Melanie Pülm

ZWITTERIONIC PENTACOORDINATE SILICON COMPOUNDS

Publisher Summary The chemistry of zwitterionic pentacoordinate silicon compounds is developed systematically. These compounds contain a silicate moiety with five covalent siliconelement bonds and an onium group. This chapter discusses a completely different class of pentacoordinate silicon compounds: zwitterionic λ 5 Si -silicates. These molecular compounds contain a pentacoordinate silicon atom and a tetracoordinate nitrogen atom. The syntheses, structures, and properties of compounds of zwitterionic λ 5 Si -silicates are discussed. The zwitterionic λ 5 Si -silicates are high-melting crystalline solids that are almost insoluble in nonpolar organic solvents and exhibit poor solubility in polar organic solvents. The crystals of these compounds consist of highly polar molecular building blocks that undergo intermolecular dipolar interactions. Different methods are used for their syntheses, some of these methods involving remarkable Si-C cleavage reactions. The products isolated directly from the reaction mixtures are well-crystallized solids, suitable for characterization by single-crystal X-ray diffraction and solid-state nuclear magnetic resonance (NMR) spectroscopy. The zwitterions are neutral molecules and are chiral, with a chiral λ 5 Si -silicates skeleton. Zwitterionic compounds are characterized by unique properties that arise from the charge separation in these molecules.

New Chiral Zwitterionic λ5Si-Silicates with an SiO5 or SiO4C Framework: Syntheses, Crystal Structures, and Properties

The syntheses of the zwitterionic (molecular) spirocyclic λ5Si-silicates 7 and 8 (SiO5 skeletons) and 10–12 (SiO4C skeletons) are described. These chiral compounds contain a pentacoordinate (formally negatively charged) silicon atom and a tetracoordinate (formally positively charged) nitrogen atom. The pairs 7/11 and 8/12 are isoelectronic O/CH2 analogues and 10–12 represent a series of homologous compounds containing different alkylene spacers that connect the ate and onium center of these zwitterions. Compound 7, 8, and 10–12 were studied by single-crystal X-ray diffraction, solution-state (1H, 13C, 29Si) and solid-state (29Si CP/MAS) NMR spectroscopy, and VT 1H-NMR spectroscopy. The experimental investigations were completed by ab initio studies of the related anionic λ5Si-silicate 13.

A Zwitterionic λ5Si-Silicate with an Almost Ideal Square-Pyramidal Si Coordination Polyhedron: Synthesis and Crystal Structure Analysis

Summary. The pentacoordinate silicon compound ((morpholinio)methyl)bis(3,4,5,6-tetrachlorobenzene-1,2-diolato(2−))silicate–acetonitrile (4ċCH3CN), a zwitterionic spirocyclic λ5Si-silicate, has been synthesized and structurally characterized by single-crystal X-ray diffraction analysis. Compound 4ċCH3CN was prepared by reaction of the silanes (MeO)3SiCH2NR2, Ph(MeO)2-SiCH2NR, or Me(MeO)2SiCH2NR2 (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 4ċCH3CN in the crystal is a nearly ideal square pyramid, the basal positions being occupied by the four oxygen atoms of the two diolato(2−) ligands. Significant differences between the isotropic 29Si chemical shifts of 4ċCH3CN in the crystal (δ=−85.1) and in solution (δ=−131.9, DMSO-d6) indicate special structural features in solution.Zusammenfassung. Die pentakoordinierte Siliciumverbindung ((Morpholinio)methyl)bis(3,4,5,6-tetrachlorbenzol-1,2-diolato(2−))silicat–Acetonitril (4ċCH3CN), ein zwitterionisches spirocyclisches λ5Si-Silicat, wurde synthetisiert und mittels Einkristallröntgenstrukturanalyse strukturell charakterisiert. Verbindung 4ċCH3CN wurde durch Reaktion der Silane (MeO)3SiCH2NR2, Ph(MeO)2SiCH2NR2 oder Me(MeO)2SiCH2NR2 (NR2=Morpholino) mit zwei Mol-Äquivalenten 3,4,5,6-Tetrachlorbenzol-1,2-diol in Acetonitril bei Raumtemperatur dargestellt. Das Si-Koordinationspolyeder von 4ċCH3CN im Kristall ist eine nahezu ideale quadratische Pyramide, wobei die vier Sauerstoffatome der beiden Diolato(2−)-Liganden die basalen Positionen besetzen. Drastische Unterschiede in den isotropen 29Si-chemischen Verschiebungen von 4ċCH3CN im Kristall (δ=−85.1) und in Lösung (δ=−131.9, DMSO-d6) weisen auf Besonderheiten der Struktur in Lösung hin.

Pentafluoro[(4-methyl-1,4-bisazoniacy clohex-1 -yl)methy 1] germanate Hydrate: Synthesis and Crystal Structure of a Zwitterionic λ6Ge-Germanate with a GeF5C Framework

Abstract The zwitterionic (molecular) λ6Ge-germanate pentafluoro[(4-methyl-1,4-bisazoniacyclohexl-yl)methyl]germanate (5) was synthesized by reaction of 1-methyl-4-[(trimethoxygermyl) methyl]piperazine (8) with HF (molar ratio 1:5) in a mixture of water and ethanol at 0 °C and isolated as the hydrate 5 · H2O. The zwitterion 5 is characterized by the presence of a hexacoordinate (formally twofold negatively charged) germanium atom and two tetracoordinate (formally positively charged) nitrogen atoms. Compound 5 · H2O was structurally characterized by single-crystal X-ray diffraction. Crystal data are as follows: C6H17F5GeN2O , triclinic space group P1̄ (no. 2), a = 7.5228(11) Å, b = 12.174(2) Å, c = 12.3041(14) Å, α = 73.74(2)°, β = 82.44(2)°, γ = 74.762(10)°, V = 1041.7(2) Å3, T = 173(2) K, Z = 4, R1 = 0.0227. There are one pair each of crystallographically independent zwitterions 5 and water molecules in the asymmetric unit, the structures of the zwitterions being very similar. Their coordination polyhedra around the germanium atoms are slightly distorted octahedra.

Singly Charged λ6Si-Silicate Anions with an SiF5C Skeleton: Syntheses and Crystal Structure Analyses of the Ionic Hexacoordinate Silicon Compounds [Me3NH][F5SiCH2NMe2H]·H2O and [Me3NH][F5SiCH2NMe2H]·H2O

The zwitterionic λ5Si-tetrafluorosilicates F4SiCH2NMe2H (1) and F4SiCH2NMe3 (2) behave as Lewis acids and react with [Me3NH]F (molar ratio 1:1) in aqueous solution to yield the ionic λ6Si-pentafluorosilicates [Me3NH][F5SiCH2NMe2H] (3) and [Me3NH][F5SiCH2NMe3] (4), respectively. These hexacoordinate silicon compounds contain singly charged λ6Si-silicate anions ([F5SiCH2NMe2H]- , [F5SiCH2NMe3]- ) with an SiF5C skeleton. Compounds 3 and 4 were isolated as the crystalline hydrates 3·H2O (yield 80%) and 4·H2O (yield 82%) which were structurally characterized by single-crystal X-ray diffraction. The Si-coordination polyhedra in the crystals of 3·H2O and 4·H2O are slightly distorted octahedra