Silicates anions

In a sophisticated variation of the Knoevenagel condensation ("Panizzi ) methyl 3,3-dimethoxypropanoate (from ketene and dimethoxymethenium tetrafluoroborate D.J. Crosby, 1962) is used as a d -reagent. Because only one carbonyl group activates the methylene group, a strong base with no nucleophilic properties (p. 10) has to be used. A sodium-sand mixture, which presumably reacts to form silicate anions in the heat, was chosen... 

Potassium silicate solutions are equally complex for example, an aqueous solution prepared from KOH and Si02 in which the ratio K Si is 1 1 contains 22 different discrete silicate anions as identified by Si COSY nmr studies. 

At pH levels above 9.0, there is an increased tendency for the soluble silica to form silicate anions and react with magnesium (and to a lesser extent with calcium) to form insoluble forms of magnesium silicate that precipitate to form crystalline scales and sludge. 

The synthesis of T8[OSiMe2H]8 was first reported by Hoebbel et al. from the reaction between TgLONMeJs and chlorodimethylsilane in dimethylformamide (DMF)/heptane (Table 18, entry 1). Other authors have modified the procedure by changing the solvents to tetrahydrofuran (THF) or hexane, or by using TslONMes (C2H40H)]g as the silicate anion source, but without significant improvement in the yield (Table 18). 

The most abundant compounds of silicon are Si02 and the related silicate anions, all of which contain Si—O bonds. See Sections 9.3 and 10.3 for descriptions of the structure and bonding of these compounds, which involve a bond networks and tetrahedral geometry. As already mentioned, many minerals are combinations of hard silicate... 

Silicagel. Silicagel is usually prepared from sodium silicates, which are produced as glasses with a stoichiometry of Si02/Na20 = 0.6-0.25. At the right pH they arc soluble in water and the solution contains mixtures of silicate anions monomeric, dimeric, trimeric species, etc. (Fig. 3.13). 

It was found that treatment of a mixture of 120 and 121 with tris(diethylamino-sulfonium) trimethyldifluorosilicate [TASF(Et)] resulted in smooth addition-elimination to the naphthoquinone to form the y-alkylation product 125 (85 %). TASF(Et) is a convenient source of soluble, anhydrous fluoride ion [47]. It is believed that exposure of 121 to TASF(Et) results in fluoride transfer to generate a hypervalent silicate anion, as depicted in structure 124. The transfer of fluoride between TASF(Et) and 121 may be driven by stabilization of the anionic species 124 by delocalization of the carbon-silicon bond into the LUMO of the unsaturated ketone. 1,4-Addition-elimination of this species to the naphthoquinone 120 would then form the observed product. 

M ions (among which Na+ from NaCl) interact with the aluminosilicate gel (and eventually direct its structure towards a zeolite framework by template effect), more readily than the Na+ which were originally associated with the silicate anions. 

The study of compounds containing pentacoordinate silicon atoms currently represents one of the main areas of research in silicon chemistry. This is evident from the numerous reviews and proceedings published on this topic in recent years.112 Most of the pentacoordinate silicon compounds described in the literature are either salts with A5.S7-silicate anions or neutral silicon complexes with a 4+1 coordination to silicon. This review deals with a completely different class of pentacoordinate silicon compounds zwitterionic A S /-silicatcs. These molecular compounds contain a pentacoordinate (formally negatively charged) silicon atom and a tetracoordinate (formally positively charged) nitrogen atom. 

Figure 1. Scheme for the liquid crystalline templating mechanism proposed by Kresge et al 1 for synthesis of mesoporous silica MCM-41. Formation of a hexagonal array of cylindrical micelles possibly mediated by silicate anions followed by condensation of the silicate anions from the silicate source (tetraethylorthosilicate) leads to templated framework structure. Calcination or extraction of the template produces hexagonally ordered mesoporous silica. 

The mechanism of silicon etching in alkaline solutions is a process of material dissolution with a simultaneous hydrogen evolution. The main soluble product is a silicic anion Si02(0H)2 that can further be condensed to form polysilicic anions. In fact, due to the acido-basic ionization of OH radicals in a highly alkaline solution, Eq. (19) should be modified as follows ... 

Zeolitic structures with pore sizes of 2000 to 10000 pm are known as mesoporous solids, and can be formed by a method known as liquid crystal templating (LCT). The combination of a suitable cationic surfactant together with silicate anions form arrays of rod-like surfactant micelles (Figure 3.7) surrounded by a polymeric siliceous framework. On calcination the mesoporous structure is formed. 

Figure 7.2 Silicate anion structures (o) orthosilicate, (6) pyrosilicate, (c) three-silicate ring, (d) six-silicate ring, (e) pyroxene, (/) amphibole, and (g) phyllosilicate.

Silica and silicates are soluble in aqueous alkali.9,15-17 The silicate anions produced in solution may be regarded as derived from silicic acid [H4Si04 or Si(OH)4], which has a first pK of 9.47 at 0.6 mol L"1 ionic strength ... 

The aqueous chemistry of aluminum(III) above pH 6 differs from that of silicates in that the only important species, other than solid Al(OH)3 at pH 5-8, is Al(OH)4, which, although isoelectronic with Si(OH)4, shows no tendency to catenate. On the other hand, below pH 5 Alm, unlike the poorly soluble Si(OH)4, is freely soluble as Al3+(aq) [actually Al(OH2)63+, Section 13.2], while at intermediate pH hydrolytic A1 species, including the ion Ali304(0H)24(0H2)127+ referred to above, predominate in solution. However, Al(OH)4 units can readily insert themselves into silicate anion species in solution. The result is usually the prompt precipitation of an aluminosilicate gel (a typical zeolite precursor), although over some limited Al, Si, and OH- concentration ranges quite high concentrations of dissolved aluminosilicates can be maintained over many months.9... 

These are of great importance in the makeup of various minerals since about three-fourths of the earth s crust is silicon and oxygen. Simple silicate anions. SiO -... 

Polymeric chain, band- and sheet silicate structures have been discussed previously (page 742), and it should not be surprising to learn that cyclic silicate anions, such as [Si3Oy]ft and [Si601B]l2 (Fig. 16.28) are known. These anions are sometimes referred to as metasilicates in line with the older system of nomenclature, which assigned vrtlto to the most fully hydrated species [as in orthosilicic acid." Si(QH)J and metu to the acid (and anion) from which one mole of water has been removed [either in fact or formally for example, "metasilicic acid," OSi(OH)2]. 

As has been mentioned above, a minor part of the TMA content of omega consists of material imbibed in the wide channel system and associated with hydroxyl and or silicate anions. We designate this cation site II. It is not really a cation site in the usual sense of the word, but merely an ill-defined place where salt can be trapped. 

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