NPs.
Figure 4. FTꢀIR spectrum of aqueous extract of fruits of piper longum.
Figure 5. UVꢀvis spectrum of green synthesized Pd NPs using extract of
fruits of Piper longum.
Reduction of PdII to Pdo
In this work, we have employed extract of fruits of piper
longum as a reducing and stabilizing agent for the synthesis of Pd
NPs. The mechanism for the formation of Pd NPs can be
explained as shown in Scheme 2. On the basis of this mechanism,
metal ions were reduced to nano zero valent (NZV) metallic
particles using flavonoid (FlOH) contents of the plant as green
10 (without any hazardous impact on the environment) chemicals
(reducing agents) as an alternative to the commonly used hydride
reducing agent, sodium borohydride.
5
Furthermore, the FTꢀIR of extract after adding PdCl2 while
35 formation of Pd NPs shows demonstrative differences in the
shape and location of signals indicating the interaction between
PdCl2 and involved sites of phytochemicals for production of
nanoparticles (Figure 6). Changing the location of peaks at 3500
to 3200, 1695, 1452, 1381 and 1074 cmꢀ1 represent the OH
40 functional groups, carbonyl group (C=O), stretching C=C
aromatic ring and CꢀOH stretching vibrations, respectively.
Polyphenolics could be adsorbed on the surface of metal
nanoparticles, possibly by interaction through πꢀelectrons
interaction in the absence of other strong ligating agents. In fact
45 the πꢀelectrons of carbonyl group (C=O) from C ring of
flavonoids in a Red/Ox system can transfer to the free orbital of
metal ion and convert that to the free metal.
Characterization of Pd NPs
15
The stable Pd NPs obtained were fully characterized by UVꢀ
Vis, TEM and FTꢀIR.
The formation of Pd NP was monitored with the help of UVꢀ
vis spectrometry. The UVꢀvis spectrum of green synthesized Pd
NPs using piper longum fruits extract (Figure 5) showed the
Figure 6. FTꢀIR spectrum of aqueous extract of fruits of piper longum.
20 significant changes in the absorbance maxima due to surface
plasmon resonance demonstrating the formation of Pd NPs. Also,
the progression of the reaction, formation and stability of
palladium nanoparticles were controlled by UVꢀvis spectroscopy.
The yellow color of the PdII solution (λmax 415 nm) immediately
25 changed dark brown (λmax 270ꢀ330 nm) indicating reduction of
PdII to Pdo and formation of Pd NPs as characterized by UVꢀvis
50
Particles morphology and size of Pd NPs were studied by
TEM. Figure 7 shows TEM image of typical nanoparticles
synthesized from extract of fruits of piper longum as both
reducing and stabilizing agent. Particle sizes ranged from 5 to 40
nm and most were near spherical. The methanolic plant extract
55 was involved as capping and stabilizing the size of the Pd NPs.
Figure 7. TEM image and size distributions of Pd nanoparticles.
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DOI: 10.1039/C4RA12875C
Entry
Aryl halide
Alkyne
Time (h)
4
Yield b (%)
91
Catalytic ability and application of the synthesized Pd NPs
for the Sonogashira coupling reaction
1
2
3
After characterization of the Pd NPs, their catalytic activities
were examined in the Sonogashira coupling reaction. In order to
test the catalytic ability and application of synthesized Pd NPs,
we first investigated their catalytic effects for ligandꢀ, amineꢀ and
copperꢀfree Sonogashira coupling between pꢀiodoanisole and
phenylacetylene under aerobic conditions. As shown in Table 1,
no product was obtained in the absence of the base (Table 1,
10 entry 1). We examined different bases, such as, KOH, Na2CO3,
DMAP, K3PO4, NaOH, Et3N and K2CO3. The results revealed
that the inorganic bases used were more effective than organic
bases like DMAP or Et3N and hence the economically cheaper
K2CO3 was chosen as a base for these coupling reactions. The
15 effect of the amount of the catalyst was determined for
Sonogashira coupling reaction. It can be seen that with an
increase in the amount of catalyst from 1.0 mol% to 3.0 mol%, a
considerable increase in the yield was observed (Table 1, entries
2 and 3). No significant improvement on the yield was observed
20 using higher amounts of the catalyst (Table 1, entry 11). Thus, the
quantity of 3.0 mol% was found to be the best weight of catalyst
for the condensation of pꢀiodoanisole (1.0 mmol) with
phenylacetylene (1.2 mmol) in H2O (4.0 mL) at 80 °C (Table 1,
entry 3). No surfactant, ligand or organic coꢀsolvent was required.
