STUCTURAL AND SPECTROSCOPIC STUDY OF Cu(II) – TAURINE –HEXAMINE TERNARY COMPLEX: DETERMINATION OF THE STABILITY CONSTANTS IN METHANOL – WATER MIXTURES

Stoichiometry and apparent stability constant (K_C) of the complex formed between Cu(II), hexamine and taurine were determined in water and methanol-water mixtures (% methanol w/w: 3.11; 6.15; 10.4; 15.2; 19.9 and 25.3) by UV–vis spectroscopy at 25^o C and constant ionic strength (0.05 M, sodium chloride). Stoichiometry of the ternary complex (1:1: 2, metal: hexamine: taurine) is not modified with an increase in methanol percentage in the analyzed interval. The value of K_C in water is greater than in the binary solutions. The effects of changing solvent composition on K_C data were explained by linear solvation free energy relationships using the solvatochromic parameter of Kamlet and Taft (α, ß and π*). Multiple linear regression analysis indicates that the hydrogen bond donating ability (α) of the solvent and non-specific interactions (π*) play an important role in the degree of occurrence of the reaction. The effect of temperature on K_C was also analyzed by assessing standard entropy and enthalpy variations of the reaction in water. Finally, the structure of the complex was investigated using IR spectroscopy, where the ligand exhibits small structural changes upon complexation.