Nature of the Si(SiMe 3 ) 3 + Cation in Aromatic Solvents
1996; American Chemical Society; Volume: 15; Issue: 26 Linguagem: Inglês
10.1021/om960179f
ISSN1520-6041
AutoresCarl‐Henrik Ottosson, Dieter Cremer,
Tópico(s)Organoboron and organosilicon chemistry
ResumoThe 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 Å, 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 silylium cation from interacting with aromatic solvent molecules.
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