*Article* **Synthesis of Ionizable Calix[4]arenes for Chelation of Selected Divalent Cations**

**Markus Blumberg 1,2, Karrar Al-Ameed 3,†, Erik Eiselt <sup>1</sup> , Sandra Luber <sup>3</sup> and Constantin Mamat 1,2,\***


**Abstract:** Two sets of functionalised calix[4]arenes, either with a 1,3-crown ether bridge or with an open-chain oligo ether moiety in 1,3-position were prepared and further equipped with additional deprotonisable sulfonamide groups to establish chelating systems for selected cations Sr2+, Ba2+, and Pb2+ ions. To improve the complexation behaviour towards these cations, calix[4]arenes with oligo ether groups and modified crowns of different sizes were synthesized. Association constants were determined by UV/Vis titration in acetonitrile using the respective perchlorate salts and logK values between 3.2 and 8.0 were obtained. These findings were supported by the calculation of the binding energies exemplarily for selected complexes with Ba2+ .

**Keywords:** calixarenes; barium; chelation

**Citation:** Blumberg, M.; Al-Ameed, K.; Eiselt, E.; Luber, S.; Mamat, C. Synthesis of Ionizable Calix[4]arenes for Chelation of Selected Divalent Cations. *Molecules* **2022**, *27*, 1478. https://doi.org/10.3390/ molecules27051478

Academic Editors: Petra Martini and Alessandra Boschi

Received: 28 January 2022 Accepted: 17 February 2022 Published: 22 February 2022

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