Biosynthesis of poly(ethylene glycol)-supported palladium nanoparticles using Colocasia esculenta leaf extract and their catalytic activity for Suzuki-Miyaura cross-coupling reactions

Article abstract of DOI:10.1039/c5ra12657f

A simple and green protocol for the synthesis of poly(ethylene glycol) stabilized palladium nanoparticles under ambient conditions from the aqueous extracts of Colocasia esculenta leaves has been reported. The nanoparticles are characterized using UV-visible spectroscopy, FTIR spectroscopy, XRD and SEM analysis. The prepared Pd NPs showed excellent catalytic activity towards Suzuki-Miyaura cross coupling reactions for a wide variety of aryl halides and phenyl boronic acid substrates. The catalytic system was found to be recyclable and could be reused in subsequent catalytic runs without significant loss of activity.

Full text of DOI:10.1039/c5ra12657f

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Biosynthesis of Poly(ethylene glycol)- supported Palladium
nanoparticles using Colocasia esculenta leaf extract and their
catalytic activity for Suzuki -Miyaura cross- coupling reaction
Received 00th January 20xx,
Accepted 00th January 20xx
Raju Kumar Borah,a Hirak Jyoti Saikia,a Abhijit Mahanta,a Vijay Kumar Das,a Utpal Boraa and Ashim
Jyoti Thakura*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A simple and green protocol for the synthesis of poly(ethylene glycol) synthesis of NPs. Biological materials such as plant extracts,
stabilized palladium nanoparticles under ambient conditions from the microorganisms etc can be used as nanofactories for the
aqueous extracts of Colocasia esculenta leaves has been reported. The synthesis of NPs as they have the reduction potential required
nanoparticles are characterized by UV-visible spectroscopy, FTIR for the synthesis and also in addition, they act as
spectroscopy, XRD and SEM analysis. The prepared Pd NPs showed stabilizers.10,11 The use of biological materials such as plant
excellent catalytic activity towards Suzuki-Miyaura cross coupling reaction extract for the synthesis of NPs offer several advantages of
for a wide variety of aryl halides and phenyl boronic acid substrates. The eco-friendliness and are compatible for pharmaceutical and
catalytic system was found to be recyclable and could be reused in other biomedical applications as they do not use any toxic
subsequent catalytic runs without significant loss of activity.
chemical for their synthetic protocols.22 Also, the processes are
cost effective, environment friendly and can easily be scaled
up for large scale synthesis. Hence, there is a huge demand for
these types of simple, inexpensive and easily accessible
catalysts which can be generated through green procedure for
these coupling reactions. As far as biosynthesis of Pd NPs is
concerned; very few reports are available for their synthesis.
Few recently reported biological synthesis for Pd NPs are peel
extract of Annona squamosa (sugar-apple),12 leaf extract of
Anacardium occidentale (cashew nut),13 Cinnamom zeylanicum
(cinnamon) bark extract,14 and Musa paradisiaca (banana) peal
extracts.15 The biologically synthesized Pd NPs have been used
for various applications in recent years such as in azo dye
From the inception, the palladium-catalyzed carbon−carbon
bond forming reactions developed by Heck, Negishi and Suzuki
have made a vital impact in the development of modern
synthetic organic chemistry.1,2 Such coupling reactions have
wide applications in the production of polymers,
agrochemicals, and pharmaceutical intermediates.3,4 Their
widespread use is mainly due to the mild conditions associated
with these reactions and also with their compatibility to a wide
variety of functional groups. Nowadays, metal nanoparticles
(NPs) are considered to be very attractive and efficient
catalysts compared to their bulk counterparts since they have
a high surface to volume ratios and also their surface atoms
are very active. In recent years, much attention had been
focused by the researchers on palladium nanoparticles (Pd
NPs) synthesis because of their remarkable physical, chemical,
optical and thermodynamic properties.21 Because of these
unique properties, Pd NPs find many applications in the field of
catalysis5 and drug delivery6. Nowadays, Pd NPs of various
sizes and shapes are synthesized by different wet synthetic
processes such as chemical,7 sonochemical8 and polyols
reduction.9 These types of synthetic processes are generally
simple, provide high growth rate and high yield. However,
these methods are not environment friendly and therefore
some alternative methods are in continuous search for the
decolorization16 and as
a
catalyst for Heck reaction.17
Therefore, immense opportunities are available for
synthesizing Pd NPs using biological materials which can be
used for various environmental and biological applications.
The present work deals with biosynthesis of Pd NPs using
Colocasia esculenta Linn. (C. esculenta) leaf extract as reducing
agent followed by evaluation of catalytic activity of
biosynthesized Pd NPs in Suzuki cross coupling reactions. C.
esculenta, locally known as ‘kochu’ (Figure 1) in Assam, a
North Eastern State of India is widely available and used by
folks in traditional system of medicine in north-eastern India.
