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(NH 2 )   3 +( 1 ) and derivatives thereof. The equilibrium structure 1a has D 3 symmetry with planar amino groups rotated out of the SiN 3 plane by 19.6 0 . The Si–N bond length of 1 a (1.658 Å) 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 1 a 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(NH 2 ) 3 ‐(H 2 O) + ( 6 ) (28.8 kcal mol −1 ) than for SiMe 3 (H 2 O) + (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 ‐(H 2 O) + ( 10 ) is only 17.3 kcal mol −1 . IGLO calculations of the 29 Si N M R chemical shifts predict that 1 a and 8 should not show the same extremely low shielding that is calculated for alkyl‐sub‐stituted silylium ions. The calculated 29 Si resonances for 8 are in reasonable agreement with the experimental N M R spectrum of (Me 2 N) 3 SiB(C 6 F 5 ) 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.