Selective oxidation of thiacalix[4]arenes to the sulfinyl and sulfonyl counterparts and their complexation abilities toward metal ions as studied by solvent extraction

Practical methods for the synthesis of sulfinyl-(5,6) and sulfonylcalix[4]arenes (7,8) were provided by the selective oxidation of thiacalix[4]arenes (3,4) with controlled amounts of an oxidant such as NaBO3 or hydrogen peroxide under mild conditions. The coordination ability of thiacalix[4]arene 4 and the sulfinyl and sulfonyl analogs (6,8) toward a wide variety of metal ions was investigated by solvent extraction and compared to that of the conventional methylene-bridged calix[4]arene 2. It was shown that the metal-ion selectivities of the sulfur-containing ligands 4, 6, and 8 were controlled by the oxidation state of the bridging sulfur moiety, which was best understood based on the hard and soft acid-base (HSAB) principle by assuming the critical role of the bridging groups in coordination to the metal center. Thus, 4 preferred soft metal ions by binding with S, while 8 did hard ones by ligating with sulfonyl O in addition to the adjacent two phenoxide oxygens, respectively. In good accordance with this hypothesis, 6 could bind to both hard and soft metal ions by using sulfinyl O and S, respectively. These made sharp contrast to the parent methylene-bridged 2, which could not essentially extract any metal ions at all, lacking lone pair electrons on the methylene bridge for coordination. (C) 2001 Elsevier Science Ltd. All rights reserved.