Synthesis, structural, spectroscopic and thermal characterization of cobalt complexes with 3-and 6-methylpicolinic acid. Voltammetric and spectrophotometric study in solution

Cobalt complexes of 3- and 6-methylpicolinic acid, namely [Co(3-Mepic)(3)] (1) and [Co(6-Mepic)(2)(H2O)(2)] center dot 2H(2)O (2) were prepared and characterized by spectroscopic methods (IR, UV-Vis, NMR), their molecular and crystal structures were determined by X-ray crystal structure analysis and their thermal stability by TGA/DTA methods. Square-wave voltammetry showed that on mercury electrode the oxidation of 2 requires higher potential than the oxidation of complex anion [Co(3-Mepic)(3)], the most probable product of the reduction of 1. The reduction of 1 and the oxidation of 2 depend on the kinetics of electron transfer and the electrode material. X-ray structural analysis revealed octahedral coordination polyhedron in both 1 and 2 and the same N,O-chelated coordination mode for both ligands. C-13, H-1 and N-15 NMR spectroscopy confirmed that coordination mode of 3-methylpicolinic acid in 1 in DMSO solution. UV-Vis spectrophotometric measurements were used to study the complexation of cobalt with 3- and 6-methylpicolinic acid in aqueous solution and to determine the composition of the formed complexes by Job method of continuous variation. The stoichiometry of the complex with 3-methylpicolinic acid is 2:3, while it is 2:3 and 3:2 for the complex with 6-methylpicolinic acid, indicating the possibility of the formation of more than one complex species. (C) 2008 Elsevier B.V. All rights reserved.