Green chemical synthesis of Pd nanoparticles for use as efficient catalyst in Suzuki-Miyaura cross-coupling reaction

Article abstract of DOI:10.1002/aoc.4758

Herein, we report the synthesis of tiny spherical Pd nanoparticles (NPs) by green chemical method under ambient conditions using flower extract of Lantana camara plant. The size of the Pd NPs is tunable from 4.7 to 6.3?nm by systematically controlling the concentration of either metal ions or plant extract. The synthesized Pd NPs were well characterized by different spectroscopic, microscopic and diffractometric techniques. The Pd NPs offered good size-dependent catalytic activity in the Suzuki-Miyaura C-C coupling reaction under mild reaction conditions in (1: 1) water-ethanol mixture. The catalyst is stable and exhibited excellent reusability up to three cycles of coupling reaction after which the catalytic activity decreases.

Full text of DOI:10.1002/aoc.4758

Received: 31 July 2018
DOI: 10.1002/aoc.4758
Revised: 2 November 2018
Accepted: 17 November 2018
F U L L P A P E R
Green chemical synthesis of Pd nanoparticles for use as
efficient catalyst in Suzuki‐Miyaura cross‐coupling reaction
Shreemoyee Phukan1 | Abhijit Mahanta1,2 | Dwipen Kakati1 | Md. Harunar Rashid1
1 Department of Chemistry, Rajiv Gandhi
Herein, we report the synthesis of tiny spherical Pd nanoparticles (NPs) by
green chemical method under ambient conditions using flower extract of Lan-
tana camara plant. The size of the Pd NPs is tunable from 4.7 to 6.3 nm by sys-
tematically controlling the concentration of either metal ions or plant extract.
The synthesized Pd NPs were well characterized by different spectroscopic,
microscopic and diffractometric techniques. The Pd NPs offered good size‐
dependent catalytic activity in the Suzuki‐Miyaura C‐C coupling reaction
under mild reaction conditions in (1: 1) water‐ethanol mixture. The catalyst
is stable and exhibited excellent reusability up to three cycles of coupling reac-
tion after which the catalytic activity decreases.
University, Rono Hills, Doimukh 791 112,
Arunachal Pradesh, India
2 Department of Chemical Science, Tezpur
University, Tezpur 784 028, Assam, India
Correspondence
Md. Harunar Rashid, Department of
Chemistry, Rajiv Gandhi University, Rono
Hills, Doimukh 791 112, Arunachal
Pradesh, India.
Email: mhr.rgu@gmail.com
Funding information
Science and Engineering Research Board,
Grant/Award Number: EMR/ 2015/
001912; Council of Scientific and Indus-
trial Research, Grant/Award Number:
01(2910)/17/EMR‐II
KEYWORDS
catalyst, C‐C coupling, green synthesis, Lantana camara, Pd NPs
1 | INTRODUCTION
surface of Pd NPs and thereby imparted adverse effects
in biological applications.[19] Therefore, the choice of
Synthesis of palladium (Pd) nanoparticles (NPs) has
received considerable attention in recent years due to
their interesting physical, chemical, optical and thermo-
dynamic properties in the nanoscale region.[1] Because
of these unique properties, Pd NPs find various applica-
tions in different areas such as catalysis,[2–5] sensing,[6,7]
biomedical[8] and plasmonic waveguiding.[9,10] Conse-
quently, much attention has been focused on the synthe-
sis of Pd NPs of controlled sizes and shapes. Generally, Pd
NPs are synthesized by various wet syntheses processes
such as chemical reduction,[11–14] sonochemical,[15]
polyol reduction methods,[7,16] etc. However, most of the
conventional chemical methods involve a reducing agent
and a stabilizer in a suitable solvent.[11–13,17] Wherein the
chemicals used are quite harmful and toxic enough to
cause environmental hazard while adopting for large‐
scale production of Pd NPs.[18] Also, most of the conven-
tional reducing agents involved in chemical methods pro-
duce toxic byproducts which might get adsorbed on the
the need of reducing and capping agents is to be consid-
ered seriously from both an economic and green chem-
istry perspective.[20–22] Hence, there is immense interest
in recent years in developing environmentally friendly
and sustainable methods for the synthesis of Pd NPs.
