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

The enthalpies for formation of five Eu3+ complexes with N-piperidinomethan e-1,1-diphosphonic acid (H(4)pmdp) have been determined by titration calori metry in 2 M (Na,H)ClO4 media at 25 degrees C. Both protonation enthalpies and entropies, and the structure of H(4)pmdp in the solid state indicate pr otonation of the piperidine nitrogen to give a zwitterion in the free ligan d. 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 liberate s an average of 3.3 +/- 0.1 water molecules per ligand from the Eu inner hy dration sphere. The number of water molecules in the inner coordination sph ere and the thermodynamic parameters of the complexes suggest that the stru cture of the Eu3+-pmdp complexes and the strength of the Eu3+-pmdp bonds ar e similar to those of other methane-1,1-diphosphonic acids despite the pres ence of the piperidine ring. However, the complexation entropies argue for weaker solvation of the Eu-pmdp complexes probably due to the hydrophobic c haracter of the piperidine ring.