Cleft Receptor for the Recognition and Extraction of Tetrahedral Oxyanions

Abstract

We report the design and synthesis of a new cleft-shaped bis-urea receptor for the recognition of tetrahedral oxyanions from a foldameric N,N′-dimethyl-N,N′-diphenylurea scaffold. The receptor forms 3:1 host-guest complexes with sulfate, phosphate, and arsenate anions in the solid state by readily binding them through hydrogen bonding. 1H NMR analysis of binding events in a competitive medium (DMSO-d6/H2O, 90:10) revealed that the receptor has a strong and selective affinity for arsenate (Ka(3:1) = 2.41 (±0.79) × 108 M-3) in comparison with phosphate and no affinity for sulfate. The receptor extracts the three oxyanions from water into chlorinated organic solvents when the tetra-n-butyl ammonium ion is used as a cationic support. The receptor also crystallizes out the arsenate anion from water with faster kinetics in acetonitrile solution with a similar cationic support.