1,4-Dihydroxyanthraquinone–copper(II) supported on superparamagnetic Fe₃O₄@SiO₂: an efficient catalyst for N-arylation of nitrogen heterocycles and alkylamines with aryl halides and click synthesis of 1-aryl-1,2,3-triazole derivatives

Abstract

1,4-Dihydroxyanthraquinone–copper(II) supported on a superparamagnetic Fe₃O₄@SiO₂ catalyst was employed for the N-arylation of nitrogen heterocycles and alkylamines with aryl halides to afford the corresponding coupled products in good to excellent yields without using external ligands or additives as promoters. Also, we have reported this recyclable catalytic system for efficient synthesis of 1-aryl-1,2,3-triazole derivatives in excellent yields. The desired triazoles were obtained from the reaction of the corresponding aryl boronic acid derivatives, alkyne, NaN₃, and 0.5 mol% catalyst in water/acetonitrile as the solvent at room temperature without the additional use of an external reducing agent. These methods show notable advantages such as the heterogeneous nature of the catalyst, low catalyst loading, easy preparation, excellent yields, short reaction times and simplicity of operation. Also, the catalyst can be separated from the reaction mixture by applying a permanent magnet externally and can be reused in six consecutive reaction cycles without significant loss of activity.

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1. Introduction

Currently the design of heterogeneous metal catalysts that preserve stability and high metal dispersion during the catalytic reactions is of great importance.^1–4 These heterogenized catalysts can be easily separated from the reaction mixtures, but, homogeneous catalysts show higher catalytic activities than their heterogeneous counterparts because of their solubility in reaction media, which increases catalytic site accessibility for the substrate.^5–7 However, recycling homogeneous catalysts is often tedious and also it is complicated and costly to recycle homogeneous catalysts, especially when noble and toxic metal complexes are used.^8,9 To solve these problems, the particles are often immobilized on various inorganic and organic supports, such as metal oxides, mesoporous silica, polymers, carbon materials, ionic liquids, and so on.^10–14

The use of magnetic materials as immobilized supports for heterogeneous catalysis has attracted a great deal of attention because of the merits of magnetic separation.^15–17 Among transition metal nanoparticle catalysts, Fe₃O₄ nanoparticles have recently received a lot of attention as excellent supports because of their unique properties including low toxicity, large surface-to-volume ratio, biocompatibility, superparamagnetism and their potential applications in various fields.^18,19 Fe₃O₄ magnetic nanoparticles have been used in wide range of disciplines, including magnetic resonance imaging, magnetic data storage, targeted gene therapy, magnetic fluids, drug delivery systems, biotechnology/biomedicine, hyperthermia treatment of cancer cells, biosensors, environmental remediation, ion exchange separation, detoxification of biological fluids and catalysis.^20–28 Also, magnetic separation could be regarded as an attractive alternative to centrifugation or alternative as it prevents the loss of catalyst and enhances its reusability.²⁹

However, pure MNPs such as Fe₃O₄ nanoparticles trend to aggregate due to their nanoscale and poorly dispersed in aqueous medium due to their hydrophobic surface.³⁰ This problem can be solved by coating of MNPs with inorganic shells or hydrophilic polymers. Among these, coating with a silica layer as the stabilizer, which prevents direct contact between the nanoparticles has attracted great attention in recent years. Furthermore, the abound hydroxyl groups on the surface of silica layer provide the opportunity to conjugate various function molecules for many special applications.^31,32

The formation of carbon–nitrogen bonds via cross-coupling reactions represents a powerful means for the preparation of various compounds important in pharmaceutical, biological and material interest.^33,34 Copper-catalyzed Ullmann-type reactions are traditional methods to assemble these compounds.^35,36 Nucleophilic aromatic substitution with aryl halides can be mediated by copper or palladium catalysts.³⁷ The high cost and toxicity of palladium catalysts restrict their use on the industrial applications. Thus, researchers have turned their attention toward the use of more efficient metals, less expensive, and less toxic to replace palladium. The lower cost of copper-based catalytic systems makes them particularly attractive for large-scale industrial applications.³⁸ During the past years, several reports describing copper-catalysed methods have appeared in the literature.^39,40 However, these copper-mediated methods often suffer from limitations such as drastic reaction conditions, lack of generality, moderate yields, high cost and requirement for stoichiometric or greater amounts of copper catalyst. Therefore, the development of a new catalytic system to overcome these shortcomings and fulfill the criteria of a mild, efficient protocol for C–N bond formation is still desirable and is in demand.

1,2,3-Triazoles are an important class of nitrogen heterocyclic compounds which have biological activities and also wide applications in medicinal chemistry, material science and synthesis.^41,42 The main method for the synthesis of 1,2,3-triazoles is the Huisgen 1,3-dipolar cycloaddition reaction of azides with alkynes. This cycloaddition reaction has become the model for click reactions.⁴³ However, the disadvantage of Huisgen's reaction was the production of a mixture of 1,4 and 1,5-disubstituted products.⁴³ Consequently, finding new protocols for stereoselective triazole synthesis is of interest.

In 2002, Rostovtsev et al. reported the efficient synthesis of 1,2,3-triazoles form the Cu^I-catalyzed azide/alkyne cycloaddition (CuAAC). Their methodology is completely stereoselective which delivered 1,4-disubstituted triazoles under mild reaction conditions.⁴⁴ Since then, the “Click Chemistry” reaction between azide and alkyne (CuAAC) has gained much attention and many copper catalysts were reported for this transformation specially the heterogeneous ones.⁴⁵ Therefore, variety of supports such as zeolites,⁴⁶ activated charcoal,⁴⁷ silica⁴⁸ and amine functionalized polymers⁴⁹ were used to immobilize Cu^I onto their surface. The problem frequently encountered with applying these catalysts is the oxidation tendency of Cu^I to Cu^II or disproportion to Cu⁰ and Cu^II which reduces the catalytic activity.⁵⁰ To overcome this matter, other groups have applied Cu^II in the presence of reducing agents or ligands⁵¹ and the most general one is the use of CuSO₄/sodium ascorbate in aqueous media.⁵² In addition, variety of immobilized Cu^II were reported for 1,2,3-triazoles synthesis.⁵³ Recently, hydroxyapatite-supported Cu^II (ref. 54) or Cu^II-hydrotalcite⁵⁵ were reported for formation of 1,2,3-triazoles in the absence of base or reducing agents. They have reported that NaN₃ can reduce Cu^II to Cu^I beyond being an azidonation reagent. Many of the reported synthetic protocols for the synthesis of triazole derivatives suffer from one or more disadvantages such as the harsh reaction conditions, long reaction times, low yields, tedious workup of the reaction mixture and difficulty in separation and recovery of the catalyst. Thus, the development of an alternate milder and cleaner procedure, which surpasses those limitations, is very much relevant for the synthesis of 1-aryl-1,2,3-triazole derivatives.