In this regard, the biogenic method involving plant
extract has received considerable attention because;
such methods are considered to be very effective in
developing a rapid, clean, nontoxic, and eco‐friendly
technology.[23–27] Another advantage associated with
the use of plant extract is that no additional capping
agent or stabilizer is required as phytochemicals present
in the plant extract or their by‐product formed during
reaction stabilize the formed Pd NPs.[28,29] Additionally,
the phytochemicals are also useful in controlling the
size and shape of Pd NPs.[30] The most prominent exam-
ples of plant used so far include Pulicaria glutinosa,[4]
Delonix regia,[23] Euphorbia thymifolia,[24] Stachys
lavandulifolia,[27] Sapindus mukorossi,[30] Glycyrrhiza
Appl Organometal Chem. 2018;e4758.
wileyonlinelibrary.com/journal/aoc
© 2018 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4758

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PHUKAN ET AL.
glabra,[31] Pistacia atlantica kurdica,[32] Origanum
vulgare,[33] watermelon rind[34] and so on. However
most of the above‐mentioned plants are either econom-
ically important or edible or have medicinal values.
However, to the best of our knowledge, reports on the
synthesis of Pd NPs using wild weeds are not available
yet.
on the improvements in the activity and selectivity of
these catalysts where Pd NPs are formed during the reac-
tion that acts as the active catalyst.[46,49,50] However,
ligands used in the system and the corresponding cata-
lysts have several disadvantages such as the use of expen-
sive, toxic phosphine ligands and organic solvents, harsh
reaction condition, low yield, environmental problems
caused by the side product formation, inert atmosphere
conditions. Also, the phosphine ligands are readily oxi-
dized to their corresponding phosphine oxides, which
can prevent the easy recovery and recycling of the cata-
lyst. For these reasons, the focus is to make the experi-
mental conditions more environmentally benign and to
develop ligand‐free palladium catalysts. Consequently,
many studies have been engaged in recent times to
search for the effective solvents in the context of green
chemistry.[51–53] Water is commonly considered as a
benign solvent in view of its non‐toxicity and abundant
natural occurrence. Around 80% of the chemical waste
from a reaction mixture corresponds to the solvent.
Therefore, the use and release of clean water instead of
organic solvents is the significant concern with respect
to environmentally friendly organic synthesis.[52–54]
Recently, many reports have been published on the
successful use of Pd NPs synthesized by plant extract
as catalysts in cross‐coupling reactions in water and/or
a water‐alcohol mixture.[4,24,27,30] Due to the ability to
dissolve bases in water for activating aryl boronic acids,
the reaction proceeds much faster in an aqueous
medium.[55] Taking into the consideration of the
effectiveness of Pd NPs in Suzuki‐Miyaura reaction,
herein we also report the use of plant extract reduced
Pd NPs as a catalyst for Suzuki‐Miyaura coupling
reaction in 1: 1 water‐ethanol mixture using K2CO3 as
a base at room temperature.
Considering the benefit of plant extracts, herein; we
intend to design a facile, cheap and environmentally
friendly method to synthesize size‐tunable spherical Pd
NPs using the flower extract of Lantara camara plants.
