Synthesis and electrochemical studies of Ni(II) complexes with tetradentate Schiff base ligands

A series of tetradentate Schiff base ligands; [1,2-bis(naphthylideneimino)ethane, 1,3-bis(naphthylideneimino)propane, 1,4-bis(naphthylideneimino)butane, and 1,5-bis(naphthylideneimino)pentane] and their Ni(II) complexes have been synthesized. The properties of these ligands and their Ni(II) complexes have been characterized by elemental analysis, IR, MMR, UV-vis spectra, molar conductance, and thermogravimetric analysis. The mole ratio of Schiff base to Ni(II) metal was found to be 1:1. The electrochemical redox process of the ligands and their Ni(II) complexes in DMF and DMSO solution containing 0.1 M tetraethyl ammonium perchlorate (TEAP) as a supporting electrolyte have been investigated by cyclic voltammetry, chronoamperometry, differential pulse voltammetry, and controlled potential coulometry at glassy carbon electrode. The redox process of the ligands was highly irreversible, whereas redox process of Ni(II) complexes were observed as one electron transfer process in quasi-reversible and diffusion-controlled reaction. The electrochemical redox potentials of the Ni(II) complexes were affected by the chelate ring size of ligands. The diffusion coefficients of Ni(II) complexes containing 0.1 M TEAP in DMSO solution were determined to be 5.7-6.9 X 10(-6) cm(2)/sec. Also the exchange rate constants were determined to be 1.8-9.5 X 10(-2) cm/sec. These values were affected by the chelate ring size of ligands.