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.