North‐East region of India is rich in biodiversity and Lan-
tana camara is readily available throughout Arunachal
Pradesh as wild sage. The population of Lantana camara
is very high in this region which is spreading very fast
due to their invasive nature. However, such plants are
not utilized properly by any researchers for their gainful
utilization. It is reported that the extract of flowers of Lan-
tana camara is composed mainly of sugars, phospholipids,
glycolipids, alkaloids, phenolics, tannins, etc. and the
amount of these substances are more in the extract of
flowers than that of other parts of the plant.[35,36] Also it
is reported that polyphenolics can be extracted efficiently
using water as extracting solvent which also increases the
versatility of the process as it further minimizes the use
of toxic solvents.[36] The polyphenolics act as better green
reducing agents for noble metal and other transition metal
ions.[25,26,37] Considering these aspects, we believe that
the use of flower extract of Lantana camara as
reducing‐cum‐stabilizing agent for the preparation of
Pd NPs can provide a new platform to this noxious plant
making it a value‐added weed for nanotechnology‐based
industries in future. So, we report in this manuscript a
fast, low‐cost and environmentally safe synthesis
method for Pd NPs using phytochemicals present in
flower extract of Lantana camara plant both as reducing
and capping agent.
In chemistry, Pd is regarded as one of the most effi-
cient metal catalysts.[2,4,5,14,38] The high surface‐to‐
volume ratio in the nanometer dimension even makes
Pd NPs more effective catalyst.[3,24,39] Subsequently Pd
NPs are widely used as catalyst in low‐temperature
oxidation/reduction of automobile pollutants,[40–42] cou-
pling reactions,[2,4,24,30–32,43–45] hydrogenation reac-
tion[23,46] as well as other organic reactions. However,
Pd earned a reputation as a catalyst for their remarkable
success in C‐C coupling reactions which is considered to
be one of the most powerful and a versatile method for
the synthesis of asymmetric biaryl compounds from aryl
boronic acids and aryl halides. These asymmetric biaryl
compounds find wide applications in the production of
agrochemicals, natural products, pharmaceuticals and
functional materials.[47,48] Many studies have reported
2 | EXPERIMENTAL
2.1 | Materials and method
Palladium chloride (PdCl2), phenyl boronic acids and
their substituted products were purchased from Sigma–
Aldrich. Flowers of Lantana camara were collected from
Papumpare district of Arunachal Pradesh, India. Solvents
used were purchased from Merck India and were distilled
before use. All other reagents were used without
further purification. All the glassware's were cleaned
in a bath of freshly prepared aqua–regia solution
(HCl: HNO3 = 3:1, V/v) and then rinsed thoroughly with
double distilled water. All solutions were prepared in
double distilled water.

PHUKAN ET AL.
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concentration of the extract, we have carried out similar
sets of reaction by varying the concentration of PdCl2
and extract maintaining the other reaction conditions
same (Table 1).
2.2 | Isolation of flower extract
The phytochemicals from the flowers of wild growing
Lantana camara were isolated by following the previ-
ously reported method.[26] For the synthesis purpose, a
4.0 gL−1 solution of the extract was prepared in distilled
water. The amount of total phenolics in the flower extract
of Lantana Camara plants were determined with the
Folin–Ciocalteu reagent using gallic acid as a standard
following our previously reported method.[25] A calibra-
tion curve was plotted (R2 = 0.994) with the series of gal-
lic acid standards from which the total phenolic contents
values were determined. Results are expressed as mg of
gallic acid equivalents (GAE). The amount of phenolic
contents present in the flower extract is 69 mg GAE/g
of the extract (GAE: gallic acid equivalent).
2.4 | General procedure for Suzuki–
Miyaura coupling reaction
In
a 50 ml round‐bottomed flask, a mixture of
4‐bromonitrobenzene (1 mmol), phenylboronic acid
(1.2 mmol), K2CO3 (1.5 mmol) and Pd NPs catalyst
(0.007 mol%) was added and stirred in ethanol‐water
(1: 1) solvent system at room temperature for the required
time. The reaction was monitored by thin layer chroma-
tography (TLC). After completion of the reaction, the
mixture was diluted with water, and the product was
extracted with ethyl acetate (two times). The extracted
product was dried over Na2SO4. The product was purified
in a column chromatography using silica gel (100–200
mesh) as stationery and ethyl acetate‐hexane solvent mix-
ture as mobile phase respectively. The purified product
was used for characterization and identification.
