Catalytic reduction of oxygen to water by non-heme iron complexes: exploring the effect of the secondary coordination sphere proton exchanging site

Abstract

In this study, we prepared non-heme Fe^III complexes (1, 2, and 3) of an N₄ donor set of ligands (H₂L, Me₂L, and BPh₂L). 1 is supported by a monoanionic bispyridine-dioxime ligand (HL). In 2 and 3, the primary coordination sphere of Fe remained similar to that in 1, except that the oxime protons of the ligand were replaced with two methyl groups and a bridging −BPh₂ moiety, respectively. X-ray structures of the Fe^II complexes (1a and 3a) revealed similar Fe–N distances; however, they were slightly elongated in 2a. The Fe^III/Fe^II potential of 1, 2, and 3 appeared at −0.31 V, −0.25 V, and 0.07 V vs. Fc⁺/Fc, respectively, implying that HL and Me₂L have comparable donor properties. However, BPh₂L is more electron deficient than HL or Me₂L. 1 showed electrocatalytic oxygen reduction reaction (ORR) activity in acetonitrile in the presence of trifluoroacetic acid (TFAH) as the proton source at E_cat/2 = −0.45 V and revealed selective 4e⁻/4H⁺ reduction of O₂ to H₂O. 1 showed an effective overpotential (η_eff) of 0.98 V and turnover frequency (TOF_max) of 1.02 × 10³ s⁻¹. Kinetic studies revealed a k_cat of 2.7 × 10⁷ M⁻² s⁻¹. Strikingly, 2 and 3 remained inactive for electrocatalytic ORR, which established the essential role of the oxime scaffolds in the electrocatalytic ORR of 1. Furthermore, a chemical ORR of 1 has been investigated using decamethylferrocene as the electron source. For 1, a similar rate equation was noted to that of the electrocatalytic pathway. A k_cat of 6.07 × 10⁴ M⁻² s⁻¹ was found chemically. Complex 2, however, underwent a very slow chemical ORR. Complex 3 chemically enhances the 4e⁻/4H⁺ reduction of O₂ and exhibits a TOF of 0.24 s⁻¹ and a k_cat value of 2.47 × 10² M⁻¹ s⁻¹. Based on the experimental observations, we demonstrate that the oxime backbone of the ligand in 1 works as a proton exchanging site in the 4e⁻/4H⁺ reduction of O₂. The study describes how the ORR is affected by the tuning of the ligand scaffold in a family of non-heme Fe complexes.