Dioxygen Activation by Thiolate Ligated Complexes with Tunable pi-Acceptor N-Heterocyclic Ligands

Abstract

Dioxygen reduction and water oxidation are critical processes for aerobic life. Developing a better understanding of O-O bond activation and formation mechanisms mediated by metalloenzymes involving first-row transition metals such as Fe and Mn, would provide valuable insights for catalyst and drug development. While synthetic iron and copper oxo species generated from O2 have been extensively studied, the process of manganese dioxygen activation and its intermediates is comparatively unexplored. This dissertation focuses on the synthesis, characterization, and dioxygen reactivity of structurally analogous Mn- and Fe-thiolate complexes with tunable N-heterocyclic amine ligands. Chapter one provides an introduction to oxygen-evolving and oxygen-dependent metalloenzymes containing Mn and Fe metallocofactors. Chapter two highlights how metal ion Lewis acidity and steric properties influence the kinetics and thermodynamics of O2 activation by a series of structurally analogous Mn-thiolate complexes. A total of four high-valent Mn-oxo intermediates were observed along the O2 activation pathway, including MnIIIMnIV(-oxo)(-OH) intermediates. Chapter three covers the hydrogen atom transfer activity by the MnIIIMnIV(-oxo)(-OH) intermediate characterized in chapter two. Chapter four focuses on the synthesis, characterization, and preliminary O2 reactivity of Fe-thiolate complexes that possess a tunable N-heterocyclic amine, wherein a less Lewis acidic FeII species within the structurally analogous series led to the observation of a new O2 activation intermediate. Chapter five discusses the comparison between the structures and reactivities of alkoxide- and thiolate-ligated Mn alkylperoxo species with otherwise identical ligand frameworks, which demonstrates key insight into how the primary coordination sphere of active sites could alter their reactivity.