Matching the metal oxides with a conjugated and confined N-oxyl radical for the photocatalytic C(sp3)-H bond activation

Generating controllable N-oxyl radicals is an efficient method for the C-H bonds activation. Here the promotion effect of N-hydroxyphthalimide (NHPI) on the photocatalytic C(sp3)-H bond activation over various metal oxides was systemically studied with control experiments, UV-Vis DRS, FT-IR, ESR, and PCR. Two photocatalytic mechanisms were highlighted for the generation of critical PINO radical from NHPI: (1) The hole oxidation mechanism, the photo-induced hole (h+) of metal oxides directly oxidize the adsorbed NHPI* to a confined PINO*; (2) Ligand-to-metal charge transfer (LMCT) mechanism, the light induces the electron transfer from adsorbed NHPI* to oxides with a wide-band gap and provides a confined PINO*. Furthermore, this work also highlights a lattice-molecular structure matching effect to explain the success of certain oxides in building the h+ oxidation or LMCT systems with NHPI. The corresponding results can inspire further photocatalysis of semiconductors in C-H bond activation with versatile N-oxyl radicals.