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

Abstract

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.

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1. Introduction

In recent years, nanoparticles (NPs) of noble metals in a size range of 1–10 nm have attracted much attention in the field of catalysis due to their excellent physical and chemical properties.¹ Catalysis over metal NPs is of great interest as their size, structure and composition can enable tuning of the activity and selectivity of the catalyst. It is known that the catalytic properties of NPs can be changed by the 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, which can be attributed to ensemble and electronic effects.^3–5 Until now, various methods have been utilized to synthesize metal NPs such as γ-ray radiolysis,⁶ evaporation–condensation,⁷ a galvanic replacement reaction,⁸ ethylene glycol methods^9,10 and co-reduction.¹¹ Many bimetallic NPs, such as Au–Cu,¹² Pd–Cu,¹³ Fe–Cu,¹⁴ Ni–Cu,¹⁵ Pd–Au,¹⁶ Pt–Pd,¹⁷ Pt–Co¹⁸ and Pt–Ni,¹⁹ have been employed for aerobic oxidation,¹² the 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 reports in the literature have shown that bimetallic nanoparticle catalysts based on Cu exhibited a much-enhanced activity compared to monometallic Cu NPs in catalytic reactions. For example, Cu–Au NPs enhanced the reduction of carbon dioxide,²⁰ Cu–Co NPs acted as a potential catalyst for higher alcohol synthesis,²¹ Cu–Ag performed as an effective catalyst for the oxidation of methanol to CO₂ (ref. 22) and for the hydrolytic dehydrogenation of ammonia borane²³ and bimetallic Cu–Pd was used as an active catalyst for formic acid oxidation.²⁴

The enamination of 1,3-dicarbonyl compounds to form β-enamino ketones and esters synthesized from low-cost raw materials with a very stable structural pattern is considered to be a very important process as these are valuable precursors in organic synthesis.^25,26 These compounds are utilized as significant intermediates for the preparation of various heterocyclic compounds.^27,28 They are also known to have medicinal applications as anticonvulsants^29,30 and anti-inflammatory^31,32 and antitumor agents.³³ Because of their extensive variety of action and potency, a straightforward and high yielding one-pot approach for the synthesis of β-enaminones and β-enaminoesters is highly desirable. The environmentally benign protocol used for the synthesis of β-enaminones is found to be a time-consuming method.³⁴ Several improved procedures have been reported for the reaction between dicarbonyl compounds and amines using catalysts such as metal triflates by microwave and ultrasound irradiation.^35–38 Other synthetic processes to yield β-enaminones and β-enaminoesters includes the cyclization of amino acids, the acylation of amines and condensation reactions.³⁹ The synthesis of some fine chemicals has been reported using different catalysts such as Yb(OTf)₃,⁴⁰ perchlorates,⁴¹ [Hbin]Tf],⁴² HClO₄–SiO₂,⁴³ montmorillonite K10,⁴⁴ silica gel,⁴⁵ natural clays,⁴⁶ silica chloride,⁴⁷ InBr₃ ⁴⁸ and CoCl₂.⁴⁹ All the methods discussed so far have their own drawbacks and limitations such as moisture sensitivity and lengthy workups;⁴⁰ the requirement of harsh conditions and the use of harmful reagents;⁴¹ the use of homogenous⁴⁸ or non-recyclable catalysts;⁴⁹ and longer reaction times and low yields.⁴⁹ Hence, there is sufficient scope for the development of a heterogeneous and reusable catalytic system that is able to catalyse the synthesis of β-enaminones and β-enaminoesters at milder operating conditions.

