Siliconium chlorides

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 partial and reversible ionization of binuclear hexacoordinate silicon complexes 55a,c,j is described in Section III.A.5.ii. Like the mononuclear siliconium chloride salts, these can also form stable binuclear disiliconium salts (57a,c,j) by replacement of the chloride by other counterions, which are better leaving groups (triflate, bromide, or iodide, Eq. 37).69... 

Summary The first ionic dissociation of the Si-Cl bond in neutral hexacoordinate silicon complexes is reported. An equilibrium reaction between the ionic siliconium chloride and its neutral precursor (dissociation-recombination) is observed. The population ratio can be controlled by temperature or by replacement of the chloro ligand by a triflate group. The reaction enthalpy and entropy of the dissociation are both negative, suggesting that solvent organization facilitates dissociation at low temperature. 

A convenient method for the preparation of neutral bis(N->Si) hexacoordinate silicon complexes has been developed and reported recently, consisting of ligand exchange between a polychlorosilane (1) and 0-trimethylsilyl derivatives of hydrazides (2, Eq. 1) [2]. An attempt to utilize this synthetic route for the preparation of isomeric 0->Si coordinated chelates did not lead to the expected hexacoordinate complexes, but to ionic siliconium chloride salts stabilized by two (O—>Si) dative bonds (5, Eq. 2) [3]. 

This unexpected difference between two apparently similar reactions raises the question of whether the N->Si coordinated octahedral 3 can also, under certain conditions, dissociate to form pentacoordinate siliconium chloride salts. To answer this question we have re-investigated the chemistry of 3 in some detail. We now report on the reversible ionic dissociation of 3 to siliconium salts, and the resulting control of the coordination number of silicon by various means temperature,... 

Table 1. Si chemical shifts for the equilibrium mixtures of 3 and the corresponding siliconium chlorides 6 at two temperatures, and for siliconium triflates (7).

However, the studies of Fujiyama and Munechika55 on the solvolyses of 2-(aryldimethylsilylethyl) chlorides 33 revealed aryl substituent effects which were consistent with the formation of a cyclic siliconium ion 34. 

Stable siliconium-ion salts can also be prepared without exchange of ions, in the case of the bulky -butyl ligand, directly from the TMS-hydrazide and t-BuSiCl3 (Eq. 35).66 The t-butylsiliconium chlorides 93(0) are the only known stable chloride salts of this kind. The chloride can still be exchanged with other anions, as shown in Eq. (18), to produce 93a,c,j bromide, iodide, and triflate.83... 

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... 

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). 

Summary Aryl ligands carrying oxygen, phosphorus or sulfur donor atoms in the side chain are introduced at silicon. Here we report on the synthesis of silyl chlorides and silyl triflates. The interaction between the silicon centre and the donor atom are discussed on the basis of NMR and X-ray crystallographic data. In the reaction of bis[2-(methoxymethyl)phenyl]methylsilyl triflate 8 with nucleophiles, reactivity of oxonium as well as of siliconium ions is observed. 

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