Selective Aerobic Oxidation of Toluene to Benzaldehyde by Parallel Photoinduced Proton-Coupled Electron Transfer and Energy Transfer Through Multiphoton Excitation.

Selectively aerobic oxidation of toluene to benzaldehyde is an important but challenging task due to the high proclivity of aldehydes to undergo over-oxidation to acids. Herein, we report a new multiphoton excitation approach for synthesizing benzaldehyde from toluene by integrating oxygen vacancy and proton-coupled electron transfer (PCET) events into one coordination polymer (Zn-AQ) to enhance efficiency and selectivity. Under light irradiation, the ligand facilitates photoinduced PCET and photoinduced energy transfer (EnT), generating radical species from the C(sp)─H bonds and highly active O species. The π-stacking interactions between the ligands enforced the metal centers to form Zn⋅⋅⋅Na⋅⋅⋅Zn⋅⋅⋅Na polyoxometalate chains with oxygen vacancy sites for the selective oxidation of the radical intermediate. Zn-AQ endows the efficient and selective oxidation of aromatic benzyl C(sp)─H bonds into aldehydes, including the conversion of toluene into benzaldehyde with the selectivity up to 94%, by combining the energies of three photons in one catalytic cycle. The first reported multiphoton catalytic strategy combines oxygen vacancy catalysis with PCET C(sp)─H bond activation mode, providing new ideas for the design of aerobic oxidation photocatalysts for target C(sp)─H bonds oxidation in a mild, efficient, and highly selective manner.