Due to the compositional abundance of ascorbic acid,
thiamine, riboflavin, niacin, carbohydrates, fats etc. in its
leaves may be responsible for its reductive potency.18 In this
context, one of our laboratory groups has recently reported
the utilization of C. esculenta leave extract for the preparation
of poly(ethylene glycol) (PEG)-supported silver NPs and they
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assessed the free radical scavenging, antibacterial and carbonate (1.5 mmol), nanocatalyst (5 wt% with respect to
larvicidal activity of the prepared NPs.19
boronic acid substrate) were added and stirred in
isopropanol:water (1:1) solvent system at room temperature
for the required time. The reaction was monitored by TLC.
After completion of the reaction, mixture was diluted with
water and the product was extracted with distilled ethyl
acetate (3 times). The combined extract was washed with
brine (3 times) and dried over anhydrous Na2SO4. The product
was purified using column chromatography technique [60-120
mesh silica gel, ethyl acetate-hexane solvent mixture.
Scheme 1 Model reaction
The synthesis of Pd NPs using C. esculenta leaf extract was
first monitored visually and then with the help of UV-visible
spectroscopy (Figure 2). Many reports showed that basic
condition was indispensible for the synthesis of NPs using
plant extract.20 Keeping this in mind, we had initially adjusted
the pH of the reaction solution within 10 and 11 using 0.1 M
NaOH solution.
Figure 1. Image of Colacasia esculenta Linn plant
In the work presented here, the authors described the
reduction potency of C. esculenta Linn. Leaves for the
preparation of poly(ethylene glycol)-supported palladium NPs.
The prepared NPs were characterized using various
spectroscopic and microscopic tools. The Suzuki cross coupling
reaction between boronic acids and aryl halides with different
types of functional groups were assessed with the prepared
NPs resulting good to high yield of the product.
For the preparation of PEG supported Pd NPs, about 10 gm
of the leaves was washed properly with deionized water to
remove dust and grounded by using domestic blender. The
whole content was stirred in a round bottom flask for about 20
0
minutes in 60 ml distilled water at 50 C and was filtered with
muslin cloth at ambient temperature. The filtered aqueous
extract was kept for further experiments.
Figure 2. UV-Visible spectra of B) plant extract, C) PdCl2 solution, D) Pd NPs
solution after bioreduction, and E) dry Pd NPs.
For the preparation of Pd NPs@PEG, 25 mL of 0.001 M
PdCl2 was prepared in 5% (w/v) PEG-4000 solution. To it, 5 mL
of the aqueous extract (obtained as above) was then added.
The pH of the solution was maintained within 10 and 11 by
using 0.1 M NaOH. Then, the solution was refluxed at 60 0C for
one hour. The formation of the NPs was indicated by the color
change from light brown to dark brown. The matrix supported
NPs were separated by centrifugation at 12,000 rpm for 15
minutes. The separated NPs were washed three times with
distilled water and ethanol in order to make the NPs free from
biomaterials and dried in an oven at 60 0C for two hours.
To investigate the effectiveness of the catalyst in Suzuki-
During visual monitoring of the reaction a gradual shifting
of the color from light brown to dark brown was observed
0
within 1 hr at moderate temperature (45 C). Metal NPs are
associated with free electrons that are responsible for Surface
Plasmon Resonance (SPR) peaks, due to combined vibration of
electrons under alkaline condition might be effective for the
utilization of the active component of the leaf extract. The
change of color from light brown to dark brown was observed
due to excitation of SPR of Pd NPs formed. In the UV-Vis
spectroscopy, a distinct peak near 300 nm was observed for
PdCl2 solution due to Pd2+ ions present in the solution.
However, as time proceeded, with the formation of Pd NPs,
the peak near 300 nm began to disappear and vanished within
2 h. This was due to the metal charge transfer from Pd2+ to
Pd(0). The appearance of a broad continuous spectrum, which
gradually increases in intensity from visible to UV region
suggests the complete reduction of Pd2+ to Pd(0).
Miyaura
reaction,
phenylboronic
acid
and
4-
bromonitrobenzene were chosen as model substrates using
isopropanol:water (1:1) as the solvent system, K2CO3 as a base
and the reactions were performed under aerobic condition
and at room temperature without adding any ligands or
additive (Scheme 1).
