Electrocatalytic CO₂ Reduction to Formate with Molecular Fe(III) Complexes Containing Pendent Proton Relays

Previously, we reported an iron(III) complex with 6,6'-([2,2'-bipyridine]-6,6'-diyl)bis(2,4-ditertbutyl-phenol) as a ligand (Fe(^tbudhbpy)Cl, 1) as catalytically competent for the electrochemical reduction of CO₂ to formate (Faradaic efficiency FE_HCO = 68 ± 4%). In mechanistic experiments, an essential component was found to be a pre-equilibrium reaction involving the association of the proton donor with the catalyst, which preceded proton transfer to the Fe-bound O atoms upon reduction of the Fe center. Here, we report the synthesis, structural characterization, and reactivity of two iron(III) compounds with 6,6'-([2,2'-bipyridine]-6,6'-diyl)bis(2-methoxy-4-methylphenol) (^mecrebpy[H]₂, Fe(^mecrebpy)Cl, 2) and 6,6'-([2,2'-bipyridine]-6,6'-diyl)bis(4-(tert-butyl)benzene-1,2-diol) (^tbucatbpy[H]₄, Fe(^tbucatbpy), 3) as ligands, where pendent -OMe and -OH groups are poised to modify the protonation reaction involving the Fe-bound O atoms. Differences in selectivity and activity for the electrocatalytic reduction of carbon dioxide (CO₂) to formate (HCO₂^-) between complexes 1-3 were assessed via cyclic voltammetry and controlled potential electrolysis (CPE) experiments in N,N-dimethylformamide. Mechanistic studies suggest that the O atoms in the secondary coordination sphere are important for relaying the exogenous proton donor to the active site through a preconcentration effect, which leads to the J_HCO (partial catalytic current density for formate) increasing by 3.3-fold for 2 and 1.2-fold for 3 in comparison to the J_HCO of ₁. These results also suggest that there is a difference in the strength of the interaction between the pendent functional groups and the sacrificial proton donor between 2 and 3, resulting in quantifiable differences in catalytic activity and efficiency. CPE experiments demonstrate an increased FE_HCO = 85 ± 2% for 2, whereas 3 had a lower FE_HCO = 71 ± 3%, with CO and H₂ generated as co-products in each case to reach mass balance. These results indicate that using secondary sphere moieties to modulate metal-ligand interactions and multisite electron and proton transfer reactivity in the primary coordination sphere through reactant preconcentration can be a powerful strategy for enhancing electrocatalytic activity and selectivity.