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.