Boron polyhedral boranes

As a result of the systematic application of coordination-chemistry principles, dozens of previously unsuspected stnicture types have been synthesized in which polyhedral boranes or their anions can be considered to act as ligands which donate electron density to metal centres, thereby forming novel metallaboranc elusters, ". Some 40 metals have been found to act as acceptors in this way (see also p. 178). The ideas have been particularly helpful m emphasizing the close interconnection between several previously separated branches of chemistry, notably boron hydride clu.ster chemistry, metallaboranc and metallacarbaborane chemistry (pp. 189-95). organometallic chemistry and metal-metal cluster chemistry. All are now seen to be parts of a coherent whole. 

The chemistry of polyhedral borane ions. L. J. Todd, Prog. Boron Chem., 1970,2,1-35 (81). 

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. 

The polyhedral boranes and carboranes discussed above may be regarded as boron clusters in which the single external orbital of each vertex atom helps to bind an external hydrogen or other monovalent atom or group. Post-transition main group elements are known to form clusters without external ligands bound to the vertex atoms. Such species are called bare metal clusters for convenience. Anionic bare metal clusters were first observed by Zintl and co-workers in the 1930s [2-5], The first evidence for anionic clusters of post-transition metals such as tin, lead, antimony, and bismuth was obtained by potentiometric titrations with alkali metals in liquid ammonia. Consequently, such anionic post-transition metal clusters are often called Zintl phases. 

Another very useful rule for classifying the structures of polyboranes and hetero-boranes as well as many metal boron cluster compounds and their derivatives has been developed by Rudolph, Williams, Mingos and Wade (see Chapter 1.1.2) [4]. Today these are generally termed the Wade rules. They can be derived from the structures and electronic requirements of closed polyhedral boranes, such as an octahedron or an icosahedron, which are present in the anions B6H62 and B,2 H, 22. Since there are only exopolyhedral B-H bonds the number of electron... 

In addition to these polyhedral boranes there exist polyboranes where two (or more) polyboranes are connected by single B-B bonds or by sharing three or four boron atoms of two (or more) borane units by 3c2e bonds. In the case of B10H16 there are two B5H8 units where the apical boron atoms of B5H9 are joined by a B-B bond (l,l -isomer) or by a B-B bond between the apical boron atom of one pentaborane unit with a basal boron atom of a second pentaborane fragment... 

Geometrically it is impossible for polyhedral boranes where two closo-polyboranes share two common boron atoms, i.e., a common edge, to exist, because the hydrogen atoms of the neighboring BH groups would interfere sterically. For instance, if two B12H12 units are joined by a common edge this would result in H-H distances of less than 1.5 A, well below the van der Waals contacts. However,... 

In contrast to the polyhedral boranes B H +m there exist a number of neutral boron duster molecules B X (X = Cl, Br, I, NR2, R) all of them having closed deltahedral structures in spite of the fact that the number of bonding electron pairs is only n. For this reason these homonudear cluster compounds of boron are called hypercloso polyboranes. However, there also exist anions of type B X 2 which fit Wade s rules. 

Polyhedral Boranes, Marcel Dekker, New York, 1968 J. F. Liebmann, A. Greenberg, R. E. Williams, eds., Advances in Boron and the Boranes, VCH Publishers, Weinheim, New York, 1988. 

Boron seems to have an affinity for cancerous tumors, and this property has been exploited in radiation therapy (Hamada et al. 1983 Hatanaka 1986). Boron-10 has been used in neutron capture therapy to cure malignant sarcomas implanted in the hind legs of mice, as well as spontaneous malignant melanomas in pigs (Slatkin etal. 1986). The sulfhydral borane monomer (Bi2HnSH) is used as a B-10 carrier in neutron therapy of malignant human brain tumors and seems to be most effective at 30 pg B-lO/kg tissue (Hatanaka 1986). Polyhedral boranes attached to monoclonal antibodies that are tumor specific may become useful in tumor therapy by neutron irradiation (Parry... 

