Reactivity of Transition Metal Hydride Complexes in Energy Conversion Processes

A majority of our energy (ca. 74%) comes from fossil fuels linked to harmful emissions of CO2. Many alternatives to fossil fuels, such as solar and wind, require the collection and storage of energy. However, the current energy storage infrastructure is not sufficient to store the energy for later use. A promising solution to the energy storage problem is to store the energy in chemical bonds as energy-dense gases or liquid fuels. Energy-dense fuels can be formed through CO2 reduction, N2 reduction, and H+ reduction reactions mediated by catalysts. Transition metal hydride complexes are key intermediates in many of these catalytic fuel forming reactions. They mediate the movement of protons and electrons to form energy-dense fuels. Often the movement of these protons and electrons is coupled in proton-coupled electron transfer (PCET) reactions. It is important to understand the PCET mechanisms for these transition metal hydride complexes as catalyst to determine ways to optimize conditions while minimizing harsh conditions. This work probes the reactivity of transition metal hydride complexes and interogates how they operate in order to develop design principles for efficient catalysts. First, the mechanism and reactivity for the light driven formation of a transition metal hydride complex was investigated through in situ photo 1H¬–NMR monitoring and quantum yields. Next, a series of tungsten hydride complexes and the putative intermediates formed by PCET reactivity were synthesized and characterized. This is followed by a preliminary investigation into the PCET mechanism of one of the tungsten hydride complexes using 2D exchange spectroscopy (EXSY) and stopped-flow rapid mixing techniques coupled with optical spectroscopy. Lastly, an iridium catalyst was investigated to determine its viability in the electrochemical Guerbet reaction by upgrading a lower chain alcohol to a higher chain alcohol avoiding the harsh conditions associated with the thermal Guerbet reaction.