To check the reusability of the catalyst, the aqueous
part containing the catalyst was centrifuged at 7000 rpm
for 15 min after completion of the reaction. The isolated
mass was re‐dispersed in the solvent system and placed
in a round‐bottomed flask, and a fresh reaction with the
same amount of starting materials was carried out.
2.3 | Synthesis of palladium nanoparticles
(Pd NPs)
The Pd NPs were synthesized by the reduction of palla-
dium salt in an aqueous medium using phytochemicals
present in the flower extract of Lantana camara plant
as reducing agent–cum–stabilizer. In a typical synthesis,
2.0 ml of 0.4 wt% aqueous extract solution was taken in
a cleaned glass vial containing 1.6 ml of water and was
heated to 55 °C under constant magnetic stirring. To the
stirring extract solution, 0.4 ml of 0.01 M aqueous PdCl2
solution was added dropwise, and the stirring was contin-
ued for 2.0 hr at the same temperature. The progress of
the reaction was monitored visually from the color
change of the reaction mixture from light yellow corre-
sponding to the mixture of extract and PdCl2 ions to
greenish within a short span of time. The resulted colloi-
dal suspension was then purified by dialysis using cellu-
lose membrane (HiMedia; MW cut‐off between 12000 to
14000) against distilled water. The purified colloidal sus-
pension was used for different spectroscopic and micro-
scopic analysis. This set of reaction and the resultant
suspension was designated as sample Ex‐Au‐1 (Table 1).
To study the effect of metal ion concentration or the
2.5 | Characterization
For UV–vis spectroscopic analysis, requisite volume of an
aqueous suspension of Pd NPs was transferred into a
quartz cuvette of path length 1.0 cm, and the spectrum
was recorded in an Agilent Cary60 spectrophotometer in
the wavelength region of 300–800 nm against water as a
blank. For transmission electron microscopic (TEM) stud-
ies, a drop of an aqueous suspension of Pd NPs was cast on
a carbon‐coated copper grid. The excess solutions were
soaked with a tissue paper followed by drying in air. The
micrographs were then recorded in a high–resolution
JEOL electron microscope (JEM 2100EM) at an accelerat-
ing voltage of 200 kV. ImageJ software (National Institute
of Health) was used to measure the size and interplane
spacing values of the nanostructures. X‐ray photoelectron
spectroscopy (XPS) analyses of the dried powder samples
were performed in Thermo Fisher Scientific, UK make
ESCALB Xi+ X‐ray photoelectron spectrometer using Al
kα radiations with incident energy of 1486.61 eV. The
instrument was operated at 15 kV and 300 W at ambient
temperature under ultrahigh vacuum. X–ray diffraction
(XRD) study of the Pd NPs deposited on microscopic glass
TABLE 1 Reaction recipe for the synthesis of Pd NPs using
flower extract of Lantana camara
Entry
No.
Sample
code
[PdCl2]
(M)
[Extract]
(wt%)
(dTEM av
)
1.
2.
3.
5.
Ex‐Pd‐1
Ex‐Pd‐2
Ex‐Pd‐3
Ex‐Pd‐4
1.00 × 10−3 0.20
0.75 × 10−3 0.20
0.50 × 10−3 0.20
0.75 × 10−3 0.30
5.1 nm
5.6 nm
6.3 nm
4.7 nm

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PHUKAN ET AL.
transforms infrared spectra (FTIR) of the powder samples
were collected in a Thermo Scientific Nicolet iS5 spectrom-
eter in the range of 4000–400 cm−1. The pellets for record-
ing the FTIR spectra were prepared by mixing the powder
sample with dried KBr in the weight ratio of 1:100. 1H and
13C NMR spectra were recorded in a JNM ECS 400 MHz
NMR spectrophotometer (JEOL) using tetramethylsilane
(TMS) as the internal standard. Chemical shift values are
expressed in ppm. Coupling constants are expressed in
Hertz.
