A novel environmentally sustainable synthesis of Au-Ag@AgCl nanocomposites and their application as an efficient and recyclable catalyst for quinoline synthesis

Article abstract of DOI:10.1039/c7nj00764g

This paper reports a facile, additive free, cost effective, green approach to the synthesis of Au-Ag@AgCl nanocomposites (NCs) using the tuber extract of Nephrolepis cordifolia. Au-Ag@AgCl NCs were fabricated by reducing cationic Ag and Au and simultaneou

Full text of DOI:10.1039/c7nj00764g

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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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ARTICLE
Novel environmentally sustainable synthesis of Au-Ag@AgCl
nanocomposites and their application as an efficient and
recyclable catalyst for quinoline synthesis
Received 00th January 20xx,
Accepted 00th January 20xx
Kanti Sapkotaa,b Sung Soo Hana,b
*
DOI: 10.1039/x0xx00000x
This paper reports a facile, additive free, cost effective, green approach to the synthesis of Au-Ag@AgCl nanocomposites
(NCs) using tuber extract of Nephrolepis cordifolia. Au-Ag@AgCl NCs were fabricated by reducing cationic Ag and Au and
simultaneously generating AgCl in presence of plant material. The sizes of dispersed NCs ranged from 10 to 50 nm
(average diameter 30 nm). The synthesized NC was found to catalyze multicomponent domino annulation-aromatization
for quinoline synthesis. UV−visible spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, powder X-ray diffraction
(XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), energy-dispersive X-ray (EDX)
spectroscopy and thermogravimetric analysis (TGA) were used to determine the optical activity, functional groups present,
crystallinity, size, oxidation state, weight loss and thermal stability, respectively, of biosynthesized Au-Ag@AgCl NCs. These
NCs exhibited excellent catalytic activities and high yields when used to synthesize pharmaceutically important quinolines
via the three-component coupling/hydroarylation/dehydrogenation of arylaldehyde, aniline, and phenyl acetylene
derivatives. The nanocatalyst was easily recovered and recycled five times without significant loss of catalytic activity.
www.rsc.org/
Introduction
characteristics as compared with their components. Hybrid
metal nanoparticles not only provide the multifunctionality
Metallic nanoparticles have been used in catalysis, electronics,
and in biological, chemical, optical, and biomedical fields. They
exhibit wide ranging properties, including anticancer,
antimicrobial, antidiabetic, antioxidant, and antibacterial
activities,1-4 and have even been used for wastewater
treatment.5 Chemical methods have generally been used to
synthesize metallic nanoparticles, but the utilization of harmful
and hazardous chemicals for chemical synthesis causes
environmental pollution and raises toxicity concerns.
Therefore, there is a need for environmentally benign methods
for the production of nanomaterials to diminish environmental
impacts and safety concerns.6-11 Plant materials can be used
for the green preparation of various metallic nanoparticles,12-15
and they have the advantages of being readily available,
environmentally sustainable, economical, and renewable.
Furthermore, the chemicals present in plant materials act as
reducing and stabilizing agents for the synthesis of metallic
nanoparticles.16-18
endowed by their components, but can also exhibit synergistic
effects. Moreover, these nano materials have remarkable
catalytic and optical properties.19-22 In particular, Au−Ag based
alloyed bimetallic nanoparticles have unique composition-
tunable plasmonic properties that can be utilized in sensor and
catalytic systems. Due to the importance and perceived
potentials of these bimetallic nanomaterials, various efforts
were devised for their synthesis. Recently, porous Au-Ag alloy
particles decorated AgCl on-chip membranes were produced
via a two-step facile immersion reaction and annealing process
and the synthesis of one dimensional Ag/Au/AgCl NCs from Ag
nanowires has also been reported.23,24 Very recently, alloyed
crystalline Au-Ag hollow nanostructures exhibiting high
catalytic performance and structural and chemical stabilities
were described.25 In addition, the synthesis of monodispersed
Au, Ag, and Au-Ag alloy nanoparticles using a hot-colloidal
method was reported.26 Other recent studies described the
size-controlled synthesis of Au@Ag core−shell nanoparticles
capped with citrate and the eco-friendly synthesis of bimetallic
Au-Ag nanoparticles.27,28 A number of authors have described
the efficient synthesis of Au@AgCl or Ag@AgCl hybrid
nanoparticles,29-35 but no report has yet described the facile,
efficient, ecofriendly synthesis of Au-Ag@AgCl nanomaterials.
