Bimetallic Ag-Cu alloy nanoparticles as a highly active catalyst for the enamination of 1,3-dicarbonyl compounds

Article abstract of DOI:10.1039/c6ra04569c

Bimetallic nanoparticles, particularly those based on copper, have recently attracted a great deal of attention for the development of low cost and highly active catalysts due to the synergistic interaction between individual metal components. In this work, bimetallic Ag-Cu alloy nanoparticles were explored as a highly active and reusable catalyst for the enamination of 1,3-dicarbonyls using diverse amines. The nanocatalysts were intensively characterized by ultraviolet-visible (UV-Vis) spectroscopy, X-ray diffraction (XRD), high-resolution transmission electron microscopy-energy-dispersive spectroscopy (HRTEM-EDS) and valence band and core level X-ray photoelectron spectroscopy (XPS) to study the effect of the bimetallic structure and composition. In comparison to monometallic Ag and Cu nanoparticles, the alloyed Ag-Cu nanoparticles showed a high catalytic performance and the resultant catalytic activity was dependant on the Ag to Cu ratio. This enhanced catalytic activity should be related to the electronic interaction between Ag and Cu nanoparticles formed due to the intimate contact between them. Our study may serve as a foundation for designing efficient alloyed nanocatalysts for fine chemical synthesis via enamination reactions.

