*2.2. The Influence of the Crown Type*

The influence of lower-rim crown modifications on the metal-calixcrown-coordination was next checked. For this purpose, reaction binding energies of Ba2+ with four selected crown-bearing calixarenes were calculated (Figure 1). That involved calix[4]crown-6 **20** and additionally benzo-crown derivative **23** with the modification on adding an aromatic six-membered ring at the bottom of the crown, propylene derivative **22** with an extended crown by a CH<sup>2</sup> group, and calix[4]crown-5 **28** lacking an ethoxy unit (CH2CH2O). As the result of the calculated binding energies (corrected for basis set superposition error) shown in Table 1, the energetically most favored environment to host the metal ion is found for calix[4]crown-6 **20**, while in the other three compounds, the metal-crown binding is weakened up to about 10%. Furthermore, the lowest binding energy is found for the barium ion and calix[4]crown-5. At first glance, the crown-5 might appear to feature an optimal size to host ions like Ba2+, as the size of the macrocycle cavity relative to the ionic radius is often used as a common parameter to rationalize and design new ligands in host-guest chemistry [59]. Considering the geometrical parameters given in Table S1, the crown-5 cavity seems to enclose the Ba2+ ion better than the crown-6; however, the calculated binding energies show a reversed trend. Our calculations are in accordance with previous studies in the field of host-guest chemistry of crown ethers [60]. Islam et al., for instance, showed that Na<sup>+</sup> binds more tightly to a crown-6 body although the crown-5 hole's size matches the sodium ion radius better compared to the one of crown-6 [61].

**Figure 1.** Calix[4]arene derivatives with modified crown ethers were used in the calculations.

**Table 1.** Computed binding energies of different types of calixcrowns (B3LYP-D3/def2-TZVP).

