Kinetics and mechanism of a catalytic chloride ion effect on the dissociation of model siderophore hydroxamate-iron(III) complexes

Proton-driven ligand dissociation kinetics in the presence of chloride, bro mide, and nitrate ions have been investigated for model siderophore complex es of Fe(III) with the mono- and dihydroxamic acid ligands R1C(=O)N(OH)R-2 (R-1 = CH3, R-2 = H; R-1 = CH3, R-2 = CH3; R-2 = C6H5, R-2 = H; R-1 = C6H5, R-2 = C6H5) and CH3N(OH)C(=O)[CH2](n)C(=O)N(OH)CH3 (H2Ln; n = 2, 4, 6). Si gnificant rate acceleration in the presence of chloride ion is observed for ligand dissociation from the bis(hydroxamate)- and mono(hydroxamate)-bound complexes. Rate acceleration was also observed in the presence of bromide and nitrate ions but to a lesser extent. A mechanism for chloride ion catal ysis of ligand dissociation is proposed which involves chloride ion depende nt parallel paths with transient Cl- coordination to Fe(III). The labilizin g effect of Cl- results in an increase in microscopic rate constants on the order of 10(2)-10(3). Second-order rate constants for the proton driven di ssociation of dinuclear Fe(III) complexes formed with H2Ln were found to va ry with Fe-Fe distance. An analysis of these data permits us to propose a r eactive intermediate of the structure (H2O)(4)Fe(Ln)Fe(HLn)(Cl)(OH2)(2+) fo r the chloride ion dependent ligand dissociation path. Environmental and bi ological implications of chloride ion enhancement of Fe(III)-ligand dissoci ation reactions are presented.