Oral Presentation 21st International Conference on Biological Inorganic Chemistry 2025

Research on molecular catalysts is significant to provide fundamental knowledge on reaction mechanisms and structure–function relationships which is essential for the development of new catalysts with improved performance. In this work, three pairs of metalloporphyrins (M = Fe^III and Zn^II/Co^II) with different covalently tailed imidazole ligands which are rationally designed to build comparable axial conformations due to various chain length (n =6 or 9) and axial steric hindrance (imidazole, 2-methylimidazole or benzimidazole), namely “chain effect”, were synthesized. Aided by crystal structure and EPR characterizations, different catalytic behaviors of three iron species in electrocatalytic O₂ reduction reaction (ORR) are comprehensively investigated which revealed that key conformation parameters (e.g. iron out-of-plane displacements (Δ_Fe, 0.36~0.43 Å) of high spin state (S = 5/2) iron(III) are reversely correlated to the catalytic activities that the species with larger Δ_Fe showing the lower catalytic activity, though the positive impact of “push effect” is also confirmed referenced to the four-coordinate [Fe(TFPP)]⁺. Theoretical studies found that such differences are attributed to the varying oxygen bonding capabilities which are efficiently regulated by the iron displacements through orbital interplay between iron and dioxygen, analogues to the oxygenation mechanisms of R and T states of Hemoglobin (Hb). The work presents a fresh perspective through structural parameters e.g., the metal out-of-plane displacement, for the studies of molecular catalysts.