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

• Published in: Journal of the American Chemical Society 2023-01, Vol.145 (2), p.811-821
• Main Authors: Huang, Wenshuang, Peng, Ling-Ya, Zhang, Jiayu, Liu, Chenrui, Song, Guoyong, Su, Ji-Hu, Fang, Wei-Hai, Cui, Ganglong, Hu, Shaowei
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.2c08000

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(N x H y ) intermediate derived from proton/electron transfer reactions of coordinated N2 remains unknown. Here, we report a dinitrogen-bridged divanadium complex bearing POCOP (2,6-( t 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 (BDFEN–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.