Keto-Oxygen on Graphitic Surface with Downshifted p-Band Center Achieves Efficient Metal-Free Transfer Hydrogenation of Nitroarenes

The critical challenge in utilizing carbon-based nanomaterials is identifying the active site. Herein, we demonstrate the keto-oxygen on the graphitic surface as active sites for catalytic transfer hydrogenation (CTH) and present an efficient nanocrystalline diamond (ND)-derived carbon-based catalyst for metal-free CTH of nitroarenes to imine with 99.9% nitrobenzene conversion and exclusive selectivity (99.9%). By selectively deconstructing the graphitic surface or eliminating carbonyl groups, the graphite-conjugated carbonyl group is confirmed as the catalytically active site. Moreover, kinetic studies display the lower activation barrier of benzylalcohol than that of nitrobenzene (88.8 vs 119.1 kJ mol^-1, respectively), indicating that alcohol dehydrogenation occurs prior to the activation of nitrobenzene. Density functional theory calculations reveal the downshifted p-band center of keto-oxygen on the sp² hybrid C surface affords moderate adsorption of benzaldehyde intermediates, which accelerates the formation of active H for the following hydrogenation step and is responsible for the high catalytic activity.