Understanding Proton Shuttling with Pendent Bases in Catalyzing the Interconversion of Dioxygen and Water
Tronic, Tristan Andrew
MetadataShow full item record
The electrocatalyzed interconversion of dioxygen and water is critical to the development of non-carbon based renewable fuels, because these are half-reactions in hydrogen fuel cells/water electrolyzers. These reactions require the coordinated transfer of four protons and electrons. Transition metal complexes are known to catalyze these reactions by facilitating electron transfer, but methods to facilitate proton transfer have not been explored. In this work, model systems with pendent bases are studied, in order to understand the ability of pendent bases to improve catalyst efficiency by acting as proton shuttles. In Chapters 2 and 3, the oxidation of phenols with pendent pyridines is explored. These molecules are models for the oxidation of Tyr 161 in photosystem II, the protein responsible for biological catalytic water oxidation. Oxidation of these phenols is coupled to concerted transfer of the phenolic proton to the pendent pyridine. In Chapter 1, the basicity of the pendent pyridine is tuned synthetically to determine the critical parameters in predicting the kinetics of oxidation. The contribution of the driving force for oxidation, determined by the basicity of the pendent pyridine, is found to far outweigh other parameters in determining oxidation rates. In Chapter 2, a model system is synthesized with an alcohol positioned between the phenol and pyridine in an effort to observe oxidation-coupled proton transfer through a hydrogen-bonding chain. Chapters 4-6 are concerned with developing transition metal complexes with pendent bases in the second coordination sphere of the metal and studying how these bases affect catalysis of oxygen reduction. In Chapter 4, cobalt complexes with tetraazamacrocyclic ligands with pendent bases are synthesized. These ligands are found to be difficult to synthesize and insufficiently rigid, failing to restrict the pendent bases from binding to the cobalt. In Chapters 5 and 6, ruthenium complexes with more rigid diazadiphosphine ligands are synthesized. These complexes are found by X-ray diffraction and NMR spectroscopy to bind dioxygen, and their pendent amines direct protons in a hydrogen bonding interaction towards the dioxygen. Electrochemical studies indicate that the pendent bases facilitate reduction, but these complexes do not catalyze four-electron reduction to water.
- Chemistry