Mechanistic studies of water oxidation and carbon dioxide reduction using transition metal catalysts with protic ligands
The majority of energy produced in the world is derived from fossil fuels which are finite and have deleterious environmental effects. For a sustainable and environmentally-friendly energy future, alternative, renewable energy sources are desired. Two reactions that could have applications towards developing renewable energy sources are water oxidation to produce hydrogen and carbon dioxide reduction to form various products (e.g. formic acid or carbon monoxide); however, these reactions require catalysts to efficiently produce the desired products. Efforts to synthesize, characterize, and study catalysts for these reactions are discussed in this dissertation. The first chapter serves as an introduction to energy-related catalytic reactions. In Chapter 2, 6,6ʹ-dihydroxybipyridine (6,6ʹ-dhbp)—a protic ligand used with several metals to produce catalysts for energy-related reactions—is studied to determine its thermodynamic acidity. In the following chapter, 6,6ʹ-dhbp is used as a ligand with copper to form complexes that are water oxidation catalysts. Chapters 4 and 5 focus on iridium and ruthenium complexes containing new bidentate ligands composed of pyridinol and N-heterocyclic carbenes (NHCs). These complexes, along with an iridium complex of 6,6ʹ-dhbp, were used as catalysts for the hydrogenation of carbon dioxide to formate and the reverse dehydrogenation of formic acid to carbon dioxide and hydrogen. However, the complexes containing the new bidentate pyridinol-NHC ligands were found to be precatalysts as they undergo transformations and decomposition during the course of the reaction. A nickel-pincer complex with a protic CNC-pincer derived of pyridinol and NHCs was used as a photocatalyst for carbon dioxide reduction in Chapter 6. The protic state of the hydroxy group in the 4-position of the pyridine ring was determined to be important for catalysis, as the deprotonated hydroxy group results in 10 times the catalytic ability as the protonated form. In the penultimate chapter, ruthenium-pincer complexes that are active carbon dioxide photoreduction catalysts are studied mechanistically by UV/vis and IR spectroscopies. The most active catalyst was studied in greater detail with real-time IR spectroscopy to help elucidate potential reaction pathways. The final chapter serves as a conclusion to summarize the results discussed in the dissertation.