Oxidative electrochemistry of iron-selenocarbonyl porphyrins

The oxidation of selenocarbonyl(5,10,15,20-tetraphenylporphinato)iron(II), (TPP)FeCSe, was studied in 1,2-dichloroethane solution at a Pt-button electrode. Results from detailed voltammetric and spectroelectrochemical experiments (both electronic and infrared) indicated that (TPP)FeCSe can be oxidized in two chemically reversible, one-electron transfers. This is comparable to thiocarbonyl iron porphyrin but is in marked contrast with the analogous carbonyl iron(II) porphyrin derivative. The latter complex loses CO concomitantly with the removal of the first electron. The product of the first oxidation of (TPP)FeCSe is a selenocarbonyl iron(III) porphyrin whereas the second oxidation step occurs at the periphery of the porphyrin ring, producing the selenocarbonyl iron (III) porphyrin cation radical. Addition of nitrogenous bases to solution generated the monoadduct, (TPP)FeCSe (nitrogenous base). This adduct was also oxidized in two discrete, one-electron transfers. The product of the first oxidation, ((TPP)Fe/sup III/CSe(nitrogenous base))/sup +/, was stable for hours. The product of the second oxidation, however, readily underwent nucleophilic attack by uncomplexed nitrogenous base present in solution. The electronic effects generated by the ligand trans to the selenocarbonyl moiety are discussed in the context of their influence on the spectral, electrochemical, and thermodynamic properties of the selenocarbonyl iron porphyrin.