Hypervalent silicon compounds structure

The most recent comprehensive review focused primarily on the reactivity aspects of hypervalent silicon complexes6, while the latest review covers only silicon-oxygen coordination7. The present chapter focuses on synthesis and structure, silicon-29 NMR spectroscopy, and on the nonrigidity of hypervalent silicon compounds and the resulting kinetic and stereochemical studies. 

The single NMR resonance moved to low frequency with each addition of HMPA, and finally remained as a singlet at 8 = -78 ppm at all ratios of HMPA to silane of 3 1 or greater. The only reasonable structure for this new species is 5. These complex changes for a relatively simple system illustrate the subtle relationship between coordination and reactivity for silicon. It was observations such as these and others that stimulated us to try to make quantitative measures on hypervalent silicon compounds. 

In this context, the existence of stable penta- and hexacoordinate silicon derivatives and their structure have elicited considerable interest. Isomerization processes of these compounds are also of importance, since penta- or hexacoor-dinate intermediates are implicated in the substitution and racemization of tetracoordinate silicon derivatives. For all these reasons, we include here (i) some available structural data for hypervalent silicon compounds (the review is not intended to be exhaustive), (ii) recent reports concerning the nucleophilically induced substitution and racemization reactions at silicon, and (ii) a criti-... 

In spite of the long history of hypervalent silicon compounds, and the considerable amount of structural investigation devoted to them, relatively little of their chemistry has... 

Hypervalent silicon compounds attract interest from both the structural and reactivity point of view [1]. The azomethine AJV -ethylene-bis(2-hydroxyacetophenoneimine) (salen H2 1), was formed by condensation of ethylenediamine with 2-hydroxyacetophenone. We set out to synthesize hexacoordinate silicon complexes containing the salen ligand. The anion salen is able to chelate the silicon atom through four donor atoms. There are some rare examples of salen-silicon compounds known from the literature [2], but characterization of these compounds seems to be doubtful [3]. Structural aspects are uncertain due to the lack of crystal structure data. 

Summary Our investigations on silicon compounds of etfaylene-MiV"-bi (2 -hydroxyacetophenoneiminate) led to the synthesis and X-ray structure analysis of a new kind of salen complex — hypervalent silicon compounds with a threefold deprotonated salen ligand and an enamine structure. This stmctural unit provides access to new routes for synthesizing hypercoordinate silicon complexes. Addition reactions between various Brpnsted acids and these new pentacoordinate silicon compounds were carried out to precipitate complexes bearing hexacoordinate silicon atoms. 

Si NMR spectroscopy in solution and, increasingly, in the solid state has become an indispensable tool in the multi-faceted silicon chemistry. The Si NMR parameters are ideally suited to complement direct structural information from X-ray crystallography. Moreover they are extremely valuable in the investigation of dynamic properties both intermolecular and intramolecular. This includes the attractive chemistry of hypervalent silicon as well as the fascinating playground of low-coordinated silicon compounds. The excellent performance of modern NMR spectrometers has overcome most problems associated with the determination of Si NMR parameters. The trend towards a multinuclear approach in NMR spectroscopy is certainly fortified by the availability of Si NMR parameters. 

Silicon-based Polymeric Materials Mechanistic Organosilicon Chemistry (a) Gas Phase and Photochemical Reactions (b) Hypervalent Silicon, Nucleophilic Substitution, and Biotransformations Structural Organosilicon Chemistry and New Organosilicon Compounds Organic Synthesis using Siiicon. 

While the last elass of eomplexes considered in this section, the compounds 145, closely resemble the usual silane a-complexes, other multicenter H Si interactions discussed above have spectroscopic and structural features common to both the IHI and a-complexes. This enigmatic situation can be explained well by the structure 132 in terms of a a-coordination of the Si-H bonds of the hypervalent ligand (H +iSiX3)" 1) to metal, which thus includes both the hypervalent interaction of the silicon with the hydride atoms and the a-complexation of the Si-H bonds to metals. The key features of complexes with multicenter H Si interactions are summarized in Table VIII, where a comparison with the IHI and the residual H-Si interactions in silane a-complexes is given. 

J. C. Martin Structural factors influencing stability in compounds of hypervalent carbon, silicon, phosphorus and iodine [5]... 

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