Hence, the described green synthesis of Au-Ag@AgCl NCs is
novel, highly efficient, cost-effective, environmentally
sustainable, and compatible for catalytic, biomedical and
industrial applications.
In particular, bimetallic hybrid NCs exhibit distinct, unique
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Nephrolepis cordifolia belongs to the family Nephrolepidaceae
and commonly known as the erect sword fern or tuber sword
fern. It has suberect rhizomes with small hairy tubers, and is
widely distributed throughout Asia and Australia. The tubers of
Nephrolepis cordifolia are used as a traditional medicine for
the treatment of coughs, fever, and hematuria and its oil
exhibits cytotoxic, antibacterial, and antifungal activities.
Furthermore, reported GC/MS analysis of volatile chemical
constituents of Nephrolepis cordifolia demonstrates that
it contains several bioactive molecules, such as, phytol (3.93%),
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DOI: 10.1039/C7NJ00764G

- cadinol (3.30%), thymol (0.42%), ethyl linolenate (6.33%),
ionone (5.59%), ethyl stearate (3.19%), methyl oleate (0.54%),
ethyl palmitate (8.0%), -cadrene epoxide (1.48%), methyl
-
Scheme 1 Green synthesis of recyclable Au-Ag@AgCl NCs and their catalytic activity on
multicomponent domino annulation-aromatization for quinoline synthesis.

palmitate (1.54%), dibutyl phthalate (1.85%),benzyl butyl
phthalate (1.34%) and dibenzofuran (0.77%).36 (Table S1 ESI†).
Electron rich compounds especially polyols and phenolic
hydroxyl compound can be used as reducing or capping agents
for the synthesis of nanoparticles from metal ions, and
phenolic components can chelate metal ions by donating
electron and be used to produce metal nanoparticles.37,38
Nitrogen containing heterocycles are important natural
products and can be pharmaceutically active. In particular, the
quinoline nucleus is present in many natural products,
biologically active molecules, and functional materials,39-44 and
has been found to be associated with wide ranging
pharmaceutical activities, such as, anticancer, antimalarial,
antimicrobial, antituberculosis, and anti-HIV activities.45-48
Functionalized quinolines can also be used as fluorophores and
potential organic semiconductors.49,50 Because of their
importance and usefulness in diverse fields, the syntheses of
quinolines have attracted the interest of synthetic organic
chemists.51-55 Representative reported reactions of quinoline
synthesis include: the Skraup, Povaron, Friedlander, Doebner-
Von Miller, Combes and Conrad-Limpach reactions.56,57
Although a number of valuable approaches have been
developed to construct quinolines, the limitations of existing
synthetic methodologies, such as, limited availability of
starting materials, costs of catalysts, harsh reaction conditions,
and lower yields, new synthetic methods are required to
produce quinolines. To the best of our knowledge, no previous
report has described the synthesis of polyfunctionalized
quinoline derivatives using environmentally compatible and
recyclable Au-Ag@AgCl nano-catalyst from commercially
available arylaldehyde, aniline and phenyl acetylene
derivatives.
derivatives were purchased from Sigma-Aldrich. The air dried
tubers of Nephrolepis cordifolia (L.) C. Presl was obtained from
the Kaski district of Nepal. All purchased chemicals were used
without further purification. Double distilled water was used
throughout the experiments. The glassware used was cleaned
with aqua-regia (3:1 concentrated solution of HCl: HNO3),
rinsed with distilled water and dried in an oven. The tubers of
Nephrolepis cordifolia were washed with distilled water and
dried at room temperature. Then, 10 gram of tuber was cut
into small pieces and 100 mL of sterile water was added. The
mixture was boiled for 15 minutes with continuous stirring,
centrifuged at 3000 rpm for 10 minutes, and filtered.
