Silicenium ions solvation

Chojnowski and co-workers have studied the polymerization of octamethyltetrasila-l,4-dioxane, a monomer more basic than cyclosiloxanes, which is capable of forming more stable oxonium ions, and thus being a useful model to study the role of silyloxonium ions.150-152 In recent work, these authors used Olah s initiating system and observed the formation of oxonium ion and its transformation to the corresponding tertiary silyloxonium ion at the chain ends.153 The 29Si NMR spectroscopic data and theoretical calculations were consistent with the postulated mechanism. Stannett and co-workers studied an unconventional process of radiation-initiated polymerization of cyclic siloxanes and proposed a mechanism involving the intermediate formation of silicenium ions solvated by the siloxane... 

The high electrophilicity of the silicenium ion can be modified by intramolecular electron donation from remote substituents. Corriu and co-workers668 were the first to synthesize species 269 with the bidentate nitrogen ligand. Cations with other structural units and donor atoms (O, S, P) were subsequently generated.669-674 Due to the interaction called internal solvation,675 the silicon atom becomes pentacoordinated and largely loses its ionic character, that is, such ions differ in principle from the trivalent silicenium ions. 

The question as to whether non-decomposing [M — Me]+ ions generated from bis-trimethyl ethers of acyclic diols remain acyclic or cyclize has been addressed by appearance energy (AE) measurements for the reactions M + [M — Me] + of a series of homologues 10974. From this analysis it was concluded that no measurable effects of the chain length (CH2) on the appearance energy was noticeable. If intramolecular solvation of the silicenium ions by the ether oxygen lone pairs, 110 — 111, would have occurred,... 

Basically, halocarbons should be capable of entering the silicon coordination sphere with a lone pair of one of their halogen atoms, as has been shown for the 1,2-dichlorobenzene solvate of a silicenium ion (/Pr3Si. .. Cl o-CeHiCl) [76]. To the best of our knowledge, this kind of solvate formation has not been proven relevant in hypercoordinated silicon chemistry yet. 

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