Polyhedral frameworks

Class (a). The analogy between the structural chemistry of H2O and Si02 is illustrated by the structure of HPFg. 6 H2O. In this crystal the H2O molecules are situated at the apices of Fedorov s space-filling by truncated octahedra, that is, at the same positions as the Si(Al) atoms in the framework of ultramarine (p. 832). Each H2O molecule in the framework (Fig. 15.4) is hydrogen-bonded to its four neighbours at a distance of 2-72 A, and the PFg ions occupy the interstices (H20-F.2-74 A P-F, 1-73 A). 

Much less symmetrical polyhedral cavities are found in (CH3)3C. NH2. 9 H2O. The framework represents a space-filling by 8-hedra ((4 = 4, /s = 4) and 17-hedra (fi = 3, /s = 9, /g = 2,f j = 3). The unit cell contains 156 H2O (16 formula units), and the amine molecules occupy the 16 large voids (17-hedra) the 8-hedra are not occupied. 

Class (b). In this class the (4-connected) networks are the edges of space-filling arrangements of pentagonal dodecahedra and one or more of the related polyhedra /s = 12,/e = 2, 3,4 (Table 15.3). 

Hydrate Z Vertices Voids (n-hedra) Total of large voids 

The hydrate of chlorine is of special interest as the solid phase originally thought to be solid chlorine but shown (in 1811) by Humphry Davy to contain water. It was later given the formula CI2. 10 H2O by Michael Faraday. The unit cell of this (cubic) structure (a 12 A) contains 46 H2O which form a framework (Fig. 15.6) in which there are 2 dodecahedral voids and 6 rather larger ones (14-hedra). If all 

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Our discussion will be restricted to metallacarboranes in which the r/-block metal is incorporated into the polyhedral framework of the carborane. No attempt will be made to cover those compounds where the transition metal atom is in a bridging group linking several carborane polyhedra together or is involved solely as a member of a substituent group. Since much of the insight into the chemistry of the metallacarboranes is obtained from structural and bonding... 

Synthetic polyhedral subrogation for the preparation of polymetallo-carboranes from monometallocarboranes is an offshoot of polyhedral contraction in that, after degradative removal of a BH vertex, a transition metal ion is reacted with the m do-metallocarborane produced rather than with an oxidizing agent. In this way, a new transition metal vertex is incorporated into the polyhedral framework without a change in the number of vertices between reactant and product (Fig. 6) ... 

Spherical siloxanes (RSiOi 5), with polyhedral frameworks, also called silsesquioxanes, can be used as defined oligomeric models for surface-modified silica gels or polysiloxanes. Whereas silsesquioxanes with alkyl-, aryl-, hydrido-, and trimethylsiloxy groups are known for a long time [1], functionalized octa-[propyl-silsesquioxanes], [X-(CH2)3]g(SiOi.5)g, were synthesized for the first time in 1990 by Weidner, Zeller, Deubzer, and Frey [2]. Octa[(3-chloropropyl)-silsesquioxane], [Cl-(CH2)3]g(SiO, 5)g, could be obtained by hydrolysis of (3-chloropropyl)-trichlorosilane. 

More complex examples of isomerism include that of carboranes, in which C atoms are introduced into polyhedral frameworks of B atoms, and of compounds such as S (NH)8 n These form cyclic molecules similar to the Sg molecule of elementary sulphur but with some S atoms replaced by NH groups. Three of the four possible isomers of S6(NH)2 have been characterized, namely the 1,3, 1,4, and 1,5 isomers. 

The structure of 2,6-di-T]-cyclopentadienyl-octahydro-l,10-dicarba-2,6-dicobalta-closo-decaborane, 2,6-(ti-C5H5)-2,6-Co2-1,10-C2B6FI8, has been deduced from X-ray diffraction. The polyhedral framework is a distorted, bicapped square antiprism, with carbons at the caps and one cobalt in each tropical plane. The cobalt atoms are bonded to each other [2.489(1) A apart], this being the first confirmed metal-metal bond in a bimetallocar-baborane.112... 

Figure 7.14 Representations of octahedra (a) ball and stick , in which a small cation is surrounded by six anions (b) a polyhedral framework (c) a solid polyhedron...

Mechanisms for optical and geometrical isomerization reactions similar to those employed for substitution reactions can be envisaged. Additionally possible is a twist mechanism, involving distortion of the polyhedral framework in the activated state but in which no ligands depart or join the coordination sphere. 

Table 1. The closed polyhedral frameworks usually observed for metal clusters with nuclearities between four and twelve, and the relative dimensions of their internal cavities.

Following our strategy, it was an important target to investigate substitution reactions at the silicon and titanium atoms of the polyhedral frameworks. Earlier experiments in titanasiloxane chemistry have already shown that TiOSi units are very sensitive towards acid- and base-catalyzed oligomerizations. Thus, the choice of reactants for selective functionalization under conservation... 

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