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

## TL;DR

This study explains how different ligands affect the way cobalt(II) complexes generate reactive oxygen species through Fenton-like reactions.

## Contribution

The study reveals a new 'second-sphere H2O2-assisted' Fenton-like reaction mechanism and highlights the role of thiolate coordination.

## Key findings

- Second-sphere H2O2-assisted reactions generate •OOH as the major ROS in NTA– and EDTA–Co(II) complexes.
- GSH–Co(II) complexes favor conventional Fenton-like reactions producing •OH as the major ROS.
- Thiolate coordination in GSH promotes conventional Fenton-like reactions kinetically and thermodynamically.

## 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 H
2
O
2
-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.