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.