Silicenium ion

Molecules with trivalent positive silicon (R3Si+) are termed either silicenium ions (in analogy to carbenium ions) or silylenium ions (derived from silylene as carbenium is derived from carbene). Silicenium ions have been known and studied in the gas phase for over 20 years302. In solution, silicenium ions have been sought for more than 40 years258 

Theoretical aspects of organosilicon compounds TABLE 36. Citations to miscellaneous silylenes 

Silicenium ions received some theoretical attention, but to a smaller extent than silylenes. Interest centered on their thermodynamic stabilities relative to the analogous carbenium ions and on substituent effects. These are important questions, because the low thermodynamic stability of silicenium ions was believed by many workers to be one of the major factors frustrating their experimental preparation258. 

The parent H3Si + cation is calculated to be planar (D3h symmetry)307 with r(Si-H) = 1.454 A307c. At all levels of calculations H3Si+ is substantially more stable than H3C +, relative to the corresponding hydrides (equation 43). 

The most accurate calculations for the heat of formation of H3Si + were carried out at the MP4SDTQ/6-31G + G(2d, f J//6-31G level giving AH° 298 = 234.1 kcal mol 1 307b c, 

Olah and Field612 were able to obtain only a polarized complex 245 from methylsilyl bromide and aluminum bromide in methylene bromide solution. However, they were able to correlate 29 Si NMR chemical shifts with 13C NMR chemical shifts of analogous compounds. B ased on such an empirical relationship, they have been able to predict 29 Si chemical shift of trivalent silicenium ion. 

Lambert and Schulz613 have prepared triisopropylthiosilicenium ion 246 by hydride abstraction from triisopropylthiosilane using trityl perchlorate in dichloro-methane solution [Eq. (4.160)]. 

Since the late 1980s a variety of approaches has been employed to generate silicenium ions.606,617-621 Most of the results, however, have been challenged.606,620,622-628 

A report by Jprgensen and co-workers642 about the synthesis of the first chiral tertiary alkylsilicenium ion 248 was also questioned. DFT/IGLO NMR studies showed627 that it is a silylated acetonitrilium ion. Calculations (B3LYP/6-31G level) to find a minimum energy structure for the free silicenium ion failed. Instead, the intramolecularly silylated spirocyclopropylarenium ion was found. 

It became clear after these efforts that the successful synthesis of free silicenium ion requires the use of a counterion as inert as possible. Furthermore, it is necessary to hinder the Si+ environment with bulky, sterically demanding groups and electronically stabilize the cation by appropriate substituents to suppress any interaction with the anion or the solvent.643,644 

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The existence of trivalent silicenium cations as reactive species in solution is more controversial. Many early attempts to demonstrate the solution-phase existence of stable silicenium ions by using techniques analogous to those successfully applied to carbocation formation failed.34-36 Other reports of attempts... 

The results indicate that the formation of long-lived trimethyl substituted silyl cations, in the presence of aromatic solvents, as claimed by Lambert et al.95 is not feasible under these conditions. Persistent silicenium ions require sterically more shielding substituents at silicon or hypercoordinative stabilization.96 98 13C and 29Si NMR chemical shifts were calculated for a series of disilylated arenium ions 85 using density functional theory (DFT). The calculations predict consistently the unsaturated carbon atoms to be too deshielded by 8-15 ppm. Applying an empirical correction, the deviation between experiment and theory was reduced to -0.4 to 9 ppm, and the 13C NMR chemical shift of the highly diagnostic cipso is reproduced by the calculations (Ad = -3.8 to 2.7 ppm).99... 

Chojnowski and co-workers have studied the polymerization of octamethyltetrasila-l,4-dioxane, a monomer more basic than cyclosiloxanes, which is capable of forming more stable oxonium ions, and thus being a useful model to study the role of silyloxonium ions.150-152 In recent work, these authors used Olah s initiating system and observed the formation of oxonium ion and its transformation to the corresponding tertiary silyloxonium ion at the chain ends.153 The 29Si NMR spectroscopic data and theoretical calculations were consistent with the postulated mechanism. Stannett and co-workers studied an unconventional process of radiation-initiated polymerization of cyclic siloxanes and proposed a mechanism involving the intermediate formation of silicenium ions solvated by the siloxane... 

The photochemical reduction of 1-methylquinolinium ions by (TMS)3SiH proceeds regioselectively to afford the corresponding 1,4-dihydroquinones in a water-acetonitrile solvent system (Reaction 4.47) [83]. Mechanistic studies demonstrated that the reactions are initiated by photoinduced electron transfer from the silane to the singlet excited states of 1-methylquinolinium ions to give the silane radical cation-quinolinyl radical pairs, followed by hydrogen transfer in the cage to yield 1,4-dihydroquinones and silicenium ion. Silyl cations are quenched by water. 

The silicenium ion (LXXXII) has been proposed as an alternative to the oxonium ion as the propagating center [Sigwalt and Stannett, 1990],... 

Many other examples of this combination of techniques will be found in various parts of this review. Probably the most persuasive example of this combination are the criteria applied in attempts to establish the structure of silicenium ions. This topic will be discussed in the next section. 

B. Towards Free Silicenium Ions in Solution How Can They be... 

Equation 1 compares the stability of substituted silicenium ions versus the parent SiH3+ (1). Equation 2 compares the stabilities of the correspondingly substituted carbocations versus CH3+. Equation 3 compares the stabilities of silicenium ions with those of the corresponding carbocations. The calculated energies of equations 1-3 are presented in Table 1, and the following conclusions can be drawn51 ... 

Let us first have a closer look at trialkyl-substituted silicenium ions2,3 since, four years ago, the X-ray structures of these species started a lively debate on silicenium ions. 

The first X-ray structures The first stable tricoordinate silicenium ions ... 

X-ray crystal structures of silicenium ion candidates have been reported as early as in 198369, but in all cases it turned out that the silicon atom formed a covalent bond to a neighbouring solvent molecule or to the counterion70,71. In 1993, both Lambert s2 and... 

Olah and coworkers77 further characterized Reed s silicenium ion 3 as a silylated bromonium ion6 [exp. R(Si—Br) = 2.479 A]. Despite all the criticism and theoretical evidence, both Lambert and Reed defended their own interpretation of structures 2 and 3 as being chemical entities with predominant silicenium ion character75,76. 

Quantum theory sheds more light on the character of such silicenium ion-Lewis base complexes, e.g. on the nature of the bonds formed. Are they covalent Do d-orbitals contribute Flow large are the complexation energies and how do they change upon substitution Can free tricoordinate silicenium ions be formed at all in 7r-donor or aromatic solvents ... 

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