Cooperatively Enhanced Ion Pair Binding with a Hybrid
Receptor
- Publication date
- Publisher
Abstract
A simple
18-crown-6-based bis-urea receptor <b>R</b><sup><b>1</b></sup> 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 <sup>1</sup>H NMR, in solid
state with single-crystal X-ray diffraction, and in gas phase with
mass spectrometry. In 4:1 CDCl<sub>3</sub>/dimethyl sulfoxide solution
the receptor’s binding preference of halide anions is I<sup>–</sup> < Br<sup>–</sup> < Cl<sup>–</sup> following the trend of the hydrogen-bonding acceptor ability of
the anions. The receptor shows a remarkable positive cooperativity
toward halide anions Cl<sup>–</sup>, Br<sup>–</sup>,
and I<sup>–</sup> when complexed with Na<sup>+</sup>, K<sup>+</sup>, or Rb<sup>+</sup>. The solid-state binding modes of <b>R</b><sup><b>1</b></sup> with alkali and ammonium halides
or oxyanions were confirmed by the X-ray structures of <b>R</b><sup><b>1</b></sup> with KF, KCl, KBr, KI, RbCl, NH<sub>4</sub>Cl, NH<sub>4</sub>Br, KAcO, K<sub>2</sub>CO<sub>3</sub>, and K<sub>2</sub>SO<sub>4</sub>. 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 <b>R</b><sup><b>1</b></sup> 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