Unraveling Reaction Mechanisms using Iron and Cobalt Complexes Supported by a Dianionic Pentadentate Ligand

Abstract

The search for carbon-neutral alternatives to fossil fuels has led to the investigation of fundamental reactions mediated by first-row transition metal complexes, such as the reduction of dioxygen to water or the oxidation of ammonia to dinitrogen. In living organisms, metalloenzymes can mediate these transformations under mild conditions, through complex mechanisms that are often difficult to study under biological conditions. Therefore, natural systems represent a constant source of inspiration for synthetic chemists who wish to develop artificial catalysts and understand their reaction mechanisms. In this context, strategic ligand design yielded a plethora of classes of ligands, which vary the reactivity and properties of transition metal complexes. In particular, pentadentate ligands have been widely employed across the periodic table to allow a single site for reactivity, thus providing a well-defined system for mechanistic studies. These systems represent a modular tool for chemists to study detailed mechanistic steps of chemical reactions and to understand the role of what are often fleeting intermediates in life-sustaining reactions. This thesis presents a concrete example of using a pentadentate ligand combined with iron and cobalt to access and stabilize high-valent metal-oxo and metal-imido complexes, which have been proposed as key intermediates in a variety of catalytic transformations. The syntheses of various iron and cobalt complexes supported by the tetrapodal pentadentate Pz4PyB2 ligand are discussed, and their reactivity is explored. As a result, these complexes have been investigated for nitrene transfers; dioxygen reduction to water, ammonia oxidation to dinitrogen, and as mimics to biological related systems. The mechanistic details of these processes were studied extensively, both experimentally and theoretically, and reveal a unique platform for reactivity.