Regulating electron transfer over asymmetric low-spin Co(II) for highly selective electrocatalysis

Modulating the steric-electronic configuration of metal-organic centers is key for tuning the activity and selectivity of heterogeneous reactions, especially multi-electron transfer reactions. Here, three different asymmetric metal-organic complexes with unique steric-electronic structures are immobilized on nanocarbon for an electron-transfer-controlled oxygen reduction reaction. The strong-field ligand-induced low-spin (LS) CoII creates a necessary steric configuration for regulating reaction selectivity through ligand's proton transfer ability, for which acidic diamine ligands facilitate a four-electron transfer (94%), whereas basic ligands drive a highly selective two-electron route (97%). The steric-electronic regulation of the reaction selectivity at catalytic sites is characterized using X-ray absorption spectroscopy, reaction kinetic path analysis, and density functional theory calculation. Our results indicate that an LS state of CoII with asymmetric coordination is necessary to form a unique "flytrap'' structure to promote O-2 capture for the subsequent proton-coupled electron transfer, which is regulated by the Bronsted acidity of coordinating ligands.