Directionality in cation–anion interactions involving chalconium cations

Abstract

The directionality of bonds is an important feature for many compounds relevant in biochemistry, crystal engineering and materials chemistry. It is widely accepted that a good number of contacts between ions (including ionic bonds) are nondirectional. A CSD survey is employed to demonstrate that significant directionality of cation⋯anion interactions is observed for systems which include a chalconium cation. Nearly 70% of all found structures contain a R–Y⋯LB angle (Y = S, Se, Te; LB = atom from anionic Lewis base) within 10° of linearity. Such structural motifs are commonly termed charge-assisted chalcogen bonds. Quantum chemical analysis traces this angular tendency to a compromise between coulombic forces and exchange repulsion, which pull in opposite directions. Charge transfer, polarization, and dispersion terms are much smaller in magnitude and thus exert a lesser influence.