Optical Response of Two Azo Ligands Containing Salicyaldimine-based Ligand as Side Chains Towards Some Divalent Metal Ions and Their Antioxidant Behavior

Abstract

According to applicability of azo-azomethine compounds in chemical sensors and biological activities, two receptors: 1,2-[1-(3-imino-4-hydroxophenylazobenzene)]-4-nitrobenzene (1) and 1,2-[1-(3-imino-4-hydroxophenylazo-4-nitrobenzene)]-4-nitrobenzene (2) are investigated for detection of nickel, cobalt, copper, lead, mercury, zinc and cadmium divalent metal ions by UV-vis spectroscopy. With the addition of all metal ions to the DMSO solution of ligands, the peaks at 558 and 549 nm increase in intensity with hypsochromic or bathochromic shifts except Zn2+ ions and 2, while the peaks at 388 and 391 nm dramatically decrease in intensity. In both cases, the largest shift is observed after addition of copper ions. In solution, both receptors produce a cation blue shift from 558 and 549 nm to 503 and 497 nm with the sensible color change of solutions from purple-red to orange. Therefore, both compounds can highly recognize copper ions in DMSO solution. In the next step, Benesi-Hildebrand plot and Job's method are used for determination of binding constant (Ka) and stoichiometry of formed complexes, respectively. Also, the investigation of solvent effect in the UV-vis spectra of ligands shows that the generation of hydrazine and enaminone tautomers increases in highly polar solvents such as DMF and DMSO. Finally, the antioxidant activity of ligands is studied by DPPH method. The results show that NO2 withdrawing groups in 1,2-[1-(3-imino-4-hydroxophenylazo-4-nitrobenzene)]-4-nitrobenzene probably affect keto∆enol equilibrium. As a result, this ligand reduces free radicals to non-reactive species by donating hydrogen.