Efficient interconversion of chemical fuels and electricity will be key to developing a carbon neutral economy. Important conversion reactions of this type are the CO2 reduction reaction and the hydrogen and formate oxidation reactions. Inorganic molecular electrocatalysts can perform these reactions, but currently lack the selectivity and efficiency required for widespread use. Metal hydride complexes are often key intermediates in these reductive and oxidative electrocatalysis reactions. The thermodynamic parameter of hydricity can be utilized to tune electrocatalysis reactions that go through hydride intermediates. For CO2 reduction, complexes with appropriate hydricity under specific conditions can thermodynamically select for formate product. However, these conditions require low proton activity – either high pH or use of acids with high pKa. As an alternative approach, kinetic barriers can be adjusted to select for formate over parasitic H2 evolution. We hypothesize installation of a local cationic field would impart a kinetic barrier for H2 evolution, allowing for selective CO2 reduction to formate in the presence of more acidic protons. Understanding how local cationic fields effect reduction potentials and hydricity can also allow better design of homogeneous hydrogen, formate, and methanol oxidation catalysts. To explore these effects, we synthesized derivatives of commonly studied nickel bis(diphosphine) electrocatalysts with cationic groups near the metal center. This series of neutral, monocationic, and dicationic complexes were successfully characterized in multiple solvents. A trend was found where increasing the overall charge of the complex induces a positive shift in the reduction potentials of the nickel center. Further, the local cation influence on hydricity and relevance for catalysis were studied.

Cationic effects on nickel bis(diphosphine) electrocatalysts

ACS Spring 2022

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