4 Conclusions
In this paper, theoretical
calculations have bene carried out for the binary and ternary complexes
involving the substituted benzonitrile and PO2F to study
how the substituent and cooperative effect tune the π-hole pnicogen
bonding interaction. In the binary complexes of
R-Ph-CN…PO2F, it is found that these pnicogen
bonds can be classified as medium strength interaction, possessing
partially covalent characteristics, and the stability of the complexes
increases with respect to substituent R in the order of
CN<Br≈Cl<F<H<CH3<NH2.
The electron-donating ability of the substituent adjoined with the
benzene ring give rise to an increase of the interaction energy of the
complexes. All the halogen atoms behave as electron-withdrawing
substituents and weaken the pnicogen bond compared with
Ph-CN…PO2F complexes. The geometrical analysis
indicates that the dihedral angle F-P-O-O of the PO2F
molecule in the complexes can well describe the strength of the
intermolecular interaction. The bonding nature of the intermolecular
interactions has been further explored by means of AIM and NBO methods.
As for the ternary complexes, the mutual interplay between pnicogen bond
and halogen bond indicated that the negative/positive synergistic effect
is consistent with the geometrical changes and cooperative energy upon
formation of the trimer. The effect of a pnicogen bond on a halogen bond
is more pronounced than that of a halogen bond on a pnicogen bond. The
results indicated that the interplay between the two interaction is
strongly influenced by bonding characteristic of the bridge molecule, in
addition to the strength of both interactions. When the ternary
complexes contain triel bond, however, notable differences are observed
for BH3 and BF3, including structural
parameter and the interaction energy, which may be arising from the
greater π electron density on the BF3 moiety. The MEP,
AIM and NBO methodologies are used to analyze the nature of mutual
interactions of the complexes. It is hoped that the results of our study
are useful in a deep understanding of the nature of π-hole pnicogen
bonding and how to tune it in more complex systems.