Nonheme High-Valent Mono- and Dinuclear Oxoiron(IV) Complexes: Generation, Characterization, Electrochemistry and Reactivity

High-valent oxoiron(IV) intermediates have been identified as key oxidants responsible for substrate oxidation and functionalization in several nonheme mono- and dinuclear metalloenzymes such as soluble methane monooxygenase ( s-MMO), Class Ia ribonucleotide reductase (RNR) and taurine/2-oxoglutarate dioxygenase (TauD). These intermediates all have high-spin S = 2 FeIV center(s), as determined by spectroscopic studies. The diiron(IV) intermediate Q (MMO-Q) and the Fe IV=O intermediate J (TauD-J), found in the catalytic cycles of s-MMO and TauD, respectively, effect substrate oxidation by cleaving a C-H bond of the corresponding substrate via hydrogen atom transfer (HAT), while the diiron(III,IV) intermediate X identified for RNR oxidizes the O-H bond of the nearby tyrosine residue to generate the tyrosyl radical used for DNA synthesis. These high-valent intermediates are of great importance for biomimetic studies using synthetic model complexes that provide valuable insights into structures, spectroscopic features and reactivities of enzymatic targets. In the past two decades, outstanding progress has been achieved in generating and characterizing synthetic analogs of these high-valent oxoiron(IV) enzymatic intermediates.;This thesis describes work on both mononuclear and dinuclear oxoiron(IV) complexes. The diiron section first describes the synthesis of a series of (μ-oxo)diiron(III) complexes supported by polydentate ligands. Two sets of ligands modified according to different strategies were designed to tune the ligand field of the iron coordination sphere. Diiron(III,IV) complexes were then generated by 1-e- oxidation of the corresponding diiron(III) precursors, and characterized by a variety of spectroscopies. The effect of ligand modifications was demonstrated by the properties (e.g., redox potential and electronic structure) of these (μ-oxo)diiron(III,IV) complexes. This work not only provides insights into the 2-His-4-carboxylate binding mode of the enzymatic active sites, but also contributes to our understanding of synthetic high-valent diiron chemistry.;One ligand supports the formation of a novel (μ-oxo)diiron(IV) complex with the highest 1-e- redox potential described to date. This diiron(IV) species can be generated in 85% yield with the unique combination of experimental conditions, thus being an excellent demonstration of strategies adopted to stabilize high-valent high-potential species. Although this diiron(IV) complex has low-spin S = 1 FeIV centers, a feature different from the S = 2 character of MMO-Q, it exhibited highly oxidative reactivity towards different types of substrates, including C-H bonds of hydrocarbons, the O-H bond of aliphatic alcohols and water, and aromatic rings. These reactions appear to proceed via different mechanisms, including hydrogen atom transfer (HAT), proton coupled electron transfer (PCET) and electron transfer (ET). Kinetic studies were performed to obtain activation parameters for those reactions. These studies demonstrated that the PCET pathway has a kinetic advantage over HAT and ET. Moreover, the activation barrier of a PCET reaction can be further reduced by incorporating a third molecule in the rate-determining step to facilitate the proton transfer. The generation and characterization of the (μ-oxo)diiron(IV) complex is a significant achievement towards our goal of making synthetic analogs of MMO-Q and set the stage for future efforts aimed at more efficient hydrocarbon oxidation by synthetic iron complexes.;In the mononuclear oxoiron(IV) section, redox properties of a series of FeIV=O complexes supported by pentadentate N5 ligands have been investigated by electrochemical methods in aqueous solution. In particular, reversible cyclic voltammetric behavior was observed for the first time for these FeIV=O complexes. Their redox potentials were found to shift along with the change of the pH value of the reaction medium, a clear demonstration that the transfer of one electron in the reduction of the FeIV=O unit is associated with the transfer of one proton to generate the FeIII-OH species. These results further allow the O-H bond dissociation energy (DO-H) of the FeIII-OH species to be calculated, which established the thermodynamic driving force for these oxoiron(IV) complexes in carrying out C-H bond cleaving reactions. Furthermore, the comparison of C-H bond reaction rates of these oxoiron(IV) complexes with those of other metal-oxo complexes revealed a kinetic advantage for the FeIV=O unit in carrying out C-H bond cleavage, a feature presumably endowed by two state reactivity (TSR) in the course of the rate-determining hydrogen atom abstraction step.