We are currently developing an adsorbent to capture harmful organic mercury compounds in environmental water. Previously, we reported that tert-butylcalix[4]thiacrown-5 (CA4SC5) is a promising adsorbent. Therefore, we are now exploring a simple method to evaluate its adsorption ability. Recent studies have shown that the chemical shifts of signals corresponding to the calixarene moiety in the NMR spectrum of CA4SC5 change depending on the solvent used for dissolution. For example, when acetonitrile and chloroform-methanol solvents were used, significant differences were observed in the chemical shifts of the tert-butyl groups and phenolic ring protons. To investigate this phenomenon, X-ray crystal structure analyses of CA4SC5 dissolved in different solvents were performed. In acetonitrile solution, acetonitrile molecules were encapsulated in the hydrophobic cavity of the calix[4]arene (CA4) moiety, forming an isotropic cavity. In contrast, in chloroform-methanol, no solvent molecules were observed within the CA4 moiety. Instead, the benzene rings connected to the thiacrown moiety adopted a more vertical orientation, while the other pair of benzene rings tilted more horizontally, resulting in an anisotropic cavity. These results indicate that the CA4 moiety can freely change its conformation when chloroform-methanol is used as the solvent.
Next, the affinities of CA4SC5 for HgCl₂, HgBr₂, Hg(CH₃COO)₂, and Hg(Pic)₂ were investigated using ¹H-NMR spectroscopy. The results showed that the three-dimensional structure of the calix[4]arene moiety changes depending on the strength of its binding to mercury(II) compounds. By performing ¹H-NMR titration using the degree of transition of the tert-butyl and phenolic ring protons, the binding constants of CA4SC5 with HgCl₂, HgBr₂, and Hg(CH₃COO)₂ were estimated. The results showed that CA4SC5 exhibited the highest affinity for Hg(CH₃COO)₂ among the three compounds, with a binding constant of 7.5 × 10⁴ M^-1. This indicates that CA4SC5 has a strong affinity for Hg(CH₃COO)₂. Furthermore, in cases where the solubility of the mercury compound in the solvent was too low to determine the binding constant, a qualitative comparison was made by analyzing the degree of transition of the tert-butyl and phenolic ring proton signals. These findings will be presented at this meeting.

Hiroki Takahashi

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