The silaguanidinium cation and the search for a stable silylium cation in condensed phases

Quantum mechanical calculations at the MP2/6-31 G(d) level are reported for the silaguanidinium cation Si(NH2)(3)(+) (1) and derivatives thereof. The equilibrium structure 1 a has D-3 symmetry with planar amino groups rotated out of the SiN3 plane by 19.6 degrees. The Si-N bond length of 1 a (1.658 Angstrom) is intermediate between a single and a double bond. Isodesmic reactions show that the stabilization of the silylium cation 1 a by the amino groups (63.5 kcal mol(-1)) is about 40 % of the resonance stabilization of the guanidinium cation (159.3 kcal mol(-1)), but la is clearly better stabilized than alkyl-substituted silylium cations. The electronic stabilization of 1 a by the amino groups is also made obvious by the calculated complexation energy with one molecule of water. The calculated stabilization through complexation of water at HF/6-31 G(d) is markedly lower for SI(NH2)(3)-(H2O)(+) (6) (28.8 kcal mol(-1)) than for SiMe(3)(H2O)(+) (40.6 kcal mol(-1)). The tris(dimethylamino) silylium cation Si(N-Me(2))(3)(+) (8) is even more stable than 1 a. The complexation energy of Si(NMe(2))(3)-(H2O)(+) (10) is only 17.3 kcal mol(-1). IGLO calculations of the Si-29 NMR chemical shifts predict that 1 a and 8 should not show the same extremely low shielding that is calculated for alkyl-substituted silylium ions. The calculated Si-29 resonances for 8 are in reasonable agreement with the experimental NMR spectrum of (Me(2)N)(3)SiB(C6F5)(4). AM 1 calculations predict that the substituted tripyrrolidino silylium cation 12 would be an even better candidate for a stable tricoordinate silylium cation in condensed phases. One of the pyrrolidine rings of 12 has tert-butyl groups in the 2 and 5 positions, which serve as a steric fence around the silicon atom.