3 | RESULTS AND DISCUSSION
A series of reaction between the flower extract of Lantana
camara plant and PdCl2 were carried out in an aqueous
medium at 55 °C for 2.0 hr. The formation of Pd NPs
was monitored initially by visual observation of the
change in color of the reaction mixture from light yellow
to greenish with time which was then further confirmed
by UV–vis spectroscopy. Figure 1 shows the UV–vis spec-
tra of an aqueous solution of the suspension of colloidal
FIGURE 1 UV–vis spectra of colloidal Pd NPs recorded after
2.0 hr of reaction
slide was carried out in a Phillips X'pert Pro multipurpose
diffractometer at an accelerating voltage of 40 kV using Cu
kα radiation (λ = 1.54 Å) as X–ray source. Fourier
FIGURE 2 Low (a) and high (b) magnification TEM images of Pd NPs recorded from sample Ex‐Pd‐1. (c) High resolution TEM image and
(d) selected area electron diffraction (SAED) pattern recorded from single polygonal Pd NPs

PHUKAN ET AL.
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Pd NPs. All the spectra are dominated by absorption peak
at 327 nm due to surface plasmon resonance (SPR) indi-
cating the formation of Pd NPs. Nasrollahzadeh et al.
reported similar observations for Pd NPs synthesized
using extracts of Euphorbia granulate plant.[56]
Ex‐Pd‐2 and Ex‐Pd‐3 prepared with Pd2+ ion concentra-
tion 0.75 and 0.5 mM, respectively, keeping the concen-
tration of extract constant at 0.2 wt%. The TEM image
(Figure 3A) of sample Ex‐Pd‐2 again shows the formation
of spherical Pd NPs. In this case, the average size of the
Pd NPs is 5.6 nm. Whereas the average size of Pd NPs
observed in sample Ex‐Pd‐3 is 6.3 nm as measured from
the respective TEM image shown in Figure 3B. The com-
parison of the microscopic results of samples Ex‐Pd‐1, Ex‐
Pd‐2, and Ex‐Pd‐3 indicate that at constant extract con-
centration decrease in the concentration of Pd2+ ions
did not show any significant change in the size or mor-
phology of Pd NPs. The particle size distribution of these
samples (Ex‐Pd‐2 and Ex‐Pd‐3) are shown in Figure S1b
and S1c in the SI. Another set of the reaction was carried
out to check the effect of the extract on the size of Pd NPs,
with the concentration of extract and Pd2+ ion of 0.3%
and 0.75 mM, respectively. The TEM image of this sample
(Figure 3C) shows the formation of again spherical Pd
NPs. However, the average size of these spherical Pd
NPs is 4.7 nm. The particles size distribution of this sam-
ple is shown in Figure S1d in the SI. To know the
The microscopic analyses of the samples were done in a
transmission electron microscope (TEM). Figure 2A shows
the TEM image of sample Ex‐Pd‐1 recorded after 2.0 hr of
reaction. The micrograph shows the presence of nearly uni-
form ultra‐small sized spherical Pd NPs. The average size of
these Pd NPs as measured from the magnified TEM image
(Figure 2B) is 5.1 nm. The particle size distribution of the
sample is shown Figure S1a in the Supporting Information
(SI). The high‐resolution TEM (HRTEM) image of such
spherical Pd NP (Figure 2C) shows the presence of lattice
fringes those are more likely wavelike with an inter‐planner
spacing of 0.23 nm. This d‐spacing value corresponds to the
(111) lattice plane of fcc Pd.[5,30] The SAED pattern
recorded from such samples show bright spots indicating
the formation of crystalline Pd NPs.
