*2.3. NMR Investigations*

To determine the stability constants for the complexation of Ba2+, Sr2+, and Pb2+ , a reliable method was developed in the past by our group using <sup>1</sup>H NMR spectroscopy. Due to the different <sup>1</sup>H NMR spectra, which were recorded for the respective complexes in comparison with the ligands, a <sup>1</sup>H NMR titration method was established [26,29,30]. For this purpose, the compounds were dissolved in acetonitrile-d<sup>3</sup> and treated in portions with an acetonitrile-d<sup>3</sup> solution containing Ba(ClO4)2. It was observed, that after addition of 0.5 equivalents of Ba salt, a new set of signals appeared, belonging to the respective Ba-complex exemplarily shown for calix **24** in Figure 2. This leads to two separate species: ligand **24** and complex **Ba-24** in the <sup>1</sup>H NMR spectrum (see, for instance, the difference of the methylene protons of the calix skeleton in the box of Figure 3). After the addition of 1 eq. of Ba(ClO4)2, no ligand was detectable anymore, which leads to the assumption of a complex with 1:1 stoichiometry. The situation in Figure 2 is a consequence of a slow exchange on the NMR time scale and is therefore not suitable for a logK determination by

NMR titration for our calix compounds [62]. Next, a more reliable titration method based on UV/vis spectroscopy was used instead to determine the association constants.

**Figure 2.** The optimized geometrical structure of the Ba-complex **Ba-20** (hydrogen atoms are hidden for clarity; the colors of the atoms are: carbon; grey, fluorine; green, sulfur; yellow, nitrogen; blue, oxygen; red, barium; brown).

**Figure 3.** <sup>1</sup>H NMR spectra showing the change in chemical shifts by treatment of calix **24** with different equivalents of Ba(ClO<sup>4</sup> )2 in acetonitrile-d<sup>3</sup> .
