Green synthesis and characterization of silver nanoparticles (Ag NPs) from extract of plant Radix Puerariae: An efficient and recyclable catalyst for the construction of pyrimido[1,2-b]indazole derivatives under solvent-free conditions

Article abstract of DOI:10.1016/j.catcom.2017.06.006

We have developed a green and environmentally friendly approach for the synthesis of silver nanoparticles (Ag NPs) using Radix Puerariae plant extract via a novel chemical route. The prepared Ag NPs has been characterized by TEM, XRD, SEM, DLS, EDX, and UV–Vis. Further, catalytic application of this fascinating nanomaterial has been utilized in the synthesis of biologically important pyrimido[1,2-b]indazole derivatives via one-pot three-component coupling reaction between aldehydes, alkynes and amines with excellent yields under solvent-free conditions. After completion of the catalytic reaction, the Ag NPs could be easily recovered and reused for four times without significant loss in its catalytic activity.

Full text of DOI:10.1016/j.catcom.2017.06.006

Catalysis Communications 99 (2017) 121–126
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Green synthesis and characterization of silver nanoparticles (Ag NPs) from
extract of plant Radix Puerariae: An efficient and recyclable catalyst for the
construction of pyrimido[1,2-b]indazole derivatives under solvent-free
conditions
a
a
b
b
Sandip Gangadhar Balwe , Vijay Vilas Shinde , Ashish A. Rokade , Seong Soo Park ,
Yeon Tae Jeong
a
a,⁎
Department of Image Science and Engineering, Pukyong National University, Busan 608-737, Republic of Korea
Department of Industrial Chemistry, Pukyong National University, Busan 608-739, Republic of Korea
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
We have developed a green and environmentally friendly approach for the synthesis of silver nanoparticles (Ag
NPs) using Radix Puerariae plant extract via a novel chemical route. The prepared Ag NPs has been characterized
by TEM, XRD, SEM, DLS, EDX, and UV–Vis. Further, catalytic application of this fascinating nanomaterial has
been utilized in the synthesis of biologically important pyrimido[1,2-b]indazole derivatives via one-pot three-
component coupling reaction between aldehydes, alkynes and amines with excellent yields under solvent-free
conditions. After completion of the catalytic reaction, the Ag NPs could be easily recovered and reused for four
times without significant loss in its catalytic activity.
Silver nanoparticles (Ag NPs)
Pyrimido[1,2-b]indazole
Solvent-free
One-pot synthesis
1. Introduction
reactions [8]. In this direction, synthetic communities paying more
attention to the development of new, efficient and environmentally
In the past few years, the preparation and application of noble metal
benign alternatives for traditionally environmentally unfriendly pro-
cesses. Now-a-days researchers are focusing on the green and shape
controlled synthesis of Ag nanoparticles for various practical applica-
tions and to minimize the effect of hazardous chemicals. Therefore, the
creation of nanoparticles using green synthesis approaches has
achieved significant research interests due to their relative simplicity,
environmental sustainability, cost-effectiveness and reproducibility
compared to physical and chemical based methods [9]. In recent years,
the natural polymers like starch, leaf extracts, root extracts, fruit ex-
tracts [10] etc. have been extensively used for the green nanoparticles
synthesis as plants are widely available, safe to handle and possess a
variety of metabolites that function as reducing agents in nanoparticle
synthesis [11]. The Radix Puerariae has been used historically for the
treatment of diarrhea, acute dysentery, and cardiovascular disease in
China, Japan, Korea [12]. Several studies have been carried out to
identify the chemical constituents of Radix Puerariae which confirms
that polyphenolic compounds, especially isoflavones are in abundant
content and responsible for the reduction [13]. A motive to exploit the
unique catalytic properties of Ag nanoparticles led us to flourish a new
method to synthesize green Ag nanoparticles using the dried root of
nanoparticles in catalysis have attracted emerging attention from the
scientific community. Owing to their fabulous properties and diversity
of applications [1]. As research in this area moves forward, scientists
are discovering novel application possibilities by varying size, shape,
composition, or local environment presents them with unusual cap-
abilities [2]. By manipulating the chemical composition of the materials
at the nanoscale, their electrical, chemical, optical, and other properties
can be manipulated precisely. Importantly, among the metal-based
nanoparticles, silver nanoparticles catalysis have been of great interest
in organic synthesis and has expanded promptly in the past few years
because of nanosilver catalysts unique reactivity, selectivity, and sta-
bility, as well as recyclability in catalytic reactions [3–5]. Therefore,
silver nanoparticles have become the focus of intensive research due to
their catalytic properties for some important organic reactions such as
3
multi component (A -coupling) reactions [6], Diels − Alder
[
4 + 2]Cycloaddition reactions [7] etc. One of the main important aims
of the green chemistry is environmental protection. Green chemistry
emphasizes the deployment of a set of principles that reduce or elim-
inate the use or generation of hazardous substances in chemical
⁎
Corresponding author.
