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 °C. The proton-dependent rate constants k3, k2, and k1 for dissociation of the tris, bis, and mono complexes are 8.6 × 103 M−1 s−1, 1.02 × 102 M−1 s−1, and 3.2 × 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 k1′ = 7.1× 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)3-ll 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. © 1994, American Chemical Society. All rights reserved.
Caudle, MT; Crumbliss, AL
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