A series of pentamethylcyclopentadienyl iridium(III) (Cp*Ir) complexes bearing 2-pyridinecarboxamidate ligands have been prepared as transfer hydrogenation catalysts that are capable of converting aldehydes to alcohols, using sodium formate as the hydride donor. Because the catalytic activity of different organoiridium catalysts can vary greatly, we wanted to identify the thermodynamic and kinetic factors that are responsible for their reactivity. The hydricity (ΔG<sup>o</sup><sub>H-</sub>) of several representative hydride complexes in aqueous media was determined using a hydride exchange method. The reaction kinetics were measured by product analysis using gas chromatography. The thermodynamic parameters for each discrete step in transfer hydrogenation, including enthalpy, entropy, and activation energies, were obtained from variable temperature studies. These data allowed us to construct reaction coordinate diagrams for several organoiridium catalysts, which provided us with new insights into their catalytic properties. Our results are useful for designing better transfer hydrogenation catalysts and predicting how structural and electronic changes will affect their catalytic efficiency.

Understanding the thermodynamic and kinetic factors that contribute to transfer hydrogenation efficiency by organoiridium complexes

255th National Meeting (2018)

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