Thermodynamic vs Kinetic Control of the Oxygen Reduction Reaction with Iron and Cobalt Porphyrin Atropisomers

Identifying factors that can control the oxygen reduction reaction (ORR) is of fundamental significance to understanding this vital reaction and to designing efficient catalysts. Although catalysts with designed steric properties have been recently shown to display improved ORR, a fundamental understanding of steric effects on catalytic ORR is limited. Herein, we report on the thermodynamic and kinetic control of catalytic ORR with atropisomers of Fe and Co tetra(2-pivalamidophenyl)porphyrins. All these metalloporphyrin atropisomers are active for homogeneous catalytic ORR in dimethylformamide with decamethylferrocene and trifluoroacetic acid. However, Co isomers displayed an activity order of αααα-Co > αααβ-Co > ααββ-Co > αβαβ-Co, while Fe counterparts showed an opposite activity order of αααα-Fe < αααβ-Fe < ααββ-Fe < αβαβ-Fe. Kinetic studies revealed that all four Co isomers have the same ORR mechanism with acid-promoted binding of the O₂ with Co^II as the rate-determining step (rds), while all four Fe isomers have the O₂ binding with Fe^II as the rds. Based on kinetic studies, we proposed that the opposite activity trend of Fe and Co atropisomers originates from the different reactivity of Fe^II and Co^II porphyrins toward O₂. In general, Fe^II porphyrins have a larger driving force than the Co^II counterparts to react with O₂. As a result, the kinetic factor plays a dominant role in the O₂ binding with Fe^II, but the thermodynamic factor becomes dominant in the case of Co^II. Therefore, this work provides a new understanding of steric effects on catalytic ORR and highlights thermodynamic and kinetic control strategies for catalyst design.