Anomalous π-backbonding in complexes between B(SiR₃)₃ and N₂: catalytic activation and breaking of scaling relations

Chemical transformations of molecular nitrogen (N₂), including the nitrogen reduction reaction (NRR), are difficult to catalyze because of the weak Lewis basicity of N₂. In this study, it is shown that Lewis acids of the types B(SiR₃)₃ and B(GeR₃)₃ bind N₂ and CO with anomalously short and strong B-N or B-C bonds. B(SiH₃)₃·N₂ 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 N₂ 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 N₂. This interaction is strengthened by electron donating substituents on Si or Ge. The π-backbonding interaction is predicted to activate N₂ for chemical transformation and reduction, as it decreases the electron density and increases the length of the N-N bond. The binding of N₂ and CO by the B(SiR₃)₃ and B(GeR₃)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 NH₃ and F^-, more strongly than binding of N₂ and CO. Molecules of the types B(SiR₃)₃ and B(GeR₃)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.