DISSOCIATION KINETICS OF (N-METHYLACETOHYDROXAMATO) IRON (III) COMPLEXES - A MODEL FOR PROBING ELECTRONIC AND STRUCTURAL EFFECTS IN THE DISSOCIATION OF SIDEROPHORE COMPLEXES

The proton-initiated dissociation kinetics and equilibria of the mono, bis, and tris complexes of iron(III) with N-methylacetohydroxamic acid (NMHA) were studied under conditions of 2.0 M NaClO4/HClO4 at 25-degrees-C. The proton-dependent rate constants k3, k2, and k1 for dissociation of the tris, bis, and mono complexes are 8.6 x 10(3) M-1 s-1, 1.02 x 10(2) M-1 s-1, and 3.2 x 10(-3) M-1 s-1, respectively. The corresponding equilibrium constants log K3, log K2, and log K1 are 1.06, -0.9, and -2.75, respectively. An acid-independent dissociation pathway is observed in the dissociation of the mono complex with a rate constant k'1 = 7.1 x 10(-3) s-1. The solution NMR spectrum of the ligand shows split methyl peaks indicating hindered rotation about the C-N bond. The equilibrium ratio for the C-N rotation was found to be 3.5 in favor of the Z isomer and the minimum lifetime of the rotation was estimated to be 0.3 s. The rates and mechanism of Fe(NMHA)l3-l dissociation are compared to corresponding processes observed for dissociation of (acetohydroxamato)iron(III) complexes and the natural trihydroxamate siderophore ferrioxamine B. Differences in rate and mechanism between the model systems and ferrioxamine B are discussed in terms of solvent effects, electrostatic effects, and C-N bond rotation in the hydroxamate group.