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J. Phys. Chem. C 2008, 112, 3334-3340
PVP-Stabilized Copper Nanoparticles: A Reusable Catalyst for “Click” Reaction between Terminal Alkynes and Azides in Nonaqueous Solvents A. Sarkar,* T. Mukherjee, and S. Kapoor* Radiation & Photochemistry DiVision, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India ReceiVed: September 21, 2007; In Final Form: December 17, 2007
Highly stable poly(N-vinyl-2-pyrrolidone) (PVP) protected copper nanoparticles were prepared using a simple chemical reduction route in different solvents {N, N-dimethyl formamide (DMF) and formamide (FA)} under aerated condition. The particles were characterized using TEM, SEM, XRD, and UV-visible spectroscopic techniques. Copper nanostructures of varying shapes and sizes were obtained using hydrazine hydrate as the reducing agent in both DMF and FA. However, reducing by ascorbic acid in FA leads to formation of mostly spherical copper nanoparticles with a narrow size distribution. The Cu nanoparticles serve as effective catalyst for 1,3-dipolar cycloaddition reactions between terminal alkynes and azides to synthesize 1,2,3-triazoles in excellent yields under mild reaction conditions. The nanocatalysts can be recycled and reused several times without significant loss of their catalytic activity.
1. Introduction 1,2,3-triazoles1 are basically five-membered nitrogen heterocyclic compounds, which have tremendous applications in biology.2,3 They have also found wide industrial applications such as corrosion inhibitors, agrochemicals, optical brighteners, and photographic materials.4 Therefore, development of new protocols for the synthesis of 1,2,3-triazoles has gained immense importance. There are several different methods available in literature for the synthesis of 1,2,3-triazoles. Cycloaddition reaction of azides derivatives with various substituted alkynes gives a variety of triazoles.5,6 The formation of 1,4-disubstituted 1,2,3-triazoles using copper (I)-catalyzed [3 + 2] cycloaddition of terminal alkynes and organic azides is considered as the best click reaction7 to date. It has enabled bio-conjugations involving 60 steps (with >99.8% yield per step)8 and has been used in activity-based protein profiling (ABPP) of crude proteome homogenates,9 for selective labeling of modified bacterial cell walls,10 and in the synthesis of novel biologically active compounds11 and materials.12 Recently, there are also reports on formation of triazoles on solid support or without the use of any catalyst.13,14 Chassaing et al.15 have reported a simple and efficient method for the [3 + 2] cycloaddition of terminal alkynes with azides using Cu(I)-modified zeolites as catalysts. Typically, such 1,3-dipolar cycloaddition reaction involves a copper(I) acetylidene intermediate generated from Cu(I) and the terminal alkyne, which then participates in an annealing process upon its coordination with the reacting azide.1 Although Cu(I) could be introduced directly in the form of different copper salts, the presence of a nitrogen containing base as well as prior exclusion of oxygen from the reaction are usually required in order to minimize the formation of undesired byproducts, primarily diacetylenes.16 Alternately, catalytic Cu(I) could also be generated in situ from copper sulfate and sodium ascorbate.1 The later eliminates the problem of byproducts. Recently, much attention has been attracted to the use of nanoparticle as catalyst in organic reactions.17-22 The size of * Authors for correspondence. Phone: (+) 91-22-25590298. Fax: (+) 91-22-25505151. E-mail:
[email protected] (A.S.) and
[email protected] (S.K.).
