Acacia concinna pods: a natural and new bioreductant for palladium nanoparticles and its application to Suzuki–Miyaura coupling

Article abstract of DOI:10.1007/s13738-019-01682-7

Abstract: In this study, efficient, simple and greener approach is applied for the synthesis of palladium nanoparticles using aqueous extract of Acacia concinna, i.e., soap pods. The extract is mainly composed of saponin terpenoid which contains hydroxyl groups in its structure. This performs a versatile role as surfactant, reducing and stabilizing agent for palladium nanoparticles. The average particle size is found about 20?nm by using 30% extract (v/v) of A. concinna (soap-pod tree). Diverse techniques are used to characterize synthesized nanoparticles such as UV–visible, TEM and XPS analysis. The synthesized nanoparticles showed excellent activity in Suzuki–Miyaura coupling. The synthesized Pd NPs can be effectively reused for at least five times with excellent yield of product. Results confirmed this protocol as eco-benign, environmental-friendly, nontoxic and alternative to conventional other chemical methods. Graphical abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s13738-019-01682-7

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-019-01682-7
ORIGINAL PAPER
Acacia concinna pods: a natural and new bioreductant for palladium
nanoparticles and its application to Suzuki–Miyaura coupling
D. S. Gaikwad1 · K. A. Undale1 · R. A. Kalel1 · D. B. Patil1
Received: 26 September 2018 / Accepted: 22 April 2019
© Iranian Chemical Society 2019
Abstract
In this study, efcient, simple and greener approach is applied for the synthesis of palladium nanoparticles using aqueous
extract of Acacia concinna, i.e., soap pods. The extract is mainly composed of saponin terpenoid which contains hydroxyl
groups in its structure. This performs a versatile role as surfactant, reducing and stabilizing agent for palladium nanopar-
ticles. The average particle size is found about 20 nm by using 30% extract (v/v) of A. concinna (soap-pod tree). Diverse
techniques are used to characterize synthesized nanoparticles such as UV–visible, TEM and XPS analysis. The synthesized
nanoparticles showed excellent activity in Suzuki–Miyaura coupling. The synthesized Pd NPs can be efectively reused for
at least fve times with excellent yield of product. Results confrmed this protocol as eco-benign, environmental-friendly,
nontoxic and alternative to conventional other chemical methods.
Graphical abstract
Keywords Acacia concina · Bioreductant · Palladium nanoparticles · TEM · XPS
Introduction
The twelve principals of green chemistry suggests for
using solvents which will not be harmful to all kinds of
living things. So, according to the principal and along
* D. S. Gaikwad
with environmental and economical point of view, water
can be considered as green and efcient solvent for carry-
ing out the organic reactions [1–3]. Some of the research
groups and review articles that have focused the unique
dgchemistry@gmail.com
1
Department of Chemistry, Vivekanand College, Kolhapur,
Maharashtra 416003, India
Vol.:(0123456789)
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H
reactivity and selectivity of organic reactions which cannot
be achieved in organic solvents are often obtained in water
[4–6]. In addition to this, several important and useful
reactions have been exploited using water as the solvent.
But sometimes, the development of methods in water is
strongly limited by (1) the low solubility of organic sub-
strates in water and (2) the sensitivity of chemical groups
toward hydrolytic degradations. One of the solutions to
circumvent these issues comprises the addition of a phase
transfer agent (surfactants), or to use salting-in agents
[7–9]. On commercial scale, many such synthetic cationic,
anionic and nonionic surfactants are available for carrying
out the reactions in aqueous medium. Some of the sur-
factants are biodegradable in nature, and hence, there is no
harm using it in organic transformation [10–12].
H
OH
HO
OH
O
N
H
HO
HO
H
O
H
H
O
O
OH
O
HO
OH
OH
Fig. 1 Structure of saponin
in, we wish to report the role of extract of Acacia concinna
in the synthesis of Palladium nanoparticles.
