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Zwitterionic Bis[citrato(2−)-O3,O4](morpholiniomethyl)germanate Hydrate and Its Silicon Analogue:  Syntheses and Crystal Structure Analyses

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journal contribution
posted on 23.12.1997, 00:00 by Reinhold Tacke, Joachim Heermann, Melanie Pülm
The zwitterionic spirocyclic λ5Ge-germanate bis[citrato(2−)-O3,O4](morpholiniomethyl)germanate (2) and its silicon analogue bis[citrato(2−)-O3,O4](morpholiniomethyl)silicate (3) were synthesized by reaction of citric acid with trimethoxy(morpholinomethyl)germane (4) and trimethoxy(morpholinomethyl)silane (5), respectively. Both reactions were carried out at room temperature in acetonitrile and the products isolated, after crystallization from water, as the hydrates 2·H2O and 3·H2O. In addition to NMR spectroscopic (solution-state 1H, 13C, and 29Si NMR; solid-state 29Si CP/MAS NMR) and mass spectrometric (FAB MS) characterization of 2, 2·H2O, 3, and 3·H2O, the hydrates were structurally characterized by single-crystal X-ray diffraction. The zwitterions 2 and 3 contain a pentacoordinate (formally negatively charged) germanium and silicon atom, respectively, and a tetracoordinate (formally positively charged) nitrogen atom. The central atom of 2 (Ge) and 3 (Si) is surrounded by four oxygen atoms and one carbon atom. In the crystal of 2·H2O and 3·H2O, the coordination polyhedra around the germanium and silicon atom can be best described as distorted trigonal bipyramids, the carboxylate oxygen atoms occupying the axial sites. In terms of the Berry pseudorotation coordinate, the geometries are displaced by 14.0% (2·H2O) and 13.4% (3·H2O) from the ideal trigonal bipyramid toward the ideal square pyramid. According to NMR spectroscopic studies, the zwitterions 2 and 3 also exist in solution ([D6]DMSO).