Characterizing the multiple non-covalent interactions in N, S-heterocycles–diiodine complexes with focus on halogen bonding

Abstract

The multiple non-covalent interactions in alkenylthioquinolines–diiodine complexes with conformationally flexible S-alkenyl fragments are studied from the viewpoint of the mutual influence of closely-located N and S donor sites. Despite several electron donor sites in an alkenylthioquinoline molecule (N, S and π-systems), which are capable of forming halogen bonds, the most stable complexes are observed if sulfur atom is a single donor center only. It corresponds to the most favorable mutual arrangement of the electron lone pairs of donor atoms and the depleted areas in electron localization function, which reflects the reduced shielding of iodine nucleus charge and coincides with σ-holes in the molecular electrostatic potential. Quinolines substituted at position 2 prefer to form the complexes with three-atomic non-covalent interactions: N···I, S···I and I···H. The bifurcated halogen bond with two-atomic interactions N···I and S···I turns out to be preferable for quinolines substituted at position 8. We have concluded that iodine participation in the bifurcated halogen bonding simultaneously with N and S atoms leads to reducing the binding strength in the complexes. It is found that the dipole polarization of an iodine atom provides the quantitative estimation of interaction energy in the complexes with multiple non-covalent interactions. The possibility to distinguish between the halogen bond S···I and van der Waals interaction S···I is also discussed.