In nature, there are several such surfactants available
which can be used for carrying out the organic reactions
[13, 14]. One of the best nonionic, natural surfactant, sap-
onin, is emerging as an efective surfactant. The aqueous
extract of Acacia concinna, i.e., soap pods mainly com-
posed of terpenoid saponin, and they are generally biode-
gradable and considered in manufacturing of detergents,
cosmetics, and many other products [15, 16]. The outer
pericarp of A. concinna pods has been traditionally used
in India for bathing purpose due to the formation of good
lather or foam in water. It has an excellent cleansing action
on dirt like detergents or soaps [17]. In addition to this
they have some medicinal properties such as anti-infam-
matory and antimicrobial activities [18, 19]. However,
there has not been much scientifc study to understand its
surfactant activities and micellar behavior.
Synthesis of metal nanoparticles using plant extracts is
very cost-efective and therefore can be used as an economic
and valuable alternative for the large-scale production of
MNPs. Extracts from plants may act both as reducing and
capping agents in nanoparticle synthesis. The major part of
the soap-pod extract consists of saponin; it has found that it
contains mainly two functionalities, i.e., hydroxyl and ether
[30, 31] and that could acts as reducing and stabilizing agent
for MNPs. As per our earlier concept with nonionic sur-
factant Triton × 100 [32], hydroxyl functional groups can
act as a reducing and stabilizing agents in the synthesis of
palladium nanoparticles and so started experiment on Pd
NPs synthesis by using plant extract.
Bandgar and his group [16] showed the use of aqueous
extract of soap pods in the synthesis of some coumarin
derivatives. Adding up to this, saponin extract has been used
for the synthesis of metal nanoparticles (MNPs). Kim et al.
used the aqueous extract of Sapindus mukorossi for the syn-
thesis of gold nanoparticles illustrating the efectiveness of
the saponins [20]. Balakrishnan et al. [21] studied the micel-
lar characteristics of a nonionic, natural surfactant, saponin,
and it was found to be 0.045 wt% UV–visible study. But
till date, micellar behavior of these surfactants is not much
studied. Looking toward these applications and its efective-
ness, we decided to fnd out its use in the synthesis of Pd
NPs and Suzuki–Miyaura coupling. The extract may help
in solubilizing the organic molecules due to good micellar
properties (Fig. 1).
Materials and methods
Materials
The pods of Acacia concinna obtained from the crop felds
of West Maharashtra, India. Pd(OAc)2 was purchased from
Spectrochem India.
Method
The outer pericarp of the fruits was separated from the seeds
and dried at room temperature for 2 days. About 30 g of
it was then soaked in 100 mL of water for 24 h, and the
solution was fltered through Whatman flter paper No. 1.
The fltrate was used as it is for further study without any
prier treatment or purifcation. To synthesize the Pd NPs,
Pd(OAc)2 was used as precursor and 10 mL fltrate of 30%
w/v as aqueous extract. It was stirred at room temperature
for 8 h. The bioreduction of the Pd(OAc)2 was confrmed by
the slow color change from yellow to black after 8 h (Fig. 2).
Numerous various solid materials such as chitosan, cellu-
lose, starch and bio-macromolecule Schif-based supported
homogenous or heterogeneous palladium catalysts or palla-
dium nanoparticles have been designed for Suzuki reactions
[22–28]. They constitute an important class of catalysts. But
in continuation with our earlier study on efect of surfactant
[29] and role in the synthesis of metal nanoparticles, here
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Result and discussion
After the initial success in the synthesis of Pd NPs, we
next focused attention toward exploring its efectiveness
in Suzuki–Miyaura coupling. The reaction carried out by
two ways, i.e., by separating the Pd NPs and to carry out
the reaction in aqueous conditions, another way to gener-
ate the Pd NPs in situ and to continue Suzuki–Miyaura
coupling (Scheme 1). First, NPs were separated from the
aqueous extract and used in a model reaction with iodo-
benzene and phenyl boronic acid by employing diferent
bases such as NaOH, KOH, K3PO4, K2CO3, Na2CO3 and
TEA (Table 1, Entry 1–7) at room temperature. Among
the bases used, Na2CO3 was found to be the best choice
for the present conditions giving 98% of the desired biaryl
and with rest of the bases obtained inferior results. Reac-
tion did not much work well under the absence of basic
conditions. Another procedure, i.e., in situ generation of
Pd NPs and continuing the Suzuki coupling, also gave the
same yield by utilizing Na2CO3 as a base. Product was
separated from reaction mixture by extracting with it by
ethyl acetate, and the catalyst was recovered by centrifu-
gation of aqueous layer. Hence, we followed the second
procedure, i.e., in situ generation of NPs and to carry out
the reaction in aqueous extract. The product obtained was
Fig. 2 Visual eye detection of Pd NPs
1
characterized by H and 13C NMR spectroscopy; it con-
frms the formation of desired product.