Full text of DOI:10.1039/c6ra04569c

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Bimetallic Ag-Cu Alloy Nanoparticles as Highly Active Catalyst for
Enamination of 1, 3-dicarbonyl Compounds
Received 00th January 20xx,
Accepted 00th January 20xx
Lipeeka Rout, a Aniket Kumar a, Rajendra S. Dhaka b and Priyabrat Dash*a
DOI: 10.1039/x0xx00000x
a Department of Chemistry, National Institute of Technology, Rourkela, Orissa, 769008, India.
www.rsc.org/
b
Novel Materials and Interface Physics Laboratory, Department of Physics, Indian Institute of
Technology Delhi, HauzKhas, New Delhi-110016, India.
Bimetallic nanoparticles, particularly copper-based, have recently attracted a great deal of attraction for the
development of low cost and highly active catalyst due to the synergistic interaction between individual
metal components. In this work, bimetallic Ag-Cu alloy nanoparticle was explored as highly active and
reusable catalyst for the enamination of 1,3-dicarbonyl using diverse amines. The nanocatalysts were
intensively characterized by UV-vis, XRD, HRTEM-EDS, valence band and core level XPS to study the effect of
bimetallic structure and composition. In comparison to monometallic Ag and Cu nanoparticles, alloyed Ag-Cu
nanoparticles showed high catalytic performance and the resultant catalytic activity was dependant on the
Ag to Cu ratio. This enhanced catalytic activity should be relevant to the electronic interaction between Ag
and Cu nanoparticles formed due to the intimate contact between them. Our study may serve as a
foundation for designing efficient alloyed nanocatalyst for fine chemical synthesis via enamination reaction.
9, 10
galvanic replacement reaction,8 ethylene glycol method
and
co-reduction.11 Many bimetallic nanoparticles such as Au-Cu,12 Pd-
1. Introduction
15
16
17
18
19
Cu,13 Fe-Cu,14 Ni-Cu, Pd-Au, Pt-Pd, Pt-Co and Pt-Ni have
been employed for aerobic oxidation,12 sonogashira reaction,13
degradation of organic contaminants,14 hydrogen production,15
ethanol electro-oxidation,17 hydrogenolysis18 and the oxygen
reduction reaction, respectively.19 In general, several kinds of
literature displayed that bimetallic nanoparticle catalyst based on
Cu exhibited much-enhanced activity than monometallic Cu NPs in
catalytic reactions. For example, Cu-Au NPs enhanced the reduction
of carbon dioxide, 20 Cu-Co NPs act as a potential catalyst for higher
In recent years, nanoparticles (NPs) of noble metals ranging in size
1-10 nm have attracted much attention in the field of catalysis due
to their enormous physical and chemical properties.1 Catalysis over
metal nanoparticles is of great interest as their size, structure and
composition can offer to tune the activity and selectivity of the
catalyst. It is known that the catalytic properties of the NPs can be
changed by alloying or mixing of two metals with different dielectric
constants. 2 This mixing, in principle, would lead to the formation of
bimetallic NPs. In many cases, compared to monometallic NPs,
bimetallic NPs have shown better optical, electronic and catalytic
properties that can be attributed to ensemble and electronic
effect.3-5 Till now, various methods have been utilized to synthesize
metal NPs such as γ-ray radiolysis,6 evaporation-condensation,7
alcohol synthesis,21 Cu-Ag perform as an effective catalyst for the
22
oxidation of methanol to CO2
and for the hydrolytic
23
dehydrogenation of ammonia borane, and bimetallic Cu-Pd as an
active catalyst for formic acid oxidation.24
Enamination of 1,3-dicarbonyl compounds to β-enamino ketones
and esters synthesized by low-cost raw material with a very stable
structural pattern are considered as valuable precursors in organic
synthesis.25,26 They are utilized as significant intermediates for the
a.Address here.
b.Address here.
28
preparation of various heterocyclic compounds.27 They are also
c. Address here.
known to have medicinal applications as anticonvulsant,29, 30 anti-
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
32
inflammatory,31,
and antitumor agents.33 Because of their
extensive variety of action and vitality, a straightforward and high
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yielding one-pot approach for the synthesis β-enaminones and β- and β-enaminoesters. The catalyst was found to be recyclable up to
enaminoesters is highly desirable. The environmentally benign 4 cycles without any significant loss of activity and yield of desired
protocol used for the synthesis of β-enaminones is found to be a product. We envision that current strategy can provide useful clues
tedious method.34 Several improved procedures have been for the design of novel bimetallic NPs and bimetallic NP-based
reported for the reaction between dicarbonyl compound with systems as potential catalyst for other fine chemical syntheses.
amines using catalysts such as metal triflates by microwave and
ultrasound irradiation.35-38 Other synthetic process to β-
2. Experimental
enaminones and β-enaminoesters has included the cyclization of
amino acids, acylation of amines and condensation reaction.39
2.1. Materials
Synthesis of some fine chemicals has been reported using different
Silver nitrate, AgNO3 (A.R. Grade, 98% pure) and Sodium
catalysts such as Yb(OTf)3,40 perchlorates,41 [Hbin]Tf],42 HClO4-
borohydride, NaBH4 were purchased from Sigma-Aldrich, copper
43
SiO2, montmorillonite K10,44 silica gel,45 natural clays,46 silica
sulphate, CuSO4.5H2O (A. R. grade) was purchased from HI media.
48
chloride,47 InBr3, and CoCl2.49 All these methods discussed so far
All other chemicals were purchased from CDH chemicals and were
have their own drawbacks and limitations such as moisture
used without further purification. TLC plates (20X20, 0.2 mm) were
sensitive and tedious workup,40 requirements of drastic conditions
purchased from SD-fine chemical. Column chromatography was
and the use of harmful reagents,41 use of homogenous catalyst,48
carried out with the use of silica gel (100–200 mesh), purchased
non-recyclable catalysts,49 and longer time and low yield.49 Hence,
from Hi-media. A cellulose dialysis membrane (diameter-17.5 mm)
there is sufficient scope for the development of heterogeneous and
was purchased from Hi-media. 18 MΩ Milli-Q water was used
reusable catalytic system which could catalyse the synthesis of β-
throughout the synthesis.
enaminones and β-enaminoesters at milder operating conditions.
2.1.1. Preparation of Ag-Cu NPs via chemical reduction method
In recent years, transition metal nanoparticles have been used for
various organic transformations due to their high surface atoms and
2, 50
Ag1-Cu3 bimetallic NPs were synthesized by co-complexion method
using AgNO3 (0.11 mmol, 5 ml) and CuSO4.5H2O (0.35 mmol, 5ml) as
precursors. NaBH4 (4.6 mmol, 10 ml) was used as reducing agent for
the reduction of the two salts. The metal precursor of the two NPs
was mixed in a round bottom flask using distilled water covered
with aluminium foil to avoid the photochemical reaction of AgNO3.
The above reaction mixture was stirred for 10 minutes using a
magnetic stirrer in ice cold condition under nitrogen atmosphere.
To this mixture, NaBH4 was added drop wise for 30 min. The whole
reaction was kept for further stirring for 2 hours under nitrogen
atmosphere. The dark green precipitate was obtained which was
further centrifuged and dried in schlenk line. Then the NPs were
dialyzed overnight to remove excess salts before any use. Ag, Cu
and other Ag-Cu nanoparticles were prepared using the same
procedure above, keeping the total metal salt concentration
constant.
co-ordination sites.
However, these catalysis still suffer from
complicated synthesis protocol, less efficiency and are prone to
oxidation. Because of the presence of the synergistic effect in
bimetallic catalysts and the vast opportunities for engineering the
particle size, shape and composition, it is expected that the
bimetallic catalysts may have high potentiality for fine chemical
synthesis. Moreover, bimetallic nanoparticles may also provide
better stability and functionality at lower cost.51, 52 In this regard,
the design of bimetallic nanoparticles involving silver and copper
will provide a stable catalyst where silver will help in mitigating the
oxidation of copper. In addition, the activity can be controlled by
simply changing the Ag to Cu ratio. Surprisingly, to the best of our
knowledge, use of bimetallic nanoparticles as an efficient catalyst
for the synthesis of fine chemicals has not been demonstrated so
far.
Herein, we report the synthesis of bimetallic Ag-Cu nanoparticles
2.2. Characterization of catalyst
via
a
low-temperature simple co-reduction method. The
composition of Ag-Cu nanoparticles was tuned by changing the
ratio of the metal precursors (3:1, 1:1, and 1:3). The formation of
bimetallic Ag-Cu nanoparticles was examined using EDS
measurements which help to determine the distribution of Ag and
Cu components in the bimetallic structure. More importantly,
change in the electronic properties of the bimetallic nanoparticles
