Ditopic Receptors Based on Dihomooxacalix[4]arenes Bearing Phenylurea Moieties With Electron-Withdrawing Groups for Anions and Organic Ion Pairs

Abstract

Two bidentate dihomooxacalix[4]arene receptors bearing phenylurea moieties substituted with electron-withdrawing groups at the lower rim via a butyl spacer (CF₃-Phurea 5b and NO₂ Phurea 5c) were obtained in the cone conformation in solution, as shown by NMR. The X-ray crystal structure of 5b is reported. The binding affinity of these receptors toward several relevant anions was investigated by ¹H NMR, UV-Vis absorption in different solvents, and fluorescence titrations. Compounds 5b and 5c were also tested as ditopic receptors for organic ion pairs, namely monoamine neurotransmitters and trace amine hydrochlorides by ¹H NMR studies. The data showed that both receptors follow the same trend and, in comparison with the unsubstituted phenylurea 5a, they exhibit a significant enhancement on their host-guest properties, owing to the increased acidity of their urea NH protons. NO₂-Phurea 5c is the best anion receptor, displaying the strongest complexation for F^-, closely followed by the oxoanions BzO^-, AcO^-, and HS ${\text{O}}_4^{-}$ . Concerning ion pair recognition, both ditopic receptors presented an outstanding efficiency for the amine hydrochlorides, mainly 5c, with association constants higher than 10⁹ M^-2 in the case of phenylethylamine and tyramine.

Keywords: UV-Vis absorption studies; X-ray diffraction; biogenic amine hydrochlorides; dihomooxacalix[4]arenes; ditopic receptors; fluorescence studies; phenylurea anion receptors; proton NMR titrations.