EFFICIENTONE-POT SYNTHESIS OF IMIDAZOLES CATALYZED BY SILICA- SUPPORTED La0.5Pb0.5MnO3 NANO PARTICLES AS ANOVEL AND REUSABLE PEROVSKITE OXIDE

Silica-supported La0.5Pb0.5MnO3 nanoparticles was prepared and used as a new perovskite-type catalyst for rapid and efficient synthesis of substituted imidazoles by an one-pot three-component condensation of [9,10]-phenanthraquinone, aryl aldehydes and ammonium acetate in excellent yield under reflux, and also solventfree conditions.


INTRODUCTION
Imidazole derivatives are an important group of heterocyclic compounds that have biological and chemical properties [1]. Nowadays these compounds are used in pharmacology. They have different capacity such as anti-bacterial, anti-viral, and anti-cancer, anti-inflammatory [2][3][4]. Also, the imidazole ring system is one of the main substructures found in most of natural products and pharmacologically active compounds, such as the hypnotic agent [5]. Therefore, a great number of synthetic methods have been reported for the synthesis of multi substituted imidazoles [6].
Perovskite-type oxides with the chemical formula ABO 3 can be crystallized with cubic structure. The level of oxygen and defects is affected by the composition because of a wide constancy range of the structure. The greater A-site cation mostly represents a metal cation belonging to one of groups of rare earth metals (La, Sm, Pr), alkaline earth metals (Sr, Ba, Ca), or alkali metals (Na, K) coordinated to 12 oxygen anions. The B-site cation represents typical normally a smaller transition metal cation, which occupying occupies octahedral spaces in the oxygen framework. Some hybrids of A-and B-site cations can create a steady perovskite-like structure [18]. The catalytic activity of perovskite-type catalysts is mainly because of the existence of the multiplicity of oxidation states [19]. Perovskite-type oxides possess adsorption, acid-base, as well as redox properties, which leads to attractive catalytic activities. They can fulfil the requirements (excellent activity, constancy, and potential to be processed into structured catalysts) [20].

General
All materials and reagents were purchased from Merck and Aldrich and utilized with no more purification. Determination of melting points were carried out using an Electro thermal type 9100 melting point apparatus and are uncorrected. The IR spectra were recorded on a Thermo Nicolet AVATAR-370 FT-IR spectrophotometer. A Bruker DRX250 spectrometer was used to obtain 1H NMR spectra.

Preparation of La 0.5 Pb 0.5 MnO 3 nano particles
In current study, the perovskite precursor was synthesized using the citrate-based sol-gel improved Pechini technique [21]. Chemicals of La(NO 3 ) 3 .6H 2 O, C 6 H 9 MnO 6 .2H 2 O, Pb(NO 3 ) 2 and citric acid (99.5%) were utilized as raw materials. Next, a mixture of metal nitrates solutions with nominal La:Pb:Mn ratios of 0.5:0.5:1 (LPMO) was prepared in deionized water. The addition of citric acid to the metal solution was performed proportionally to have the similar equivalents. The solution was concentrated through evaporation at nearly 50 o C with agitating for 1 h for converting them to steady (La, Pb)/CA complexes. The solution was agitated on a hot plate at about 75 o C for removing excess water and accelerating polyesterification reaction. Next, the dry gel was achieved thought heating gradually to 120 o C in an oven. The gel pieces were milled to create a fine powder. Finally, La 0.5 Pb 0.5 MnO 3 NPs were synthesized through thermal treating the precursor at 650 o C for 9 h in air. Most of the residual carbon was removed trough annealing of the amorphous precursor and the hexagonal perovskite phase was achieved.

Preparation of silica-supported La 0.5 Pb 0.5 MnO 3 nanoparticles (30% w/w)
Initially silica was put into oven for 2 hours with 90 o C, then catalyst (0.3 g) with silica (0.7 g) were worn out into glass mortar about 1 hour when mixture were bright and unify.

Typical procedure synthesizing imidazole derivatives in reflux condition
A mixture of aldehyde (1 mmol), 9,10-phenanthraquinone (1 mmol), ammonium acetate (2.5 mmol) and S-LPMO nanoperovskite (0.04 g) in 10 mL ethanol was stirred. The resulted mixture was refluxed for described time. At the end of the reaction (the reaction progress was monitored by TLC using n-hexan:ethylacetateas eluent solution) the catalyst was separation with filtering, and solid perovskite catalyst was isolated and could be reused. The solid product resulted after evaporation of organic phase, was washed with cold water (3 × 20 mL), and recrystallized in ethanol to provide pure product.

