Anomalous Pi-backbonding in Complexes between B(SiR3)3 and N2: Catalytic Activation and the 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 was found 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 at the M06-2X/jun-cc-pVTZ level. The selective binding enhancement of N2 and CO is due to pi-backbonding from Lewis acid to Lewis base, as demonstrated by orbital analysis and density difference plots. The pi-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 pi-regions of N2. This interaction is strengthened by electron donating substituents on Si or Ge. The pi-backbonding interaction is predicted to activate N2 for chemical transformation and reduction, as it decreases the electron density and increases the N-N bond length. The binding of N2 and CO by the B(SiR3)3 and B(GeR3)3 types of Lewis acids also has a strong pi-bond contribution. The relatively high pi-bond strength is connected to the high positive surface electrostatic potential [VS(r)] above the B atom at the pyramidal binding conformation. Electron withdrawing substituents increase the potential and the sigma-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 B-doping of nanostructured silicon or germanium compounds. B-doped silicene show promising properties as catalyst for the electrochemical NRR.