due to alloying has been studied using core and valence band XPS
analysis. Later on, these bimetallic catalysts were used as efficient
catalyst for one pot synthesis of β-enaminones and β-
enaminoesters. Compared with monometallic nanoparticles,
bimetallic Ag1-Cu3 exhibited much enhanced activity, providing high
yield of the desired product in less interval of time. In this study,
various amine precursors such as cyclic and branched amine with
bulkier group were employed for the synthesis of β-enaminones
The catalyst was analyzed by powder X-Ray diffractometer (XRD)
study (RIGAKU JAPAN/ULTIMA-IV) using Cu Kα radiation (λ=0.154
nm) and in the 2Ɵ range of 20°-80°. XRD was used to determine the
crystal phase of prepared catalysts. The UV-vis absorbance spectra
were obtained on UV-2450, Shimadzu. Transmission electron
micrographs (TEM) of the sample were recorded using PHILIPS CM
200 equipment using carbon coated copper grids. High angle
annular dark field scanning transmission electron microscopy
(HAADF-STEM) images were taken in a Bruker TEM/STEM operated
1
at 200-300 keV. H NMR and 13C NMR spectra were recorded on a
Bruker spectrometer at 400 MHz using TMS as an internal standard.
A commercial electron energy analyser (PHOIBOS 150 from Specs
GmbH, Germany) and a non-monochromatic Mg Kα X-ray source
(hν=1253.6 eV) have been used to perform XPS measurements with
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the base pressure of <1*10-9 mbar. Catalytic reactions were absorption peak at 415-440 nm (Fig. 1d-f) which is in between the
monitored by thin-layer chromatography on 0.2 mm silica gel F-254 monometallic Ag and Cu NPs. The red shift of surface plasmon
plates. All the reaction products are known compounds and have adsorption peaks from 410 nm to 520 nm was attributed to
been identified by comparing their physical and spectral increasing of Cu content in Ag–Cu alloy nanoparticles.56 The above
characteristics with the literature reported values.
results indicate the co-existence of two elements in one particle
and form an alloy structure.2 To further confirm this, the Ag-Cu
bimetallic nanoparticles were examined by XRD, HRTEM, with EDS
mapping and EDS line scan.
2.3. Typical method for synthesis of β-enaminones and β-
enaminoesters
In a 100 ml round bottom flask, dicarbonyl compound (1 mmol),
amine (1 mmol) and Ag-Cu bimetallic NPs (20 mg) were added in 5
ml of methanol and stirred at 60 °C for the desired time given in
Table 4. The progress of the reaction was monitored using thin layer
chromatography (TLC) using ethyl acetate and hexane as eluent. On
completion of the reaction, 5 ml of water was added to the reaction
mixture and centrifuged at 4000 rpm for 10 min to separate the
bimetallic NPs. The supernatant was slowly decanted and the
sediment nanoparticle was dried and collected for further cycle of
reaction. The crude product was extracted from the mixture by
adding ethyl acetate and dried using sodium sulphate (Na2SO4). All
products were characterized using NMR spectroscopy.
Fig. 2. shows the XRD results of the synthesized monometallic Ag,
Cu, and bimetallic Ag-Cu nanoparticles. The XRD analysis was
conducted with
a fully dried powder of the nanoparticles.
Prominent peak in the XRD pattern indicates that the samples
obtained is of high crystallinity. Major peaks at 2θ values of 38.5°,
44.5°, 64.7° and 77.7° for Ag nanoparticles corresponds to the
lattice plane of (111), (200), (220) and (311) of metallic Ag (JCPDS
card No. 03-0931), respectively. In case of copper nanoparticles,
diffraction peaks formed at 2θ value of 43.5°, 50.7° and 74.4°
corresponds to the lattice plane of (111), (200), and (220) from
metallic Cu (JCPDS card No. 02-1225). In bimetallic Ag-Cu
nanoparticles, all the above Ag and Cu diffraction peaks were
observed, suggesting that bimetallic nanoparticle consists both Ag
and Cu phases. It is important to note that typical oxides peak at
61.7 ° due to CuO and 37.5 ° due to Cu2O phase are both absent in
the bimetallic spectra demonstrating the formation of an oxide free
Ag-Cu bimetallic system.57 From the major diffraction peak, lattice
parameter were obtained and shown in Table 1. The lattice
parameters were found to be in agreement with the selected area
electron diffraction (SAED) results as described in the later part of
the discussion. Later on, taking into (111) reflection of the XRD
spectra the average crystallite size was calculated using Debye
3. Results and Discussion
Co-reduction method was used to synthesize the monodisperse Ag-
Cu bimetallic nanoparticles in an ice cold condition under the
nitrogen atmosphere. The final nanoparticles were dialysed
overnight to make the nanoparticle free from impurities. In order to
fabricate the alloy nanoparticles (i.e. coexistence of Ag and Cu in
each nanoparticle), the possible effect of reducing agent was taken
into consideration. This is because the standard electrode potential
of Ag+/Ag0 (0.78 eV) is higher than that of Cu+2/Cu0 (0.34 eV).
Therefore, Ag+ is reduced more rapidly than Cu+2. This was avoided
by the addition of an excess of reducing agent (NaBH4), which helps
in the simultaneous reduction of Cu+2 and Ag+ to Cu0 and Ag0,
respectively, forming the Ag-Cu alloy nanoparticle. To confirm the
composition, structure and size of the Ag-Cu bimetallic
nanoparticles as synthesized above, various characterization
techniques such as UV-vis, XRD, HRTEM with energy dispersive
spectroscopy (EDS) mapping/line scan and XPS were carried out as
discussed below.
59
Scherrer formula.58 Crystallite size of Ag1-Cu3 bimetallic NPs was
found to be around 10 nm which nearly matches to the particle size
obtained from TEM analysis discussed in the later part.
The morphology and structural behaviour of the Ag1-Cu3 catalyst
(chosen because of its higher catalytic activity as described in
catalysis section) were analysed by TEM. Fig. 3a shows the
representative TEM image which shows that the product consists of
uniform spherical nanoparticles. The histogram (inset) shows an
average size of 7.5 ± 1 nm for the nanoparticles. The crystalline
nature of the Ag1-Cu3 NPs was confirmed by SAED analysis which
depicts a ring like structure (Fig. 3b). The (111), (200), (220) and
(311) rings are indexed to the face centred cubic (fcc) structure of
Ag and the (111), (200) and (220) rings are attributed to the fcc
structure of Cu.60 These patterns indicated that the nanoparticle is
polycrystalline. For further demonstration of alloy nanostructure,
HRTEM and EDS analysis were carried out. The HRTEM image of
Ag1-Cu3 nanoparticle shown in Fig. 3c can be resolved into two
groups of lattice fringes. The lattice distance of 0.23 nm
corresponds to the lattice fringe distance of (111) plane of Ag while
the lattice distance of 0.20 nm corresponds to the (111) plane of Cu.
The EDX analysis of Ag1-Cu3 nanoparticles shows the presence of Ag
and Cu with the concentration of 25.8 and 74.2 %, respectively (Fig.
3.1. Characterizations of synthesized NPs
Fig. 1 shows the changes in the absorption spectra during the
formation of monometallic and bimetallic nanoparticles. A strong
absorption peak at about 400 nm for silver nanoparticles was found
53
which is due to the surface plasmon absorption (Fig. 1a).
For
comparison, UV-vis spectra CuSO4.5H2O and monometallic Cu
nanoparticles are also shown (Fig. 1b). It has been found that only
weak absorption peak exist >600 nm for CuSO4.5H2O (Fig. 1c),
whereas monometallic Cu particles give surface plasmon resonance
55
(SPR) band with peak at ~560 nm.54,
The Cu nanoparticles
displayed a monotonic spectrum increasing exponentially towards
shorter wavelength.2 The bimetallic Ag-Cu NPs exhibit an
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3d). All the above results confirm the successful synthesis of of Ag: Cu, which found to be 1:2.8 (Table 2), very close to the
bimetallic Ag1-Cu3 nanoparticles.
original value of 1:3. This further suggests the oxidation stability of
the Ag-Cu alloy nanoparticle along with the presence of significantly
smaller amount of CuO, possibly formed due to the chemisorption
of small amount of O2 on the nanoparticle surface. Due to this, the
core-level peaks of Ag3d5/2 and Cu 2p3/2 found to be asymmetric
towards higher BE side. In addition, it can be observed that the BE
of Ag 3d5/2 and Cu 2p3/2 core level peak position in bimetallic Ag1-
Cu3 nanoparticles shifted to lower energies (~1.3 for Ag and ~1.4 for
Cu) in comparison to their bulk values (368.3 eV for Ag and 932.4 eV
for Cu). Abrikosov et al. observed the shift in the core-level peaks
when the alloy formation occurs, which was explained in terms of
intra-atomic charge re-distribution caused by valence electron
hybridization.76 Therefore, the observed shift in the Ag 3d5/2 and Cu
2p3/2 core-levels suggests alloy formation of Ag1-Cu3 nanoparticles.
Also, these peak shifts in the XPS spectra further suggest the
possible electronic interaction (synergistic effect) between Ag and
Cu in Ag1-Cu3 bimetallic nanoparticle. This is because, in comparison
