Proton-directed redox control of O-O bond activation by heme hydroperoxidase models.

Hangman metalloporphyrin complexes poise an acid-base group over a redox-active metal center and in doing so allow the "pull" effect of the secondary coordination environment of the heme cofactor of hydroperoxidase enzymes to be modeled. Stopped-flow investigations have been performed to decipher the influence of a proton-donor group on O-O bond activation. Low-temperature reactions of tetramesitylporphyrin (TMP) and Hangman iron complexes containing acid (HPX-CO2H) and methyl ester (HPX-CO2Me) functional groups with peroxyacids generate high-valent Fe=O active sites. Reactions of peroxyacids with (TMP)FeIII(OH) and methyl ester Hangman (HPX-CO2Me)FeIII(OH) give both O-O heterolysis and homolysis products, Compound I (Cpd I) and Compound II (Cpd II), respectively. However, only the former is observed when the hanging group is the acid, (HPX-CO2H)FeIII(OH), because odd-electron homolytic O-O bond cleavage is inhibited. This proton-controlled, 2e- (heterolysis) vs 1e- (homolysis) redox specificity sheds light on the exceptional catalytic performance of the Hangman metalloporphyrin complexes and provides tangible benchmarks for using proton-coupled multielectron reactions to catalyze O-O bond-breaking and bond-making reactions.