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 O 2 with Co II as the rate-determining step (rds), while all four Fe isomers have the O 2 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 O 2 . In general, Fe II porphyrins have a larger driving force than the Co II counterparts to react with O 2 . As a result, the kinetic factor plays a dominant role in the O 2 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.