Global climate change has been driven in part by the combustion of fossil fuels. A more efficient means of extracting energy from chemical fuels is critical for mitigating further CO2emissions. Direct formic acid fuel cells offer higher theoretical efficiencies than standard combustion engines and thus represent a promising alternative to combustion. The proposed mechanism for formate oxidation with molecular catalysts generally involves hydride transfer from formate to the metal center; therefore, the metal-hydride complex must be less hydridic than formate to favor this key step. We have prepared a series of [CpCo(P-P)(MeCN)]2+complexes bearing bidentate phosphine ligands and evaluated their thermochemical parameters and reactivity with formate as a function of ligand properties. While these complexes are predicted to favor formate oxidation from a thermodynamic standpoint, cyclic voltammetry studies in the presence of 10 equivalents of formate do not show electrocatalytic current enhancement; instead, rapid formation of a Co(III)-formate complex occurs that appears to be stable on this short experimental timescale. By 1H and 31P NMR, quantitative production of the Co(III)-formate species occurs upon mixing in the presence of a slight excess of formate. This rapid reaction is then followed by slow generation of a paramagnetic species, followed by gradual conversion to the Co(III)-hydride complex. We present a preliminary mechanistic scheme to explain this complicated reactivity. Importantly, our results suggest that formate oxidation is indeed accessible with this series of complexes. Future work will focus on the introduction of pendent functional groups onto the phosphine ligands in order to lower the kinetic barrier for hydride transfer.

Developing molecular electrocatalysts for formate oxidation

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.