To study the effect of concentration of Pd2+ on the
size of Pd NPs, we recorded the TEM images of samples
FIGURE 3 TEM images of Pd NPs recorded from samples (a) Ex‐Pd‐2, (b) Ex‐Pd‐3 and (c) Ex‐Pd‐4. (d) EDX spectrum recorded from
sample Ex‐Pd‐3

6 of 10
PHUKAN ET AL.
composition of the formed Pd NPs, we have recorded the
EDX spectra of sample Ex‐Pd‐3 during TEM analysis. The
spectra (Figure 3D) confirmed the presence of peak due
to elemental Pd indicating the formation of pure Pd NPs
as peak due to any other impurities could not be noticed.
Further to investigate the surface compositions and chem-
ical state of the plant extract derived Pd NPs, X‐ray photo-
electron spectra (XPS) were recorded from sample Ex‐Pd‐1.
Wide survey scans were recorded first followed by a
detailed scanning of the edge of Pd‐3d (Figure 4). The
charging effect on the sample was corrected by setting the
binding energy of the carbon (C‐1 s) at 284.6 eV and this
carbon peak was used as a reference position for scaling
all the other peaks. Figure 4b shows that the peak appears
(Table 1). The diffractograms show the presence of diffrac-
tion peaks at 2θ = 38, 46, and 66° which are characteristic
peaks of Pd as reported earlier.[50] The additional peaks
appeared in the diffractogram might be due to some com-
plex formed between Pd (II) and the organic moiety pres-
ent in the extract as reported earlier.[33]
Further, to confirm the capping of Pd NPs by biomol-
ecules, we recorded the FTIR spectra of plant extract
reduced Pd NPs and the plant extract which are presented
in Figure 5. FTIR spectra of the extract showed IR bands
at 3276, 2933, 2361, 1590, 1399 and 1022 cm−1. The bands
observed at 3276 and 1590 cm−1 correspond to –O‐H and
–C=O stretching modes, respectively. On the other hand,
the bands at 1399 and 1022 cm−1 are due to –C‐O and ‐C‐
O‐C stretching modes, respectively. Whereas the FTIR
spectra of Pd NPs (sample Ex‐Pd‐3) showed the IR bands
at 3399, 1627, 1433 and 1022 cm−1 which are due to –O‐
H, ‐C‐H, –C=O and –C‐O‐C stretching modes, respec-
tively. This observation confirmed that the phytochemi-
cals present in the extract are anchored on the surface
of Pd NPs providing stability to them from aggregation.
To probe the effectiveness of the synthesized Pd NPs
in the Suzuki‐Miyaura reaction, we chose 4‐bromo nitro-
benzene (1 mmol) and phenylboronic acid (1.2 mmol) as
model substrates. For the initial investigation, the model
reaction was performed at room temperature with 2 ml
of an aqueous suspension of catalyst (0.014 mol%; sample
Ex‐Pd‐1), using 1.5 mmol of K2CO3 as a base under aero-
bic and ligand‐free condition. However, 62% (Table 2,
entry 1) of yield was isolated after a span of 12 hr. The
low yield of the product led us to check the effect of co‐
solvent on the reaction because; it has been reported that
the addition of co‐solvent to an aqueous system assist in
the complete dissolution of the organic substrates, and
as a spin‐orbit doublet at 337 eV (3d5/2) and 343.1 eV (3d3/2
)
for Pd0 in the product. The peaks due to C‐1 s, O‐1 s and N‐
1 s in the survey spectrum (Figure 4a) arise due to the pres-
ence of extract on the surface of Pd NPs.
The crystalline properties and phase structures of the
synthesized Pd NPs were studied using powder XRD tech-
nique. Figure S2 in the SI shows the XRD patterns of the
FIGURE 4 X‐ray photoelectron spectra of Pd NPs recorded from
sample Ex‐Pd‐1: (a) full scan, and (b) Pd 3d region
FIGURE 5 FTIR spectra of pure extract and extract reduced Pd
NPs

PHUKAN ET AL.
