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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). These homogeneous electrocatalysts are the first cobalt example and second first-row transition metal example for formate oxidation. Thermodynamic measurements reveal 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 with the strongest cobalt- hydride bond, given by its thermodynamic hydricity, is the fastest electrocatalyst in this series, with an observed rate constant for formate oxidation of 135 ± 8 h−1 at 25 °C. Electrocatalytic turnover is not observed for the complex with no pendent amine groups: decomposition of the complex structure is evident in the presence of high formate concentrations.
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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.
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- Award ID(s):
- 2117792
- PAR ID:
- 10581489
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- ACS Catalysis
- Volume:
- 15
- Issue:
- 3
- ISSN:
- 2155-5435
- Page Range / eLocation ID:
- 1771 to 1781
- Subject(s) / Keyword(s):
- cobalt complexes formate oxidation electrocatalysis hydricity pendent amines
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1021/acscatal.4c03189
