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

In this study, we prepared non-heme Fe III complexes (1, 2, and 3) of an N 4 donor set of ligands (H 2 L, Me 2 L, and BPh 2 L). 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 -BPh 2 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 Me 2 L have comparable donor properties. However, BPh 2 L is more electron deficient than HL or Me 2 L. 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 O 2 to H 2 O. 1 showed an effective overpotential ( η eff ) of 0.98 V and turnover frequency (TOF max ) of 1.02 × 10 3 s -1 . Kinetic studies revealed a k cat of 2.7 × 10 7 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 × 10 4 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 O 2 and exhibits a TOF of 0.24 s -1 and a k cat value of 2.47 × 10 2 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 O 2 . The study describes how the ORR is affected by the tuning of the ligand scaffold in a family of non-heme Fe complexes.