Supraicosahedral carboranes

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. 

Supraicosahedral Carboranes in Detail 3.02.17.1 Eleven-vertex Carboranes... 

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. 

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... 

Figure 3.26. Routes to supraicosahedral carboranes using tethers to keep cage carbon atoms adjacent.

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... 

The structure of the bimetallic 10-vertex cluster was shown by X-ray diffraction to be (84). When the icosahedral carborane l,2-C2BioHi2 was used, the reaction led to the first supraicosahedral metallocarboranes with 13- and 14-vertex polyhedral structures (85)-(89). Facile isomerism of the 13-vertex monometallodicarbaboranes was observed as indicated in the scheme above (in which = CH and O = BH). 

Closo-carboranes with 5 to 12 cage vertices have been known for several decades (Figure 1, COMC (1982) 5.4.2.2 and 5.4.2.3). However, the first report of a 13-vertex c/ttro-carborane only appeared in 2003. 2 The synthesis of the first supraicosahedral closo-carborane was made possible using a tether linking both carbon atoms of ort o-carborane with... 

Unlike the subicosahedral geometrical pattern, the supraicosahedral pattern is complex as shown in Figure 13 (COMC (1982) 5.4.2.7.2 and COMC (1995) 6.2.3.1, 6.2.4.4 and 6.2.5.1). For example, there are eight distinct geometries known with a nido-12-vertex count. This pattern is expected to expand further as the interest in 13-and 14-vertex closo-carboranes, which were only identified in the last three years, grows. 

Polyhedral carboranes (continued) seven-vertex carboranes, 3, 58 six-vertex carboranes, 3, 56 small carborane syntheses, 3, 54 subicosahedral carborane geometrical patterns, 3, 50 subicosahedral and icosahedral, 3, 51 supraicosahedral, characteristics, 3, 96 ten-vertex carboranes, 3, 60 thirteen-vertex carboranes, 3, 100 tricarbaborane synthesis, 3, 54 twelve-vertex carboranes, 3, 98 Polyhedral skeletal electron pair theory, Ru and Os tetranuclear clusters, 6, 874-875 Poly(iV-heterocyclic carbene) ligands... 

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. 

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