Structures of Si-Adamantanes

The results of the crystal structure analysis of compound are presented in Fig. 25, including bond lengths and a projection of the unit cell to (010). 

This molecule consists of two condensed 1,3,5,7-tetrasilaadamantanes connected through atoms C(123), Si(l) and C(145) (all designations of atoms in this section refer to Fig. 26a, and not to lUPAC nomenclature). 

Regarding these as fused non-distorted adamantane skeletons with tetrahedral angles and bond distances of Si—C =188 pm, the distances between the methylene C atoms C(24) and C(35), as well as those between the methyl groups on Si atoms Si(2) and Si(3) or Si(4) and Si(5) would be considerably too small. This would cause proton contacts of about 130 pm. It was therefore important and informative to measure the distortion in the region of the Si bridged eight-membered ring Si(2), C(123), Si(3), C(35), Si(5), C(145), Si(4), C(24) and its effect on the total adamantane skeleton. 

Carbosilanes that have been clarified structurally thus far are of two classes either carborundanes (Si—C six-membered rings existing in the chair conformation) or scaphanes (Si—C six-membered rings in the boat conformation). Compound represents a further type of carbosilane, namely, one in which the six-membered rings exist in both the chair and boat conformations. The results of the X-ray crystal structure analysis are presented in Fig. 27 and Table 71. 

Certain differences exist between the bond distances in the adamantane moiety (187.8 pm) and to its substituents (187.9 pm), on the one hand, and the distances in the wurtzitane moiety (190.0 pm in barrelane 189.1pm in iceane) on the other hand, indicating a somewhat larger strain existing in the six-membered rings in the boat conformation (cf. IV. 1.1.3). 

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