Iron-containing enzymes can utilize dioxygen to accomplish a variety of oxidative transformations with high efficiency and specificity. Protons play crucial roles in the formation of high-valent iron species that serve as active oxidants through facilitating the O–O bond cleavage of their peroxoiron precursors. This involvement of proton in the O–O bond cleavage process is well documented for cytochrome P450 enzymes and its synthetic mimics, where the protonation of the distal oxygen of the FeIII–OOH species Compound 0 leads to O–O bond heterolysis and the formation of a powerful oxidant Compound I. However, the process of proton-assisted O–O bond cleavage is much less well defined in the context of nonheme iron chemistry. In this study, we present evidence that the addition of a protic acid promotes O–O bond cleavage of [(BnTPEN)FeIII(OOH)]2+ (BnTPEN = N-benzyl-N,N’,N’-tris(2-pyridylmethyl)-1,2-diamino-ethane), an FeIII–OOH species with a saturated coordination sphere and sluggish oxidation reactivity. The course of O–O bond lysis depends on the pKa of the added acid in CH3CN. Addition of acids with pKa values higher than 8.5 leads to the formation of hydroxyl radicals. However, the introduction of acids with pKa values lower than 8.5 results in O–O bond heterolysis, as evidenced by A/K ratios as high as 9 in catalytic cyclohexane hydroxylation, which is unprecedented for this class of complexes in its more than 20-year history. Kinetic studies suggest that the addition of strong acids results in up to a million-fold faster activation of the FeIII–OOH species, leading to the formation of an FeV=O oxidant capable of cyclohexane oxidation in seconds at -40 °C, as well as aromatic hydroxylation. This study sheds light on Compound-I-like formation in the context of nonheme iron chemistry and opens the possibility for using the large reservoir of pentadentate-ligand-supported iron complexes to be used as catalysts.

Acid pKa-dependence O–O bond lysis of a nonheme FeIII–OOH species: Analogy to the formation of heme compound I

2019 Fall National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.