Dioxygen reduction and Superoxide dismutase mimicry using first row transition metal complexes with redox active organic ligands.

Abstract

The efficient activation of small molecules, such as dioxygen (O2) and superoxide (O2-), is key to solving several energy- and health-related challenges. The selective reduction of O2 to water (H2O) is integral to the development of state-of-the-art fuel cell electrocatalysts that could supplant fossil fuel sources as an alternative energy source. The effective dismutation of O2- to O2 and hydrogen peroxide (H2O2) can alleviate the oxidative stress correlated with aging and other health disorders. Although several transition metal complexes have been designed and studied to promote dioxygen reduction and O2- degradation, most of these metal complexes rely exclusively on transition metals to supply and accept electrons during these reactions. Prior work from the Goldsmith lab has found that covalently attaching quinols to coordination complexes can augment the redox activity of transition metals and even allow redox-inactive metal ions to catalyze new sorts of chemical reactions. This dissertation is primarily focused on using coordination complexes with quinol-containing ligands to catalyze the oxygen reduction reaction (ORR). Chapter one provides an overview of recent advances in the development of molecular catalysts for the ORR. This chapter also details the current electrochemical and spectrophotometric methods that are used to study these catalysts as well as several performance metrics used to assess the activity, selectivity, and efficiency of electrocatalysts for the ORR. Chapter two discusses how pendent quinol or phenol groups tune the selectivity of molecular Co(II)-based ORR electrocatalysts. Although the activities and effective overpotentials of the quinol- and phenol-containing catalysts are similar, the use of the quinol shifts the product selectivity from H2O2 to H2O. Chapter three explores the ORR activity of several Fe(II) and Fe(III) complexes with additional quinol- and phenol-containing ligands. This chapter begins to establish structure-function relationships for this class of catalyst and illustrates how the numbers of pyridine and quinol groups on the electrocatalysts can influence their activity, product selectivity, and efficiency for the ORR. An additional quinol bolsters the activity and selectivity for water production, albeit at the cost of a higher effective overpotential. In chapter four, Density Functional Theory (DFT) calculations are performed to better understand the mechanism of O2- dismutation catalyzed by a Zn(II) complex with a pendent quinol group. It also reports a Zn(II) complex with a ligand that instead contains a pendent phenol. The phenolic complex is catalytically inactive, confirming that the redox-active quinol is essential for the degradation of O2- by the Zn(II)-quinol catalyst.