Characterization and Reactivity of Synthetic Nonheme Oxoiron(IV) Complexes

Abstract

Nonheme iron enzymes are prevalent throughout nature and utilize oxygen as the oxidant. While some intermediates are proposed, such as iron(IV)-oxo and iron(III)-peroxo species, the nature of the reactivity of these species has not yet been fully explored. Nonheme synthetic iron model complexes allow for easy modification to probe the reactivity of such species, and allow for characterization for later comparison with enzymes. This dissertation explores the reactivity of iron(IV)-oxo species supported by a tetramethycyclam framework. Interesting, in contrast to what had been reported previously, the electron-donating properties of the axial ligand do not correspond to the hydrogen atom transfer (HAT) reactivity, while a consistent trend for oxygen atom transfer (OAT) is observed. Ligand tethering is found to have a large impact on the enthalpy of activation. It is proposed that the iron(IV)-oxo moiety rises out of the plane to react, which forces tethered ligands to weaken the axial bond. The activation of oxygen by synthetic iron complexes, in the presence of either a hydrogen atom donor or an acid and a proton source, has been proposed to mimic enzymatic activity. However, the reexamination of mechanisms previously reported to follow an enzyme-mimicking pathway are instead due to peroxyl radicals. This highlights the importance of testing such mechanisms, as autooxidation is a common problem with many compounds in the presence of dioxygen. Finally, species such as iron(IV)-oxo and iron(III)-peroxo complexes, as well as related complexes, are characterized by resonance Raman spectroscopy. Many of these complexes have a ligand with a carboxylate moiety, as seen in nonheme enzymes. Characterization of these complexes show similarities between iron(IV)-oxo and iron(III)-peroxo and chromium(IV)-peroxo species reported previously, having similar vibration values, while major differences exist in vibrations between previously reported iodosylarene-iron(III) complexes and new iodosylarene-iron(III) complexes.