The efficient interconversion between electrical energy and chemical fuels is critical for our future sustainable energy economy. We are developing molecular first-row transition metal catalysts for the electro-oxidation of fuels such as formic acid and hydrogen. The oxidation of both substrates by a transition metal catalyst likely involves hydride transfer from the substrate to the metal center, setting a thermodynamic requirement for the hydricity of the resulting metal-hydride intermediate. Here, we have identified a series of cobalt(III) complexes with bidentate phosphine ligands that exhibit strong hydride affinities to promote formate and hydrogen oxidation. We have isolated the corresponding cobalt(III)-hydride complexes and found that their acidity and hydricity values depend on the phosphine ligand properties. In all cases, treatment of the parent cobalt(III) complex with formate leads to rapid formation of a cobalt(III)-formate adduct. Subsequent hydride transfer from formate to the metal is kinetically accessible, but this reaction is significantly faster for systems containing phosphines with pendent amine groups. Currently, efforts are underway to optimize reaction conditions for achieving electrocatalytic formate oxidation under controlled-potential electrolysis conditions. We have also found that the pendent amine complexes are capable of heterolytic hydrogen cleavage at ambient temperature and pressure, setting the stage of electrochemical hydrogen oxidation under mild conditions. Preliminary results suggest there is a direct dependence of hydrogen activation conversion on the base strength, and we working to fully elucidate the relationships between the thermodynamic and kinetic favorability of H<sub>2</sub> cleavage with these complexes. We have also shown that complexes featuring unfunctionalized phosphine ligands exhibit significantly slower rates of hydrogen splitting under comparable conditions, highlighting the important role that the outer coordination sphere plays in lowering the kinetic barrier for hydride transfer.

Developing molecular electrocatalysts for fuel oxidation

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ACS Fall 2022

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