In recent years, transition metal NPs have been used for various organic transformations due to their high surface area and availability of numerous co-ordination sites.^2,50 However, these catalysts still suffer from complicated synthesis protocols and low 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 bimetallic catalysts will have a high potential for applications in fine chemical synthesis. Moreover, bimetallic NPs may also provide better stability and functionality at a lower cost.^51,52 In this regard, the design of bimetallic NPs 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. To the best of our knowledge, the use of bimetallic NPs 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 NPs via a low-temperature simple co-reduction method. The composition of the Ag–Cu NPs was tuned by changing the ratio of the metal precursors (3 : 1, 1 : 1 and 1 : 3). The formation of the bimetallic Ag–Cu NPs was examined using energy dispersive spectroscopy (EDS) measurements, which helps to determine the distribution of the Ag and Cu components in the bimetallic structure. More importantly, the change in the electronic properties of the bimetallic NPs due to alloying has been studied using core and valence band (VB) X-ray photoelectron spectroscopy (XPS) analysis. Later on, these bimetallic nanoparticles were used as efficient catalysts for the one pot synthesis of β-enaminones and β-enaminoesters. Compared with monometallic NPs, the bimetallic Ag₁–Cu₃ NPs exhibited a much enhanced activity, providing a high yield of the desired product in less time. In this study, various amine precursors, such as cyclic and branched amines with bulkier groups, were employed for the synthesis of β-enaminones and β-enaminoesters. The catalyst was found to be recyclable up to 4 cycles without any significant loss of activity or yield of desired product. We envision that the current strategy will provide useful clues for the design of novel bimetallic NPs and bimetallic NP-based systems as potential catalysts for other fine chemical syntheses.

2. Experimental

2.1. Materials

Silver nitrate, AgNO₃ (A.R. Grade, 98% pure), and sodium borohydride, NaBH₄, were purchased from Sigma-Aldrich and copper sulphate, CuSO₄·5H₂O (A.R. Grade), was purchased from HiMedia. All other chemicals were purchased from CDH chemicals and were used without further purification. Thin-layer chromatography (TLC) plates (20 × 20, 0.2 mm) were purchased from SD-fine chemicals. Column chromatography was carried out with the use of silica gel (100–200 mesh), purchased from HiMedia. A cellulose dialysis membrane (diameter 17.5 mm) was purchased from HiMedia. 18 MΩ Milli-Q water was used throughout the synthesis.

2.1.1. Preparation of Ag–Cu NPs via a chemical reduction method. Ag₁–Cu₃ bimetallic NPs were synthesized by a co-complexion method using AgNO₃ (0.11 mmol, 5 mL) and CuSO₄·5H₂O (0.35 mmol, 5 mL) as precursors. NaBH₄ (4.6 mmol, 10 mL) was used as the 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 and 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 conditions under nitrogen atmosphere. To this mixture, NaBH₄ was added dropwise for 30 min. The whole reaction was stirred further for 2 hours under nitrogen atmosphere. A dark green precipitate was obtained, which was further centrifuged and dried in a Schlenk line. Then the NPs were dialyzed overnight to remove excess salts before further use. Ag, Cu and the other Ag–Cu NPs were prepared using the same procedure as above, keeping the total metal salt concentration constant.

2.2. Characterization of the catalysts

The catalysts were analyzed with powder X-ray diffraction (XRD) (RIGAKU JAPAN/ULTIMA-IV) using Cu Kα radiation (λ = 0.154 nm) in the 2θ range of 20–80°. XRD was used to determine the crystal phase of the prepared catalysts. The ultraviolet-visible (UV-Vis) absorbance spectra were obtained on a UV-2450, Shimadzu. Transmission electron microscopy (TEM) analysis of the sample was carried out using PHILIPS CM 200 equipment with carbon coated copper grids. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images were taken with a Bruker TEM/STEM operated at 200–300 keV. ¹H nuclear magnetic resonance (NMR) and ¹³C NMR spectra were recorded on a Bruker spectrometer at 400 MHz using tetramethylsilane (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) were used to perform XPS measurements with a base pressure of <1 × 10⁻⁹ mbar. Catalytic reactions were monitored by TLC on 0.2 mm silica gel F-254 plates. All the reaction products are known compounds and have been identified by comparing their physical and spectral characteristics with the literature reported values.

2.3. The typical method for the synthesis of β-enaminones and β-enaminoesters

In a 100 mL round bottom flask, the dicarbonyl compound (1 mmol), the amine (1 mmol) and the Ag–Cu bimetallic NPs (20 mg) were added to 5 mL of methanol and stirred at 60 °C for the desired time given in Table 4. The progress of the reaction was monitored by 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 NPs were dried and collected for a 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.

3. Results and discussion

A co-reduction method was used to synthesize the monodisperse Ag–Cu bimetallic NPs in ice cold conditions under a nitrogen atmosphere. The final NPs were dialysed overnight to make the NPs free from impurities. In order to fabricate the alloy NPs (i.e. to ensure the coexistence of Ag and Cu in each nanoparticle), the possible effect of the reducing agent was taken into consideration. This is because the standard electrode potential of Ag⁺/Ag⁰ (0.78 eV) is higher than that of Cu²⁺/Cu⁰ (0.34 eV). Therefore, Ag⁺ is reduced more rapidly than Cu²⁺. This was avoided by the addition of an excess of the reducing agent (NaBH₄), which helps in the simultaneous reduction of Cu²⁺ 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 NPs as synthesized above, various characterization techniques, such as UV-Vis, XRD, high-resolution transmission electron microscopy (HRTEM) with EDS mapping/line scans and XPS, were carried out as discussed below.

