Controlling Oxygen Reduction Selectivity through Steric Effects: Electrocatalytic Two‐Electron and Four‐Electron Oxygen Reduction with Cobalt Porphyrin Atropisomers

Abstract Achieving a selective 2 e − or 4 e − oxygen reduction reaction (ORR) is critical but challenging. Herein, we report controlling ORR selectivity of Co porphyrins by tuning only steric effects. We designed Co porphyrin 1 with meso ‐phenyls each bearing a bulky ortho ‐amido group. Due to the resulted steric hinderance, 1 has four atropisomers with similar electronic structures but dissimilar steric effects. Isomers αβαβ and αααα catalyze ORR with n =2.10 and 3.75 ( n is the electron number transferred per O 2 ), respectively, but ααββ and αααβ show poor selectivity with n =2.89–3.10. Isomer αβαβ catalyzes 2 e − ORR by preventing a bimolecular O 2 activation path, while αααα improves 4 e − ORR selectivity by improving O 2 binding at its pocket, a feature confirmed by spectroscopy methods, including O K‐edge near‐edge X‐ray absorption fine structure. This work represents an unparalleled example to improve 2 e − and 4 e − ORR by tuning only steric effects without changing molecular and electronic structures.