Synthesis of Au-Ag@AgCl NCs
10 mL of prepared tuber extract was mixed with 100 mL of 1
mmol aqueous AgNO3 solution in a 100 mL sterile Erlenmeyer
flask, stirred for 5 min, and sonicated for 30 min when the
color of the solution turned dark yellow indicating the
formation of silver nanoparticles. At the same time, 10 mL of
tuber extract was added to 100 ml of 1 mmol of an aqueous
solution of HAuCl4 in a 500 mL sterile Erlenmeyer flask and
sonicated for 30 min when the color of the solution became
pink/red, indicating the formation of Au nanoparticles. The
dark yellow colored solution of Ag nanoparticles was then
mixed with the gold nanoparticle solution. This mixture was
then sonicated for 4 hours at 50 oC and kept in sunlight for 1 hr
when the color of the mixture became reddish orange,
indicating the formation of Au-Ag@AgCl NCs. AgCl was
supposed to be generated in situ in the reaction mixture due
to the combination of Ag+ ions of AgNO3 and Cl- ions of HAuCl4.
The prepared nanoparticles were centrifuged at 10,000 rpm
for 20 minutes using a Beckman coulter’s centrifuge (Model:
Avanti® J-E, USA), washed four times with distilled water and
collected in a sterile Petri dish. Finally, they were vacuum dried
Herein, we report an efficient, facile and green synthetic
route for the fabrication of Au-Ag@AgCl NCs using the tuber
extract of Nephrolepis cordifolia as a reducing and stabilizing
agent. The synthesized nanoparticles were used as recyclable,
efficient catalysts for the multicomponent tandem synthesis of
various quinoline derivatives from arylaldehyde, aniline, and
phenyl acetylene derivatives (Scheme 1).
o
in an oven for 20 hours at 50 C. Alternatively, we tried to
make the Au-Ag@AgCl NCs in one-pot procedure using 10 mL
of plant extract solution with 100 mL of 1 mmol AgNO3 and
HAuCl4. The desired Au-Ag@AgCl NCs were formed which was
confirmed by XRD and TEM analysis (See Figure S4 and Figure
S5 ESI†). However, TEM analysis shows that the nanoparticles
formed were more aggregated compared to sequentially
synthesized Au-Ag@AgCl NCs.
Experimental
Materials and methods
Gold (III) chloride trihydrate (HAuCl4·3H2O, 99.9%), silver
nitrate (AgNO3, 99%), aniline, phenyl acetylene and aldehyde
2 | J. Name., 2012, 00, 1-3
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Characterization of NCs
ARTICLE
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DOI: 10.1039/C7NJ00764G
The UV-visible spectra of Au-Ag@AgCl NCs were recorded to
confirm the formation of nanoparticles using an UV-visible
spectrophotometer (Shimadzu UV-2600) and a quartz cuvette.
Fourier transform infrared (FT-IR) spectra were examined
using a Perkin–Elmer FT-IR spectrometer in transmittance
mode between 400 and 4000 cm-1. The crystalline properties
of Au-Ag@AgCl NCs were investigated by powder X-ray
diffraction using a PAN alytical X'Pert PRO MPD unit (operated
at an accelerating voltage of 40 kV and 30 mA with a Cu Kα X-
ray source (λ = 1.5406 Å) over the 2θ range of 20° to 90°, and
using a scanning rate of 1.2° min−1). The sizes and shapes of
the Au-Ag@AgCl NCs produced were determined by field
emission transmission electron microscopy (FE-TEM, FEI Tecnai
F20). The copper grid of the TEM was dipped into prepared
diluted samples three times and then dried at room
temperature. X-ray photoelectron spectroscopy was used to
determine surface compositions and electronic states using a
Thermo Scientific K-Alpha system an Al Ka X-ray source and a
ion source energy range of 100 V to 3 keV. Thermal analysis
was conducted by monitoring percentage of weight loss using
a differential scanning calorimeter (TG-DTA, SDT-Q600 V20.5
Fig. 1 UV–vis spectra and XRD patterns of Au-Ag@AgCl NCs (a) UV spectra of AuNPs,
AgNPs and Au-Ag@AgCl NCs, and (b) UV spectra of Nephrolepis cordifolia tuber extract
(c) XRD patterns of Au-Ag@AgCl (d) XRD patterns of Au-Ag@AgCl after recycle.
