Chelates of silicon

Most pentacoordinate bis-chelates of silicon with two N Si dative bonds have the TBP1 geometry. Clearly this geometry is preferred. It is only... 

Summary Treatment of 81X4 (X = Cl, Br) with bidentate ligands 2-[(7V-benzylimino)-phenylmethyl]-5-methoxyphenol l H and 8-hydroxyquinoline L H leads to hypercoordinate bis- or tris-chelates of silicon. These complexes were characterized by means of multinuclear NMR spectroscopy. The X-ray crystal structure of (L )2SiCl2 was determined. Furthermore the molecular structure of dichlorobis(8-oxoquinolinato)-silicon (L )2SiCl2 is presented, which is concluded from the comparison of calculated with experimental Si CP/MAS NMR data. 

Chelates of silicon have already been discussed in some detail in Chapter I in connection with the effects of organic compounds on the solubility of silica and also on the rate of dissolution of silica. Further discussion will be limited to some of the compounds that have been formed. 

Chelates of silicon with triethanolamine are classed as atranes by Veronkov (176). These are essentially esters with an additional R group attached to the silicon atom which is coordinated with the nitrogen atom, as shown by the high dipole moment infrared and nmr spectra N(C2H 0)3Si-R. 

Kaufman suggested (97d) that silica polymerization is inhibited in regions where the hormone gibberellic acid causes a lowering of the pH from 6.5 to 5.0 or less, as noted in elongating cells, for example (97e). It may be significant that such a drop in pH would stabilize the tropolone-type chelates of silicon and thus inhibit release of monomer (see Weiss, in Ref. 127). 

In animals it is possible that silicon metabolism involves another type of chelate (i.e., catechol type anionic complexes). In this case, according to Baumann (390b), the chelate is stable only above pH 7 and liberates silicic acid below this pH. No chelates of silicon have been bolated from animal tissues. However, the presence of a variety of molecules with catechol-like structure such as catecholamines makes it possible that compounds of this type could be involved in animals. Such chelates have been discussed in Chapters 1 and 2. 

A bis(chelate) structure was found for the closely related germylene [MeC(NPr )2]2Ge, which was also made from GeCl2(dioxane) and 2 equivalents of the lithium amidinate (colorless crystals, 81%). The same synthetic approach was used to make bis(amidinato) metal dichlorides of silicon and germanium in high yields (83-95%). Rapid oxidative addition of chalcogen atom sources (styrene sulfide and elemental Se) to the germylene derivatives resulted in a series... 

FIGURE 18. The temperature gradient of the 29Si chemical shift in Si—O chelate complexes (in solution) plotted against the deviation (A) of silicon from the equatorial plane (in the solid state). The gradient K is expressed as K = (<529SiTj — <529Six2)/(Ti — T2)... 

Axially chiral bis-isoquinoline N,N -bisoxide (S)-17 has been reported to promote the addition of Me3SiCN (1.5-2.0 equiv.) to imines 78, derived from aromatic aldehydes (Scheme 7.17) here, CH2CI2 was identified as an optimal solvent. The reaction is stoichiometric in 17, and exhibits partial dependence on the imine electronics (62-78% ee), but much less than that observed for the allylation of PhCHO catalyzed by QINOX (24) (vide supra). The o-substitution had a positive effect in the case of Cl (95% ee), but a very negative effect in the instance of MeO (12% ee). Chelation of the silicon by the N-oxide groups was suggested to account for the stereochemical outcome. Analogues of 17 were much less successful [75]. 

Dichloro neutral bis-chelates of hexacoordinate silicon were obtained by two methods directly (35) by transsilylation of Af-dimethylamino-O-(trimethylsilyl)acylimidates (1) with tetrachlorosilane, or by rearrangement as described in the previous section (70). The presence of two chloro ligands was utilized for further substitution, to form hexacoordinate silicon tris-chelates, by reaction with bis(trimethylsilyl)-precursors 71 and 72, as shown in Eqs. (31) and (32). The products (73, 74, and 75), are hexacoordinate neutral silicon tris-chelates, and 75 is the first reported zwitterionic tris-chelate with three different chelate rings.73... 

Summary The rich variety of the coordination chemistry of silicon is discussed and some theoretical issues are raised. In an attempt to understand further the underlying chemistry, some thermodynamic and kinetic parameters for the formation and substitution of pentacoordinate silicon compounds have been measured by NMR methods. Values of -31 3 kJ mol for SHand -100 10 J K mor for A5-were measured for the intramolecular coordination of a pyridine ligand to a chlorosilane moiety. A detailed kinetic analysis of a nucleophilic substitution at pentacoordinate silicon in a chelated complex revealed that substitution both with inversion and retention of configuration at silicon are taking place on the NMR time-scale. The substitution with inversion of configuration is zero order in nucleophile but a retentive route is zero order in nucleophile at low temperature but shows an increasing dependence on nucleophile at higher temperatures. These results are analysed and mechanistic hypotheses are proposed. Some tentative conclusions are drawn about the nature of reactivity in pentacoordinate silicon compounds. 

Reaction of functionalized ketones such as a-hydroxy ketones or 1,3-ketones with allyltrifluorosilane in the presence of Et3N has also been investigated [95]. Although catecholate does not play a key role in the pentacoordination of silicon, chelating hexacoordinate intermediates are formed to give allylation products with high yields and selectivity (Sch. 54). 

---