| Description |
The development of cost-effective, efficient, and selective catalysts for the hydrogenation of CO2 and CO2-amine adducts to alternative fuels is a desirable target for renewable energy. While a healthy number of catalysts have been developed for the purpose of producing fuels such as methanol and formic acid, thorough thermodynamic and kinetic analyses to correlate catalytic performance with structural features such as pKa, Keq, and hydricity (ΔGH-) are rarely reported. Additionally, catalysts are often subjected to optimization studies that present a double-edged sword: turnover number (TON) to maximize product formation is achieved; however it is all the more difficult to compare catalysts' inherent hydrogenation ability as no unified set of conditions is used. In the pursuit of understanding how structure impacts reactivity, a series of four catalysts varying in their ligand structure were studied. All ligands have the base structure ‘PNX,' where X can represent either nitrogen (diethylamine or pyridine) or phosphorous and the central N is either an aliphatic amine or aromatized pyridine. These ligands around bound to ruthenium in a pincer fashion to give complexes of the general form (R2PNX)Ru(H)(Cl)(CO), which can be deprotonated to yield the active precatalysts, (R2P*NX)Ru(H)(CO). Using the dearomatized/ deprotonated precatalysts, reactivity with substrates H2, CO2, and various HX species present in catalysis were determined via nuclear magnetic iv resonance (NMR), ultraviolet-visible (UV-vis), and infrared (IR) spectroscopies. The equilibria of each substrate was measured to obtain Keq as a function of temperature. The ease of deprotonation of (R2PNX)Ru(H)(Cl)(CO) was determined by UV-Vis and was used to determine pKa. Hydrogen equilibria in combination with pKa was used to determine ΔGH-. Finally, high-temperature high-pressure hydrogenations were conducted using identical conditions to compare the four PNX catalysts. In an additional project, the attempted synthesis of four MRI contrast agents, ‘N3' and ‘N6' pyridine complexes of iron and cobalt revealed protodeboronation of a pinacolborane moiety under oxidative conditions. |
