Vanadium-Catalyzed Dinitrogen Reduction to Ammonia via a [V]═NNH₂ Intermediate

The catalytic transformation of N₂ to NH₃ by transition metal complexes is of great interest and importance but has remained a challenge to date. Despite the essential role of vanadium in biological N₂ fixation, well-defined vanadium complexes that can catalyze the conversion of N₂ to NH₃ are scarce. In particular, a V(NH) intermediate derived from proton/electron transfer reactions of coordinated N₂ remains unknown. Here, we report a dinitrogen-bridged divanadium complex bearing POCOP (2,6-(Bu₂PO)₂-C₆H₃) pincer and aryloxy ligands, which can serve as a catalyst for the reduction of N₂ to NH₃ and N₂H₄. Low-temperature protonation and reduction of the dinitrogen complex afforded the first structurally characterized neutral metal hydrazido(2-) species ([V]═NNH₂), which mediated ¹⁵N₂ conversion to ¹⁵NH₃, indicating that it is a plausible intermediate of the catalysis. DFT calculations showed that the vanadium hydrazido complex [V]═NNH₂ possessed a N-H bond dissociation free energy (BDFE_N-H) of as high as 59.1 kcal/mol. The protonation of a vanadium amide complex ([V]-NH₂) with [Ph₂NH₂][OTf] resulted in the release of NH₃ and the formation of a vanadium triflate complex, which upon reduction under N₂ afforded the vanadium dinitrogen complex. These transformations model the final steps of a vanadium-catalyzed N₂ reduction cycle. Both experimental and theoretical studies suggest that the catalytic reaction may proceed via a distal pathway to liberate NH₃. These findings provide unprecedented insights into the mechanism of N₂ reduction related to FeV nitrogenase.