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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.

SUBMITTER: Santra A

PROVIDER: S-EPMC10935699 | biostudies-literature | 2024 Mar

REPOSITORIES: biostudies-literature

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

Santra Aakash A Das Avijit A Kaur Simarjeet S Jain Priya P Ingole Pravin P PP Paria Sayantan S

Chemical science 20240213 11

In this study, we prepared non-heme FeIII 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 FeII comp ...[more]

PMID: 38487234

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Project description:Transforming CO2 into valuable materials is an important reaction in catalysis, especially because CO2 concentrations in the atmosphere have been growing steadily due to extensive fossil fuel usage. From an environmental perspective, reduction of CO2 to valuable materials should be catalyzed by an environmentally benign catalyst and avoid the use of heavy transition-metal ions. In this work, we present a computational study into a novel iron(I) porphyrin catalyst for CO2 reduction, namely, with a tetraphenylporphyrin ligand and analogues. In particular, we investigated iron(I) tetraphenylporphyrin with one of the meso-phenyl groups substituted with o-urea, p-urea, or o-2-amide groups. These substituents can provide hydrogen-bonding interactions in the second coordination sphere with bound ligands and assist with proton relay. Furthermore, our studies investigated bicarbonate and phenol as stabilizers and proton donors in the reaction mechanism. Potential energy landscapes for double protonation of iron(I) porphyrinate with bound CO2 are reported. The work shows that the bicarbonate bridges the urea/amide groups to the CO2 and iron center and provides a tight bonding pattern with strong hydrogen-bonding interactions that facilitates easy proton delivery and reduction of CO2. Specifically, bicarbonate provides a low-energy proton shuttle mechanism to form CO and water efficiently. Furthermore, the o-urea group locks bicarbonate and CO2 in a tight orientation and helps with ideal proton transfer, while there is more mobility and lesser stability with an o-amide group in that position instead. Our calculations show that the o-urea group leads to reduction in proton-transfer barriers, in line with experimental observation. We then applied electric-field-effect calculations to estimate the environmental effects on the two proton-transfer steps in the reaction. These calculations describe the perturbations that enhance the driving forces for the proton-transfer steps and have been used to make predictions about how the catalysts can be further engineered for more enhanced CO2 reduction processes.

Dataset Information

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

Publications

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

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