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Magnetic studies indicate that the Cu 4 clusters exhibit ferromagnetic coupling interactions. Investigation on their catalytic activity for the oxidation of ethylbenzene suggests that catalytic activity of 1—3 is closely related to the acidity of complexes and the existence of unsaturated coordination sites in the complex. Dalton Transactions — Royal Society of Chemistry.
Continue with Facebook. Sign up with Google. Log in with Microsoft. Bookmark this article. You can see your Bookmarks on your DeepDyve Library. European Journal of Inorganic Chemistry , 25 , Inorganic and Nano-Metal Chemistry , 48 6 , Journal of Structural Chemistry , 59 3 , Inorganic and Nano-Metal Chemistry , 48 3 , Synthesis, crystal structures, and catalytic properties of two oxidovanadium V complexes with tridentate Schiff bases.
Russian Journal of Coordination Chemistry , 43 6 , European Journal of Inorganic Chemistry , 19 , Gomes , Filipe A.
Almeida Paz , Anabela A. Molecular Catalysis , , Montilla , A. Metal oxide-triazole hybrids as heterogeneous or reaction-induced self-separating catalysts. Journal of Catalysis , , Amarante , Anabela A. Valente , Martyn Pillinger , Isabel S. Catalysis Letters , 4 , Shen , Z. Zhang , L. Shao , Q. Lian , C. Synthesis, crystal structure, and catalytic property of a vanadium V complex with mixed ligands. Russian Journal of Coordination Chemistry , 41 6 , Three-Membered Ring Systems.
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More by Zeba N. Cite this: ACS Omega , 3 , 8 , — ACS AuthorChoice. Article Views Altmetric -. Citations Abstract High Resolution Image. In recent years, catalysis has become an attractive methodology to make organic reactions environmentally benign and economically feasible.
Heterogeneous catalysts are of great interest because they are easily recoverable from reaction mixture by simple filtration and can be reused. Further, to achieve high surface area and stability in polar solvents, STA is impregnated on to different supports, such as polymers and silica. It is worthy to mention here that among the synthesized organic scaffolds, imidazoles are very important motifs in a number of organic molecules and have gained significant attention due to their varied biological and pharmacological activities, such as antiulcerative agent, 12 selective benzodiazepine receptor antagonist, 13 proton pump inhibitor, 14 platelet coagulation drug in animal and human beings 15 Figure 1 , antibacterial agent, 16 antitumor agent, 17 as pesticides, 18 CB 1 cannabinoid receptor antagonist, 19 modulators of P-glycoprotein-mediated multidrug resistance, 20 and as a glucagon receptor.
High Resolution Image. Figure 2. Synthetic methods for imidazoles derived from isatin. Results and Discussion. The protons of heteropoly acids are highly acidic and can be exchanged easily by metal ions. Scheme 1. A weak band in the fingerprint region at cm —1 was assigned for Zr—O—H bond. When [email protected] was supported on ZrO 2 , a broad peak at cm —1 had appeared due to merging of stretching and bending modes of vibrations of STANPs and ZrO 2 at , , , and , as these values lie closely in the same range with respect to each other Figure S1D.
The surface morphology of the zirconia, fresh catalyst, and recycled catalyst was determined by scanning electron microscope SEM analysis and is shown in Figure 3. The SEM image of the catalyst showed an even distribution of [email protected] on the surface of zirconia Figure 3 a. Elemental mapping of the catalyst Figure 4 b confirmed the uniform distribution of all elements i. Further, mapping of individual elements, viz.
The particles of ZrO 2 were obtained free from aggregation, and most of the particles were well separated and have defined grain boundaries, as shown in Figure S2a. Thermal stability of the catalyst was observed by thermogravimetric TG analysis. The TG curve Figure S3 showed two-stage decomposition due to changes occurring in the catalyst at different temperatures. The curve showed a weight loss of 5. Another weight loss of about 2. The total acidity and presence of acidic sites on the surface of the catalyst was measured by TPD-NH 3 analysis.
To get insight of the presence of paramagnetic cerium ion in the catalyst, X-band EPR spectrum of the powdered sample was recorded.
The calculated g value 2. ICP-AES analysis was performed to confirm the actual concentration of cerium incorporated in the catalyst. The optimum reaction conditions were developed using several parameters, such as screening of different catalysts and solvents, catalyst loading on different supports, and effect of catalyst amount and temperature on the model reaction.