Comparison with diferently synthesized Pd NPs by
diverse methods is shown in Table 2; it highlights qualities
as well as demerits of the earlier literature methods. How-
ever, it is noteworthy that the in situ generated Pd-NPs
using Acacia concinna pods extract in the present method
has a comparatively similar/excellent activity compared
with other known Pd NPs.
Fig. 3 UV study of reaction mixture
After screening the base and method of reaction, we
tried diferent substituted aryl halides and various boronic
acids using synthesized Pd NPs. All the results are men-
tioned in Table 3.
Characterization of Pd NPs
A variety of functional groups can be tolerated for aryl
halides including methyl, nitro, cyano, methoxy as well
as aryl boronic acids. The results conclude that electronic
character of aryl halides or boronic acids has no obvious
efect on the Suzuki–Miyaura coupling under optimized
reaction conditions. It is noteworthy that less activated
aryl chlorides aforded corresponding product in excellent
yield (Table 3, Entry 3b).
The UV–visible measurements were carried out to iden-
tify the formation of Pd NPs under same reaction con-
ditions. Figure 3 shows the UV–visible spectra of the
aqueous extract, Pd(OAc)2 and reaction mixture. It con-
firms the disappearance of peak of Pd(OAc)2 from the
reaction mixture and formation of Pd NPs. The NPs was
separated from reaction mixture by centrifugation method
and was analyzed by TEM analysis. The average size of
the nanoparticles found to be 20 nm (Fig. 4). Again, the
formation of NPs was further confirmed by XPS analysis
(Fig. 5); the analysis found to correct one. High content
of saponins present in aqueous extract of soap pod has a
hydroxyl functionality which is responsible for reduction
in Pd(OAc)2.
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Fig. 4 TEM images of Pd nanoparticles with SEAD pattern
OH
B
R'
X
Pd NPs
HO
+
R
R
R'
Na2CO3
Soap-pod
Extract
3
1
2
Scheme 1 Typical Suzuki–Miyaura coupling
Reusability study
Reusability of catalyst is imperative aspect of green chem-
istry. Hence, the reusability of Pd NPs was investigated for
model reaction iodobenzene (1 mmol), phenyl boronic acid
(1.1 mmol), Na2CO3 (3 mmol), Pd(OAc)2 (2 mol%) and
10 mL plant extract at room temperature. After completion
of reaction on TLC, product was isolated by extraction
with diethyl ether and Pd NPs were separated from aque-
ous by centrifugation method. Then, for next subsequent
run, fresh reactants were taken, the separated Pd NPs was
Fig. 5 XPS analysis of Pd NPs
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Journal of the Iranian Chemical Society
Table 1 Efect of diferent bases on the Suzuki–Miyaura coupling
respectively (Fig. 6). After reusability study, the Pd NPs
are analyzed by TEM analysis (Fig. 7), and no any signif-
cant change in the structure is observed.
reaction
Entry
Base (mmol)
Time (h)
Yield (%)*
1
2
3
4
5
6
7
8
NaOH (3)
KOH (3)
K3PO4 (3)
K2CO3 (3)
Na2CO3 (3)
Na2CO3 (2)
TEA (3)
3
3
5
2
1
3
3
12
60
62
40
55
98
80
70
–
Experimental
General
NMR spectra were recorded on a BrukerAC-300 spectrom-
eter in CDCl3 using tetramethylsilane as internal standard.
