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

✍ Scribed by Reinhold Tacke; Rüdiger Bertermann; Andreas Biller; Olaf Dannappel; Martin Penka; Melanie Pülm; Reiner Willeke

• Publisher: John Wiley and Sons
• Year: 2000
• Tongue: German
• Weight: 364 KB
• Volume: 626
• Category: Article
• ISSN: 0372-7874
• DOI: 10.1002/(sici)1521-3749(200005)626:5<1159::aid-zaac1159>3.0.co;2-1

✦ Synopsis

The syntheses of the zwitterionic spirocyclic k 5 Si-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 k 5 Si-silicates 7±14 were studied by solution ( 1 H, 13 C, 29 Si) and solid-state ( 13 C, 15 N, and 29 Si 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 1 H 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 (K)-and (D)-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 (K)-and (D)-enantiomers of 7±14 are configurationally stable in solution on the NMR time scale ([D 6 ]DMSO, room temperature). They undergo an intramolecular (K)/(D)-enantiomerization (twist-type mechanism), with an activation free enthalpy of DG { = 72± 73 kJ mol ±1 (experimentally established for 7 and 13; calculated energy barrier for the model species 15: 66.0 kJ mol ±1 ).