Therefore, in continuation of our studies and to promote the methods of new synthetic using recyclable catalysts,⁵⁶ we focused our attention on a simple and efficient method for N-arylation of nitrogen heterocycles and alkylamines with aryl halides and click synthesis of 1-aryl-1,2,3-triazole derivatives in the presence of 1,4-dihydroxyanthraquinone–copper(II) immobilized on superparamagnetic Fe₃O₄@SiO₂ nanoparticles as illustrated in Scheme 1.

Scheme 1 N-Arylation of nitrogen heterocycles and alkylamines with aryl halides and click synthesis of 1-aryl-1,2,3-triazole derivatives is using the Fe₃O₄@SiO₂–DAQ–Cu(II) nanoparticles.

2. Results and discussion

First of all, the preparation of magnetic nanocatalyst with core–shell structure was presented by using nano Fe₃O₄ as the core, TEOS as the silica source and PVA as the surfactant. Then, Fe₃O₄@SiO₂ was coated with 1,4-dihydroxyanthraquinone–copper(II) nanoparticles.⁵⁷ The Fe₃O₄, Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂–DAQ–Cu(II) nanocatalysts were characterized by various methods such as Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction analysis (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), dynamic light scattering (DLS), thermogravimetric analysis (TGA), vibration sample magnetometer (VSM), X-ray photoelectron spectroscopic (XPS), N₂ adsorption–desorption isotherm analysis and inductively coupled plasma analyzer (ICP).⁵⁷

To optimize the conditions for the N-arylation reaction, initially the reaction between iodobenzene and imidazole as the model reaction was examined in the presence of various amount of the catalysts and the results are presented in Table 1.

Table 1 Optimization of the amount of catalyst, solvent, base and temperature on the N-arylation reaction^a

Entry	Catalyst amount (mol%)	Solvent	Base	Temp (°C)	Time (h)	Yield (%)
a Reaction conditions: iodobenzene (1 mmol), imidazole (1.2 mmol), base (2 mmol), Fe3O4@SiO2–DAQ–Cu(II) catalyst and solvent (3 mL).
1	0.45 mol%	EtOH	Cs2CO3	Reflux	8	21
2	0.45 mol%	MeOH	Cs2CO3	Reflux	8	29
3	0.45 mol%	H2O	Cs2CO3	Reflux	8	0
4	0.45 mol%	MeCN	Cs2CO3	Reflux	8	56
5	0.45 mol%	DMF	Cs2CO3	100 °C	4	92
6	0.45 mol%	DMSO	Cs2CO3	100 °C	4	89
7	0.45 mol%	Toluene	Cs2CO3	110 °C	8	75
8	None	DMF	Cs2CO3	100 °C	8	0
9	0.2 mol%	DMF	Cs2CO3	100 °C	8	34
10	0.3 mol%	DMF	Cs2CO3	100 °C	8	69
11	0.4 mol%	DMF	Cs2CO3	100 °C	4	84
12	0.5 mol%	DMF	Cs2CO3	100 °C	4	91
13	0.45 mol%	DMF	K2CO3	100 °C	8	63
14	0.45 mol%	DMF	K3PO4	100 °C	8	71
15	0.45 mol%	DMF	Et3N	100 °C	8	26
16	0.45 mol%	DMF	NaOAc	100 °C	8	37
17	0.45 mol%	DMF	NaOH	100 °C	8	65
18	0.45 mol%	DMF	Cs2CO3	r.t	8	Trace
19	0.45 mol%	DMF	Cs2CO3	60 °C	8	42
20	0.45 mol%	DMF	Cs2CO3	80 °C	6	74
21	0.45 mol%	DMF	Cs2CO3	90 °C	4	82
22	0.45 mol%	DMF	Cs2CO3	110 °C	4	90

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The results show clearly that catalyst is effective for this transformation and in the absence of it; the reaction did not take place even after higher reaction time (Table 1, entry 8). The use of a lower catalyst loading resulted in incomplete reaction (Table 1, entries 9–11). Increasing the amount of catalyst gave no substantial improvement in the yield (Table 1, entry 12).

Among all the solvents tested, including EtOH, MeOH, H₂O, MeCN, DMF, DMSO and toluene, DMF gave the highest yield (Table 1, entries 1–7). The screening of the reaction time indicates that 4 h reaction time is the most suitable to afford the highest yield of product 3a (Table 1, entry 5). We then decided to screen various bases. In general, we obtained superior results with cesium carbonate (Cs₂CO₃). Replacement of cesium carbonate by potassium carbonate (K₂CO₃), tripotassium phosphate (K₃PO₄), triethylamine (Et₃N), sodium acetate (NaOAc) or sodium hydroxide resulted in significantly decreased yields (Table 1, entries 5, 13–17).

Also, during our optimization studies, various temperatures were examined and it was found that the temperature plays a significant role in terms of reaction rate and isolated yield and the best results were obtained at 100 °C (Table 1, entries 5, 18–22). Thus, the optimum conditions were found to be 1.0 mmol of iodobenzene, 1.2 mmol of imidazole, 0.45 mol% of the Fe₃O₄@SiO₂–DAQ–Cu(II) catalyst and 2 mmol of Cs₂CO₃ in DMF (3 mL) at 100 °C.

