| Abstrakt: |
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 CO2to formate (Faradaic efficiency FEHCO2–= 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 1–3were 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 JHCO2−(partial catalytic current density for formate) increasing by 3.3-fold for 2and 1.2-fold for 3in comparison to the JHCO2−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 2and 3, resulting in quantifiable differences in catalytic activity and efficiency. CPE experiments demonstrate an increased FEHCO2–= 85 ± 2% for 2, whereas 3had a lower FEHCO2–= 71 ± 3%, with CO and H2generated 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. |