In Au-Ag@AgCl NCs, the plasmonic absorption of Au
nanoparticles was red shifted from 530nm to 520nm. The
surface plasmon resonance of the tuber extract was observed
at 278 nm, which indicated the presence of polyols in the
tubers of Nephrolepis cordifolia (Figure1b).58 The Au-Ag@AgCl
NCs were found to be stable for 2 months when the same
surface plasmon resonance at 520 nm was observed.
o
Build 15) in the range 30 to 800 C using a heating rate 10 oC
per minute in N2. Zeta potential measurement was performed
using a Zetasizer Nano ZS (Malvern Instrument Ltd.,UK).
Catalytic activity
The synthesized Au-Ag@AgCl NCs were used as catalysts for
the synthesis of various quinoline derivatives via three-
component coupling/hydroarylation/dehydrogenation of
arylaldehyde, aniline, and phenyl acetylene derivatives.
Powder X-ray diffraction analysis was carried out to
determine the crystal structure and phase of the Au-Ag@AgCl
NCs. XRD pattern confirmed the presence of Au-Ag with AgCl
(Figure 1c). Braggs peaks of Au-Ag observed at 38.2o, 44.4o,
64.6o, 77.6o and 81.7o, which indexed with the (111), (200),
(220), (311), and (222) planes, respectively, of face-centered
cubic Au (JCPDS No. 04-0784) and Ag (JCPDS No. 04-0783).59
Similarly, XRD peaks at 27.8o, 32.2o, 46.2o, 54.8o, 57.4o, 67.4o,
74.4o, 76.7o and 85.7o were assigned to the (111), (200), (220),
(311), (222), (400), (331), (420) and (422) crystal planes,
respectively, of crystalline AgCl (JCPDS file : 31-1238).59,60
FT-IR analysis was performed in order to identify the
capping agents and functional groups responsible for the
formation of NCs. Figure 2a shows the FT-IR spectra of the
tuber extract and Au-Ag@AgCl NCs. The major absorption
peaks of the tuber extract and Au-Ag@AgCl NCs were
observed at 3300 cm-1, 2918 cm-1, 1610 cm-1, 1349 cm-1, 1042
cm-1, 527 cm-1 and at 3280 cm−1, 2918 cm−1, 1636 cm−1, 1365
cm−1, 1016 and 569 cm−1, respectively (Figure 2a). These
absorptions were slightly shifted for Au-Ag@AgCl NCs though
the peak at 2918 cm-1 remained unchanged. The appearance of
absorption peak at ~3300 cm−1 was assigned to the O–H
stretch of alcohols or phenols and the N–H stretch observed in
tuber extract, which moved slightly to 3280 cm−1 in Au-
Ag@AgCl NCs. Similarly, absorption bands at ~1610 cm−1 and
General procedure for the syntheses of quinoline derivatives 4a-
4f.
2 mol% Au-Ag@AgCl NCs and 10 mol% of p-toluenesulfonic
acid (PTSA) were added to a stirred solution of aldehyde
1 (1.0
mmol), amine (1.0 mmol) and alkyne (1.0 mmol) in 1,2-
2
3
dichloroethane (5.0 mL). The reaction mixture was refluxed for
9-12 h and the progress of the reaction was monitored by TLC.
After reaction completion, the solvent was removed under
reduced pressure and the residue was subjected to silica gel
column chromatography to isolate the pure products 4a-4f.
