New Zwitterionic Pentacoordinate Silicates with SiONFC2, SiONF2C, and SiO2N2C Frameworks:  Synthesis, Structure, and Dynamic Stereochemistry

The zwitterionic monocyclic λ5 Si-monofluorosilicates 3a−c were synthesized by reaction of the zwitterionic λ5 Si-trifluorosilicates 1a−c with 1 molar equiv of (Z)-Me3SiO(Ph)CNN(H)SiMe3 (2). The zwitterionic monocyclic λ5 Si-difluorosilicate 8 and the zwitterionic spirocyclic λ5 Si-silicate 9 (isolated as the isomer 9-I) were prepared in an analogous manner, starting from the zwitterionic λ5 Si-tetrafluorosilicate 7 and using 1 (→ 8) or 2 (→ 9) molar equiv of 2. Single-crystal X-ray diffraction studies showed that the Si-coordination polyhedra of 1a, 3a, 8, 9-I·CHCl3, and 9-I·CH3CN are distorted trigonal bipyramids. The axial positions are occupied by two fluorine atoms (1a), one fluorine atom and one oxygen atom (3a, 8), or two oxygen atoms (9-I·CHCl3, 9-I·CH3CN). These results are in agreement with the NMR data obtained for these compounds in solution. The chiral monocyclic compounds (3a−c and 8) undergo two distinct rate processes, inversion of absolute configuration of the chiral λ5 Si-silicate skeleton (enantiomerization) and a novel “ammonium-nitrogen inversion”, measured by variable-temperature and selective inversion recovery NMR experiments (activation barrier for “ammonium-nitrogen inversion”: 3a−c, 17.0 ± 0.3 kcal mol-1; 8, 20.8 ± 0.3 kcal mol-1; activation barrier for inversion of absolute configuration at the silicon atom: 3c, 20.7 ± 0.3 kcal mol-1; 8, 11.7 ± 0.3 kcal mol-1). In the spirocyclic λ5 Si-silicate 9-I only one of these processes, the “ammonium-nitrogen inversion” (ΔG ⧧ = 20.8 ± 0.3 kcal mol-1), could be observed, while inversion of absolute configuration at the silicon atom could not be detected by NMR spectroscopy. Furthermore, no interconversion of diastereomers 9-I ⇄ 9-II was observed.