The reaction of isatin 1 , benzaldehyde 2d , and ammonium acetate 3 , affording 2-phenyl-3,4-dihydroimidazo[4,5- b ]indole 4d , was chosen as the model reaction Scheme 2. Efficiency of the catalyst on the model reaction was investigated by a set of experiments using different catalysts.
First of all, the model reaction was carried out without any catalyst and trace amount of the product was obtained after a long period of time Table 1 , entry 1. Then, lanthanide salts were used as catalyst, which afforded moderate yield of the product Table 1 , entries 2—4.
Among lanthanide salts, cerium chloride showed better catalytic activity Table 1 , entry 4. However, catalytic activity was enhanced and yield was improved with heteropoly acids, phosphotungstic acid PTA and STA Table 1 , entries 5 and 6.
However, [email protected] showed good result among the three Table 1 , entry Concerning the surface area of the catalyst, support materials were also used to optimize the reaction conditions.
On the basis of these observations, the model reaction was carried out using [email protected] with different support materials such as SiO 2 , Al 2 O 3 , and ZrO 2 Table 1 , entries 12— Table 1.
Effect of Different Catalysts on Model Reaction a. To investigate the effect of solvents on the model reaction, different green organic solvents were used in various conditions.
Using ethanol and isopropanol, good results were obtained Table 2 , entries 1 and 2. When water was mixed with isopropanol and ethanol, i. Table 2. Effect of Solvents on Model Reaction a. To see the efficiency of catalyst loading on support, variable amounts of [email protected] were supported on ZrO 2. It was observed that yield was improved with increasing amount of [email protected] on support material Table S1.
Further increasing the amount of [email protected] on support, slightly reduced the yield of the product Table S1 , entry 5. The result showed that yield was improving sequentially by increasing the amount of catalyst from 20 to 80 mg Table S2.
Considering this observation, 80 mg of catalyst was chosen for the catalytic activity. The model reaction was also examined with different temperatures to know the effect of temperature on multicomponent synthesis of isatin-based imidazoles.
Table 3. Scheme 3. Table 4. The plausible mechanism for the synthesis of isatin-based imidazoles is shown in Scheme 4. This accelerated the nucleophilic attack of ammonia from dissociation of ammonium acetate on II to form intermediate III.
Another molecule of ammonia reacted with III and liberated a water molecule to form intermediate IV that reacted with activated isatin molecule V to afford intermediate VI. Scheme 4. Plausible Mechanism for the Synthesis of 4a — l. There was no appreciable change in the FTIR spectrum of the recovered catalyst. All characteristic bands , , and cm —1 were retained in the spectrum, confirming that there was no loss in the structure of the recycled catalyst Figure S1E.
The SEM image Figure 3 b showed that surface morphology of the recovered catalyst did not change after the seventh cycle. The XRD pattern of the recycled catalyst Figure S4e remained the same, showing no structural change in the catalyst after the seventh cycle.
The ICP-AES analysis showed almost the same percentage of Ce after the seventh cycle, indicating that no leaching of Ce occurred during the reaction course. The advantageous features of the present protocol include reusability of the catalyst up to the seventh cycle, excellent product yield, shorter reaction time period, and green reaction condition. Experimental Section. The catalyst was prepared in three steps according to Scheme 1. This mixture was dehydrated at the same temperature to obtain dried hydrogel.
The hydrogel was then extracted with methanol in Sohxlet apparatus for 2 days. Zirconia-supported [email protected] were prepared by employing impregnation method. Reaction progress was monitored through TLC. The precipitate was then extracted with ethyl acetate, washed with double distilled water, dried over anhydrous sodium sulfate, and evaporated under reduced pressure to get crude product, which was then recrystallized with ethanol to get pure product 4a — l.
The catalyst in aqueous phase was filtered and washed with ethanol for further catalytic use. After each 60 s, intermittent cooling was done and meanwhile reaction mixture was thoroughly mixed. Reaction progress was monitored by TLC.
After completion of reaction, reaction mixture was withdrawn from the microwave oven, allowed to cool, and acidified with 1 N HCl to get precipitate. The precipitate obtained was then extracted with ethyl acetate, washed with double distilled water, dried over anhydrous sodium sulfate, and evaporated under reduced pressure to obtain crude product, which was then recrystallized with ethanol to get pure product 4a — l.
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