摘要
Fe complexes 1 and 2, bearing tetraamido macrocyclic ligands (TAMLs) with different pi-conjugation arrangements, were prepared as molecular catalysts to investigate the influence of the ligand on the efficacy of the electrochemical C(sp(3))-H-oxidation reaction. Pourbaix diagrams revealed that catalyst 2, with extended pi-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(k(cat))) on the C(sp(3))-H bond dissociation energy (BDE(Csp-H)3) of organic substrates. The slope of the relationship between log(k(cat)) and BDE(Csp-H)3 was notably shallower for catalyst 2 compared to catalyst 1, reflecting a reduced sensitivity to substrate BDE(Csp-H)3. This behavior is attributed to the greater ligand pi-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 k(cat) 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(sp(3))-H-oxidation by iron-based molecular catalysts.