Kinetically Inert Macrocyclic Europium(III) Receptors for Phosphate

The significant role that phosphate plays in environmentalwaterpollution and biomedical conditions such as hyperphosphatemia highlightsthe need to develop robust receptors that can sequester the anioneffectively and selectively from complex aqueous media. Toward thatgoal, four macrocyclic tris-bidentate 1,2-hydroxypyridonate (HOPO)europium(III) complexes containing either a cyclen, cyclam, TACN,or TACD ligand cap were synthesized and evaluated as phosphate receptors.The solubility of Eu-III-TACD-HOPO in water was insufficientfor luminescent studies. Whereas Eu-III-cyclen-HOPO is eightcoordinate with two inner-sphere water molecules, both Eu-III-cyclam-HOPO and Eu-III-TACN-HOPO are nine coordinate withthree inner-sphere water molecules, suggesting that the two coordinationstates are very close in energy. As observed previously with linearanalogues of tripodal HOPO complexes, there is no relationship betweenthe number of inner-sphere water molecules and the affinity of thecomplex for phosphate. Whereas all three complexes do bind phosphate,Eu-III-cyclen-HOPO has the highest affinity for phosphatewith the anion displacing both of its inner-sphere water molecules.On the other hand, only one or two of the three inner-sphere watermolecules of Eu-III-TACN-HOPO and Eu-III-cyclam-HOPOare displaced by phosphate, respectively. All three complexes arehighly selective for phosphate over other anions, including arsenate.All three complexes are highly stable. Eu-III-cyclen-HOPOand, to a lesser extent, Eu-III-TACN-HOPO are more kineticallyinert than the linear Eu-III-Ser-HOPO. Eu-III-cyclam-HOPO,on the other hand, is not. This study highlights the significant effectthat minor changes in the ligand cap can have on both the ligand exchangerate and affinity for phosphate of tripodal 1,2-dihydroxypyridinonatecomplexes. The incorporation of eithera cyclen or a TACN ligand capinto tripodal 1,2-hydroxypyridinonate ligands increases the kineticinertness of the corresponding europium(III) complex by an order ofmagnitude without affecting the stability of the complex. Both complexeshave high affinity and high selectivity for phosphate, highlightingtheir potential for the further development of sequestration technology.