Single-Atom Co-N₄ Sites Mediate C=N Formation via Reductive Coupling of Nitroarenes with Alcohols

It remains challenging to construct C=N bonds due to their facile hydrogenation. Herein, a single Co atom catalyst was discovered to be active for the selective construction of C=N bonds toward the synthesis of imines and N-heterocycles via reductive coupling of nitroarenes with various alcohols, including inert aliphatic ones. DFT calculations and experimental data revealed that the transfer hydrogenation proceeded via the intramolecular hydride transfer and the transfer of H from the α-C_sp3-H bond to the nitro group was the rate-determining step. The single Co atoms served as a bridge to transfer the electrons from the catalyst to the adsorbed alcohol molecules, resulting in the activation of the α-C_sp3-H bond. Unlike metal nanoparticles, the C=N bonds in imine products can be reserved due to the large steric hindrance from substituents on C and N. DFT calculation also confirmed that transfer hydrogenation of the C=N bonds in imines is thermodynamically unfavored with a much higher energy barrier compared with the transfer hydrogenation of the -NO₂ group (1.47 vs 1.15 eV).