Without base
Typical procedure for Suzuki–Miyaura coupling
Reaction conditions: iodobenzene (1 mmol), phenyl boronic acid
(1.2 mmol), Pd NPs (10 mg by wt), Na2CO3 (3 mmol) and soap-pod
extract (10 mL 30%V/V) at r.t.
To a 25-mL round bottom fask, a mixture of aryl halide
(1 mmol), arylboronic acid (1.2 mmol), Pd(OAc)2 (2 mol%)
and Na2CO3 (3 mmol) was added in soap-pod extract
(10 mL). The mixture was stirred at room temperature
and monitored by TLC. After completion of reaction, the
mixture was extracted with diethyl ether (3×10 mL). The
*Isolated yield
added into that, and reaction continued till completion of
it. It was found that NPs can be efectively reused for at
least fve times by giving 98, 94, 92, 89 and 85% yield,
Table 2 Comparison with other synthesized Pd NPs
Entry Catalyst
Conditions
Time (h) % Yield* [Refs.]
1
2
Pd-NPs
Pd NPs (10 mg by wt), Na2CO3 (3 mmol), soap-pod extract (10 mL,
30%V/V) at r.t.
1
1
98 [This work]
98 [33]
Pd-NPs in Ionic liquid
Pd(OAc)2 (2 mol%), TSIL (20 mol%), Na2CO3 (1 mmol), Water (5 mL),
80 oC
3
4
5
Pd-NPs using surfactant
Pd-NPs in PEG
Pd-NPs in water
PdCl2 (0.015 mmol) TBAF (1.5 mmol) at 40–100 °C argon atmosphere
Pd(OAc)2 (2 mol%) K2CO3 (1 mmol), PEG (4 g), 45 °C
0.5-12
1-5
4
90 [34]
92 [35]
96 [36]
PdCl2 (1.2 mol%), IL (1.0 ml), Na2CO3 (1.05 mmol), water (0.5 ml),
110 °C
6
Pd-NPs in CH3OH:CH3CN (1:1) K2CO3 (2 mmol), CH3OH:CH3CN (1:1) at r.t.
2-6
99 [37]
*Isolated yield
Table 3 Combinatorial library
of biaryls synthesized by
Suzuki–Miyaura coupling
Entry
Aryl halide (1)
Boronic acid (2)
Time (h)
Yield (%)*
TON
TOF
3a
3b
3c
3d
3e
3f
3 g
3 h
3i
R=H, X=I
R=H, X=Cl
R=H, X=Br
R′=H
1
2
1
1
1.5
1.5
1
1.5
2
1.5
1
2
98
80
95
92
89
88
85
84
87
90
87
80
48
40
47.5
46
44.5
44
42.5
42
43.5
45
43.5
40
48
20
R′=4-OMe
R′=4-Me
R′=H
R′=4-OMe
R′=H
R′=4-OMe
R′=H
R′=H
47.5
46
29.66
29.33
42.5
28
21.75
30
43.5
20
R=4-Me, X=Br
R=4-Me, X=I
R=4-OMe, X=I
R=4-OMe, X=I
R=4-CN, X=Br
R=4-NO2, X=I
R=4-COMe, X=Br
R=4-OMe, X=Br
1-Naphthyl iodie
3j
3 k
3 l
R′=H
R′=4-Me
R′=H
Reaction conditions: aryl halide (1 mmol), boronic acid (1.2 mmol), Pd(OAc)2 (2 mol%), Na2CO3
(3 mmol) and soap-pod shell extract (10 mL 30%V/V) at r.t.
*Isolated yield
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Journal of the Iranian Chemical Society
environmentally friendly and high yielding with all kinds
of aryl halide and aryl boronic acids.
Acknowledgement One of authors DSG gratefully acknowledges
Department of Science and Technology, New Delhi, India, for fnancial
assistance under Start-up Research Grant [No. SB/FT/CS-145/2014].
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