AMMONIOMETHYL)BIS[SALICYLATO(2-)-O-1,O-2]SILICATES - ZWITTERIONIC SPIROCYCLIC LAMBDA-SI-5-SILICATES WITH 2 6-MEMBERED SIO2C3 RING-SYSTEMS

The zwitterionic spirocyclic lambda(5)Si-silicate pholiniomethyl)bis[s alicylato(2-)-O-1,O-2]silicate (5) was synthesized by reaction of trim ethoxy(morpholinomethyl)silane and dimethoxy(morpholinomethyl)phenylsi lane, respectively, with salicylic acid (molar ratio 1:2) in acetonitr ile at room temperature. 2][(2,2,6,6-tetramethylpiperidinio)methyl]sil icate (6) was obtained analogously, starting from hoxy[(2,2,6,6-tetram ethylpiperidino)methyl]silane. Compound 5 was isolated as a crystallin e product consisting of enantiomorphic crystals, each containing exclu sively zwitterions with the same absolute configuration. Crystallizati on of this product from acetonitrile yielded the crystalline solvate 5 . CH3CN (crystals containing a 1:1 mixture of enantiomers with opposi te absolute configuration). Compounds 5 (Delta-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 t he crystal of 5, 5 . CH3CN, and 6 contain a pentacoordinate (formally negatively charged) silicon atom (SiO4C skeleton) and a tetracoordinat e (formally positively charged) nitrogen atom. The coordination polyhe dra around the silicon atoms are distorted trigonal bipyramids, the ax ial positions being occupied by the carboxylate oxygen atoms. Solid-st ate MAS NMR studies revealed the following isotropic Si-29 chemical sh ifts: delta -120.6 (5), -123.1 (5 . CH3CN), -120.4 (6). The zwitterion 6 exists also in solution (CDCl3; H-1, C-13, and Si-29 NMR studies).