High-Valent Iron Oxidant Formation through O–O Cleavage of Peroxoiron Species

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High-Valent Iron Oxidant Formation through O–O Cleavage of Peroxoiron Species

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2020-09

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This thesis revolves around the generation of high-valent iron species through O–O bond cleavage of peroxoiron intermediates and performing hydrocarbon oxidation catalysis taking advantage of these potent metal-based oxidants. Chapter 1 introduces how iron-containing enzymes activate O2-derived O–O bonds to generate high-valent oxoiron oxidants, followed by mechanistic studies of bio-inspired synthetic catalysts that use H2O2 as the terminal oxidant to generate high-valent iron species. Chapter 2 provides direct evidence for an FeV(O)(OH) species, which has been proposed to be generated through water-assisted FeIII–OOH O–O bond heterolysis and serve as the active oxidant for a nonheme iron catalyst Fe(TPA). Using an ambient mass spectrometry technique (rTMn-DESI-MS), we were able to trap this FeV(O)(OH) species under catalytic conditions, lending further support to its role as the active oxidant. Chapter 3 revisits the mechanism of Fe(TPA) catalysis with carboxylic acids as additives in light of a recently characterized [(PyNMe3)FeV(O)(OOCR)]2+ capable of reversible O–O bond cleavage, allowing it to embody some characters of FeV, FeIV and FeIII, constituting the so-called “catalytic troika”. We carried out comprehensive spectroscopy, reactivity and kinetic studies and found that substrates can affect the decay of the FeIII–OOH intermediate, which contradicts previous proposals where the O–O bond cleavage is rate-determining and substrate oxidation occurs afterwards. We revamped our mechanistic model with “Messi”-like species serving as the key oxidant to reconcile with some of the inconsistencies found in the previous model. Chapter 4 looks at the role of protons in FeIII–OOH O–O bond cleavage in synthetic nonheme iron systems. We demonstrated for (BnTPEN)FeIII–OOH that stronger acids with pKa values lower than 8.5 facilitate O–O bond heterolysis and the formation of a putative FeV=O oxidant, enabling the complex to perform cyclohexane hydroxylation catalysis for the first time the complex’s 25-year history. Chapter 5 revisits a nonheme diiron complex FeIII2(6-HPA), which serves as a functional model for sMMOH. Based on Mӧssbauer studies of isolated solid of [(6-HPA)FeIII2(µ-O)(µ-1,2-O2)](ClO4)2, it is proposed that it can undergo temperature-dependent reversible O–O bond homolysis to give rise to [(6-HPA)FeIV2(µ-O)(O)2](ClO4)2, to serve as the active oxidant in catalytic reactions. We carried out detailed spectroscopic and reactivity studies in solution and found that the chemistry in solution does not necessarily parallel that observed for isolated solids. We identified a new diferric-µ-oxo-hydroxo-peroxo species that would be a better candidate for the precursor to the active oxidant.

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University of Minnesota Ph.D. dissertation. September 2020. Major: Chemistry. Advisor: Lawrence Que, Jr.. 1 computer file (PDF); xx, 205 pages.

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Xu, Shuangning. (2020). High-Valent Iron Oxidant Formation through O–O Cleavage of Peroxoiron Species. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/250039.

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