E-mail address: ytjeong@pknu.ac.kr (Y.T. Jeong).
http://dx.doi.org/10.1016/j.catcom.2017.06.006
Received 4 May 2017; Received in revised form 30 May 2017; Accepted 10 June 2017
Available online 12 June 2017
1566-7367/ © 2017 Elsevier B.V. All rights reserved.

S.G. Balwe et al.
Catalysis Communications 99 (2017) 121–126
Fig. 3. The EDX spectrum of Ag NPs synthesized with Radix Puerariae extract.
Fig. 1. UV–Vis spectra of Ag NPs synthesized with Radix Puerariae extract.
plant Radix Puerariae by the simple sonication method. And its catalytic
3
activity has been tested for one-pot multi-component (A -coupling)
reactions for the synthesis of biologically important pyrimido[1,2-b]
indazoles under solvent-free conditions.
Pyrimido[1,2-b]indazole which incorporate both indazole and pyr-
imidine moieties, have aroused considerable interests in the fields of
medicinal chemistry and material science [14]. For instance, some
pyrimido[1,2-b]indazole considered to be active as protein kinase in-
hibitors [15], cancer cell proliferative disorders [16], Alzheimer's dis-
eases [17], viral infections [18], auto-immune diseases, and neurode-
generative disorders. These examples emphasize the vital importance of
pyrimido[1,2-b]indazole as key pharmacophores in bioactive mole-
cules. Although these fused indazole and pyrimidine moieties are im-
portant, efficient synthetic approaches to substituted pyrimido[1,2-b]
indazole have rarely been documented. However, these methodologies
suffer from some shortcomings due to their tedious procedures [19],
restricted efficacy [20], uses of corrosive and toxic reagents [21], high
reaction temperature, longer reaction time, non-compatible solvents
and low yields. Therefore, developing convenient and efficient ap-
proaches to a diverse range of substituted pyrimido[1,2-b]indazoles
from simple and readily available starting materials are highly desir-
able.
Fig. 4. The XRD spectrum of Ag NPs synthesized with Radix Puerariae extract.
amine, acetylene and various aldehydes under solvent-free conditions
(Table 2).
2. Experimental
2.1. Synthesis and characterization of catalyst
2.1.1. Preparation of Radix Puerariae extract
1 g of Radix Puerariae powder was dissolved in 100 mL of water and
boiled at 100 °C for 2 h. After cooling the mixture at room temperature,
the mixture was centrifuged at 3000 rpm for 15 min followed by fil-
tering the mixture. The filtrate was used as Radix Puerariae extract for
the synthesis of Ag nanoparticles.
In continuation of our studies for the development of new and ef-
ficient methods for heterocyclic synthesis [22] and nanocatalysts pre-
paration, we here report an efficient Ag NPs catalyzed three component
synthesis of pyrimido[1,2-b]indazole, by the coupling of reaction of
Fig. 2. TEM images of Ag NPs synthesized with Radix
Puerariae extract. (a) With magnification 100 nm and (b)
with 50 nm.
122

S.G. Balwe et al.
Catalysis Communications 99 (2017) 121–126
Table 1
a
Optimization of the reaction conditions for the synthesis of compound 4a .
Entry
Catalyst (mol%)
Solvent
Time (h)
Temp (°C)
Yieldb (%)
1
2
3
4
5
6
7
Ag NPs (0.5)
Ag NPs (0.5)
Ag NPs (0.5)
Ag NPs (0.5)
Ag NPs (0.5)
Ag NPs (0.5)
Ag NPs (0.5)
Ag NPs (0.5)
Ag NPs [1]
Acetonitrile
Toluene
Ethanol
THF
Dioxane
6
8
6
6
8
6
8
1
1
1
1
1
3
6
6
6
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
80
80
80
80
110
74
67
69
58
56
44
63
Water
DMF
c
8
9
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
96,91,89,85
95
96
78
94
95
24
48
61
1
1
1
1
1
1
1
0
1
2
3
4
5
6
Ag NPs [5]
Ag NPs (0.1)
Ag NPs (0.5)
Ag NPs (0.5)
CuI [10]
80
80
80
80
CuSO
4
. 5H
2
O [10]
Cu(OTf)
2
(10)
a
b
c
Reaction conditions: 3-aminoindazoles (1, 1 mmol), 2-methoxybenzaldehyde (2, 1 mmol) and ethynylbenzene (3, 1 mmol).
