Critical Factors in Determining the Heterolytic versus Homolytic Bond Cleavage of Terminal Oxidants by Iron(III) Porphyrin Complexes

Abstract

Heterolytic versus homolytic cleavage of the metal-bound terminal oxidant is the key for determining the nature of reactive intermediates in metalloenzymes and metal catalyzed oxygenation reactions. Here, we study the bond cleavage process of hypochlorite by iron(III) porphyrin complexes having 4-methoxy-2,6-dimethylphenyl (1), 2,4,6-trimethylphenyl (2), 4-fluoro-2,6-dimethylphenyl (3), 2-chloro-6-methylphenyl (4), 2,6-dichlorophenyl (5), and 2,4,6-trichlorophenyl (6) groups at the meso position. Oxoiron(IV) porphyrin π-cation radical complexes (CompI) are characterized from the reactions of 1-4 with tetra- n-butylammonium hypochlorite (TBA-OCl) in dichloromethane at -80 °C, while oxoiron(IV) porphyrin complexes (CompII) are characterized for 5 and 6 under the same conditions. For all of 1-6, we find the formation of an epoxidation product in good yields from the catalytic reactions with TBA-OCl, suggesting heterolytic cleavages of the O-Cl bonds. CompI of 5 and 6 are reduced to the corresponding CompII by both chloride and hypochlorite, while CompI of 1-4 are not. The reduction reactions with hypochlorite are much faster than those with chloride. These results provide a mechanism where the O-Cl bond of the iron-bound hypochlorite is cleaved heterolytically to form CompI for all of 1-6, but the subsequent reduction reaction with remaining hypochlorite affords CompII for 5 and 6. The E(OCl•/OCl-) value is the boundary to discriminate the identity of the final product: CompI or CompII. Thermodynamic analysis based on the redox potential is successfully applied for explaining the bond cleavage processes of the hypochlorite, hydroperoxide, and tert-butyl peroxide complexes.