Abstract
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.