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

Achieving a  s elective 2e -  or 4e -  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 2e -  ORR by preventing a bimolecular O 2  activation path, while  αααα  improves 4e -  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 2e -  and 4e -  ORR by tuning only steric effects without changing molecular and electronic structures.