Supraicosahedral (metalla) carboranes*
Alan S.F. Boyd1, Anthony Burke1, David Ellis1, Daniel Ferrer1, Barry
T. Giles1, Miguel A. Laguna1, Ruaraidh McIntosh1, Stuart A. Macgregor1,
Daniel L. Ormsby2, Georgina M. Rosair1, Frank Schmidt1, Neil M.M.Wilson1,
and Alan J. Welch1
1 Department of Chemistry, Heriot-Watt University,
Edinburgh EH14 4AS, UK; 2 Department of Chemistry, Imperial College,
London SW7 2AZ, UK
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.
*Lecture presented at the XIth International Meeting
on Boron Chemistry (IMEBORON XI), Moscow, Russia, 28 July - 2 August
2002. Other presentations are published in this issue,
pp. 1157-1355.
