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-MeC 6 H 4 Cl, 4-MeC 6 H 4 Br, 4-MeOC 6 H 4 Cl, 1,2-Br 2 C 6 H 4 ) 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-MeOC 6 H 4 Cl (featuring an electron-rich arene moiety and exhibiting very poor σ-hole-(Cl) ability) increases significantly in the stacked trimer (TCB) 2  ⋅ 4-MeOC 6 H 4 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.