Silicenium cations

When the counterion is complex, for example metal-halogen anions such as BF4-, the most electronegative portion of the counterion becomes attached to the silicon center. Because of this attachment, it is natural to consider the intermediacy of a silicenium cation (silylium or silylenium ion) intermediate in such reactions (Eq. 4). Bond energies derived from electron impact studies indicate that Eq. 4 is exothermic in the gas phase by about 8 kcal/mol.26,29 There seems little doubt that trivalent silicon-centered cationic species do exist in the gas phase30,31 or that processes similar to that shown in Eq. 4 do occur there.32,33... 

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... 

In a recent publication, Lambert and Zhang40 have presented a promising new procedure for isolating silicenium cations (equation 2)... 

The postulated silicenium cations are covalently bonded C-centered -complexes between SiR3+ and aromatic solvents. Formally, these may be described as Wheland a complexes, in which the silicenium cation character is totally lost. 

It must be added that a new theoretical calculation of triarylsilicenium moieties nicely reproduced the experimental results of 14d48. The calculated 29Si chemical shift is 251.4. The 25.0 ppm difference may be the result of the applied computational method. Thus, the long debated controversy of the silicenium cation problem seems to be solved49. 

Since 1989, when the first volume of The Chemistry of Organosilicon Compounds appeared1, large strides have been made in preparing silicenium cations. The first X-ray structures2,3 of solvent stabilized silicenium cations are among the most notable successes. Most recently, a free silicenium cation has been prepared in solution4 5. In addition... 

IV. QUANTUM MECHANICAL STUDIES OF SILICENIUM CATIONS A. Thermodynamic Stability of Silicenium Cations... 

TABLE 6. Estimated ranges of <529Si chemical shifts (ppm) for silicenium cations in the gas phase and in solution0 b... 

A high-level theoretical investigation of the structure of the experimentally studied compound 160a and some of its analogs is described245. The question of solvation of a silicenium cation is addressed and 160a served as a model245. 

Silyl migration in chloromethylpentamethyldisilane 33 X=Y=Si has been known to occur thermally or more easily in the presence of aluminum chloride70. Similar rearrangements promoted by a Lewis acid were observed in (chloromethyl)pentamethylgermasilanes71. Presumably, the reaction does not involve a carbenium or a silicenium cation as a discrete reactive intermediate but proceeds via the cyclic transition state 34 involving aluminum chloride participation (equation 22). 

A similar coordirmtion sphere at silicon is found in unsaturated Si=E compounds (E = element of group 14-16) and their donor adducts (coordination numbers 3 and 3+1). Attached to coordination number three at silicon the existence of silicenium cations is still controversely discussed in the literature. While some experiments are interpreted as evidence for their existence in the condensed phase, theoretical calculations do not support the formulation of donor free silicenium cations. However, there seems to be no doubt about the verfication of inter- and intramolecular donor stabilized cationic species (coordination numbers 3+1 and 3+2, respectively). 

The photolysis of an equimolar amount of SiL 2 and Cp2Mo(PEt3) in hexane results in the formation of the molybdenum-silylene complex Cp2Mo(SiL 2) (3) (Eq. 5). Complex 3 is a deep red, extremly air- and moisture-sensitive solid showing a Si-NMR signal at 139.9 ppm. This is significantly downfield-shifted in comparison both with the free silylene SiL (78.3 ppm [11] and with other silylene complexes ((CO)2Ni(SiL 2)2i 97.5 ppm (CO)5Mo iPhAr 97.5 ppm [12]) indicating a distinct silicenium cation character of 3. 

Relative to hydrogen, all the first-row substituents, with the exception of fluorine, stabilize the silicenium ion. The greater stability of a-substituted silicenium cations versus their corresponding carbocations can be attributed in part to the lower electronegativity of silicon (1.7) compared to carbon (2.5). Equation 3 also takes into account the relative stability of alkanes versus silanes, i.e. the stability of C—H versus Si—H bonds. The Si—H bond is weaker than the C—H bond by ca 13 kcalmoU in the gas phase, e.g. the bond dissociation energy D//2Qg(C—H) in CH4 is 104.9 kcalmoU, compared... 

Lambert has used ethyl substituents R, a weakly coordinating anion and toluene as the solvent to stabilize and crystallize the silicenium cation SiRj. The synthesis of SiRj" is favored by a driving force, the formation of a C-H bond. - ... 

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