Solvent-Free Microwave-Assisted Green Synthesis and Antimicrobial Evaluation of 2-(1H-Benzimidazol-2-yl) Benzoic Acid and its Analogues Using Recyclable PPA-SiO₂ Catalyst

A sustainable and environmentally benign synthetic approach for the preparation of 2-(1H-benzimidazole-2-yl) benzoic acid (BZ–BA) and a structurally modified derivative (BZ–BA–X) is reported here. The target benzimidazole framework was efficiently synthesized via a solvent-free microwave-assisted cyclization of o-phenylenediamine with 2-formylbenzoic acid in the presence of PPA-SiO₂ reusable catalyst, affording the desired products in good yield (from 35% to 80%) within a significantly reduced reaction time (from 1-2 hours to 6-10 minutes). The structures of the synthesized compounds were confirmed by FT–IR, ¹H & ¹³C NMR, HRMS spectra, and elemental analysis. The antimicrobial potential of the compounds was evaluated in vitro against two selected Gram-positive and two Gram-negative bacterial strains, as well as two fungal strains, using the broth microdilution method. The results revealed moderate antimicrobial activity, particularly against Gram-positive bacteria, with the electron-withdrawing substituted derivatives exhibiting enhanced potency. The overall potency ranking, based on the average MIC values of the compounds studied, is BZ-BA-NO2 ˃ BZ-BA-Cl ˃ BZ-BA-Br ˃ BZ-BA-Me ˃ BZ-BA ≈ BZ-BA-Ome. The present study highlights the advantages of the developed green synthetic protocol in terms of efficiency, sustainability, and biological relevance.