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

• Main Authors: Alexandre S. Miranda, Paula M. Marcos, José R. Ascenso, Mário N. Berberan-Santos, Rachel Schurhammer, Neal Hickey, Silvano Geremia
• Format: Article
• Language: English
• Published: MDPI AG 2020-10-01
• Series: Molecules
• Subjects: dihomooxacalix[4]arenes; naphthyl(thio)urea anion receptors; alkylammonium hydrochlorides; ditopic receptors; chiral recognition; NMR studies
• Online Access: https://www.mdpi.com/1420-3049/25/20/4708

Fluorescent dihomooxacalix[4]arene-based receptors <b>5a</b>–<b>5c</b>, bearing two naphthyl(thio)ureido groups at the lower rim via a butyl spacer, were synthesised and obtained in the cone conformation in solution. The X-ray crystal structures of 1,3- (<b>5a</b>) and 3,4-dinaphthylurea (<b>5b</b>) derivatives are reported. Their binding properties towards several anions of different geometries were assessed by ¹H-NMR, UV-Vis absorption and fluorescence titrations. Structural and energetic insights of the naphthylurea <b>5a</b> and <b>5b</b> complexes were also obtained using quantum mechanical calculations. The data showed that all receptors follow the same trend, the association constants increase with the anion basicity, and the strongest complexes were obtained with F⁻, followed by the oxoanions AcO⁻ and BzO⁻. Proximal urea <b>5b</b> is a better anion receptor compared to distal urea <b>5a</b>, and both are more efficient than thiourea <b>5c</b>. Compounds <b>5a</b> and <b>5b</b> were also investigated as heteroditopic receptors for biologically relevant alkylammonium salts, such as the neurotransmitter γ-aminobutyric acid (GABA·HCl) and the betaine deoxycarnitine·HCl. Chiral recognition towards the guest <i>sec</i>-butylamine·HCl was also tested, and a 5:2 selectivity for (<i>R</i>)-<i>sec</i>-BuNH₃⁺·Cl⁻ towards (<i>P</i>) or (<i>M</i>) enantiomers of the inherently chiral receptor 5a was shown. Based on DFT calculations, the complex [(<i>S</i>)-<i>sec</i>-BuNH₃⁺·Cl⁻/(<i>M</i>)-5a] was indicated as the more stable.