Results and discussion
Characterization of Au-Ag @AgCl NCs
UV-visible spectroscopy was performed to confirm the
formation of NCs. The UV-visible spectra of the synthesized
Au-Ag@AgCl nanoparticles exhibited the characteristic surface
plasmon resonance (SPR) peak of metallic Ag and Au due to
the oscillation of electrons in metal nanoparticles in an applied
electromagnetic field (Figure 1a). This peak is dependent on
the sizes, shapes, and the microenvironments of NCs.37
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First, the removal of 1.88% by weight arVoieuwnAdrticle1O0n0lionCe
DOI: 10.1039/C7NJ00764G
corresponding to the evaporation of moisture and volatile
compounds in the nanocomposite. Second, 20.82% weight loss
was observed at ~150 oC to 400 oC, which may have been due
to the loss of capping agents present in the bio synthesized Au-
Ag@AgCl NCs. Third, 2.88% weight loss occurred beyond 800
oC, which may be due to removal of phytochemicals present in
Au-Ag@AgCl NCs. Overall loss in mass along with prominent
endothermic peaks at 51.59, 252.17 and 312.53oC were
observed in TGA graph. After testing the residue amounted to
74.42% of the original weight of Au-Ag@AgCl NCs, which
represented the total amount of Au, Ag and AgCl present.
Au-Ag@AgCl NC particle morphology and size were
determined by TEM. Figure 3 shows a TEM image, size
distribution histogram, HR-TEM, and SAED patterns of the Au-
Ag@AgCl NCs. Most of the nanocomposites were spherical
though a few hexagonal particles were also observed (Figure
3a). TEM micrographs showed polydispersed particles sized
from 10 to 50 nm and average diameter 30 nm (Figure 3b).
Fig. 2 (a) FT-IR spectra of tuber extract and Au-Ag@AgCl NCs and (b) TGA and DSC curve
of and Au-Ag@AgCl NCs.
Fig. 3 TEM images of Au-Ag@AgCl NCs (a) at scale bar 50 nm, (b) size distribution
histogram (c) at 5 nm scale bar and (d) SAED pattern
1349 cm−1 were assigned to asymmetric stretch of C=C and
symmetric stretch of C–O, which shifted to 1635 cm−1 and 1365
cm−1, presumably due to the presence of other biomolecules in
the tuber extract. The absorption peak at 2918 cm-1
corresponded to C–H stretch. Slight shifting of peaks in IR
absorption bands corresponding to -OH and -NH2 functional
groups was caused by their participation in the reduction and
Fig. 4 (a) HRTEM image at 2 nm with d-spacing (b) EDS peaks of Au-Ag@AgCl NCs.
1
capping of Au-Ag@AgCl NCs. Moreover, H NMR analysis of
crude CHCl3 extract of Nephrolepis cordifolia shows the
characteristic peaks of hydroxyl and amine groups at δ 9-12
and 4-5 ppm region (Figure S6 ESI†).
A number of phytochemicals, such as, ethyl palmitate,
ethyl linolenate, β-ionone, phytol, diterpene alcohol, eugenol,
anethol and α-cadinol have been reported in Nephrolepis
cordifolia.36 These polyols and flavonoids played the key role
for the reduction and capping of Au-Ag@AgCl NCs.
Thermogravimetric analysis was performed to examine the
thermal stability of Au-Ag@AgCl NCs produced. Figure 2b
presents differential scanning calorimetry- thermogravimetric
analysis (DSC–TGA) results for percentage of weight losses
while heating from room temperature to 800 C at 10 C min-1
under N2. The overall weight loss was found to be 25.58%.
