Multiple-path dissociation mechanism for mono- and dinuclear tris(hydroxamato)iron(III) complexes with dihydroxamic acid ligands in aqueous solution

Linear synthetic dihydroxamic acids ([CH3N(OH)C=O)](2)(CH2)(n); H2Ln) with short (n = 2) and long (n = 8) hydrocarbon-connecting chains form mono- and dinuclear complexes with Fe(III) in aqueous solution. At conditions where the formation of Fe-2(L-n)(3) is favored, complexes with each of the two li gand systems undergo [H+]-induced ligand dissociation processes via multipl e sequential and parallel paths, some of which are common and some of which are different for the two ligands. The pH jump induced ligand dissociation proceeds in two major stages (I and II) where each stage is shown to be co mprised of multiple components (I-x, where x = 1-3 for L-2 and L-8 and IIy, where y = 1-3 for L-2 and y = 1-4 for L-8). A reaction scheme consistent w ith kinetic and independent ESI-MS data is proposed that includes the tris- chelated complexes (coordinated H2O omitted for clarity) {Fe-2(L-n)(3), Fe- 2(L-2)(2)((LH)-H-2)(2), Fe((LH)-H-n)(3), Fe(L-8)((LH)-H-8)}, bis-chelated c omplexes {Fe-2(L-n)(2)(2+), Fe((LH)-H-n)(2)(+), Fe(L-8)(+)}, and monochelat ed complexes {Fe((LH)-H-n)(2+)). Analysis of kinetic data for ligand dissoc iation from Fe-2(L-n)((LH)-H-n)(3+) (n = 2, 4, 6, 8) allows us to estimate the dielectric constant at the reactive dinuclear Fe(III) site. The existen ce of multiple ligand dissociation paths for the dihydroxamic acid complexe s of Fe(III) is a feature that distinguishes these systems from their biden tate monohydroxamic acid and hexadentate trihydroxamic acid counterparts an d may be a reason for the biosynthesis of dihydroxamic acid siderophores, d espite higher environmental molar concentrations necessary to completely ch elate Fe(III).