to Cu, Ag has a higher redox potential, which leads to lower
electron densities of Cu atom in the bimetallic Ag1-Cu3 nanoparticle.
Though the results from XRD, TEM, SAED and HRTEM are
consistent, they are not sufficient to fully confirm the bimetallic
alloyed structure and distribution of the Ag and Cu nanoparticles. In
order to get conclusive confirmation of the alloy structure of the
Ag1-Cu3 NPs, the HAADF-STEM, EDS elemental mapping patterns
and EDS line scanning profiles were performed and are shown in
Fig. 4. HAADF-STEM image (Fig. 4a) shows a clear luminance
between Ag and Cu, suggesting the alloy structure of the as-
prepared Ag1Cu3 nanoparticles rather than a core–shell structure of
63
Ag and Cu.61, 62 EDS elemental mapping pattern further revealed
that Ag and Cu elements are uniformly distributed through the
whole bimetallic Ag1Cu3 nanostructure, indicating the formation of
Ag-Cu alloy without phase segregation, as shown in Fig. 4b (overlap,
Ag-Cu), 4c (red, Ag) and 4d (green, Cu). The results demonstrated
that Ag and Cu are well overlapped in the entire part. The line scan
along the direction derived in Fig. 4e demonstrated that Ag and Cu
are mixed well in the nanoparticle.64-67 The EDS analysis indicates an
average composition of approximately 1:3 of Ag and Cu, which is in
agreement with the molar percentage of respective salts during the
synthesis. This further shows the high compositional uniformity of
the bimetallic nanoparticles.
Besides the shift in the core-level peak positions in metal
nanoparticles, the changes in the valence band (VB) spectra are
crucial to understand the formation of alloy nanoparticles.77
Therefore, we have recorded the XPS valence band (VB) spectra of
Ag1-Cu3 nanoparticle (as shown in Fig. 6), which shows a broad peak
centred around 3 eV below the Fermi level. The characteristic
features of Ag1-Cu3 VB can be seen as: a broad peak at ~2.5 eV and
a shoulder ~4.5 eV. To understand the contribution from Ag and Cu,
the VB spectrum is deconvulated with two peaks at around 2 eV
and 4.5 eV, which are attributed to the Cu 3d and Ag 4d,
respectively. These values are relatively in agreement with the alloy
nanoparticles reported in the literature, further demonstrating the
We have performed x-ray photoelectron spectroscopy (XPS)
measurements to find the chemical composition and the oxidation
state of Ag1-Cu3 nanoparticles. A survey scan, as shown in Fig. 5a,
revealed the presence of elemental Ag, Cu, and O in the sample. In
Fig. 5(b-c), we show the core-level spectra of Ag 3d and Cu 2p,
respectively. The binding energy (BE) value of Ag 3d5/2 and Ag 3d3/2
core levels for Ag1-Cu3 nanoparticles appears at 367.1 and 373.3 eV,
respectively, associated with pure Ag nanoparticles. Absence of
peaks at 367.3 and 367.6 eV suggest that there is no significant
formation of AgO and Ag2O species, respectively in our nanoparticle
sample.68-70 In the Cu 2p core-level spectrum, we observed two
strong peaks at 931.0 eV and 950.7 eV, which are associated with
Cu 2p3/2 and Cu 2p1/2, respectively. These values are found to be
consistent with those reported for Cu (0).71 Both Ag and Cu data
suggested the metallic nature of Ag and Cu in Ag1-Cu3 bimetallic
nanoparticles, which is consistent with the XRD and TEM analysis.
Though the Cu 2p spectrum demonstrated that most of the Cu exist
2
formation of alloy Ag1-Cu3 nanoparticle. Moreover, there is a shift
of valence band peak position of 1.5 and 0.5 for Cu 3d and Ag 3d,
respectively compared to their bulk values (Cu = 3.5, Ag = 5 eV). 78 79
This shift of valence band energy indicates the charge separation
behaviour of Ag and Cu and suggest the possibility of electronic
interaction between Ag and Cu in the nanoparticle alloy
formation.78, 80, 81
in the form of metallic Cu(0) (931.0 eV and 950.7 eV) state, the 3.2. Catalytic studies for synthesis of β-enaminones and β-
enaminoesters
presence of small amount of Cu(II) (934.1 eV and 954.5 eV) can also
be found. The presence of Cu(II) was further confirmed by a
satellite peak at 942.0 eV. This can be attributed to the oxidation of
surface Cu atoms in air.72-74 It is interesting to find that while XRD
does not show any evidence of CuO phase, XPS analysis
demonstrates the surface presence of Cu+2 ion which suggests that
CuO is present only on the surface.75 Similar characteristic XPS
spectra containing shakeup satellite peaks were also reported in
literature in which an excess of copper was used in relation to other
species.22 Moreover, by fitting the spin-orbit splitting peaks and
taking area under the curve, we calculated the surface composition
The catalytic reactivity of Ag-Cu bimetallic nanoparticle was
evaluated for the synthesis of β-enaminones and β-enaminoesters
by one-pot condensation of diketones and amines at 60 °C using
methanol as a solvent. At first, the emphasis was led on optimizing
the reaction conditions. Condensation of acetylacetone and aniline
served as the model reaction for optimization (Scheme 1). Initially,
monometallic Ag, Cu, and different composition of Ag-Cu bimetallic
NPs (1:1, 1:3, 3:1) synthesized in this work were evaluated for their
activity using the model reaction.
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It was observed that in comparison to monometallic nanoparticles, reaction conditions were found to be, catalyst: Ag1-Cu3 bimetallic
bimetallic nanoparticles of different ratios showed best NPs, solvent: methanol, catalyst loading: 20 mg, temp: 60 °C and
performance for the catalytic reaction providing higher yields of the time: 1 h 45 min.
desired product (Table 3). Fig. 7 shows the catalytic activity of the
bimetallic series of Ag-Cu nanoparticles for the model reaction;
After optimizing the reaction conditions, we further investigated
bimetallic nanoparticles which were rich in Cu (Ag1-Cu3) showed the
the scope of the optimized protocol using different dicarbonyl
highest activity. Such electronic enhancement effects in bimetallic
compounds and different substituted aliphatic and aromatic
nanoparticle catalysts have been previously documented in various
amines. Excellent yield and high purity of products were obtained in
Ag-Cu bimetallic catalysts and other bimetallic systems (Ag-Pd, Au-
all the cases (Table 6, entries 1-40). All the condensation reactions
Pd).82-85 Therefore, Ag1-Cu3 nanocatalyst was chosen as the
were completed within 45-180 min at 60 °C without formation of
preferred catalyst for optimizing the reaction conditions. As shown
any side products. Acetylacetone was found to give good yield of 95
in Table 3, entry 1, for the control experiment (without metallic
% with aniline (Table 6, entry 14). Aliphatic and alicyclic amines
nanoparticles), only trace products were formed, confirming that
such as methyl amine, ethyl amine, propyl amine, butyl amine, and
no condensation takes place without metal nanoparticles.
morpholine were found to give an excellent yield of the condensed
product with acetylacetone using Ag1-Cu3 bimetallic NPs as catalyst
Further, the influence of various reaction parameters such as (Table 6, entries 13-16, and 18). It was found that the reaction with
solvent, temperature and catalyst loading was studied by using the aliphatic amine proceeds smoothly in a short period of time as
model reaction. In order to verify the effect of different solvent compared to aromatic amines that can be attributed to the higher
media on the condensation of the model reaction, solvents with nucleophilicity of aliphatic amines than aromatic amine. 87 Later on,
different polarity such as acetonitrile, tetrahydrofuran (THF), a variety of amines possessing a wide range of functional groups
dichloromethane (DCM), hexane, ethanol and toluene were used to were selected for further studies. It was observed that the amines
optimize the reaction conditions (Table 4). It was noticed that for a containing electron donating group provide better yield in less time
nonpolar solvent such as hexane the yield of product is very less. as compared to the amines containing electron withdrawing group.
88
The yield of the product was found to be higher on using polar
For example, anisidine and p-nitroaniline containing electron
-
-
solvents such as methanol, ethanol, and acetonitrile. Among withdrawing groups OCH3 and NO2 respectively, have strong
various solvents tried, the highest yield of β-enaminones and β- deactivating effect resulting in lower yields (Table 6, entries 20-21).
enaminoesters were obtained in methanol solvent which can be
attributed to the interaction of strong hydrogen bond in methanol
In addition, other bulkier and cyclic amines were tested in the
that stabilises the reaction intermediates and increases the rate of
condensation reaction. It was observed that the condensation of
86
reaction. Therefore, methanol was used for further study of the
dicarbonyl compound with 2, 5-dimethyl aniline produce a very low
catalytic reaction. The reaction was also performed under solvent
yield of product (75%) even when the reaction was carried out for a
