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

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 ), 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 activation path, while αααα improves 4 e ORR selectivity by improving O 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.