Six of acetylacetone-S-R-thiosemicarbazones (R: methyl, ethyl, propyl, butyl, pentyl or hexyl) were synthesized, and the template reactions resulting in N2O2 type complexes of the thiosemicarbazones with 4-methoxy-salicylaldehyde were carried out in the presence of iron(III) or nickel(II). The thiosemicarbazones and metal complexes were defined by elemental analysis, and infrared, UV-Vis, H-1 NMR spectra. Three complexes bearing butyl-pentyl- and hexyl-groups were analyzed by X-Ray diffraction technique. The crystal data demonstrated that the iron complexes are in square pyramidal structure and nickel complexes in square planar structure. To explicate the relationship between the S-alkyl chain length and antioxidant ability, total antioxidant capacity (as TEAC value) and free radical scavenging activity of the compounds have been tested using CUPRAC and DPPH methods, respectively. Furthermore, the antioxidant potential of the thiosemicarbazones were determined in vitro measuring the scavenging activity of hydroxyl radical (center dot OH), superoxide anion radical (center dot O-2(-)), and hydrogen peroxide (H2O2). The results revealed that the compounds have an antioxidant features comparable to ascorbic acid reference. Synthesized compounds with the remarkable antioxidant properties have a potential to examine in development of new anticancer agents. (C) 2018 Elsevier B.V. All rights reserved.