_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Cluster compounds
mixture of the more stable closo-boranes is produced (Fig. 11.5 ). Nido carboranes
can also be pyrolysed or photolysed to closo species. Carboranes often exhibit high
thermal stability, the closo compounds being more stable than the nido. Most closo
boranes are stable to at least 400°C under static conditions. At or above this
tempera ture rearrangements to geometrica! isomers may be observed. It is found
that those closo compounds in which the two carbon atoms are separated as far as
possible in the structure are the most stable isomers and are often the final
products of pyrolysis.
Carboranes may contain one, two, three or four carbon atoms, although those
with two have been most thoroughly studied, being readily accessible by the
alkyne route. Insertion of ethyne into the arachno borane B 4 H 10 gives a mixture of
nido-carboranes including 1, 2-C 2 B3H 9 and alk:yl derivatives of nido-2, 3, 4-
C3B3H7. Treatment of the former product with more ethyne affords closo-2, 3, 4,
5-C 4 B 2 H 6 (Fig. 11.6).
HC=CH
+ other products
Fig. 11.6 Reactions of tetraborane (10) with ethyne
c/oso-2,3,4,5-C4 B2 H6
Carboranes show characteristic absorptions in the infrared at
2900-3160cm- 1 v(C-H), 2500-2660cm- 1 v(B-Hterminat) and
1800-2050 cm - 1 , 1460-1500 cm - 1 v(B-Hbrtdging), which are useful in characterization.
N.m.r. spectroscopy is also valuable, information being available not
only from 1 H and 13 C spectra, but also from 11 B (I=f) (80% abundance).
Interpretation of 11 B spectra is simplified as 11 B- 11 B coupling is not detected. The
other naturally occurring isotope ofboron {I 0 B) has I = 3. Both 11 B and 10 B nuclei
couple to attached protons, but as proton signals from 10 B- 1 H are oflow relative
intensity on account of the isotopic abundance, tend to be rather broad and are
split into seven components their contribution is usually ignored. The resonance
from a proton attached to 11 B consists of four lines of equal intensity.
The eicosahedral closo-carborane 1. 2-C 2 B 10 H 14 is prepared by the action of
ethyne with nido-B 10 H 14 in the presence of a Lewis base:
342
nido-B, 0 H 14 + 2SEt 2 ---> B, 0 H 12 (SEt,) 2 + H 2
B, 0 H 12 (SEt 2 ) 2 + C 2 H 2 ---> closo-1, 2-C 2 B 10 H, 2 + 2SEt 2 + H 2