Tetrasilatetrahedranes

In 1993, Wiberg et al. obtained the first tetrasilatetrahedrane (/-Bu3SiSi)4 764 from the reduction of the 2,2,3,3-tetrabromo-tetrasilane f-Bu3SiBr2SiSiBr2Si/-Bu3 765 with /-Bu3SiNa (Scheme 100). 

The first well characterized tetrasilatetrahedrane lsi (cf. Scheme 2.5-1) was synthesized from R SiX3 or R SiX2-SiX2R (R = SiBu 3, X = halogen) and NaR in tetrahydrofuran (THF) [18]. It was obtained as orange crystals and has high ther-... 

The phases of the hB elements are largely unused, e.g. NaSi etc. with the tetrasilatetrahedrane Si, " unit or LiiaSiv and LiizGev with the trigonal planar Sii and Gez stars (28). We were already able to observe reactions of these compounds, but upto now the products have not been characterized. Reactions of NasSijte, Naj Siiae (x=3-12) and LiaSns with benzophenone proceed in the way described above, but so far only amorphous precipitations of silicon and tin have been obtained. 

The reductive reaction of l,2-bis(2,6-diisopropylphenyl)-l,l,2,2-tetrachlorodisilane (1) with LiNp led to several products from which 4 was isolated after hydrolytic workup (Scheme l)23. Compound 4 is believed to be formed by the hydrolysis of the intermediate 3, which arises from the cleavage of an Si—Si bond of the tetrasilatetrahedrane 2. 

FIGURE 5. Possible conversion of the bond-stretch isomer to a tetrasilatetrahedrane upon replacement of the substituents in circles by more bulky silyl groups. Reproduced with permission from Reference 10. Copyright 1995 American Chemical Society... 

Tetrasilatetrahedrane 11 is unexpectedly stable to water, air and light. It cannot be reduced by sodium, but reacts with TCNE and Br232. Unlike t-butyl substituted tetrahe-drane of carbon35, 11 is thermally stable and its crystals do not melt below 350 °C. The bulky /-BusSi substituents evidently prevent the collapse of the tetrahedrane skeleton. UV-Vis absorptions were observed at 210 (e = 76000), 235 (e = 71 000), 310 (e = 20000) and 451 (e = 3600) nm. 

FIGURE 6. ORTEP drawing of tetrasilatetrahedrane (11). Reprinted with permission from Reference 32. Copyright 1993 VCH... 

NMR spectroscopy is a powerful tool for structural analysis. The chemical shifts of polyhedral silicons range from —22 to 39 ppm. The 29Si chemical shifts of tetrasilatetrahedrane ll32, hexasilaprismane 1237 and octasilacubanes (163, 18a44b, 18b45, 2046 and 2247) are listed in Table 10. 

Recently, some attention has been focused on the synthesis and on the properties of strained cage compounds made up exclusively of Si atoms. As a consequence a number of derivatives of tetrasilatetrahedrane, hexasilaprismane and octasilacubane have been synthesized. The current knowledge in the field up to 1994 has been summarized... 

The highly strained tetrasilatetrahedrane structure can only be stabilized using extremely bulky substituents like the tri-i-butylsilyl (= supersilyl) group. Thus the only tetrasilatetrahedrane derivative known so far has been synthesized by coupling (t-Bu)3SiSiBr2SiBr2Si(Bu-t)3 with two equivalents of (i-Bu SiNa92 (equation 24). 

The only known example of the tetrasilatetrahedrane molecule, tetrakis(tri-terf-butylsilyl)tetrasilatetrahedrane 1, was reported by Wiberg in 1993 by the reaction of f-Bu3Si—SiBr2—SiBr2—Si(Bu-f)3 with f-Bu3SiNa in THF at — 20°C16. The details of its synthesis as well as its physico-chemical characteristics are summarized in a recent review14 and will not be considered in the present article. 

As in the case of monocyclic 3- and 4-MRs (see Section V.E.l.a.ii), electropositive substituents stabilize also M H cage systems which include such rings, e.g. tetrahedranes, prismanes and cubanes (Table 24)191,192,195. The only isolated tetrasilatetrahedrane indeed carries the electropositive Si(Bu-t)3 groups as substituents238. Nevertheless, the steric protection afforded by the bulky silyl ligands is probably the most important factor allowing the isolation of the tetrasilatetrahedrane179 195. 

The synthesis of the title compound was reported several years later by the same authors, who had previously prepared tetrasilatetrahedrane in a similar way. Thus, f-BusSi—GeCl2—GeCl2—Si(Bu-f)3, which was prepared by the reaction of GeCLj and f-Bu3SiNa in THF at room temperature, was reacted with f-Bu3SlNa in THF at —78°C. The tetragermatetrahedrane 2 was formed in a low yield together with some other products (Scheme 1). 

In fact, an isolation of compounds - such as the substituted tetrasilatetrahedrane shown in Scheme 1 -is in many cases not possible without introducing overloaded groups like the rBu3Si group, that is the supersilyl group [2]. In addition, compounds which possess underloaded as well as overloaded silicon atoms may become isolable but retain - like the silaneimine shown in Scheme 1 - their chemical potency... 

Scheme 16. Preparation of bis(supersilyl)tetrabromdisiIane as well as bis(supersiIyl)dibromdisilene, and trapping the latter compound with diphenylacetylene as well as transforming it in tetrakis(supersilyl)tetrasilatetrahedrane...

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