Fluorosilanes structure

More illustrative is the structure of the fluorosilane which exhibits the same tricapped tetrahedron geometry. The three N-Si interactions take place in the frontal position towards the Si-F bond instead of the rear coordination observed in penta- and hexacoordinated structures. Furthermore, the benzylamino groups have the possibility not to be coordinated at silicon since these groups have the capability of free rotation around the carbon-nitrogen bond. 

In contrast to fluorosilanes with pentacoordinate silicon systems such as 235-237, it is very difficult to find similar compounds of chlorosilanes to establish the Si—Cl bond length in similar cases. X-ray crystal structure is available for only few of these compounds and only typical cases of the shortest and longest Si—Cl bonds are given. The shortest bond lengths are found in compounds where the silicon is bonded to a metal such as... 

The lithium compound has a polymeric lattice structure in the crystal, formed via Li F contacts. The LiNSiF part of the compound forms an eight-membered ring. The lithium is three-coordinated (sum of the angles = 359.3°)96,97. In the reactions of the lithium derivative with fluorosilanes, exocyclic substitution occurs, e.g., to give 119 (equation 33). The lithium compounds are stable up to about 100 °C. At higher temperatures Li—F elimination occurs and silyl coupled cyclodisilazanes (120) are obtained (equation 34)96,97. 

Krisher, L. C., and L. Pierce Second Differences of Moments of Inertia in Structural Calculations Application to Methyl-Fluorosilane Molecules. J. chem. Physics 32, 1619—1625 (1960). 

Isomeric products are formed in the reaction of lithiated di-rm-butyl-methylsilylhydrazine with fluorosilanes. The formation of these isomers requires prior coordination of the Li+ ion with the two N atoms of the hydrazine unit. The crystal structure of the lithiated di-tert-butylmethyl-silylhydrazine 4 (9)14 exhibits two different silylhydrazide units I and II, which are bound by six Li+ ions to form a hexameric entity. 

The hydrosilane reagents used, where available, were obtained from commercial suppliers such as Pierce Chemical (Rockford, 111.), Peninsular Chem Research (Gainesville, Fla.), and Matheson, Coleman and Bell (Norwood, Ohio). Others were synthesized by the lithium aluminum hydride or deuteride (Alfa Chemical, Beverly, Mass.) reduction of appropriate organic chloro- or fluorosilane, either purchased from the above-mentioned sources or synthesized by conventional routes involving organomagnesium or -lithium condensations with halosilanes. All such compounds were at least 98% pure as determined by vapor-phase chromatography (VPC). Structural identity was established by molecular analysis, infrared, and NMR spectroscopy where necessary. 

An in stereoisomer of fluorosilaphane 260 was obtained as depicted in Scheme 44 in overall 0.4% yield. First, tri(o-tolyl)fluorosilane 258 was fluorinated at silicon with AgF and then brominated with NBS to give tris[2-(bromomethyl)phenyl]fluorosilane 259. Condensation of 259 with l,3,5-tris(mercaptomethyl)benzene yielded the final product 260 <1998JA6421, 1999JOC5626>. The inside location of the fluorine atom was established by X-ray crystallographic analysis. The Si-F bond distance is very short (1.591 A) compared to the mean Si-F distance for all tetracoordinate tris[(alkyl(aryl)]fluorosilanes found in the Cambridge Structural Database. The 19F NMR resonance for 260 appears at a = 5.3 ppm, that is, 155 ppm upfield from that of tri(o-tolyl)fluorosilane 258. 

The synthesis of array L7 is reported in Fig. 8.22. Compound 8.38 was reacted simultaneously with amines (Mi, two representatives), aldehydes (Mi, five representatives), and isonitriles (Ms, two representatives) to give 10 compounds (not all the combinations were reacted). The reaction was performed in trifluoroethanol (TFE), another hybrid fluorous-organic solvent (step a. Fig. 8.22), and after evaporation of the TFE, the crude product 8.39 was purified by two-phase extraction between fluorous solvents and benzene (step b). After evaporation of the solvent, the fluorous tag was cleaved with TBAF (step c) and a triphasic extraction (step d, Eig. 8.22) was performed to remove the fluorosilane tag and acid 8.38-related impurities extracted into the fluorous layer. Excess TBAE and TBAE-related impurities partitioned into the acidic aqueous layer. Yields and purities of the synthetic protocol are reported together with the structures of the library members L7a-j in Table 8.2. 

Careful purification of solutions of fluorosilane and fluorosilicate anions and a decrease in concentration and temperature result in a fine structure of F NMR spectra due to a non-equivalent arrangement of ligands (Table 16). For example, in the F NMR spectra of octahedral complexes of the type RSiFj" a doublet (equatorial atoms) and a quintuplet are observed whose mutual arrangement depends on the nature of the ligand In fluorine derivatives of penta- or hexa-coordinate silicon with slow exchange, the Hgip values (if they can be determined) are greater for the fluorine atoms with chemical shifts in low field. 

The molecular structure is estimated by comparison with SiH I, Sil. and the various bormo-, chloro-, and fluorosilanes (1). 

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