To examine the scope for this coupling reaction, we have investigated the reactions using a variety of aryl halides and a wide range of N(H)-heterocycles and amines as the substrates under the optimized reaction conditions and the results are outlined in Table 2. We found that aryliodides and aryl bromides containing electron releasing groups (3b, 3e) as well as electron withdrawing groups (3c, 3g) reacted with imidazole to give corresponding N-arylated imidazoles. Presence of electron-withdrawing group such as nitro groups on aryl halides (Table 2, 3c, 3f) increased the yield of the coupling reaction whereas an electron releasing groups such as methyl and methoxy groups on aryl halides (Table 2, 3b, 3e, 3i, 3m) at para position decreased reaction yield for the N-arylation reaction. To test further the scope of this catalyst, reactions with indole, 2-methyl-1H-indole, 1H-benzimidazole and 1H-pyrrole are conducted, which resulted in a very good yield (Table 2, 3h–3o). When diamines such as piperazine were employed, the reaction was complete within 6 h affording 81% yield (Table 2, 3p). Moreover, aryliodides underwent N-arylation of aliphatic amines such as piperazine, phenylpiperazine, dibutylamine and diethylamine under similar conditions to afford the corresponding products in high yields (Table 2, 3p–3t). Further we have screened the chloro-derivatives for the N-arylation of imidazole (Table 2, 3u–3w). N-Arylation of imidazole with chlorobenzene gave no coupled product under the above conditions. However, an increase of catalyst loading (0.6 mol%) with longer reaction time at 130 °C provided 43% yield of the coupled product (Table 2, 3u). Among chloro-derivatives, p-nitrochlorobenzene with a strong electron-withdrawing group resulted in quantitative yield (Table 2, 3v), whereas 2-chloropyridine resulted in lower yield (Table 2, 3w).

Table 2 N-Arylation of N(H)-heterocycles and amines with aryl halides using Fe₃O₄@SiO₂–DAQ–Cu(II) nanocatalyst^a,e

a Reaction conditions: aryl halide (1 mmol), amine (1.2 mmol), Cs₂CO₃ (2 mmol), Fe₃O₄@SiO₂–DAQ–Cu(II) catalyst (0.45 mol%), DMF (3 mL), 100 °C.b Reaction conditions: aryl halide (2 mmol), amine (1.2 mmol), Cs₂CO₃ (3 mmol), Fe₃O₄@SiO₂–DAQ–Cu(II) catalyst (0.8 mol%), DMF (5 mL), 100 °C.c Reaction temperature: 130 °C.d Reaction with 0.6 mol% of Fe₃O₄@SiO₂–DAQ–Cu(II) catalyst.e Isolated yield.

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For optimization of the reaction parameters for efficiently synthesis of 1-aryl-1,2,3-triazole derivatives, the reaction of phenyl acetylene (1.1 mmol), phenylboronic acid (1 mmol) and sodium azide (2 mmol) were investigated in the presence of Fe₃O₄@SiO₂–DAQ–Cu(II) as a catalyst in various solvents and also in the presence of various amounts of catalyst. The results are summarized in Table 3. The optimized reaction conditions were: 1.1 mmol of phenyl acetylene, 1.0 mmol of phenylboronic acids, 2.0 mmol of sodium azide, 0.5 mol% of Fe₃O₄@SiO₂–DAQ–Cu(II) catalyst and 3 mL of H₂O/ACN (Table 3, entry 11).

Table 3 Optimization of the amount of catalyst and solvent in a one-pot synthesis of the model reaction^a

Entry	Catalyst (mol%)	Solvent	Time (h)	Yield^b (%)
a Reaction conditions: phenylboronic acid (1 mmol), sodium azide (2 mmol), phenyl acetylene (1.1 mmol), Fe3O4@SiO2–DAQ–Cu(II) catalyst and solvent (3 mL).b Isolated yield.
1	0.5 mol%	CH3CH2OH	10	82
2	0.5 mol%	CH3OH	10	78
3	0.5 mol%	i-PrOH	10	35
4	0.5 mol%	CH2Cl2	10	36
5	0.5 mol%	CH3CN	10	78
6	0.5 mol%	DMSO	10	71
7	0.5 mol%	DMF	10	67
8	0.5 mol%	THF	10	26
9	0.5 mol%	Toluene	10	33
10	0.5 mol%	H2O	10	82
11	0.5 mol%	H2O/ACN	4	95
12	None	H2O/ACN	24	—
13	0.1 mol%	H2O/ACN	10	31
14	0.2 mol%	H2O/ACN	10	64
15	0.3 mol%	H2O/ACN	6	81
16	0.4 mol%	H2O/ACN	4	91
17	0.6 mol%	H2O/ACN (1 : 1)	5	92

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After optimizing the reaction conditions, scope of the Fe₃O₄@SiO₂–DAQ–Cu(II) catalyzed one-pot CuAAC was investigated for variety of boronic acids and alkynes. The results are summarized in Table 4. Various substituted aromatic and heteroaromatic boronic acids and different aromatic and aliphatic alkynes were investigated (Table 4). It was noticed that the triazole products were formed in excellent yields with varying reaction times. The substrate scope of on pot synthesis clearly shows that the reaction time depended on both aryl boronic acid as well as alkyne (Table 4). However, reaction of phenyl boronic acid with phenyl acetylene required 4 h for completion of the reaction (Table 4, 7a). 4-Phenoxyphenyl boronic acids as well as formyl substituted phenyl boronic acid produced corresponding triazoles in 5 h (Table 4, 7d, e). In contrast, as seen in Table 4, the present method is not only suitable for aromatic alkynes but can also successfully be applied to aliphatic alkynes (7g–l). Heteroaromatic boronic acids such as thiophen-3-ylboronic acid and indol-5-yl boronic acid participated well in these reactions producing corresponding triazoles in excellent yields (Table 4, 7f, j).

Table 4 Fe₃O₄@SiO₂–DAQ–Cu(II) catalyzed one-pot synthesis of 1-aryl-1,2,3-triazoles^a

a Reaction conditions: aryl boronic acid (1 mmol), sodium azide (2 mmol), alkyne (1.1 mmol), Fe₃O₄@SiO₂–DAQ–Cu(II) catalyst (0.5 mol%), H₂O/ACN (1 : 1), rt.

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A mechanism for the catalytic activity of Fe₃O₄@SiO₂–DAQ–Cu(II) in the synthesis of 1-aryl-1,2,3-triazole derivatives is shown in Scheme 2. The mechanism of CuAAC revealed that sodium azide, which is used as azidonating reagent in one-pot protocol reduces Cu(II) to click-active Cu(I).⁵⁸ Thus, the plausible mechanism for one-pot Fe₃O₄@SiO₂–DAQ–Cu(II) catalyzed CuAAC reaction is depicted in the Scheme 2.

