Kinetics of Aqueous Acid Hydrolysis of Iron(III) 5-Substituted-8-Hydroxyquinoline Complexes: Mechanistic Implications

Synopsis : The acid-driven stepwise dissociation kinetics of tris (8-hydroxy-5-sulfonated-quinoline) iron(III) complex and two other 8-hydroxyquinoline derivatives are reported and compared to literature data. The main finding is that, in the rate determining step, the iron-oxygen bond cleavage (oxygen of the hydroxyl group of the ligand) occurs in the transition state with proton transfer to the oxygen of the incipient free ligand oxine in relation with its structure (C-OH). Comparison with literature data shows that for the hydroxamate ligand with its coordinating oxygen involved in C=O, there is no proton transfer during the iron-oxygen bond cleavage in the transition state.The acid hydrolysis reaction rate constants of the mono -oxine iron(III) complexes, with oxine=8-hydroxyquinoline, sulfoxine=8-hydroxy-5-sulfonated-quinoline and fluoro-oxine=5-fluoro-8-hydroxyquinoline, were measured in aqueous solution, 2.0 u M in NaClO 4 at 25°C. Under these conditions, for iron(III)-sulfoxine, the dissociation r...