Isolated yield.
Catalyst was reused three times.
2
.1.2. Synthesis of Ag nanoparticles
In a typical synthesis, 5 mL of 20 mM AgNO was mixed with 2 mL
3
be due to decrease in size of Ag NPs. Similarly the absorption intensities
were also increased as the time increased, this may be attributed to
increase in concentration of Ag NPs with time.
of Radix Puerariae extract in a glass vial. The pH of the reaction mixture
was adjusted to 9 by adding sufficient quantity of 100 mM NaOH so-
lution. Then the vial was kept in the sonication bath for 9 h. The
synthesis of Ag nanoparticles was monitored by UV–Vis spectroscopy at
regular time intervals. Reaction was stopped after 9 h. and the product
was centrifuged at 12500 rpm for 20 min, (three times) to remove the
Radix Puerariae extract from Ag nanoparticles.
2.2.2. TEM analysis
The morphological studies of Ag NPs were performed on Tunneling
Electron Microscopy instrument operating at 120 kV. The TEM samples
were prepared by freshly prepared Ag NPs washed three times with
distilled water and centrifuged at 13500 rpm for 20 min. The Ag NPs
were prepared by dropping a small amount of Ag NP solution on copper
grid a drying at room temperature. The morphology of Ag NPs syn-
thesized with Radix Puerariae extract was investigated by TEM. Fig. 2
shows the TEM image of Ag NPs synthesized with different magnifi-
cation (a) 100 nm and (b) 50 nm. The TEM image demonstrates
monodisperse Ag NPs with spherical and oval shape, with average
particle size ranging from 10 to 35 nm. Monodispersity might be due to
capping of Radix Puerariae extract and sonication effect.
2
2
.2. Characterization techniques
.2.1. UV–Vis spectroscopy
The synthesis of Ag NPs by using Radix Puerariae extract was
monitored by UV–Vis spectroscopy. For the UV–Vis analysis the sample
was taken out after regular time intervals from the reaction solution
exactly 200 μL and diluted up to 2 mL. The UV–Vis spectroscopy ana-
lysis was carried in range 300–900 nm.
UV–Vis spectroscopy is an essential analytical technique for de-
termining the AgNP formation, which is related to LSPR of Ag NPs size,
shape and dielectric medium surrounding the Ag NPs. The UV–Vis
spectra were analyzed at different time intervals during the synthesis
are shown in Fig. 1. The spectra of Radix Puerariae extract do not show
any peak in the range 300–900 nm. When, NaOH was added to the
Radix Puerariae extract, a peak at 350 nm was appeared. After, 30 min
the characteristic peak of Ag nanoparticles at 430 nm was appeared.
The peak at 350 nm was shifted to 370 nm, and disappeared as the
reaction progresses. The absorption peak is observed around
2.2.3. EDX analysis
The morphology studies and EDX of Ag NPs synthesized with Radix
Puerariae extract were analyzed on the instrument (JAM-6700F Jeol
Ltd.). The SEM samples of Ag NPs were prepared by dropping a small
amount of Centrifuged Ag NPs solution on a silicon wafer and drying it
at room temperature. The EDX analysis of Ag NPs synthesized with
Radix Puerariae extract is shown in Fig. 3. The EDX spectrum shows a
strong peak at 3 keV, corresponding to Ag as, reported by Rokade et al.
[11] The peak for the silicon is due to the substrate used for sample
preparation. The EDX spectrum confirms the formation of Ag NPs
synthesized with Radix Puerariae extract.
400–430 nm, indicating the formation of Ag NPs. The reaction peaks
shifted towards the shorter wavelength with increasing time, this may
123

S.G. Balwe et al.
Catalysis Communications 99 (2017) 121–126
Table 2
a
Synthesis of pyrimido[1,2-b]indazole derivatives .
