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 center dot ArX (ArX=PhCl, PhBr, 4-MeC6H4Cl, 4-MeC6H4Br, 4-MeOC6H4Cl, 1,2-Br2C6H4) which were studied by X-ray diffraction. In these systems, the strong collective effect of pi center dot center dot center dot pi stacking interactions and lone pair-(X)center dot center dot center dot pi-hole-(C) bondings between TCB and ArX promote the strength of X center dot center dot center dot N-cyano halogen bonding (HaB). Theoretical studies showed that the stacking interactions affect the o-hole depth of the haloarenes, thus significantly boosting their ability to function as HaB donors. According to the molecular electrostatic potential calculations, the sigma- hole-(Cl) value (1.5 kcal/mol) in the haloarene 4-MeOC6H4Cl (featuring an electron-rich arene moiety and exhibiting very poor sigma-hole-(CI) ability) increases significantly in the stacked trimer (TCB)(2)center dot 4-MeOC6H4Cl (12.5 kcal/mol). Theoretical DFT calculations demonstrate the dramatic increase of X center dot center dot center dot N-cyano HaB strength for stacked trimers in comparison with parent unstacked haloarenes.