Closo-carboranes

As with the simple boranes, the closo 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 closo carborane. For example, pyrolysis of 2,3-C2B4Hg is another route to 2,3-C2B3H2 [30347-95-6], l,2-C2B4Hg [20693-68-9] and l,6-C2B4Hg [20693-67-8], and 1,5-C2B3H3 [20693-66-7] (123). 

In most cases the prolonged treatment of a closo 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. 

Cluster expansion reactions with diborane provide an alternative route to intermediate closo-carboranes, e.g. ... 

The reaction, which is quite general for closo-carboranes, involves the reductive opening of an n-vertex c/o50-cluster followed by metal... 

Several reports of carboranylated porphyrins have appeared for utilization in the BNCT of cancer.150 The examples include both nido- (126) (Fig. 73) and closo-carborane cluster-containing (124 and 125) (Fig. 73) materials. Metal-free nido-carboranylporphyrins were able to deliver a higher amount of boron to cells than the corresponding zinc complexes. 

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. 

Reaction of 13a with elemental Li in Et20 yields 15a, the first derivative of the simplest polyhedral closo-carborane of the anionic series [CH(BH) ] (n = 4) (Scheme 3.2-9). Polyhedral members with n = 3 are transition states of type 5 as shown in Section 3.2.2.1. 

Scheme 3.2-24. Reaction of 44 with BBr3 gives a mixture of closo-carboranes 45, 46 and 47, and with FeCI2 the metalla-carborane/sandwich complex 48.

For a recent photoelectron spectroscopic study of some closo-carboranes C2B 2Hn n = 5, 6, 7, or 12) which supports this skeletal bonding treatment, see Ref. 82a). 

The alkaline degradation of closo-carboranes, used to prepare smaller carboranes from icosahedral starting materials, also occurs by closo- to nido-, possibly even arachno-, cage opening, e.g. 107),... 

Onak s CB5H7 (III-C4) 101) is under investigation by Beaudet 3), who has determined that the lone 666-bridge hydrogen is almost certainly above a triangular face. This undesirable situation (a violation of rule 2p) probably accounts for its instability, i.e., in the presence of a Lewis base (proton acceptor) the related closo-anion CBsHe" is produced 116). More recently, R. R. Rietz has reported (private communication, 1974) a second closo-carborane, CBgHg. 

Almost all of the ido-carboranes have bridge hydrogens, and the organization of their structures is much more complicated than closo-carborane structures because the bridge hydrogens (rule 2) apparently... 

Homoleptic gold(III) derivatives with dithiolate, ligands of the type [Au(S-S)2] are well known and are usually prepared from [AuC14] with the dithiol some examples are with 1,2-benzene dithiolate, maleonitriledithiolate, dmit, and so on, [164, 336]. Similar complexes have been reported for bidentate sulfur ligands such as dithiocarbamates, dithiophosphates, and so on, [41]. Other derivatives as the trinuclear species [Au(C6F5)(S2C6H4)]3 [337] or the complex with one nido and one closo-carborane dithiolate are known [338]. Figure 1.72 collects some of these complexes. 

The present review will be based on the classical views of aromaticity [14-16], confining it to two dimensions, although it has been customary to ascribe the exceptional stability of icosahedral closo-carboranes to their three-dimensional aromaticity [17, 18], The discovery of fullerenes, nanocones, and nanotubes has opened new vistas for bent and battered benzene rings. Also, it is possible to obtain benzenic rings... 

Three-dimensional aromaticity has been demonstrated, for instance in the case of Cram s cyclophane in 1951 [46, 47] and, more recently, for von Rague Schleyer s adamantane dication [48], half-sandwich carbocations, and nido-carboranes. Ferrocene and various other or-ganometallic compounds also are three-dimensional aromatics. Spherical aromaticity is exhibited by closo-carboranes, borane anions, and, to a lesser extent, C60, although these compounds are isotropic. 

A separate series of closo-carboranes, isoelectronic with the BnH2 dianions and the neutral QB 2H compounds, consists of the monocarbon anions CB iH (Table 5-3). The preparative routes to these species vary widely, but a commonly used method for incorporating a single carbon atom into a borane cage involves the conversion of cyanoboranes to amino derivatives the cyano carbon atom satisfies its valence requirements via absorption into the cluster framework. 

The structures of these compounds seem to dictate their a closed polyhedral structure. The closo carboranes, such... 

Some carboranes have formulae compatible with both classically bonded and nonclassically bonded structures and exhibit valence isomerism. The tet-racarbaborane Me4C4B6Et6 is one such. As prepared by dimerization of the small closo carborane Me2C2B3Et3 [Eq. (3.2)], it has a classical adamantane-type structure, with its four CMe units linked through BEt units (1). However, when heated it isomerizes to the expected nido carborane structure (2) with a skeletal structure like that of decaborane. ... 

Figure 3.8. B-C and B-B bond orders calculated and CH and BH group charges for closo boranes and closo carboranes [B H ]2 [1-CB iHJ" and 1. //-(CB LI, show the skeletal electron distributions in these systems.

---