Electrocatalytic Formate Oxidation by Cobalt–Phosphine Complexes

We report a family of cobalt complexes based on bidentate
phosphine
ligands with two, one, or zero pendent amine groups in the ligand
backbone. The pendent amine complexes are active electrocatalysts
for the formate oxidation reaction, generating CO2 with
near-quantitative faradaic efficiency at moderate overpotentials (0.45–0.57
V in acetonitrile). Thermodynamic measurements reveal that these complexes
are energetically primed for formate oxidation via hydride transfer
to the cobalt center, followed by deprotonation of the resulting cobalt-hydride
by formate acting as a base. The complex featuring a single pendent
amine arm is the fastest electrocatalyst in this series, with an observed
rate constant for formate oxidation of 135 ± 8 h–1 at 25 °C, surpassing the activity of the bis-pendent amine
analogue. Electrocatalytic turnover is not observed for the complex
with no pendent amine groups: decomposition of the complex is evident
in the presence of high formate concentrations. Thus, the application
of thermodynamic considerations to electrocatalyst design is demonstrated
as a successful strategy, while also highlighting the delicate balance
of ligand properties necessary for achieving productive turnover.