Metal-Ion Extractability of Sulfur-Bridged Oligomers of Phenol; Distinct Effect of the Number of Sulfur Bridges Rather Than the Cyclic/Acyclic Form

The solvent-extraction behavior of 2,8,14,20-tetrathiacalix[4]arene 2 was compared with those of 2-monothiacalix[4]arene 3 as well as linear tetramer (4) and dimer (5) of p-tert-butylphenol linked by sulfide bonds at the ortho positions in order to clarify the effect of the cyclic and acyclic structure and the number of sulfide bonds. The linear tetramer 4 showed almost the same extraction behavior as that of the cyclic counterpart 2, showing the dependence on the pH for the extraction of Co(II), Ni(II), Cu(II), and Zn(II) ions. On the other hand, monothiacalix[4]arene 3 and acyclic dimer 5 showed far inferior extractability as compared to the former two, suggesting that the number of sulfide linkages, rather than the cyclic/acyclic structure, is predominant to determine the extractability. An equilibrium study on the extraction behavior of open-chain 4 (H4L) toward Co(II), Cu(II), and Zn(II) ions revealed that the composition of the extracted species can be written as [Mi(H2L)i] (i: integer), as was the case obtained by cyclic 2. Furthermore, the extraction constants of 4 for those metal ions were almost the same as those obtained by 2. The similarity of the extraction behavior between 2 and 4 suggests that the acyclic tetramer 4 may act as a pseudo-thiacalix[4]arene to have the same complexation manner as cyclic 2 by the O^-,S,O^--donor set, in which the presence of a phenol unit flanked by sulfide bonds at the both ortho positions is essential. In other words, the effect of the bridging sulfide in the phenol oligomers may be reasonably explained by the electron-withdrawing effect of the sulfide moiety to provide the phenol unit with a higher acidity to form [M2(H2L)2]-type complexes having a double-cone structure stabilized by two sets of O^-,S,O^--ligated metal bridges.