A zwitterionic lambda Si-5-silicate with an almost ideal square-pyramidal Si coordination polyhedron: Synthesis and crystal structure analysis

The pentacoordinate silicon compound ((morpholinio)methyl)bis(3,4,5,6-tetra chlorobenzene-1,2-diolato(2-))silicate-acetonitrile (4 CH3CN), a zwitterion ic spirocyclic lambda(5)Si-silicate, has been synthesized and structurally characterized by single-crystal X-ray diffraction analysis. Compound 4.CH3C N was prepared by reaction of the silanes (MeO)(3)SiCH2NR2, Ph(MeO)(2)-SiCH 2NR, or Me(MeO)(2)SiCH2NR2 (NR2 = morpholino) with two mol equivalents of 3 ,4,5,6-tetrachlorobenzene-1,2-diol in acetonitrile at room temperature. The Si coordination polyhedron of 4 CH3CN in the crystal is a nearly ideal squ are pyramid, the basal positions being occupied by the four oxygen atoms of the two diolato(2-) ligands. Significant differences between the isotropic Si-29 chemical shifts of 4.CH3CN in the crystal (delta = -85.1) and in sol ution (delta = -131.9, DMSO-d(6)) indicate special structural features in s olution.