Silicenium ions stabilities

The first few pentacoordinate cationic silicon complexes (153-155) were discussed in an earlier review2. The first two were described as trivalent silicenium ions stabilized by mtermolecularcoordination183 184, while 155 is an intramolecular silicenium ion21a. Since then, a few other compounds were reported. Oxidation of the dihydrido-hexacoordinate chelate 156 by the addition of excess iodine produced 157185. The X-ray structure of 157 showed a slightly distorted TBP for the cation, with a well separated (Si—I distance 5.036 A) I8 2 anion for every two cations. The nitrogens are in the apical positions, and both Si-N distances (2.08 and 2.06 A) are longer than covalent bonds but well within the coordination distance. 

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... 

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 ... 

Miscellaneous Halonium Ions. The Si-containing ring system 194 analogous to 188 has been reported by Muller and co-workers.422 The silyl cation is stabilized by intramolecular interactions with the F atoms increasing the coordination number of Si and, consequently, Si becomes considerably shielded. In the 29Si NMR spectrum, the resonance at 829 Si 77 indicate that cation 194 has only a small silicenium ion character... 

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... 

Free tricoordinate silicon cations (silicenium) are exceedingly unstable, and have only recently been first realized.74,75 A silicenium ion can be stabilized by coordination with two intramolecular donors, to form a stable (pentacoordinate) siliconium complex. These have been reported with... 

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