Date of Award

12-31-2022

Document Type

Campus Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry/Inorganic Chemistry

First Advisor

Jonathan Rochford

Second Advisor

Jason Evans

Third Advisor

Niya Sa

Abstract

Electrocatalytic carbon dioxide (CO2) reduction is an efficient way to mitigate the excess of CO2 in our atmosphere. However, CO2 reduction does not come without its challenges. In order to lower energy requirements of electrocatalytic CO2 reduction, we must turn to nature and utilize proton-coupled electron transfer. By coupling protonation and reduction events, the formation of highly charged intermediates along the reaction coordinate is avoided, lowering the overall energy requirements, and increasing kinetic efficiency for catalysis.

This dissertation serves as a study of steric and electronic influence of the secondary coordination sphere (SCS) of [fac–Mn(I)CH3CN(R2bpy)(CO)3]+ complexes in tuning product selectivity and lowering potential requirements for catalysis. Mn(I) metal complexes are of interest as CO2 reduction catalysts as they are earth abundant, inexpensive, and efficient. Chapter 1 serves to provide the reader with an overview of Mn(I) metal complexes in the literature as they pertain to CO2 reduction.

The incorporation of Lewis basic ether groups in the SCS has been demonstrated to not only increase kinetic efficiency of CO2 reduction, but also to lower potential requirements due to stabilization of critical transition states along the reaction coordinate. Chapter 2 builds upon the latter work to investigate the influence of steric bulk at 6,6′-bisaryl substituted bpy ligands in [fac–Mn(I)CH3CN(R2bpy)(CO)3]+ pre-catalysts. In Chapter 3, ligands are functionalized to study structure-activity relationships in [fac–Mn(I)CH3CN(R2bpy)(CO)3]+ pre-catalysts to form each desired product, carbon monoxide (CO) and formic acid (HCO2H). Selective CO formation has been observed in the presence of aryl ether functionalities, until additional basic groups are introduced in the SCS, where a mixture of products are formed. Included herein are an array of complexes, each with unique functionalization in the SCS, including protic and aprotic groups. In Chapter 4, these unique functionalizations are further expanded to the outer coordination sphere (OCS). OCS functionalization is observed in nature to assist in protonation and hydrogen-bond stabilization of key transition states important for CO2 reduction.

It was found that protic groups, as well as basic amino groups in the secondary coordination sphere often contribute to metal-hydride formation, and therefore encourage hydrogen evolution, or HCO2H production. These effects were further studied in Chapter 5, where Brønsted acid sources of decreasing pKa were added, and voltametric and controlled potential electrolysis experiments were conducted in the presence and absence of CO2.

Comments

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