Study of Binuclear Silicon Complexes of Diketopiperazine at S_N2 Reaction Profile

Abstract

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.

Sigma-Hole Interactions in Anion Recognition

“…In a study of S N 2 reactions on silicon complexes of diketopiperazine substituted with different leaving groups (F, Cl, OTf, Br, and I) on the Si centers, the presence of a good leaving group (e.g., iodine) was found to promote the formation of a Si-O covalent bond in compound 47 (Figure 22A). 74 The resulting electrophilic Si atom was found to engage in strong TrB interactions with the displaced iodide anion (77% P VdW Si-I radii), which is almost collinear with the Si-O bond. Separately, a disiloxane compound containing highly electron-deficient Si atoms covalently bonded to several electron-withdrawing groups (48) was found to bind a Cl À bridged between both Si centers via two convergent TrB interactions in the solid state (Figure 22B).…”

Tetrel‐Bonding Interaction: Rediscovered Supramolecular Force?

“…Furthermore, in Figure 3 (bottom) we show the X‐ray structures of two binuclear pentacoordinate silicon complexes of diketopiperazine. By means of variable‐temperature NMR spectroscopy and X‐ray crystallography experiments the S N 2 reaction mechanism has been studied for five differently substituted analogous structures (X=F, Cl, OTf, Br, and I) 14. Two extreme cases are shown in Figure 3, wherein a covalent bond (F‐Si, OYOMIZ) is observed for a very poor leaving group (F − ) and a tetrel bond (I⋅⋅⋅Si, OYOMOZ) is clearly formed for a good leaving group (I − ).…”

Tetrel Bonding Interactions

“…Shown in Figure are the X‐ray structures of two binuclear pentacoordinate silicon complexes of diketopiperazine. These compounds have been used to study the S N 2 reaction mechanism in five differently substituted analogous structures (leaving group = F, Cl, OTf, Br, and I) . In Figure , we show two extreme cases that were characterized by X‐ray analysis.…”