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.