Computational Mechanistic Study of Oxygen Reduction by an Asymmetric Pacman Dicobalt Porphyrin Catalyst

Asymmetric Pacman dinuclear cobalt-porphyrin complex 1, featuring triphenylporphyrin (TPP) and tri(pentafluorophenyl)porphyrin (TPFP) ligands, outperforms its symmetric analogue 2 [(Co-TPFP)2], exhibiting larger oxygen reduction reaction (ORR) currents and lower overpotentials. Density functional theory (DFT) calculations are conducted to investigate the reaction mechanisms and the origin of this performance difference. Two O2-coordination models (μ-bridging and end-on) and eight pathways were examined. For both complexes 1 and 2, the catalytic cycle begins with the initial [CoIIPor•+---CoIIPor•+]2+ catalyst (with the ligand abbreviated as Por), which undergoes two reductions to form a dinuclear CoII species, facilitated by electron acceptance from the redox-noninnocent porphyrin radical cations. The subsequent endergonic O2 binding is followed by two proton-coupled electron transfer (PCET) steps, yielding the key intermediate [CoII(μ-η1:η1-HOOH)CoII]. Crucially, H2SO4-mediated O–O cleavage exhibits a lower barrier than nonacid-assisted pathways, producing the [H2O–CoIII---CoIII–OH]+ species; protonation of the CoIII–OH moiety then leads to electron rearrangement and releases the second H2O molecule, and the [CoIIPor•+---CoIIPor•+]2+ catalyst is regenerated for the next cycle. The enhanced efficiency of complex 1 stems from its asymmetric structure: The Co-TPP unit serves as the primary site for O2 binding and reduction, while the positively charged Co-TPFP moiety acts as a Lewis acid, promoting O2 activation. Additionally, Bro̷nsted acid H2SO4 facilitates O–O cleavage via proton donation and hydrogen bonding, lowering the barrier for this critical step and stabilizing intermediates. Conversely, the strong electron-withdrawing pentafluorophenyl groups of complex 2 hinder electron transfer to O2, diminishing its catalytic activity. This work establishes asymmetric active-site engineering as a vital design principle for biomimetic ORR catalysts.