Figure 10. The linear relationship between the interaction
energy of the halogen bonding in dimer and the percentage ofE coop to the total interaction of trimer.
When X is BH3, although the B…N distance shows a
slight contraction upon the formation of trimer which is opposite to
that expected, the interaction energy between BH3 and
CN-Ph-CN vary as expected, from -35.85 kcal/mol in the case of
BH3…CN-Ph-CN dimer decreasing to -34.21 kcal/mol
for BH3…CN-Ph-CN…PO2F
trimer. Similar inconsistent changes of structural parameter and the
interaction energy have been obtained in earlier investigations of a
series of RCN…BH3 complexes[59]. The
calculated results via MP2/aug-cc-pVTZ indicated that the binding
energies are 17.4 and 22.6 kcal/mol for
F3CCN…BH3 and
CH3CN…BH3, while the
corresponding B…N distance is 1.576 and 1.584 Å, respectively. It
is counterintuitive that the electron-withdrawing substituent on the
nitrile exhibits shorter distances. Herein, similar cases occur in the
BH3 complexes. That is, the shorter B…N distances
in the trimer compared to that in dimer is also contrary to the
expectation, given CN-Ph-CN acts as both Lewis base in the two
interactions in the complex. We attribute the observation to the
inductive effect on the spacial extent of the sp lone pair of the
N atom, i.e. , the negative synergistic effect of the pnicogen
bond in CN-Ph-CN…PO2F results in the lone pair
contracts, and the optimal overlap can only be achieved at even shorter
B…N distances. When X is BF3, however, the
B…N distance varies in the expected manner, i.e., uniformly
increasing with the binding energies decreasing. The different behaviors
shown by BH3 and BF3 upon formation of
the trimers may be due to the greater π electron density on the
BF3 moiety[59], which
interacts more strongly with the π electron density with CN-Ph-CN. The
resulted different Pauli repulsion can be used to explain the different
structural variations between the complexes of the BH3and BF3.
In order to deeply understand the interplay between the two interactions
in the ternary complexes, the most negative MEP (Vmin)
on the free N atom in X…CN-Ph-CN and Y…Br-Ph-CN dimers,
and the changes of Vmin (ΔVmin) in
comparison with that in CN-Ph-CN (-32.23 kcal/mol) and Br-Ph-CN (-36.54
kcal/mol) monomers are summarized in Table 4. The Vminon the free N atom decreases in all the X…CN-Ph-CN dimers, and
thus it forms a weaker pnicogen bond in the
X…CN-Ph-CN…PO2F complexes. On the other
hand, the increasing Vmin on the free N atom in the
Y…Br-Ph-CN dimer enables Br-Ph-CN to be a stronger base in
forming the ternary complexes. Furthermore, a good linear correlation
between the ΔVmin and the cooperative energyE coop has been found with
R2=0.991, as shown in Figure 11, except for the values
of the BF3…CN-Ph-CN dimer. This suggests that the
electrostatic interaction is the main driving force in forming the
complexes. As for
BF3…Br-Ph-CN…PO2F, the
cooperative energy amounts to be 13.45 kcal/mol, which is largest among
the systems we studied. This may be caused by the abnormal great
geometrical changes upon formation of the ternary complexes, which has
been described above.