The basicity of silylamines and alkylamines: An equilibrium study of the competitive borane adduct formation

In order to investigate the Lewis acid/base chemistry of silylamines in solution, the equilibrium reactions of Me₃SiNMe₂ / Me₃SiNMe₂(BH₃) and different amines NR₃ / amineboranes NR₃(BH₃) were monitored by ¹H NMR spectroscopy in C₆D₆ and CDCl₃ at 60 °C. For the equilibrium of the reference silylamine/-borane and Me₃CNMe₂ / Me₃CNMe₂(BH₃) values K = 0.35 (ΔG = 3.1 kJ mo^-1) in C₆D₆ and K = 0.50 (ΔG = 2.0 kJ mol^-1) in CDCl₃ have been estimated, showing that both the silylamine and the C/Si-analogous organic amine have very similar donor properties. Small amines like NMe₃ proved to be much more powerful donors than the silylamine Me₃SiNMe₂, whereas bulky amines are less efficient donors, indicating that steric effects play an important role. The formation of BH₃ adducts of Me₂NSiH₃ and (H₃Si)₃N which could not be followed experimentally owing to the low stability of the products, has been studied by quantum chemical calculations. A comparison with data for Me₃N(BH₃) shows that the silylamines are poorer donors, but the reason for this remains unclear.