Scheme 2 Plausible mechanism of one-pot synthesis of 1-aryl-1,2,3-triazoles using Fe₃O₄@SiO₂–DAQ–Cu(II) catalyst.

The recycling of the catalyst is an important issue in heterogeneous catalysis system. Thus, we turn our attention to reusability of our Fe₃O₄@SiO₂–DAQ–Cu(II) catalyst in the preparation of 1-phenyl-1H-imidazole (3a) and 1,4-diphenyl-1H-1,2,3-triazole (7a) under optimized conditions. It is noteworthy that the catalyst could be used six times without significance loss of its activity (Fig. 1a). Thus, it is reasonable to believe that the immobilized catalyst can be repeatedly used for large-scale production without significant loss of its catalytic activity.

Fig. 1 (a) Recyclability of Fe₃O₄@SiO₂–DAQ–Cu(II) in the synthesis of 3a and 7a under the optimized conditions; (b) photograph of a magnet attracting Fe₃O₄@SiO₂–DAQ–Cu(II) in solution; (c) and (d) TEM and DLS images of Fe₃O₄@SiO₂–DAQ–Cu(II) nanoparticles after six reaction cycles.

The superparamagnetic property of the particle is very important for its application.

As shown in Fig. 1b, the magnetic Fe₃O₄@SiO₂–DAQ–Cu(II) particles in solution were black emulsion, and it could be separated from solution in a short period under an external magnetic field. Thus, the attraction and dispersion processes can be readily altered by applying and removing an external magnetic field, showing good dispersion and magnetic separation.

Also, the morphology of the recycled Fe₃O₄@SiO₂–DAQ–Cu(II) particles after six runs was analyzed. Fig. 1c show representative TEM image of the catalyst after 6 cycles of catalytic reaction. As revealed by TEM, the obtained magnetite particles possess approximately spherical shapes and a mean diameter of 60 nm (Fig. 1c).

We did not observe significant change in the morphology of the catalyst, but some particles might have aggregated. DLS measurements were also carried out to measure the hydrodynamic diameter of the nanoparticles after six reaction cycles. This size distribution is centered at a value of 70 nm (Fig. 1d). Generally, the size of nanoparticles will be increased after each cycle and leaching of copper, and increasing of catalyst size lead to a decrease in the yield.

To determine the exact species responsible for the observed reactions and to measure the extent of Cu leaching after the reactions, we have used the hot filtration test.^59,60 For this aim, we have studied the reaction of iodobenzene, imidazole and cesium carbonate in the presence of Fe₃O₄@SiO₂–DAQ–Cu(II) catalyst under optimized conditions. When the reaction was half complete, the nanoparticles were removed in situ at the reaction temperature, and the reactants were allowed to undergo further reaction in the solution. We confirmed that there was no further conversion of the reactants in the absence of the nanoparticles. These results indicate that there was little of the active metal species in the solution phase and confirmed that the catalyst acts heterogeneously in the reaction.

Also, in order to determine the absolute amount of the copper species dissolved in solution caused by leaching, the crude reaction mixtures were evaporated to dryness and analyzed using Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). It was shown that 2.67 wt% (0.0061 mmol g⁻¹) of the total amount of the original copper species was lost into solution during the course of the reaction after six cycles. Therefore, the analysis of the reaction mixture by the ICP technique showed that the leaching of Cu was negligible.

3. Conclusion

In conclusion, we developed a mild and practical Fe₃O₄@SiO₂–DAQ–Cu(II) catalyzed N-arylation of N(H)-heterocycles and alkylamines with aryl halides to afford corresponding coupled products in good to excellent yields without using any external ligands or additives as promoters. N-Arylated products were isolated in good to excellent yields, demonstrating the versatility of the protocol. This heterogeneous catalyst, also efficiently works for the efficient triazole synthesis stating from NaN₃, alkynes, and aryl boronic acid derivatives in H₂O/ACN at room temperature. Heterogeneous nature, thermal stability, easy separation of the catalyst, simple procedure, excellent yields, short reaction time, easy product separation and lower loading of catalyst makes this method an instrumental alternative to the previous methodologies for the scale up of these reactions. In addition, these methods offer the competitiveness of recyclability of the catalyst without significant loss of catalytic activity, and the catalyst could be easily recovered by external magnetic field and reused for at least 6 cycles.

4. Experimental

4.1. Materials and physical measurements

All chemicals were of analytical grade and used as received. The progress of the reactions was followed by TLC using silica gel SILG/UV 254 plates. IR spectra were recorded with KBr pellets using a Shimadzu 8300 spectrophotometer. X-ray diffraction (XRD) pattern was recorded on a Bruker AXS D8-advance X-ray diffractometer using Cu Kα radiation (λ = 1.5418 Å). The morphology and size of the particles were examined by transmission electron microscopy (TEM) using a Philips EM208 transmission electron microscope. Field emission scanning electron microscopy (FE-SEM) was recorded on a Hitachi S-4160 instrument.

Particle sizes of MNPs were measured using a HORIBA-LB550 dynamic light scattering. The X-ray photoelectron spectra (XPS) were recorded on the XR3E2 (VG Microtech) twin anode X-ray source using AlKα = 1486.6 eV. The ¹H and ¹³C NMR spectra were recorded on a Bruker Avance DPX 250 MHz spectrometer (using CDCl₃ with TMS as the standard). The Cu analysis and leaching test was carried out by ICP analyzer (Varian, Vista-pro). The element analyses (C, H, N) were obtained from a Thermo finigan Flash EA-1112 CHNSO rapid elemental analyzer. Melting points were recorded with an Electrothermal Type 9100 melting point apparatus. Therefore, all of the products were characterized by FT-IR, ¹H NMR and ¹³C NMR, and also by comparison with authentic samples.

4.2. General procedure

4.2.1. Preparation of Fe₃O₄@SiO₂ core–shell. The preparation of Fe₃O₄ nanoparticles were performed according to the previous reports.⁶¹ Briefly, FeCl₃·6H₂O (1.3 g, 4.8 mmol), 0.9 g of FeCl₂·4H₂O (4.5 mmol) and polyvinyl alcohol (PVA 15000) (1 g) as a surfactant were dissolved in 30 mL of water. The mixture was stirred vigorously for 30 min in 80 °C. Then, to this mixture, hexamethylenetetramine (HMTA) (1.0 mol L⁻¹) was added with vigorous stirring. After the color of bulk solution turned to black (pH: 10.0), the magnetite precipitates were continuously stirred with N₂ protection at 60 °C for 2 h. Subsequently the obtained magnetite particles were separated from the suspension using a permanent magnet and washed with ethanol three times and dried at 80 °C for 10 h.

