Closo Carboranes species

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). 

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. 

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. 

Caution. Hydrogen peroxide must not come in contact with organic material or solvents, due to the possibility of fire or explosion. Extreme care must be taken to ensure the identity of the cesium dodecahydro-closo-dodecaborate reagent. The exposure of other polyhedral borane or carborane species to the reaction conditions described herein could result in spontaneous explosion. All reactions were conducted in well-ventilated hoods using additional polycarbonate blast shields. The product, Cs[B12(OH)12], must not be brought to dryness in the presence of peroxide solutions as the solid peroxide has been observed to be a shock-sensitive explosive. 

Nonicosahedral carboranes can be prepared from the icosahedral species by similar degradation procedures or by reactions between boranes such as B H q and B H with acetylenes. The degradative reactions for intermediate C2B H 2 species (n = 6-9) have been described in detail (119). The small closo-Qr Yi 2 species (n = 3-5 are obtained by the direct thermal reaction (500—600°C) of B H using acetylene in a continuous-flow system. The combined yields approach 70% and the product distribution is around 5 5 1 of 2,4-C2B3H2 [20693-69-0] to l,6-C2B Hg [20693-67-8] to 1,5-C2B3H3 [20693-66-7] (120). A similar reaction (eq. 60) employing base catalysts, such as 2 6-dimethylpyridine at ambient temperature gives nido-2 >-(Z, ... 

The discovery of polyhedral boranes and polyhedral heteroboranes, which contain at least one atom other than in the cage, initiated a new era in boron chemistry.1-4 Most commonly, of the three commercially available isomeric dicarba-closo-dodecaborane carboranes(l,2-, 1,7-, and 1,12-), the 1,2-isomer 1 has been used for functionalization and connection to organic molecules. The highly delocalized three-dimensional cage bonding that characterizes these carboranes provides extensive thermal and kinetic stabilization as well as photochemical stability in the ultraviolet and visible regions. The unusual icosahedral geometry of these species provides precise directional control of all exopolyhedral bonds. 

The structures of nido- and amcAwo-boranes and carboranes can be rationalized in a similar manner to those of the closo species if one considers the hypothetical anions, and B Hn , from which they... 

As Table 5-3 indicates, isomers exist for several of the closo systems (although not all of the possible isomers have been found), and thermal rearrangements of the cage skeleton are common in these species, as described below. The lower members are colorless volatile liquids, while the 1,2-, 1,7-, and 1,12-QB10H12 isomers (known informally as ortho-, meta-, and para-carborane, respectively) are white nonvolatile solids. 

For polyhedral clusters (sometimes called deltahedral, because the faces are all triangles resembling the Greek letter delta) the ancestor of all electron counting schemes is the correlation proposed by Wade between borane (or carborane) cages and metal carbonyl cages. Wade first drew attention to the similarity of a M(CO)3 unit and a BH (or CH) unit, a relationship that we would now call isolobality (Section 1-6). He then proposed that the 2n + 2 rule for closo boranes (Chapter 5) would also apply to closo metal cluster species such as [Os CO) ]2, and that 2n + 4 and 2n + 6 electron counts would, similarly, be appropriate for stable M clusters with nido and arachno structures. Hydrogen atoms are assumed to contribute one electron each, an interstitial carbon atom four electrons, and so on. 

This is basically the same as the one in procedure A but using the closo species instead of the nido one.l8, It is far simpler than the former one since the nido species does not need to be synthesized. For [(rf-NC4H4)Co(C2B,Hu)] the procedure is as simple as the reaction of o-carborane with KCNCJHJ and CoCl2 in the ratio 1 12 5 in dme yields after working up [(rf-NC4H4)Co(C2B,Hll)] in 76% yield. The reaction is shown in Figure 3. 

The electron-counting schemes can be extended to isoelectronic species such as the carboranes (also known as carbaboranes). The CH" unit is isoelectronic with BH many compounds are known in which one or more BH groups have been replaced by CH (or by C, which also has the same number of electrons as BH). For example, replacement of two BH groups by CH" " in yields closo-C- Bayif, a neutral compound. 

As an alternative to the reductive generation of a nido borane or carborane cluster from a closo parent, one can regard a nido species as the product of decapitation or deboronation of a closo parent. Formal removal of a B kf unit, normally from a high connectivity site in a closo borane or carborane, followed by protonation of the anionic residue around the open face generated, leaves a neutral nido residue. 

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