Supraicosahedral (metalla) carboranes

Abstract

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.