Icosahedral carboranes properties

Garboranes provide another interesting class of WGAs (see Figure 19). The 12-vertex icosahedral carboranes are very weakly donating, stable, and quite soluble in non-polar media. They are perhaps the least coordinating anions known. Because of these properties, substituted carboranes have been used to prepare bona fide examples of such... 

Carboranes, in general, and icosahedral carboranes, in particular, in their neutral and anionic forms are also known for their exceptional characteristics such as low nucleophihcity, high hydrophobicity, and their electron-withdrawing properties having highly polarizable o-aromatic character [48,49]. 

In contrast to the previously known boron hydrides, the polyhedral boron hydrides were shown to be exceptionally stable. One of the most striking features of the carboranes is the capability of the two carbon atoms and ten boron atoms to adopt the icosahedral geometry in which the carbon and boron atoms are hexacoordinated. This feature of carboranes gives rise to the unusual properties of these molecules and their carbon and boron derivatives. Due to these features of icosahedral carboranes, their chemistry has developed extensively, and the results obtained are summarized in R. Grimes monograph and review [5]. Investigation of the properties of polyhedral boron hydrides resulted in the conclusion that these compounds have aromatic properties. It was the first example of nonplanar three-dimensional aromatic compounds and resulted in the development of the eon-cept of three-dimensional aromaticity that is generally accepted at the present time [6,7]. 

Carboranes and phosphacarboranes, like the boranes themselves, may give rise to charged anionic species as well as neutral molecules. The best-known examples are those derived from the icosahedral BijHi j ion such as BnHnCH, B,oHioCH, B,oH,o2C and BioHioCHP. Compounds of this kind are extremely stable and they all tend to have similar chemical and physical properties. 

Over the past decades dodecahydro-c/oio-dodecaborate(2-) derivatives have been investigated experimentally and theoretically studies have shown that the cluster has favorable properties for application as nonlinear optical materials, compounds in which the dodecaborate acts as an electron donor and is connected to an electron-withdrawing substituent. In contrast to dodecaborate, neutral icosahedral c/oTO-carboranes behave as strong electron acceptors several investigations and theoretical studies confirm this effect. 

Boranes and carboranes are comprised of a-aromatic cage-like structures with boron and carbon vertices ranging from small tetrahedral to supra-icosahedral clusters. Despite more than four decades of work with this class of inorganic compound, there have been few reports of their photophysical properties, with a distinct paucity of information regarding luminescence. Carboranes have nevertheless been incorporated into photophysically active metal complexes, usually as ancillary components. But what of boranes, carboranes, and metallacarboranes (where a transition metal atom can either be an integral vertex in the polyhedral skeletal framewoik or an exterior component thereof) as chromophores themselves and their resultant optoelectronic behavior In this arena, there are very few contributors. 

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