Then, Fe₃O₄@SiO₂ core–shell nanospheres were prepared through a modified Stöber method.⁶² In a typical process, 0.05 g of synthesized Fe₃O₄ nanoparticles were dispersed in solution composed of 5 mL of deionized water, 50 mL of ethanol and 5 mL of NaOH (10 wt%). Subsequently, 0.2 mL TEOS (tetraethoxysilane) was slowly added and the mixture was stirred vigorously. After being stirring for 30 min, the Fe₃O₄@SiO₂ nanoparticles were magnetically collected, washed with ethanol and deionized water, and then dried under vacuum at 80 °C for 10 h.

4.2.2. General procedure for the preparation of functionalized Fe₃O₄@SiO₂ nanoparticle with 3-(triethoxysilyl)-propylamine (Fe₃O₄@SiO₂–NH₂). The obtained Fe₃O₄@SiO₂ nanoparticles (1 g) were dispersed in 10 mL ethanol solution by sonication for 20 min, and then 3-aminopropyl (triethoxy) silane (1 mmol, 0.176 g) was added to the mixture. The above mixture was stirred vigorously and refluxed for 12 h under nitrogen atmosphere. Then, the as prepared functionalized MNPs nanoparticles were separated by an external magnet and washed thoroughly with ethanol and water to remove the unattached substrates. Finally, the precipitated product (Fe₃O₄@SiO₂–NH₂) was dried in oven at 80 °C for 6 h.²⁷

4.2.3. Preparation of (4-hydroxy-9,10-dioxo-9,10-dihydroanthracen-1-yloxy)-acetic acid ethyl ester (2). In a typical run, a mixture of DMF (10 mL), ethyl 2-bromoacetate (0.5 mmol), 1,4-dihydroxyanthraquinone (1.0 mmol) and t-BuOK (0.5 mmol) was intensively stirred at room temperature. The reaction was monitored by TLC and it was found to be completed by 18 h. After completion of the reaction, 20 mL water was added, the mixture was filtered, and the residue was dried. The crude product (2) was purified by silica gel column and eluted with EtOAc : nhexane (1 : 1) to afford pure compound.⁶⁰

4.2.4. Preparation of Fe₃O₄@SiO₂–DAQ nanoparticles. The modification of Fe₃O₄@SiO₂–NH₂ with (4-hydroxy-9,10-dioxo-9,10-dihydro-anthracen-1-yloxy)-acetic acid ethyl ester (2) was carried out via ligand formation between the amino group and the carbonyl group of ester (2). In brief, the as-prepared Fe₃O₄@SiO₂–NH₂ (0.5 g) nanoparticles were suspended in ethanol (10 mL). After addition of (4-hydroxy-9,10-dioxo-9,10-dihydro-anthracen-1-yloxy)-acetic acid ethyl ester (2) (1 mmol, 0.325 g), the mixture was refluxed stirring under inert atmosphere for 24 h. Then, the resulting product (Fe₃O₄@SiO₂–DAQ) was collected by a magnet, washed with distilled water and ethanol and dried under vacuum at 70 °C.⁶⁰

4.2.5. Preparation of Fe₃O₄@SiO₂–DAQ–Cu(II) nanoparticles. Fe₃O₄@SiO₂–DAQ (0.5 g) was dispersed into 10 mL of absolute ethanol, and 0.182 g of Cu(OAc)₂ (1 mmol) was added into this dispersion, then this mixed system was refluxed for 24 h at the nitrogen atmosphere. The obtained Fe₃O₄@SiO₂–DAQ–Cu(II) NPs were collected by magnetic separation and washed with deionized water and ethanol three times and finally dried under vacuum at 80 °C for 10 h.⁶⁰

4.2.6. General procedure for the N-arylation of amines with aryl halides. In an oven dried 10 mL round-bottom flask, aryl halide (1 mmol), amine (1.2 mmol), Cs₂CO₃ (2 mmol), Fe₃O₄@SiO₂–DAQ–Cu(II) magnetic catalyst (0.02 g, 0.45 mol%) and DMF (3 mL) were stirred under nitrogen atmosphere at 100 °C and the reaction was monitored by TLC. After completion of the reaction and separation of the nanocatalyst with an external magnetic field, water (20 mL) was added and the mixture was extracted with ethyl acetate. The organic phase was washed with water (2 × 5 mL) and dried over anhydrous Na₂SO₄ and concentrated to get the crude product. The resulting crude product was purified by flash chromatography to give the desired products.

4.2.7. General procedure for one-pot synthesis of 1,4-disubstituted 1,2,3-triazoles. To a flask equipped with a magnetic stir bar, aryl boronic acid (1.0 mmol), sodium azide (2.0 mmol), alkyne (1.1 mmol), Fe₃O₄@SiO₂–DAQ–Cu(II) catalyst (0.022 g, 0.5 mol%), and H₂O/ACN (1 : 1) (3.0 mL) were added and the mixture was stirred at room temperature. The progress of the reaction was monitored by thin layer chromatography (TLC). After stirring for the appropriate time, the catalyst was removed using magnetic field. Then, the filtrate was extracted with EtOAc (3 × 10 mL) and dried over anhydrous sodium sulfate. Combined organic layer was evaporated to dryness, and the solid residue recrystallized from EtOH/H₂O (1 : 3 v/v) to give a crystalline pure product.

4.3. Spectral data

4.3.1. 1-Phenyl-1H-imidazole (3a). Dark brown oil, ¹H NMR (250 MHz, CDCl₃): δ = 7.14–7.45 (m, 7H), 7.80 (s, 1H); ¹³C NMR (62.9 MHz, CDCl₃): δ = 118.2, 121.4, 127.5, 129.9, 130.4, 135.5, 137.4.

4.3.2. 2-Imidazol-1-ylpyridine (3g). Dark brown oil, ¹H NMR (250 MHz, CDCl₃): δ = 7.14–720 (m, 2H), 7.28–7.34 (m, 1H), 7.25–7.34 (m, 1H), 7.58 (s, 1H), 7.73–7.80 (m, 1H), 8.28 (s, 1H), 8.41–8.43 (m, 1H); ¹³C NMR (62.9 MHz, CDCl₃): δ = 112.3, 116.7, 122.0, 130.6, 132.2, 134.9, 139.0, 149.1; anal. calcd for C₈H₇N₃: C, 66.19; H, 4.86; N, 28.95%. Found: C, 65.89; H, 4.97; N, 29.14%.