the nanocatalyst is of utmost importance in catalysis for providing highly active catalyst surface, which maximizes the reaction rates and minimizes consumption of the catalyst. Orgueira et al.23 have reported the use of Cu(0) nanosize activated powder as catalyst for cycloaddition between terminal alkynes and azides. However, addition of secondary or tertiary amine is essentially required in order to dissolve this nanosized powder. During the course of the reaction, zero-valent copper gets oxidized to the Cu(II) state via the Cu(I) state, which preclude the use of catalyst for further use. Three-component coupling reactions for the synthesis of 1,4-disubstituted-1,2,3triazoles using bimetallic Pd(0)-Cu(I)24a and microwaveassisted Cu(0)-Cu(I)24b catalyst have also been reported. Kantam et al.25 have reported use of alumina supported copper nanoparticles as catalyst for preparation of 1,2,3-triazoles by the reaction of terminal alkynes, sodium azide, and alkyl/allyl halides. Molteni et al.26 have also reported the use of mixture of Cu/Cu-oxide nanoparticles as an effective catalyst for the ‘‘click’’ 1,3-dipolar cycloaddition between azides and terminal alkynes. Similarly, Pachon et al.27 have shown the efficiency of Cu nanoclusters as catalysts for 1,3-dipolar cycloadditions of azides to terminal alkynes. All of these reactions catalyzed using copper nanoparticles or copper salt require long reaction time (2-12 h). Lipshutz and Taft28 have reported an effective method to synthesize triazole using Cu/C catalyzed cycloaddition of alkynes and azides. It has been shown that the reaction can be accelerated with stoichiometric Et3N or by increasing the reaction temperature or using microwave irradiation where the reaction times and reduces from several hours to few minutes. In majority of the above-mentioned protocols, copper nanocatalyst are prepared, isolated, and then redispersed into the reaction medium before the addition of reactants. Further, for the reusability of the catalyst, the particles were separated, washed, and then added into fresh batch of reaction medium. This makes the entire process cumbersome and may also lead to loss of the catalyst during the workup procedure. For the economic and environmental importance in chemical and pharmaceutical industries, recycling of homogeneous nanocatalyst is becoming a task of great relevance.22 With an approach
10.1021/jp077603i CCC: $40.75 © 2008 American Chemical Society Published on Web 02/12/2008
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to improve the catalytic utility of the nanocatalyst, the preparation of the copper nanoparticles in the reaction medium itself was carried out. Copper nanoparticles have been widely studied because of their role in electronics, catalysts, resins, and so forth.29-31 There are many reports on the synthesis and characterization of copper nanoparticles using various stabilizers in aqueous and nonaqueous solvents.32-44 Since DMF and FA have been widely used as a reaction medium for several organic reactions, we have attempted the synthesis of Cu nanoparticles in these solvents. In the present study, a bottom-up approach for the synthesis of PVP coated Cu nanoparticles in DMF and FA is reported. The as-prepared particles show high catalytic activity toward 1,3- dipolar cycloaddition reactions carried out at room temperature under aerated conditions, reducing the reaction time enormously. These nanoparticles maintain their catalytic efficiency for several cycles and serve as a potential reusable catalyst. 2. Experimental Section 2.1. Materials. Copper(II) acetate monohydrate, copper (II) sulfate, and hydrazine hydrate were purchased from Sigma and used as received. Poly(N-vinyl-2-pyrrolidone) [PVP] (360 000) was purchased from Aldrich and used as received. Ascorbic acid; dichloromethane [DCM], N, N-dimethyl formamide [DMF], and formamide [FA] were of UV spectroscopy grade and obtained from Spectrochem, India. Water purified through a Millipore system was used. 2.2. Method. 2.2.1. Synthesis of Copper Nanoparticles Using Hydrazine Hydrate as Reducing Agent: (Solution A). Dispersion of colloidal copper nanoparticles was prepared by adding freshly prepared 1 × 10-1 mol dm-3 hydrazine hydrate to 1 × 10-2 mol dm-3 of copper acetate or copper sulfate in DMF or FA. 1% (w/v) PVP was added as a stabilizing agent. The reaction mixture was agitated to obtain a homogeneous Cu sol under aerated condition. 2.2.2. Synthesis of Copper Nanoparticles Using Ascorbic Acid as Reducing Agent: (Solution B). Copper ions were introduced in the form of copper salt, that is, either copper acetate or copper sulfate. In a typical experiment, copper sulfate (3 × 10-2 mol dm-3) and ascorbic acid (6 × 10-2 mol dm-3) was added to 1% (w/v) PVP in FA. Formation of copper nanoparticles takes place in two stages using ascorbic acid as the reducing agent under aerated condition. In the first step on addition of ascorbic acid, large clusters of Cu particles are formed because of rapid reduction of copper ions. In the second step, slow formation of stable copper nanoparticles takes place, thus rendering the solution red color. 2.2.3. Synthesis of 1,2,3-Triazoles Using Cu Nanoparticles Catalyst. A typical experimental procedure was followed for the synthesis of triazole. Incorporation of different terminal alkynes and azides to the as-prepared PVP protected copper nanoparticles lead to formation of desired respective triazoles. In one typical case phenyl propargyl ether (∼0.16 mol dm-3) and benzyl azide (∼0.16 mol dm-3) were added to either solution A or solution B. This heterogeneous mixture was vigorously stirred till the product separated out as a white solid within 15 min. The product formed was then extracted with ether and then dried under reduced pressure to obtain the desired triazole as a white crystalline solid. 2.3. Characterization. Samples for transmission electron microscopy (TEM) were prepared by putting a drop of the colloidal solution on a copper grid coated with a thin amorphous carbon film kept on a filter paper. Excess of solvent was removed from the sample. Samples were dried and kept under
Figure 1. (a) Transmission electron micrograph of copper nanoparticles in DMF obtained from reduction of 1 × 10-2 mol dm-3 of copper acetate monohydrate by 1 × 10-1 mol dm-3 hydrazine hydrate containing 1% PVP. (b) Electron diffraction pattern from the metallic Cu nanoparticles in DMF with sample condition the same as a.