3.1. Characterizations of the synthesized NPs

Fig. 1 shows the changes in the absorption spectra during the formation of the monometallic and bimetallic NPs. A strong absorption peak at about 400 nm for the silver NPs was found, which is due to the surface plasmon absorption (Fig. 1a).⁵³ For comparison, the UV-Vis spectra of CuSO₄·5H₂O and monometallic Cu NPs are also shown (Fig. 1b). It has been found that only weak absorption peaks exist >600 nm for CuSO₄·5H₂O (Fig. 1c), whereas monometallic Cu particles show a surface plasmon resonance (SPR) band with a peak at ∼560 nm.^54,55 The Cu NPs displayed a monotonic spectrum, increasing exponentially towards shorter wavelengths.² The bimetallic Ag–Cu NPs exhibit an absorption peak at 415–440 nm (Fig. 1d–f), which is in between the monometallic Ag and Cu NPs. The red shift of the surface plasmon adsorption peaks from 410 nm to 520 nm was attributed to the increase in the Cu content in the Ag–Cu alloy NPs.⁵⁶ The above results indicate the co-existence of two elements in one particle and the formation of an alloy structure.² To further confirm this, the Ag–Cu bimetallic NPs were examined by XRD, HRTEM with EDS mapping and EDS line scans.

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 shows the XRD results of the synthesized monometallic Ag, Cu and bimetallic Ag–Cu NPs. The XRD analysis was conducted with a fully dried powder of the NPs. The prominent peak in the XRD pattern indicates that the samples obtained were of high crystallinity. Major peaks at 2θ values of 38.5°, 44.5°, 64.7° and 77.7° for the Ag NPs correspond to the lattice planes of (111), (200), (220) and (311) of the metallic Ag (JCPDS card no. 03-0931), respectively. In the case of the copper NPs, diffraction peaks formed at the 2θ values of 43.5°, 50.7° and 74.4° correspond to the lattice planes of (111), (200) and (220) from metallic Cu (JCPDS card no. 02-1225). In bimetallic Ag–Cu NPs, all the above Ag and Cu diffraction peaks were observed, suggesting that the bimetallic NPs consist both of Ag and Cu phases. It is important to note that the typical oxide peaks 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, the lattice parameters were obtained and are 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. Taking the (111) reflection of the XRD spectra, the average crystallite size was calculated using the Debye–Scherrer formula.^58,59 The crystallite size of the Ag₁–Cu₃ bimetallic NPs was found to be around 10 nm, which nearly matches with the particle size obtained from the TEM analysis discussed later.

Fig. 2 XRD pattern of (a) Ag NPs, (b) Cu NPs, (c) Ag₁Cu₁ NPs, (d) Ag₃Cu₁ NPs and (e) Ag₁Cu₃ NPs.

Table 1 XRD data for the lattice parameters

	Peak position (2θ)		Lattice parameter (Å)
	[111]	[200]	d111	d200
Ag	38	44.1	2.32	1.98
Cu	43.1	49.5	2.08	1.76

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The morphology and structural behaviour of the Ag₁–Cu₃ catalyst (chosen because of its higher catalytic activity, as described in the catalysis section) were analysed by TEM. Fig. 3a shows the representative TEM image, which shows that the product consists of uniform spherical NPs. The histogram (inset) shows an average size of 7.5 ± 1 nm for the NPs. 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 the alloy nanostructure, HRTEM and EDS analysis was carried out. The HRTEM image of the 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 the (111) plane of Ag, while the lattice distance of 0.20 nm corresponds to the (111) plane of Cu. The EDS analysis of the Ag₁–Cu₃ NPs shows the presence of Ag and Cu with a concentration of 25.8 and 74.2%, respectively (Fig. 3d). All the above results confirm the successful synthesis of the bimetallic Ag₁–Cu₃ NPs.

