ZnS/CuFe₂O₄: Magnetic Hybrid Nanocomposite to Catalyze the Synthesis of 2,4,5-triaryl-1H-imidazole Derivatives ^†

Abstract

Integration of nanomaterials is an entirely new method to synthesis efficient catalysts. These compounds provided new characteristics and distinctive application which is not accessible in the single-particle nanostuctures. Although there is little catalytic activity in each component of the hybrid material, their hybrid displays much higher activity. Indeed, the presence of intermediate metal and their oxides in the framework of hybrid catalyst caused a synergistic effect, thus facilitate the organic reaction more effectively. The extensive biochemical and pharmacological activities of imidazole-containing compounds have required the development of efficient methods for synthesizing these compounds, which is a significant topic in organic chemistry. The imidazole nucleus function as a main scaffold for constructing of biologically important molecules. The ZnS/CuFe₂O₄ magnetic hybrid nanocatalyst was synthesized by a simple co precipitation and characterized by conventional analyses successfully. Synthesized nanocomposite was utilized as a magnetic and heterogeneous catalyst for the one-pot synthesis of 2,4,5-triaryl-1H-imidazole derivatives with condensation of various aromatic aldehydes, benzil and ammonium acetate. The presented method shows some advantages such as mild conditions, good yields, and simple separation of products from the reaction mixture and cost-effective catalyst. The experimental data showed ZnS/CuFe₂O₄ nanocatalyst were easily separated from the reaction mixture using an external magnetic field and use again five times in subsequent reactions without appreciable reduction in catalytic activity.

1. Introduction

Within the last decades nonmagnetic catalysts have gained significant attention, since they possess merits of both homogeneous and heterogeneous catalysts like high reactivity and ease of separation from the reaction mixture [1]. For manufacturing magnetic catalyst, iron seems to be the most suitable case because of its more natural abundance, ease of availability, low-cost and less toxicity than other metals with magnetic nature [2]. Nanomaterial combination is a new approach for the synthesis of effective catalyst. Such compounds provided new features and distinctive applications that are not present in single-particle nanoparticles [3]. ferrites are important magnetic material with unique features that has wide applications in a variety of fields such as sensors, environmental filtration, magnets and electrical materials, magnetic resonance imaging, ceramic coating. CuFe₂O₄ has inverse spinel structure which in that Cu²⁺ cations and half of the Fe³⁺ cations occupy octahedral sites, while the other half of the Fe³⁺ cations occupy tetrahedral sites. Inverse spinel ferrites have been utilized as catalysts in reactions such as azide-alkyne cycloaddition, epoxide ring-opening and the Ullmann coupling [4,5].

The synthesis of imidazole as main scaffold for constructing of biologically active molecules is a noteworthy topic in organic chemistry. They have diverse medicinal and biological activities like antibacterial, antifungal, reducing inflammation, anti-cancer antiviral, anti-diabetic, anti-allergic, analgesic and herbicidal [6,7]. In continues to our research on MCRs and nanomaterials and due to the importance of heterocyclic compounds [6,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24], in this work, ZnS/CuFe₂O₄ was used as a hybrid nanocatalyst in the synthesis of 2,4,5-triaryl-1H-imidazole derivatives via one-pot condensation of various aromatic aldehydes, benzil and ammonium acetate (Scheme 1).

2. Experimental

2.1. General

All solvents and chemicals were purchased from Merck and Aldrich companies. EDX spectra were provided with a Numerix DXP–X10P. FESEM image was provided with Hitachi S-480 instrument. The XRD pattern of the nanocatalyst was recorded with the X′Pert Pro diffractometer operating with (40 mA, 40 kV). Melting points were measured with an Electrothermal 9100 apparatus.

2.2. Preparation of ZnS/CuFe₂O₄

At first thioacetamide (1.6 mmol) was dissolved in 20 ml distilled water. Then FeCl₃.6H₂O (2 mmol), CuCl₂.4H₂O (1 mmol) and Zn (NO₃)₂.2H₂O (1 mmol) were dissolved in 20 mL distilled water separately, and gradually added to the aqueous solution of thioacetamide. The resulting mixture was first dispersed using ultrasonic and then stirred at room temperature. Next, ammonia was added drop by drop to above mixture and stirring continued at room temperature for 30 min. The resulting precipitate was separated from the reaction mixture using magnets, washed several times with distilled water and ethanol, and dried at room temperature for 24 h to provide a ZnS/CuFe₂O₄ magnetic catalyst.

2.3. General Procedure for the Synthesis of 2,4,5-triaryl-1H-imidazole Derivatives

The mixture of 4-chlorobenzaldehyde (1 mmol), ammonium acetate (4 mmol), benzil (1 mmol) and ZnS/CuFe₂O₄ (0.03 g) as a catalyst were stirred at 80 °C in ethanol. The reaction progress was monitored by thin layer chromatography. After completion of the reaction, the crude precipitate was washed with deionized water and dried. Pure products were obtained by recrystallization from ethanol.

3. Results and Discussion

Elemental analysis of the ZnS/CuFe₂O₄ nanocatalyst was carried out using EDX to study the chemical composition of the synthesized catalysts. As is observed in Figure 1, the presence of high-intensity peaks related to Zn, Cu, S, Fe and O elements represent the components of the synthesized catalyst.

FESEM image was used to study morphology and particle size of the synthetic nanocatalyst. Figure 2 shows spherical like nanoparticles with uniform distribution. The average of particle size in the ZnS/CuFe₂O₄ catalyst is less than 20 nm.

Furthermore, X-ray diffraction analysis to investigate the crystalline nature of catalyst was employed. As is depicted in Figure 3, the XRD pattern of ZnS/CuFe₂O₄ was compared with CuFe₂O₄ and ZnS. Prepared nanocatalyst has distinct peaks in 2θ = 29.80°, 32.21°, 35.71°, 43.31°, 48.40°, 57.25°, 59.66°, 63.62° which are consistent with the XRD pattern of ZnS and CuFe₂O₄.

Catalytic Application of ZnS/CuFe₂O₄ in the Synthesis of 2,4,5-triaryl-1H-imidazole Derivatives

Synthesizing of 2,4,5-triaryl-1H-imidazole derivatives using some aromatic aldehydes in optimum condition was studied in order to evaluate generality, applicability and limitation of this protocol. As can be observed in

Table 1. Synthesis of 2,4,5-triaryl-1H-imidazole derivatives.

Entry	R	Product	Yielda (%)	Mp (°C)
1	4-Cl	4a	92	259–261
2	3-NO2	4b	89	264–266
3	4-OMe	4c	83	228–230
4	3-OMe	4d	89	257–259
5	4-Me	4e	82	229–231

^a Isolated yield.

, all aromatic aldehydes with electron withdrawing and electron donating substitution produced the satisfying corresponding products.

4. Conclusions

ZnS/CuFe₂O₄ nanocomposite was prepared via a convenience procedure and was utilized as cost-effective hybrid heterogeneous catalyst in the synthesis of biologically active 2,4,5-triaryl-1H-imidazole derivatives. This protocol offers some merits such as mild reaction conditions, simple separation of products from the reaction mixture and retrievable catalyst.