Benzylic C_{sp³}–H Bond Oxidation on the (111) Facets of Octahedral Cu₂O Nanocrystals

Octahedral-shaped Cu2O nanocrystals (o-Cu2O NCs) have been synthesized by the wet chemical method. By powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), and high-resolution transmission electron microscopy (HRTEM), it was confirmed that the synthesized material was Cu2O NCs with octahedral morphology with the (111) facets exposed. The synthesized o-Cu2O NCs have been explored as the catalyst for benzylic sp3 C–H bond oxidation. Reaction parameters like oxidants, solvents, temperature, and reaction time were optimized for the model oxidation reaction involving diphenylmethane (DPM) as a substrate. The results obtained indicate that the substrate having benzylic sp3 C–H bond energy less than 90 kcal/mol can be easily oxidized to the corresponding carbonyl compound in the presence of these nanocrystals at RT with stirring the reaction mixture over the bar magnet (up to 92% conversion). The oxidation of Cu2O to CuO occurs all together with the progress of the reaction when carried out under typical aerial atmosphere in the presence of light (daylight and mercury compact fluorescent lamp (CFL) tubes prevailing in the laboratory). However, this can be avoided when the reaction was carried out in the dark or inert atmosphere or continuous flow of oxygen gas. A reaction mechanism has been proposed based on the free radical scavenger experiments and other special experiments. Also, the chemical events that occurred in the proposed reaction mechanism have been supported by mass spectral and electrochemical analysis, density functional theory (DFT) calculations, and some other special experiments. Furthermore, Brunauer–Emmett–Teller (BET) surface area analysis was carried out to confirm the catalytic effect due to facets. The experimental results also lead to proving the activation of O2 molecule through single electron transfer (SET) mechanism on the (111) facets of semiconducting Cu2O NCs in light.