O₂ reduction by iron porphyrins with electron withdrawing groups: to scale or not to scale

Iron porphyrins are synthesized by systematically introducing electron withdrawing groups (EWGs) on pyrroles to evaluate the relationship between rate (k) and overpotential (η). The results indicate that while EWGs lead to a rise in the thermodynamic Fe^III/II reduction potential (E⁰), the potential of the O₂ reduction reaction (ORR) does not scale with E⁰. More importantly, the iron porphyrins with higher E⁰ show an order of magnitude higher rate of ORR than unsubstituted iron tetraphenyl porphyrin. This contests the scaling relationship often offered to predict rates of ORR by iron porphyrins based on their E⁰. Mechanistic investigations reveal that the rate-determining step (rds) of ORR change between these iron porphyrins with EWG's, as the pK_a and E⁰ of several key intermediate species likely change on altering the macrocycle. These results suggest that linear dependence of log(rate) on E⁰ or η may only be valid for complexes where the rds of ORR remains the same.