Syntheses and Ion Selectivity of Conformational Isomers Derived from Calix[4]arene

Three conformational isomers (cone, partial cone, and 1,3-alternate) of 5,11,17,23-tetra-tert-butyl-25,27-bis-[(ethoxycarbonyl)methoxy]-26,28-bis(2-pyridylmethoxy)calix[4]arene (3) were synthesized from 5,11,17,23-tetra-tert-butyl-25,27-dihydroxy-26,28-bis(2-pyridylmethoxy)calix[4]arene (6). The examination of the metal selectivity in two-phase solvent extraction established that the cone conformer predominantly results when the base contains template metal cations, whereas the partial cone and 1,3-alternate conformers result when the base contains nontemplate metal cations. The solvent extraction data indicated that cone-3 shows the strong metal affinity as comparable with that of cone-5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis[(ethoxycarbonyl)methoxy]calix[4]arene (cone-2) and binds not only Na⁺ but Li⁺. On the other hand, partial-cone-2 shows a poor metal affinity. The difference was discussed on the basis of spectroscopic and X-ray crystallographic data. This paper demonstrates for the first time that the metal selectivity of ionophoric calix[n]arenes can be changed not only by the change in the ring size but also by the conformational change.