*2.4. UV Titration Studies*

To determine the association constants by UV titration, the open-chain calix[4]arenes **7a**,**b** as well as the calix[4]crowns **20**–**22** and benzocrown compounds **23**–**25** and **27** were dissolved in acetonitrile and aliquots of M(ClO4)<sup>2</sup> with M = Ba, Sr, Pb in acetonitrile were added, which caused a clear change in the UV absorption. This behavior is shown in Figure 4 based on the example of compound **21** and its complexes **Ba-21**, **Sr-21** and **Pb-21**. Considering one wavelength with a high change in absorption, the stoichiometry of the complex was determined from the diagram titration curve (Figure 4, exemplarily for **Ba-21**). All titration experiments showed the presence of a complex with a 1:1 stoichiometry, formed by functionalized calix[4]crowns with M(ClO4)<sup>2</sup> except for calix **6** (see Supplementary Materials), as the slope of the graph is changing at the 1:1 molar ratio. The results from the evaluation of the association constants are summarized in Table 2 (all UV titration experiments are expressed in the Supplementary Materials).

**Figure 4.** UV/Vis spectra of calix[4]arene **21** at different M(ClO<sup>4</sup> )<sup>2</sup> concentrations (**A**) M = Ba, (**B**) M = Sr, (**C**) M = Pb for titration measured in acetonitrile. (**D**) Titration curve with change of the absorption using compound **21** at different Ba(ClO<sup>4</sup> )<sup>2</sup> concentrations, measured in acetonitrile at λ = 275 nm.



(a) No value available from UV titration, (b) see ref. [29], (c) Value over the upper method detection limit, n.d.—not determined.

The association constants of the bridged derivatives **20**–**25** and **27** with Sr2+, Ba2+ and Pb2+ showing values up to 8.0 and were higher as for the basic complexes **Ba-14a** (logK 4.6), **Sr-14a** (logK: 4.3), and **Pb-14a** (logK 3.3) [26] without additional side functions due to the increasing number of donor atoms and the additional macrocyclic effect. Interestingly, higher association constants were obtained for the benzocrown derivatives **23**–**25** in comparison to the basic crown derivatives **20**–**22**. The calix derivatives with benzocrown bridge led to more stable complexes showing higher stability constants of approximately one magnitude (e.g., **Sr-21**: 5.0 vs. **Sr-24**: 6.4 (C3F7) or **Ba-20**: 5.7 vs. **Ba-23**: 6.6 (CF3)) for the group 2 metal ions. Moreover, the propylene bridge in calix **22** has only a marginal influence on the association constant regarding the respective Ba-complexes **Ba-21** and **Ba-22**, but a strong influence when comparing the Pb-complexes (logK(**Pb-21**) = 5.5 vs. logK(**Pb-22**) = 7.4).

Additionally, a high value is even found for calix **5** with morpholine modification and its respective complexes with Ba2+ and Pb2+ possibly due to the higher number of donor atoms. The constants for the open-chain derivatives **Ba-7a**,**b** are lower or equal to the value found for **14a** in contrast to the values found for **Pb-7a**,**b**, which were higher. Differences between the M2+ complexes from the same ligand arose from the different ion radii as well as from the chemical behavior (HSAB concept), as Sr2+ and Ba2+ belong to the alkaline earth metals (group 2) and Pb2+ is a group 4 metal ion and therefore, show a softer ion character.
