RESUMO
Stoichiometry and apparent stability constant (K(C)) of the complex formed between Al(III) and 3-hydroxyflavone were determined in methanol and water-methanol mixtures (% water w/w: 3.11; 6.15; 10.4; 15.2; 19.9 and 25.3) by UV-vis spectroscopy at 25.0°C and constant ionic strength (0.05 M, sodium chloride). Stoichiometry of the complex (1:2, metal:ligand) is not modified with an increase in water percentage in the analyzed interval. The value of K(C) in methanol 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 methanol. Finally, the structure of the complex was investigated using FTIR spectroscopy and DFT calculations. The ligand exhibits small structural changes upon complexation, localized on the chelating site. The calculated vibrational frequencies of the complex were successfully compared against the experimental values.
Assuntos
Alumínio/química , Flavonoides/química , Metanol/química , Água/química , Absorção , Cinética , Conformação Molecular , Concentração Osmolar , Soluções , Solventes/química , Espectrofotometria Infravermelho , Temperatura , VibraçãoRESUMO
The complexation of Al(III) by 2,4(OH)(2)-acetophenone in methanol and ethanol was investigated using spectroscopic methods (UV-vis) and theoretical procedures (DFT) in order to determine its stoichiometry and stability constant and to analyze the effect of temperature, ionic strength and solvent on the reaction rate. The stoichiometric composition of the complex is 1:1 and the stability constant in methanol is greater than in ethanol. The parameters obtained by the Arrhenius equation and the transition-state theory permitted to explain why the reaction proceeds more rapidly in EtOH. The reaction is favoured when the ionic strength and the reaction medium permittivity decrease. Taking into account the kinetic results and the theoretical calculations performed, a reaction mechanism in three steps is proposed that considers the interaction between ionic species of opposite unitary charge to generate the metal complex. In the formation of the complex, the aluminium atom interacts forming a covalent bond with the oxygen atom of the hydroxyl group of the ligand and its carbonyl group by means of a strong Coulombic interaction.