Water Exchange on [Ln(DO3A)(H2O)2] and [Ln(DTTA–Me)(H2O)2]− Studied by Variable Temperature, Pressure, and Magnetic Field NMR

Water exchange kinetics of [Ln(L)(H2O)2]x complexes (Ln = Pr, Nd, Dy, Tm, and Yb; L = DO3A and DTTA–Me) were studied by 17O NMR spectroscopy as a function of temperature, pressure, and frequency and by 1H nuclear magnetic relaxation dispersion. Water exchange rate constants of both complexes show a maximum at dysprosium. Water exchange on negatively charged complexes of the acyclic DTTA–Me ligand is much faster than on the neutral complexes of the macrocyclic DO3A. Small activation volumes |ΔV⧧| < 1 cm3 mol–1 measured for water exchange on [Ln(DO3A)(H2O)2] indicate an interchange type of mechanism (I) for the lanthanide complexes studied. In the case of [Ln(DTTA–Me)(H2O)2]−, a change in mechanism is detected from a dissociative mechanism (D, ΔV⧧ = 7 cm3 mol–1) for complexes with larger ions (Pr to Gd) to an interchange mechanism (Id, I; ΔV⧧ = +1.8 and +0.4 cm3 mol–1) for complexes with smaller ions (Dy and Tm).