Acid-base properties of sulfur-bridged calix[4]arenes

The acid-base properties of sulfur-bridged calixarenes, thiacalix[4]arenetetrasulfonate (4) and its sulfonyl analogue (5), have been investigated via potentio- and spectrophoto-metric studies, comparing them with the conventional methylene-bridged calix[4]arenetetrasulfonate (3). The titration curves revealed the acidity of the phenolic OH groups in the calix[4]arenes 3-5 to be in the order: 3 < 4 ≪ 5. In particular, the oxidation of the bridging sulfur to sulfone strongly enhanced the acidity of the sulfur-bridged calix[4]arene; the pK_a,n values (n = 2-4) of the sulfone-bridged 5 were lower by 7 pK units than those of the sulfide-bridged 4. The effects of the linkage groups on the acidity of the calix[4]arenes are discussed from the structural and electronic viewpoints, such as the macrocyclic ring size, the hydrogen bonding manner, and the resonance of the linkage moiety.