Entry
R
1
R
2
Product
Time (h)
Yield (%)b
M.p (°C)
1
2
3
4
5
6
7
8
9
2-OCH
3-OCH
4-OCH
3
3
3
C
C
C
6
6
6
H
H
H
4
4
4
H
H
H
H
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
4p
4q
4r
1
1
1
1
1
1
1
1
1
1
1
1
1.2
1.2
1.2
1
1
1
96
93
92
95
90
91
90
89
92
88
89
87
92
91
90
89
88
87
88
89
90
88
92
93
88
142–144
138–140
202–204
147–149
130–132
164–166
158–160
185–187
142–144
152–154
157–159
191–193
205–207
193–195
182–184
172–174
172–174
196–198
177–179
210–212
168–170
181–183
246–248
240–242
198–200
4-OC
2
H
5
C
6
H
4
4-C(CH
3
)
2
C
6
H
4
4
H
2-OCH
3-OCH
4-OCH
3
C
C
C
6
6
6
2
H
H
H
4
4-CH
4-CH
4-CH
4-CH
H
4-CH
H
H
4-CH
H
H
4-CH
H
H
H
H
H
H
H
4-CH
3
3
3
3
3
3
4
4
4-C(CH
2,5-CH
2,5-CH
3
)
C
6
H
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
3
C
C
6
6
H
H
3
3
3
3
3
3
2-Cl-6-FC
6
H
3
4-ClC
4-ClC
6
H
H
4
6
4
4-FC
3-ClC
4-BrC
6
H
4
6
H
4
6
H
4
3-FC
2-FC
6
H
H
4
6
4
4s
4t
1
1
1
1
1
1
1.2
4-BrC
2-ClC
3-BrC
6
H
H
H
4
6
4
4u
4v
4w
4x
4y
6
4
4-CNC
4-CNC
6
H
H
4
6
4
3
26
27
28
H
H
H
4z
1.2
1.1
1.2
89
86
84
188–190
202–204
184–186
4ab
4ac
2
9
H
4ad
1.2
76
178–180
3
3
0
1
Benzaldehyde
3,4,5-OCH
H
H
4ae
4af
1
1
92
89
148–150
230–232
3
C
6
H
2
The structures of the products synthesized in the current study were determined using 1HNMR, and 13C NMR spectroscopies, and HRMS analysis.
a
Reaction conditions: 3-aminoindazoles (1, 1 mmol), 2-methoxy-benzaldehyde (2, 1 mmol), (3, 1 mmol) and ethynylbenzene(3, 1 mmol), Ag NPs (0.5 mol%), solvent-free, 80 °C.
Isolated yields.
b
2.2.4. X-ray diffraction analysis
2.2.5. Dynamic Light Scattering (DLS) and zeta potential
The Crystalline phase of Ag NPs synthesized with Radix Puerariae
The hydrodynamic diameter and stability of Ag NPs was analyzed
on the DLS. The Ag NPs synthesized with Radix Puerariae extract were
filtered through a membrane filter (0.2 μm) in order to remove any dust
particle. Dynamic Light Scattering analysis of Ag NPs synthesized with
Radix Puerariae extract is shown in (Supporting information) Fig. 5 (a)
From DLS analysis the average particle size of Ag NPs was determined
to be 25 nm, which is in accordance with TEM results. (b) shows the
Zeta potential values of Ag NPs synthesized with Radix Puerariae ex-
tract. The Zeta potential analysis determines the stability of Ag NPs, the
zeta potential values higher than −30 mV and less than +30 mV are
considered to be stable Ag NPs. The zeta potential values of Ag NPs
extract was analyzed by X-ray diffraction. The sample of Ag NPs syn-
thesized with Radix Puerariae extract was prepared by drying Ag NPs
overnight at room temperature. The XRD pattern of Ag NPs synthesized
with Radix Puerariae extract is shown in Fig. 4. To confirm the UV–Vis
analysis and EDX analysis XRD study was done. The XRD pattern con-
tains clear peaks at 38.2 , 44.20 , 64.36 and 77.30o which can be in-
dexed to 〈111〉, 〈200〉, 〈220〉, and 〈311〉 respectively con-
firming crystalline Ag NPs.
o
o
o
124

S.G. Balwe et al.
Catalysis Communications 99 (2017) 121–126
synthesized with Radix Puerariae extract were −31.9 mV, which in-
2.4. Gram scale preparation of the compound (4a)
dicate the Ag NPs are highly stable.