Fig. 3 (a) TEM micrograph of the Ag₁–Cu₃ NPs with the size distribution curve of Ag₁–Cu₃ NPs shown in the inset. (b) SAED pattern of the Ag₁–Cu₃ bimetallic NPs, (c) HRTEM image of the Ag₁–Cu₃ NPs and (d) EDS spectra of the Ag₁–Cu₃ bimetallic NPs.

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 NPs. In order to obtain conclusive confirmation of the alloy structure of the Ag₁–Cu₃ NPs, HAADF-STEM, EDS elemental mapping patterns and EDS line scanning profiles were performed and are shown in Fig. 4. The HAADF-STEM image (Fig. 4a) shows a clear luminance between Ag and Cu, suggesting an alloy structure of the as-prepared Ag₁Cu₃ NPs rather than a core–shell structure of Ag and Cu.^61–63 EDS elemental mapping patterns further revealed that Ag and Cu elements are uniformly distributed throughout the whole bimetallic Ag₁Cu₃ nanostructure, indicating the formation of an 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 NPs.^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 the respective slats taken during the synthesis. This further shows the high compositional uniformity of the bimetallic NPs.

Fig. 4 (a–d) HAADF-STEM-EDS mapping of the Ag₁–Cu₃ NPs and (e) EDS line scan of the Ag₁–Cu₃ bimetallic NPs.

We performed XPS measurements to determine the chemical composition and oxidation state of the Ag₁–Cu₃ NPs. A survey scan, as shown in Fig. 5a, revealed the presence of elemental Ag, Cu and O in the sample. In Fig. 5b and c, we show the core-level spectra of Ag 3d and Cu 2p, respectively. The binding energy (BE) values of the Ag 3d_5/2 and Ag 3d_3/2 core levels for the Ag₁–Cu₃ NPs appear at 367.1 and 373.3 eV, respectively, which are associated with pure Ag NPs. The 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 2p_3/2 and Cu 2p_1/2, respectively. These values are found to be consistent with those reported for Cu(0).⁷¹ Both the Ag and Cu data suggested the metallic nature of Ag and Cu in the Ag₁–Cu₃ bimetallic NPs, which is consistent with the XRD and TEM analysis. Though the Cu 2p spectrum demonstrated that most of the Cu exists in the form of the metallic Cu(0) (931.0 eV and 950.7 eV) state, the presence of a 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 the CuO phase, XPS analysis demonstrates the surface presence of Cu²⁺ ions, which suggests that CuO is present only on the surface.⁷⁵ Similar characteristic XPS spectra containing shakeup satellite peaks were also reported in the 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 the area under the curve, we calculated the surface composition of Ag : Cu, which was found to be 1 : 2.8 (Table 2), very close to the original value of 1 : 3. This further suggests the oxidation stability of the Ag–Cu alloy NPs, along with the presence of significantly smaller amounts of CuO, possibly formed due to the chemisorption of small amounts of O₂ on the nanoparticle surface. Due to this, the core-level peaks of Ag 3d_5/2 and Cu 2p_3/2 were found to be asymmetric towards the higher BE side. In addition, it can be observed that the BE of the Ag 3d_5/2 and Cu 2p_3/2 core level peak position in the bimetallic Ag₁–Cu₃ NPs 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 alloy formation occurs, which was explained in terms of the intra-atomic charge re-distribution caused by valence electron hybridization.⁷⁶ Therefore, the observed shift in the Ag 3d_5/2 and Cu 2p_3/2 core-levels suggests the alloy formation of the Ag₁–Cu₃ NPs. Also, these peak shifts in the XPS spectra further suggest the possible electronic interaction (synergistic effect) between Ag and Cu in the Ag₁–Cu₃ bimetallic nanoparticle. This is because, in comparison to Cu, Ag has a higher redox potential, which leads to lower electron densities in the Cu atom in the bimetallic Ag₁–Cu₃ nanoparticle.

Fig. 5 (a) XPS survey spectra of the Ag₁–Cu₃ bimetallic NPs, (b) Ag 3d core level XPS spectra of the Ag₁–Cu₃ bimetallic NPs, (c) Cu 2p core level XPS spectra of the Ag₁–Cu₃ bimetallic NPs.

