Carborane definition

While related to its carbon analogs, the existence of the RsSi species as a free ion in condensed phases had been doubted for a long time. However, NMR characterization using bulky aryl substituents has provided evidence for the triply coordinated silicon cation. " However, definitive evidence was recently reported by the groups of Reed and Lambert with a silyl cation species bound to three mesityl groups and a carborane [HCBnMesBrg] counterion (Eig. 7.5). It was suggested that... 

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... 

HMPA (hexamethylphosphoramide) OP(NMe2)3 4 does not deboronate the C2B10H12 isomers at room temperature but acts as an hydrogen bond acceptor for the acidic hydrogens at the carbon atoms of these clusters to yield 1 1 carboranerHMPA adducts on crystallization.6 These C-H--0 hydrogen bonded supramolecular structures provide the first definitive X-ray structures of the unsubstituted carboranes. 

The structural patterns illustrated in Figure 3.1, and the shapes shown in column 3 for arachno species, are essentially those first suggested by Williams in 1971,2 and have stood the test of time remarkably well. However, the shapes shown should be regarded as illustrative rather than definitive. As with the nido species, alternative shapes to those shown in Figure 3.1 are possible for arachno species, with different sites left vacant on the parent deltahedron. We have already noted that many nido boranes and carboranes with eight skeletal atoms show deviations in their atom connectivities from those shown in Figure Nevertheless, apart from these particular exceptions,... 

As a result of numerous B NMR studies, chemical shift values have been reported for various boron-containing species. What follows is a compilation of chemical shifts corresponding to different common boranes and related species as recorded in the original bodies of work (Figure 2.1). The list includes single-boron compounds, polyboranes, carboranes, and borates. Historically, chemical shift values have been reported relative to numerous reference standards, and the definition of upheld versus down-field has not always been consistent. All chemical shift values herein are given in relation to the external reference standard BF3-Et20 and rounded to within 0.1 ppm. 

Despite the amount of effort that has been applied, a definitive mechanism is still lacking, perhaps suggesting that several mechanisms may be operative. In the most recent work, Wu and Jones favor the triangular-face rotation mechanism, Edvenson and Gaines favor a closo —> nido mechani.sm, and Roberts and Johnson favor an anticubeoctahedron mechanism. Wales has computed a number of intermediates and transition states on the C2B10H12 surface with full optimizations and frequency characterization at the STO-3G level. The highest activation barrier from o-carborane on the pathway to m-carborane was 311 kJ mol" and the highest activation barrier from m-carborane toward p-carborane was 494 kJ mol". Many of the intermediates had only C1 symmetry. 

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