Zwitterionic Spirocyclic λ5Si-Silicates with Two Bidentate Acetohydroximato(2-) or Benzohydroximato(2-) Ligands: Synthesis, Structure, and Dynamic Stereochemistry

Abstract

The syntheses of the zwitterionic spirocyclic λ5Si‐silicates 7–14 are described. The chiral zwitterions contain a pentacoordinate (formally negatively charged) silicon atom and a tetracoordinate (formally positively charged) nitrogen atom, the ate and onium center being connected by an alkylene group. The zwitterions each contain two identical bidentate diolato(2–) ligands that formally derive from acetohydroximic acid or benzohydroximic acid. The stereochemistry and dynamic behavior of these compounds were investigated by experimental and theoretical methods. For this purpose, the zwitterionic λ5Si‐silicates 7–14 were studied by solution (1H, 13C, 29Si) and solid‐state (13C, 15N, and 29Si CP/MAS) NMR experiments. In addition, compounds 7, 8, 10, 11, and 13 were structurally characterized by single‐crystal X‐ray diffraction. The dynamic behavior (intramolecular enantiomerization) of 7 and 13 in solution was studied by VT 1H NMR experiments. These experimental studies were completed by ab initio investigations of the related anionic model species 15. The chiral compounds 7–14 exist as (λ)‐ and (δ)‐enantiomers in the solid state and in solution. The trigonal‐bipyramidal structure of the respective Si‐coordination polyhedra, with the two carbon‐linked oxygen atoms in the axial sites, is the energetically most favorable one. The (λ)‐ and (δ)‐enantiomers of 7–14 are configurationally stable in solution on the NMR time scale ([D6]DMSO, room temperature). They undergo an intramolecular (λ)/(δ)‐enantiomerization (twist‐type mechanism), with an activation free enthalpy of δG{ = 72–73 kJ mol–1 (experimentally established for 7 and 13; calculated energy barrier for the model species 15: 66.0 kJ mol–1).

Zwitterionic λ5Si-Silicates withSiO2S2C Skeletons: Syntheses and Structural Characterization in the Solid State

“…Compounds 1 [2], 2 [3], 3 [4], 4 [5], 5 [6], 6 [6], 7 [7], and 8 [8] are examples of this. We have now succeeded in synthesizing the first zwitterionic k 5 Si-silicates with SiO 2 S 2 C skeletons, compounds 9 and 10.…”

Optically Active Zwitterionic λ5Si,λ5Si′‐Disilicates: Syntheses, Crystal Structures, and Behavior in Aqueous Solution

“…The precursor 9 was synthesized by the treatment of (chloromethyl)trimethoxysilane with ammonia in an autoclave (32 % yield; Scheme ). The starting materials 10 ,4 11 ,5 12 ,6 and 16 7 were prepared according to previous reports, and 13 was commercially available. The precursor 14 was synthesized by the reaction of 12 with iodomethane in acetonitrile (92 % yield; Scheme ), and the treatment of (iodomethyl)trimethoxysilane8 with 1‐methylimidazole in cyclohexane afforded 15 (70 % yield; Scheme ).…”

“…Compounds 10 – 12 and 16 were prepared according to procedures given in refs. 4–7. Compound 13 was purchased from ABCR.…”

[Benzene-1,2-diolato(2−)][benzene-1,2-diolato(1−)]methyl[(2,2,6,6-tetramethylpiperidinio)methyl]silicate: Isolation, Structural Characterization, and Thermally Induced Methane Elimination

“…Further studies are planned to get more information about this remarkable type of reaction. These investigations will also include related Si−C bond cleavage reactions mediated by aceto- and benzohydroxamic acid and by benzoin …”