Table 2 Compositional analysis (atomic ratio) of the Ag₁–Cu₃ alloy NPs

Sample	Ag	Cu	Ag : Cu ratio
Ag : Cu 1 : 3	26.4	73.6	1 : 2.8

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Besides the shift in the core-level peak positions in the metal NPs, the changes in the VB spectra are crucial to understand the formation of alloy NPs.⁷⁷ Therefore, we have recorded the XPS VB spectra of the 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 the 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 deconvoluted 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 NPs reported in the literature, further demonstrating the formation of the alloy Ag₁–Cu₃ nanoparticle.² Moreover, there is a shift in the VB peak position of 1.5 and 0.5 for Cu 3d and Ag 3d, respectively, as compared to their bulk values (Cu = 3.5 and Ag = 5 eV).^78,79 This shift of VB energy indicates the charge separation behaviour of Ag and Cu and suggests the possibility of an electronic interaction between Ag and Cu in the nanoparticle alloy formation.^78,80,81

Fig. 6 XPS of the VB of the Ag₁–Cu₃ bimetallic NPs.

3.2. Catalytic studies for the synthesis of β-enaminones and β-enaminoesters

The catalytic reactivity of the Ag–Cu bimetallic NPs was evaluated for the synthesis of β-enaminones and β-enaminoesters by a one-pot condensation of diketones and amines at 60 °C using methanol as a solvent. At first, the emphasis was placed on optimizing the reaction conditions. Condensation of acetylacetone and aniline served as the model reaction for optimization (Scheme 1). Initially, monometallic Ag and Cu and the different composition of Ag–Cu bimetallic NPs (1 : 1, 1 : 3 and 3 : 1) synthesized in this work were evaluated for their activity using the model reaction.

Scheme 1 One pot synthesis of β-enaminones and β-enaminoesters using a Ag₁–Cu₃ bimetallic nanoparticle catalyst.

It was observed that in comparison to monometallic NPs, bimetallic NPs of different ratios showed the best performance for the catalytic reaction, providing higher yields of the desired product (Table 3). Fig. 7 shows the catalytic activity of the bimetallic series of Ag–Cu NPs for the model reaction; bimetallic NPs that were rich in Cu (Ag₁–Cu₃) showed the highest activity. Such electronic enhancement effects in bimetallic nanoparticle catalysts have been previously documented in various Ag–Cu bimetallic catalysts and other bimetallic systems (Ag–Pd and Au–Pd).^82–85 Therefore, the Ag₁–Cu₃ nanocatalyst was chosen as the preferred catalyst for optimizing the reaction conditions. As shown in Table 3, entry 1, for the control experiment (without metallic NPs), only trace products were formed, confirming that almost no condensation takes place without metal NPs.

Table 3 Comparison of different catalysts for the condensation of the model reaction^a

Entry	Catalyst	Solvent	Temp. (°C)	Catalyst loading (mg)	Yield^b (%)
a Reaction conditions: acetyl acetone (1 mmol), aniline (1 mmol), catalyst loading (20 mg), methanol (5 mL), temperature (60 °C) and time (1 h 45 min).b Isolated and unoptimized yield.
1	Without catalyst	Methanol	60	20	Traces
2	Ag NPS	Methanol	60	20	30
3	Cu NPs	Methanol	60	20	50
4	Ag3Cu1	Methanol	60	20	85
5	Ag1Cu3	Methanol	60	20	95
6	Ag1Cu1	Methanol	60	20	92

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Fig. 7 Catalytic activities of the monometallic and bimetallic NPs over a series of catalytic reactions.

