Nature of the Si(SiMe3)3+Cation in Aromatic Solvents

Abstract

The silylium cation Si(SiMe3)3+ (1) was investigated by HF, B3LYP, PISA-HF, SCRF, and the NMR/ab initio/IGLO approach in the gas phase and in benzene solution employing the 6-31G(d) basis set. In the gas phase, the δ(29Si) value of the central Si atom of 1 is 920 ppm relative to that of TMS according to IGLO/[7s6p2d/5s4p1d/3s1p]//HF/6-31G(d) calculations, which is in line with the corresponding δ(29Si) values calculated for other silyl-substituted silylium cations and results from large paramagnetic contributions to the δ(29Si) shift value. In benzene solution, 1 forms the Wheland σ-complex Si(SiMe3)3-C6H6+ (3), in which the silylium cation character of 1 is lost despite the fact that 3 is the weakest silylium cation−benzene complex so far investigated. According to NMR/ab initio/IGLO calculations, complex 3 is characterized by a weak covalent SiC bond of 2.293 A, a coordination energy of 13.6 kcal/mol, and a δ(29Si) value of 111 ppm. Steric blocking by three SiMe3 groups is not sufficient to prevent a si...