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 HSO4-. 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.

Dihomooxacalix[4]arene-Based Fluorescent Receptors for Anion and Organic Ion Pair Recognition

“…With regard to the trigonal planar and tetrahedral anions, urea 5b displays the same behaviour, showing the highest affinity for the carboxylate AcO − and the inorganic oxoanion H 2 PO 4 − , respectively (log K ass = 3.17 and 2.67). In the case of urea 5a , and as observed before with other dihomooxa bidentate [ 19 , 23 ] and tetraurea receptors [ 21 ], there is a slight inversion of the basicity order (AcO − /BzO − and H 2 PO 4 − /HSO 4 − ). π stacking interactions may contribute to the increased binding of BzO − over that of AcO − .…”

“…Thus, only slight differences are evident for the B (14°) and D (−11°) rings. For comparison, we have previously reported two analogous dihomooxacalix[4]arenes with a 1,3-substitution pattern on the lower rim and which differ from 5a for the presence of two p -CF 3 -phenylurea moieties [ 23 ] or unsubstituted phenylurea moieties [ 20 ] in place of the naphthyl urea groups. All structures with the 1,3-substitution pattern on the lower rim exhibit conformations which are comparable ( Table 1 ).…”

“…The spectral changes were interpreted using the HypSpec 2014 program [ 41 ]. Details concerning the photophysical properties determination has already been described [ 23 ].…”

“…As part of our on-going interest on the host-guest properties of substituted dihomooxacalix[4]arenes (calix[4]arene analogues in which one CH 2 bridge is replaced by one CH 2 OCH 2 group) with (thio)urea units [ 19 , 20 , 21 , 22 , 23 ], we have extended our research into the study of fluorescent receptors for anion [ 24 ] and ion-pair recognition. Thus, dihomooxacalix[4]arene-based fluorescent sensors bearing (thio)urea groups as a binding site and naphthalene moieties as a fluorophore unit were obtained for the first time.…”

Recognition of Anions, Monoamine Neurotransmitter and Trace Amine Hydrochlorides by Ureido‐Hexahomotrioxacalix[3]arene Ditopic Receptors

“…These receptors form 1:1 complexes and exhibit their highest affinity towards the spherical and linear anions F – , Cl – and CN – , and also for the trigonal planar AcO – and BzO – . Di‐phenylurea derivatives have also been tested as ditopic receptors for alkylammonium salts and biogenic amine hydrochlorides, showing very high association constants and selectivity for phenylethylamine, tyramine, dopamine and serotonin, and no interaction with histamine and norepinephrine , . With this in mind, we planned to exploit the structural features of the hexahomotrioxacalix[3]arene macrocycles (a cavity formed by a 18‐membered ring, with only two basic conformations: cone with C 3 v ‐symmetry and partial cone with C s ‐symmetry) and study them as anion and organic ion‐pair receptors.…”