Kinetics of the substitution of dehydroacetic acid in tris (dehydroacetato) Fe(III) complex by 8-hydroxyquinoline, di- and tetra-hydroxyquinone

The ligand substitution reactions of dehydroacetic acid (Hdha) in [Fe(dha)(3)] with second ligand such as 8-hydroxyquinoline (Hquin), 1,4-dihydroxyanthraquinone (H(2)dhaq) and 1,4,5,8-tetra-hydroxyanthraquinone (H(4)thaq) were investigated spectrophotometrically by in low polarity solvents like benzene, chloroform and dichloromethane. It is deduced that the substitution reaction takes place through one successive step. The reaction was performed at four different temperatures (5-25)degrees C, and it exhibits a first order dependence on the concentration of the starting complex. The observed rate constant depends on the concentration of both leaving and entering ligands. The evaluation of the kinetic data gives activation parameters which support an associative mechanism in the transition states and the higher rate of substitution of the dha in Fe(dha)3 complex is due to entropy effect. The solid complexes were synthesized and characterized by elemental analysis, IR and UV-vis spectral techniques. (C) 2006 Elsevier B.V. All rights reserved.