Three H2O2 molecules are involved in the “Fenton-like” reaction between Co(H2O)62+ and H2O2

Abstract

Co(II) complexes and Co(H₂O)₆²⁺ are used as catalysts in advanced oxidation processes. Therefore it was decided to study the kinetics of reaction of Co(H₂O)₆²⁺ with H₂O₂. Surprisingly, the kinetic results point out that the process involves three consecutive reactions, each of them requiring an H₂O₂ molecule, i.e. three H₂O₂ molecules ligate to the central cobalt cation prior to the formation of radicals. DFT analysis suggests that the transient (H₂O)₃Co^II(OOH)₂(H₂O₂) decomposes via: (H₂O)₃Co^II(OOH)₂(H₂O₂) → (H₂O)₃Co^II(OOH)(˙OOH)(OH) + ˙OH ΔG⁰ = −5.975 kcal mol⁻¹, with no evidence for the formation of a Co(III) transient. It is proposed that analogous mechanisms are involved whenever the redox potential of the central cation is too high to enable the reaction: M(H₂O)₆ⁿ⁺ + H₂O₂ → M⁽ⁿ⁺¹⁾⁺_aq + ˙OH + OH⁻.