7 of 10
TABLE 2 Optimization of the reaction condition for catalyst, solvent and basea
Entry
Pd NPs (mL; mol%)
Solvent (vol)
Time (hr)
Yield (%)b
1
2
3
4
5
6
7
8
2.0; 0.014
2.0; 0.014
2.0; 0.014
2.0; 0.014
1.5; 0.0105
1.0; 0.007
0.5; 0.0035
1.0; 0.007
‐
12
3
62
93
93
93
93
93
85
88
MeOH (2 ml)
EtOH (2 ml)
3
i‐PrOH (2 ml)
3
EtOH (2 ml) + Water (0.5 ml)
EtOH (2 ml) + Water (1 ml)
EtOH (2 ml) + Water (1.5 ml)
EtOH (2 ml) + Water (1 ml)
3
3
3
3
aPhenylboronic acid (1.2 mmol), 4‐bromo nitrobenzene (1 mmol). bIsolated yield.
thereby increase the efficiency of the reaction.[4,32,52,53]
Thereby, in the next assessment we checked the effective-
ness of different alcoholic co‐solvents like ethanol,
isopropanol, methanol, etc. (Table 2, entries 2–4). We
were pleased to note that the addition of the alcoholic
co‐solvents dramatically increased the % yield of the reac-
tion in our system up to 93% within 3 hr. All the alcoholic
solvents delivered the same yield; hence we preferred
ethanol as the most apposite solvent for the transforma-
tion (Table 2, entry 3) due to its more environmentally
friendly behavior compared to other alcohols. After this
satisfactory result, we decided to optimize the reaction
condition for the amount of catalyst. For this, we
decreased the amount of aqueous suspension of catalyst
gradually from 2 ml to 1.5 ml, 1 ml, and 0.5 ml, respec-
tively (Table 2, entries 4, 5, 6 and 7). After completion
TABLE 3 Suzuki‐Miyaura cross‐coupling reactions of various aryl halides and aryl boronic acids catalyzed by Pd NPs (sample Ex‐Pd‐1)a
Entry
R1
R2
X
Time
Yield (%)b
TOFc (h−1
)
1
H
4‐NO2
4‐COCH3
4‐OMe
H
Br
Br
Br
I
3 hr
93
88
90
94
94
94
95
92
92
91
85
82
84
82
85
2105.6
913.2
2
H
2 hr
3
H
90 min
40 min
90 min
100 min
35 min
50 min
60 min
60 min
90 min
3 hr
1245.2
2926.4
1300.6
1170.5
3380.0
2291.3
1909.4
1888.6
1176.1
588.0
4
4‐OMe
4‐OMe
4‐OMe
4‐OMe
4‐Me
4‐Me
4‐tertbutyl
3‐OMe
4‐Cl
5
H
Br
Br
Br
Br
Br
Br
Br
I
6
4‐NO2
4‐OMe
H
7
8
9
4‐OMe
4‐NO2
H
10
11
12
13
14
15
H
4‐Cl
4‐COCH3
4‐NO2
4‐OMe
Br
Br
Br
4 hr
435.8
4‐Cl
3 hr
567.2
4‐F
2 hr
882.0
c
aReaction conditions: aryl halide (1 mmol), aryl boronic acid (1.2 mmol), in air. bIsolated yield. Turnover frequency (TOF): moles of aryl halide converted per
mole of Pd per h.

8 of 10
PHUKAN ET AL.
TABLE 4 A comparison between the yields and catalyst loading for different plant extract mediated synthesized Pd NPs and their activity
towards Suzuki‐Miyaura cross coupling reaction
Entry
Catalyst loading
Solvent system
Time
Yield (%)
Reference
[57]
1
2
3
4
5
6
7
0.1 mmol%
2 mol%
5 mg
EtOH‐H2O
H2O
30 min
2 hr
97%
95
[34]
[31]
[50]
[30]
[32]
H2O
2 hr
93
1 mol%
5 wt%
iPrOH‐H2O
iPrOH‐H2O
H2O/60 °C
EtOH‐H2O
3.5 hr
30 min
55 min
3 hr
89
94
0.002 g
95
0.007 mol%
93
This work
and purification, it was noticed that 2 ml, 1.5 ml and 1 ml
of the catalyst resulted in the same conversion of 93%, but
0.5 ml of the catalyst offered 85% yield (Table 2, entries 4,
5, 6 vs. 7). So, this series of experiments concluded that
1 ml of the catalyst suspension (0.007 mol%) is enough
for the desired transformation in 3 hr (Table 2, entry 6).
