Die reversible Einelektronen-Oxydation von Metallochlorinen und ein Vergleich der Liganden Octaäthylporphin und -chlorin : The One-Electron Oxidation of Metallodilorins and a Comparison of the Ligands Octaethylporphin and Octaethylchlorin

Many metallochlorins can be reversibly oxidized in a one electron step. The midpoint oxidation potentials of magnesium-octaethyl-chlorin (MgOAC), ZnOAC, CuOAC, NiOAC and PdOAC were found to be approximately 107, 197, 331, 356, and 422 mV vs. a saturated calomel electrode in chloroformmethanol. As with analogous metalloporphyrins the order of the oxidations potentials increases approximately linearly with the electronegativity values of the metal ions. The midpoint potentials of the one-electron oxidation of metallochlorins were found to be about 300 mV lower than those of analogous metallporphyrins. Sn (IV) -OAC-dihydroxide and the monocation of OAC could not be reversibly oxidized. A simple electrostatic argument can explain the oxidation behavior of these and other porphyrin derivatives. The more negative the ligand ring, the easier it is to remove an electron and vice versa. Thus experimental results point to a more ionic character of the metal-ligand bonds in metallochlorins as compared to metalloporphyrins. The unusual behavior of chlorophyll a and the optical and ESR-spectra of the resulting radicals are discussed.Some general properties of the chlorin- and porphyrin macrocycles are compared. Characteristic differences between the two ligand-systems are explained by the low basicity of the pyrrolin-nitrogen and the decreased symmetry of the ligand field in the chlorins.