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

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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 /[D6 ]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) ⋅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.