Equilibrium Studies on the Gd 3+ , Cu 2+ and Zn 2+ Complexes of BOPTA, DTPA and DTPA‐BMA Ligands: Kinetics of Metal‐Exchange Reactions of [Gd(BOPTA)] 2–

The stability and protonation constants of the complexes formed between the ligand BOPTA [H 5 BOPTA: 4‐carboxy‐5,8,11‐tris(carboxymethyl)‐1‐phenyl‐2‐oxa‐5,8,11‐triazatridecan‐13‐oic acid] and Gd 3+ , Cu 2+ , and Zn 2+ have been determined by pH potentiometry (Gd 3+ , Zn 2+ ) and spectrophotometry (Cu 2+ ) at 25 °C and at constant ionic strength maintained by 0.15 M NaCl. For comparison, the stability and protonation constants of the complexes of Gd 3+ , Cu 2+ , and Zn 2+ formed with DTPA and DTPA‐BMA {H 5 DTPA: diethylenetriamine‐ N , N , N′ , N″ , N″ ‐pentaacetic acid, H 3 DTPA‐BMA: 2,2′‐[(carboxymethyl)imino]bis[ethylenebis(methylcarbamoylmethyl)imino]diacetic acid} have also been determined under similar conditions. The stability constants (log  K ML ) of the complexes of BOPTA and DTPA are very similar, but in 0.15 M NaCl the protonation constants of the ligands (log  K i H ) and the log  K ML values are lower than those obtained in 0.1 M KCl or Me 4 NCl by 0.3–0.9 log  K units. The order of selectivity of the ligands for Gd 3+ over Zn 2+ is BOPTA > DTPA > DTPA‐BMA. The complex [Cu(DTPA‐BMA)] – deprotonates in the pH range 7–10 with the dissociation of an amide NH group and the coordination of the amide N atom. The kinetics of the exchange/transmetallation reactions between the complex [Gd(BOPTA)] 2– and the metal ions Cu 2+ , Zn 2+ , and Eu 3+ have been studied by spectrophotometry (Cu 2+ , Eu 3+ ) and relaxometry (Zn 2+ ) in the pH range 3.3–6 at 25 °C in 0.15 M NaCl. The reactions with Cu 2+ and Zn 2+ occur predominantly with direct attack of the metal ions on the [Gd(BOPTA)] 2– complex. The kinetic activity of Eu 3+ is lower, and in the exchange reactions with Eu 3+ , the proton‐assisted dissociation of [Gd(BOPTA)] 2– (which is followed by fast reaction between the free ligand and Eu 3+ ) could be also investigated. The rate constants, characterizing the proton‐assisted dissociation of [Gd(BOPTA)] 2– and the exchange reactions occurring with the direct attack of Cu 2+ , Zn 2+ , and Eu 3+ on the complex, are lower by about 30–90 % than the rate constants obtained earlier for similar transmetallation reactions of [Gd(DTPA)] 2– . The half‐time for the dissociation of [Gd(BOPTA)] 2– at pH 7.4 and at 25 °C in the presence of 1 × 10 –5 M Zn 2+ and 1 ×10 –6 M Cu 2+ is 169 h.