Further, the influence of various reaction parameters, such as solvent, temperature and catalyst loading, was studied by using the model reaction. In order to verify the effect of different solvent media on the condensation of the model reaction, solvents with different polarity, such as acetonitrile, tetrahydrofuran (THF), dichloromethane (DCM), hexane, ethanol and toluene, were used to optimize the reaction conditions (Table 4). It was noticed that for a nonpolar solvent, such as hexane, the yield of the product is much lower. The yield of the product was found to be higher when polar solvents such as methanol, ethanol, and acetonitrile were used. Among the various solvents tested, the highest yields of β-enaminones and β-enaminoesters were obtained in methanol solvent, which can be attributed to the interaction of the strong hydrogen bond in methanol that stabilises the reaction intermediates and increases the rate of reaction.⁸⁶ Therefore, methanol was used to further study the catalytic reaction. The reaction was also performed under solvent free conditions to investigate the influence of the solvent parameters, but only traces of the product were found (Table 4, entry 8). These observations indicated that the solvent plays an important role in the condensation reaction, in which bimetallic Ag₁–Cu₃ NPs promote the mass diffusion and transport of the reactants.⁶⁹ Later on, the amount of catalyst loading was varied from 5 mg to 25 mg. It was observed that the yield of the reaction involving the condensation of 1 mmol of acetylacetone and 1 mmol of aniline increases up to a catalyst dose of 20 mg. Further increasing the catalyst dose only marginally affects the yield of the product (Table 5, entries 1–5). The best result with respect to yield was obtained for 20 mg of Ag₁–Cu₃ catalyst (Table 5, entry 4). Pure Ag and Cu NPs showed very low activity (a yield of 30% and 50%, respectively) with the same catalyst loading (Table 5, entry 6–7). Hence, 20 mg of catalyst was utilised for further catalytic studies. Finally, the influence of temperature on the activity of the Ag₁–Cu₃ catalyst for the model reaction was studied varying the temperature from room temperature to reflux conditions. As shown in Fig. 8, it can be observed that upon increasing the temperature, the yield of the desired product initially increased. Upon further increasing of the temperature, i.e. at reflux conditions, the yield of the product significantly decreased. Based on these findings, the optimum reaction conditions were found to be, catalyst: Ag₁–Cu₃ bimetallic NPs, solvent: methanol, catalyst loading: 20 mg, and temperature: 60 °C.

Table 4 The effect of solvents on the Ag₁–Cu₃ NPs-catalyzed synthesis of β-enaminones and β-enaminoesters^a

Entry	Solvent	Yield^b (%)
a Reaction conditions: acetyl acetone (1 mmol), aniline (1 mmol), Ag1–Cu3 NPs (20 mg), temperature (60 °C) and time (1 h 45 min).b Isolated and unoptimized yield.
1	Ethanol	86
2	Methanol	95
3	Acetonitrile	93
4	DCM	80
5	THF	89
6	Toluene	77
7	Hexane	60
8	Solventless	Traces

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Table 5 Effect of the catalyst dose on the Ag₁–Cu₃ NPs-catalyzed synthesis of β-enaminones and β-enaminoesters^a

Entry	Solvent	Catalyst	Catalyst loading (mg)	Temperature	Yield^b (%)
a Reaction conditions: acetyl acetone (1 mmol), aniline (1 mmol), methanol (5 mL), time (1 h 45 min) and temperature (60 °C).b Isolated and unoptimized yield.
1	Methanol	Ag1–Cu3 NPs	5	60 °C	58
2	Methanol	Ag1–Cu3 NPs	10	60 °C	72
3	Methanol	Ag1–Cu3 NPs	15	60 °C	88
4	Methanol	Ag1–Cu3 NPs	20	60 °C	95
5	Methanol	Ag1–Cu3 NPs	25	60 °C	95.5
6	Methanol	Ag NPs	20	60 °C	30
7	Methanol	Cu NPs	20	60 °C	50

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Fig. 8 Effect of temperature on the percentage yield of enaminoesters synthesized by the one pot condensation of ethylacetoacetate and methyl amine.

After optimizing the reaction conditions, we further investigated the scope of the optimized protocol using different dicarbonyl compounds and different substituted aliphatic and aromatic amines. Excellent yields and a high purity of products were obtained in all cases (Table 6, entries 1–40). All the condensation reactions were completed within 45–180 min at 60 °C without formation of any side products. Acetylacetone was found to give a good yield of 95% with aniline (Table 6, entry 14). Aliphatic and alicyclic amines, such as methyl amine, ethyl amine, propyl amine, butyl amine, and morpholine, were found to give an excellent yield of the condensed product with acetylacetone using Ag₁–Cu₃ bimetallic NPs as the catalyst (Table 6, entries 13–16, and 18). It was found that the reaction with aliphatic amine proceeded smoothly in a short period of time as compared to aromatic amines, which can be attributed to the higher nucleophilicity of aliphatic amines than aromatic amines.⁸⁷ Later on, a variety of amines possessing a wide range of functional groups were selected for further studies. It was observed that the amines containing electron donating groups provided better yields in less time as compared to the amines containing electron withdrawing groups.⁸⁸ For example, anisidine and p-nitroaniline, containing the electron withdrawing groups OCH₃⁻ and NO₂⁻, respectively, have a strong deactivating effect, resulting in lower yields (Table 6, entries 20–21).

