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 CO₃/²-/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.