New Zwitterionic Pentacoordinate Silicates withSiONFC₂,SiONF₂C, andSiO₂N₂C Frameworks:  Synthesis, Structure, and Dynamic Stereochemistry

Abstract

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.

Zwitterionic λ5Si-Silicates withSiO2S2C Skeletons: Syntheses and Structural Characterization in the Solid State

“…Compounds 1 [2], 2 [3], 3 [4], 4 [5], 5 [6], 6 [6], 7 [7], and 8 [8] are examples of this. We have now succeeded in synthesizing the first zwitterionic k 5 Si-silicates with SiO 2 S 2 C skeletons, compounds 9 and 10.…”

Dimethyl-, diphenyl- and cyclotetramethylene silylchloride derivatives of guanidine, their synthesis and structure. Formation of pentacoordinated silicon compounds and an unexpected diionic disila-diguanidinium dichloride

“…Compounds with cyclic frameworks, strained silacarbon rings, spirobisilafluorenes, silaphenanthrolines, silanaphthyls and amidinates with k 5 Si-coordination can readily be assembled [8][9][10][11][12][13][14][15][16]. Additionally structures with zwitterionic frameworks and k 5 Si atoms have been made [17,18]. Pentacoordinated silicon compounds can be utilised in various organic syntheses [19][20][21][22] as well as when studying reaction pathways and transition stages in organic reactions [23][24][25][26].…”

Neutral Mononuclear and Binuclear Hexacoordinate Silicon Complexes withSiO5C Skeletons

“…Several hitherto unknown binding situations of pentacoordinate silicon could be realized and studied experimentally by taking advantage of the “zwitterion trick” 3. Compounds 1 ,4 2 ,5 and 3 6 with their Si O 2 N 2 C, Si ONFC 2 , or Si S 4 C skeleton are examples of this 7. To extend this successful concept, we decided to aim for novel zwitterionic pentacoordinate silicate systems with formally positively charged bidentate ligands.…”