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

Two bidentate dihomooxacalix[4]arene receptors bearing phenylurea moieties substituted with electron-withdrawing groups at the lower rim via a butyl spacer (CF ₃ -Phurea 5b and NO ₂ 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 ¹ H 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 ¹ H 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. NO ₂ -Phurea 5c is the best anion receptor, displaying the strongest complexation for F ^- , closely followed by the oxoanions BzO ^- , AcO ^- , and HS O 4 - . Concerning ion pair recognition, both ditopic receptors presented an outstanding efficiency for the amine hydrochlorides, mainly 5c , with association constants higher than 10 ⁹ M ^-2 in the case of phenylethylamine and tyramine.
(Copyright © 2019 Miranda, Serbetci, Marcos, Ascenso, Berberan-Santos, Hickey and Geremia.)