A Missing Link in Dinitrogen Fixation Enabled by Hydrazido Isomerization

Dinitrogen (N₂) fixation via an associative mechanism universally follows the distal pathway, while the alternating or enzymatic pathways involving diazene (HN═NH) are scarce, thereby making it difficult to elucidate associated pathways. Here, we present an η²-hydrazido (the diazene's resonance form) involved hybrid pathway, in which the η¹- to η²-hydrazido isomerization fills a missing link in N₂ fixation for the generation of diazenes. This contrasts with the classic alternating pathway, where η²-hydrazido is considered to form via direct functionalization of a diazenido's N_α (metal-bound nitrogen) atom. Moreover, oxidation triggers the reverse η²- to η¹-hydrazido isomerization, a previously unknown transformation, establishing a redox-switchable interconversion. Mechanistic and computational studies reveal a dominant intramolecular mechanism for both isomerizations, governed by SiMe₃ group migration between the two nitrogen atoms. Electronic structures identify hydrazyl radical anion character as essential for facilitating isomerization, positioning these species as a versatile platform for accessing diverse η²-hydrazido complexes and designing new transformations of reduced dinitrogen in synthetic systems.