Hierarchically Porous Mn3O4 Nanocrystals for Aerobic, Solvent-Free Dehydrogenative C─N Cross-Coupling Reactions.

Synthesis of a robust redox metal oxide based material with nanoscale porosity is very demanding in the context of heterogeneous catalysis. Hereby, we report a convenient synthetic approach for designing hierarchically porous Mn3O4 nanocrystals (HP-Mn3O4-NCs) using a small organic molecular entity, sodium salicylate, as a structure-directing agent. The porosity and the crystallinity were evaluated through the powder XRD, TEM, and N2 sorption. TEM analysis revealed the presence of mesopores, which agrees well with the N2 sorption analysis. XPS analysis revealed the presence of a mixed-valent Mn2+ and Mn3+. The aerial dehydrogenative C─N cross-coupling of aromatic diamines and alcohols for the selective production of benzimidazole derivatives proceeds smoothly in the presence of HP-Mn3O4-NCs as a heterogeneous catalyst. The catalyst displayed a clean reaction profile with excellent product yields of up to 94% for various substrates. Noticeably, the reaction is accomplished under base-free, solvent-free, and aerial conditions. The existence of mesopore enables a high internal surface area, thereby increasing the exposure of surface-active sites. The mesopores of the material promote the facile diffusion of reactants and products. Large specific surface area, abundant surface-active sites, stability, and structural robustness collectively endow the HP-Mn3O4-NCs with high catalytic activity and recycling efficiency.