Study of Binuclear Silicon Complexes of Diketopiperazine at S_N2 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.本研究合成了一系列二酮哌嗪（diketopiperazine）类双核五配位硅配合物，并通过对比五种不同取代的同类结构（X=F、Cl、OTf、Br及I），探究了取代基（离去基团）对Si-O键配位作用的影响。借助X射线晶体学辅助的变温核磁共振波谱技术，本研究首次在双核五配位硅配合物体系中观测到：以硅为中心的三角双锥结构的轴向键同时存在非离子型（O-Si）与离子型（Si-X）解离过程，进而形成复杂平衡体系。上述两种解离路径与双核五配位硅化合物中硅原子位点的亲核取代反应模型高度吻合。