Vanadium-Catalyzed Dinitrogen Reduction to Ammonia via a [V]═NNH 2 Intermediate.

The catalytic transformation of N 2 to NH 3 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 N 2 fixation, well-defined vanadium complexes that can catalyze the conversion of N 2 to NH 3 are scarce. In particular, a V(N x H y ) intermediate derived from proton/electron transfer reactions of coordinated N 2 remains unknown. Here, we report a dinitrogen-bridged divanadium complex bearing POCOP (2,6-( t Bu 2 PO) 2 -C 6 H 3 ) pincer and aryloxy ligands, which can serve as a catalyst for the reduction of N 2 to NH 3 and N 2 H 4 . Low-temperature protonation and reduction of the dinitrogen complex afforded the first structurally characterized neutral metal hydrazido(2-) species ([V]═NNH 2 ), which mediated 15 N 2 conversion to 15 NH 3 , indicating that it is a plausible intermediate of the catalysis. DFT calculations showed that the vanadium hydrazido complex [V]═NNH 2 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]-NH 2 ) with [Ph 2 NH 2 ][OTf] resulted in the release of NH 3 and the formation of a vanadium triflate complex, which upon reduction under N 2 afforded the vanadium dinitrogen complex. These transformations model the final steps of a vanadium-catalyzed N 2 reduction cycle. Both experimental and theoretical studies suggest that the catalytic reaction may proceed via a distal pathway to liberate NH 3 . These findings provide unprecedented insights into the mechanism of N 2 reduction related to FeV nitrogenase.