(Ammoniomethyl)bis[salicylato(2−)-O¹,O²]silicates:  Zwitterionic Spirocyclic λ⁵Si-Silicates with Two Six-Membered SiO₂C₃ Ring Systems

The zwitterionic spirocyclic λ⁵Si-silicate (morpholiniomethyl)bis[salicylato(2−)-O¹,O²]silicate (5) was synthesized by reaction of trimethoxy(morpholinomethyl)silane and dimethoxy(morpholinomethyl)phenylsilane, respectively, with salicylic acid (molar ratio 1:2) in acetonitrile at room temperature. Bis[salicylato(2−)-O¹,O²][(2,2,6,6-tetramethylpiperidinio)methyl]silicate (6) was obtained analogously, starting from trimethoxy[(2,2,6,6-tetramethylpiperidino)methyl]silane. Compound 5 was isolated as a crystalline product consisting of enantiomorphic crystals, each containing exclusively zwitterions with the same absolute configuration. Crystallization of this product from acetonitrile yielded the crystalline solvate 5·CH₃CN (crystals containing a 1:1 mixture of enantiomers with opposite absolute configuration). Compounds 5 (Δ-enantiomer), 5·CH₃CN (racemate), and 6 (racemate) were structurally characterized by single-crystal X-ray diffraction. The spirocyclic frameworks of 5 and 6 are built up by two six-membered SiO₂C₃ ring systems. The zwitterions in the crystal of 5, 5·CH₃CN, and 6 contain a pentacoordinate (formally negatively charged) silicon atom (SiO₄C skeleton) and a tetracoordinate (formally positively charged) nitrogen atom. The coordination polyhedra around the silicon atoms are distorted trigonal bipyramids, the axial positions being occupied by the carboxylate oxygen atoms. Solid-state MAS NMR studies revealed the following isotropic ²⁹Si chemical shifts:  δ −120.6 (5), −123.1 (5·CH₃CN), −120.4 (6). The zwitterion 6 exists also in solution (CDCl₃; ¹H, ¹³C, and ²⁹Si NMR studies).