Cooperatively Enhanced Ion Pair Binding with a Hybrid Receptor
datasetposted on 21.09.2015, 00:00 by Toni Mäkelä, Elina Kalenius, Kari Rissanen
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.