25 Since no copper salt used, the undesired formation of Glaserꢀtype
oxidative homocoupling product, a diyne, was also avoided.
4
4
92
91
5
4
5
6
4
3
3
93
96
92
7
4
90
4
3
6
89
93
84
8
9
10
6
92
11
12
13
6
5
8
94
95
93
Table 1. Palladium nanoparticles catalyzed coupling reaction of pꢀ
iodoanisole and phenylacetylene.a
Entry
Pd NPs (mol%)
Base
ꢀ
Yield (%)b
0.0
1
2
3
4
5
6
7
8
9
1.0
1.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
5.0
a
Reaction conditions: aryl halide (1.0 mmol), terminal alkyne (1.2
K2CO3
K2CO3
DMAP
Na2CO3
NaOH
KOH
K3PO4
KOAc
Et3N
56
93
0.0
80
18
20
75
Trace
Trace
93
mmol), Pd NPs (3.0 mol%), K2CO3 (2.0 mmol), water (4.0 mL), 80 °C.
45 b Isolated yield.
Catalyst recyclability
One of the advantages of heterogeneous catalysts is their easy
separation from the reaction mixture. After each cycle, the Pd
50 NPs catalyst could be separated and recovered conveniently by
centrifugation from the reaction mixture and washed with ethanol
and dried in an oven, and then, new substrates and K2CO3 were
added to set up a new reaction. Before being employed in the
next run, the recovered catalysts were washed with ethanol and
55 distilled water several times respectively. As shown in Figure 8,
recovered catalyst was recycled five times for the coupling
reaction of pꢀiodonitrobenzene and phenylacetylene. This
reusability demonstrates the high stability and turnover of catalyst
under operating conditions.
To check the heterogeneity of this catalyst, which is an
important factor, the filtrate of each cycle was analyzed by
Inductively Coupled Plasma Atomic Emission Spectroscopy
(ICPꢀAES) technique. Very low palladium contamination was
observed during the course of a reaction. According to the ICP
10
K2CO3
a
30 Reaction conditions: pꢀiodoanisole (1.0 mmol), phenylacetylene (1.2
mmol), Pd NPs (3.0 mol%), base (2.0 mmol), water (4.0 mL), 80 °C, 4 h.
b Isolated Yield.
Sonogashira reactions of different aryl halides and terminal
35 alkynes with 3.0 mol% of Pd NPs as catalyst were investigated.
Reactions were carried out in H2O at 80 °C and different times.
The results are listed in Table 2. It can be seen from Table 2 that
most of the aryl halides with electronꢀdonating or electronꢀ
withdrawing substituents resulted in excellent yields.
40
60
Table 2. Sonogashira coupling reaction of aromatic aryl halides and
terminal alkynes catalyzed by Pd NPs.a
65 analysis, less than 0.2% of the initial amount of palladium was
detected.
4
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Preparation of extract of fruits of piper longum
50 g of dried fruits powdered of piper longum was added to
300 mL 30% methanolic solution in 500 mL flask and well
mixed. The preparation of extract was done by using magnetic
55 heating stirrer at 70 °C for 30 min. The extract obtained was
centrifuged in 6500 rpm then filtered and filtrate was kept at
refrigerator to use further.
Preparation of sample for HPLC analysis
60
The 30% methanolic extract of fruits of Piper longum was
prepared by soaking in for 24 h. The extract was centrifuged at
5000 rpm and filtered through 0.2 µ membrane filter using highꢀ
pressure vacuum pump and the clear solutions were used for
HPLC fingerprint analysis.
