The Hexacoordinate Silicate Dianions <i>m</i><i>er</i>-Tris[glycolato(2−)-<i>O</i>¹,<i>O</i>²]silicate and <i>f</i><i>ac</i>-Tris[benzilato(2−)-<i>O</i>¹,<i>O</i>²]silicate: Syntheses and Structural Characterization

Treatment of tetramethoxysilane with glycolic acid and morpholine (molar ratio 1:3:2) in methanol, followed by crystallization from methanol/tetrahydrofuran, yielded morpholinium mer-tris[glycolato(2−)-O1,O2]silicate (mer-7). Treatment of benzilic acid with sodium hydride, followed by addition of tetrachlorosilane and triethylamine (molar ratio 3:4:1:2), afforded, after crystallization from 1,4-dioxane/acetonitrile/diethyl ether/n-pentane, triethylammonium fac-tris[benzilato(2−)-O1,O2]silicate−hemi-1,4-dioxane (fac-8·1/2C4H8O2). Single-crystal X-ray diffraction studies showed that the Si-coordination polyhedra of the hexacoordinate silicon(IV) complexes mer-7 and fac-8·1/2C4H8O2 are distorted octahedra. Both compounds were additionally characterized by solid-state VACP/MAS NMR studies (13C, 29Si), and fac-8·1/2C4H8O2 was studied in solution by 1H, 13C, and 29Si NMR experiments. The structural investigations were complemented by computational studies (MP2 studies, TZP level) of the dianions of fac-7 and mer-7.