A silylium ion is a reactive
silyl-containing cation with the formula . With three rather than the usual four bonds to Si, silylium ions are the silicon analogues of
carbenium ion
The carbenium ion is a kind of cation, positive ion with the structure RR′R″C+, that is, a chemical species with carbon atom having three covalent bonds, and it bears a +1 formal charge. Carbenium ions are a major subset of carbocations, whic ...
s. They can be viewed as protonated
silylene
Silylene is a chemical compound with the formula SiR2. It is the silicon analog of carbene. Silylene rapidly when condensed.
Silylenes are formal derivatives of silylene with its hydrogens replaced by other substituents. Most examples feature ...
s. Early efforts to generate these cations produced salts of the pyridine complex
CH3)3Si-NC5H5sup>+, the hydride-bridged species
Et3Si)2Hsup>+, and the toluene complex
mes)3Si(toluene)sup>+.
Well-characterized silylium salts with well-defined three-coordinate silicon cations trimesitylsilylium and tris(pentamethylphenyl) . These cations are related to
trityl
Triphenylmethane or triphenyl methane (sometimes also known as Tritan), is the hydrocarbon with the chemical formula, formula (C6H5)3CH. This colorless solid is soluble in nonpolar organic solvents and not in water. Triphenylmethane is the basic sk ...
(), with the extra methyl groups providing steric protection, compensating for the greater size of Si vs C. Its
29Si NMR chemical shift is 225.5 ppm, downfield of TMS, which indicates that the cation is quite "naked".
Trimethylsilyl trifluoromethanesulfonate
Trimethylsilyl trifluoromethanesulfonate (TMSOTf) is an organosilicon compound with the formula . It is a colorless moisture-sensitive liquid. It is the trifluoromethanesulfonate derivative of trimethylsilyl. It is mainly used to activate ketone ...
(Me
3SiOTf), normally considered a source of electrophilic silicon, has a
29Si NMR shift of 43 ppm.
Salts of and have been crystallized with the
carborane
Carboranes (or carbaboranes) are electron-delocalized (non-classically bonded) clusters composed of boron, carbon and hydrogen atoms.Grimes, R. N., ''Carboranes 3rd Ed.'', Elsevier, Amsterdam and New York (2016), . Like many of the related boron ...
CB11Me5Br6sup>− and decaborate
12Cl12sup>2-, respectively.
Weakly coordinating anion
Anions that interact weakly with cations are termed non-coordinating anions, although a more accurate term is weakly coordinating anion. Non-coordinating anions are useful in studying the reactivity of electrophilic cations. They are commonly found ...
s are essential for the isolation of these highly electrophilic cations.
See also
*
Silanide
A silanide is a chemical compound containing an anionic silicon(IV) centre, the parent ion being . The hydrogen atoms can also be substituted to produce more complex derivative anions such as tris(trimethylsilyl)silanide (hypersilyl), tris(''tert ...
References
Cations
Reactive intermediates
Silicon
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