Cobalt(II)-Mediated Fenton-like Reactions: Effects of Second-Sphere H2O2 and Thiolate Coordination

Abstract

While the Co(II) aqua complex is not a good catalyst for H₂O₂ decomposition due to its high redox potential, the Fenton-like activity of Co(II) can be promoted by chelation with suitable ligands. Previous experiments have shown that different reactive oxygen species (ROS) are generated in the presence of different ligands, but the underlying mechanism is unclear. In this study, density functional theory calculations are used to investigate the decomposition of H₂O₂ mediated by Co(II) complexes containing nitrilotriacetate (NTA), ethylenediaminetetraacetate (EDTA), and glutathione (GSH). For the NTA- and EDTA-Co(II) complexes, the formation of free ^•OH via the conventional Fenton-like pathway is thermodynamically unfavorable. However, H₂O₂ accumulated in the second coordination sphere via hydrogen bonding with carboxylate groups can readily undergo hydrogen atom transfer with ^•OH produced from the coordinated H₂O₂, generating ^•OOH as the major ROS. This reaction step provides a thermodynamic driving force for the H₂O₂ decomposition, which we call the second-sphere H₂O₂-assisted Fenton-like reaction. On the other hand, the conventional Fenton-like reaction of the GSH-Co(II) complex is kinetically and thermodynamically favorable, generating ^•OH as the major ROS. Detailed analysis reveals that the thiolate group of GSH plays a dominant role in promoting the conventional Fenton-like reaction.

1

Free energy profile of the conventional Fenton-like reaction mediated by the NTA complex of Co­(II). Selected bond distances and spin densities (ρ) are given for quartet/doublet states. The energy unit is kcal/mol, and the bond distance unit is Å.

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Electronic structure changes during Fenton-like reactions mediated by five-coordinate trigonal bipyramidal and six-coordinate octahedral Co­(II) complexes.

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Free energy profile of the second-sphere H₂O₂-assisted Fenton-like reaction mediated by the NTA complex of Co­(II). Selected bond distances and spin densities (ρ) are given for quartet/doublet states. The energy unit is kcal/mol, and the bond distance unit is Å.

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Free energy profile of the conventional Fenton-like reaction mediated by the EDTA complex of Co­(II). Selected bond distances and spin densities (ρ) are given for quartet/doublet states. The energy unit is kcal/mol, and the bond distance unit is Å.

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Free energy profile of the second-sphere H₂O₂-assisted Fenton-like reaction mediated by the EDTA complex of Co­(II). Selected bond distances and spin densities (ρ) are given for quartet/doublet states. The energy unit is kcal/mol, and the bond distance unit is Å.

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Optimized structures of [(GSH)­Co^II(H₂O)]⁻. The energy unit is kcal/mol, and the bond distance unit is Å.

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Free energy profile of the conventional Fenton-like reaction mediated by the GSH complex of Co­(II). Selected bond distances and spin densities (ρ) are given for quartet/doublet states. The energy unit is kcal/mol, and the bond distance unit is Å.

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Effects of the thiolate group on the Fenton-like reaction. Black and red indicate quartet and doublet states, respectively.

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Free energy profile of the reaction between [(NTA)­Co^III(OH)]⁻ and DMPO. The energy unit is kcal/mol, and the bond distance unit is Å.