To evaluate the effect of ligand geometry on the coordination number, number of inner-sphere water molecules, and affinity for anions of the corresponding lanthanide complex, six tris-bidentate 1,2-hydroxypyridonate (HOPO) europium(III) complexes with different cap sizes were synthesized and characterized. Wider or more flexible ligand caps, such as in EuIII-TREN-Gly-HOPO and EuIII-3,3-Gly-HOPO, enable the formation of nine-coordinate europium(III) complexes bearing three inner-sphere water molecules. In contrast, smaller or more rigid caps, such as in EuIII-TREN-HOPO, EuIII-2,2-Li-HOPO, EuIII-3,3-Li-HOPO, and EuIII-2,2-Gly-HOPO, favor eight-coordinate europium(III) complexes that have only two inner-sphere water molecules. Notably, there is no correlation between the number of inner-sphere water molecules and the affinity of the Eu(III) complexes for phosphate. Some q = 2 (EuIII-TREN-HOPO, EuIII-3,3-Li-HOPO, and EuIII-2,2-Gly-HOPO) and some q = 3 (EuIII-TREN-Gly-HOPO) complexes have no affinity for anions, whereas one q = 2 complex (EuIII-2,2-Li-HOPO) and one q = 3 complex (EuIII-3,3-Gly-HOPO) have a high affinity for phosphate. For the latter two systems, each inner-sphere water molecule is replaced with a phosphate anion, resulting in the formation of EuLPi2 and EuLPi3 adducts, respectively.
Bibliographical noteFunding Information:
The authors acknowledge the support of the National Science Foundation provided by INFEWS N/P/H2O:SusChEM CHE-1610832. S.-Y.H. was supported in part by a John Wertz Fellowship from the Department of Chemistry of the University of Minnesota. M.Q. was supported in part by a UROP fellowship from the University of Minnesota.
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