Mn(III)-ligand complexes as a catalyst in ligand-assisted oxidation of substituted phenols by permanganate in aqueous solution

Ligands can significantly increase the oxidation rates of phenolic compounds by MnO₄⁻. This was often explained by the in situ formed Mn(III)- or Mn(X)-ligand complexes that can oxidize phenols faster than MnO₄⁻ can. This work discovered that Mn(III)-ligand complexes also acted as a catalyst for the oxidation of phenolic compounds by MnO₄⁻ (i.e., the catalytic role of Mn(III)-ligand). First, when phenol was mixed with MnO₄⁻ and pyrophosphate (PP, a representative ligand), Mn(III)-PP was found to form while phenol was quickly oxidized. However, the amount of phenol that was directly oxidized by Mn(III)-PP only accounted for ∼25% of phenol that was oxidized in the mixture, indicating that there were other pathways. Then, when pentachlorophenol (PCP) was used as another phenolic probe, the externally prepared Mn(III)-PP was observed to only slightly oxidize PCP, but its addition significantly accelerated PCP oxidation by MnO₄⁻. The Mn(III)-PP concentration also remained unchanged during the above reaction, thus suggesting the catalyst role of Mn(III)-PP. This new pathway was further validated by successfully explaining all the experimental observations obtained so far, including the effect of pH, effects of different ligand amounts and types, product patterns, and the induction period. Finally, possible catalytic mechanisms of Mn(III)-ligand were discussed based on the experimental results.