Unveiling the potential of tailored β-substituted iron-porphyrins for highly efficient oxygen reduction reactions (ORR)†

A series of β-substituted iron porphyrins, including Fe(iii)ClTPPMe₄, Fe(iii)ClTPPPh₄, and Fe(iii)ClTPPBr₄, featuring electron-withdrawing and electron-donating groups, were synthesized, characterized, and evaluated for their catalytic activity in the oxygen reduction reaction (ORR). These catalysts exhibited onset potentials of 0.77 V, 0.98 V, and 0.79 V (RHE), comparable to platinum-based electrodes, along with high stability over 11 000 s and a 4e⁻/4H⁺ transfer pathway. Electron-withdrawing β-substituents combined with steric effects enhance ORR performance, with the phenyl-substituted porphyrin exhibiting unique catalytic behavior due to steric distortions in the porphyrin ring. DFT calculations revealed that a reduced HOMO–LUMO gap facilitates electron transfer which in turn improves catalytic efficiency.