Polyhedral cages

CO)3-9-7 -Cp-2,8-Me2-l,4,9-Fe2CoC2B4H4, a nine-vertex polyhedral cage system . ... 

This chapter concerns carboranes (carbaboranes), which are boron clusters with at least one carbon atom as part of the polyhedral cage. Published studies on carboranes before 1981 were reviewed in GOMC (1982) and between 1982 and 1992 in COMC (1995). The present review covers the period of 1992 to early 2005. Unlike in previous chapters, boron hydrides with organic substituents attached to a boron atom, organopolyboron hydrides, are not discussed in this chapter. Borane clusters containing at least one non-carbon atom as part of the cage framework are reviewed in Chapters 3.03, 3.04 and 3.05 of this volume. 

The formation of polyhedral cages and clusters is a widespread phenomenon examples can be found in nearly all parts of the Periodic Table. 

Fig. 1 Examples of polyhedral cage molecules and metalloid clusters (first row) compared to related Zintl ions and intermetalloid clusters that are obtained from the reaction of Zintl ions (second row)...

The cation plays a prominent structure-directing role in zeolite crystallization. The unique structural characteristics of zeolite frameworks containing polyhedral cages (62, 63) have led to the postulate that the cation stabilizes the formation of structural subunits which are the precursors or nucleating species in crystallization. The many zeolite compositions and complex cation base systems studied allow a test of the structuredirecting role of the cation and the cation templating concept. Table I summarizes the cation base systems from which zeolites have been synthesized. The systems used before 1969 are indicated to illustrate the number and complexities of new cation systems investigated since that time. Table II presents a summary of zeolite framework structure types, the cation systems in which they have been formed, and a proposal for a cation specificity for the formation of each framework type. A similar... 

Tables II and III are presented as an initial attempt to establish a broad correlation between crystallization and structure in terms of cation composition. The extensive assumptions and uncertainties involved are well recognized including acceptance of assignment of framework type based on similarity of x-ray powder diffraction patterns, exclusion of some polyhedral cages found in zeolite structures (62) f the relative concentrations of cations in mixtures, variables other than cation, and the possible presence of impurity cations not reported but derived from reagents or reaction vessels.

Another class of compounds that has attracted much interest are the sesquioxides and sesquichalcogenides of the type (RE)2nY3 [(M2Y3) ] (E = Si, Ge, Sn Y = O, S, Se), due to their unique structural properties. These molecules have polyhedral cage-like structures analogous to those of bicyclo[l.l.l]pentane (n = 1) (1), adamantane (n = 2)... 

Tetrameric hydrazinoboranes exhibit a polyhedral cage structure involving tetra-coordinated boron (structure 18) which was confirmed by X-ray studies 203>. 

The boron hydrides are one of most beautiful classes of polyhedral compounds whose representatives range from simple to rather complicated systems. Our description here is purely phenomenological. Only in passing is reference made to the relationship of the characteristic polyhedral cage arrangements of the boron hydrides and the peculiarities of multicenter bonding that has special importance for their structures. 

The formation of polyhedral cages and clusters is now recognized as an important and widespread phenomenon, and examples may be found in nearly all parts of the periodic table. A cage or cluster is in a certain sense the antithesis of a complex yet there are many similarities due to common symmetry properties. In each type of structure a set of atoms defines the vertices of a polyhedron, but in a complex these atoms are each bound to one central atom and not to each other, whereas in a cage or cluster there need not be a central atom and the essential feature is a system of bonds connecting each atom directly to its neighbors in the polyhedron. 

In some cases, it is easiest to describe a zeolite by relating the framework to a series of component polyhedra, each polyhedral cage being a clustering of linked tetrahedra. The... 

While the chemistry of other types of silanols has been explored in somewhat detail [30], the reactivity studies of silanetriols, until recently, have mainly been limited to a few esterification reactions [31], The recent objective in this area is to see whether the three OH groups on silicon can be reacted in concert with suitable metal and metalloid precursors to result in three-dimensional polyhedral cages containing a high metal to silicon ratio. 

This difference in behavior is even more pronounced when the metalla-carbaborane salts 1 are protonated with HX acids, as once again the polyhedral cage becomes intimately involved in the reactivity. Thus, treatment of dichloromethane solutions of lb, la, or If with aqueous HI yields, within seconds, the salts [Y][c/oso-l,8-Me2-ll-(CH2R)-2-I-2,2,2-(CO)3-2,l,8-WC2B9Hg] (R = Me, 35a C6H4Me-4, 35b C6H4OMe-2, 35c) (32,39). The salt 35d was likewise prepared from la and aqueous (or dry... 

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