Iron porphyrin electrocatalysts for carbon dioxide reduction: investigations of the importance of Brønsted acids and hydrogen bonding

The capture and conversion of the greenhouse gas carbon dioxide (CO2) is a widely researched strategy to addressing the climate crisis and mitigating CO2 emissions stemming from the use of fossil-fuels. The field of CO2 conversion to value-added products using catalysts has been extensively explored since about the 1970s. Those early works used molecular catalysts to reduce CO2 to carbon monoxide, a valuable precursor to fuels synthesis, but the reactions were slow and/or lacked selectivity. Modern catalysts, like those described in this thesis, specifically assist in mediation of the input of multiple protons and electrons to reduce CO2 rapidly and selectively to desired products. Functional groups positioned in the periphery of catalyst active sites enhance catalytic activity and selectivity by stabilizing key intermediates and providing an internal source of protons. Understanding how these functional groups work and their role in CO2 reduction is crucial to the development of scalable catalyst designs. This thesis describes investigations of molecular catalysts, particularly metalloporphyrins functionalized with hydroxyphenyl groups, to better understand how pre-positioned functional groups promote catalysis. Chapter 1 covers a brief history of molecular catalyst functional groups and their roles in CO2 reduction. The subsequent Chapters provide a deeper understanding of these previously investigated catalyst functional groups with a specific focus on pendent hydroxyphenyl groups. Of particular interest in this thesis is the role of hydrogen bonding interactions between solvent and the internal pendent groups, as well as the potential for internal protonation via pendent hydroxy groups. Chapter 2 presents a systematic study of the positions and orientations of 2,6-dihydroxyphenyl groups surrounding an iron porphyrin active site and their effects on CO2 reduction. In Chapter 3, a series of hydroxyphenyl substituted iron porphyrin catalysts that present a range of pKa values (Brønsted acidity) are investigated in combination with a range of analogous added phenols. That work revealed the interplay between internal and external acid sources. Chapter 4 delves further into the study of pendant proton sources, focusing on a formally catechol group to elucidate the role of a proton and electron donation. Chapter 5 involves a study of an iron tetraphenyl porphyrin catalyst to understand the role of hydrogen bonding in a catalyst lacking hydroxyphenyl groups. Finally, Chapter 6 provides some forward-looking comments and proposals for future experiments.