New 5-methylisatin-thiosemicarbazones: preparation, spectroscopic study and antioxidant properties

Abstract

New isatin-thiosemicarbazone (1–6) were obtained from various 5-methylisatin and various isothiocyanates with good yields and efficient methods. The structures of the synthesized compounds were clarified by FT-IR, ¹H-NMR, and ¹³C-NMR spectroscopic techniques and elemental analysis. The antioxidant activity properties of the compounds were investigated by 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging method and total reducing power determinations. In our total reducing power study, where we chose Trolox as the standard antioxidant, it was found that the reducing powers of compounds containing different substituent groups and positions were close to each other and lower than Trolox. In the study using butylated hydroxytoluene (BHT) as a reference antioxidant, DPPH antiradical scavenging capacities of the synthesized compounds were determined. The free radical scavenging effects of the synthesized compounds were compared with the IC₅₀ values obtained from the concentration equations and were found in the order BHT > 6 > 3 > 5 > 1 > 2 > 4. Structural and electronic properties of the compounds were examined by density functional theory (DFT) calculations and their relationship with the antioxidant properties of the compounds was discussed. DFT calculations played an important role in determining how the electronic properties of the synthesized compounds, especially in the presence of different substituents and in the ortho and meta positions, and accordingly the antioxidant properties of the compounds, changed. Experimental and theoretical results confirmed that methoxy-containing structures in the synthesized compounds exhibit higher antioxidant effects compared to halogen-containing structures, regardless of their position.

Graphical abstract