Comprehensive insights into synthetic nitrogen fixation assisted by molecular catalysts under ambient or mild conditions

N₂ is fixed as NH₃ industrially by the Haber–Bosch process under harsh conditions, whereas biological nitrogen fixation is achieved under ambient conditions, which has prompted development of alternative methods to fix N₂ catalyzed by transition metal molecular complexes. Since the early 21st century, catalytic conversion of N₂ into NH₃ under ambient conditions has been achieved by using molecular catalysts, and now H₂O has been utilized as a proton source with turnover frequencies reaching the values found for biological nitrogen fixation. In this review, recent advances in the development of molecular catalysts for synthetic N₂ fixation under ambient or mild conditions are summarized, and potential directions for future research are also discussed.