Anomalous π-backbonding in complexes between B(SiR) and N: catalytic activation and breaking of scaling relations

Chemical transformations of molecular nitrogen (N 2 ), including the nitrogen reduction reaction (NRR), are difficult to catalyze because of the weak Lewis basicity of N 2 . In this study, it is shown that Lewis acids of the types B(SiR 3 ) 3 and B(GeR 3 ) 3 bind N 2 and CO with anomalously short and strong B-N or B-C bonds. B(SiH 3 ) 3 ·N 2 has a B-N bond length of 1.48 Å and a complexation enthalpy of −15.9 kcal mol −1 at the M06-2X/jun-cc-pVTZ level. The selective binding enhancement of N 2 and CO is due to π-backbonding from Lewis acid to Lewis base, as demonstrated by orbital analysis and density difference plots. The π-backbonding is found to be a consequence of constructive orbital interactions between the diffuse and highly polarizable B-Si and B-Ge bond regions and the π and π* orbitals of N 2 . This interaction is strengthened by electron donating substituents on Si or Ge. The π-backbonding interaction is predicted to activate N 2 for chemical transformation and reduction, as it decreases the electron density and increases the length of the N-N bond. The binding of N 2 and CO by the B(SiR 3 ) 3 and B(GeR 3 ) 3 types of Lewis acids also has a strong σ-bonding contribution. The relatively high σ-bond strength is connected to the highly positive surface electrostatic potential [ V S ( r )] above the B atom in the tetragonal binding conformation, but the σ-bonding also has a significant coordinate covalent (dative) contribution. Electron withdrawing substituents increase the potential and the σ-bond strength, but favor the binding of regular Lewis acids, such as NH 3 and F − , more strongly than binding of N 2 and CO. Molecules of the types B(SiR 3 ) 3 and B(GeR 3 ) 3 are chemically labile and difficult to synthesize. Heterogenous catalysts with the wanted B(Si-) 3 or B(Ge-) 3 bonding motif may be prepared by boron doping of nanostructured silicon or germanium compounds. B-doped and hydrogenated silicene is found to have promising properties as catalyst for the electrochemical NRR. Lewis acids of the types B(SiR 3 ) 3 and B(GeR 3 ) 3 bind N 2 and CO with anomalously short and strong B-N or B-C bonds.