Enzyme-mimicking single atoms enable selectivity control in visible-light-driven oxidation/ammoxidation to afford bio-based nitriles†

Abstract

Nitriles are versatile nitrogen-containing scaffolds that exist widely in medicines, dyes and functional materials, and are typically synthesized over homogeneous catalysts under relatively harsh conditions. Herein, through mimicking the key active species of hemoglobin p450 heme monooxygenase in most organisms, a Fe–N₄ single atom-doped sulfur-containing carbon nitride (Fe/CNS) photocatalyst was facilely prepared by one-step thermal polymerization, which was illustrated to be efficient for renewable conversion of bio-based alcohols and NH₂OH·HCl to a wide range of nitriles (80–92% yields) via cascade oxidation–ammoxidation at room temperature, or to quantitatively furnish benzaldehyde via oxidation in the absence of a nitrogen source. Theoretical calculations showed that the isolated Fe–N₄ sites directly capture photogenerated electrons (e⁻) and molecular oxygen (O₂) to generate superoxide radicals (˙O₂⁻), while the surrounding S atoms confine photogenerated holes (h⁺). The high efficiency of Fe/CNS in the photo-generation of ˙O₂⁻ and holes may contribute to the smooth formation of nitriles by cascade photocatalytic oxidation of alcohol and ammoxidation via an in situ formed oxime, respectively. Moreover, Fe/CNS was also applicable to the selective synthesis of various imines (83–98% yields) from the oxidation of benzylamines or heterocyclic amines under visible-light irradiation, and could be recycled at least 5 times with no evident decline in catalytic activity. The strategy of rationally constructing atomic sites to spatially isolate paired electron–holes and form specific reactive species for enhanced photocatalytic activity/selectivity provides an efficient and green approach for biomass valorization.