Molecular Cobalt Complexes with Pendant Amines for Selective Electrocatalytic Reduction of Carbon Dioxide to Formic Acid

We
report here on a new series of CO2-reducing molecular
catalysts based on Earth-abundant elements that are very selective
for the production of formic acid in dimethylformamide (DMF)/water
mixtures (Faradaic efficiency of 90 ± 10%) at moderate overpotentials
(500–700 mV in DMF measured at the middle of the catalytic
wave). The [CpCo­(PR2NR′2)­I]+ compounds contain diphosphine ligands,
PR2NR′2, with two pendant amine residues that act as proton relays during
CO2-reduction catalysis and tune their activity. Four different
PR2NR′2 ligands
with cyclohexyl or phenyl substituents on phosphorus and benzyl or
phenyl substituents on nitrogen were employed, and the compound with
the most electron-donating phosphine ligand and the most basic amine
functions performs best among the series, with turnover frequency
>1000 s–1. State-of-the-art benchmarking of catalytic
performances ranks this new class of cobalt-based complexes among
the most promising CO2-to-formic acid reducing catalysts
developed to date; addressing the stability issues would allow further
improvement. Mechanistic studies and density functional theory simulations
confirmed the role of amine groups for stabilizing key intermediates
through hydrogen bonding with water molecules during hydride transfer
from the Co center to the CO2 molecule.