Catalytic Nitrogen Fixation Using Well-Defined Molecular Catalysts under Ambient or Mild Reaction Conditions

Ammonia (NH₃) is industrially produced from dinitrogen (N₂) and dihydrogen (H₂) by the Haber-Bosch process, although H₂ is prepared from fossil fuels, and the reaction requires harsh conditions. On the other hand, microorganisms have fixed nitrogen under ambient reaction conditions. Recently, well-defined molecular transition metal complexes have been found to work as catalyst to convert N₂ into NH₃ by reactions with chemical reductants and proton sources under ambient reaction conditions. Among them, involvement of both N₂-splitting pathway and proton-coupled electron transfer is found to be very effective for high catalytic activity. Furthermore, direct electrocatalytic and photocatalytic conversions of N₂ into NH₃ have been recently achieved. In addition to catalytic formation of NH₃, selective catalytic conversion of N₂ into hydrazine (NH₂NH₂) and catalytic silylation of N₂ into silylamines have been reported. Catalytic C-N bond formation has been more recently established to afford cyanate anion (NCO^-) under ambient reaction conditions. Further development of direct conversion of N₂ into nitrogen-containing compounds as well as green ammonia synthesis leading to the use of ammonia as an energy carrier is expected.