Percentage mass losses were observed to occur in three stages.
o
o
Fig. 5 HAADF-STEM image at 100 nm with the EDS elemental mapping of Au, Ag and Cl
4 | J. Name., 2012, 00, 1-3
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Fig. 7 (a) UV–vis spectra under various parameters (after 4 months, 10%NaCl, pH 8.0,
pH 7.4, pH 6.0, and autoclave) and (b) Zeta potential distribution, of the Au-Ag@AgCl
NCs
Stability testing of Au-Ag@AgCl NCs
The stability of Nephrolepis cordifolia stabilized Au-Ag@AgCl
NCs was studied by evaluating the plasmon resonance (SPR)
peaks using various parameters. For example, the plasmon
wavelength measured after 4 months, using 10% NaCl,
Fig. 6 XPS full scan survey showing (a) Au-Ag@AgCl NCs, (b) Au 4f-electrons of gold, (c)
Ag 3d-electrons of silver, and (d) Cl 2p-electrons of Cl.
o
autoclaved at 120 C for 50 minutes and in phosphate buffer
solution at pH 6, 7.4 and 8 shows only slight shift of λmax (1 –
5 nm). These results demonstrate that Au-Ag@AgCl NCs were
stable in biological fluids at physiological pH (Figure 7a).
Moreover, zeta potential distribution Au-Ag@AgCl NCs in
aqueous solution was found to be -37.5 mV (Figure 7b). Thus
large negative zeta potential value confirmed the higher
stability of Au-Ag@AgCl NCs.65,66
Figure 3c showed the HRTEM image of the nanocomposite
(scale bar =5 nm). The SAED (selected area diffraction) pattern
demonstrated the crystalline nature of bio-synthesized Au-Ag
@AgCl NCs by the presence of multiple diffraction rings (Figure
3d). Figure 4a shows the HRTEM image with d-spacing of 0.221
nm and EDS confirmed the presence of Au, Ag, Cl, Cu (from the
grid), C and O (from biomoleules) (Figure 4b). The high-angle
annular dark-field image along with scanning transmission
electron microscopy (HAADF-STEM) results showed the
elemental mapping of Au, Ag and Cl in the Au-Ag@AgCl NCs
(Figure 5).
X-ray photoelectron spectroscopy (XPS) was carried out in
order to determine the oxidation state and surface chemical
composition of the as synthesized Au-Ag@AgCl NCs (Figure 6).
Figure 6a displays the XPS spectrum of Au-Ag@AgCl NCs,
which confirmed that the synthesized nanocomposite contains
gold, silver and chloride. The presence of C and O in the XPS
spectrum could be due to XPS instrumentation and bioorganic
molecule present in Au-Ag@AgCl NCs. The XPS peak at 285 eV
corresponds to the C 1s region (Fig. S1, ESI†). Figure 6b
exhibits the characteristic 4f bands of metallic gold. The
doublet peaks represent Au 4f7/2 and Au 4f5/2 at binding
energies 84.38 and 88.18 eV respectively. The 4f bands were
splited into a pair of peaks with a prominent spectrum of 4f7/2
at a binding energy 84.38 eV, which is characteristic of Auo.61,62
The two XPS peaks observed at 367.98 eV and 373.98 eV
corresponded to Ag 3d5/2 and Ag 3d3/2 for Ag+ of AgCl
respectively (Figure 6c), which was also confirmed by XRD.
Furthermore, two small peaks observed at 368.98 eV and
374.98 eV confirmed the binding energies of Ag 3d5/2 and Ag
Catalytic activity
The selection and optimization of the catalyst is presented in
Table 1. The highest yield of the product 4a was obtained using
Au-Ag@AgCl NCs compared to other catalysts such as AuNPs,
AgNPs, AgNO3, HAuCl4 and plant extract. Based on the
optimization study, quinolines 4a
tandem annulation/aromatization of aryl aldehydes (1a or 1b
and aniline derivatives (2a or 2b) with various phenyl
acetylene derivatives (3a 3d) as summarized in Table 2.
