Mahmoud Nasrollahzadeh*a,
S. Mohammad Sajadib and
Mehdi Mahamc
aDepartment of Chemistry, Faculty of Science, University of Qom, PO Box 37185-359, Qom, Iran. E-mail: mahmoudnasr81@gmail.com; Fax: +98 25 32103595; Tel: +98 25 32850953
bDepartment of Petroleum Geoscience, Faculty of Science, Soran University, PO Box 624, Soran, Kurdistan Regional Government, Iraq
cDepartment of Chemistry, Aliabad Katoul Branch, Islamic Azad University, Aliabad Katoul, Iran
First published on 29th April 2015
We report the green synthesis of CuO nanoparticles (CuO NPs) using Tamarix gallica leaf extract and their catalytic activity for N-arylation of nitrogen-containing heterocycles with aryl halides under ligand-free conditions. Tamarix gallica leaves are used as a bio-material for the first time as reducing agent. The green synthesized CuO NPs was characterized using the powder XRD, TEM, UV-vis and FT-IR. This method offers several advantages, viz. high yields, simple methodology, recyclability of the catalyst and a simple workup procedure.
In the past years, a great deal of attention has been focused on the development of metal catalyzed arylation.2 However, these methodologies suffer from numerous limitations such as high reaction temperature (>200 °C), long reaction times, the high cost of catalysts, use of expensive, toxic and sensitive to air ligands, the need for stoichiometric amounts of catalyst and a major challenge lies in the separation of product from expensive catalyst. In addition to these problems, the major drawback is that the majority of catalysts cannot be reused. To overcome from these problems, we have investigated a new and ligand-free catalytic system for N-arylation of nitrogen-containing heterocycles. In contrast with homogeneous catalysts, heterogeneous catalysts are easy to separate and can be recycled. This is very beneficial for industrial process in the green chemistry domain.
Due to high surface-to-volume ratio and metal NPs highly active surface compared to the bulk metals, heterogeneous catalysts are more and more used in the form of NPs.3 Among various heterogeneous nanocatalysts, CuO NPs containing high surface area have received considerable attention because of their good chemical and thermal stability, low cost, low toxicity, ease of handling and potential applications in organic synthesis as an efficient and recyclable catalyst.4 The CuO NPs are attractive both from economic and industrial points of view, as compared to those of the precious metals. Also, according to the thermodynamic calculations in literature,5 among various copper forms in aqueous media such as Cu(OH)2, Cu2O, etc., CuO is thermodynamically more stable.
Various exploitable methods exist for the synthesis of CuO NPs by conventional physical and chemical synthetic procedures. But some of the methods serious drawbacks and limitations such as the use of toxic, expensive chemicals and flammable organic solvents, and formation of toxic by-product make it very difficult to meet the requirements of environmentally friendly chemical processes.6 In addition, during the chemical synthesis, the residuals of some toxic chemicals may adsorb on the surfaces of the NPs, which prevents their application in biomedical equipment. Therefore, the development of simple, highly efficient methodologies for the preparation of NPs remains desirable. In recent years, environmental friendly and green synthesis of metallic nanoparticles (NPs) has attracted tremendous attention in the scientific community, due to the growing environmental contamination caused by the conventional physical and chemical methods.7 The biosynthetic techniques for the preparation of metal NPs has several advantages over chemical synthesis, such as simplicity, cost effectiveness as well as compatibility for biomedical and pharmaceutical applications. Among biological methods for the preparation of metal NPs plant extracts have attracted significant attention, due to easy and simple sampling, and cost effectiveness of this method facilitates the large scale biosynthesis of NPs.7i,7j Plant extracts may act both as reducing agent and stabilizing agents for the synthesis of metal NPs.
The Tamarix gallica from the family of Tamaricaceae is a tree or shrub halophyte from coastal regions and desert, relatively long-living plant that can tolerate a wide range of environmental conditions and resist abiotic stresses such as salt, high temperature, and drought stresses (Fig. 1).8
As a part of our ongoing research on the green synthesis and catalytic applications of various metal NPs in organic reactions,9 we herein report a novel and simple method for the green synthesis of CuO NPs using Tamarix gallica leaf extract as a reducing and stabilizing agent. The CuO NPs were employed as a recyclable catalyst for N-arylation of nitrogen-containing heterocycles with aryl halides under ligand-free conditions (Scheme 1). The N-arylation reaction has been successfully achieved with good to excellent yields. The catalyst exhibited remarkable reusable activity.
The leaves of this species is extensively used as a source of herbal medicine for presence of medicinal phytochemicals. Fig. 2 shows the HPLC fingerprint of leaf extract of the plant demonstrating the presence of potent antioxidants such as gallic acid (1), Epicatechin (2), syringic acid (3), vanillic acid (4), rosmarinic acid (5), p-coumaric acid (6), ferrulic acid (7), quercitin (8), trans-cinnamic acid (9) and flavone (10) respectively. These phyto-constituents confirmed the application of Tamarix gallica leaf extract as a suitable source for synthesis of nanoparticles using the reducing ability of its antioxidant contents.
