Polyhedral silanes

FIGURE 5. 3-21G( ) structures of bicyclo[1.1.0]tetrasilanes. Reproduced by permission of VCH Verlagsgesellschaft from Ref. 121. 

The strain energy of 21a (using appropriate homodesmic reactions) is estimated at MP2/6-31G //3-21G( ) + ZPE as 60.3 kcal mol by 6.6 kcal mol-1 lower than in bicyclo[1.1.0]butane122 (at 3-21G( ) this conclusion is reversed121). Boatz and Gordon also find that 21a is by 3.4 kcal mol -1 more stable than the isomeric 23122. 

Boatz and Gordon s paper includes also a comprehensive study of the structures, relative energies, bent bonds, and strain energies of silabicyclo[ 1.1.0]butanes and silacyclobutanes of the general formula CnSi4 H6 (n=0-4)122. Some of these molecules are discussed in Section IV.C.2. 

TABLE 9. Strain energies (kcalmol ) calculated from homodesmic reactions 8 at the HF/6-31G level  

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Inspection of Table 14.4.1 shows thatE(C-C) > E(Si-Si),. E(C-H) > (Si-H) and E(C-C) > E(C-O), but (Si-Si) < < (Si-0). Thus alkanes are much more stable than silanes, and silanes react readily with oxygen to convert the Si-Si bonds to stronger Si-O bonds. Although silanes do not exist in nature, some compounds with Si-Si and Si=Si bonds have been synthesized in the absence of air and in non-aqueous solvents. The silanes Si H2 +2 (n = 1 — 8), cyclic silanes Si H2n (n = 5,6), and some polyhedral silanes are known. The structures of tetrahedral Si Si ButU, trigonal-prismatic Si6(2,6- PrC6H3)6, and cubane-like Sis(2,6-Et2C6H3)8 are shown below ... 

Sprung and Guenther were the first to postulate that the hydrolysis of organyltrifunctional monomers XSiYj involves a consecutive formation of linear, cyclic, polycyclic and, finally, polyhedral siloxanes. They assumed the chain growth to be of a random character. This hypothesis was further developed by Brown and Vogt who studied the hydrolysis of cyclohexyl- and phenyltrichloro-silane They suggested the formation of polyhedral silsesquioxanes and their homo... 

The most simple molecular topology of such systems reported so far is a tetrahedral supermolecule obtained by reacting tetrakis(dimethylsiloxy)-silane with alkenyloxy-cyanobiphenyls (Fig. 22), as discussed previously. Such tetramers exhibit smectic A liquid crystal phases [179]. For such end-on materials, microsegregation at the molecular level favors the formation of the smectic A phases in preference to the nematic phase exhibited by the mesogenic monomers themselves. The use of different polyhedral silox-ane systems (Fig. 24) or the Ceo polyhedron as the template for multi- and polypedal hexakis(methano)fullerenes (Fig. 70) substituted with a large number of terminally attached mesogenic groups confirm the same tendency to the formation of smectic A phases (vide supra). 

This approach has also been employed for synthesis of highly enan-tiomerically enriched cychc silanes with sihcon-centered chirahty [141]. Zhang et al. [142] prepared second-generation dendrimers based on polyhedral silsesquioxane cores with up to 72 terminal groups at their surface (Scheme 13). 

Another striking example is the T symmetry of tetrakis[trimethylsilyl] silane (25), the discussion of which is deferred to the last section, dealing with high-symmetry chiral polyhedral symmetry. An interesting conformational deformation in a crystal lattice has been analyzed by X-ray crystallography coupled with force-field calculations (26). 

Figure 9.2 Surface modification chemistries of nanocellulose for PLA/nanocellulose biocomposites, a, Acetylation b, Esterification with various organic acids c, d, e, Grafting of PCL, PLA, P(CL-fi-LA) f, Silanization g, Silylation h, Carbojymethylation combined with hexanoation i, PEG grafting j, Modified with polyhedral oligomeric silsesquioxane (POSS).

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