Spectroscopic and Computational Studies of a Small Molecule Functional Mimic of Iron Superoxide Dismutase Fe-dapsox

摘要

Fe-dapsox [dapsox=2,6-diacetlypyridinebis(semioxamazide)] is a heptacoordinate pentagonal bipyramidal, functional mimic of iron-dependent superoxide dismutase that has been well characterized on the basis of kinetics and mechanistic studies; however, prior to our studies, its electronic structure had yet to be examined. This paper details our initial characterization of Fe-dapsox in both its reduced and oxidized states, using electronic absorption (Abs) and low temperature magnetic circular dichroism spectroscopies. Density functional theory (DFT) geometry optimizations have yielded models in good agreement with the published crystal structures. Time-dependent DFT and INDO/S-CI calculations performed on these models successfully reproduce the experimental Abs spectra and identify intense, low energy transitions in the reduced complex (Fe^II(H2dapsox)) as metal-to-ligand charge transfer transitions, suggesting the presence of π-back-bonding in this complex. This back-bonding along with the proton uptake accompanying metal ion reduction provide a compelling mechanism by which the metal-centered redox potential is correctly tuned for catalytic superoxide disproportionation.