Ca²⁺- and Ba²⁺-Selective Receptors Based on Site-Selective Transmetalation of Multinuclear Polyoxime−Zinc(II) Complexes

Ca2+-selective recognition was achieved by using the site-selective transmetalation of homotrinuclear metallohost [LZn3]2+ containing a linear tetraoxime ligand. The selectivity (log(KCa/KMg) > 5.1) is comparable to those of the excellent Ca2+ receptors or sensors such as BAPTA, Quin2, and K23E1. X-ray crystallography revealed that the Ca2+ complex [L1Zn2Ca]2+ has a helical structure. On the other hand, the larger analogue H6L gave a mixture of [L2Zn4]2+ isomers, which selectively recognizes Ba2+ to give a single tetranuclear complex, [L2Zn3Ba]2+.