The overall objective of the research project is to evaluate electron transfer reactivity of low, intermediate, and high spin iron porphyrins. Porphyrins with Fe(I), Fe(II), Fe(III), and Fe(IV) central metal ions will be investigated. It is proposed to use primarily electrochemical methodologies to characterize to the potentials, rates and mechanisms of electron transfer for various synthetic complexes which vary in 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 Fe(I), Fe(II), Fe(III), and Fe(IV) 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 by diatomic molecules such as NO, CO, CS, and 02. 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. We will investigate iron porphyrins in both high and low oxidation states as well as evaluate redox reactions of porphyrins which function in the catalytic reduction of N02 to N0 or NH3. We will investigate the site of electron transfer for each electrode reaction and monitor the spectral characteristics of each electrogenerated complex. Finally, we will also investigate the spin state and any spin equilibria associated with the various iron complexes. Extensive use will be made of variable temperature electrochemistry, thin- layer spectroelectrochemistry, and combined ESR/electrochemistry techniques, three techniques routinely used in our laboratory. The utlimate aim of this proposed research is the modeling of biological redox reactions involving iron porphyrin complexes. By accomplishing the goals outlined in this proposal we hope to gain insight into factors influencing the oxidation- reduction reactions of various heme proteins.
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