Typical process for synthesizing imidazole derivatives in solvent-free condition
A mixture of aldehyde (1 mmol), 9,10-phenanthraquinone (1 mmol), ammonium acetate (2.5 mmol) and S-LPMO nano perovskite (0.04 g) was stirred. Then, the mixture was heated at a temperature of 100 o C for described time (the reaction progress was monitored by TLC using nhexan:ethylacetate as eluent). After cooling, the filtration of mixture was carried out and the filtrated mixture was washed using ethanol to separated catalyst. The filtrate was concentrated through evaporation under reduced pressure using a rotary evaporator. To obtain pure products, the recrystallization of remaining solid from ethanol was carried out.
Analytical data for selected compounds

RESULTS AND DISCUSSION
After synthesis of La 0.5 Pb 0.5 MnO 3 nanoparticles [21], for characterization of this catalyst, the morphology of the sample was determined using SEM analysis. Figure 1 demonstrates the micrograph related to the samples prepared through the sol-gel modified Pechini technique and calcined 650 o C. According to the SEM images, the surface is obviously porous and it appears that the size of grown particles is uniform. The size of pores is in the range of 30-188 nm. Moreover, SEM 160 image shows that besides the presence of bigger particles, there were relatively smaller particles on the surface. However, the main particles on the surface were the bigger ones. The aggregation of smaller particles (in the nm scale) may cause the creation of larger LPMO NPs on the surface. FT-IR spectra were obtained for single LPMO and S cm -1 . Figure 2 shows the FT-IR spectrum of LPMO perovskite nanoparticle, and Fig the FT-IR spectrum of S-LPMO stretching and bending vibration of H 1113 cm -1 is usually assigned to the Si cm -1 corresponding to LPMO perovskite coated on SiO [18,21].
For the purpose of optimization of the amount of catalyst and the reaction time, were executed various amount catalyst in presence ethanol solvent. A out on the synthesis of 2 condensing [9,10] phenanthraquinone acetate (2.5 mmol) in ethanol. For establish effectiveness o catalyst at ambient temperature. We were performed trace catalyst (entry 1). We increased temperature up to reflux condition, but final product was appreciable (entry 2). After th catalyst. Results summarized in Table 1 indicated the best result was with 0.04 g amount of catalyst but increasing the amount of catalyst when we supported LPMO on silica (S IR spectra were obtained for single LPMO and S-LPMO samples in range of 400 IR spectrum of LPMO perovskite nanoparticle, and Figure 3 shows LPMO sample. The IR bands at 3430 and 1638 cm -1 is due to the stretching and bending vibration of H 2 O molecules, respectively. The very strong and IR band at is usually assigned to the Si-O-Si asymmetric vibrations. The picks at 668 and 592 sponding to LPMO perovskite coated on SiO 2 are clearly observed in this spectrum For the purpose of optimization of the amount of catalyst and the reaction time, initially we were executed various amount catalyst in presence ethanol solvent. A model study was carried out on the synthesis of 2-(4-chlorophenyl)-1H-phenanthro[9,10-d]imidazole (3 g) via [9,10] phenanthraquinone (1 mmol), 4-choloro benzaldehyde (1 mmol), ammonium acetate (2.5 mmol) in ethanol. For establish effectiveness of LPMO, a test reaction done without catalyst at ambient temperature. We were performed trace amount of product in the absence of catalyst (entry 1). We increased temperature up to reflux condition, but final product was appreciable (entry 2). After that, reactions were reacted with different amounts of LPMO Results summarized in Table 1 indicated the best result was with 0.04 g amount of increasing the amount of catalyst did not improve the yield (entry 6 and 7). Also, supported LPMO on silica (S-LPMO), the amount of product was increased (entry 8 LPMO samples in range of 400-4000 3 shows is due to the O molecules, respectively. The very strong and IR band at Si asymmetric vibrations. The picks at 668 and 592 are clearly observed in this spectrum initially we model study was carried g) via ammonium f LPMO, a test reaction done without amount of product in the absence of catalyst (entry 1). We increased temperature up to reflux condition, but final product was not of LPMO Results summarized in Table 1 indicated the best result was with 0.04 g amount of ). Also, entry 8). Considering solvent plays a key role in reactions, reaction was reacted in presence different solvents. Results are presented in Table 2. We observed protic solvent including ethanol and methanol have higher yield in this method. The best result was achieved using ethanol(entry 3). This reaction had proper result in free solvent condition. Thus this reaction was considered in free solvent reaction similarly. At the end of the model reaction, the nano catalyst was separated from the reaction mixture, washed with acetone and dried at 100°C under vacuum for 2 h and reapplied four times for the same reaction. As presented in Table 3, the catalyst can be reused at least four times with no change in its activity.  Finally, after optimizing the reaction conditions, we prepared a range of replaced imidazoles (Table 4). In all cases, aldehydes reacted effectively with replaced carrying either electrondonating (entries 7-10) or electron-withdrawing (entry 2-4) groups and provided the estimated products in good to yields.
The mechanism of the reaction was proposed in Scheme 2. Ammonia molecules are obtained from ammonium acetate. We think that the aldehyde and 1,2-dicarbonyl compounds including 9,10-phenanthraquinone at first activated by S-LPMO as Lewis acid, in the rate determining step.

CONCLUSION
In conclusion, we benefit from one pot synthesis for the preparation of substituted imidazoles through three-component condensation of aldehyde, 9,10-phenanthroquinone, and ammonium acetate in presence of silica-supported La 0.5 Pb 0.5 MnO 3 nanoparticles as an efficient, re-usable and green solid acid catalyst, in reflux and solvent-free conditions. This catalyst has enhanced specific surface area, therefore increasing the contact between the catalyst and reactant. Excellent yields, short reaction times, simplicity of operation and easy separation and reusability of catalyst are several advantages of this technique. Recovery of both products and inorganic support/catalyst is generally possible, leading to an effective and low-waste route to a range of products.