Metallacarboranes supraicosahedral

Application of the polyhedral expansion methodology to C2B10H12 leads to supraicosahedral metallacarboranes such as closo-(t C HB) Co C2B1 -H 12 [33340-90-8] (194—199). Further expansion of 13-vertex species or thermal metal transfer reactions leads to the 14-vertex cluster [(T C H )Co]2C2B1 ( H 12 [52649-56-6] and [52649-57-7] (199). Similar 14-vertex species have been obtained from tetracarbaboranes (203) and show unusual structures. The isomeric bimetallic cobaltacarborane complexes c/oso-(r 3 -CpCo)2C2BgH10 (cp = C H ) can be formed by either polyhedral expansion or contraction reactions. Six isomers of this cluster are formed in the thermally-induced intermolecular metal transfer and polyhedral expansion of the 11-vertex Ao-(t b-CbHb)CoC2B8H10. 

A question of interest in deltahedral borane chemistry is whether supraicosahedral boranes B H 2 (n > 13) can be prepared and whether the Frank-Kasper polyhedra (Figure 1) are suitable models for their structures. The supraicosahedral metallacarborane (C5H5)2Co2B10C2H,2 is known21 and has a structure based on the Frank-Kasper bicapped... 

It is now well established experimentally that the range of deltahedral structures possible for metallaboranes and metallacarboranes extends beyond the icosahedron to include 13- and 14-vertex deltahedra,as will be outlined later (see Section 3.10). It is also known, from calculations and some experiments, that polyhedral structures with some nontriangular faces (and so not fully deltahedral) may be preferred for boranes and supraicosahedral carboranes.What structures are experimentally possible for carboranes themselves beyond the icosahedron has yet to be fully explored. We focus here on systems with up to 12 skeletal atoms. 

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. 

Routes to such supraicosahedral metallacarboranes have typically involved the RedMet reaction sequences already mentioned [Eq. (3.10)], sequences first used by M, F. I lawthorne in the 1970s since then, well over a hundred closo MC2B10 species, but many fewer M2C2B9 and M2C2B10 species, have been characterized. Further information on such systems is to be found in References 50 and 52. 

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... 

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