Metal-hydride complexes play a crucial role in many catalytic transformations including the hydrogenation or electrocatalytic reduction of CO2 to formic acid. Hydride transfer from the metal hydride to CO2 is a key step for these reactions; therefore, hydricity ΔG°H− of the metal-hydride bond is an important parameter for describing and predicting the reactivity of such complexes. Here, a comprehensive dataset of hydricity values in acetonitrile is presented, which consists of data for 56 transition metal-hydride complexes [M-H](n-1)+ that exhibit a reversible one-electron reduction for the analogous parent complex [M]n+. We show that hydricity varies approximately linearly as a function of E1/2(Mn+/(n-1)+) across the entire dataset, regardless of metal center, ligand architecture, and coordination geometry. The linear trend is well described using a thermochemical cycle relating hydricity and the homolytic bond dissociation free energy of the metal-hydride bond. This relationship enables for the rapid estimation of hydricity simply based on the reduction potential of the parent metal complex [M]n+. Furthermore, in the context of CO2 hydrogenation, we use this model to construct a schematic guide depicting the necessary base strength required for H2 activation under ambient conditions based on the hydricity of a given metal hydride. Another schematic guide is established that shows the necessary Brønsted acid strength and associated overpotential for electrocatalytic CO2 reduction to formate based on the metal-hydride hydricity.

Hydricity and other thermodynamic considerations for CO2 reduction

255th National Meeting (2018)

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