**4. Conclusions**

Two sets of new functionalized calixarene ligands were prepared and analysed via UV/vis titration regarding their potential to chelate divalent cations like Ba, Sr and Pb. These ions are of high importance, e.g., as diagnostic or therapeutic radionuclides in radiopharmacy, or they are found in environmental chemistry. Thus, there exists a keen necessity for a strong complexation of these ions. In summary, the additional protonionizable side functions connected together with a benzocrown ether structure to the calixarene skeleton led to a considerable improvement of the complex stability resulting in high association constants. There is still room to improve the stability of these complexes for future application as chelators for radiometal ions in radiopharmacy.

**Supplementary Materials:** The following supporting information can be downloaded online. Table S1: Calculated bond lengths for Ba-O bonds of calix[4]crown-6 (20) and calix[4]crown-5 (BP86-D3/def2- TZVP), Figure S1: Partly labelled schemes of calix[4]crown-6 (20) and calix[4]crown-5, Table S2: the optimized coordinates (in Angstrom) of Ba-calix[4]crown complex calculated using BP86/TZVP, Table S3: The optimized coordinates (in Angstrom) of Ra-calix[4]crown complex calculated using BP86/TZVP, Figures S2–S34: <sup>1</sup>H and <sup>13</sup>C NMR spectra of compounds, Figures S35–S64: UV spectra and UV titration plot of compounds with Ba2+, Sr2+ and Pb2+ .

**Author Contributions:** Conceptualization, C.M. and S.L.; methodology, C.M. and S.L.; software, S.L. and K.A.-A.; validation, C.M., S.L. and K.A.-A.; formal analysis, M.B., E.E. and K.A.-A.; investigation, C.M., K.A.-A. and S.L.; resources, C.M. and S.L.; writing—original draft preparation, C.M. and S.L.; writing—review and editing, all authors; visualization, M.B., E.E., K.A.-A.; supervision, C.M. and S.L.; project administration, C.M.; All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the University of Zurich, and the Swiss national science foundation, grant no. PP00P2\_170667. K.A.-A. received a Swiss Government Excellence Scholarship.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All data are included in this paper.

**Acknowledgments:** The authors thank Linda Belke for their extensive experimental support.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of the compounds are not available from the authors.
