Spirosilicates

The zwitterion X5-spirosilicate bis[2,3-naphthalendiolato][2-(dimethylammonio)phe-nyl]silicate (56 isolated as 57 = 56-1/2 MeCN) was synthesized by reaction of [2-(dimethylamino)phenyl]dimethoxyorganosilanes with 2,3-dihydroxynaphthalene in acetonitrile at room temperature. Reaction of 57 or of [2-(dimethylamino)phenyl]trimetho-xysilane with water in acetonitrile yielded the cage-like silasesquioxane 58 (R = 2 — Me2NCgH4). The crystal structures of 57 and 58 were studied by X-ray diffraction. In addition, 57 and 58 were characterized by solid-state 29Si CPMAS NMR142 (Figure 34). 

Several synthetic pathways for the preparation of spirosilicates have been reported ... 

The reaction of polyalkoxysilicate anion with pinacol (equation 10). When catechol was used instead of pinacol, a hexacoordinate spirosilicate was formed. 

The dilithio derivatives of catechol and hexafluorocumyl alcohol react with organo-trichlorosilanes to give spirosilicates (equations 11 and 12)34,35. 

The most intriguing synthetic approach is the conversion of inorganic silica to spirosilicate by the reaction with a 1,2-diol and alkali hydroxide (equation 13)36. 

Anionic five-coordinated silicates have been prepared also with oxygen-containing six- and seven-membered chelate rings (16-18), using suitable aromatic diols, 1,8-dihydroxynaphthalene and 2,2 -dihydroxybiphenyl, respectively37. However, spirosilicates... 

A compilation of 29 Si chemical shifts for bis-spirosilicates and related acyclic oxo-silicates is given in Table 3. It can be seen that (529Si is primarily dependent on the monodentate ligand attached directly to silicon, in a manner similar to that found in neutral silanes40. 

No simple correlation can be found between the solid state calculated percent (TBP — SP) deviation and the 29Si chemical shifts in solution. Table 4 demonstrates this analysis for complexes sharing the same ligand environment at silicon, four oxygens and a phenyl group the crystal structures of four closely related spirosilicates (19, 21, 49, 50) were... 

TABLE 5. Comparison of ligand-exchange barriers with percent (TBP — SP) deviation in the crystal for spirosilicates 5135 ... 

The crystal structures of several members of the zwitterionic complex family were determined. Like in the anionic spirosilicates, the structures were found to vary continuously between TBP and SP, with Si—O bond lengths essentially equal to those in anionic analogs. More than one crystalline modification was found for some of the zwitterions, and even those (even for the same complex ) were substantially different in molecular geometry for 60 the percent deviation from TBP - SP was calculated to be 34.9% in the monoclinic crystal, 70.0% in the orthorombic, 86.2% in the crystalline hydrate and 96.3% in another crystal form of the monohydrate46 47. These results were interpreted in terms of variation in hydrogen bonds in the different crystal modifications. In the hydrates... 

A bridged bis-spirosilicate with an ethylenedioxy bridge (85) (i.e. based on the SiOs ligand framework) was prepared directly from inorganic silica and ethylene glycol in the presence of potassium hydroxide (equation 13)36. 

The preparation of a novel neutral zwitterionic k5-spirosilicate and its characterization was reported238. 

This pattern was anticipated by Holmes14 on the basis of the tendency to structural distortion in the solid state displayed by the spirosilicates 25-27 and related compounds, compared with the corresponding phosphoranes, and may be attributed to the greater effective radius of Si" over P, typically73 some 0.07-0.15 A. 

The short reaction time, the mild reaction conditions as well as the high yields and the selectivity proved the hydrolytic oligocondensation of zwitterionic A, -spirosilicates to be an appropriate method for the synthesis of organo-functionalized octasilsesquioxanes [5]. The synthesis of 1 required a modification of the reaction conditions and an extension to ionic A, -spirosilicates. 

Pyridiniumbis(2,3-naphthalenediolato)-4-(trimethylsilylethynyl)phenylsilicate (3), a new ionic A, -spirosilicate, was synthesized in seven steps starting from 4-bromoacetophenone in an overall... 

Scheme 1. Synthesis of I via hydrolytic oligocondensation of the ionic X -spirosilicate 3 (dmso = dimethyl-sulfoxide).

Conversely, the spirosilicates 16 and 19 (Table 4), having five- and six-membered chelate rings, respectively, are reported to have essentially the same degree of deviation from TBP toward SP structure in the crystal (32.5 and 29.5%, respectively), while their Si chemical shifts (22 is taken as an analog of 19) are dramatically different (—132.35 and —87.51 ppm, respectivelyNo explanation has been offered for this difference. The Si chemical shift in 16 compares favorably with shifts measured in acyclic oxo-silicates, 46 and (Table 3). It is tempting to conclude that steric strain has a... 

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