The Silylium Ion Problem

Several reviews appeared on the heavier congeners of the carbenium ions. Clearly, the silylium ion problem has received the most attention, and both theoretical as well as experimental aspects have been reviewed. The chemistry of cationic germanium, tin and lead is covered by a recent review by Zharov and Michl. We will concentrate in this review on the description of the progress made during the last 4 years and will try to give an account on the synthesis, the properties and the structure of organosubstituted three-coordinated, tiivalent group 14 element cations and closely related species in the condensed phase. 

In Sections 3 and 4, the history of the silylium ion problem and the basis for a systematic description of silylium ions in solution are presented. So far, none of the protagonists of the scientific feud on the nature of silylium ions in solution has tried to clarify what is meant by terms such as silylium ion character, free, nearly free, or coordinated silylium ions, degree of complexation, and the type of interactions between solute and solvent. Of course, this has to do with the fact that depending on the experiments performed just one or two properties of the solvated silylium ions can be measured, and this is not sufficient to support basic definitions. However, calculations provide a variety of different silylium ion properties and, thereby, a basis can be established to give a rather conclusive description of silylium ions in solution. 

Reed, C. A. The Silylium Ion Problem, RjSP. Bridging Organic and Inorganic Chemistry, Acc. Chem. Res. 1998, 31, 325-332. A review by a major contributor to the field. 

Until the year 2002 no experimental data existed on the structures of unperturbed R3E+ cations, the exact analogues of the carbenium ions. Computational data combined with NMR chemical shift calculations, which could be compared to experiment, were the only source of reliable structural information for silylium ions6,7,13,77,121 while for germylium, stannylium and plumbylium ions this combined approach was not attractive due to either the non-existence of the experimental data (Ge) or the complexity of the computational problem (Sn, Pb). On the other hand, a series of excellent experimental studies demonstrated, for example, the high coordination tendency of small trialkylsilylium ions either toward the counteranion38,114,127,138 or toward the solvent.36,37,67,116,127 The solid state structures of these silyl cation salts showed clear indications either of cation/anion or cation/ solvent coordination. Thus, the nature of the observed cation, i.e. the degree of silylium ion character remained disputable.10,11,13... 

In Sections 5, 6 and 7, three different approaches to the problem of silylium ions in solution are described. First, the typical gas phase versus solution phase ab initio (DFT) description of silyl compounds and silylium ions is given (Section 5). In Section 6, the NMR/ab initio/IGLO and NMR/DFT/IGLO methods are used to investigate solvation of silylium ions in different solvents. This work demonstrates how complex the solvation process of a silylium ion can be and, therefore, there is a need to generate silylium ions under well-defined situations in solution which simplify investigations. Out of this necessity, the idea of intramolecular solvation of silylium ions was born, which is discussed in Section 7. 

The major problem encountered during the synthesis of silylium ions is their inherent high electrophilicity and reactivity, which leads such species to interact in solution or the solid state with solvents or counteranions that are found to be innocent in other areas of chemistry. The ability of silicon to expand its coordination... 

The oxidation of sterically hindered disilanes by trityl cation in pivaloyl nitrile was shown to yield silylium ions which form with the solvent silylnitrilium ions, e.g., 38 (Scheme 14) [53]. The generatitMi of silyl cations was, however, only observed for sterically overloaded disilanes, such as hexa-tcrt.-butyldisilane. Hexaethyldisilane for example was found to be inert under the applied reaction conditions [53], In view of the problems with bulky silanes in the standard hydride-transfer reaction, the oxidation of disilanes is a complementary addendum to the synthetic methodology. The very limited preparative access to stable radicals restricts the oxidation of silyl radicals to give silylium ions to only a few selected examples. The most prominent example is the oxidation of the stable radical 39 to the homoaromatic silylium ion 40 by trityl cation (Scheme 15) [23]. 

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