Crystal Environment Effects on Halogen-Bonded para-Substituted Pyridines in Their I–Cl Adducts

Strong halogen bond (HaB) interactions between six different para-substituted pyridines [R–Py] and I–Cl (R = CF₃, CO₂Me, H, Me, NMe₂, and pyrrolidin-1-yl) lead to the formation of [R–Py]•I–Cl adducts, which can be considered as a unique molecular entity, as deduced from both energetic and topological analyses, with the strongest HaB observed with the most electron-rich pyridines. All six adducts at optimized geometries show HaB interactions weaker than those found at experimental geometries, highlighting the effect of the crystalline environment on such HaB systems. Indeed, when neighboring adducts associated through secondary C–H···Cl interactions are explicitly considered in the geometry optimizations, the experimental N···I···Cl geometries are then convincingly reproduced, paralleling the effect of an implicit PCM solvent model. The effects of such weaker secondary hydrogen bond interactions are also well reproduced by considering one single [Py]•I–Cl adduct interacting with HF molecules through weak F–H···Cl hydrogen bonds along the series [Py]•ICl < [Py]•ICl•HF < [Py]•ICl•2HF. The topological properties at both N_Py···I/I···Cl bond critical points follow the evolving behavior of the concomitant increase/decrease of both interaction strengths while paralleling the shortening/lengthening of their internuclear distances. Furthermore, the analysis of the topological Cl and I atomic net charges, as well as the dipole moment of the adducts, points out the increase of charge separation [N–I^δ+–Cl^δ–] within the adduct with that of the N_Py···I interaction strength and its concomitant shortening. Complementary ELF and Feynman force calculations (to determine the ionic vs. covalent vs. charge-shift components) show that the halogen-bonded [R-Py]•ICl adducts switch from a closed-shell interaction with an incipient covalent character to a dative covalent bond, when considering explicitly the molecular crystalline environment.