The UV spectrum of plant extract shows bonds due to the transition localized within the ring of cinnamoyl and benzoyl system. Although, they are generally related the π → π* transitions of double bounds but this absorbent bonds are specifications of flavones nuclei inside the extract as supported by scientific report.10 Therefore, the UV spectrum of extract (Fig. 3) shows bonds at λmax 350 nm (bond I) due to the transition localized within the ring of cinnamoyl system; whereas the one centered at 231 nm (bond II) is for ring related to the benzoyl system. They are related to the π → π* transitions and these absorbent bonds demonstrate the presence of polyphenolics.
In our research there is a focus on the synthesis of NPs in aqueous media using reducing properties of antioxidant phytochemicals inside the plant specially polyphenolics as a major reducing and polyhydroxyl highly polar agents in Tamarix gallica leaf extract according the below possible mechanism (Scheme 2). As indicated in HPLC chromatogram, there are many antioxidant compounds inside the plant extract therefore based on the proposed mechanism Cu(II) are reduced to Cu(0) then because of the highly oxidation potential of Cu(0) to combination with oxygen and reach the higher oxidation states it spontaneously converts to the CuO NPs, Scheme 2, steps 1 and 2.
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Scheme 2 Reducing ability of antioxidant phenolics of Tamarix gallica leaf extract to produce nano particles. |
As demonstrated using X-ray diffraction analysis, the produced CuO NPs has a crystal structure which the formation of these nanocrystals can be explained via the mechanism briefly as formation of CuO nanocrystals using the coagulation of smaller particles to produce the large nanocrystals through quasi-spherical particles as transition state (Scheme 3). Firstly, the produced CuO NPs undergoes nucleation from a required critical number of nanoparticles in the solvent. Step 3 indicates the growth of this nucleus by diffusion process onto the surface of the nucleated particle cause to produce interparticles; in step 4, interparticles growth by collision and fusion of two particles via the oriented attachment route; and in final step interparticles growth via exchange (dissolution and diffusion) of molecules between various particles, commonly referred to ripening of CuO nanocrystals.11
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Scheme 3 Formation of CuO nanocrystals using the coagulation of smaller particles to produce the large nanocrystals through quasi-spherical particles as transition state. |
The green synthesized CuO NPs was characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), FT-IR and UV-vis techniques.
The formation of CuO NPs was controlled by UV-vis spectroscopy. Fig. 4 shows the UV-vis spectrum of CuO NPs formation. The reaction was completed after 3 min. Our results showed that the maximum absorbance of green synthesized CuO NPs was due to the surface plasmon absorption of nanosized cupric oxide particles. Changing the color of the reaction (yellow to dark brown, Fig. 5) and appearance the maxima at 250 nm indicates the reduction process and formation of nanoparticles. As monitored by UV-vis, the synthesized CuO NPs by this method are approximately stable with a few variances in the shape, position and symmetry of the absorption peak even after one month which indicates the relative stability of product.
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Fig. 4 UV-vis spectrum of green synthesized CuO NPs using the aqueous extract of leaves of the Tamarix gallica in 3 min to one month. |
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Fig. 5 Photograph of Tamarix gallica leaf extract (A) and green synthesized CuO NPs produced after bioreaction (B). |
The FT-IR analysis was carried out to identify the possible biomolecules responsible for the reduction of Tamarix gallica and CuO NPs. The FT-IR spectrum of the crude extract (Fig. 6) depicted some peaks at 3393, 2925, 1661, 1442, 1300 to 1000 cm−1 which represent free OH in molecule and OH group forming hydrogen bonds, saturated hydrocarbons (Csp3–H), carbonyl group (CO), stretching C
C aromatic ring and C–OH stretching vibrations, respectively. Because of presence the mentioned functional groups inside the structure of polyphenolics, the spectrum can demonstrate the presence of phenolics in the plant leaf extract. The presence of phenolics in the extract could be probably responsible for the reduction of metal ions and formation of nanoparticles.
Furthermore, FT-IR spectrum of CuO NPs is shown in Fig. 7. The appeared bands are due to lattice vibration modes indicating the functional groups of samples. The broad band in 3305 cm−1 is OH stretching bond. The band around 1435 cm−1 is generally attributed to the bending vibration of sp2-carbon groups for aromatic and 1622 cm−1 for carbonyl functional group.
The X-ray diffraction analysis of prepared CuO NPs was carried to identify the product. The XRD spectrum shown in Fig. 8 contains all the peaks associated with the crystalline planes of pure CuO confirming the crystallinity and phase purity of the NPs.12 No other diffraction peaks arising from metallic Cu or Cu2O present in the XRD pattern, which indicate the high phase purity of synthesized sample.
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Fig. 8 XRD powder pattern of CuO NPs synthesized using aqueous extract of leaves of Tamarix gallica. |
TEM studies of the CuO NPs were carried out to investigate the shape and the size of the particles. A typical TEM image of CuO NPs is shown in Fig. 9. The image clearly shows the formation of CuO NPs of different sizes and shapes, mainly spherical. The particles are of various sizes.
