Kinetics and Mechanism of Interactions Between Iron(lll) and Desferrioxamine B. The Formation and Hydrolysis of Ferrioxamine B in Acidic Aqueous Solution

The kinetics of the formation and hydrolysis of ferrioxamine B complexes have been studied in acidic aqueous solution (0.001- -1.0 M HCl) at 25.0 °c, μ = 1.0 M (maintained by NaCl). Two stage kinetics have been observed in both the formation and in the hydrolysis reactions. The proposed reaction model involves formation/ hydrolysis of bidentate, tetradentate, and hexadentate bonded desferrioxamine B to iron(III). The formation of the bidentate complex occurs by two paths: one involving unhydrolysed ferric ion yields the rate constant k1 = 282 M-1 s-1, and the other via hydrolysed ferric species gives the rate constant k1\u27 = 4.1 X 103· M-1 s-1• The analogous rate constants for the reverse hydrolysis reactions were obtained; k - 1 = = 0.65 M-1 s-1 for the acid dependent and k _ 1\u27 = 0.016 s·1 for the acid independent path. The formation and hydrolysis rate constants of the tetradentate linked complex have been calculated using the steady-state approximation as k 2 = 9 s·1 and k _2 = 2 M-1 s-1, respectively. The conversion of tetradentate to hexadentate as well as hydrolysis of the hexadentate to tetradentate bonded species have been characterized by the rate constants k 3 = 2.2 s-1 and k _ 3 = = 18 M-1 s-1, respectively. These results are compared with kinetic data previously reported. Chloride media increases the rate of formation and hydrolysis of the bidentate species, and possibly has the same effect on the tetradentate species but does not affect hydrolysis of the hexadentate complex. These effects were analyzed in terms of a labilization of iron(III) coordinated ligands by inner-sphere coordinated chloride