Pentacoordinate silicon complexes with dynamic motion resembling a pendulum on the SN2 reaction pathway

A series of glutarimide derivatives which has two carbonyl coordination sites for intramolecular pentacoordination at silicon with a X1+nSiC3-nO moiety have been synthesised and characterized. The substituent (leaving group) effects on the Si-O bond exchange between the two coordination sites (resembling a pendulum) have been studied by comparison of the differently substituted (X = F, Cl, OTf, Br and I) structures. The activation parameters for the Si-O bond exchange process were measured by NMR and separately computed and are consistent with the strength of Si-O bond coordination and the nature of the leaving group, X. The temperature-dependent Si-29 NMR spectroscopy is supported by X-ray crystallography and shows that the tetrahedral reactant is converted into pentacoordinate intermediates by intramolecular O-Si association followed by reversion to a tetrahedral geometry by Si-X dissociation. The two association/dissociation patterns offer a model for nucleophilic substitution at a silicon atom. A continuum of structures on the S(N)2 reaction profile from the glutarimide derivatives correlates reasonably well with the structural data obtained from derivatives of lactams, diketopiperazines and quinolones

Synthesis and preliminary characterization of octakis (chloropropyldimethylsiloxy) octasilsesquioxane

Octakis (hydridodimethylsiloxy) octasilsesquioxane was hydrosilated with allyl chloride using Spiers catalyst (H2PtCl6). This reaction was monitored using FT-IR spectroscopy. The synthesized product was characterized by 13C, 29Si NMR (MAS), SEM, FT-IR, Thermogravimetric techniques. The three propyl groups alpha, beta, gamma, (to the terminal silicon atom), associated of an allyl chloride, were clearly seen in the 13C NMR (alpha-CH2 at 31.8; beta-CH2 at 37.7; gamma-CH2 at 50.1 ppm). In addition, the 29Si NMR spectrum of the final product, exhibits three Q signals for Q type silicon attributed to Q² (-90.1) Q³ (-100.2) and Q4 (-111.3ppm). The presence of allyl chloride substitutes in octameric cube confers a relative porosity and thermal stability to the material