Study of Thermodynamic and Kinetic Stability of Transition Metal and Lanthanide Complexes of DTPA Analogues with a Phosphorus Acid Pendant Arm

The thermodynamic and kinetic stabilities of the complexes of phosphonate and phenylphosphinate analogues of H 5 dtpa with selected transition‐ and lanthanide‐metal ions are presented. Both phosphorus‐containing ligands form thermodynamically very stable complexes, with stability constants comparable with or even higher than those reported for the parent H 5 dtpa. However, the kinetic inertness of their gadolinium( III ) complexes against acid‐ and metal‐assisted decomplexations is surprisingly much lower. The half‐life times of gadolinium( III ) complexes of the new ligands in the presence of excess of concurrent metal ions [copper( II ) or europium( III )] are about 2–3 orders of magnitude shorter compared to H 5 dtpa and its amide derivatives. The behaviour can probably be attributed to steric strain in the new complexes, to the high affinity of phosphonate ligand for proton, and/or to easy formation of binuclear complexes, which act as intermediates in the complex dissociation. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)