ONE-ELECTRON OXIDATION OF METALLOPORPHYRINS

Many metalloporphyrins can be reversibly oxidized in a one-electron step. The midpoint oxidation potentials of magnesium octaethylporphyrin (MgOEP), ZnOEP, CuOEP, NiOEP, and PdOEP were found to be approximately 427, 525, 601, 636, and 725 mV vs. saturated calomel electrode in methanol or chloroform-methanol. Plots of E1 vs. log (Ox/Red) were 59 ± 3 mV, indicating a one-electron oxidation. The order of the oxidation potentials increases with the qualitative stability constants of the porphyrin complexes and also with the electronegativity values of the metal ions. The stannic dication (Sn(IV)OEP diacetate) and the diprotonated dication (OEPH42+) porphyrins were resistant to potentials greater than 1 V. A simple electrostatic argument can explain the reduction and oxidation behavior of these and other porphyrin derivatives. The more negative is the porphyrin ring, the easier it is to remove an electron, and vice versa. Bacteriochlorophyll gave a one-electron titration curve wi th a midpoint of 270 mV. Chlorophyll was changed to a different chlorin which itself had a midpoint potential of ∼550 mV. Optical and esr spectra of the resulting radicals are reported. © 1969, American Chemical Society. All rights reserved.