UV-visible light-induced photochemical synthesis of benzimidazoles by coomassie brilliant blue coated on W-ZnO@NH2 nanoparticles

Abstract

Heterogeneous photocatalysts proffer a promising method to actualize eco-friendly and green organic transformations. Herein, a new photochemical-based methodology is disclosed in the preparation of a wide range of benzimidazoles through condensation of o-phenylenediamine with benzyl alcohols in the air under the illumination of an HP mercury lamp in the absence of any oxidizing species catalyzed by a new photocatalyst W-ZnO@NH₂-CBB. In this photocatalyst, coomassie brilliant blue (CBB) is heterogenized onto W-ZnO@NH₂ to improve the surface characteristics at the molecular level and enhance the photocatalytic activity of both W-ZnO@NH₂ and CBB fragments. This unprecedented heterogeneous nanocatalyst is also identified by means of XRD, FT-IR, EDS, TGA-DTG, and SEM. The impact of some influencing parameters on the synthesis route and effects on the catalytic efficacy of W-ZnO@NH₂-CBB are also assessed. The appropriate products are attained for both the electron-withdrawing and electron-donating substituents in the utilized aromatic alcohols. Furthermore, preparation of benzimidazoles is demonstrated to occur mainly via a radical mechanism, which shows that reactive species such as ·O₂ ^-, OH˙ and h⁺ would be involved in the photocatalytic process. Stability and reusability studies also warrant good reproducibility of the nanophotocatalyst for at least five runs. Eventually, a hot filtration test proved that the nanohybrid photocatalyst is stable in the reaction medium. Using an inexpensive catalyst, UV-vis light energy and air, as a low cost and plentiful oxidant, puts this methodology in the green chemistry domain and energy-saving organic synthesis strategies. Finally, the anticancer activity of W-ZnO nanoparticles is investigated on MCF7 breast cancer cells by MTT assay. This experiment reveals that the mentioned nanoparticles have significant cytotoxicity towards the selected cell line.

Scheme 1. The schematic diagram for the synthesis of benzimidazoles in the presence of W–ZnO@NH₂–CBB nanophotocatalyst.

Fig. 1. FT-IR spectra of (a) W–ZnO@NH₂, (b) CBB and (c) W–ZnO@NH₂–CBB.

Fig. 2. FESEM images of (a) W–ZnO@NH₂ and (b) W–ZnO@NH₂–CBB.

Fig. 3. The FE-SEM map pictures of W–ZnO@NH₂–CBB.

Fig. 4. TGA profile of W–ZnO@NH₂–CBB nanocomposite.

Fig. 5. XRD patterns of (a) simulated standard ZnO and (b) real W–ZnO@NH₂–CBB.

Fig. 6. N₂ adsorption–desorption isotherms (A) and pore size distribution according to the BJH method (B) for W–ZnO nanocomposite.

Fig. 7. Optical band-gap analysis of W–ZnO@NH₂–CBB and W–ZnO.

Fig. 8. Effect of different photocatalysts on the reaction progress. Benzyl alcohol and o-phenylenediamine selected as the starting materials and reactions were performed under an open atmosphere at room temperature in ethanol (10 mL) using an HP-Hg lamp for 2 h. 0.02 g of photocatalyst was used in all cases.

Fig. 9. Effect of photocatalyst amount on the efficiency of condensation reaction of benzyl alcohol with ortho-phenylenediamine at 25 °C in ethanol (10 mL) under air and UV-vis irradiation for 2 h.

Fig. 10. Effect of reaction time on the yield%. Reaction conditions are similar to that in the caption of Fig. 8. 0.02 g of photocatalyst was used in all cases.

Fig. 11. Effect of W–ZnO@NH₂ : CBB ratio on the efficacy of condensation reaction. Reaction condition is similar to that in the caption of Fig. 8. Reaction time was 2 h. 0.02 g of photocatalyst was used in all cases.

Fig. 12. Effect of solvent on the efficacy of condensation reaction at 25 °C in 10 mL solvent under air and UV-vis irradiation. Reaction time was 2 h. 0.02 g of photocatalyst was used in all cases.

Fig. 13. Effect of temperature on the efficacy of the condensation reaction. Reaction time was 2 h in 10 mL ethanol under air and UV-vis irradiation. 0.02 g of photocatalyst was used in all cases.

Fig. 14. Effect of some familiar scavengers on the reaction progress under the standard reaction conditions described below Table 2. 0.02 g of photocatalyst was used in all cases.

Fig. 15. MTT assay of MCF7 cells exposed to different concentrations of W–ZnO nanoparticles after 24 h.

Fig. 16. Studying recyclability of W–ZnO@NH₂–CBB under the optimized reaction conditions. 0.02 g of photocatalyst was used in all cases.

Fig. 17. XRD patterns of the fresh (a) and final reused photocatalyst (b).

Fig. 18. FT-IR spectra of the fresh (a) and final reused photocatalyst (b).

Fig. 19. FESEM images of the final reused photocatalyst.

Scheme 2. A suggested mechanism for the synthesis of 2-substituted benzimidazoles catalyzed by W–ZnO@NH₂–CBB.