Ion-pair recognition based on halogen bonding: a case of the crown-ether receptor with iodo-trizole moiety

Abstract

The development of halogen-bond-based ditopic receptors capable of binding simultaneously both a cation and an anion has attracted recent research interest. In this work, the crown-ether receptor 1, which consists of an iodo-trizole moiety for anion recognition through halogen bonding and a Lewis-basic center for cation binding, was investigated using density functional theory calculations. The structural and energetic features for the complexes of 1 with single cations, single halide anions, and ion pairs were explored. Intermolecular interactions in these complexes were systematically analyzed by the atoms in molecules and noncovalent interaction index methods. The presence of the coordinated cation significantly increases the anion-binding affinity, while the binding of halide anions has a slight influence on the cation-binding affinity. Anti-cooperative effects were found in the ion-pair recognition of 1, due to the strong attraction between the two counterions in the complexes. The solvent weakens the interaction strength considerably, and anti-cooperativity becomes very small in solvent. The results reported in this work are of fundamental importance in the design of ion-pair receptors based on halogen bonding.