Nitrogen Fixation Catalyzed by Well-Defined Molecular Catalysts

The Haber–Bosch process to synthesize ammonia from dinitrogen and dihydrogen under harsh conditions has been one of the fundamentals to the modern chemical industry for over a century, where most of dihydrogen is provided by reforming fossil fuels. On the other hand, diazotrophs have fixed dinitrogen as ammonium derivatives under ambient conditions for billions of years. For the last two decades, a series of well-defined molecular complexes have been developed to catalyze the conversion of dinitrogen into ammonia or hydrazine on treatment with chemical reductants and proton sources under ambient or mild conditions, where the highest catalytic activity has been achieved by introducing direct splitting of dinitrogen and proton-coupled electron transfer systems into catalyses. Ammonia has been also shown to be obtained via direct electrocatalytic or photocatalytic reduction of dinitrogen, providing new synthetic methods for green ammonia synthesis. Selective reduction of dinitrogen into hydrazine has been recently reported by using well-defined molecular complexes as catalysts, whereas catalytic Si–N, C–N, and B–N bond forming reactions have been also achieved to afford nitrogen-containing compounds.