Table 6 Synthesis of β-enaminones and β-enaminoesters using the Ag₁–Cu₃ bimetallic NPs^a

Entry	R	Amine (R1)	Product	Time	Yield^b (%)
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, the catalyst was recycled for four consecutive cycles and showed the same activity without any significant loss of yield. Amine (R1) – entry (1–3, 13–16, 26–28, 38) alkyl amine, entry (4, 7–8, 17, 20–21, 29, 32–33, 39) aryl amine.
1	OC2H5	CH3		1 h	91
2	OC2H5	C3H7		2 h	90
3	OC2H5	C4H9		1 h 10 min	88
4	OC2H5	C6H5		2 h 30 min	93
5	OC2H5			1 h 45 min	85
6	OC2H5			1 h 30 min	88
7	OC2H5	PhNO2		2 h 15 min	75
8	OC2H5	C7H9NO		2 h	82
9	OC2H5			1 h 45 min	91
10	OC2H5			1 h	92
11	OC2H5			6 h	72
12	OC2H5			2 h	83
13	CH3	CH3		45 min	93
14	CH3	C2H5		45 min	92
15	CH3	C3H7		1 h 15 min	92
16	CH3	C4H9		45 min	91
17	CH3	C6H5		1 h 45 min	95^c
18	CH3			1 h	85
19	CH3			50 min	90
20	CH3	PhNO2		2 h 45 min	82
21	CH3	C7H9NO		2 h 30 min	85
22	CH3			2 h	90
23	CH3			45 min	95
24	CH3			5 h	75
25	CH3			1 h 30 min	85
26	OCH3	C2H5		2 h 30 min	92
27	OCH3	C3H7		2 h 15 min	90
28	OCH3	C4H9		45 min	91
29	OCH3	C6H5		1 h	90
30	OCH3			1 h 30 min	88
31	OCH3			1 h 45 min	91
32	OCH3	PhNO2		3 h	80
33	OCH3	C7H9NO		2 h 30 min	87
34	OCH3			1 h 45 min	91
35	OCH3			1 h 30 min	90
36	OCH3			6 h 30 min	70
37	OCH3			3 h	80
38	Ph	C4H9		2 h	80
39	Ph	C6H5		3 h	85
40	Ph			2 h 15 min	83

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In addition, other bulkier and cyclic amines were tested in the condensation reaction. It was observed that the condensation of dicarbonyl compounds with 2,5-dimethyl aniline produce a very low yield of product (75%) even when the reaction was carried out for a longer reaction time. This lower yield of β-enaminones by the condensation reaction can be attributed to the steric hindrance between the two adjacent methyl groups (Table 6, entry 11). Amines with cyclic group such as cyclohexyl amine gave good yields in the condensation reaction (Table 6, entry 23). Substituted aromatic amines with electron donating substituents were more reactive and provide a better yield of the corresponding condensed product. The condensation of amines to other dicarbonyl compounds, such as ethyl acetoacetate, methylacetoacetate and benzoyl acetone, were also studied (Table 6, entry 1–12, 26–37 and 38–40). Among all the entries, the minimum yield was observed when benzoyl acetone was used as the β-dicarbonyl compound in the optimized condition. In benzoyl acetone, the presence of the electron withdrawing group (–COC₆H₅) generates weak acidic protons compared to its ester counterparts. Therefore, a lower yield of product was obtained in the case of benzoyl acetone.

From all the catalysis data, it can be concluded that bimetallic NPs of different ratios showed more activity in comparison to their monometallic counterparts. For the bimetallic Ag–Cu NPs, the electron density on the surface was higher than that of the monometallic NPs because of electron transfers from Cu to Ag owing to higher redox potential of the later. The core level peak shifts of Ag and Cu in the XPS analysis clearly suggest this electron transfer, resulting in a lower electron density of Cu in the bimetallic Ag–Cu NPs. This synergistic electronic effect behaviour enhances the catalytic activity in the bimetallic Ag–Cu NPs. In addition, the chemisorption strength (BE) of the organic moieties plays an important role in controlling the catalytic activity that depends on the d-band centre of the metal surface. According to the Hammer–Norskov model, the d-band centre of Ag is located at −3.9 eV, whereas that of Cu is located at −2.4 eV.^89–91 When the organic moiety (diketones or amines) interacts with the d-band of Ag and Cu, an overlap of the adsorbate state with the metal state happens. In the case of the monometallic Ag and Cu NPs, this overlap results in weak binding, thereby lowering the activity of the catalyst. However, in the case of the bimetallic Ag–Cu NPs, an alloy structure is formed (confirmed by HRTEM-EDS and XPS) due to the close contact between Ag and Cu. This generates a stronger BE and higher activity in the case of the bimetallic Ag–Cu NPs. Therefore, the structural effects and composition effects in the bimetallic NPs play an important role in their enhanced catalytic performance.

