Pentacoordinate siliconium complexes

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... 

Selected Bond Lengths and Angles from Crystal Structures of Pentacoordinate Siliconium Complexes and Elimination Product, and Calculated Percent TBP... 

Novel Pentacoordinate Siliconium Complexes Stabilized by Oxygen and Nitrogen Donors Highly Sensitive and Unusual Equilibrium between Ionic Penta- and Neutral Hexacoordinate Compounds ... 

Binuclear Ethylene-Bridged Silicon Chelates Equilibrium between Neutral Hexacoordinate and Ionic Pentacoordinate Siliconium Complexes... 

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. 

Compound 86 exists in two diastereomeric forms due to the chiral silicon centers, which are evident in the various NMR spectra (Table XXV). The crystal structure of 86 shows a molecular inversion center, and hence belongs to the meso diastereomer. The geometry around silicon in the crystal is a distorted TBP and the chloride is more than 7 A away from silicon, in accord with a pentacoordinate siliconium chloride structure. The dative Si bonds are exceptionally short in 86 (1.802 and 1.807 A) relative to those in other pentacoordinate complexes.2,3,1115... 

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). 

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... 

In the presence of bulky X ligands, a facile methyl halide elimination reaction is observed (Eq. 2) [3]. In this elimination the siliconium ion complex 2, with its two N—>Si dative bonds, is converted into a neutral pentacoordinate complex 3, with only one remaining dative bond (Fig. 1, Table 1). The reaction is probably driven by partial release of steric interaction, caused by the removal of one of the A-methyl groups. This is indicated by a decrease in elimination rate in the presence of less bulky ligands, cyclohexyl and isobutyl, and the failure to observe elimination when X = methyl. The reactivity order of the halide ions follows their nucleophilicities F > Br > CF, while the less nucleophilic ttiflate ion does not react at all. 

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