Transmission
of Electronic Substituent Effects across
the 1,12-Dicarba-closo-dodecaborane Cage: A Computational
Study Based on Structural Variation, Atomic Charges, and 13C NMR Chemical Shifts
The ability of the 1,12-dicarba-closo-dodecaborane
cage to transmit long-range substituent effects has been investigated
by analyzing the structural variation of a phenyl probe bonded to
C1, as caused by a remote substituent X at C12. The geometries of
41 Ph–CB10H10C–X molecules, including
11 charged species, have been determined by MO calculations at the
B3LYP/6-311++G** level of theory. The structural variation of the
phenyl probe is best represented by a linear combination of the internal
ring angles, termed SFCARB.
Multiple regression analysis of SFCARB, using appropriate explanatory variables, reveals the
presence of resonance effects, superimposed onto the field effect
of the remote substituent. The ability of the para-carborane cage to transmit resonance effects is, on average, about
one-half of that of the para-phenylene frame in coplanar
para-substituted biphenyls. Analysis of the π-charge variation
of the phenyl probe confirms that the para-carborane
frame is less capable than the coplanar para-phenylene
frame of transmitting π-electrons from the remote substituent
to the phenyl probe, or vice versa. The para-carborane
cage is a better π-acceptor than π-donor; this makes π-donor
substituents less effective than π-acceptors in exchanging π-electrons
with the phenyl probe across the cage. When the remote substituent
is an uncharged group, the para-carborane cage acts
as a very weak π-acceptor toward the phenyl probe. The structural
variation of the para-carborane cage has also been
investigated. It consists primarily of a change of the C1···C12
nonbonded separation, coupled with a change of the five B–C–B
angles at C12. This concerted geometrical change is controlled by
the electronegativity of the substituent and the resonance interactions
occurring between substituent and cage. These, however, appear to
be important only when π-donor substituents are involved. The 13C NMR chemical shifts of the para-carbon
of the phenyl probe correlate nicely with SFCARB, pointing to the reliability of these quantities
as measures of long-range substituent effects. On the contrary, the 11B and 13C chemical shifts of the cage atoms do
not convey information on electronic substituent effects.