Separations of soft heavy-metal cations by lower rim-functionalized calix[4]arenes

Calix[4]arenes functionalized on the lower rim with various donor groups are used for recognition and separation of soft heavy metal cations, e. g., Ag(I), Au(III), Cd(II), Hg(II), Pd(II), and Pt(II). Recently, calix[4]arenes containing proton-ionizable N-X-sulfonyl carboxamide functionalities on the lower rim have been found to efficiently extract Hg(II), a soft metal ion, from acidic aqueous solutions with excellent selectivity over alkali, alkaline earth, and many transition and heavy metal cations, including Ag(I), Cd(II), Pb(II), Pd(II), and Pt(II). This unexpected favoring of Hg(II) complexation by ionophores containing hard donor groups has been used to obtain the first calixarene-based, mercury-selective fluorogenic reagent by incorporating a dansyl fluorophore moiety as a part of the N-X-sulfonyl carboxamide groups.