Stacking Interactions: A Supramolecular Approach to Upgrade Weak Halogen Bond Donors

The co-crystallization of tetracyanobenzene (TCB) with haloarenes ArX provided six new co-crystals TCB ⋅ ArX (ArX=PhCl, PhBr, 4-MeC₆ H₄ Cl, 4-MeC₆ H₄ Br, 4-MeOC₆ H₄ Cl, 1,2-Br₂ C₆ H₄ ) which were studied by X-ray diffraction. In these systems, the strong collective effect of π⋅⋅⋅π stacking interactions and lone pair-(X)⋅⋅⋅π-hole-(C) bondings between TCB and ArX promote the strength of X⋅⋅⋅N_cyano halogen bonding (HaB). Theoretical studies showed that the stacking interactions affect the σ-hole depth of the haloarenes, thus significantly boosting their ability to function as HaB donors. According to the molecular electrostatic potential calculations, the σ- hole-(Cl) value (1.5 kcal/mol) in the haloarene 4-MeOC₆ H₄ Cl (featuring an electron-rich arene moiety and exhibiting very poor σ-hole-(Cl) ability) increases significantly in the stacked trimer (TCB)₂  ⋅ 4-MeOC₆ H₄ Cl (12.5 kcal/mol). Theoretical DFT calculations demonstrate the dramatic increase of X⋅⋅⋅N_cyano HaB strength for stacked trimers in comparison with parent unstacked haloarenes.