Optically Active Zwitterionic λ⁵Si,λ⁵Si′‐Disilicates: Syntheses, Crystal Structures, and Behavior in Aqueous Solution

Abstract

The zwitterionic lambda(5)Si,lambda(5)Si'-disilicates 1-8 were synthesized and characterized by solid-state and solution NMR spectroscopy. In addition, compounds 26 H(2)O, 32 CH(3)CN, 45/2 CH(3)CN, 6CH(3)OH, 7, and 8CH(3)OHCH(3)CN were studied by single-crystal X-ray diffraction. The optically active (Delta,Delta,R,R,R,R)-configured compounds 1-8 contain two pentacoordinate (formally negatively charged) silicon atoms and two tetracoordinate (formally positively charged) nitrogen atoms. One (ammonio)alkyl group is bound to each of the two silicon centers, and two tetradentate (R,R)-tartrato(4-) ligands bridge the silicon atoms. Although these lambda(5)Si,lambda(5)Si'-disilicates contain SiO(4)C skeletons, some of them display a remarkable stability in aqueous solution as shown by NMR spectroscopy and ESI mass spectrometry.

Four independent structures of a pentacoordinate silicon species at different points on the Berry pseudorotation pathway

“…As a consequence of their structural and dynamic diversity pentacoordinate silicon compounds are of great current interest. [1][2][3][4][5][6][7] There are two particularly challenging aspects to the chemistry of pentacoordinate silicon: their use as transition state and intermediate models in the commercially and academically important nucleophilic substitution reactions at tetracoordinate silicon; [8][9][10][11][12][13] and the study of their stereochemical non-rigidity. 1,7,14,15 The stereochemical non-rigidity of pentacoordinate compounds and the mechanism of stereomutation can frequently be related to the X-ray crystal structures of a series of closely related compounds.…”

Study of Binuclear Silicon Complexes of Diketopiperazine at SN2 Reaction Profile

“…Evidence for the double Si-X ionization in 3b is presented in Figures 5 and 6: the low-temperature 1 H and 13 C NMR spectra of the 3b feature two signals for the ring CH 2 , one typical of pentacoordination (δ 13 C 51.2 ppm, δ 1 H 4.34 ppm; assigned to the CH 2 adjacent to a pentacoordinate silicon) and the other of tetracoordination (δ 13 C 49 ppm, δ 1 H 4.20 ppm; assigned to the CH 2 adjacent to the tetracoordinated silicon). Similar temperature dependence of the 1 H and 13 C NMR chemical shifts was also observed for the SiMe 2 group. The intensity ratio of these two signals is temperature dependent in a fully reversible fashion, such that at lower temperatures the relative population of the tetracoordinate species (3 000 ) increases.…”

“…Pentacoordinate Binuclear Silicon Complexes. An NMR spectroscopic study of the binuclear pentacoordinate silicon compounds 3b (X = Cl) and c (X = OTf) derived from diketopiperazine examined in CDCl 3 /CD 3 OD at room temperature shows broadening in the 1 H, 13 C, and 29 Si NMR spectra, Table 2. Methanol is an unusual solvent for NMR studies of reactive silanes.…”

“…The intensity ratio of these two signals is temperature dependent in a fully reversible fashion, such that at lower temperatures the relative population of the tetracoordinate species (3 000 ) increases. If this were a result of Si-O bond dissociation or siloxane hydrolysis, there would be two CdO chemical shifts in 13 C NMR spectra at lower temperature.…”

Synthesis and Hydrolysis–Condensation Study of Water-Soluble Self-Assembled Pentacoordinate Polysilylamides

“…In contrast to carbon, silicon displays the ability to form hydrolyzable hypercoordinate silicon species with five, six, and seven substituents. Some such compounds 2 contain two pentacoordinate silicon atoms in a single molecule. − Shea and co-workers − reported the first examples of linear and cyclic polymers 3 with pentacoordinate siliconate groups, using catechol-type ligands. Lambert and co-workers have prepared and characterized pentacoordinate silicon macrocycles 4 based on Tacke ligands, − containing protonated amine groups.…”