Halogen bonding interactions in the XC₅H₄N···YCF₃ (X = CH₃, H, Cl, CN, NO₂; Y = Cl, Br, I) complexes

The noncovalent interactions between the σ-hole region outside the halogen atom and the nitrogen atom of pyridine and its para-substituted derivatives are the focus of this work. Based on the analyses of the electrostatic potentials, YCF₃ (Y = Cl, Br, I) act as halogen bond donors, XC₅H₄N (X = CH₃, H, Cl, CN, NO₂) act as halogen bond acceptors, and the binary halogen-bonded complexes XC₅H₄N···YCF₃ have been designed and investigated by B3LYP-D3/aug-cc-pVDZ calculations together with the aug-cc-pVDZ-PP basis set for iodine. When the halogen bond acceptor remains unchanged, the interactions between C₅H₅N and YCF₃ (Y = Cl, Br, I) increase with the order of Y = Cl, Br, and I. When the halogen donor ICF₃ is fixed, the halogen bonding interactions decrease along the sequence of X = CH₃, H, Cl, CN, NO₂. Therefore, the halogen bond of the CH₃C₅H₄N···ICF₃ complex is the strongest. The interactions between Lewis acid YCF₃ (Y = Cl, Br, I) and pyridine and para-substituted pyridine are closed-shell and noncovalent interactions. On the one hand, when the halogen bond acceptor XC₅H₄N is fixed, with the increase of halogen atomic number, the strength of halogen bond increases; on the other hand, when the halogen bond donor ICF₃ is fixed, as the electron-withdrawing ability of the electron-withdrawing group (X) increases, the halogen bond gradually weakens.