Theoretical investigation of borane compounds mimicking transition metals for N2 fixation and activation

N-2 fixation is very difficult because of the nonpolarity and high stability of N-2. Traditionally, it is achieved by transition metal (TM) systems utilizing the back donation from the d orbitals of the TM to the antibonding pi* orbitals of N-2 to activate N-2. This back donation is rare for main group compounds due to the lack of high-lying valence d orbitals. In the present study, we show that borane compounds with weak B-X (X = H, Si, Ge, and Sb) bonds can mimic TM systems and be used to fix and activate N-2. This is achieved by the back donation from the sigma bonding orbitals of the B-X bonds to the antibonding pi* and sigma* orbitals of N-2. There is even a linear relationship between the number of B-X bonds and the binding potential energy of N-2 with (BRRR3)-R-1-R-2 (R-1, R-2, R-3 = H, CH3, SiH3, GeH3, and SbH2). Based on these findings, we designed several stable silylborane compounds that are feasible for N-2 fixation and activation under mild reaction conditions, i.e., room temperature and 1 atm. In some sandwich-like complexes formed between N-2 and silylborane compounds, N-2 is even activated from the triple bond to double bond.

Automated summary

N-2 fixation is traditionally achieved by transition metal systems through back donation from the d orbitals to activate N-2, which is rare for main group compounds. However, this study shows that borane compounds with weak B-X bonds can mimic TM systems and fix and activate N-2. Based on these findings, stable silylborane compounds were designed for N-2 fixation and activation under mild reaction conditions.