Ion-Pair Complexation with Dibenzo[21]Crown-7 and Dibenzo[24]Crown-8 bis-Urea Receptors

Synthesis and ion-pair complexation properties of novel ditopic bis-urea receptors based on dibenzo[21]crown-7 (R-1) and dibenzo[24] crown-8 (R-2) scaffolds have been studied in the solid state, solution, and gas phase. In a 4:1 CDCl3/[D-6]DMSO solution, both receptors clearly show positive heterotropic cooperativity toward halide anions when complexed with Rb+ or Cs+, with the halide affinity increasing in order I- < Br- < Cl-. In solution, the rubidium complexes of both receptors have higher halide affinities compared to the caesium complexes. However, Rb+ and Cs+ complexes of R-2 show stronger affinities toward all the studied anions compared to the corresponding cationic complexes of R-1. Similar selectivity of the receptors toward the studied ion pairs was also observed also in the gas phase by competition experiments with mass spectrometry. A total of eight crystal structures with different rubidium and caesium halides and oxyanions were obtained in addition to the crystal structure of R-2 center dot BaCl2. The selectivity observed in solution and in the gas phase is explainable by the conformational differences observed in the crystal structures of ion-pair complexes with R-1 and R-2. In the solid state, R-1 has an open conformation due to the asymmetric crown-ether scaffold, whereas R-2 has a compact, folded conformation. Computational studies of the ion-pair complexes of R-2 show that the interaction energies of the complexes increase in the order CsI < CsBr < CsCl < RbCl, supporting the selectivity observed in solution and the gas-phase.