65
Preparation of palladium nanoparticles
In a typical synthesis of Pd NPs, 15 mL extract of the plant
fruits was added dropwise to 50 mL of 0.003 M aqueous solution
of PdCl2 with constant stirring at 80 °C. Reduction of palladium
Conclusion
We have successfully synthesized Pd NPs via the reduction of
5
aqueous Pd2+ ions using extract of fruits of piper longum without
any stabilizer or surfactant. The flavonoids present in extract of
fruits of piper longum act as both reducing and
capping/stabilizing agents. The nanoparticles have been
characterized by UVꢀvis, FTꢀIR and TEM. In addition, the
70 ions (PdII) to palladium (Pdo) was completed around 30 min,
using monitoring by UVꢀvis and FTꢀIR spectra of the solution.
The color of the reaction mixtures gradually changed in 30 min at
80 °C indicated the formation of palladium nanoparticles. Then
the colored solution of palladium nanoparticles was centrifuged at
75 7000 rpm for 45 min to absolute precipitation of Pd NPs.
10 synthesized Pd NPs capped by biomolecules showed potent
catalytic application for the ligandꢀ and copperꢀfree Sonogashira
coupling reaction in aqueous medium. The present method has
the advantages of readily available starting materials,
straightforward and simple workꢀup procedures, elimination of
15 ligand, high yields, tolerance for a wide variety of functionality,
and excellent reusability of the catalyst. It is also observed that
the catalyst was recycled five times without any significant loss
of catalytic activity. The described strategy for Pd NPs is
straightforward, robust, environmentally friendly, and costꢀ
20 effective, and shows considerable great potential. The
synthesized Pd NPs by this method are quite stable and can be
kept under inert atmosphere for several months.
General procedure for Sonogashira coupling reaction
An aryl halide (1.0 mmol), a terminal alkyne (1.2 mmol) and
K2CO3 (2.0 mmol) were added to a freshly prepared solution of
80 palladium in water (3.0 mol%) in a glass flask under vigorous
stirring. The mixture was stirred at 80 °C for the appropriate time
under aerobic conditions. After completion of the reaction
(monitored by TLC or GC), the reaction mixture was cooled and
the organic layer was extracted with EtOAc, washed with water,
85 dried over Na2SO4, filtered and evaporated under reduced
pressure. The residue was subjected to column chromatography
to afford the pure product. The desired pure products were
characterized by NMR, FTꢀIR and from their melting points. All
the products are known and the spectroscopic data and melting
90 points were consistent with those reported in the literature.1,6ꢀ9
Experimental
25
Highꢀpurity chemical reagents were purchased from the Merck
and Aldrich chemical companies. All materials were of
commercial reagent grade. Melting points were deterꢀmined in
open capillaries using a BUCHI 510 melting point apparatus and
are uncorrected. 1H NMR and 13C NMR spectra were recorded on
Acknowledgement
We gratefully acknowledge the Iranian Nano Council and
University of Qom for the support of this work.
95
30 a Bruker Avance DRXꢀ400 spectrometer at 400 and 100 MHz,
respectively. FTꢀIR spectra were recorded on a Nicolet 370 FT/IR
spectrometer (Thermo Nicolet, USA) using pressed KBr pellets.
The element analyses (C, H, N) were obtained from a Carlo
ERBA Model EA 1108 analyzer carried out on PerkinꢀElmer
35 240c analyzer. TLC was performed on silica gel polygram SIL
G/UV 254 plates. Transmission electron microscopy (TEM) was
recorded on a ZeissꢀEM10Cꢀ80 KV. UVꢀvisible spectral analysis
was recorded on a double‐beam spectrophotometer (Hitachi,
U‐2900) to ensure the formation of nanoparticles. The RPꢀ
40 HPLCꢀDAD instrument consisted of auto injector, sample cooler,
pumps, and column oven and diode array detector. An ODS 5µ
(3) column (250 x 4.60 mm, particle size 5 µm) was applied. The
mobile phase containing methanol: water was filtered through 0.2
µ membrane filter before use. Flow rate of the effluent was 1
45 mL/min, the detector wavelengths 270 nm and 370 nm and the
volume of injection 10 µL. Identification was based on coꢀ
injections of reference compounds and comparisons of absorption
spectra. All solvents were HPLCꢀgrade and chemicals were
analytical grade.
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