Free base porphyrin reduction potentials: A useful structure-reactivity parameter

Reduction potentials for a series of free-base porphyrins of widely differing basicity have been measured in DMF (dimethylformamide) solution using phase-selective fundamental harmonic and second harmonic ac polarography. The potentials show a good linear correlation with porphyrin properties, such as electrophilic substitution rates and reduction potentials for the corresponding manganese(III) and iron(III) metalloporphyrins, regardless of the nature of the porphyrin substituent or its position on the porphyrin periphery. Examples provided show that the reduction potentials, which are rapidly and accurately obtained, are a reasonable measure of porphyrin electron density, which can be useful in structure-reactivity correlations.