Ionizable (thia)calix[4]crowns as highly selective [sup.226][Ra.sup.2+] ionophores

The [sup.226][Ra.sup.2+] selectivity of the ionizable (thia)calix[4]-crowns 1-4 was determined in the presence of a large excess of the most common alkali and alkaline earth cations. Selective [sup.226][Ra.sup.2+] (2.9 x [10.sup.-8] M) extraction occurs even at extremely high [M.sup.n+]/[sup.226][Ra.sup.2+] ratios of 3.5 x [10.sup.7] [[M.sup.n+] = [Na.sup.+], [K.sup.+], [Rb.sup.+], [Cs.sup.+], [Mg.sup.2+], [Ca.sup.2+], and [Sr.sup.2+] (1 M)] and an ionophore concentration of [10.sup.-4] M. The selectivity coefficients log([K.sup.Ra.sub.ex]/[K.sup.M.sub.ex]) are ~3.5 for [Mg.sup.2+], [Ca.sup.2+], and [Sr.sup.2+]. In the presence of [Ba.sup.2+], which has very similar chemical properties, only the thiacalix[4]crown-6 derivative 4 showed a selectivity for [sup.226][Ra.sup.2+]. In addition to the remarkable [sup.226][Ra.sup.2+] selectivities, the effective pH range (pH 8-13) of the thiacalix[4]crown dicarboxylic acids (3 and 4) allows for full regeneration of the ionophores at lower pH values (pH