The enthalpies for formation of five Eu3+ complexes with N-piperidinomethane-1,1-diphosphonic acid (H4pmdp) have been determined by titration calorimetry in 2 M (Na,H)ClO4 media at 25 °C. Both protonation enthalpies and entropies, and the structure of H4pmdp in the solid state indicate protonation of the piperidine nitrogen to give a zwitterion in the free ligand. Luminescent lifetime measurements show that the formation of the 1∶1 and 1∶2 Eu∶L complexes of a variety of methane-1,1-diphosphonic acids liberates an average of 3.3 ± 0.1 water molecules per ligand from the Eu inner hydration sphere. The number of water molecules in the inner coordination sphere and the thermodynamic parameters of the complexes suggest that the structure of the Eu3+–pmdp complexes and the strength of the Eu3+–pmdp bonds are similar to those of other methane-1,1-diphosphonic acids despite the presence of the piperidine ring. However, the complexation entropies argue for weaker solvation of the Eu–pmdp complexes probably due to the hydrophobic character of the piperidine ring.
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