Carboranate anion

The discovery (116) of the base-promoted degradation of the isomeric closo-Q,. . cages provided one of the most important carborane anion systems, the isomeric nido-Q,. ][ , anions,... 

Figure 6.20 Degradation of o/oso-carboranes to the corresponding debor-mWo-carborane anions.

Metallocarboranes and metalloboranes, discussed in 6.5.4, may be prepared from metal halides and carborane anions. Selected examples of complexes formed by thermal—or otherwise—induced polyhedral rearrangements of existing metallocarboranes are given in a few equations, and some other individual compounds are listed in tables. 

The original synthesis of metallacarborancs is effected by deprotonation of nido-carborane anions in a manner like that employed to prepare ferrocene, e.g. ... 

Because the complexes listed in Tables 1-8 are all prepared with metal halides and carborane anions, only one example for each type of metallocarborane (number of B atoms) is given in the following representative equations for clarity, the skeletal framework in the equations have numbered positions and H atoms are omitted ... 

Another milestone discovery in the held of silyl cations chemistry was achieved by Reed and co-workers in the same year, 1993, when Lambert published his Et3Si study. Reed synthesized his t-PrjSi (CBuHsBrg), (2+ (CBnH5Br5), by the hydride transfer reaction of /-PrjSiH and Ph3C (CBnH6Br6) in toluene,taking advantage of the very low nucleophilicity of the carborane anion" (Scheme 2.9). 

Monoanionic carborane anions based around [c/ow-l-CBnHi2]- are among the most inert and least coordinating anions currently known. The high chemical stability and low nucleophilicity allows the stabilization of cationic species not isolable with other anions. The silver salt of this compound was crystallized as Ag(CB11H12) 2C6H6 (236), in which the silver is bonded to two... 

At this point the hygroscopic potassium salt may be isolated and dried, or, more conveniently, the potassium salt may be dissolved in water and the carborane anion precipitated with one of a variety of large cations, such as the rubidium, cesium, tetramethylammonium, or trimethylammonium ions. The tri-methylammonium salt of the carborane anion is useful because it is readily purified by recrystallization from water and may be easily converted in solution to salts containing other counterions. ... 

Fig. 6 A view down the c-axis of the (BEDT-TTFlFe BgHjoC SE salt, illustrating the very thick anionic layers which result from the thiophene derivatized iron carborane anions. Hydrogen atoms and disordered thiophene groups have been omitted for clarity...

Ab initio computations have so far proved to be reliable structural tools for neutral carboranes and their anions. They have been used to predict the carborane geometries for six-vertex, eight-vertex, and 11-vertex nido-carboranes,67-69 and for 5-12 vertex r/aro-carboranes.70,71 However, carborane anions in the presence of a group I metal cation may form ion-pair anions with one cation tightly bound to the carborane anion (see Chapter 3.03).72,73 Even computations on a salt with a carborane anion in a non-coordinating solvent would need to include the cation for computed shifts that tally with observed NMR data.74... 

The nido-carborane anion, 7,8-C2B9H12, and derivatives are generally formed by deboronating ort o-carboranes with suitable bases (COMC (1982) 5.4.2.6.6) but a direct route to two derivatives of the anion has been developed in good yields from decaborane and polarized alkynes.78... 

Figure 4 Synthesis of carborane anion, CB-nH12, by carbene insertion. 

Thus, further refinement of this route to the carborane anion CBnHi2 from B11H14 would represent a significant advance in opening up CBnH12 chemistry. 

The Brellochs reaction represents a significant breakthrough in monocarbaborane chemistry, the exploration of which was previously limited by multistep routes. For example, the important carborane anion CBnFI12 can now be made in two steps with an overall yield of 66%.88 Its synthesis previously required three steps involving undesirable compounds such as hydrogen cyanide, dimethylsulfate, and trialkylamine. (COMC (1982) 5.4.2.6.2) The arachno-cztbotznc... 

LnCl3, only the full-sandwich complexes were formed.204 The reactions of the larger, nido-C2B9-carborane, anion... 

The controlled synthesis of carbons adjacent nido- and arachno-carborane anions can be achieved by the reduction of C (cage)-C (cage)-linked o-carboranes with group 1 metals.234-237 Typical examples are illustrated in Scheme 22. These have been found to undergo metallation reactions similar to their carbons apart analogs, forming full- and half-sandwich complexes of lanthanide metals.238... 

In solution, the TPFPB salt of the methyl-substituted vinyl cation 19 has only limited stability. Decomposition into unidentified products took place during several days. In contrast, the steric and electronic effects provided by the /-butyl substituent confer indefinite stability to the vinyl cation 20 in the absence of nucleophiles. That is, according to the NMR experiments, a solution of 20 TPFPB in benzene showed no sign of decomposition even after weeks at room temperature. With the carborane anion [CBnH6Br6] suitable crystals for... 

In a series of papers Reed and colleagues present the solid state structures of a series of simple trialkyl-substituted silyl cations with halogenated carborane anions... 

Reed and co-workers utilized the silylium carboranate salts to generate novel Bronstedt superacids based on carborane anions as conjugate bases by reaction of the salts with liquid HCl [Eq. (7)]. These carborane superacids are able to cleanly protonate Cgo and benzene at room temperature to yield and benzenium... 

Larsen, A. S., Holbrey, J. D., Tham E S., Reed, C. A., Designing ionic liquids Imidazolium melts with inert carborane anions, /. Am. Chem. Soc., 122, 7264-7272,2000. 

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