New insights into the process of phenolic degradation by high-valent iron - a proton-coupled electron process

Degradation of phenolic compounds by Fe(IV) occurs through the cleavage of the phenolic hydroxyl group, accompanied by the generation of phenoxy radicals. Previous studies have attributed the destruction of the phenolic hydroxyl group by Fe(IV) to electron transfer or hydrogen atom transfer (HAT). The current study experimentally and theoretically revisits the role of proton transfer in the oxidation of phenolic compounds. It was found that the proton-coupled electron transfer (PCET) process leads to a change in the second-order rate constant for the degradation of phenolic compounds by Fe(IV). The PCET process consists of the concerted electron-proton transfer (CEPT) process and the stepwise proton-electron transfer process. Based on experimental and theoretical results, it is proposed that OH⁻ plays a key role in the pH-dependent behavior on phenolic oxidation as a proton acceptor. Electronic structure analysis elucidates the orbital contributions of the electron transfer in the CEPT process, providing a theoretical distinction between CEPT and HAT mechanisms. Considering the complexity of the reaction and the significant solvent effects, ab initio molecular dynamics simulations were employed for a more comprehensive discussion. A PCET-based model was proposed to describe the competition between stepwise and concerted PCET pathways well. With increasing pH, the CEPT process becomes the dominant process for protonated phenolic degradation. Overall, this study deepens our understanding of proton transfer in Fe(IV)-dominated systems, offering new insights for future research on PCET processes.