Initially, we chose to study the reaction between iodobenzene (3.3 mmol) and 1,2,4 triazoles (3.0 mmol) to optimize the reaction conditions (Table 1). The effect of catalyst loading, base and solvent on the model N-arylation reaction was investigated. Control experiments show that there is no reaction in the absence of catalyst or base. The effect of reaction solvent was also investigated. According to data given in Table 1, DMF was the most efficient solvent for this reaction (Table 1, entry 6). After choosing DMF as the solvent, we examined several different bases. Among the solvents examined, K2CO3 was found to be the most effective. No significant improvement on the yield was observed with using higher amounts of the catalyst (entry 13). Thus, the optimized reaction conditions for this N-arylation reaction of 1,2,4 triazoles are the CuO NPs (0.2 mmol) in 10 mL of DMF using K2CO3 (3.0 mmol) as base at room temperature under ligand-free conditions (Table 1, entry 6).
Entry | CuO NPs (mmol) | Solvent | Base | Reaction time (h) | Yieldb (%) |
---|---|---|---|---|---|
a Reaction condition: 3.3 mmol of iodobenzene, 3.0 mmol of 1,2,4 triazoles, 3.0 mmol of base and 10 mL of solvent, room temperature.b Isolated yield. | |||||
1 | 0.0 | MeCN | K2CO3 | 12 | 0.0 |
2 | 0.2 | MeCN | — | 12 | 0.0 |
3 | 0.2 | MeCN | K2CO3 | 12 | 45 |
4 | 0.2 | Toluene | K2CO3 | 12 | 71 |
5 | 0.2 | MeOH | K2CO3 | 12 | 28 |
6 | 0.2 | DMF | K2CO3 | 2 | 92 |
7 | 0.2 | DMF | NaOH | 5 | 38 |
8 | 0.2 | DMF | Cs2CO3 | 5 | 48 |
9 | 0.2 | DMF | DBU | 12 | 0.0 |
10 | 0.2 | DMF | Et3N | 12 | 21 |
11 | 0.2 | DMF | t-BuOK | 5 | 58 |
12 | 0.2 | DMF | K2CO3 | 10 | 92 |
13 | 0.3 | DMF | K2CO3 | 2 | 91 |
With having the optimum conditions in hand, we examined the scope of this reaction with a series of 1,2,4 triazoles. According to the results summarized in Table 2, in all cases, this protocol afforded the desired products in good to excellent yields. As shown in Table 2, reaction of aryl halides with other nitrogen-containing heterocycles afforded the corresponding N-arylated products in good to excellent yield under ligand-free conditions.
Entry | Het-NH | Aryl halide | Product | Temperature/time (h) | Yieldb (%) | Natureref. |
---|---|---|---|---|---|---|
a Reaction condition: 3.3 equiv. of aryl halide, 3.0 equiv. of Het-NH, 0.2 mmol of catalyst, 3.0 equiv. of K2CO3 and 10 mL of DMF.b Isolated yield. | ||||||
1 | ![]() |
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r.t./4 | 92 | Oil13a |
2 | ![]() |
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r.t./4 | 82 | Solid, M.P.: 70–72 °C (Lit.13b: 71 °C) |
3 | ![]() |
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r.t./4 | 93 | Solid, M.P.: 171–173 °C (Lit.13a: 171–173 °C) |
4 | ![]() |
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r.t./6 | 78 | Solid, M.P.: 171–173 °C (Lit.13a: 171–173 °C) |
5 | ![]() |
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r.t./6 | 60 | Solid, M.P.: 171–173 °C (Lit.13a: 171–173 °C) |
6 | ![]() |
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Reflux/5 | 91 | Oil13a |
7 | ![]() |
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Reflux/5 | 87 | Oil13a |
8 | ![]() |
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Reflux/5 | 97 | Oil13a |
9 | ![]() |
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Reflux/5 | 97 | Solid, M.P.: 86–88 °C (Lit.13c: 85–87 °C) |
10 | ![]() |
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Reflux/5 | 96 | Oil13d |
11 | ![]() |
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Reflux/5 | 95 | Oil13d |
12 | ![]() |
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Reflux/5 | 95 | Solid, M.P.: 129–130 °C (Lit.13g: 128–129 °C) |
13 | ![]() |
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Reflux/6 | 88 | Solid, M.P.: 129–130 °C (Lit.13g: 128–129 °C) |
14 | ![]() |
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Reflux/6 | 61 | Solid, M.P.: 129–130 °C (Lit.13g: 128–129 °C) |
The present method offers several notable features, compared with the other literature works13 on the N-arylation of nitrogen-containing heterocycles, i.e., (1) the use of plant extract as an economic and effective alternative represents an interesting, fast and clean synthetic route for the large scale synthesis of CuO NPs without use of toxic, hazardous and dangerous materials or surfactant template, (2) avoidance of the toxic ligands and homogeneous catalysts for the N-arylation reactions, (3) wide substrate scope and generality, (4) higher yields, (5) in our method, because of the heterogeneous nature of the CuO NPs catalyst, it can be easily recovered and reused and (6) according to the UV-vis results, the synthesized CuO NPs by this method are quite stable and can be kept for one month.
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