4.3.3. 1-Phenyl-1H-indole (3h). Dark brown oil, ¹H NMR (250 MHz, CDCl₃): δ = 6.59–6.63 (m, 1H), 7.06–7.17 (m, 2H), 7.25–7.30 (m, 2H), 7.41–7.52 (m, 5H), 7.59–7.64 (m, 1H); ¹³C NMR (62.9 MHz, CDCl₃): δ = 103.6, 110.5, 120.4, 121.1, 122.4, 124.4, 126.4, 128.0, 129.3, 129.6, 135.8, 139.8; anal. calcd for C₁₄H₁₁N: C, 87.01; H, 5.75; N 7.25%. Found: C, 86.91; H, 5.86; N, 7.23%.

4.3.4. 1-p-Tolyl-1H-indole (3i). Dark brown oil, ¹H NMR (250 MHz, CDCl₃): δ = 2.35 (s, 3H), 6.58 (dd, J = 4.2 Hz, 1H), 7.04–7.32 (m, 7H), 7.43–7.47 (m, 1H), 7.58–7.62 (m, 1H); ¹³C NMR (62.9 MHz, CDCl₃): δ = 21.1, 103.2, 110.5, 120.1, 121.1, 122.2, 124.3, 128.1, 129.2, 130.1, 136.3, 137.3; anal. calcd for C₁₅H₁₃N: C, 86.92; H, 6.32; N, 6.76%. Found: C, 86.57; H, 6.55; N, 6.88%.

4.3.5. 1-Pyridin-2-yl-1H-indole (3j). Dark brown oil, ¹H NMR (250 MHz, CDCl₃): δ = 6.56 (d, J = 3.5 Hz, 1H), 6.97 (t, J = 6.0 Hz, 1H), 7.00–7.25 (m, 3H), 7.49–7.56 (m, 3H), 8.09 (d, J = 8.2 Hz, 1H), 8.36–8.39 (m, 1H); ¹³C NMR (62.9 MHz, CDCl₃): δ = 105.7, 113.3, 114.6, 120.2, 121.2, 121.5, 123.3, 126.1, 130.6, 135.2, 138.5, 149.0, 152.5; anal. calcd for C₁₃H₁₀N₂: C, 80.39; H, 5.19; N, 14.42%. Found: C, 80.57; H, 5.09; N, 14.34%.

4.3.6. 2-Methyl-1-phenyl-1H-indole (3k). Dark brown oil, ¹H NMR (250 MHz, CDCl₃): δ = 2.22 (s, 3H), 6.32 (s, 1H), 7.00–7.07 (m, 3H), 7.25–7.29 (m, 2H), 7.36–7.51 (m, 4H); ¹³C NMR (62.9 MHz, CDCl₃): δ = 13.4, 101.3, 110.0, 119.6, 120.0, 121.0, 127.7, 128.0, 128.2, 129.4, 137.0, 138.2; anal. calcd for C₁₅H₁₃N: C, 86.92; H, 6.32; N, 6.76%. Found: C, 86.72; H, 6.45; N, 6.83%.

4.3.7. 1-Phenyl-1H-benzimidazole (3l). Brown oil, ¹H NMR (250 MHz, CDCl₃): δ = 7.32–7.36 (m, 2H), 7.44–7.61 (m, 6H), 7.89–7.90 (m, 1H), 8.17 (s, 1H); ¹³C NMR (62.9 MHz, CDCl₃): δ = 110.5, 120.6, 122.8, 123.7, 124.0, 128.0, 130.0, 136.3, 142.8, 144.2; anal. calcd for C₁₃H₁₀N₂: C, 80.39; H, 5.19; N, 14.42%. Found: C, 80.51; H, 5.01; N, 14.48%.

4.3.8. 1-Pyridin-2-yl-1H-benzimidazole (3n). Brown oil, ¹H NMR (250 MHz, CDCl₃): δ = 7.26–7.39 (m, 3H), 7.58 (d, J = 8.2 Hz, 1H), 7.86–7.93 (m, 2H), 8.06 (d, J = 7.8 Hz, 1H), 8.58–8.62 (m, 2H); ¹³C NMR (62.9 MHz, CDCl₃): δ = 112.6, 114.3, 120.6, 121.8, 123.3, 124.2, 132.1, 138.9, 141.3, 144.7, 149.5, 149.9; anal. calcd for C₁₂H₉N₃: C, 73.83; H, 4.65; N, 21.52%. Found: C, 73.62; H, 4.76; N, 21.62%.

4.3.9. 1-Phenyl-1H-pyrrole (3o). Dark brown oil, ¹H NMR (250 MHz, CDCl₃): δ = 6.27 (t, J = 4.5 Hz, 2H), 7.00 (t, J = 4.5 Hz, 2H), 7.12–7.18 (m, 1H), 7.28–7.33 (m, 4H); ¹³C NMR (62.9 MHz, CDCl₃): δ = 110.6, 119.4, 120.6, 125.7, 129.7, 140.9; anal. calcd for C₁₀H₉N: C, 83.88; H, 6.34; N, 9.78%. Found: C, 83.64; H, 6.43; N, 9.93%.

4.3.10. 1,4-Diphenylpiperazine (3p). Dark brown solid, mp 149–150 °C, ¹H NMR (250 MHz, CDCl₃): δ = 3.36 (s, 8H), 6.88–7.02 (m, 6H), 7.28–7.34 (m, 4H); ¹³C NMR (62.9 MHz, CDCl₃): δ = 49.5, 116.4, 120.1, 129.2, 151.3; anal. calcd for C₁₆H₁₈N₂: C, 80.63; H, 7.61; N, 11.75%. Found: C, 80.89; H, 7.52; N, 11.59%.

