DIRECT OBSERVATION OF THE PUSH EFFECT ON THE O-O BOND-CLEAVAGE OF ACYLPEROXOIRON(III) PORPHYRIN COMPLEXES

The first direct observation of the push effect on heterolytic and homolytic O-O bond cleavage steps of acylperoxoiron(III) porphyrin complexes is reported for a series of acylperoxoiron(III) porphyrins (5) having substituents at the meso-positions of the porphyrin ring. Transformation of 5 to the corresponding oxoferryl (O=Fe(IV)) porphyrin cation radicals (6) in methylene chloride at -80-degrees-C by heterolytic O-O bond cleavage was found to be first order in [5]. Introduction of electron-donating substituents at the meso-positions of the porphyrin ring facilitates the O-O bond cleavage in 5. Addition of 1 equiv of imidazole derivatives to a methylene chloride solution of 5 immediately gave an acylperoxoiron(III) porphyrin-imidazole adduct (9). The conversion of 9 to 6 was also found to be first order in [91, and the coordination of electron-rich imidazole derivatives strongly encouraged the O-O bond cleavage of 9. On the other hand, the push effect on the homolytic O-O bond cleavage reaction has been examined in toluene at -6-degrees-C to about -40-degrees-C. The homolytic O-O bond cleavage of 9 afforded the imidazole adduct of oxoferryl porphyrin complex 7 when phenylperacetic acid was employed. Homolysis of the O-O bond is enhanced by the imidazole ligation; however, the push effect on homolysis is much less than that on heterolysis. These results explain the biological utilization of strong electron-donor ligands in heme enzymes such as peroxidase, cytochrome P-450, and catalase.