M. Hofmann et al., EXISTENCE OF C,3-ME(2)-CLOSO-1,2-C2B3H3 REFUTED BY THE AB-INITIO IGLO, GIAO-MP2 NMR METHOD - ATTEMPTED REPETITION OF THE SYNTHESIS/, Inorganic chemistry, 35(21), 1996, pp. 6170-6178
The B-11 chemical shift assignment for the closo-carborane C,3-Me2-1,2
-C2B3H3 structure (1a or 1b) is refuted by ab initio/IGLO, GIAO/NMR ev
aluations. Attempts to reproduce the literature NMR data by examining
the computed chemical shifts of many alternative structures failed. Re
arrangement of the 1,2-C2B3H5 isomer into the much more stable 1,5-C2B
3H5 is computed to involve only a modest energy barrier. Repetition of
the thermal (but not the electric discharge) experiments failed to gi
ve a compound with the spectroscopic data reported earlier. The origin
al experimental data remain unexplained. Nevertheless, the elimination
of the C,3-Me(2)-1,2-C2B3H3 structural assignment removes the sole ex
ception to four established structural patterns of closo-carboranes: (
a) carbons tend to occupy sites with the lowest coordination, (b) carb
ons tend to be nonadjacent in the most thermodynamically stable closo-
carboranes, (c) the B-11 NMR of boron atoms with exo-terminal alkyl gr
oups are always found at lower field than those of otherwise identical
boron atoms with exo-terminal hydrogens, and (d) exo-terminal delta(H
-1)'s normally parallel the delta(B-11) of the boron nuclei to which t
hey are bound.