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

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