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

Abstract

N₂ fixation is very difficult because of the nonpolarity and high stability of N₂. Traditionally, it is achieved by transition metal (TM) systems utilizing the back donation from the d orbitals of the TM to the antibonding π* orbitals of N₂ to activate N₂. 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 N₂. This is achieved by the back donation from the σ bonding orbitals of the B–X bonds to the antibonding π* and σ* orbitals of N₂. There is even a linear relationship between the number of B–X bonds and the binding potential energy of N₂ with BR¹R²R³ (R¹, R², R³ = H, CH₃, SiH₃, GeH₃, and SbH₂). Based on these findings, we designed several stable silylborane compounds that are feasible for N₂ fixation and activation under mild reaction conditions, i.e., room temperature and 1 atm. In some sandwich-like complexes formed between N₂ and silylborane compounds, N₂ is even activated from the triple bond to double bond.