4.3.11. 1-Phenyl-4-pyridin-2-ylpiperazine (3r). Light brown solid, mp 100–101 °C, ¹H NMR (250 MHz, CDCl₃): δ = 3.24 (dd, J¹ = 7.0 Hz, J² = 5.0 Hz, 4H), 3.64 (t, J = 5.2 Hz, 4H), 6.56–6.65 (m, 2H), 6.79–6.93 (m, 3H), 7.18–7.25 (m, 2H), 7.40–7.47 (m, 1H), 8.14–8.16 (m, 1H); ¹³C NMR (62.9 MHz, CDCl₃): δ = 45.3, 49.2, 107.2, 113.6, 116.3, 120.1, 129.2, 137.6, 148.0, 151.3, 159.4; anal. calcd for C₁₅H₁₇N₃: C, 75.28; H, 7.16; N, 17.56%. Found: C, 75.38; H, 7.27; N, 17.35%.

4.3.12. N,N-Dibutylaniline (3s). Black oil, ¹H NMR (250 MHz, CDCl₃): δ = 0.88 (t, J = 7.5 Hz, 6H), 1.20–1.32 (m, 4H), 1.46–1.55 (m, 4H), 3.18 (t, J = 7.5 Hz, 4H), 6.54–6.58 (m, 3H), 7.09–7.17 (m, 2H); ¹³C NMR (62.9 MHz, CDCl₃): δ = 14.0, 20.4, 29.4, 50.8, 111.6, 115.0, 129.2, 148.2.

4.3.13. N,N-Diethylaniline (3t). Dark brown oil, ¹H NMR (250 MHz, CDCl₃): δ = 1.18 (t, 6H), 3.37 (q, 4H), 6.63–6.73 (m, 3H), 7.20–7.27 (m, 2H); ¹³C NMR (62.9 MHz, CDCl₃): δ = 12.6, 44.3, 111.8, 115.3, 129.3, 147.8; anal. calcd for C₁₀H₁₅N: C, 80.48; H, 10.13; N, 9.39%. Found: C, 80.25; H, 10.03; N, 9.72%.

4.3.14. 1,4-Diphenyl-1H-1,2,3-triazole (7a). White solid; mp: 174–175 °C (175–177 °C);^{58 1}H NMR (250 MHz, CDCl₃) δ: 7.26 (d, J = 1.6 Hz, 1H), 7.38 (t, J = 7.6 Hz, 1H), 7.47 (t, J = 7.6 Hz, 2H), 7.56 (t, J = 8.0 Hz, 2H), 7.81 (d, J = 8.0 Hz, 2H), 7.93 (d, J = 7.6 Hz, 2H), 8.20 (s, 1H); ¹³C NMR (62.9 MHz, CDCl₃) δ: 117.6, 120.6, 125.9, 128.4, 128.8, 128.9, 129.8, 130.3, 137.1, 148.5; IR (KBr, cm⁻¹): 3118, 2919, 2356, 1729, 1463, 1236, 1070.

4.3.15. 1-(4-Hydroxyphenyl)-4-phenyl-1H-1,2,3-triazole (7b). White solid; mp: 207–208 °C (206–208 °C);^{58 1}H NMR (250 MHz, CD₃OD) δ: 6.96 (d, J = 9.2 Hz, 2H), 7.37 (t, J = 6.0 Hz, 1H), 7.46 (t, J = 7.6 Hz, 2H), 7.68 (d, J = 6.4 Hz, 2H), 7.91 (d, J = 7.2 Hz, 2H), 8.75 (s, 1H); ¹³C NMR (62.9 MHz, DMSO-d₆) δ: 116.0, 119.5, 121.9, 125.3, 128.1, 128.8, 128.9, 130.4, 146.9, 157.8; IR (KBr, cm⁻¹): 3135, 1603, 1520, 1229, 1055.

4.3.16. 1-(4-Methoxyphenyl)-4-phenyl-1H-1,2,3-triazole (7c). White solid; mp: 156–157 °C (155–159 °C);^{58 1}H NMR (250 MHz, CDCl₃) δ: 3.89 (s, 3H), 7.06 (d, J = 7.2 Hz, 2H), 7.35 (t, J = 7.2 Hz, 1H), 7.46 (t, J = 7.4 Hz, 2H), 7.71 (d, J = 7.4 Hz, 2H), 7.92 (d, J = 7.2, 2H), 8.11 (s, 1H); IR (KBr, cm⁻¹): 3133, 2956, 2356, 1722, 1517, 1229, 1041.

4.3.17. 1-(4-Phenoxy-phenyl)-4-phenyl-1H-1,2,3-triazole (7d). White solid; mp: 170–171 °C (171–173 °C);^{58 1}H NMR (250 MHz, CDCl₃) δ: 7.08 (d, J = 8.5 Hz, 2H), 7.16 (m, 3H), 7.34 (m, 3H), 7.44 (t, J = 7.7 Hz, 2H), 7.74 (d, J = 8.9 Hz, 2H), 7.92 (d, J = 8.3 Hz, 2H), 8.15 (s, 1H); ¹³C NMR (62.9 MHz, CDCl₃) δ: 117.8, 119.3, 119.5, 122.3, 124.2, 125.9, 128.4, 128.9, 130.0, 132.3, 148.4, 156.4, 158.0; IR (KBr, cm⁻¹): 3125, 2923, 2356, 1585, 1511, 1247.

4.3.18. 1-(3-Formylphenyl)-4-phenyl-1H-1,2,3-triazole (7e). White solid; mp: 164–166 °C (162–165 °C);^{58 1}H NMR (250 MHz, CDCl₃) δ: 7.41 (t, J = 4.0 Hz, 1H), 7.50 (t, J = 5.6 Hz, 2H), 7.77 (t, J = 8.0 Hz, 1H), 7.94 (d, J = 7.2 Hz, 2H), 7.99 (d, J = 7.6 Hz, 1H), 8.19 (d, J = 7.2 Hz, 1H), 8.30 (d, J = 7.61 Hz, 2H), 10.13 (s, 1H); ¹³C NMR (62.9 MHz, CDCl₃) δ: 117.4, 120.1, 125.9, 128.7, 129.0, 129.9, 130.6, 130.8, 137.7, 137.8, 148.9, 190.8; IR (KBr, cm⁻¹): 3137, 2817, 1703, 1597, 1238, 1011.

