## 3 Results and Discussion
### Function of Groups and Coordination Feature
\({}^{1}\)H-NMR spectrum is a powerful tool to analyze the conformation of calixarene derivatives. In the \({}^{1}\)H-NMR spectra of SC4A-Cs(I) and SC4A-Sr(II) complexes in Figure S4 (in Supplementary Materials), the downfield shift of the aromatic protons signal confirming the involvement of phenoxy oxygen atoms in complexation with metal ions [39]. This is consistent with the formation of the complex in which the metal is located within the hydrophilic cavity of SC4A surrounded by phenoxy and sulfonyl oxygen atoms. Furthermore, in \({}^{1}\)H-NMR spectrum of SC4A-Sr(II) complex, the appearance of the two signals in the region of ArH protons indicates the cation is restricted upon cavity of SC4A [40]. SC4A-Cs(I) complex did not cause notable changes in the \({}^{1}\)H-NMR except peak shift. This may because the size of the Cs(I) ion, which is larger than the Sr(II) ion, does not fit in the cavity of SC4A. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Conditions** & \multicolumn{3}{c}{**DCs**} & \multicolumn{3}{c}{**DSr**} \\ \hline
**pH (\(\pm\)0.05)** & **SC4A** & **TC4A** & **SC4 AOEt** & **SC4A** & **TC4A** & **SC4 AOEt** \\ \hline
2 & 0.13 & 0.07 & 0.07 & 0.04 & 0 & 0.25 \\
3 & 1.60 & 0.06 & 0.09 & 0.39 & 0 & 0.28 \\
4 & 4.86 & 0.04 & 0.10 & 5.49 & 0.01 & 0.30 \\
5 & 19.32 & 0.02 & 0.12 & 20.04 & 0 & 0.31 \\
6 & 27.62 & 0.04 & 0.17 & 20.67 & 0 & 0.35 \\ \hline \hline \end{tabular} Conditions: \(T=\) 298 K, [SC4A]\({}_{\text{[o]}}=\) [TC4A]\({}_{\text{[o]}}=\) [SC4 AOEt]\({}_{\text{[o]}}=\) 1 \(\times\) 10\({}^{-3}\) M, [Metal]\({}_{\text{[a]}}=\) 1 \(\times\) 10\({}^{-4}\) M, contact time = 60 min, shaking speed: 150 rpm.  
\end{table} 
Table 3: Distribution ratio of metals with a change in pH. Thus, the mechanism of the extraction of Cs(I) with SC4A is illustrated in Figure 8a. The sulfonyl and phenol moiety of SC4A take part in coordination to Cs(I) ion in the extraction process. For the 2:1 (SC4A-Sr) complex, its coordination feature in the extraction process arouses our interest due to the excellent extraction ability of SC4A towards alkaline earth metals such as Mg(II), Ca(II) and Ba(II). The feature of crown ether-Sr(II) complex had been obtained according to X-ray structural studies [41]. Unfortunately, structures of the extracted species in the organic phase could not be obtained. In order to further examine the formation of the SC4A-Sr(II) complex, the extractions of alkaline earth metals Mg(II), Ca(II) and Ba(II) with SC4A were conducted. The relationship of log \(D\) and log [SC4A] is illustrated in Figure S5 (in Supplementary Materials). The stoichiometry of these extracted complexes determined using the slope analysis method revealed a 2:1 molar ratio of SC4A to the metals even though the ionic radii of Mg(II), Ca(II) and Ba(II) are 57, 100, and 135 pm, respectively [42]. On the basis of the above discussion, it was further deduced that two SC4A molecules formed an adjustable cavity for complexation of one alkaline earth metal ion, and the size of the adjustable cavity changes depending on the ion within an appropriate range. This coordination feature is similar to the sandwich complex of Cs(I) with 18-crown-6 [43]. The possible mechanism of the extraction of Sr(II) with SC4A is illustrated in Figure 8b. During the extraction process, an adjustable cavity that can capture alkaline earth metal with different radii is expected to be formed. Figure 8: (**a**) Extraction mechanism of Cs(I) with SC4A from aqueous solution; (**b**) Extraction mechanism of Sr(II) with SC4A from aqueous solution. Figure 7: IR spectra (KBr) of SC4A and extracted species: (**a**) SC4A; (**b**) SC4A-Cs(I) complex; and (e) SC4A-Sr(II) complex. Conditions: \(T\) = 298 K, [SC4A]\({}_{\text{(o)}}\) = 1 \(\times\) 10\({}^{-3}\) M, [Metal]\({}_{\text{(a)}}\) = 1 \(\times\) 10\({}^{-4}\) M, contact time = 60 min, pH = 6, shaking speed: 150 rpm. SC4A-Cs(I) complex and SC4A-Sr(II) complex were dried in air.