Enhanced Cooperative Lithium Halide Recognition by Heteroditopic Halogen Bonding (XB) Macrocycles

A series of macrocyclic heteroditopic receptors was synthesized to investigate cooperative recognition of alkali-metal halide ion pairs. The receptors combine either a 1,3-bis-iodotriazole (XB) or 1,3-bis-prototriazole (HB) benzene scaffold for halide binding with poly(ethylene glycol)-based macrocyclic moieties for cation coordination. Intensive ¹H NMR binding studies revealed that XB-functionalized macrocycles exhibit significantly higher halide affinities than their HB analogues, while increased macrocycle size enhances alkali-metal cation binding strength. Notably, the halide-bound 1·XB macrocycle induced strong positive cooperativity in lithium-ion recognition, with up to a 7-fold increase in binding affinity. Density functional theory (DFT) calculations suggest that electrostatic stabilization between cobound ions underlies this effect, with the most pronounced enhancement observed for the 1·XB@LiI complex. Solid-liquid extraction experiments further demonstrated the practical potential of the XB system, achieving efficient transfer of lithium halide salts into organic solution. These findings establish halogen-bonded macrocycles as effective platforms for cooperative ion-pair recognition and highlight their promise for applications in lithium salt recovery and recycling.