Exploring and Engineering the Surface Microenvironment of Single-Atom Catalysts for Suzuki Cross-Couplings

Single-Atom Catalysts (SACs) are an emerging class of materials capable of bridging the gap between homogeneous and heterogeneous catalysts by mitigating disadvantages and providing the benefits of both. One field where SACs are receiving attention is for carbon-carbon cross-couplings, such as Suzuki reactions. While capable of performing these reactions, SACs lack the fine control afforded to homogenous catalysts through the use of ligands. Much of the SACs surface will have little interaction with reactants, and there is no way to effectively control the microenvironment surrounding the active site. Our approach seeks to understand the interactions between an SAC and the reaction substrates by modifying the surface of the SAC to enhance surface/substrate interaction and through understanding the factors that drive the selectivity of a reaction in comparison to a reaction with a homogeneous catalyst. A brief exploration of SACs in given in Chapter 1, before Chapter 2 begins by discussing the synthesis and characterization of our SAC, Pd/CeO¬2. Chapter 2 then investigates our SAC modification approach, where catechol monolayers are deposited on the surface of an SAC and increase the activity through π-stacking effects. The enhancement effect is lost as bulkier ligands are used, presumably due to steric hinderance. Chapter 3 begins our exploration of regioselectivity, where a reaction with two reactive sites is studied, to determine which product is favored. In this study, the catalysts Pd/CeO2 and Pd(OAc)¬2/4PPh3 are systematically compared under a multitude of conditions to assess the driving forces behind selectivity and activity, as well as deepening the understanding of the differences between a typical homogeneous catalyst and an SAC. Chapter 4 brings together the knowledge gained from Chapters 2 and 3 by utilizing carboxylic acid monolayers to control the regioselectivity of a reaction through π-stacking interactions between pyridine carboxylic acids and nitrogen containing aryl halides. Chapters 5 draws conclusions from the entire body of work, and Chapter 6 discusses possible future directions for investigating surface interactions on SACs and for using monolayers to enhance SAC performance.