4.3.19. 1-Thiophen-3-yl-4-phenyl-1H-1,2,3-triazole (7f). White solid; mp: 166–167 °C (164–166 °C);^{58 1}H NMR (250 MHz, CDCl₃) δ: 7.32–7.38 (m, 1H), 7.44 (m, 1H), 7.48–7.50 (m, 1H), 7.52–7.53 (m, 1H), 7.61–7.62 (q, J = 1.2, 3.2 Hz, 1H), 7.68 (m, 1H), 7.90 (s, 1H), 7.92 (d, J = 1.6 Hz, 1H), 8.11 (s, 1H); ¹³C NMR (62.9 MHz, CDCl₃) δ: 114.1, 118.0, 120.9, 125.9, 127.3, 128.5, 128.9, 130.2, 135.9, 148.0; IR (KBr, cm⁻¹): 3107, 1559, 1445, 1229, 1072.

4.3.20. 1-(4-Hydroxyphenyl)-4-cyclohexyl-1H-1,2,3-triazole (7g). White solid; mp: 169–170 °C (167–169 °C);^{58 1}H NMR (250 MHz, CDCl₃) δ: 1.23–1.35 (m, 2H), 1.38–1.50 (m, 3H), 1.76 (m, 1H), 1.85 (m, 2H), 2.14 (m, 2H), 2.86 (m, 1H), 6.25 (brs, OH), 7.03 (d, J = 8.8 Hz, 2H), 7.56–7.60 (m, 3H); ¹³C NMR (62.9 MHz, CDCl₃) δ: 26.0, 26.1, 33.0, 35.2, 116.6, 118.1, 122.2, 130.0, 154.0, 157.4; IR (KBr, cm⁻¹): 3141, 2928, 2358, 1601, 1517, 1225, 1063.

4.3.21. 1-(4-Methoxyphenyl)-4-cyclohexyl-1H-1,2,3-triazole (7h). White solid; mp: 91–92 °C (90–93 °C);^{58 1}H NMR (250 MHz, CDCl₃) δ: 1.26–1.30 (m, 1H), 1.40 (m, 4H), 1.80–1.90 (m, 3H), 2.18 (m, 2H), 2.82 (m, 1H), 3.82 (s, 3H), 6.98 (d, J = 6.8 Hz, 2H), 7.62 (m, 3H); ¹³C NMR (62.9 MHz, CDCl₃) δ: 26.2, 29.4, 33.1, 35.8, 55.0, 114.7, 117.2, 121.8, 132.5, 154.2, 159.6; IR (KBr, cm⁻¹): 3130, 2924, 2361, 1725, 1519, 1254, 1046.

4.3.22. 1-(Naphth-2-yl)-4-cyclohexyl-1H-1,2,3-triazole (7i). White solid; mp: 141–142 °C (139–141 °C);^{58 1}H NMR (250 MHz, CDCl₃) δ: 1.22–1.36 (m, 1H), 1.39–1.55 (m, 4H), 1.79 (m, 1H), 1.88 (m, 2H), 2.16 (m, 2H), 2.90–2.92 (m, 1H), 7.51–7.58 (m, 2H), 7.81 (s, 1H), 7.87–7.92 (m, 3H), 8.00 (d, J = 8.8 Hz, 1H), 8.15 (s, 1H); ¹³C NMR (62.9 MHz, CDCl₃) δ: 26.1, 26.2, 29.7, 33.0, 35.4, 117.7, 118.1, 119.0, 126.8, 127.3, 127.9, 128.2, 129.9, 132.7, 133.3, 134.8, 154.5; IR (KBr, cm⁻¹): 3128, 2926, 1566, 1501, 1239, 1038.

4.3.23. 1-(Indol-5-yl)-4-cyclohexyl-1H-1,2,3-triazole (7j). Light pink solid; mp: 138–139 °C (136–139 °C);^{58 1}H NMR (250 MHz, CDCl₃) δ: 1.21–1.37 (m, 2H), 1.36–1.51 (m, 3H), 1.77 (m, 1H), 1.86 (m, 2H), 2.16 (m, 2H), 2.87 (m, 1H), 6.63 (t, J = 2.4 Hz, 1H), 7.33 (t, J = 2.8 Hz, 1H), 7.47–7.56 (m, 2H), 7.68 (s, 1H), 7.91 (d, J = 1.6 Hz, 1H), 8.48 (s, 1H); ¹³C NMR (62.9 MHz, CDCl₃) δ: 26.1, 26.2, 33.1, 35.4, 103.3, 111.8, 113.2, 115.9, 118.5, 126.2, 128.0, 130.9, 135.4, 154.0; IR (KBr, cm⁻¹): 3349, 2921, 2360, 1444, 1223, 1046.

4.3.24. 1-(4-Hydroxyphenyl)-4-octyl-1H-1,2,3-triazole (7k). Light pink solid; mp: 99–101 °C (98–100 °C);^{58 1}H NMR (250 MHz, CDCl₃) δ: 0.88 (t, J = 6.8 Hz, 3H), 1.26–1.39 (m, 10H), 1.72–1.76 (m, 2H), 2.79 (t, J = 7.6 Hz, 2H), 7.06 (d, J = 7.2 Hz, 2H), 7.58 (d, J = 6.8 Hz, 2H), 7.64 (s, 1H); 14.1, 22.7, 25.5, 29.2 (2C), 29.3, 29.4, 31.9, 116.7, 119.5, 122.3, 129.9, 148.8, 157.8; IR (KBr, cm⁻¹): 3122, 2920, 1600, 1236, 1057.

4.3.25. 1-(4-Methoxyphenyl)-4-octyl-1H-1,2,3-triazole (7l). Light yellow solid; mp: 54–56 °C (53–55 °C);^{58 1}H NMR (250 MHz, CDCl₃) δ: 0.94 (t, J = 6.4 Hz, 3H), 1.27–1.43 (m, 10H), 1.72 (m, 2H), 2.82 (t, J = 7.6 Hz, 2H), 3.89 (s, 3H), 7.03 (d, J = 6.8 Hz, 2H), 7.61–7.71 (m, 3H); ¹³C NMR (62.9 MHz, CDCl₃) δ: 14.1, 22.7, 25.7, 29.4, 29.5, 29.7, 30.4, 31.9, 55.6, 114.7, 119.0, 122.4, 132.5, 149.0, 159; IR (KBr, cm⁻¹): 3127, 2919, 2357, 1725, 1521, 1256, 1048.

Acknowledgements

This work was supported by council of Iran National Science Foundation and University of Shiraz from the Iran Society for the Promotion of Science.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra16646f

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