The overall objective of the proposed project is to evaluate electron transfer reactivity of low, intermediate, and high spin iron porphyrins. Both mononuclear and binuclear ferrous and ferric complexes will be studied. It is proposed to use primarily electrochemical methodologies to characterize, in non-aqueous media, rates and thermodynamics of electron transfer as a function of the Sigma, Pi and steric effects of the axially bound ligand, the spin state of the metal, and the porphyrin ring basicity. Thermodynamics for ligand addition to iron(II) and iron(III) will be calculated. Special emphasis will be placed on Fe(II) and Fe(III) complexes axially coordinated by Sigma bonded alkyl or aryl groups, as well as diatomic molecules such as NO, CO, CS and O2. The electrochemical reactivity of several Mu-oxo, Mu-nitrido, and Mu-carbido bridged dimers containing iron in the oxidation states (IV), (III), (II) and (I) will also be investigated. Mechanisms and rates of electron transfer, as well as thermodynamics of ligand binding, will be elucidated as a function of axial ligand complexation and porphyrin structure. Extensive use will be made of variable temperature electrochemistry and thin-layer spectroelectrochemistry. Both techniques are routinely used in our laboratory. The ultimate aim of this proposed research is the prediction and control of model compound reactivity. By accomplishing the goals outlined in this proposal we hope to gain insight into factors influencing the oxidation-reduction reactions of heme proteins. Thus, by synthesis of porphyrins containing judiciously placed electron-donating or electron-withdrawing substituents and appropriate axial ligands, we will attempt to direct the site of electron transfer (i.e., metal, porphyrin ring or axial ligand). At the same time we will aim towards obtaining model compounds which exhibit desired and predicted heterogenous electron transfer rate constants and standard potentials.
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