The goal of the proposed research is to investigate the reactivity of manganese and iron complexes as models of heme metalloenzymes that utilize dioxygen. Heme enzymes such as cytochrome P450, peroxidases, catalases, heme oxygenase, nitric oxide synthase and cytochrome C oxidase all rely upon Mn or Fe active sites to perform an impressive range of critical biological functions involving O2 or its derivatives. Although the functions of these enzymes are highly diverse, there are some mechanistic commonalities involving key intermediates that bring these systems together. These key intermediates involve high-valent metal-oxo (M=O) and metal-(hydro)peroxo (M-OO(H)) species. Understanding the reactivity and spectroscopic features of these species is critically important, yet many questions remain about these species in part because of challenges associated with studying what are unstable and often short-lived intermediates. We will address fundamental questions regarding high-valent M=O species and their precursors, M-OO(H), through a synthetic analog approach. We will utilize ligands known as corrolazines (Czs) and corroles (Cors) to carry out this research. These ligands are designed to stabilize high oxidation states (e.g. MVO). They also support lower-valent complexes such as MIII and MIV, which will be used to study O-O cleavage events. Aims of this proposal are 1) to synthesize novel high-valent (Cz)M(O) and (Cor)M(O) complexes, determine their structural and spectroscopic properties, and examine their reactivity in biologically relevant transformations and 2) to determine the reactivity of (Cz)M and (Cor)M toward O-O cleavage/O2 activation. Synthetic methods, including ligand design and coordination chemistry, will be used to construct new model complexes of interest. We will correlate the structural and electronic properties of the (Cz)M and (Cor)M (M = Mn, Fe) complexes with reactivity in reactions of direct biological relevance (e.g. hydrogen-atom-transfer (HAT), oxygen-atom-transfer (OAT), and electron-transfer (ET) processes). Reactivity and mechanism will be studied through analysis of products and kinetic measurements with a range of HAT substrates containing C-H and O-H bonds, O-atom acceptor substrates, and ET agents. Comparison of our Mn and Fe chemistry should yield insights regarding why Nature chooses Fe or Mn to perform specific functions. Fundamental information regarding the mechanisms of biomimetic HAT, OAT, ET and related reactions will be obtained.