In a 50 ml round-bottomed flask, a mixture of aryl halide
(0.5 mmol), boronic acid substrate (0.55 mmol), potassium
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Powder X-ray diffraction technique was used to determine stretching vibration of ascorbic acid present in the leaf
the crystal domain size and the structure of the NPs formed. extract.28 The peak at 2102 cm-1 is due to C-H stretching
The XRD pattern for both PdCl2 and Pd NPs were taken and is vibration of sugar present in the leaf extract.29 Surface
shown below (Figure 3).
hydroxyl groups available in the plant extract of the bioactive
molecules primarily reduce Pd2+. The presence of the peaks at
3395 cm-1, 1622 cm-1 and 1073 cm-1 in the IR spectrum of Pd
NPs indicated that different polyol compounds adsorbed on
the palladium surface which might be responsible for capping
and stabilizing Pd NPs.
The SEM images for the Pd NPs are shown in the below
(Figure 5):
Figure 3. XRD patterns for (a) PdCl2 and (b) Pd NPs respectively
From the XRD pattern of Pd NPs, three peaks were
observed at 40.260, 45.780 and 68.670 corresponding to
reflection to the (111), (200) and (220) planes respectively that Figure 5. SEM images for PEG stabilized Pd NPs at (a) 10 µm, (b) 2 µm and (c) 1
µm magnification
could be indexed to fcc phase of Pd NPs (JCPDS≠89-4897). The
broadening of the XRD peak indicates that the synthesized
The surface morphology of the NPs was studied by using
particles are in the nano range. Scherrer’s formula was used
SEM analysis. The SEM images of PEG stabilized Pd-NPs are
for the determination of average crystalline size of Pd NPs.
visualised at (a) 10 µm, (b) 2 µm and (c) 1 µm scale
Since there are three distinct peaks in the XRD spectrum of Pd
respectively. From the SEM images we can say that the NPs are
NPs, the prominent peak at 40.260 corresponding to (111)
of irregular shapes and of different sizes. This might be due to
crystal plane has been considered to calculate the crystal
the agglomeration of the Pd-NPs.
domain size
To study the effect of different solvents, bases and amount
D= Kα/βcosθ
of the catalyst we have chosen 4-bromonitrobenzene and
Where, D represents average crystal domain size, K is the
phenylboronic acid as our model substrates and the results are
Scherrer’s constant, K=0.9 in this case, α= 15.74 nm which is
summarized in Table 1. Screening of solvents using K2CO3 as
the wavelength of X-Rays, β is the peak angular width (full
base showed that isopropanol : water (1:1) was found to be
width half maxima, FWHM) and θ is the diffraction angle. The
the most effective solvent system with excellent yield (Table 1,
FWHM value for this case is 0.7868. Now applying the
entry 7). Our analysis also proved that presence of base was
Scherrer’s formula for the crystal domain size of the NPs, it
very essential for the coupling reactions, since the reaction did
was found to be about 19.35 nm.
not proceed in the absence of a base (Table 1, entry 2). The
The typical FT-IR spectra for the plant extract and PEG
assisted Pd NPs are shown below (Figure 4).
Table 1: Effect of solvents and bases on Suzuki-Miyaura cross coupling reaction of 4-
bromonitrobenzene and phenylboronic acid:a
Entry
Solvent
No solvent
H2O
Base
K2CO3
No base
K2CO3
K2CO3
K2CO3
K2CO3
Time (h)
Yield(%)b
1
2
3
4
5
6
5
15
24
24
24
26
28
No reaction
H2O
35
70
45
36
DMF
MeOH:MeCN (1:1)
MeOH:H2O (1:1)
Isopropanol:H2O
(1:1)
Figure 4. FT-IR spectra of (a) biosynthesized Pd-NPs and (b) the plant extract.
7
8
9
K2CO3
Na2CO3
KOH
1.5
2.5
3
90
90
85
The composition of the leaf extracts shows the abundances
of ascorbic acid (vitamin C), thiamine, riboflavin, niacin,
carbohydrates (glucose, fructose and sucrose), fats etc. as
mentioned earlier.18 These biomolecules have numbers of
hydroxyl groups. For the plant extract, in the IR spectrum, the
peak observed at 3423 cm-1 is due to surface hydroxyl groups.
The other prominent peak at 1622 cm-1 is due to the C=C
Isopropanol:H2O
(1:1)
Isopropanol:H2O
(1:1)
aReaction Condition: 0.5 mmol 4-bromonitrobenzene, 0.55 mmol phenylboronic
acid, 1.5 mmol base, 3 wt% catalyst and 4 ml solvent. bIsolated yield.
