Synthesis, Experimental and Quantum Mechanical Investigation of the Crystal Structures of Two Triazolyl-Indole Compounds, along with the Evaluation of Their Antioxidant Activity

Abstract

In this study, two 1,2,4-triazolyl-indole compounds, 5-(1H-indol-3-yl)-4-propyl-2,4 dihydro-3H-1,2,4-triazole-3-thione (3a) and 5-(1H-indol-3-yl)-4-butyl-2,4-dihydro-3H-1,2,4-triazole-3-thione (3b), were synthesized and the structures of these compounds were determined by X-ray diffraction method. These two compounds crystallize in the orthorhombic space groups, Pbca. The structures have been determined by direct methods and refined to R, 0.0459 (3a) and 0.0714 (3b). The molecular and crystal structures are stabilized by two intermolecular hydrogen bond for both molecules. The triazole parts of the molecules are almost perpendicular to propyl and butyl groups. NMR studies was also performed to enhance comprehension of the molecular structure of the compounds. The most stable states of the structures, as determined by both DFT analysis and experimental realizations (X-ray, NMR and FT-IR), are the thione form. Although the thione forms are thermodynamically more stable than the thiol forms (Delta H degrees approximate to-15 kcal/mol), they display higher chemical reactivity, as reflected by their smaller HOMO-LUMO energy gaps (Delta E = 4.374 eV for 3a and 4.377 eV for 3b). In addition, the antioxidant activities of both compounds were evaluated using ABTS, DPPH, and FRAP assays. Although both (3a) and (3b) exhibited measurable radical scavenging and reducing power, their antioxidant capacities were found to be lower than that of the reference compound quercetin.