Mechanism of Ligand Substitution on High-Spin Iron(III) by Hydroxamic Acid Chelators. Thermodynamic and Kinetic Studies on the Formation and Dissociation of a Series of Monohydroxamatoiron(III) Complexes

Thermodynamic and kinetic studies were performed to investigate the complexation of aqueous high-spin iron(III) by six bidentate hydroxamic acids, R1C(O)N(OH)R2(R1 = CH3 or C6H5 and R2 = H, CH3, or C6H5) in acid medium. Both complex formation and dissociation (aquation) reactions were investigated over the temperature and [H+] ranges of 2-50°C and 0.025-1.1 M, respectively. Complexation occurs by coordination of both oxygen atoms of the ligand to the iron(III) center, with concomitant loss of a proton yielding 1:1 complexes of the type [Fe(R1C(O)N(O)R2)(H2O)4]2+. The thermodynamic driving force for complex formation (Qf for path 1) is an increase in entropy, with the enthalpy of formation actually opposing complex formation. The ΔSf° values are essentially constant and ΔHf° values correlate with the variations in overall thermodynamic stability. Complex stability was found to parallel systematic variations in the electron donor or acceptor ability of the R1 and R2 substituents (as defined by σ+) with R2 showing the major effect. Kinetic studies were performed in two ways: as a second-order complex formation process going to equilibrium (kexpform = ka’ + kb’/[H+]) and as a relaxation process (kexprel = ka + kb/[H+] + kc[H+]) caused by rapidly increasing the [H+]. Both methods gave equivalent results which were interpreted on the basis of a parallel path mechanism involving substitution on Fe(H2O)63+ and Fe(H2O)5OH2+ by the undissociated hydroxamic acid (HA): Fe(H2O)63+ + HA ⇌ Fe(A)(H2O)42+ + H+ k-1) (path 1); Fe(H2O)5OH2+ + HA ⇌ Fe(A)(H2O)42+ (k2, k-2) (path 2). The reaction path involving Fe(H2O)63+ and A-aq can be excluded based on experimental evidence. Most of the reaction occurs via path 2 over the [H+] range investigated. The magnitudes of the rate constants indicate that ring closure is not rate determining in either path. Activation parameters for complex formation via both paths show competing effects in ΔHǂ and ΔSǂ (isokinetic relationship) which is reflected in only small variations in the observed formation rate constants. Isokinetic behavior is also exhibited by both complex dissociation paths. The variations in complex aquation rates at 25°C via path 2 (which is independent of [H+]) are controlled by variations in ΔHǂ-2. These variations in ΔHǂ-2 show the same trends with changes in R1 and R2 as that found for the overall thermodynamic stability (Qf) of the complexes. In contrast, variations in aquation rates via path 1 at 25°C are controlled by variations in ΔHǂ-1, which correlate with ligand acid strength(pka). These results for complex aquation are interpreted on the basis of a two-path mechanism involving a common intermediate (singly bonded ligand) which can undergo further spontaneous bond cleavage (path 2), or acquire a H+aq (depending upon [H+] and ligand basicity) and then aquate (path 1). In both paths, variations in thermodynamic stability are reflected in the variations in the aquation rates. The major influence of R2 is attributed to its direct influence in stabilizing positive charge density on nitrogen which thereby enhances the degree of derealization of the lone pair of electrons on nitrogen into the carbonyl function. L.FERs suggest that this in turn stabilizes the ground state (ΔHf°) and destabilizes the transition state (ΔSǂ-2). Hence, the aquation studies indicate the presence of some degree of associative character during complex formation. Although the rate constants for complex formation via either path do not vary widely, the associated activation parameters do and are also consistent with some associative character in complex formation with Fe(H2O)5OH2+ as well as with Fe(H2O)63+. It is also concluded that stabilization of positive charge density on nitrogen in any way increases the acidity of the free ligand, while only stabilization of the resonance form corresponding to nitrogen lone pair derealization into the carbonyl function stabilizes the monohydroxamatoiron(III) complex-both thermodynamically and kinetically. © 1979, American Chemical Society. All rights reserved.

Duke Scholars

Author Alvin L. Crumbliss Chemistry