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.