Development of Homogeneous Aqueous Transition Metal Catalysts for Sustainable Chemistry

Organic solvents are often chosen in the development of homogeneous organometallic catalysts due to increased solubility, stability, and control. Various combinations of organic solvents are frequently tested for their solvent effects in order to improve catalytic efficiency or selectivity. Many organic solvents can also be dried or removed from the reaction with ease. On the other hand, homogeneous catalysis in water presents significant challenges. These problems include a narrow variety of co-solvents to chose from, low solubility of organic ligands, and relatively small temperature ranges (5-100 °C). Aqueous catalysts must also compete with protic solvents, counterions, and other catalytic species in the reaction mixture. Despite these obstacles, the use of water is environmentally and economically sustainable. The high specific heat capacity and heat of vaporization of water allows more energy to be transferred at a given temperature. The excellent hydrogen bonding ability of water, and equilibrium of hydronium and hydroxide, offer additional advantages. In this project, we studied an assortment of aqueous soluble catalysts to enhance their catalytic efficiencies. A Cp*Ir(dihydroxy-bipyridine) complex was found to be regioselective in transfer hydrogenations at room temperature. A new Cu(dihydroxy-bipyridine) catalyst was introduced for phosphate ester hydrolysis in water. Cobalt salophen catalytic systems were developed for chemoselective alkyne hydrations in methanol/water. Cobalt cyclen heterobimetallic catalytic systems showed potential for further use in nitrile hydration at low temperatures. This work has utilized principles of sustainable chemistry to develop and expand homogeneous aqueous transition metal catalysis.