As K2CO3 is a mild base, we did not feel it necessary to
check the effect of other bases on the reaction but carried
out the reaction in 1 mmol of K2CO3 keeping other
parameters constant (Table 2, entry 8). But the study
yielded a slight decrease in the % yield of the reaction.
Thus, the best condition for the desired cross‐coupling
reaction was with 0.007 mol% of Pd catalyst, 1.5 mmol
of K2CO3 and 4 ml of solvent (1: 1) ethanol: water
(Table 2, entry 6).
With the optimized reaction condition in hand, we
examined the effect of size on the catalytic activity of Pd
NPs. So, when we repeated the coupling reaction with
sample Ex‐Pd‐4, we observed the conversion of 92%
within 1 hr. This confirmed that with smaller sized Pd
NPs, the coupling reaction took place at a faster rate. Fur-
ther, we also examined the scope and limitation of the
reaction using different electron donating and withdraw-
ing group substituted arylhalides and arylboronic acids
respectively as summarized in Table 3. From the sub-
strate study analysis it was observed that the coupling
product from the electron donating group substituted
halides and boronic acids (Table 3, entries 3, 4, 5, 7, 8, 9
and 11) consumed less time and offered excellent yield
compared to the electron withdrawing group substituted
substrates (Table 3, entries 2, 13, 14 and 15). The turn
over frequency for the different reaction catalyzed by Pd
NPs is also presented in Table 3. A comparative result
of the coupling reaction of the most relevant works is pre-
sented in Table 4 which shows that our system is more
effective than some of the other reports.
recyclability, and to check this property, we have per-
formed the reusability experiment with our model system
using 4‐bromo nitrobenzene (1 mmol) and phenylboronic
acid (1.2 mmol) in water: ethanol system (1:1) at room
temperature following the previously optimized condition
(Table 2). Our study revealed (Figure 6) that the Pd
nanocatalysts could be reused at least for four cycles;
however, a gradual decrease in yield was observed which
might be due to a decrease in the activity of the catalyst
due to possible adsorption of the reactants on the surface
of Pd NPs. Another reason might be the loss of Pd NPs
during its recovery for successive uses. To understand
the fate of the used catalyst and to check the stability,
we isolated the Pd NPs after its use in the catalysis reac-
tion and analyzed by both TEM and SEM. The TEM
image (Figure 7) of the reused catalyst showed almost
similar morphology with no sign of aggregations suggest-
ing that the catalyst are highly stable and could be reused
for further runs. However, comparison of the SEM
images (Figure S3 in the SI) suggests that there are some
It is important to note that one of the key advantages
of Pd NPs based catalytic system is their potential
FIGURE 6 Catalytic performance of recovered Pd NPs in Suzuki‐
Miyaura reaction

PHUKAN ET AL.
9 of 10
ACKNOWLEDGMENTS
MHR acknowledges Council of Scientific and Industrial
Research (Grant No. 01(2910)/17/EMR‐II), India for
supporting the research. Thanks are also due to partial
support from Science and Engineering Research Board
(Grant No. EMR/2015/001912), India. We also thank
SAIF NEHU, NEIST Jorhat and SAIF GU for extending
their instrumental facilities to us.
ORCID
Md. Harunar Rashid
https://orcid.org/0000-0001-7470-
8384
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How to cite this article: Phukan S, Mahanta A,
Kakati D, Rashid MH. Green chemical synthesis of
Pd nanoparticles for use as efficient catalyst in
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