New Schiff bases based on isatin and (thio)/carbohydrazone: preparation, experimental–theoretical spectroscopic characterization, and DFT approach to antioxidant characteristics

Abstract

In this study, synthesis, spectroscopic elucidation, and investigation of antioxidant properties of new Schiff bases based on isatin and (thio)/carbohydrazone derivatives have been reported for the first time. The structures of the synthesized compounds were elucidated by FT-IR, 1H-NMR, and 13C-NMR spectroscopic methods and elemental analysis. Their DPPH, ABTS, and CUPRAC activities were evaluated as antioxidant properties. Electronic and spectral data of the compounds were obtained by DFT calculations at the B3LYP/6–311+ +G(2d,2p) level of theory. Intramolecular interactions and charge densities on the bonds were analyzed by QTAIM and IRI calculations. In addition to parameters such as frontier molecular orbital energy eigenvalues, electronegativity, nucleophilicity index, and electrodonating power, the changes in the enthalpy of the compounds for the reactions realized through the SET mechanism were calculated to elucidate the antioxidant reactions of the compounds. Most of synthesized compounds exhibited antioxidant activities with the IC50 values ranging from 27.13 to 43.35 µM for DPPH, from 6.47 to 24.96 µM for ABTS and with the A0.50 values ranging from 9.04 to 47.52 µM for CUPRAC. Among them, compound 3, containing two hydroxyl groups, showed the strongest antioxidant activity for each assay (IC50 = 27.13 µM for DPPH, 6.47 µM for ABTS, and A0.50 = 9.04 µM for CUPRAC). The antioxidant activities of compound 3 were almost two or threefold weaker than that of BHA (IC50 = 9.55 µM for DPPH, 3.42 µM for ABTS, and A0.50 = 2.24 µM for CUPRAC), used as a standard. In addition, thiocarbohydrazone compounds exhibited higher antioxidant activity than carbohydrazones. Electron donating ability and single electron transfer enthalpy calculations predicted that thiocarbohydrazone compounds can perform SET reactions more easily than carbohydrazones.