CHEMISTRY OF IRON OXOPHLORINS .3. REVERSIBLE, ONE-ELECTRON OXIDATION OF THE IRON(III) OCTAETHYLOXOPHLORIN DIMER

The oxidative behavior of the dimeric octaethyloxophlorin (OEPOH(3)) complex, {Fe-III(OEPO)(2)}, 1, has been studied in regard to its relevance toward heme catabolism and the effects of pi-pi overlap as seen in the chlorophyll special pair. Electrochemical studies show that {Fe-III(OEPO)}(2), 1, undergoes a reversible, one-electron oxidation that is readily accomplished chemically by silver perchlorate or silver tetrafluoroborate. The oxidation occurs 0.26 V more readily than oxidation of the mu-oxa dimer, (OEP)Fe-III-O-Fe-III(OEP), and 0.66 V more readily than the monomeric, meso-methoxy-substituted complex, ClFeIII(OEPOMe). This ease of oxidation is attributed at least in part to the effect of direct overlap between the two tetrapyrrole planes in 1. The H-1 NMR spectrum of the oxidized cation, 1(+), shows a number of features, particularly the variation in line widths for the eight methylene resonances, that are consistent with its formulation as a dimer. The pattern of H-1 NMR resonances and the UV/vis spectrum of 1(+) indicate that a ligand-based oxidation has occurred but that the ligand hole is delocalized over both macrocycles. The stability of It depends upon the availability of axial ligands. For example 1(+) reacts with bromide ion to form 1 and Fe-III(OEPO.)Br in a stoichiometric fashion.