Steric Effects on the Oxygen Reduction Reaction with Cobalt Porphyrin Atropisomers

The steric effects on the oxygen reduction reaction (ORR) have been rarely studied because O2 is small in size and ORR catalysts with the only difference in steric hindrance are difficult to be designed and synthesized. Herein, we report on homogeneous ORR catalyzed by four Co porphyrin atropisomers in tetrahydrofuran with decamethylferrocene and HClO4 at 298 K. All four atropisomers are active and selective for the 2H+/2e– ORR with an activity order αααα > αααβ > ααββ > αβαβ. Kinetic studies revealed that the four atropisomers have the same ORR mechanism with proton-coupled O2 binding as the rate-determining step.