free conditions to investigate the influence of the solvent
longer reaction time. This lower yield of β-enaminones by the
parameter, but traces of the product were only found (Table 4,
condensation reaction can be attributed to the steric hindrance
entry 8). This observation indicated that solvent plays an important
between two adjacent methyl groups (Table 6, entries 11). Amine
role in the condensation reaction in which bimetallic Ag1-Cu3
having cyclic group such as cyclohexyl amine gave good yield in the
nanoparticles promote the mass diffusion and transport of
condensation reaction (Table 6, entry 23). Substituted aromatic
reactants.69 Later on, the amount of catalyst loading was varied
amines having electron donating substituent were more reactive
from 5 mg to 25 mg. It was observed that the yield of the reaction
and provide a better yield of the corresponding condensed product.
involving condensation of 1 mmol of acetylacetone and 1 mmol of
The condensation of amines to other dicarbonyl compounds such as
aniline enhances upto catalyst dose of 20 mg. Further increasing the
ethyl acetoacetate, methylacetoacetate, and benzoyl acetone were
catalyst dose only marginally affects the yield of the product (Table
also studied (Table 6, Entry 1-12, 26-38, 39-40). Among all, the
5, entries- 1-5). Best result with respect to yield was obtained in 20
minimum yield was observed when benzoyl acetone, is used as β-
mg of Ag1-Cu3 catalyst amount (Table 5, entry 4). Pure Ag and Cu
dicarbonyl compound in the optimized condition. In benzoyl
nanoparticles showed very low activity (yield of 30 % and 50 %,
acetone, the presence of electron withdrawing group (-COC6H5)
respectively) with same catalyst loading (Table 5, entry 6-7). Hence,
generates weak acidic protons compared to its ester counterparts.
the catalyst amount of 20 mg was utilised for further catalytic
Therefore, less yield of product was obtained in case of benzoyl
studies. Finally, the influence of temperature on the activity of Ag1-
acetone.
Cu3 catalyst for the model reaction was studied varying the
temperature from room temperature to reflux condition. As shown
in Fig. 8, it can be observed that on increasing the temperature
yield of the desired product initially increased. On further increasing
the temperature i.e. at reflux condition, the yield of the product
significantly decreased. Based on these findings, the optimum
From all the catalysis data it can be concluded that bimetallic
nanoparticles of different ratios showed more activity in
comparison to their monometallic counterparts. For bimetallic Ag-
Cu nanoparticles, the electron density on the surface is higher than
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that of monometallic nanoparticles because electron transfers from Ag1-Cu3 bimetallic catalyst after 4 cycles. The XRD spectra reveal the
Cu to Ag owing to higher redox potential of the later. The core level existence of crystalline nature of the catalyst after the catalytic
peak shifts of Ag and Cu in the XPS analysis clearly suggest this cycle. Further, the morphology of the reused catalyst was
electron transfer, resulting in lower electron density of Cu in investigated by TEM analysis (Fig. S2a) which shows not much
bimetallic Ag-Cu nanoparticles. This synergistic electronic effect change in morphology but some agglomeration. This possibly was
behaviour enhances the catalytic activity in the bimetallic Ag-Cu one of the reason for decrease in yield of the products. EDX analysis
nanoparticles. In addition, the chemisorption strength (binding shows the surface composition of the bimetallic nanoparticle after
energy) of organic moieties plays an important role in controlling reusability (Fig. S2b). EDS mapping of reused catalyst (Fig. S2c)
the catalytic activity that depends on the d-band centre of the shows the presence of both the metal which confirms the stability
metal surface. According to Hammer-Norskov model, d-band centre of the NPs after four cycles of catalytic reaction. The minimal loss of
of Ag locates at -3.9 eV whereas Cu locates at -2.4 eV. 89, 90,91 When yield after each cycle can further be attributed to the fact that after
the organic moiety (diketones and amine) interact with the d-band each cycle minimal amount of catalyst was lost during the catalyst
of Ag and Cu, overlap of the adsorbate state with the metal state regeneration process.
happens. In the case of monometallic Ag and Cu nanoparticles, this
overlap results in weak binding, thereby lowering the activity of the
3.4. Comparison with reported results
catalyst. However, in the case of bimetallic Ag-Cu nanoparticles,
alloy structure is formed (confirmed by HRTEM/ EDS and XPS) due
to the intimate contact between Ag and Cu. This generates a
stronger binding energy and higher activity in the case of bimetallic
Ag-Cu nanoparticles. Therefore, the structural effects and
composition effects in bimetallic nanoparticles play an important
role in its enhanced catalytic performance.
In order to investigate the merit of our catalyst, we have compared
the efficiency of other reported catalysts with our catalyst for the
synthesis of β-enaminones and β-enaminoesters which are
presented in Table 7. It can be found that our catalyst shows higher
activity with minimal amount of catalyst and less interval of time
compare to other catalysts such as Cu NPs,88 Ag NPs,95 Ag NPs in
Based on the observations discussed above, a plausible Ag1-Cu3
catalysed reaction mechanism for the condensation reaction is
proposed in Scheme 2. The condensation product generally forms
through an addition-elimination reaction. Ag1-Cu3 nanoparticles co-
ordinates to the carbonyl oxygen of the enol form of acetylacetone
(enol form of acetyl acetone is more stable than the diketone),
followed by the addition of aniline.92 This generates a tetrahedron
intermediate after passing through a four-membered transition
state (II-IV) which further undergoes a water elimination reaction to
yield the imine moiety. The final product β-enaminones form after
the tautomerization of the imine intermediate. Overall, the
nanoparticles play an important role in catalyzing/activating the
reaction by properly binding or co-ordinating with the organic
substrate, resulting in lowering of the energy barrier required for all
the intermediate steps in the formation of β-enaminones and β-
enaminoesters. Similar observation has previously been
documented for the binding of Cu nanoparticle on organic
substrates. 93, 94
96
hollow magnetic spheres,34 and Zn(oAc)2.H2O. Therefore, these
results show that Ag1-Cu3 NPs system is more effective for the
synthesis of fine chemicals such as β-enaminones and β-
enaminoesters.
4. Conclusions
In conclusion, for the first time we have demonstrated the use of
bimetallic Ag-Cu nanoparticles synthesized by a simple co-reduction
method as efficient catalyst for β-enaminones and β-enaminoesters
synthesis. Among various ratios of bimetallic Ag-Cu nanoparticles,
bimetallic Ag1-Cu3 has shown highest activity for the desired
products. Several comprehensive techniques including HRTEM, EDS
line scan, EDS mapping, valence band and core level XPS indicated
the alloy formation of nanoparticles. XRD and XPS measurement
showed that Cu atom in the Ag-Cu bimetallic nanoparticles were
less oxidized. It was found that bimetallic Ag1-Cu3 can be employed
for different dicarbonyl compounds and amines. Furthermore, the
Ag1-Cu3 catalyst has stable catalytic activity, evidenced by the
reusability of the catalyst for four runs with minimal loss in activity.
This work should shed light on the design of more effective
bimetallic nanocatalyst for enamination reactions.
3.3. Reusability of catalyst
The catalyst stability was examined by conducting catalytic tests on
Ag1-Cu3 catalyst under the optimized conditions. After the
completion of the reaction the NPs were filtered from the reaction
mixture and washed using a mixture of distilled water and ethanol
and dried in schlenk line for the other cycle of reaction. The model
reaction of acetyl acetone and aniline was carried out consecutively
upto 4 times by using recycled catalyst under identical conditions.
The catalyst showed good reactivity upto 4 cycles without any
significant loss of activity and yield of the desired product (from
95% to 88 %) (Fig. 9). Fig. S1 shows the XRD spectra of the reused
Acknowledgments
The authors are thankful to Department of Science & Technology
(DST), Govt. of India, for funding. XPS facility at IIT Delhi is partially
funded by FIST grant of DST, India.
6 | J. Name., 2012, 00, 1-3
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28.
N. D. Koduri, Z. Wang, G. Cannell, K. Cooley, T. M. Lemma,
K. Miao, M. Nguyen, B. Frohock, M. Castaneda, H. Scott,
D. Albinescu and S. R. Hussaini, J. Org. Chem., 2014, 79,
7405–7414
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List of Figures
Scheme 1
One pot synthesis of β-enaminones and β-enaminoesters using Ag1-Cu3 bimetallic nanoparticle catalyst.
Scheme 2
Plausible mechanism for synthesis of β-enaminones and β-enaminoesters.

