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

Abstract

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.

Solid-state 29Si MAS NMR studies of diatoms: structural characterization of biosilica deposits

“…1 and 2) of the biosilica deposits from the different diatom species show the well known pattern typical of amorphous silica with resolved Q 2 (δ-=92), Q 3 (δ-=101), and Q 4 (δ-=111) signals [8] and do not exhibit clear hints to the presence of penta-or hexacoordinate silicon, although the existence of pentacoordinate silicon (SiO 5 units) in these samples cannot totally be excluded. 29 Si NMR shifts of pentacoordinate silicon compounds with SiO 5 skeletons are given elsewhere [9]. The spectra of the different diatom samples are virtually identical according to peak distribution and chemical shifts and compare very well to the NMR data obtained from biosilica of the diatom Navicula pelliculosa [10] and synthetic silica gel (Fig.…”

Pentacoordinate Silicon Compounds with SiO5 Skeletons Containing SiOH or SiOSi Groups: Derivatives of the Pentahydroxosilicate(1−) Anion [Si(OH)5]- and Its Anhydride [(HO)4Si−O−Si(OH)4]2-

“…We have developed simple preparative methods for the synthesis of novel pentacoordinate silicon compounds with Si O 5 skeletons containing functional SiOH and SiOSi groups, and we have studied the structures of these compounds in the solid state and in solution . Such λ 5 Si -hydroxosilicates and λ 5 Si ,λ 5 Si ‘ -μ-oxo-disilicates represent interesting tools for mechanistic investigations in silicon-oxygen chemistry and are promising synthons which may form the basis for the development of a new branch in the chemistry of pentacoordinate silicon .…”

A Zwitterionic Spirocyclic Pentacoordinate Silicon Compound Synthesized in Water by SiO and SiC Bond Cleavage

“…We have contributed to this field by systematic studies on zwitterionic λ 5 Si ‐silicates that contain a pentacoordinate (formally negatively charged) silicon atom and a tetracoordinate (formally positively charged) nitrogen atom 1d. The zwitterions 1 ,2 2 ,3 3 ,4 4 ,5 5 ,6 6 ,7 7 ,8 8 ,9 and 9 10 are examples of this type of compound Generally, these zwitterionic λ 5 Si ‐silicates are quite sensitive to water and easily undergo hydrolytic SiOC, SiSC, and SiNC bond cleavage.…”