Manganese Carbodiimide (MnNCN): A New Heterogeneous Mn Catalyst for the Selective Synthesis of Nitriles from Alcohols

Earth-abundant manganese oxides (MnO_x) were competitive candidates when screening catalysts for ammoxidation of alcohols into nitriles due to their redox property. However, over-oxidation and possible acid-catalyzed hydrolysis of nitriles into amides still limited the application of MnO_x in nitrile synthesis. In this work, manganese carbodiimide (MnNCN) was first reported to be robust for the ammoxidation of alcohols into nitriles, avoiding over-oxidation and the hydrolysis. Besides the high activity and selectivity, MnNCN demonstrated wide substrate scope including the ammoxidation of primary alcohols into nitriles, the oxidative C-C bonds cleavage and ammoxidation of secondary alcohols, phenyl substituted aliphatic alcohols, and diols into nitriles. Controlled experiments and DFT calculation results revealed that the excellent catalytic performance of MnNCN originated from its high ability in the activation of O₂ molecules, and favorable oxidative dehydrogenation of C=N bonds in the aldimine intermediates (RCH=NH) into nitriles, inhibiting the competitive side reaction of the oxidation of aldehydes into carboxylic acids, followed to amide byproducts. Moreover, the hydrolysis of nitriles was also inhibited over MnNCN for its weak acidity as compared with MnO_x. This study provided new insights into Mn-catalyzed aerobic oxidations as a highly important complement to manganese oxides.