(Ammoniomethyl)bis[salicylato(2−)-<i>O</i><sup>1</sup>,<i>O</i><sup>2</sup>]silicates:  Zwitterionic Spirocyclic λ<sup>5</sup><i>Si</i>-Silicates with Two Six-Membered SiO<sub>2</sub>C<sub>3</sub> Ring Systems

The zwitterionic spirocyclic λ<sup>5</sup><i>Si</i>-silicate (morpholiniomethyl)bis[salicylato(2−)-<i>O</i><sup>1</sup>,<i>O</i><sup>2</sup>]silicate (<b>5</b>) 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−)-<i>O</i><sup>1</sup>,<i>O</i><sup>2</sup>][(2,2,6,6-tetramethylpiperidinio)methyl]silicate (<b>6</b>) was obtained analogously, starting from trimethoxy[(2,2,6,6-tetramethylpiperidino)methyl]silane. Compound <b>5</b> 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 <b>5</b>·CH<sub>3</sub>CN (crystals containing a 1:1 mixture of enantiomers with opposite absolute configuration). Compounds <b>5</b> (Δ-enantiomer), <b>5</b>·CH<sub>3</sub>CN (racemate), and <b>6</b> (racemate) were structurally characterized by single-crystal X-ray diffraction. The spirocyclic frameworks of <b>5</b> and <b>6</b> are built up by two six-membered SiO<sub>2</sub>C<sub>3</sub> ring systems. The zwitterions in the crystal of <b>5</b>, <b>5</b>·CH<sub>3</sub>CN, and <b>6</b> contain a pentacoordinate (formally negatively charged) silicon atom (<i>Si</i>O<sub>4</sub>C 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 <sup>29</sup>Si chemical shifts:  δ −120.6 (<b>5</b>), −123.1 (<b>5</b>·CH<sub>3</sub>CN), −120.4 (<b>6</b>). The zwitterion <b>6</b> exists also in solution (CDCl<sub>3</sub>; <sup>1</sup>H, <sup>13</sup>C, and <sup>29</sup>Si NMR studies).