Siliconium salts

The siliconium salts of these counterions are stable (i.e., not in noticeable equilibrium with the hexaco ordinate species) and are discussed in Section IV.B.l. 

The equilibrium reaction between neutral hexacoordinate chelates and pentacoordinate siliconium halide salts is discussed in Section III.A.4 (Eq. 17). This reaction can be driven completely to the ionic side by replacement of the chloro ligand by better leaving groups triflate and bromide (Eq. (18), listed again with compound labels see Section III.A.4.iv). The products of this counterion-exchange reaction are stable siliconium salts 90(OTf)-92(OTf), 90(Br)-92(Br), which no... 

Even the ionization-resistant hexacoordinate complexes with two electron-withdrawing monodentate ligands form stable ionic siliconium salts when treated with A1Q3, as shown in Eq. (36).66... 

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... 

Fig. 47. Molecular structure of siliconium salt 90a(OTf) in the crystal.66 Reproduced with permission from the American Chemical Society.

Si Chemical Shifts of Various Siliconium Salts (ppm, in CDC13, 300 K)... 

The binuclear siliconium salts 57 have two chiral centers at the silicons and hence exist as dj and meso diastereomers. Only the latter isomers were isolated and characterized by crystallographic analysis mes6>-57a(OTf) and meso-51c(OTf) (Figs. 52 and 53). The characterization is further confirmed by the 29Si chemical shifts (Table XXVI) which are in agreement with those for similar mononuclear siliconium salts. 

Fig. 54. Molecular structures of the siliconium salt 93a(OTf) (left) and the elimination product 96j (right) in the solid-state.83 Reproduced with permission from Wiley-YCH.

In contrast to the nearly equal geometries of these mononuclear siliconium salts, the binuclear siliconium salts have a significantly different geometry as the data in Table XXYII show, 57a,c have equal O-Si-O and N-Si-N angles, and hence are further advanced along the TBP -> SP coordinate to 100% SP, i.e., both binuclear molecules in the solid-state have essentially pure SP geometries. 

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,... 

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. 

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... 

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 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... 

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... 

Steric Effect on the Formation, Structure, and Reactions of Pentacoordinate Siliconium Ion Salts... 

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]. 

Corriu and Henner [80] have examined the siliconium-ion question and have failed to prove the existence of siliconium ions by physiochemical methods, the preparation of stable salts, or identification of reaction intermediates. They conclude that the high affinity of silicon for nucleophiles explains the failure to prove the presence of such ions, but they point out that it is not possible to completely exclude their existence. 

Kalikhman 1, Gostevskii B, Sivaramakrishna A, Kost D, Kocher N, Stalke D (2005) Steric effect on the formation, structure, and reactions of pentacoordinate siliconium ion salts. In Auner N, Weis J (eds) Organosilicon chemistry VI from molecules to materials. Wiley, Weinheim, pp 297-302... 

The synthetic efforts toward the isolation of silylium salts with an ideal trigonal-planar coordinated positively charged silicon atom in the cation created a series of stabilized sUyl cations in which either the interaction with the solvent, the counteranion, or intramolecular donor groups pacify the high electron demand of the silyl cation. This electron donation leads to cationic species 7 in which the silicon atom adopts a distorted tetrahedral coordination environment (Scheme 2) [9]. Examples for siliconium ions 8 in which the silicon atom has expanded its coordination number to 5 by addition of two solvent molecules are known but are less frequently observed. Intermolecular species 7 and 8 as well as intramolecular variants 9 and 10 for both modes of stabilization have been structurally characterized [9, 28, 29]. 

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