Study of Binuclear Silicon Complexes of Diketopiperazine at SN2 Reaction Profile

A series of binuclear pentacoordinate silicon complexes of diketopiperazine have been synthesized and substituent (leaving group) effects on the Si−O bond coordination have been studied by comparison of the five differently substituted analogous structures (X = F, Cl, OTf, Br, and I). Variable-temperature NMR spectroscopy supported by X-ray crystallography shows, for the first time in binuclear pentacoordinate silicon complexes, a complex equilibrium with both nonionic (O−Si) and ionic (Si−X) dissociation of the axial bonds in the silicon-centered trigonal bipyramids. The two dissociation pathways are consistent with a model for nucleophilic substitution in a binuclear pentacoordinate silicon compound at the silicon atom.