Thermodynamic and Kinetic Studies on the Iron(III)-Hydroxamate Interaction in Acid Media

Thermodynamics and kinetics of 1:1 complexation of iron(III) with benzo- and several ortho-substituted (CH3, NH2, Cl and OH) benzohydroxamic acids (HL) forming FeL2+ have been investigated spectropho-tometrically in aqueous perchloric acid solution. Under the experimental conditions ([H+] « TFc « THL) complexation involves reactions of HL with Fe3+ (k1 path) and FeOH2+ (k2 path) ions leading to an equilibrium. Both k1 and k2 have been evaluated by stopped-flow spectrophotometry following the formation of FeL2+ and the dissociation of FeL2+ in acid solution. The AH* value (42.5 ± 4.8 Hmol-1) for the k2 path is close to that for the water exchange of FeOH2+ species, suggesting essentially a dissociative (Id) process, but that (54.5 + 3.8 kJmol-1) corresponding to the k1 path is perceptibly lower than that for the water exchange of Fe3+ species indicating associative (Ia) character. The reverse rate constants (k-1 and k_2) for the dissociation of the complexes by the two paths have also been evaluated from experimental values of k1, k2, and equilibrium constants (Q) for formation of FeL2+ from Fe3+ and HL, and the hydrolysis constant (Kh) of the Fe3+ ion. There is good agreement for Q values determined by equilibrium and kinetic experiments. Q follows an increasing value of k1 and a decreasing order of k-1 and thus both contribute favourably to the stability of the complex. Formation of a 1:1:1 ternary complex Fe(Nta)L- in the reaction of Fe(Nta) with benzohydroxamic acid (HL) has also been studied. Results indicate that Nta3- considerably destabilizes the iron(III)-hydroxamate interaction which is due to a considerable increase in the dissociation rate by a factor of ca 5 × 103, whereas the formation rate is enhanced by a factor of only ca 4 at 25[ddot]C. Thus the strong sigma donor Nta3- labilizes the strong sigma donor hydroxamate more than H2O bound to iron(III).