Silicenium Ions: Quantum Chemical Computations

Synthesis of Stable 2-Silanaphthalenes and Their Aromaticity

“…It is known that the silicon center of silenes has a strong Lewis acid character and is stabilized by complexation with Lewis bases such as THF, Me 3 N, F - , etc . Such complex formation results in an upfield shift in the 29 Si NMR spectrum. 1d, The 29 Si chemical shift of 2-silanaphthalene 1a ( δ Si = 87.3) in THF/C 6 D 6 (6:1), however, did not differ from that measured in C 6 D 6 , indicating that 1a does not form a complex with THF. In contrast, as mentioned earlier, the Si chemical shift of Märkl's silabenzene measured in THF/Et 2 O/petroleum ether (4:1:1) 6 is at a much higher field ( δ Si = 26.8) than that of 1a , indicating that it is coordinated with the solvent molecule, most likely THF, and hence stabilized in such a mixed solvent.…”

“…Märkl et al reported the synthesis of a “stable” silabenzene, 2,6-bis(trimethylsilyl)-1,4-di- tert -butylsilabenzene, but this survives only in solution (THF/Et 2 O/petroleum ether, 4:1:1) below −100 °C. The stabilization is, however, apparently due to coordination of the solvent Lewis base, judging from the 29 Si NMR chemical shift ( δ Si = 26.8) observed at a relatively high field (vide infra). 1d,e,, …”

The First Cyclic π-Conjugated Silylium Ion: The Silatropylium Ion Annelated with Rigid σ-Frameworks

“…On the other hand, when the reaction was conducted in dichloromethane- d 2 (CD 2 Cl 2 ) at −50 °C, the 1 H and 13 C NMR signals corresponding to silatropylium ion 2 were observed as shown in Figure and Table . Apparently, C 7 D 8 with its lower polarity and a larger molecular size could not stabilize the cationic species by coordination, 1i-j, while CD 2 Cl 2 with its medium polarity is small in size for approaching the silylium-ion center to coordinate to it. Cation 2 is stable at temperatures below −50 °C, but it slowly abstracts the chloride ion from CD 2 Cl 2 to furnish dichlorosilepin 3 at −50 °C ( t 1/2 ≈ 6 h) .…”

“…Studies on silylium ion in condensed phases have attracted much attention, particularly following the X-ray structural analyses made by Lambert's and Reed's groups. Nevertheless, only a few types of compounds having a silylium-ion character have been reported so far, − because in many cases the silylium ion forms a strong coordinate bond with the solvent or counteranion. 1e-j As one of the representative π-conjugated silylium ions, the silatropylium ion ( 1 ) is of great interest from the viewpoints of its aromaticity, structure, and reactivity. However, its presence in solution has been highly questioned …”

Triethylsilyl Perfluoro-Tetraphenylborate, [Et3Si+][F20-BPh4–], a Widely Used Nonexistent Compound

“…Over the past decade, R 3 Si + silylium ion chemistry has progressed from controversy over their existence − to an exploitation of their potent electrophilicity in stoichiometric reactions and, most impressively, in catalytic chemistry . These advances owe much to the availability of very weakly coordinating anions, , particularly carborane anions and the perfluorinated tetraphenylborate anion (abbreviated F 20 -BPh 4 – or PFTPB – ).…”