1. Tungsten Photoredox Chromophores Containing Amine-Functionalized Diphosphine and N-Heterocyclic Carbene Ligands. 2. The Chemistry of Low-Coordinate Cobalt Compounds Supported by N-Heterocyclic Carbenes and Bulky Amines.

The development of renewable and sustainable energy sources is a major challenge facing the world population. Our growing energy demand is primarily driven by a growing global population, increasing global industrialization, and overall enhancement of the quality of life in developing regions. Among alternative sources of energy, solar is perceived as one of the most promising and utilizable resources. Solar energy is disadvantaged by difficulty in storing the energy once captured. Artificial photosynthesis involves the conversion of solar energy into chemical potential via solar fuel generation from a renewable feedstock. Our group views tungsten alkylidyne chromophores as promising candidates due to their visible light absorption profile, long-lived excited states, and strong photoreduction properties. The work presented in the first half of this thesis describes design and characterization of tungsten alkylidyne chromophores for improved catalytic turnover, as well as enhanced photoreduction properties. The work presented in the second half of this thesis describes work done on low-coordinate cobalt amines and bulky N-heterocyclic carbene stabilized iron carbonyls for hydrosilylation. Chapter 2 describes the incorporation of chelating phosphine ligands with with pendant Brønsted basic groups for improved catalytic turnover utilizing hydrogen. We report the synthesis and characterization of both a model tungsten hydride compound utilizing conventional ligands, as well a second compound bearing ligands with Brønsted basic sites. Using ligands with Brønsted basic sites, we are able to change the preferred location of protonation. Chapter 3 describes our design and synthetic efforts toward using strongly sigma donating N-heterocyclic carbene ligands to increase photoredox potentials. Chapters 4 and 5 describe work done in a previous research group on the synthesis, structure, characterization, and reactivity of low- coordinate cobalt amide compounds. A detailed study of the protonolysis and aminolysis of a three-coordinate cobalt amide chloride is discussed. Chapter 6 describes work on an iron xxvi tetracarbonyl supported by an extremely bulky N-heterocyclic carbene. The synthesis, structure, and catalytic hydrosilylation chemistry are discussed. Appendix I reports the structure of a cobalt bis(anilide) discovered during work described in Chapter 4. Appendix II collects data of the structural studies reported in this thesis.