Cooperatively Enhanced Ion Pair Binding with a Hybrid Receptor

A simple
18-crown-6-based bis-urea receptor R1 was synthesized in three steps from a commercial starting
material. The receptor’s behavior toward anions, cations, and
ion pairs was studied in solution with 1H NMR, in solid
state with single-crystal X-ray diffraction, and in gas phase with
mass spectrometry. In 4:1 CDCl3/dimethyl sulfoxide solution
the receptor’s binding preference of halide anions is I– < Br– < Cl– following the trend of the hydrogen-bonding acceptor ability of
the anions. The receptor shows a remarkable positive cooperativity
toward halide anions Cl–, Br–,
and I– when complexed with Na+, K+, or Rb+. The solid-state binding modes of R1 with alkali and ammonium halides
or oxyanions were confirmed by the X-ray structures of R1 with KF, KCl, KBr, KI, RbCl, NH4Cl, NH4Br, KAcO, K2CO3, and K2SO4. They clearly present two different binding
modes, either as separated or contact ion pairs depending on the nature
and size of the bound cation and anion. Complexation capability of R1 in the gas phase was studied with
competition experiments with electrospray ionization mass spectrometry
showing preference of KCl complexation over NaCl, KBr, or KI supporting
the results obtained in solution.