Silicon complexes pentacoordinate

Pentacoordinate silicon complexes can also be prepared via the reaction of diaryldichlorosilanes with carbenes 4 (R = Et, Pr R = Me). Interestingly, a 2-(trimethylsilyl)imidazolium salt (56) was formed when Me3SiI was treated with 4 (39). Unfortunately, no information regarding the crystal structures of these species is available. 

By this method also the zwitterionic silicates 9-15 were obtained The geometry at silicon in these compounds is TBP, like in anionic and neutral pentacoordinate silicon complexes. A typical crystal structure is shown in Figure 5 for compound 9. This structure apparently also exists in solution (CD3CN), as the 29Si chemical shift for 9 in this solvent (—122.9 ppm) compares well with the solid state CP-MAS shift of -121.0 pm28 31. 

In general, pentacoordinate silicon complexes have a five-membered chelate cycle. Few exceptions are known with larger (six-membered, 102)" and smaller (four-membered 10397a, 10497b) chelate cycles. 

Nitrogen-coordinated pentacoordinate complexes have been used as stereoselective reducing agents in the preparation of erythro-(meso)- 1,2-diols from diketones and a-hydroxyketones109. The reducing agent was the (l-naphthylamino-8)trihydridosilane 92e. After formation of the dioxo chelate from the diketone (equation 32), the diol was obtained from the pentacoordinate silicon complex by reduction with LiAlILt. 29 Si NMR spectroscopy was used for the product-ratio analysis in this reaction, which was found to yield primarily the erythro diols. 

New neutral pentacoordinate silicon complexes with a dative O Si bond (207, 208) have been prepared and studied. The NMR spectral data and crystal structures were reported239. 

A study of the solvolytic and thermolytic reactions of a series of heterocyclic pentacoordinate silicon complexes was described241. 

Corriu and coworkers37 showed that alkoxysilanes R Si(OR )4-n [in particular RSi(OMe)3] give better yields for the DDB cyclization than do chlorosilanes. The reaction of anionic pentacoordinated silicon complexes [RSi(02CgH4-0)2] Na+ with DDB and subsequent LiAlPLi reduction give 1-R-diphenylsiloles (R = Me, Ph)38. Bis(silacyclopentadien-l-yl)alkanes were formed from DDB and a, a>-bis(dihalomethylsilyl)alkanes39. 

Corriu and co-workers in similar mono- and dihalo pentacoordinate silicon complexes.17a 18... 

Novel Dimeric Pentacoordinate Silicon Complexes Unusual Reactivity of Electron-Rich Aminosilane Intermediates... 

Scheme 4. The synthesis of the novel pentacoordinate silicon complexes 6 -11.

Bulky X-ligands, and in particular the /-butyl group, cause severe geomettical distortions of pentacoordinate silicon complexes. The most dramatic example is that mentioned above, in which the nitrogen ligands in 3 have moved from axial to equatorial positions upon elimination of methyl halide from 2. The crystallographic evidence for this geometry is depicted in Fig. 1. The extent of... 

Fig. 2. A model Berry pseudorotation reaction coordinate made up of crystal structures of pentacoordinate silicon complexes with varying N-Si-N and O-Si-0 angles.

Ligand Exchange via Coordinative Si-N Bond Cleavage and Pseudorotation in Neutral Pentacoordinate Silicon Complexes... 

Summary Introduction of a chiral ligand to a series of pentacoordinate silicon complexes led to the assignment of two intramolecular rate processes Si-N cleavage and pseudorotation. Linear correlations between and Si chemical shifts of the complexes, as well as between the latter and the N-methyl exchange barriers were attributed to variation in strength of N—>Si coordination. 

The reaction of highly strained allylsilacyclobutanes with aldehydes has recently been developed to produce homoallylic alcohols with a high degree of re-gio- and stereoselectivity (Scheme 10-36) [63]. These species are structurally akin to the allyltrialkylsilanes, but are more mechanistically aligned with the allyltri-halosilanes. The -2-butenylsilacyclobutane upon reaction with an aldehyde at elevated temperature will produce almost exclusively the anti homoallylic alcohol. When the Z-2-butenylsilacyclobutane is used instead, the syn homoallylic alcohol is obtained. The mechanism proposed for the reaction involves the association of aldehyde and allylsilacyclobutane to form an activated pentacoordinate silicon complex. A closed, chair-like transition structure is proposed to account for the observed stereoselectivity in the reaction (Scheme 10-36k A theoretical examina-... 

TABLE 2. 19F NMR absorptions in pentacoordinate silicon complexes at the slow exchange limit ... 

Summary The preparation of new pentacoordinate silicon complexes 4-6 from polychloro- or fluoro-silanes and 0-silylated-iV -dimethylhydrazides is described, along with a typical trigonal-bipyramidal (TBP) crystal structure. Evidence is presented that 4-6 undergo rapid N—>Si dissociation-recombination. Coupling constants are reported extending across the dative bond, even at temperatures in which N-Si dissociation is rapid. A Karplus-type correlation between dihedral angles and vicinal coupling constants is shown. 

The standard group valence is 4. For Si, Ge, and Sn, however, coordination numbers exceeding 4 occur regularly. The central atoms in such species are thus hypervalent, that is, they have more than an octet of valence electrons. For example, 2,2-bipyridyl reacts with triphenylchlorosilane to yield a relatively stable pentacoordinate silicon complex. Observe that, although the Si carries a -1 formal charge, the complex as a whole is cationic ... 

Synthesis of pentacoordinate silicon complexes from Si02. Nature. 353 642-644. Longloilert, R., Chaisuwan, T., Luengnaruemitchai, A., and Wongkasemjit, S. 2011. Synthesis of MCM-48 from sUatrane via sol-gel process. J. Sol-Gel ScL Technol. 58(2) 427-435. 

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