Formation and Dissociation Kinetics of the Complexes Gd(DOTP) 5− and Gd(DOTPMB) −

The monobutyl ester of H 8 DOTP, the ligand H 4 DOTPMB, was synthesized, and the protonation constants ( K H i ) and the stability constant of Gd(DOTPMB) were determined by pH‐potentiometry (25 °C, 0.1 M Me 4 NCl): log K H i , ( i = 1, 2, and 3) = 10.34(0.02), 7.72(0.025), and 2.42(0.030), respectively, and log K GdL = 12.19(0.05). The rates of formation of Gd(DOTPMB) and Gd(DOTP) were studied by 1 H relaxometry in the pH range 5.4−7, and also by spectrophotometry in the case of Gd(DOTP) (7 < pH < 8). For both reactions, first‐order rate constants were obtained at different concentration ratios of the reactants, which indicated the rapid formation of a reaction intermediate. The compositions of the intermediates are Gd(H i DOTPMB) ( i = 1, 2) and H n Gd(H 2 DOTP) ( n = 0−4), respectively, where one or two protons are attached to the nitrogen atoms of the ligand. The rate of rearrangement ( k r ) of the intermediate Gd(H i DOTPMB) to the product Gd(DOTPMB) increases with increasing [OH − ]: k r = k OH [OH − ] + k 2OH [OH − ] 2 , where k OH = (1.3±0.25) ×10 3 M −1 s −1 and k 2OH = (7.8±0.2) ×10 11 M −2 s −1 . For the formation reaction of Gd(DOTP), only the first term exists and k OH = (7.2±0.1) ×10 3 M −1 s −1 . For the complexation reactions, similar mechanisms were proposed in which deprotonation of the species Gd(HDOTPMB) and H n Gd(HDOTP) plays the rate‐determining role. For the deprotonation, general base catalysis was found to be satisfactory. The rate of dissociation of Gd(DOTPMB) in 0.025−1.0 M HCl solution ([HCl] + [Me 4 NCl] = 1.0 M ) was lower than that of Gd(DOTP) and the first‐order rate constants exhibited saturation curves with increasing [H + ]. Based on the assumption that the protonated species HGd(DOTPMB) and H 5 Gd(DOTP) dissociate, the rate constants (and protonation constants) were found to be (5.4±0.2) ×10 −4 M −1 s −1 ( K H GdL = 1.7±0.1) and (2.1±0.1) ×10 −4 M −1 s −1 ( K H H4GdL = 1.9±0.1), respectively.