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List of tables
Table 1. XRD data for lattice parameters.
Table 2. Compositional analysis (atomic ratio) of Ag1-Cu3 alloy nanoparticle.
Table 3. Comparison of different catalysts for condensation of the model reaction. a
Table 4. Effect of solvents on Ag1-Cu3 nanoparticles-catalyzed synthesis of β-enaminones and β-enaminoesters.a
Table 5. Effect of catalyst dose on Ag1-Cu3 nanoparticles-catalyzed synthesis of β-enaminones and β-enaminoesters. a
Table 6. Synthesis of β-enaminones and β-enaminoesters using Ag1-Cu3 bimetallic nanoparticles.a
Table 7. Comparison study with other available catalysts in literatures.
Table 1.
Peak position (2Ɵ)
Lattice parameter (Å)
[111]
[200]
44.1
d 111
d 200
Ag
Cu
38
2.32
2.08
1.98
43.1
49.5
1.76
Table 2.
Sample
Ag
Cu
Ag:Cu ratio
Ag:Cu 1:3
26.4
73.6
1:2.8
Table 3.
Entry
Catalyst
Solvent
Temp.(°C)
Catalyst
Yield b (%)
Loading (mg)
1.
2.
3.
4.
5.
6.
Without catalyst
Ag NPS
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
60
60
60
60
60
60
20
traces
30
20
20
20
20
20
Cu NPs
50
Ag3Cu1
85
Ag1Cu3
95
Ag1Cu1
92
a Reaction conditions: Acetyl acetone (1 mmol), aniline (1 mmol), Catalyst loading (20 mg), methanol (5 mL),
and temperature (60 °C) Time-1 h 45 min.
b Isolated and unoptimized yield.