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reaction can also proceed in the presence of other inorganic then the reaction requires less time to complete. However, in
bases like Na2CO3, KOH with comparable yield of the cross the presence of electron withdrawing substituents the
coupling products (Table 1, entries 8 and 9).
reaction requires more time for completion. The progress of
After optimizing the reaction condition for the solvent the reaction was monitored by TLC and the products formed
system and the appropriate base, we studied the effect of were characterized by 1H NMR and 13C NMR analyses and
amount of catalyst on the reaction rate and its efficiency. The melting point determination.
results are summarized below (Table 2). From this observation
From the green chemistry perspective, the reusability of
it was found that 5 wt% catalytic amount of the catalyst was catalysts makes them more attractive. Accordingly, we also
the optimized amount for the coupling reaction.
investigated the reusability of our catalyst. To examine the
reusability of the catalyst, the model reaction (scheme 1) was
carried out. After completion of the reaction, the reaction
mixture was filtered through Whatmann 40 filter paper. The
catalyst being insoluble and remains as residue, it was
collected and dried. The filtrate was extracted with ethyl
acetate. The collected Pd NPs were dried and taken in a round
bottomed flask and a fresh reaction with the same amount of
starting material was carried out. We find that the catalyst is
reusable up to third cycles without any significant loss of the
catalytic activity. After third cycle slight loss of activity was
observed (Figure 6). After fifth cycle a significant loss of
activity of the catalyst was observed. This may be due to the
deactivation of the catalyst during the course of the reaction
and recovery process.
Table 2: Effect of catalytic amount on Suzuki coupling reaction
Entry
Amount of the catalysta (wt/%)
Time (h)
1.5
Yieldb
90
1
2
3
4
3
5
1.5
94
6.5
10
1.3
94
1
94
awt% w.r.t. the phenylboronic acid derivative. bIsolated yield.
To evaluate the scope and limitation of the current
procedure, reactions of a wide range of electronically diverse
aryl bromides and iodides with different phenylboronic acid
substrates were examined using the prepared PEG@Pd NPs as
a catalyst (Table 3).
94
92
92
85
80
Table 3: Suzuki-Miyaura cross-coupling reactions of various aryl halides with
arylboronic acids using Pd NPs@PEG as catalyst:a
R1
X
R2
Time (h)
Yieldb (%)
1st
2nd
3rd
4th
5th
Entry
123
224
324
425
523
626
727
824
9
H
4-Br
4-Br
4-I
H
1.5
1.5
2
92
94
95
95
92
92
91
90
90
83
Run
H
NO2
MeO
H
H
Figure 6. Reusability checking of the Pd nano catalyst
2-MeO
4-MeO
2-Me
3-Me
4-CHO
4-Br
4-I
1.5
1
MeO
MeO
MeO
H
In conclusion, we have developed a very efficient and
green process for the synthesis of Pd NPs using C. esculenta
plant extract which exhibits excellent reactivity under mild
conditions for Suzuki cross-coupling reaction of aryl halides
and phenylboronic acid derivatives. The reaction also take
much lesser time and less amount of catalyst compared to
some of the conventional method.30-32 It is found that a wide
range of functional groups are compatible in the reaction
conditions and also gives very high yield of the cross-coupling
products. In the UV-Visible wavelength range, the synthesized
NPs show good surface plasmon resonance behaviour. The
polyol components and the heterocyclic compounds found in
the plant leaves were considered to be responsible for the
reduction of Pd(II) which eliminated the use of toxic reducing
agents. Therefore, we can consider the synthetic process as
green synthesis. The C. esculenta plants are easily available in
North-Eastern region which can further be applied for the
4-I
1
4-Br
4-Br
4-I
2
2
4-CHO
4-tert-
butyl
MeO
NO2
1.5
1.5
10
4-Br
1124
2-CHO
4-Br
H
1.5
96
12
1324
2-NH2
H
4-Br
4-Cl
H
2
94
15
NO2
24
aReaction Condition: 0.5 mmol halobenzene, 0.55 mmol phenylboronic acid
derivative, 1.5 mmol base, 5 wt% catalyst and 4 ml solvent. bIsolated yield.
From the substrate study, it is found that the rate of Suzuki
coupling reaction depends on the nature of the functional
group present on the aryl halide and the arylboronic acids. If
electron donating group is present on the arylboronic acid
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Notes and references
Department of Chemical Sciences, Tezpur University, Tezpur-784001,
Assam, India. Tel: +91 9435181464; E-mail:ashim@tezu.ernet.in
(A. J. Thakur, Corresponding author)
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Graphical Abstract
:
Biosynthesis of Poly(ethylene glycol)-supported Palladium nanoparticles using Colocasia esculenta leaf extract
and their catalytic activity for Suzuki –Miyaura cross-coupling reaction
Raju Kumar Borah, Hirak Jyoti Saikia, Abhijit Mahanta, Vijay Kumar Das, Utpal Bora and Ashim Jyoti Thakur*
A green and efficient method for the development of PEG supported Pd NPs has been developed from aqueous
extract of Colocasia esculenta leaf. The prepared NPs shows excellent catalytic activity for Suzuki-Miyaura cross
coupling reaction for a wide varieties of aryl halides and phenyl boronic acid substrates at room temperature.
Recycled
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