Siliconium cations

In this progress report we have reviewed the latest developments in the large area of cationic low-coordinated species and their coordination with Lewis donors. It is clear that these species are of a broad interest, in particular for catalysis. In some cases, e. g. the methylene phosphenium cation, the donor adducts also open new routes for synthesis. Regarding the mechanism for the diverse donor-addition reactions, the structural details are only poorly understood and need a better classification. In particular the variation of the Lewis-donor has to be established. Hitherto in most cases iV-donation is studied. It includes amines or pyridines. Obviously the effect of other donors, such as phosphines, thioethers needs to be studied as well. The siliconium cation for which these effects are better known could provide an understanding for further investigations within this field. 

Alternatively, the pentacoordinated dihydrosilane 791 can be reacted with suitable electrophiles, such as I2, PhCOCl, PhCOBr, Ph3C+BF4, CF3S03SiMe3, to give the same trivalent siliconium cations having different... 

The first hydrazide-based siliconium-cation salts were obtained during an attempt to prepare neutral, hexacoordinate bis-chelates with O Si coordination (80), in analogy to the extensively studied isomeric N Si coordinated 30-38 (see Section III.A.l). In analogy to Eq. (15), the A-trimethylsilylhydrazides 81 were allowed to react with polyhalosilanes 22-25 (Eq. 33). However, in contrast to Eq. (15), the expected 80 were not... 

For several of the siliconium salts crystal structure analyses were obtained, confirming the pentacoordination and the ionic nature of the compounds (well separated cations and anions). The crystal structures for 90a(OTf), 90c(OTf), 91a(OTf), 91a(AlCl4), and 93a(OTf) are depicted in Figs. 47-51, respectively. Further structural support is found in the 29Si NMR chemical shifts (Table XXVI). A remarkable observation in Table XXVI is the nearly equal 29Si chemical shifts of siliconium salts sharing the same silicon complex, but with different anions [e.g. 91a(OTf), 91a(Br), and 91a(AlCl4)] the equal shifts are the evidence that the siliconium cations are essentially independent of... 

The structural evidence for 97, 98 came from their characteristic 29Si chemical shifts (Table XXVIII), and a crystal structure analysis for 97a(OTf), the triflate salt derived from anion exchange with 97a (Fig. 56, Table XXIX). Table XXVIII shows that these two salts, the chloride and triflate, have the same 29Si chemical shifts, thus confirming the identical siliconium cation parts in both. In fact, from Table XXVIII it is also evident that the NMR spectra for the two siliconium... 

Keywords siliconium cations, Hexacoordinate silicon, binuclear silicon complexes... 

Further support comes from calculation of 1 Jch spin-spin coupling constants at IGLO-DFPT/BIII level. The data were interpreted as giving strong evidence for intramolecularly TT-stabilized silanorbomyl cation structure lacking coordination to solvent or counterion. The species can also be regarded as an almost symmetrically bridged /J-silyl carbocation with siliconium ion character. 

A special case of reversible ionization of a hexacoordinate silicon complex has been described as a novel tautomeric equilibrium.41 It differs from the formation of siliconium-ion salts in that the positive charge resides on nitrogen, in a dimethylammonium cation, and not on silicon. The transsilylation of lg with 12 in equimolar concentrations leads to the pentacoordinate zwitterionic complex 13 (Eq. (10), Section II.B.5). However, when the molar ratio was 2 1, respectively, an equilibrium mixture of tautomers (58, 59) was obtained, as shown in Eq. (21). The same mixture was also obtained when a second mole-equivalent of lg was added to 13. 

We make a distinction between two types of ionic pentacoordinate complexes those which are in dynamic equilibrium with neutral hexacoordinate complexes have been dealt with in Sections III.A.4, III.A.5.ii, and III.B.2. The second group includes those pentacoordinate siliconium-ion salts which are formed as such and are stable and do not equilibrate (to a noticeable extent) with their hypothetical neutral hexacoordinate counterparts. The present section discusses this group of persistent salts of pentacoordinate silicon cations. 

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... 

B. Silyl Cations Stabilized by Nitrogen Donor Groups 1. Dimethylaminoimidato-Coordinated Siliconium Complexes... 

There are comparatively few cationic complexes, the most important being the octahedral /3-diketonates and tropolonates of Si and Ge such as [Ge(acac)3]+ and Si trop3+. Siliconium ions can also be formed by reactions such as... 

Intramolecularly coordinated silyl cations are known since the pioniering studies of Corriu [5] and Willcott [6] both using a tridentate substituent at silicon. Siliconium ions bearing the bidentate PTMH or DMBA substituent may be obtained via different pathways (Scheme 3). 

The signal set of modification I is very similar to the spectrum of cationic complexes such as 2-OTf (compare Figs, lb and Ic), thus identifying this modification as a pentacoordinated siliconium compound. Moreover, addition of 1 equiv. ClAlMej to a solution of 4 in CDCI3, thereby transforming the chloride of 4 into the less nucleophilic Me2AlCl3 " anion, reduces the complex H NMR spectrum (Fig. lb) to that of cationic complexes (compare Fig. Ic). 

By considering the fact that the disproportionation proceeds over acidic catalysts, a possible intermediate is a cationic one. A siliconium ion and a silicenium ion are the possible intermediates. The former is produced by... 

In carbon chemistry, the term carbenium for trivalent, sextet, positively charged carbon was derived from carbene, for divalent, sextet, neutral carbon. Because divalent silicon is commonly called silylene, the appropriate term for trivalent, sextet, positively charged silicon is silylenium. The alternative but commonly used term carbocation for all positively charged carbon systems finds analogy in silyl cation (much less often, silico-cation). We will use both these naming systems in this discussion. The term siliconium, like carbonium, denotes the highest valency (pentavalency), as in SiH3. The term silicenium for the trivalent system has been discarded since silylene rather than silicene has been accepted as the appropriate term for the divalent state. 

No firm evidence is available to exclude either type of the intermediates (C) and (D). Stable examples of both are now well-documented in phosphorus159 as well as in silicon chemistry (Section II). The participation of pentacoordinate siliconium ions in chlorosila-cyclobutane isomerization160 and of analogous cationic species in the racemization of triorganotin halides161 has been supported. The possible intervention of hexacoordinate intermediates in substitution reactions at phosphorus has often been considered162. 

In summary, the silanorbomyl cations 5 and 25 are stable compounds in arene solvents at ambient temperature, lacking direct coordination to solvent or counterion. They are stabilized by intramolecular Jt-interaction and can be therefore regarded as an almost symmetrically bridged p-silyl-substituted carbocation with siliconium ion character. 

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