Single-Atoms on Crystalline Carbon Nitrides for Selective C―H Photooxidation: A Bridge to Achieve Homogeneous Pathways in Heterogeneous Materials

Single-atom catalysis is a field of paramount importance in contemporary science due to its exceptional ability to combine the domains of homogeneous and heterogeneous catalysis. Iron and manganese metalloenzymes are known to be effective in CH oxidation reactions in nature, inspiring scientists to mimic their active sites in artificial catalytic systems. Herein, a simple and versatile cation exchange method is successfully employed to stabilize low-cost iron and manganese single-atoms in poly(heptazine imides) (PHI). The resulting materials are employed as photocatalysts for toluene oxidation, demonstrating remarkable selectivity toward benzaldehyde. The protocol is then extended to the selective oxidation of different substrates, including (substituted) alkylaromatics, benzyl alcohols, and sulfides. Detailed mechanistic investigations revealed that iron- and manganese-containing photocatalysts work through a similar mechanism via the formation of high-valent MO species. Operando X-ray absorption spectroscopy (XAS) is employed to confirm the formation of high-valent iron- and manganese-oxo species, typically found in metalloenzymes involved in highly selective CH oxidations. Low-cost iron and manganese single-atoms are stabilized on poly(heptazine imides) (PHI). The resulting materials are exploited as photocatalysts for the selective oxidation of toluene into benzaldehyde, and further applied to the conversion of alkylaromatics and benzyl alcohols into the corresponding carbonyl derivatives. Operando X-ray absorption spectroscopy supports the intermediacy of metal-oxo species under heterogeneous conditions.image