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. The Ag₁–Cu₃ NPs co-ordinate to the carbonyl oxygen of the enol form of acetylacetone (the enol form of acetyl acetone is more stable than the diketone), followed by the addition of aniline.⁹² This generates a tetrahedral 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 NPs play an important role in catalyzing/activating the reaction by properly binding or co-ordinating with the organic substrate, resulting in the lowering of the energy barrier required for all the intermediate steps in the formation of β-enaminones and β-enaminoesters. A similar observation has previously been documented for the binding of Cu NPs on organic substrates.^93,94

Scheme 2 Plausible mechanism for the synthesis of β-enaminones and β-enaminoesters.

3.3. Reusability of catalyst

The catalyst stability was examined by conducting catalytic tests on the 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 a Schlenk line for the next reaction cycle. The model reaction of acetyl acetone and aniline was carried out consecutively up to 4 times by using the recycled catalyst under identical conditions. The catalyst showed a good reactivity up to 4 cycles without any significant loss of activity or yield of the desired product (from 95% to 88%) (Fig. 9). Fig. S1† shows the XRD spectra of the reused Ag₁–Cu₃ bimetallic catalyst after 4 cycles. The XRD spectra reveal the existence of the crystalline nature of the catalyst after the catalytic cycle. Further, the morphology of the reused catalyst was investigated by TEM analysis (Fig. S2a†), which shows not much change in the morphology but some agglomeration. This possibly was one of the reasons for a decrease in the yield of the products. EDS analysis shows the surface composition of the bimetallic nanoparticle after reuse (Fig. S2b†). EDS mapping of the reused catalyst (Fig. S2c†) shows the presence of both the metals, which confirms the stability of the NPs after four cycles of catalytic reactions. The minimal loss of yield after each cycle can further be attributed to the fact that after each cycle, minimal amounts of catalyst were lost during the catalyst regeneration process.

Fig. 9 Reusability study of the Ag₁–Cu₃ NPs bimetallic nanoparticle catalyst.

3.4. Comparison with reported results

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 seen that our catalyst shows higher activity with minimal amounts of catalyst and shorter reaction times compared to other catalysts such as Cu NPs,⁸⁸ Ag NPs,⁹⁵ Ag NPs in hollow magnetic spheres³⁴ and Zn(oAc)₂·H₂O.⁹⁶ Therefore, these results show that the Ag₁–Cu₃ NPs system is more effective for the synthesis of fine chemicals such as β-enaminones and β-enaminoesters.

Table 7 Comparison study with other available catalysts in the literature

S. no.	Catalyst	Product	Time	Catalyst amount (mol%)	Catalyst amount (mg)	Yield (%)	Particle size	Ref.
1	Cu NPs		2 h 50 min	10	—	92	20 nm	77
2	Ag NPs		8 h	10	—	90	40 nm	83
3	Ag NPs in hollow magnetic spheres		8 h	—	31	98	10 nm	32
4	Zn(oAc)2·H2O		48 h	5	—	86	—	84
5	Ag1–Cu3 NPs		1 h 45 min	10	20	95	8 nm	This work

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4. Conclusions

In conclusion, for the first time we have demonstrated the use of bimetallic Ag–Cu NPs synthesized by a simple co-reduction method as an efficient catalyst for β-enaminones and β-enaminoesters synthesis. Among various ratios of the bimetallic Ag–Cu NPs, bimetallic Ag₁–Cu₃ showed the highest activity for yielding the desired products. Several comprehensive techniques, including HRTEM, EDS line scan, EDS mapping, valence band and core level XPS, indicated the alloy formation of the NPs. XRD and XPS measurements showed that the Cu atom in the Ag–Cu bimetallic NPs 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 nanocatalysts for enamination reactions.

Acknowledgements

The authors are thankful to the Department of Science & Technology (DST), Govt. of India, for funding. XPS facility at IIT Delhi is partially funded by FIST grant of DST, India. We would also like to thank Mr Subhabrata Chakraborty (TEM in-charge, NIT Rourkela) for HAADF-STEM imaging.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04569c

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