The boron hydrides, including the polyhedral boranes, heteroboranes, and their metaUa derivatives, encompass an amazingly diverse area of chemistry. This class contains the most extensive array of structurally characterized cluster compounds known. Included here are many novel clusters possessing idealized molecular geometries ranging over every point group symmetry from identity (C[) to icosahedral (I[). Because boron hydride clusters may be considered in some respects to be progenitorial models of metal clusters, their development has provided a framework for the development of cluster chemistry in... 

Alternative routes to boron nitride have been examined that do not require those challenging reaction conditions. These fall into three categories, which are (1) the pyrolysis of borazines (2) pyrolysis of organic or inorganic polymers that bear borazine groups as side units and (3) pyrolysis of polyhedral borane derivatives. 

The B12 icosahedron is a regular polyhedron with 12 vertices, 30 edges, and 20 equilateral triangular faces, with B atoms located at the vertices, as shown in Fig. 13.2.2. The B12 icosahedron is a basic structural unit in all isomorphic forms of boron and in some polyhedral boranes such as h has 36... 

Boranes are recognized as clusters of boron atoms which represent triangularfaced polyhedra. It was Williams62 in 1970 who pointed out in a classic paper that the structures of all boranes are derived from those of the closed polyhedral borane anions, or carboranes, with vertices ranging from 5 to 12. These regular polyhedra are shown in Fig. 5 and represent the structures of the closo-boranes (or carboranes) from which all other borane structures are derived. 

More complex boron hydrides, the polyhedral boranes, will be described in Section 8.3.5. 

In the First Edition of Regular Polytopes, Coxeter stated, ... the chief reason for studying regular polyhedra is still the same as in the times of the Pythagoreans, namely, that their symmetrical shapes appeal to one s artistic sense [23], The success of modem molecular chemistry does not diminish the validity of this statement. On the contrary. There is no doubt that aesthetic appeal has much contributed to the rapid development of what could be termed polyhedral chemistry. One of the pioneers in the area of polyhedral borane chemistry, Earl Muetterties, movingly described his attraction to the chemistry of boron hydrides, comparing it to M. C. Escher s devotion to periodic drawings [24], Muetterties words are quoted here [25] ... 

A special case of molecular boron suboxides is found in the hydroxylated polyhedral boranes (see Boron Hydrides). For example, the icosahedral anion closo- Q 2H 2 and related compounds can be completely hydroxylated to form discrete molecular boron suboxides, including closo-[Bi2(OH)i2] These can be regarded as members of a class of formally hydrated suboxides related to compounds snch as tetrahydroxydiborane, B2(HO)4 (BO H2O). The terminal hydroxyl groups of these compounds can be fimctionahzed as esters or ethers to produce a variety of large molecule derivatives. 

In discrete molecules, the elements of group 13 are most typically found in the +3 oxidation state. There are major exceptions to this rale, such as the polyhedral boranes (see Boron Hydrides) and carboranes (see Boron Polyhedral Carboranes and Boron Metallacarbaboranes), which formally contain and are outlined elsewhere in this Encyclopedia, and the compounds of thallium (Tl), which typically contain Tl for the reasons described above. Thus, in this section, compounds containing group 13 elements in the atypical -I-1 and -1-2 oxidation states are presented. 

The chemistry described above exemplifies the great similarity between aromatic polyhedral borane chemistry and the aromatic branch of organic chemistry. Modular syntheses with carboranes and the discovery of a variety of camouflaged derivatives clearly reveals the tentacles of organic chemistry reaching into the polyhedral borane field. Thus, the conflux of boron and carbon chemistries is broadened. 

Polyhedral boranes, carboranes and carbametallic boron hydride derivatives... 

Polyhedral hydroborate anions (commonly referred to as polyhedral borane anions in the current literature) of generic formula have assumed an unusual status in inorganic chemistry within recent years, not only because of their high thermal and hydrolytic stability, but also because of the wide range of geometries which the boron atom skeletons in compounds of the series (n = 6-12) can adoptd Convenient syntheses... 

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