The oxidation of formic acid can produce H2 directly or be used in transfer hydrogenation to unsaturated substrates using transition metal catalysts. The mechanism for these transformations involves the initial transfer of a hydride from formate to a metal complex followed by either protonation or transfer of that hydride to a new substrate. The thermodynamic ability of a substrate to donate a hydride, known as hydricity, is an important parameter for predicting the reactivity of a catalyst for this process. Herein, we describe the determination of the hydricity of Ir(Cp*)(ppy)H (ppy = 2-phenylpyridine) (49 kcal/mol in acetonitrile) by the H2 heterolysis method. Following determination, using the hydricity of formate (44 kcal/mol in acetonitrile) and pKa of formic acid (20.9 in acetonitrile) as a guide, the optimization of formic acid oxidation to CO2 and H2 are described. These insights are then used to expand on the reactivity of the Ir(Cp*)(ppy)(H) with other weak hydride donors such as methanol and isopropanol as well as for transfer to olefins.

Hydricity of Ir(Cp*)(ppy)H and its implications to formate oxidation and H2 generation

ACS Spring 2022

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