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. Well‐defined molecular transition metal complexes have been found to work as catalysts to convert N2 into NH3 under ambient reaction conditions, where involvement of both N2‐splitting pathway and proton‐coupled electron transfer has been effective for high catalytic activity. Direct electrocatalytic and photocatalytic NH3 formation, selective catalytic hydrazine formation, and catalytic C−Si or C−N bond formation have been also established.