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Synthesis and characterization of cobalt complexes with 3- and 6-methylpicolinic acid. Spectrophotometric study of complexes in solution

Cobalt complexes of 3- and 6-methylpicolinic acid, namely [Co(3-Mepic)3] (1) and [Co(6-Mepic)2(H2O)2] . 2H2O 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. 1 and 2 were prepared in similar reaction conditions (mildly alkaline aqueous solution). Unexpected oxidation of cobalt(II) by air occurred in the case of 3-methylpicolinic acid, leading to the formation of 1. 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]− , which is the most probable product of the reduction of 1. Voltammetric measurements can not explain the difference in oxidation states of cobalt ions in 1 and 2, since 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 and the same N, O-chelated coordination mode for both 3- and 6-methylpicolinic acid. 13C, 1H and 15N 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 by 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 1:2, while it is 1:1.5 for the complex with 6-methylpicolinic acid, indicating the possible polymeric species.

Cobalt; 3- and 6-methylpicolinic acid; Crystal structures; UV/Vis spectroscopy; Voltammetry

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