Mechanism of Catalytic O₂ Reduction by Iron Tetraphenylporphyrin

Abstract

The catalytic reduction of O2 to H2O is important for energy transduction in both synthetic and natural systems. Herein, we report a kinetic and thermochemical study of the oxygen reduction reaction (ORR) catalyzed by iron tetraphenylporphyrin (Fe(TPP)) in N,N′-dimethylformamide using decamethyl-ferrocene as a soluble reductant and para-toluenesulfonic acid (pTsOH) as the proton source. This work identifies and characterizes catalytic intermediates and their thermochemistry, providing a detailed mechanistic understanding of the system. Specifically, reduction of the ferric porphyrin, [FeIII(TPP)]+, forms the ferrous porphyrin, FeII(TPP), which binds O2 reversibly to form the ferric-superoxide porphyrin complex, FeIII(TPP)((O2•−). The temperature dependence of both the electron transfer and O2 binding equilibrium constants has been determined. Kinetic studies over a range of concentrations and temperatures show that the catalyst resting state changes during the course of each catalytic run, necessitating the use of global kinetic modeling to extract rate constants and kinetic barriers. The rate-determining step in oxygen reduction is the protonation of FeIII(TPP)((O2•−) by pTsOH, which proceeds with a substantial kinetic barrier. Computational studies indicate that this barrier for proton transfer arises from an unfavorable preassociation of the proton donor with the superoxide adduct and a transition state that requires significant desolvation of the proton donor. Together, these results are the first example of oxygen reduction by iron tetraphenylporphyrin where the pre-equilibria among ferric, ferrous, and ferric-superoxide intermediates have been quantified under catalytic conditions. This work gives a generalizable model for the mechanism of iron porphyrin-catalyzed ORR and provides an unusually complete mechanistic study of an ORR reaction. More broadly, this study also highlights the kinetic challenges for proton transfer to catalytic intermediates in organic media.

Active-Site Modulation in an Fe-Porphyrin-Based Metal–Organic Framework through Ligand Axial Coordination: Accelerating Electrocatalysis and Charge-Transport Kinetics

“… 60 , 61 Specifically, during electrocatalytic ORR, the reduction of [Fe III –porphyrin] + forms an Fe II (porphyrin) species that is able to effectively bind O 2 (via electron donation) to form the ORR intermediate, Fe III (porphyrin)(O 2 •– ). 62 The affinity of O 2 for the Fe II (porphyrin) species could be tuned by manipulating the metal’s electronic properties. A promising route to do so is by the axial coordination of electron-donating ligands (such as MeIM) to the active metal site.…”

Combining scaling relationships overcomes rate versus overpotential trade-offs in O 2 molecular electrocatalysis

“…TOF max values were determined from the catalytic CVs by foot-ofthe-wave analysis (FOWA), Eq. 2 [ Table 1; (5,11,12)]. This widely used approach normalizes the catalytic currents (i c ) to the noncatalytic peak current of the Fe III/II couple (i p ) at potentials (E) near the onset of the catalytic wave, where complications such as substrate depletion are minimized.…”

“…The yellow shading indicates an aspirational region. (B) Fe(por) catalyzed O 2 reduction mechanism, as described in the main text (11). (C) Drawings of 1 and of the cation in the solid-state x-ray crystal structure, [1•2H 2 O]OTf 5 (Fe, orange; N, blue; C, white; O, red; H atoms omitted for clarity; thermal ellipsoids at 50% probability).…”

Encapsulation of Porphyrin‐Fe/Cu Complexes into Coordination Space for Enhanced Selective Oxidative Dehydrogenation of Aromatic Hydrazides

“…On the basis of the literature surveys, [63][64][65][66] the mechanism of reaction was tentatively proposed (Scheme 3). The reaction may be initiated by single-electron transfer between aromatic hydrazides a and Fe(III) species to give the cation radical b.…”