Thermodynamics and hydration of the europium complexes of a nitrogen heterocycle methane-1,1-diphosphonic acid

Abstract

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.