The Silylium Ion (R3Si+) Problem: Effect of Alkyl Substituents R

Abstract

The X-ray structures of four R~S~( B I -~-C B~~H~) species have been determined (R3 = Et3, i-Pr3, t-Bu3, and t-BuzMe). Br6-CBllH6-is a particularly weakly coordinating anion and leads to long Si-Br bond distances (ca. 2.43-2.48 A). The developing silylium ion character in these species is reflected in the approach of the R3Si6+ moiety toward planarity. The sum of the C-Si-C angles for each of the four derivatives lies in the range ca. 345-351". The closest approach to 360" is found in the triisopropyl derivative, not the tri-tert-butyl derivative as might be anticipated from steric considerations. One of the isopropyl groups in the i-Pr3 derivative has its carbon atoms very nearly planar with the silicon atom. This is indicative of C-H bond hyperconjugative stabilization of the developing positive charge on silicon. An "inorganic" description would be in terms of a n a-agostic C-H interaction with the electron-deficient 3pz orbital on silicon. The results suggest that hyperconjugative stabilization of cationic character, although relatively unimportant compared to that for carbon in carbenium ions, is nevertheless not negligible for silicon. Solid state CPMAS 29Si NMR data are also reported. The chemical shifts lie in the 105-115 ppm downfield region as expected for silylium ion-like character in these R3sid+(Br6-cB~~H6)+ species. (14) Xie, Z.; Liston, D. J.; Jelinek, T.; Mitro, V.; Bau, R.; Reed, C. A. (15) Kira, M.; Hino, T.; Sakurai, H.

R3E+ Carborane Salts of the Heavier Group 14 Elements: E = Ge, Sn, and Pb

“…The average Ge-C bond length is 1.943 Å . In the corresponding Si compound, 23 the sum of the C-Si-C angles is 349.0°and the Si-Br bond lengthening is ca. 0.23 Å .…”

Ring‐Opening Protonolysis of Sila[1]ferrocenophanes as a Route to Stabilized Silylium Ions

“…[30] To probe the stability of a ferrocenyl-substituted silicon cation in the presence of such stabilizing substituents the synthesis and ring-opening of fcSiiPr 2 (3 c) were undertaken.…”

Silicenium Ions: Quantum Chemical Computations

“…Surprisingly, the closest approach to planarity was found for the tri-iso-propyl derivatives, rather than for the tri-tert-butyl species. The authors also concluded that hyperconjugative stabilization of the cationic silicon centre is not negligible, although it is less pronounced than in carbocations 90 . The hexachloro carborane anion was found to be the least coordinating and, based on structural, energetic and NMR criteria, more than 50% silicenium ion character was assigned to the i-Pr 3 Si υC (Cl 6 CB 11 H 6 ) υ complex 91 21.…”

“…Reed's group reported the X-ray structures of trialkyl-silicenium ions R 3 Si C complexed to low-nucleophilic hexahalo carborane anions 90,91 . The authors varied systematically both the alkyl substituents R (R D Me, Et, i-Pr and t-Bu) and the halogen substitution of the X 6 CB 11 H 6 (X D Cl, Br, I) counterions.…”

“…The authors varied systematically both the alkyl substituents R (R D Me, Et, i-Pr and t-Bu) and the halogen substitution of the X 6 CB 11 H 6 (X D Cl, Br, I) counterions. The degree of silicenium ion character in the R 3 Si C moiety was estimated from the sum of the bond angles around silicon, which should reach the ideal 360°value in a free tri-coordinate silicenium ion 90 . Surprisingly, the closest approach to planarity was found for the tri-iso-propyl derivatives, rather than for the tri-tert-butyl species.…”