Quantum-Chemical Design of Cryptand-like Ditopic Salt Binders

Hartree-Fock, density functional, and MP2 methods are applied to the problem of designing neutral, bicyclic C3-symmetric cages incorporating interacting anion- and cation-binding sites which strongly bind NaCl as an ion contact pair. A large number of trial ligands L and their complexes L:NaCl are tested, with the focus on maximizing binding by (i) optimizing the cavity size and shape and (ii) varying the nature of the anion- and cation-binding functionalities. The corresponding complexes L:Cl(-) and L:Na(+) are also studied in some detail. An analysis of their structures and charge distributions helps to build a consistent picture of the requirements for a successful NaCl binding. The 'best' candidate ligand utilizes a tripodal triether-substituted amine N(CH2CH2OR-)3 to bind the sodium cation; three thiourea groups in a tripodal arrangement with a 1,3,5-trisubstituted benzyl spacer group {C6H3(CH2NHC [Formula: see text] XNH-)3 X=O,S} to bind chloride; and a -CH2CH2- spacer linking the two binding sites. A simple Quantitative Structure-Property analysis suggests that the binding cavity shape and size is near to the optimal one for this system.