Spin-Dependent Oxygen Reduction Electrocatalysis Is Suppressed by Electron-Withdrawing Substitution in Perfluorinated Iron Phthalocyanine

The oxygen reduction reaction (ORR) plays a key role in energy conversion systems. Perfluorinated iron phthalocyanine (16(F)FePc), a member of the MN 4 molecular catalyst family, has been reported to exhibit enhanced ORR activity compared to unsubstituted iron phthalocyanine (FePc). This is attributed to fluorine-induced electron-withdrawing effects that decrease the electron-density of the FeN 4 active site. On the other hand, because O 2 is paramagnetic, the electron spin is a relevant factor in ORR electrocatalysis through chirality-induced spin selectivity (CISS). In this work, the interplay between inductive electron-withdrawing effects and CISS-driven electrocatalysis was studied in self-assembled electrode systems based on FePc and 16(F)FePc. The molecular catalysts were immobilized on polycrystalline gold (Au _Poly ) and Au(111) substrates using l - and d -methionine as chiral linkers, which act as spin filters. Single-molecule conductance of l - and d -methionine was further evaluated by spin-polarized scanning tunneling microscope break-junction (SP-STM-BJ) measurements to assess chirality-dependent electron transport. ORR activity was examined using a rotating ring−disk electrode configuration to determine the total number of electrons transferred and H 2 O 2 yield. The ORR behavior of FePc and 16(F)FePc assembled on methionine reveals the interplay between CISS and inductive electron-withdrawing effects. FePc shows chirality-dependent electroactivity, favoring the four-electron pathway. In contrast, 16(F)FePc exhibits negligible chirality dependence for ORR, indicating that the strong electron-withdrawing effect of fluorine substituents on the Fe center dominates the reduction process. These results demonstrate that inductive substituent effects can override spin-dependent electrocatalysis, providing insight into the design of molecular catalysts in which CISS contributions are modulated or suppressed.