Supraicosahedral (metalla)carboranes

Although supraicosahedral (hetero)boranes have long been of interest to theoreticians, the area is under-developed from a synthetic viewpoint. The synthesis of supraicosahedral carboranes by reduction then capitation (RedCap) of C₂B₁₀ species is attractive, but unsuccessful as long as the cage carbon atoms are free to separate in the reduction step. Studies on 4,1,6-MC₂B₁₀ 13-vertex metallacarboranes have shown that the partial degradation of such species can be a facile process, in spite of the fact that the binding energy of the metal atom to the carborane framework can be at least as high as that of a {BH} fragment. These findings support the general concept of the kinetic instability of 1,6-C₂B₁₁ species, explaining why a supraicosahedral carborane could not be made from 1,2-C₂B₁₀H₁₂. However, tethering together the two cage C atoms with a C₆H₄(CH₂)₂ strap ultimately allowed the synthesis of the first supraicosahedral carborane. This species has a henicosahedral geometry, and there is evidence that a facile rearrangement from kinetic to thermodynamic isomer has occurred. The RedCap synthesis of this unprecedented cluster has the potential to be applied successively, yielding 14-, 15-, 16-, etc. vertex carboranes, the larger of which may be sufficiently kinetically stable to exist without a C,C tether.