Acid p Ka Dependence in O-O Bond Heterolysis of a Nonheme FeIII-OOH Intermediate to Form a Potent FeV=O Oxidant with Heme Compound I-Like Reactivity

Shuangning Xu, Apparao Draksharapu, Waqas Rasheed, Lawrence Que

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Abstract

Protons play essential roles in natural systems in controlling O-O bond cleavage of peroxoiron(III) species to give rise to the high-valent iron oxidants that carry out the desired transformations. Herein, we report kinetic and mechanistic evidence that acids can control the mode of O-O bond cleavage for a nonheme S = 1/2 FeIII-OOH species [(BnTPEN)FeIII(OOH)]2+ (2, BnTPEN = N-benzyl-N,N′,N′-tris(2-pyridylmethyl)-1,2-diaminoethane). Addition of acids having pKa values of >8.5 in CH3CN results in O-O bond homolysis, leading to the formation of hydroxyl radicals that give rise to alcohol/ketone (A/K) ratios of around 1 in the oxidation of cyclohexane. However, the introduction of acids with pKa values of <8.5 elicits a different outcome, namely the achievement of A/K ratios of as high as 9, the observation of rapid and catalytic hydroxylation of cyclohexane, and a million-fold acceleration in the decay rate of the FeIII-OOH intermediate at -40 °C. These results implicate the generation of a highly reactive FeV=O species via proton-assisted O-O bond heterolysis of the FeIII-OOH intermediate, which is unprecedented for nonheme iron complexes supported by neutral pentadentate ligands and serves as a nonheme analogue for heme enzyme compounds I.

Original languageEnglish (US)
Pages (from-to)16093-16107
Number of pages15
JournalJournal of the American Chemical Society
Volume141
Issue number40
DOIs
StatePublished - Oct 9 2019

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Heme
Oxidants
ethylenediamine
Cyclohexane
Ketones
Acids
Protons
Alcohols
Iron
Hydroxylation
Hydroxyl Radical
Enzymes
Ligands
Observation
Oxidation
Kinetics

PubMed: MeSH publication types

  • Journal Article

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Acid p Ka Dependence in O-O Bond Heterolysis of a Nonheme FeIII-OOH Intermediate to Form a Potent FeV=O Oxidant with Heme Compound I-Like Reactivity. / Xu, Shuangning; Draksharapu, Apparao; Rasheed, Waqas; Que, Lawrence.

In: Journal of the American Chemical Society, Vol. 141, No. 40, 09.10.2019, p. 16093-16107.

Research output: Contribution to journalArticle

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abstract = "Protons play essential roles in natural systems in controlling O-O bond cleavage of peroxoiron(III) species to give rise to the high-valent iron oxidants that carry out the desired transformations. Herein, we report kinetic and mechanistic evidence that acids can control the mode of O-O bond cleavage for a nonheme S = 1/2 FeIII-OOH species [(BnTPEN)FeIII(OOH)]2+ (2, BnTPEN = N-benzyl-N,N′,N′-tris(2-pyridylmethyl)-1,2-diaminoethane). Addition of acids having pKa values of >8.5 in CH3CN results in O-O bond homolysis, leading to the formation of hydroxyl radicals that give rise to alcohol/ketone (A/K) ratios of around 1 in the oxidation of cyclohexane. However, the introduction of acids with pKa values of <8.5 elicits a different outcome, namely the achievement of A/K ratios of as high as 9, the observation of rapid and catalytic hydroxylation of cyclohexane, and a million-fold acceleration in the decay rate of the FeIII-OOH intermediate at -40 °C. These results implicate the generation of a highly reactive FeV=O species via proton-assisted O-O bond heterolysis of the FeIII-OOH intermediate, which is unprecedented for nonheme iron complexes supported by neutral pentadentate ligands and serves as a nonheme analogue for heme enzyme compounds I.",
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