Comparison of the Kinetics, Mechanism, and Thermodynamics of Aqueous Iron(III) Chelation and Dissociation by Hydroxamic Oxo and Thio Acid Ligands
The kinetics and thermodynamics of aqueous iron(III) complexation by 4-CH3OC6H4C(X)N(OH)H (X = O, S) to form Fe-(H2O)4(4-CH30C6H4C(X)N(0)H)2+ are reported. These data provide a direct comparison between the iron(III) chelation chemistry of hydroxamic oxo and thio acids. A parallel-path mechanism involving Fe(H2O)63+ and Fe(H2O)5OH2+ was found to be operative for both ligands. Equilibrium quotients and microscopic rate constants for the forward and reverse directions of both paths were obtained along with the corresponding ΔH0, ΔH∗ and ΔS°, ΔS∗ values. An associative interchange (Ia) mechanism is operative for the substitution of both ligands at Fe(H2O)63+. Data are also presented that support an Ia mechanism for substitution at Fe(H2O)5OH2+, although the associative character in this path may be due to H-bonding interactions between the ligand and coordinated OH-. Initial bond formation at iron(III) for either path occurs at the >C=X (X = O, S) site. The rtoohydroxamic acid forms a more stable complex at physiological pH, and aquation by the acid-dependent and acid-independent, paths is ca. 50 times slower than for the hydroxamic acid complex. The increased kinetic stability is consistent with enhanced derealization of the N atom lone electron pair into the C-N bond in the thiohydroxamic acid complex. The chemistry of iron(III) chelation by CH3OC6H4C(S)N(OH)H suggests that siderophores may exist that use the thiohydroxamate moiety for iron(III) binding. © 1985, American Chemical Society. All rights reserved.
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