Using redox-active ligands to promote two-electron oxidation at earth-abundant cobalt complexes

Multielectron transfer is central to electrocatalytic reactions. In general, multielectron processes can be accessed at a single potential where subsequent electron transfers are easier than the first, or at a series of distinct potentials for sequential single electron transfer steps. However, the latter poses challenges in controlling reactivity at the catalytic site and usually requires a larger overpotential because of the separated redox couples. Thus, the development of electrocatalysts based on the earth-abundant first-row transition metals that can promote multielectron redox behavior at one potential has become a focus of sustainable catalysis. Despite several advances in application of redox-active ligands to access multielectron transfers at first-row transition metals, it remains rare to observe two-electron redox behavior, especially a two-electron oxidation, at mononuclear first-row metal complexes. Herein, we present a series of neutral cyclopentadienyl cobalt complexes bearing a redox-active ortho-phenylenediamido (opda) ligand that undergo two-electron oxidation, which is primarily localized on the ligand. X-ray crystallographic analysis of the two-electron oxidized species reveals significant shortening of the ligand C–N bonds, clearly indicating the change in oxidation state of the opda backbone. Our computational results based on density-functional theory demonstrate a significant structure reorganization upon oxidation and the importance of acetonitrile coordination to favor the second electron transfer. We are now exploring the reactivity of these cobalt complexes with electrophilic substrates, where bond formation at cobalt is achieved through two-electron oxidation of the opda ligand. This work will lead to new avenues for sustainable electrocatalysis where multielectron transfer reactivity is dominated by the redox-active ligand framework rather than the first-row transition metals.