Amine Groups in the Second Sphere of Iron Porphyrins Allow for Higher and Selective 4e^–/4H⁺ Oxygen Reduction Rates at Lower Overpotentials

Abstract

Iron porphyrins with one or four tertiary amine groups in their second sphere are used to investigate the electrochemical O2 reduction reaction (ORR) in organic (homogeneous) and aqueous (heterogeneous) conditions. Both of these complexes show selective 4e–/4H+ reduction of oxygen to water at rates that are 2–3 orders of magnitude higher than those of iron tetraphenylporphyrin lacking these amines in the second sphere. In organic solvents, these amines get protonated, which leads to the lowering of overpotentials, and the rate of the ORR is enhanced almost 75,000 times relative to rates expected from the established scaling relationship for the ORR by iron porphyrins. In the aqueous medium, the same trend of higher ORR rates at a lower overpotential is observed. In situ resonance Raman data under heterogeneous aqueous conditions show that the presence of one amine group in the second sphere leads to a cleavage of the O–O bond in a FeIII-OOH intermediate as the rate-determining step (rds). The presence of four such amine groups enhances the rate of O–O bond cleavage such that this intermediate is no longer observed during the ORR; rather, the proton-coupled reduction of the FeIII-O2 – intermediate with a H/D isotope effect of 10.6 is the rds. These data clearly demonstrate changes in the rds of the electrochemical ORR depending on the nature of second-sphere residues and explain their deviation from linear scaling relationships.

Meso‐substituted Metalloporphyrin‐based Composites for Electrocatalytic Oxygen Reduction Reactions

“…Porphyrin structure has great influence on the electronic status of the molecule and thus the ORR activity. Recently, much efforts have focus on the fields of the center metal optimization and substrate modification, while tailoring substituents at the peripheral of macrocycles has encountered great challenge in further enhancing the ORR performance [56,57] . In‐depth understanding on the structural effect of molecules or exploring new approaches to reduce the overpotential and to increase the response current during oxygen reduction is highly anticipated.…”

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

“…A cross-linked tyrosine residue present around the active site of CcO is thought to play an important role in cleaving the O-O bond of the Fe III -O-O-Cu II intermediate. 24 Inspired by CcO, the effect of secondary coordination sphere interactions in the biomimetic ORR catalysts, such as the presence of a proton relay site, [25][26][27][28][29][30][31][32][33][34] positively charged functional groups to exert an electrostatic effect around the redox-active metal ion, 35,36 functional groups which can facilitate hydrogen atom transfer (HAT) and proton-coupled electron transfer (PCET) reactions, 31,37 etc., have been explored to understand the O-O bond cleavage mechanism in different M-porphyrin/corrole complexes. It was established that the presence of such secondary effects could control the key factors of the ORR, such as the reaction rate, overpotential, and product selectivity.…”

Iron Dioxygen Adduct Formed during Electrochemical Oxygen Reduction by Iron Porphyrins Shows Catalytic Heme Dioxygenase Reactivity

“…Unlike normal spectro-electrochemistry where the electrode is static and electrode processes are dominated by diffusion from bulk solvent, the forced convection inherent to the rotating disc technique used in SERRS-RDE allows investigation of catalytic mechanism under steady state (Scheme ). − This has allowed the characterization of intermediates involved in ORR catalyzed by different iron porphyrins and obtain insights into factors that control the rate and selectivity of ORR for both molecular and biosynthetic catalysts. − Normally SERRS-RDE is performed at potentials where the current becomes potential independent, i.e., mass transfer controlled. In this potential region, for most synthetic and biosynthetic Fe and Fe/Cu porphyrins, an Fe III –OOH species was found to accumulate on the electrode during ORR (Scheme ).…”