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T. Base et al. / Journal of Organometallic Chemistry xxx (2015) 1e9
5
(ortho-carborane) can theoretically exist as five positional isomers.
Two of these (isomers 1-o and 9-o), with the magnitudes of their
dipole moments at the opposite extremes of the range exhibited by
the series, are used here as starting compounds for the thermolytic
isomerization experiments described in Section 1 above. The
products of the thermolysis experiments were experimentally
analysed using gas-chromatographic retention times and a corre-
lation between these times and the calculated dipole moments of
the individual isomers [29,30]. The dipole moments were calcu-
lated for the quantum-chemically energetically-optimised molec-
ular structures. All species, being isomers, had the identical
summary formula SC2B10H12 which permits direct comparison of
their calculated energies and thence rationalisation and prediction
of their relative stabilities and incidences in the conditions of the
thermolysis experiments. In agreement with their calculated dipole
moments (Table 1), the ‘extreme’ isomer 9-o (5.26 D) shows a
higher gas-chromatographic (GC) retention time (13.63 min under
the standard conditions used in this work e see the Experimental
section) than isomer 1-o (11.23 min, 3.73 D) at the other extreme.
The retention times of these two isomers not only correlate with
the strength of their dipole moments but also delineate the extent
of the region on the scale of retention times in which the other
three B-SH isomers appear, as well as delineating the extent of the
range of calculated dipole moments. There is a narrow overlapping
area on the retention-time scale between mono-SH isomers
derived from ortho-carborane and those derived from meta-car-
borane (Figs. 2 and 3), but the values for all the individual isomers
are distinct and diagnostic. Calculated dipole moment values for all
mono-SH derivatives and their respective retention times as
determined from GC analysis are displayed above in Table 1; Fig. S2
in the supplementary data demonstrates the linear correlation.
A second computationally derivable aspect that is relevant to
the isomerizations is the stability of individual isomers. As
mentioned briefly above, Fig. 4 plots their calculated absolute en-
ergies relative to the most stable one, the para derivative 2-p. In
agreement with the increasing stability of the parent unsubstituted
closo-dicarbadodecaborane cluster compounds in the order
ortho < meta < para, it is seen that their mono-thiolated derivatives
exhibit the same general trend. However, there are several addi-
tional aspects worth noting. First, in each of the three series derived
from the ortho-, meta-, and para-carboranes, the C-SH derivatives
are significantly less stable than any of their respective B-SH iso-
mers. Second, the four B-SH isomers derived from ortho-carborane
exhibit comparable stabilities, and a similar phenomenon is also
observed for the four B-SH isomers derived from meta-carborane;
the para-carborane system generates only two mono-SH isomers,
of which the B-SH one is also more stable. Third, there is a relatively
big energy difference between the B-SH isomers of ortho- and
meta-carborane, the latter being significantly more stable, while
there is not such a big difference between the B-SH isomers of
meta-carborane and the one B-SH isomer of para-carborane.
of 1,2-C2B10H12,[43e45] 9-(HS)-1,2-C2B10H11 (9-o) and 9,12-(HS)2-
1,2-C2B10H10[14,46] are in Table 3. For the dithiolated derivative
9,12-(HS)2-closo-1,2-C2B10H10, results both of a single-crystal X-ray
diffraction analysis and of a gas-phase electron-diffraction analysis
had been published previously [14,46]. It can be seen that there is a
good agreement between the experimental and computational
data. Inter-compound comparison reveals that the clusters exhibit
relatively small differences in their interatomic distances due to the
presence of the thiol substituents. In a parallel with the previously
discussed effect of the attachment of an SH group to the carbon
atoms [25e27], which increases the C(1)-C(2) interatomic distance,
it can be seen that the B(9)-B(12) interatomic distance also in-
creases upon the attachment of an SH group to the B(9) position.
However, the increase is much smaller than in the case of the C-
substituted derivatives. Specifically, the B(9)eB(12) distance in the
parent unsubstituted ortho-carborane is 1.783 Å and becomes
longer after the attachment of one (1.789 Å) and two (1.797 Å) SH
groups to the boron atoms 9 and 12, but this overall increase of
0.017 Å is an order of magnitude smaller than the 0.18 Å difference
observed for the C-substituted species [25e27]. Similar consider-
ations apply to the meta isomer 9-m when compared to the
unsubstituted parent meta-carborane, although here the compari-
son data for the corresponding dihiolated species is not available;
selected structural parameters for the meta isomer 9-m are pre-
sented in Table 4, together with previously reported data [43,45] for
the parent unsubstituted meta-carborane, closo-1,7-C2B10H10
.
We note that here is an obvious difference between the calcu-
lated and experimental data on the SH groups, including their
lengths and angles, which can be attributed to the limitations of X-
ray diffraction analysis in the location of hydrogen atoms. We
provide the values only for purposes of their comparison with the
calculated or gas-phase electron diffraction data.
2.4. Crystal packing
Medicinal and surface science chemistry involving thiolated
cluster species [4e12,14e20] will depend on both coarser and finer
intermolecular reactions, and in this context elements of the crystal
packing in both 9-o and 9-m merit a brief closer discussion. For
isomer 9-o two basic short-contact interactions are observed: first,
the interaction CeH/S(H)/HeC between the partially positive
hydrogen atoms on the carbon vertices of one molecule and the
sulphur atoms of the SH groups of two adjacent molecules, and
second, the interaction between the thiolate hydrogen atom and
the partial negative charge localized in the {B(3)B(7)B(8)} triangle
of boron atoms (Fig. 6). Another difference between the C- and B-
substituted SH isomers is the greater negative charge localized on
the sulfur atoms that are attached to the electron-donating boron-
atom positions. The full unit cell of 9-o is presented in Fig. S1
(Supporting Information). In 9-o, all CH vertices and sulphur
atoms are involved in the CeH/SeH short contact interactions.
This interaction is observed between molecules stacking in four
rows along the a axis. These rows of stacked molecules are thus
interlinked as displayed in Fig. 6. Both types of interactions can be
understood by the analysis of atomic charges on the particular
atoms involved in these interactions. Boron atoms B(3), B(7), B(8)
and sulfur atom in the eSH group exhibit negative charges while
the hydrogen atoms in CH and eSH moieties are both charged
positively. The crystal packing in the meta isomer 9-(HS)-1,7-
C2B10H11, 9-m, shows similar features to that in 9-o, but the more
distal positions of the CH vertices, and possibly also their less acidic
character in the meta-carborane skeleton, lead to a more compli-
cated structure with eight molecules per unit cell and with less
involvement of the CH vertices in the CH/SeH interaction. The full
unit cell of 9-m is presented in Fig. S1 (Supporting Information). For
2.3. Structural characterization
The isomerization experiments show that the attachment of SH
groups to the ortho-carborane skeleton lowers the temperature
necessary for skeletal rearrangements. This clear tendency could be
associated with concomitant changes of the interatomic distances
in the ortho- and meta-carborane cages caused by the attachment of
a thiol group to the cage vertices. Within attempts to obtain single-
crystals of all starting species suitable for single-crystal X-ray
diffraction experiments in order to assess this, in our hands only
isomers 9-o and 9-m have given suitable crystals (Fig. 5).
Selected interatomic dimensions for both the experimentally
obtained and quantum-chemically calculated molecular structures
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