Boron icosahedral

To date, the most extensively studied polyboron hydride compounds in BNCT research have been the icosahedral mercaptoborane derivatives Na2[B22H22SH] and Na [(B22H22S)2], which have been used in human trials with some, albeit limited, success. New generations of tumor-localizing boronated compounds are being developed. The dose-selectivity problem of BNCT has been approached using boron hydride compounds in combination with a variety of deUvery vehicles including boronated polyclonal and monoclonal antibodies, porphyrins, amino acids, nucleotides, carbohydrates, and hposomes. Boron neutron capture therapy has been the subject of recent reviews (254). 

Figure 6.1 The icosahedron and some of its symmetry elements, (a) An icosahedron has 12 vertices and 20 triangular faces defined by 30 edges, (b) The preferred pentagonal pyramidal coordination polyhedron for 6-coordinate boron in icosahedral structures as it is not possible to generate an infinite three-dimensional lattice on the basis of fivefold symmetry, various distortions, translations and voids occur in the actual crystal structures, (c) The distortion angle 0, which varies from 0° to 25°, for various boron atoms in crystalline boron and metal borides.

The discovery of polyhedral boranes and polyhedral heteroboranes, which contain at least one atom other than in the cage, initiated a new era in boron chemistry.1-4 Most commonly, of the three commercially available isomeric dicarba-closo-dodecaborane carboranes(l,2-, 1,7-, and 1,12-), the 1,2-isomer 1 has been used for functionalization and connection to organic molecules. The highly delocalized three-dimensional cage bonding that characterizes these carboranes provides extensive thermal and kinetic stabilization as well as photochemical stability in the ultraviolet and visible regions. The unusual icosahedral geometry of these species provides precise directional control of all exopolyhedral bonds. 

ICOSAHEDRAL CARBORANES AND INTERMEDIATES LEADING TO THE PREPARATION OF CARBAMETALLIC BORON HYDRIDE DERIVATIVES... 

A very large number of derivatives of S4N4 are known, and some of them are described in Chapter 15. Elemental boron exists as an icosahedral B12 molecule that has the structure... 

A black, crystalline form of boron having a density of 2.34 g/cm3 results. Boron exists in unit cells that have an icosahedral structure that has 20 faces that are equilateral triangles meeting at 12 vertices with a boron atom at each vertex, as shown in Figure 13.1. 

Pentaborane, B5H9, is unusual in having a tetragonal C4v skeleton, rather than the icosahedral fragment geometry that is typical of other boron hydrides. The unique apex atom B1 makes four short B—B bonds to the base (1.695 A), whereas the four basal B—B bonds are of more typical length (1.798 A). 

As a final example we consider the icosahedral B12H122- dianion, whose relevance to higher extended boron structures may be motivated in the following terms. 

Figure 3.110 Boron-atom numbering and the two-dimensional hexagonal grid pattern of two two-center and ten three-center bonds in the 0.10.3.0 styx structure for icosahedral Bi2Hi22. ...

Foremost among the elements that pack in icosahedral (and other Ar-chimedeans) structures is boron, its various hydrides (boranes), and related boron compounds (25 pp. 157-185). This topic, which is covered by numerous reviews and books, will not be further discussed in this chapter. Many other cage structures have been discussed in the literature, some of them are briefly discussed in (26). 

Although legions of icosahedral carboranes 49 are known, the formation of supra-icosahedral boron-containing clusters was restricted to metal carboranes (MxC2B10 or MxC4Bg, x = 1,2) [62, 63]. 

A typical building block used to construct several solid-state structures (boron-rich borides and allotropes of elemental boron) is the B12 icosahedron. According to King, (1993) an icosahedral B12 building block in which each of the 12 vertices contributes a single electron for an external two-electron two-centre (2e, 2c) bond to an external group implies the following electron count ... 

The first metallacarboranes were isolated in M. F. Hawthorne s laboratory and contained a metal ion, two carbon and nine boron atoms forming an icosahedral C/050-MC2B9 cage structure.It was immediately recognized that these species may be viewed as having metal ions coordinated in a pentahapto manner by the open face of a [nido-1 dianion. This was a useful formalism since it empha-... 

The material reviewed in this Chapter hitherto has focused on metallacarboranes in which the metal atom is a vertex in an icosahedral cage framework. Until recently, monocarbollide metal compounds with core structures other than 12 vertexes were very rare since suitable carborane precursors were not readily available." However, Brellochs recent development of the reaction of decaborane with aldehydes to give 10-vertex monocarboranes permits a considerable expansion in this area of boron cluster chemistry. As a consequence, several intermediate-sized monocarboranes are now easily accessible and we have recently begun to exploit the opportunities that these present. In particular, we have focused thus far on complexes derived from the C-phenyl-substituted species [6-Ph- zJo-6-CBgHii] It is clear from these initial studies that a wealth of new chemistry remains to be discovered in this area, not only from among the metal derivatives of PhCBg car-boranes such as those discussed in this section, but also in the metal complexes of other newly available carboranes. 

As with the icosahedral monocarbollide-metal species reviewed in the preceding sections, the boron-bound hydrides of 129 and 130 may be abstracted by Me and... 

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