C/oso-Carboranes

Although c/oso-carboranes are stable to high temperatures and to most common reagents, M. F. Hawthorne showed (1964) that they can... 

A particularly elegant route to metallacarbor-anes is the direct oxidative insertion of a metal centre into a c/oso-carborane cluster the reaction uses zero-valent derivatives of Ni, Pd and Pt in a concerted process which involves a nett transfer of electrons from the nucleophilic metal centre to the cage ... 

As with the simple boranes, the c/oso carboranes are generally more thermally stable than the corresponding nido and arachno species. Thermal decomposition of nido and arachno carboranes often leads to one or more c/oso carborane. For example, pyrolysis of 2,3-C2B4Hg is another route to 2,3-C2B5H7 [30347-95-6], 1,2-C2B4H6 [20693-68-9] and 1 6-C2B4H6 [20693-67-8], and 1,5-C2B3H5 [20693-66-7] (123). 

In most cases the prolonged treatment of a c/oso carborane or borane with strong base results in the removal of a single-boron vertex to yield a nido cluster, inert to further degradation. This principle is exploited in the polyhedral contraction and subrogation synthetic strategies. In the prototypical case,... 

Synthesis of carbon-atoms-adjacent 13- and 14-vertex c/oso-carboranes and n fo-carborane anions. From L. Deng, H.-S. Chan and Z. Xie, Angew. Chem. Int. Ed. 44, 2128—31 (2005). 

We have shown for the first time 12 that not only c/oso-carboranes, but also iodo-nido-7,8-dicarbaundecaborates containing an iodine atom in the polyhedral skeleton or in the pentagonal plane of the polyhedron enter into cross-coupling reactions with organomagnesium and organozinc compounds catalyzed by palladium complexes. It is known that ifo-7,8-dicarbaundecaborates are strong BH-acids (pKa = 21.1), due to the presence of the axial hydrogen atom. 

C/oso-carboranes are the most numerous and the most stable of the carboranes. They are colourless volatile liquids or solids (depending on mol wt.) and can be prepared from an alkyne and a borane by pyrolysis, or by reaction in a silent electric discharge. This route, which generally gives mixtures, is particularly useful for small c/oso-carboranes (n —5-1) and for some intermediate c/oso-carboranes n = 8-11), e.g. 

Figure 6.20 Degradation of c/oso-carboranes to the corresponding debor-m do-carborane anions.

Arene-Coupled Macrocyclic Systems Incorporating c/oso-Carboranes... 

Table 19 Synthetic routes to c/oso-1, 2-C2BioH- 2 (ortho-carborane) derivatives from decaborane and alkynes...

Table 23 C-H to C-X and B-H to B-X conversions in c/oso-1,7-C2Bi0Hi2 (meta-carborane) derivatives...

Table 24 Structures of c/oso-1, 7-C2BioH12 (mefa-carborane) derivatives determined experimentally by X-ray diffraction unless otherwise stated ...

Molecular Orbital Descriptions. In addition to the localized bond descriptions, molecular orbital (MO) descriptions of bonding in boranes and carboranes have been developed (4). Early work on boranes helped develop one of the most widely applicable approximate MO methods, the extended Htckel method. Molecular orbital descriptions are particulady useful for c/oso molecules where localized bond descriptions become cumbersome because of the large number of resonance structures that do not accurately reflect molecular symmetry. Such descriptions show that the highest occupied MO (HOMO) is degenerate for most deltahedral B H molecules, but that a closed shell is obtained for the corresponding [B H l2 anions. After... 

Tetraborane(lO) reacts with RC=CR (R=H, CH3) to produce mixtures of small w io-(open cage) or c/oso-(closed cage) carboranes 163> however, with C2H4, 2,4-dimethylenetetraborane(10) is formed 164>. This compound is properly classified as an alkyl substituted tetraborane(lO). 

The insertion of a tin atom into a nonicosahedral carborane framework was first reported by Grimes and co-workers (233), following approaches similar to those which had previously been successful with transition metal reagents. Reduction of c/oso-2,4-C2B5H7 with sodium naphthalenide generates the C2B5H7 dianion, which on treatment with tin dichloride yields... 

FIGURE 10. (a) (He I) PE spectrum of l,2-dimethyl-l,2-disila-cZoso-dodecaborane(12) ((H3CSi)2 BioHio) with Koopmans assignment by MNDO eigenvalues as well as cluster orbital diagrams for the three radical cation states of lowest energy (b) comparison of MNDO charge distributions for both 1,2-carborane as well as 1,2-bis(silamethyl)-c/oso-dodecaborane(l 2)... 

The first definitive crystal structures of the group 14 c/oso-metalla-carboranes were those reported by Hosmane and co-workers (38-41) from the reactions involving the monoanion of m do-2-[(CH3)3Si]-3-(R)-2,3-C2B4H6 [R = (CH3)3Si, CH3, H] with SnCl2 in THF (C4H80) to produce the corresponding stannacarboranes (see Scheme 3). The structures of the complexes (Figs. 13-15) show Sn2+ to occupy an apical position in... 

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