TWOFOLD DEPROTONATED CITRIC-ACID AS A BIDENTATE LIGAND OF PENTACOORDINATE SILICON - SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF THE ZWITTERIONIC LAMBDA-5-SI-SPIROSILICATE BIS[CITRATO(2-)-O-3,O-4][(DIMETHYLAMMONIO)METHYL]SILICATE HYDRATE

The zwitterionic lambda(5)Si-spirosilicate bis [citrate(2-)-O-3,O-4][(dimethylammonio)methyl]silicate (4) was synthesized by reaction of (MeO)(3)SiCH(2)NMe(2) (3) with citric acid (molar ratio 1:2) in acetonitrile at room temperature and isolated, after crystallization from water, as the hydrate 4 . H2O (yield 81%), The crystal structure of 4 . H2O was studied by single-crystal X-ray diffraction. The alcoxide oxygen atoms and central carboxylate oxygen atoms of two citrato(2-) Ligands and one carbon atom coordinate to the silicon atom of 4 . H2O. The coordination polyhedron around the pentacoordinate silicon atom (SiO4C framework) can be described as a distorted trigonal bipyramid, the two carboxylate oxygen atoms occupying the axial sites. The lambda(5)Si-silicon(IV) complex 4 also exists in solution (DMSO, H2O).