Oxidation of Strong C-H Bonds by a Powerful O2-Derived Iron(IV)-Oxo Species

Abstract

Dioxygen plays a crucial role in the metabolic processes of living organisms, as both its formation and activation are essential to maintaining life. Nonheme iron enzymes catalyze an amazing array of oxidation reactions utilizing O2 under mild conditions. Understanding the mechanisms by which nature is able to utilize dioxygen to perform these transformations is of great interest both on a fundamental and practical level. To this end, synthetic systems have been employed to gain in-depth mechanistic insight. The tridentate ligand TpPh2 (TpPh2 = hydrotris(3,5-diphenylpyrazol-1-yl)borate) has been previously used as a synthetic model for the 2-His-1-carboxylate active sites of nonheme iron oxygenases. Addition of an α-keto carboxylate provides a five-coordinate complex that, upon addition of oxygen, undergoes oxidative decarboxylation of the α-keto acid with concomitant self-hydroxylation of a single ortho position of a ligand phenyl ring, thus acting as a functional mimic of α-KG-dependent dioxygenases. As in α-KG-dependent dioxygenases, a Fe(IV)=O species is proposed to be the oxidant that carries out ligand self-hydroxylation. This work focuses on attempts to intercept the reactive iron-oxygen species with hydrocarbons with varying C-H bond dissociation energies (BDE), including n-butane. The reactions were analyzed to identify oxidation products and assess the oxidative power of the putative Fe(IV)=O oxidant. The first step in the activation of O2 by non-heme iron enzymes is usually the formation of an Fe(III)-superoxo species upon binding of O2 to the Fe(II) center and subsequent electron transfer. Such species are well characterized in the enzymatic and synthetic heme literature, but only recently have iron-superoxo species been trapped and characterized in nonheme iron enzymes. In this thesis is reported the characterization of the first synthetic mononuclear nonheme Fe(III)-superoxo species by bubbling O2 into a solution of Fe(II)(BDPP) (BDPP = 2,6-Bis(((S)-2-(diphenylhydroxymethyl)-1-pyrrolidinyl)methyl)pyridine) at -80 °C and its conversion to a Fe(III)-hydroperoxo species through H-atom abstraction and then to a diferric-peroxo species via comproportionation. Heterobimetallic centers are also involved in oxygen metabolism. In this thesis is reported the characterization of two nonheme heterobimetallic Fe-O-M species: a FeIII-O-CrIII species generated from dioxygen activation and a FeIV-O-CeIV intermediate generated during iron-catalyzed water oxidation.