vacuum in a desiccator before putting them in a specimen holder. TEM characterization was performed on a Philips CM 200 SUPERTWIN STEM microscope operating at 200 kV. Particle sizes and morphologies were measured from the TEM micrographs and calculated by taking at least 100 particles. The SEM characterization was carried out using JEOL-JSM 6360 scanning electron microscope. Absorption measurements were carried out on a Chemito-Spectrascan UV 2600 spectrophotometer. The
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Figure 2. SEM image of Cu nanoparticles obtained from reduction of 3 × 10-2 mol dm-3 copper sulfate by 6 × 10-2 mol dm-3 ascorbic acid in FA containing 1% PVP.
Figure 3. UV-visible absorption spectra of copper nanoparticles in FA. Sample condition same as Figure 2.
spectra were recorded at room temperature using either 0.2 cm or 1 cm quartz cuvette. However, all of the spectra are later normalized to 1 cm path length of the cuvette. X-ray powder diffractometers were recorded on a Philips X-ray diffractometer (PW 1710) with Ni filtered Cu KR radiation using silicon as an external standard. The measurements were performed in a range of 10-80° in a continuous scan mode with a step width of 0.02° and scan of 1° per min. NMR spectra were recorded on a JEOL spectrometer (300 MHz). Mass spectra were obtained using a GC/MS Shimadzu QP-5050 (EI, 70 eV). 3. Results and Discussion 3.1. Characterization of Copper Nanoparticles. DMF has been used as solvent for many triazole formation reactions mainly when carbohydrate based reactants have been used. Thus, stable PVP-coated Cu nanoparticles were prepared in DMF as a solvent using a simple chemical route. Copper ions in DMF were reduced by hydrazine hydrate in presence of PVP under aerated condition. Under vigorous stirring condition, reddish brown colored Cu sol is obtained. The solution was centrifuged to obtain reddish brown Cu solid. A typical TEM image of PVP coated copper nanoparticles is shown in Figure 1a. The image clearly shows the formation of metallic Cu nanoparticles of different sizes and shapes, mainly cubical. The particles are of various sizes. Formation of larger clusters was also observed
Figure 4. (a) Transmission electron micrograph of copper nanoparticles in FA. Sample condition same as Figure 2. (b) Electron diffraction pattern from the metallic Cu nanoparticles in FA. Sample condition same as Figure 2.
probably because of aggregation of smaller particles. Figure 1b shows the diffraction pattern of the Cu nanoparticles in DMF. FA is used as a solvent in many organic reactions. Thus, attempts were made to synthesize copper nanoparticles in FA by employing the bottom-up approach and are discussed below. In this case, copper ions were introduced in the form of copper salt either copper acetate or copper sulfate under aerated condition. Both copper sulfate and copper acetate showed similar results. (a) Reduction of Copper Ions by Hydrazine Hydrate. Stable PVP-coated copper nanoparticles were obtained using hydrazine
PVP-Stabilized Copper Nanoparticles
Figure 5. XRD Pattern of copper particles. Sample condition same as Figure 2.
hydrate as the reducing agent in FA. The particles showed various morphologies and similar to particles prepared in DMF using hydrazine hydrate.
J. Phys. Chem. C, Vol. 112, No. 9, 2008 3337 (b) Reduction of Copper Ions by Ascorbic Acid. Metallic copper nanoparticles were formed by reduction of copper sulfate, which is used as a copper source. Ascorbic acid was added to copper sulfate in FA containing 1% PVP (w/v) as the stabilizing agent. The solution was agitated to form a homogeneous solution. Formation of copper nanoparticles took place in two stages using ascorbic acid as the reducing agent. Initially, on addition of ascorbic acid, pink colored large clusters of metallic copper particles precipitates out and settles at the bottom of the reaction vessel due to rapid reduction. Figure 2 shows the SEM image of the large clusters having size around 500 nm. The EDAX analysis showed the presence of pure metallic Cu particles. However, with time slow reduction of copper ions lead to the formation copper nanoparticles and a red colored Cu sol is obtained. Figure 3 shows the surface plasmon absorption band of copper nanoparticles obtained by reduction of 3 × 10-2 mol dm-3 copper sulfate with 6 × 10-2 mol dm-3
TABLE 1: Synthesis of the Triazoles Prepared by the Copper Nanoparticle Catalyzed Cycloaddition
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SCHEME 1
TABLE 2: Comparison of Various Protocols Used for “Click” Reaction Sr. No.
protocol
reaction time
conditions
1 2 3 4 5 6 7 8
ref 1 ref 23 ref 24a ref 24b ref 25 ref 26 ref 28 Our method
∼8-36 h
