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

Abstract

A series of glutarimide derivatives which has two carbonyl coordination sites for intramolecular pentacoordination at silicon with a X_1+nSiC_3−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 ²⁹Si 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_N2 reaction profile from the glutarimide derivatives correlates reasonably well with the structural data obtained from derivatives of lactams, diketopiperazines and quinolones.