2.5. Recycling efficiency of Ag NPs catalyst
2
.3. Catalytic activity of Ag NPs for ‘one-pot’ A3 coupling reaction
.Finally, we investigated the recovery and reusability of the catalytic
system in the model reaction. After completion of the reaction, the
reaction mixture was cooled to room temperature and ethyl acetate was
added. The reaction mixture was centrifuged at 10000 rpm for 25 min.
Then separated Ag NPs was recovered and washed with triple distilled
water for several times and centrifuged at 13500 rpm for 30 min. (three
times). It was found that the catalytic system could be reused up to
three consecutive runs. Furthermore, no significant change was ob-
served in textural properties as is clear in TEM of the recycled catalyst.
Multicomponent reactions provide a very versatile and efficient
method to construct desired molecules. The effect of Ag NPs on their
3
catalytic activity for the A -coupling reaction was investigated using 3-
aminoindazoles (1, 1 mmol), 2-methoxybenzaldehyde (2, 1 mmol) and
ethynylbenzene (3, 1 mmol). Initially, we examined the effect of var-
ious solvents on the model reaction with 0.5 mol% of Ag NPs catalyst
under reflux condition. Among all the solvents tested ACN, toluene,
2
EtOH, THF, dioxane, H O, and DMF, it was found that the rate of re-
(
Fig. 6 in Supporting information). In addition, no detectable ag-
actions was slower and resulted in moderate yields of 4a in even after
prolonged reaction times (Table 1, entries 1–7). To our delight, when
the reaction was carried out under solvent-free condition, furnished the
desired product 4a in 96% yield at 80 °C in 1 h. (Table 1, entry 8).
Further increment in the catalyst loading was not found to be effective
gregation was observed in the recovered Ag NPs. However, lower yield
was found with recycled Ag NPs, it may be due to handling loss during
work-up in subsequent cycles.
3. Conclusion
(
Table 1 entries, 9 and 10). Similarly, increasing the temperature and
time of the reaction did not show improvement in the yield (Table 1,
entries 12 and 13). To confirm the catalytic activity of the Ag NPs
catalyst, a comparison study was conducted with various copper cata-
lysts under solvent-free conditions at 80 °C and observed low catalytic
properties (Table 1, entries 14–16). From all these establishments we
were satisfied to find that the reaction proceeded smoothly and almost
complete conversion of reactants was observed to afford the desired
product 4a in 96% yield using Ag NPs under solvent-free conditions.
Having established the optimized reaction conditions, the scope and
generality of this protocol were explored and representative results are
summarized in Table 2. To our delight, we found this transformation to
be very general for a wide range of aldehydes which provided easy
access to substituted pyrimido[1,2-b]indazole derivatives (4a-4af). It
was found that electronic effect of the substrate had no significant
impact on the overall yields of the products. For example, aromatic
aldehydes carrying electron-donating and withdrawing substituents,
could react efficiently to give the corresponding products without sig-
nificant difference. Moreover, when the aromatic ring was replaced by
a hindered naphthyl group, the desired product was obtained in 84%
yield. Subsequently, the heteroaromatic thiophene-2-carboxaldehyde
and furan-2- carboxaldehyde also well tolerated. We were delighted to
find that the aliphatic aldehydes such as cyclohexanecarbaldehyde, also
afforded the desired product with good yields. We also employed two
different alkyne substrates, such as 1-ethynylbenzene and 1-ethynyl-4-
methylbenzene, produced pyrimido[1,2-b]indazole in good to excellent
yields. However, aliphatic alkynes such as n-hexyne and n-pentyne, did
not afford the desired products. Therefore, the present protocol has
general applicability, accommodating a variety of substitution patterns.
In conclusion, we have developed a simple and green approach
towards the development of a new method for the synthesis of silver
nanoparticles (Ag NPs). The resulting Ag NPs showed enhanced cata-
lytic activity for the synthesis of biologically important pyrimido[1,2-b]
indazole via one-pot, three-component reaction. The promising points
of this methodology are simplicity in the catalyst preparation from in-
expensive materials, an easy work-up procedure for reactions, shorter
reaction times, higher reaction rates, reusability of catalyst and the
potential for a variety of desirable products being synthesized.
Appendix A. Supplementary data
All Compounds NMR spectra were provided as Supplementary data.
Supplementary data associated with this article can be found in the
online version, at http://dx.doi.org/10.1016/j.catcom.2017.06.006.
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