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Table 4.
Entry
1.
Solvent
Yield b (%)
Ethanol
86
2.
3.
4.
5.
6
Methanol
Acetonitrile
DCM
95
93
80
THF
89
Toluene
Hexane
77
7
60
8
Solventless
Traces
a Reaction conditions: dicarbonyl compound (1 mmol), amine (1 mmol), Ag1-Cu3 NPs (20 mg), and temperature
(60 °C), Time-1 h 45 min.
b Isolated and unoptimized yield.
Table 5.
Entry
Solvent
Catalyst
Catalyst loading
Temperature
Yield b (%)
(mg)
5
1.
2.
3.
4.
5.
6.
7.
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Ag1-Cu3 NPs
Ag1-Cu3 NPs
Ag1-Cu3 NPs
Ag1-Cu3 NPs
Ag1-Cu3 NPs
Ag NPs
60 °C
60 °C
60 °C
60 °C
60 °C
60 °C
60 °C
58%
72%
10
15
20
25
20
20
88 %
95 %
95.5 %
30 %
50 %
Cu NPs
a
Reaction conditions: Acetyl acetone (1 mmol), aniline (1 mmol), methanol (5 mL), Time- 1h 45 min and
temperature (60 °C).
b Isolated and unoptimized yield.
Table 6.
Entry
R
Amine (R1)
Product
Time
Yield b
%
O
HN
1
OC2H5
CH3
1 h
91
H3CH2CO

