Oligoether-Strapped Calix[4]pyrrole: An Ion-Pair Receptor Displaying Cation-Dependent Chloride Anion Transport

A ditopic ion-pair receptor (1), which has tunable cation- and anion-binding sites, has been synthesized and characterized. Spectroscopic analyses provide support for the conclusion that receptor 1 binds fluoride and chloride anions strongly and forms stable 1:1 complexes ([1.F]- and [1.Cl]-) with appropriately chosen salts of these anions in acetonitrile. When the anion complexes of 1 were treated with alkali metal ions (Li+, Na+, K+, Cs+, as their perchlorate salts), ion-dependent interactions were observed that were found to depend on both the choice of added cation and the initially complexed anion. In the case of [1.F]-, no appreciable interaction with the K+ ion was seen. On the other hand, when this complex was treated with Li+ or Na+ ions, decomplexation of the bound fluoride anion was observed. In contrast to what was seen with Li+, Na+, K+, treating [1.F]- with Cs+ ions gave rise to a stable, host-separated ion-pair complex, [F.1.Cs], which contains the Cs+ ion bound in the cup-like portion of the calix[4]pyrrole. Different complexation behavior was seen in the case of the chloride complex, [1.Cl]-. Here, no appreciable interaction was observed with Na+ or K+. In contrast, treating with Li+ produces a tight ion-pair complex, [1.Li.Cl], in which the cation is bound to the crown moiety. In analogy to what was seen for [1.F]-, treatment of [1.Cl]- with Cs+ ions gives rise to a host-separated ion-pair complex, [Cl.1.Cs], in which the cation is bound to the cup of the calix[4]pyrrole. As inferred from liposomal model membrane transport studies, system 1 can act as an effective carrier for several chloride anion salts of Group 1 cations, operating through both symport (chloride+cation co-transport) and antiport (nitrate-for-chloride exchange) mechanisms. This transport behavior stands in contrast to what is seen for simple octamethylcalix[4]pyrrole, which acts as an effective carrier for cesium chloride but does not operates through a nitrate-for-chloride anion exchange mechanism.