Probing the Role of the Secondary Coordination Sphere in Modulating Linear Free Energy Relationships for Oxygen Reduction Reaction Catalyzed by a Family of Non-Heme Iron Complexes

A family of non-heme iron(III) complexes (1–10) bearing bis(pyridine-dioxime) scaffolds in the primary coordination sphere and diverse functional groups in the secondary coordination sphere (SCS), including o-NHMe2+-C6H4, o-OMe-C6H4, o-OH–C6H4, –C6F5, pyridine, and pyrimidine, was investigated for the catalytic oxygen reduction reaction (ORR). All the complexes showed 4e–/4H+ selectivity toward the ORR under both electrocatalytic and chemical reaction conditions. Kinetic studies indicate that the formation of a protonated Fe(O2•) intermediate is the rate-determining step for this series of catalysts. The turnover frequency (TOF) correlates with the FeIII/FeII redox potential, revealing a linear free-energy relationship (LFER) in which more electron-rich iron centers exhibit higher catalytic activity under the chemical ORR. However, complexes bearing o-OMe-C6H4 and –C6F5 substituents in the SCS display TOF values significantly higher than predicted by the LFER, highlighting the key role of these functional groups in promoting catalysis through the formation of proton-relay sites near the active center. In contrast, the complex containing an o-NHMe2+-C6H4 substituent exhibits the lowest TOF among the series, in contrast to the beneficial effect of tertiary amines previously reported for Fe(porphyrin) catalysts. Overall, this study demonstrates the crucial role of SCS interactions in modulating the activity of non-heme iron catalysts for ORR.