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O
HN
2
OC2H5
C3H7
2 h
90
H3CH2CO
O
HN
3
4
OC2H5
OC2H5
C4H9
C6H5
1 h 10 min
2 h 30 min
88
93
H3CH2CO
O
HN
H3CH2CO
O
O
5
6
7
8
9
OC2H5
OC2H5
OC2H5
OC2H5
OC2H5
85
88
75
82
91
1 h 45 min
O
N
N
H
H3CH2CO
H2N
O
HN
1 h 30 min
H3CH2CO
NO2
OCH3
CH3
PhNO2
2 h 15 min
O
HN
H3CH2CO
H3CH2CO
H3CH2CO
C7H9NO
2 h
O
HN
NH2
1 h 45 min
O
HN
CH3
NH2
OC2H5
10
11
1 h
92
72
O
HN
H3CH2CO
NH2
OC2H5
6 h
O
HN
H3CH2CO

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NH2
O
HN
12
13
OC2H5
2h
83
93
H3CH2CO
O
HN
CH3
CH3
45 min
O
HN
14
15
CH3
CH3
C2H5
45 min
92
92
O
HN
C3H7
1 h 15 min
O
HN
16
17
CH3
CH3
C4H9
C6H5
45 min
91
1 h 45 min
95 c
O
HN
O
O
18
CH3
1 h
85
90
O
O
N
N
H
H2N
19
20
CH3
HN
50 min
NO2
CH3
PhNO2
2 h 45 min
2 h 30 min
82
85
O
HN
OCH3
21
22
CH3
C7H9NO
O
HN
NH2
CH3
CH3
2 h
90
O
HN
CH3

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NH2
CH3
CH3
CH3
23
24
O
HN
45 min
95
75
NH2
5h
O
HN
NH2
25
26
27
1h 30 min
2h 30 min
2 h 15 min
85
92
90
O
O
HN
HN
OCH3
OCH3
C2H5
H3CO
O
HN
C3H7
H3CO
O
HN
28
29
OCH3
OCH3
C4H9
C6H5
45 min
1 h
91
90
H3CO
O
HN
H3CO
O
O
30
31
OCH3
1 h 30 min
88
91
O
N
N
H
H3CO
H2N
OCH3
1 h 45 min
O
HN
H3CO
NO2
32
OCH3
PhNO2
3 h
80
O
HN
H3CO

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OCH3
33
34
OCH3
C7H9NO
2 h 30 min
87
91
O
HN
H3C
O
NH2
OCH3
CH3
1 h 45 min
O
HN
CH3
H3CO
35
OCH3
NH2
1h 30 min
90
O
HN
H3CO
NH2
36
OCH3
OCH3
O
HN
6h 30 min
70
H3CO
37
38
NH2
3h
80
80
O
HN
H3CO
O
HN
Ph
Ph
C4H9
2 h
39
C6H5
3 h
85
O
HN
H2N
O
HN
40
Ph
2 h 15 min
83
a Reaction conditions: dicarbonyl compound (1 mmol), amine (1 mmol), Ag1-Cu3 NPs (20 mg), methanol (5 mL),
and temperature (60 °C).
b Isolated and unoptimized yield.
c In this reaction catalyst was recycled for four consecutive cycles which showed the same activity without any
significant loss of yield.

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Table 7.
S.No
Catalyst
Product
Time
Catalyst Catalyst Yield %
amount amount
Particle
size
Ref.
(mol %)
(mg)
1
2
Cu NPs
2.50 h
10
-
92
90
98
86
95
20 nm
77
O
O
O
O
O
HN
Ag NPs
8 h
10
-
40 nm
10 nm
-
83
32
84
HN
HN
HN
HN
3
4
5
Ag NPs in
hollow
magnetic
spheres
8 h
-
31
Zn(oAc)2.H2O
48 h
5
-
Ag1-Cu3 NPs
1 h 45
min
10
20
8 nm
This
work
List of Figures
Fig.1. UV-vis spectrum of (a) Ag, (b) Cu, (c) CuSO4.5H2O,(d) Ag1Cu1, (e) Ag1-Cu3 and (f) Ag3-Cu1 bimetallic NPs .
Fig.2. XRD pattern of (a) Ag NPs (b) Cu NPs (c) Ag1Cu1 NPs (c) Ag3Cu1 NPs and (c) Ag1Cu3 NPs.
Fig.3. (a) TEM micrograph of Ag1-Cu3 NPs with size distribution curve of Ag1-Cu3 NPs inset (b) SAED pattern of Ag1-Cu3
bimetallic NPs, (c) HRTEM image of Ag1-Cu3 NPs and (d) EDX spectra of Ag1-Cu3 bimetallic NPs.
Fig.4. (a-d) HAADF-STEM-EDS mapping of Ag1-Cu3 NPs (e) EDS line scan of Ag1-Cu3 bimetallic NPs.

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Fig.5. (a) XPS survey spectra of Ag1-Cu3 bimetallic nanoparticle, (b) Ag 3d core level XPS spectra of Ag1-Cu3 bimetallic
nanoparticle, (c) Cu 2p core level XPS spectra of Ag1-Cu3 bimetallic nanoparticle.
Fig.6. X-ray photoelectron spectra of the valence band of Ag1-Cu3 bimetallic nanoparticles
.
Fig.7. Catalytic activities of monometallic and bimetallic nanoparticle over series of catalytic reaction.
Fig.8. Effect of temperature on percentage yield of enaminoesters synthesized by one pot condensation of
ethylacetoacetate and methyl amine.
Fig.9. Reusability study of Ag1-Cu3 NPs bimetallic nanoparticle catalyst.
Fig.1.

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Fig.2.
Fig.3.

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Fig.4.

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Fig.5.
Fig.6.

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Fig.7.
Fig.8.

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Fig.9.

Page 23 of 23
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Bimetallic AgꢀCu alloy nanoparticles were obtained via a simple coꢀreduction method, which exhibits significantly
improved activity for synthesis of βꢀenaminones and βꢀenaminoesters over their monometallic counterparts.