Exploring the mechanism of ion-pair recognition by new calix[4]pyrrole bis-phosphonate receptors: insights from quantum mechanics study

Abstract

Three diastereomeric bis-phosphonate cavitands based on an aryl extended calix[4]pyrrole can act as ion-pair receptors for alkylammonium/phosphonium chloride salts. In this contribution, the ion-pair binding mode and binding affinity were investigated using density functional theory (DFT) calculations and new nonconvalent weak interaction analysis method. One of the three receptors was chosen as the host and two guests were chosen to model quaternary phosphonium chloride salts and primary ammonium chloride salts respectively, and two types of arrangements – separated and contact – were taken into account for each guest. The binding energy suggests that contact arrangement is the favorable binding mode for this receptor and it prefers to bind primary ammonium chloride salts rather than quaternary phosphonium chloride salts both in the gas-phase and in solution, in agreement with the experiment. Moreover, geometry analysis and charge transfer based on natural bond orbital (NBO) analysis suggest that the binding modes and binding affinities of the receptor towards different ion-pairs are determined by the amount and strength of hydrogen bonds. Furthermore, nonconvalent weak interactions between the host and the guests have been explored to unravel the driving forces responsible for the ion-pair recognition. There are hydrogen bonding, van der Waals, cation–π, ion-induced dipole and charge–charge interactions in ion-pair recognition, and hydrogen bonding interactions play the dominant role in this process. This work unveils the mechanism of ion-pair recognition by new calix[4]pyrrole bis-phosphonate receptors, while opening exciting perspectives for the design of novel calix[4]pyrrole-based ion-pair receptors.