Carboranes relative stabilities

The structures, relative stabilities, and relative Lowry-Bronsted acidities of carboranes and boranes as well as related anions, Lewis base adducts, and heteroelement analogs are rationalized primarily on the basis of rudimentary coordination numbers. The principal factors, in decreasing order of importance, are (a) the various deltahedra and deltahedral fragments, (b) the placement of bridge and endohydrogens, (c) the placement of carbon and other heteroelements, and d) the resulting coordination number of boron. 

Although the existence of C2B10Hi22 ions derived from the o-, m- and p-carboranes were reported already during the 1960 ies1 4, their number and their structures have remained largely unknown. The chemistry of the dianions has been extensively explored, but they have not yet been isolated as salts. In the present study we discuss the uptake of one and two electrons by each of the three carboranes, based on ab initio calculations, and present the resulting optimized structures and relative stabilities of the neutral carboranes and their anions. 

The concept of intramolecular coordination by the use of C, Y-chelating ligands of aryl units has been well established. However, the analogous chemistry of its cousin, the three-dimensional carborane, remains relatively unexplored. Recently, o-carborane has attracted much interest due to its ease of preparation and derivati-zation, thermal stability, and steric bulk. The easy preparation of o-carborane can be utilized for preparing useful compounds similar to the C, Y-chelating ligands of... 

With the relative air and water stability of silver phosphine complexes in mind, the Frost and Weller groups reported the use of a silver(I) carborane triphenylphosphine complex as a catalyst for the aza-Diels-Alder reaction.82,83 These unique catalyst complexes were able to catalyze the reaction of Danishefsky s diene (195) with 196 in... 

Ceo is a highly electrophilic molecule and numerous addition reactions of nucleophiles and electrons (to give Cg, n = 1-6) are known. Another surprise was therefore the recent report of the protonated [HCeo]. To stabilize this unusual acid, the exceptionally inert [CBnHeXg]" carborane anion was used (X = Cl and Br). (Have we become so jaded not to consider the anion exotic because its framework interpolates the superaromatic icosahe-dral [Bi2Hi2] and its dicarbon isoelectronic relatives, the isomeric 0-, m-, and p-carboranes, B10C2H12, and their numerous and diversely substituted derivatives )... 

Overall, p-carborane interacts with the linking group L in a way similar to a saturated system such as cyclohexane. As a consequence, the choice of the linking group L has a relatively small impact on mesogenic properties of the compound. However, the bulk of the /7-carborane diminishes the effectiveness of the carboxamide group in stabilization of the mesophases by obstructing the formation of the intermolecular H-bonds. 

It was postulated that the markedly different effect of carborane derivatives on host s properties results from the poor packing and the larger size of the carborane units in the more densely packed tilted phase than in the orthogonal one (tilt of the smectic phase results in more efficient packing of molecules). The anomalous behavior of 49D (m = 1) in the FLC mixture, was ascribed to relatively strong intermolecular interactions between the dopant and the host, which would suppress molecular motions and result in the formation of a more ordered phase. A similar stabilization of the E phase in a 3 2 mixture of 28A and 28D was explained by interactions between the carborane and electron-rich benzene rings. ... 

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