Kinetics, mechanism, and thermodynamics of aqueous iron(III) chelation and dissociation: Influence of carbon and nitrogen substituents in hydroxamic acid ligands
Thermodynamic and kinetic studies were performed to investigate the complexation of aqueous high-spin iron(III) by 12 bidentate hydroxamic acids, R1C(G)N(OH)R2, (R1 = CH3, C6H5, 4-NO2C6H4, 4-CH3C6H4, 4-CH3OC6H4; R2 = CH3, C6H5, 4-CH3C6H4, 4-ClC6H4,4-IC6H4, 3-IC6H4, 4-NCC6H4, 3-NCC6H4, 4-CH3C(0)C6H4), in acid medium. Both complex formation and dissociation (aquation) reactions were investigated by stopped-flow relaxation methods over a range of [H+] and temperatures. A two-parallel-path mechanism without proton ambiguity is established for the reaction of Fe(H2G)63+ and Fe(H20)5OH2+ with R1C(O)N(OH)R2 to form Fe(H2O)4(R1C(O)N(O)R2)2+. Equilibrium quotients, ΔH° and ΔS° values, rate constants, and ΔH⋆ and ΔS⋆ values for both reaction paths in the forward and reverse directions are reported. ΔH⋆ and ΔS⋆values are found to be linearly related and compensating. On the basis of an analysis of the equilibrium quotients, rate constants, and activation parameters for the reaction in both directions, an associative interchange (Ia) mechanism is proposed for hydroxamic acid ligand substitution at Fe(H2O)63+. Similar trends for these parameters are observed for the reaction at Fe(H2O)5OH2+, suggesting an associative interchange character for this reaction path also. However, coordinated water dissociation appears to be dominant, and some associative character for this path may be the result of H-bonding interactions between the undissociated hydroxamic acid and coordinated-OH. Electron-donating and-withdrawing R1 and R2 substituents were selected in order to determine the relative influence of the C and N substituent on the hydroxamic acid and to determine the optimum hydroxamic acid structure for kinetic and thermodynamic stability of the iron(III) chelate. Kinetic and thermodynamic chelate stabilization are enhanced by increasing electron density on the carbonyl oxygen atom, which is promoted by electron donors in the R1 position and delocalization of the N atom lone pair of electrons into the C-N bond. The influence of the R2 substituent appears to be dominant with an electron-releasing alkyl group as the preferred R2 substituent for kinetic and thermodynamic stability. The optimum hydroxamic acid ligand for kinetic and thermodynamic stability of the iron(III) chelate was found to be 4-CH3OC6H4C(O)N(OH)CH3. © 1984, American Chemical Society. All rights reserved.
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