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

Abstract

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, welldefined 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.

Strategic design of VO2 encased in N-doped carbon as an efficient electrocatalyst for the nitrogen reduction reaction in neutral and acidic media

“…This could be justified with respect to a very recent report by Huang et al that experimentally and theoretically suggested that the NRR on V active sites could energetically prefer a distal pathway negating the chances of the formation of hydrazine, rightly corroborating our experimental results. 48 Thus, the NRR on our active material of VO 2 @CN was kinetically facilitated yielding 0.31 and 0.52 μmol h −1 mg cat −1 of NH 3 in Na 2 SO 4 and HCl, respectively. The concentrations of ammonia in all cases were calculated using the standard calibration curves and the corresponding linear regression plot in the relevant acidic or neutral electrolytes (Fig.…”

Strategies to achieve effective nitrogen activation

“…Only one molecular electrocatalyst (tri (phosphine) borane iron system) has been reported to reliably proved the reaction mechanism, but it needs a lower temperature (−35°C) to slow down the background HER and high reduction potential 193 . Huang et al 194 first separated and characterized the neutral hydrazide complex intermediate ([V]═NNH 2 ) in the vanadium metal complexes catalyzed NRR system. Dinitrogen compounds and NH 3 were obtained by the reduction of protonation by vanadium amino complexes ([V]–NH 2 ).…”

Snapshots of Early-Stage Quantitative N2 Electrophilic Functionalization

“…The research on N 2 reduction reactions (N 2 RRs) in molecular systems dates back to the 1960s, and ever since seminal work was published by Chatt, Hidai, and co-workers, tremendous efforts have been invested in the development of efficient catalysts for the synthesis of nitrogen-containing molecules directly from N 2 . − Metal dinitrogen complexes necessarily feature strong π-backdonation from electron-rich metal centers to N 2 π* orbitals; therefore, the following N 2 functionalization is almost invariably realized by electrophilic attacks of metal N 2 complexes with potent electrophiles, such as H + , − C + , − and Si + , − leading to diazenido and/or hydrazido complexes (Figure ). More important is that such processes initiate the vast majority of catalytic N 2 RRs .…”