The Selectivity of Reversible Oxy-Anion Binding in Aqueous Solution at a Chiral Europium and Terbium Center: Signaling of Carbonate Chelation by Changes in the Form and Circular Polarization of Luminescence Emission

Reversible anion binding in aqueous media at chiral Eu^III and Tb^III centers has been characterized by ¹H NMR and by changes in the emission intensity and circular polarization following direct or sensitized (365 nm) excitation via an alkylphenanthridinium chromophore. Using a series of heptadentate tri-amide or polycarboxylate hgands, the affinity for CO₃²-/HCO₃-, phosphate, lactate, citrate, acetate, and malonate at pH 7.4 was found to decrease as a function of the overall negative charge on the complex: citrate and malonate bound most strongly, and lactate and hydrogen carbonate also formed chelated ternary complexes in which displacement of both of the metal-bound water molecules occurred, which was confirmed by VT 17-O NMR measurements of the corresponding Gd complexes. The binding of carbonate was studied in particular, and ¹H NMR and CPL data were obtained that were consistent with the formation of a complex with a reduced helical twist about the metal center. Monohydrogen phosphate was bound in a monodentate manner, giving a mono-aqua adduct. The binding of carbonate to cationic Eu complexes in the presence of a simulated extra-cellular anionic background at pH 7.4 was monitored by variation in the emission intensity, ratio of intensities (615/594 nm), and dissymmetry factors as a function of added total carbonate.