Proton vs Electron:\nThe Dual Role of Redox-Inactive\nMetal Ions in Permanganate Oxidation Kinetics

Redox-inactive metal-ion-driven modulation of the oxidation\nbehavior\nof high-valent metal–oxo complex has garnered significant interest\nin biological and chemical synthesis; however, their role in permanganate\n(Mn(VII)) oxidation for the removal of organic pollutants has been\nlargely neglected. Here, we uncover the impact of six metal ions (i.e.,\nCa<sup>2+</sup>, Mg<sup>2+</sup>, Ni<sup>2+</sup>, Zn<sup>2+</sup>, Al<sup>3+</sup>, and Sc<sup>3+</sup>) presenting in water environments\non Mn(VII) activity. These ions uniformly boost the electron and oxygen\ntransfer capabilities of Mn(VII) while impeding proton transfer, as\nevidenced by electrochemical tests, thioanisole probe analysis, and\nthe kinetic isotope effect. The observed effects are intricately linked\nto the Lewis acidity of the metal ions. Further mechanistic insights\nreveal that Mn(VII) can interact with metal ions without direct reduction.\nSuch interactions modify the electronic configuration of Mn(VII) and\ncreate an acidic microenvironment, thus increasing its electrophilicity\nand the energy barrier for the abstraction of proton from organic\nsubstrates. More importantly, the efficacy of Mn(VII) in removing\nphenolic pollutants is regulated by these ions through changing the\ndriving force for proton and electron transfer, i.e., facilitated\nat pH > 4.5 and inhibited at lower pH. The contribution of active\nMn intermediates is also discussed to reveal the oxidative mechanism\nof the metal ion/Mn(VII) system. These findings not only facilitate\nthe rational design of Mn(VII) oxidation conditions in the presence\nof metal ions for water decontamination but also offer an alternative\nparadigm for enhancing electrophilic oxidation.