Carborane chemistry

The research field of ortho-carborane chemistry continues to expand rapidly mainly due to the commercial availability of ort/fo-carborane itself and the recent interest of these derivatives as agents for BNCT and other applications. Synthesis of these carboranes is still dominated by reactions of decaborane with various alkynes as shown in Table 19 (Figure 2). 3-Substituted derivatives are made by capping the /[Pg.71]

It is anticipated that the area of supraicosahedral carborane chemistry will expand rapidly since the materials like ort o-carborane needed to make these 13- and 14-vertex carboranes are commercially available. 

In order to recognize the patterns that pervade carborane chemistry, it is instructive to conduct a molecular census, to ascertain where and in what coordination-number environments the various groups, e.g., BH, BHj, CH, CHg, CHj, and Lewis base equivalents (B LB, BH LB, and BH2 LB, where LB = Lewis base) are most frequently found. Table II itemizes the various groups obvious trends may be observed and examples of the extreme cases are given. 

Wade also incorporates electron-rich species such as C5H5 and C4H42- as arachno species derived by removal of two nonadjacent high-coordination vertices from the pentagonal and tetragonal bipyramids, respectively, and has, moreover, correlated many aspects of seemingly unrelated classes of compounds beyond the recognized borders of carborane chemistry. 

Early in the development of polyhedral borane and carborane chemistry the need arose to degrade these materials under protolytic conditions for analytical purposes. Such reactions, for example,... 

The rapid development of carborane chemistry is mainly due to their practical applications. For instance, the potential utility of carborane polymers as gaskets, O-rings, and electrical connector inserts has been reported. Their functionality for solvent extraction of radionuchdes as well as the potential medicinal value of the isoelectronic and isostructural boron analogues of biologically important molecules has been the subject of many review articles. For example, a number of boron compounds have been found to possess anti-inflammatory and antiarthritic activity in animal model studies. Boron compounds have also been implicated in studies designed to probe the importance of the so-called anionic subsite of acetylcholine esterase and Ach receptors. But, by far the most interesting practical apphcations of carboranes are in areas of boron neutron capture therapy (BNCT) and supramolecular assembly. 

Figure 7 Application of the Barton oximation reaction to carborane chemistry...

The chemistry of the eleven-vertex nido tricarbaboranes (tricarbollides) is, in contrast to that of the dicarbollides1 one of the youngest areas of carborane chemistry. It was as late as in 1995 when the first representatives of this family were reported.2 Essentially, there are two synthetic routes leading to tricarbollide compounds cyanide and isonitrile methods.3 The cyanide method is based on a room-temperature reaction between the dicarbaborane... 

A recent addition to organolanthanide carborane chemistry is the use of hydroxyethyl- and alkoxycthyl-tf-carborane ligands. Scheme 217 illustrates typical synthetic routes leading to lanthanide derivatives of such ligands.761... 

The discovery of metallacarboranes was of major importance not only for carborane chemistry, but also for organometallic and cluster chemistry in... 

Structurally, metallacarborane chemistry is the most fully documented area of carborane chemistry. There is not enough space here for a comprehensive survey, but Tables 3.2 and 3.3 and Figure 3.22 show the formulae and skeletal structures of some representative examples. More comprehensive compilations of structural data are to be found in References 1-3 and 199. Metals have been incorporated into the whole range of known carborane deltahedra, and indeed their presence has allowed supraicosahedral systems to be made that are without precedent among metal-free carboranes or boranes, as outlined below. 

A breakthrough in 2003 into supraicosahedral carborane chemistry (as distinct from supraicosahedral metallacarborane chemistry) came from the use of external tethers to hold together the two cage carbon atoms during syntheses. - - Hie groups of A. J. Welch - - - and Z. have... 

Now that access into supraicosahedral carborane chemistry has been achieved, it appears likely that more higher carboranes will be prepared, and that further ingenious synthetic routes to such systems will be found. Though the stabilities of the products may not rival those of icosahedral carboranes, it will be interesting to see whether many of the higher carboranes or boranes envisaged by Lipscomb,--- Hoffmann,Schleyer,34,35,55,22 230... 

Recently, even dicarbollide ions have been coordinated to carbonyl compounds 217) providing an example of a triple cross-over" between carborane chemistry, coordination chemistry, and photochemistry. 

Recent research activity in carborane chemistry has been directed toward expanding the use of carborane clusters in materials science including, among others, molecular recognition systems, display devices, modular construction systems, NLO materials and special polymers. Nonetheless, their use in developing functional materials such as olefin polymerization catalysts and luminescent materials is still limited. 

Carboranes were first reported in 1962. Much of the early work was done in the USA as part of a classified project on borane chemistry, seeking novel rocket fuels, an object which proved abortive. Since then thousands of compounds of this type have been reported. Carboranes have structures which are related to those of the boranes by isoelectronic replacement of groups (e.g. B" or BH by C BH or BH by CH). They may belong to closo, nido or arachno series, although the last are relatively uncommon. The usual entry to carborane chemistry is to heat a borane with an alkyne or to subject the mixture to a silent electric discharge. Thus nido-BjH, and ethyne at 200°C give mainly nido-I S-C BJl, while at 500°C a... 

There are a very large and growing number of more complex boranes. In fact, it is fair to say that borane and related carborane chemistry (in which one or more carbon atoms replace the same number of boron atoms) have constituted one of the major areas of growth in inorganic chemistry over the last five decades. Most... 

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