Exploring Electrochemical C(sp³)–H Oxidation over Fe Complexes: Ligand Effect on the Rate–Bond Dissociation Energy Relationship and Reaction Mechanism

Fe complexes 1 and 2, bearing tetraamido macrocyclic ligands (TAMLs) with different π-conjugation arrangements, were prepared as molecular catalysts to investigate the influence of the ligand on the efficacy of the electrochemical C(sp3)–H-oxidation reaction. Pourbaix diagrams revealed that catalyst 2, with extended π-conjugation in its TAML, reached a higher oxidation state under a lower potential and generated different reaction intermediates. The catalytic performances were assessed using the linear free energy relationship; that is, the dependence of the logarithm of the second-order catalytic rate constant (log(kcat)) on the C(sp3)–H bond dissociation energy (BDE(Csp3–H)) of organic substrates. The slope of the relationship between log(kcat) and BDE(Csp3–H) was notably shallower for catalyst 2 compared to catalyst 1, reflecting a reduced sensitivity to substrate BDE(Csp3–H). This behavior is attributed to the greater ligand π-conjugation of 2 that the high-valent Fe-oxo active species is therefore more stable and less electrophilic, resulting in a smaller difference in the kcat than 1 for different substrates. This study explicitly reveals that modifications in ligand architecture can directly modulate catalytic activity and alter reaction pathways in the electrochemical C(sp3)–H-oxidation by iron-based molecular catalysts.