-4f were synthesized by
)
-
Reaction of benzaldehyde (1a) and aniline (2a) in the presence
of phenyl acetylene (3a) and 2 mol% Au-Ag@AgCl NCs in
refluxing 1,2-dichloroethane (DCE) for 9 h provided 4a in 96%
yield (entry 1, Table 2). Reaction between 4-methyl
benzaldehyde (1b) and aniline (2a) in the presence of phenyl
acetylene (3a) afforded product 4b in 97% yield (entry 2, Table
2). Similarly, diversely substituted phenyl acetylenes 1-ethynyl-
3-methylbenzene (3b), 1-chloro-3-ethynylbenzene (3c), or 1-
ethynylnaphthalene (3d) reacted with 1a and 2b in refluxing
DCE to afford the desired products 4c-4e in 95, 96 and 93%
yields, respectively (entries 3-5, Table 2). Moreover, reaction
between naphthalen-1-amine (2b) and 1a and 3a provided 2,4-
diphenylbenzo[h]quinoline (4f) in 91% yield. These results
show the Au-Ag@AgCl NCs provide efficient synthetic routes
to biologically interesting heterocycles in good yield. The
structure of 4a was identified by 1H and 13C NMR and by direct
comparison with reported data (data and spectra of the
synthesized compounds are presented in the ESI†).
3d
respectively due to metallic silver (Ag0). Previous studies
3/2
have produced similar results.63-64 Moreover, figure 6d shows
chlorine 2p bands at the binding energies of 198.38 eV and
199.98 eV, corresponding to characteristic Cl 2p3/2 and Cl 2p1/2
peaks, respectively.63 This result further confirmed the
presence of Au, Ago, Ag+ and Cl in the synthesized Au-Ag@AgCl
NCs.
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Table 1 Optimization of the reaction conditions for the synthesis of 4aa
Table 3 Recyclability of the Au-Ag@AgCl NCs a
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aReaction conditions: Aldehyde 1a (1.0 mmol), amine 2a (1.0 mmol), alkyne 3a (1.0
mmol), Au-Ag@AgCl NCs nanocatalyst (2 mol%), PTSA (10 mol%), solvent 1,2-
dichloroethane (5.0 mL), bIsolated yield.
aReaction conditions: Aldehyde 1 (1.0 mmol), amine 2 (1.0 mmol), alkyne 3 (1.0
mmol), catalyst (2 mol%), PTSA (10 mol%), solvent 1,2-dichloroethane (5.0 mL).
Table 2 Construction of diverse quinoline derivatives from aniline, benzaldehyde
and phenyl acetylene derivatives via Au-Ag@AgCl NCs catalyzed multicomponent
domino annulation-aromatizationa
Proposed reaction mechanism for the formation of 4a in
the presence of Au-Ag@AgCl NCs is illustrated in Scheme 2.
Aldehyde 1a condenses with amine 2a in presence of p-
toluenesulphonic acid (PTSA) to form imine
coordination complex with Au-Ag@AgCl NCs and alkyne 3a to
produce the propargylamine intermediate . Intermediate
then undergoes intramolecular hydroarylation of the alkyne to
afford dihydroquinoline intermediate . Finally, air oxidation of
intermediate provides the required quinoline 4a
5, which forms a
6
6
7
7
.
Recyclability of catalyst
Catalyst recyclability was examined in order to test the
stability of the synthesized Au-Ag@AgCl NCs. For this purpose,
a series of recycle experiments were conducted as shown in
Table 3. After washing with ethyl acetate and hot water five
times and drying in vacuum for 3 h, recovered Au-Ag@AgCl
NCs were recycled. Au-Ag@AgCl NC was recycled and reused
for five times without significance loss of catalytic activity. XRD
analysis of the Au-Ag@AgCl NCs after recycling
aReaction conditions: Aldehyde 1 (1.0 mmol), amine 2 (1.0 mmol), alkyne 3 (1.0
mmol), Au-Ag@AgCl NCs nanocatalyst (2 mol%), PTSA (10 mol%), solvent 1,2-
dichloroethane (5.0 mL).
Scheme 2 Proposed reaction mechanism for the formation of 4a.
Fig. 8 TEM images of recycled Au-Ag@AgCl NCs at scale bar (a) 50 nm (b) 20 nm (inset:
SAED pattern), and (c) EDS peaks after 5th catalytic cycle.
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