Hexasilaprismanes

For the synthesis of hexasilaprismane, tetrachlorodisilane (RSiCl2—SiC R) and trichlorosilane (RSiCb) are utilized as the starting compounds. The Mg/MgBr2 reagent is quite useful also for the synthesis of hexasilaprismane. A hexasilaprismane derivative, hexakis(2,6-diisopropylphenyl)tetracyclo[2.2.0.02 6.03 5]hexasilane (12), was successfully... 

Figure 7 shows the ORTEP drawing of the hexasilaprismane 1237. Its structural parameters are listed in Table 4. The crystal has a two-fold axis of symmetry. The skeleton has a slightly distorted prismane structure with two triangular units [Si-Si = 2.374-2.387 A (av. 2.380 A) and ZSi-Si-Si = 59.8-60.3° (av. 60.0°)] and three... 

FIGURE 7. ORTEP drawing of hexasilaprismane 12. Reprinted with permission from Reference 9. Copyright 1995 Academic Press, Inc. 

FIGURE 8. Electronic absorption spectra of hexasilaprismane 12 and hexagermaprismane in hexane... 

The hexasila-Dewar benzene 13 is thermally stable at —150 °C, but it gradually reverted to the hexasilaprismane 1243. The half-life is 11/2 = 0.52 min at 0 °C in 3-methylpentane. The activation parameters for the isomerization of 13 to 12 are a = 13.7 kcalmol-1, A= 13.2 kcalmol-1 and A= — 17.8 cal K-1 mol-1. The small Ea value is consistent with the high reactivity of Si=Si double bonds. Most probably, the small HOMO-LUMO gap of 13 makes it possible that the Si=Si double bonds undergo a formally symmetry forbidden [2 + 2] thermal reaction. Hexasila-Dewar benzene is a key... 

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. 

The static solid-state NMR can produce a broad pattern to give the three principal values of the shielding tensors for polyhedral silicons. Figures 15 and 16 show the static powder spectra of octasilacubane 22 and of the hexasilaprismane 12, respectively56. The values of the an, 022 and <733 tensors are given in Table 11 together with the calculated values for the methyl substituted compounds57. 

FIGURE 16. Static solid state 29 Si NMR spectrum of hexasilaprismane 12... 

TABLE 11. Experimental and calculated principal values of shielding tensors for hexasilaprismane and octasilacubane... 

In 12, the principal axis corresponding to 0-33, the most shielded principal value (—182 ppm), is perpendicular to the three-membered rings. High shielding in this direction can be found in all types of cyclotrisilanes58. The hexasilaprismane 12 has an unprecedented spread anisotropy of 280 ppm a high value of 322 ppm is also found theoretically for MegSig (Table 11). 

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... 

Because H-Si-SiHs is nearly isoenergetic with H2Si = SiH2, it is not obvious which structure would be favored for hexasilabenzene, SieHe. Whereas proponents of aromaticity may take heart from the theoretical calculations that show the hexasilabenzene structure to be 34 kcal/mol more stable than the tris(silylene) form, they will be reminded of how little aromaticity has to do with the magic of six tt electrons by the revelation that the hexasilaprismane is calculated to be 14 kcal/mol more stable yet. 

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