Investigating the Synergistic Influence of Redox‐Inactive Metal Ions at the Secondary Coordination Sphere of Non‐Heme Iron Complexes on Oxygen Reduction Reaction

The effect of different redox inactive metal ions embedded at the secondary coordination sphere (SCS) of non-heme iron(III) complexes has been examined toward the oxygen reduction reaction (ORR). The study discloses that Mg²⁺ and Ca²⁺ present at the SCS have a profound effect on enhancing ORR, whereas Ba²⁺,Sr²⁺, Nd³⁺, or Y³⁺ don't have a beneficial effect on ORR.

Abstract

The secondary coordination sphere (SCS) around the substrate binding site of a catalyst, such as a proton relay, formation of a hydrogen bonding network, and electrostatic effects, plays a critical role in controlling the selectivity and rate of oxygen reduction reaction (ORR). Herein, we report the ORR study of a series of Fe^III complexes having a uniform primary coordination sphere and an unlike SCS. The Fe(III) complex, lacking any secondary coordination sphere (SCS) modifications, exhibited slow oxygen reduction reaction (ORR) kinetics in the presence of decamethylferrocene (Fc^*) and trifluoroacetic acid (TFAH) in oxygen-saturated acetonitrile. The presence of redox-inactive metal ions (Mⁿ⁺), such as Mg²⁺ and Ca²⁺, at SCS enhances the ORR rate, which can be attributed to the presence of the electrostatic effect created by these Mⁿ⁺ ions. Strikingly, no benign effect was noticed when Sr²⁺, Ba²⁺, Y³⁺, and Nd³⁺ were present at the SCS. Interestingly, all the Fe complexes revealed catalytic H₂O₂ disproportionation and H₂O₂ reduction reaction (H₂O₂RR). The study highlights the importance of modifying the SCS on the ORR and H₂O₂RR by a series of molecular non-heme Fe(III) complexes.

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