Schiff base and metal(II) complexes containing thiophene-3-carboxylate: Synthesis, characterization and antioxidant activities

A new Schiff base ligand (L) obtained from methyl 6-acetamide-2-amino-4,5,6,7-tetrahydrobenzo[b] thiophene-3-carboxylate and 2-hydroxy-3-methoxybenzaldehyde was synthesized. Then, the metal complexes of the Schiff base ligand were prepared with FeCl(2 center dot)4H(2)O, CoCl2 center dot 6H(2)O, NiCl2 center dot 6H(2)O, [RuCl2(p-cymene)] (2) and PdCl2(CH3CN)(2) metal salts. Schiff base ligand and complexes were characterized using elemental analysis, H-1 NMR, C-13 NMR, FT-IR, LC-MS, UV-Vis., magnetic susceptibility, thermogravimetric analysis (TGA) methods. To determine the antioxidant activity of ligand and complex compounds, DPPH free radical removal activity, reduction force for ferric ions (Fe+3) to ferrous ions (Fe+2) according to the FRAP method, reducing force for cupric ions (Cu+2) to the cuprous ions (Cu+) with CUPRAC method and total antioxidant activities by ferric thiocyanate method were investigated. It was determined that Fe(II), Co(II), Ni(II) and Ru(II) complexes had octahedral structure and Pd(II) complex had a square planar structure. It was also determined that complex compounds generally showed more effective antioxidant activity than ligand. (C) 2019 Elsevier B.V. All rights reserved.