Evaluation of Water Exchange Kinetics on [Ln(AAZTAPh–NO2)(H2O)q]x Complexes Using Proton Nuclear Magnetic Resonance
Water exchange kinetics on [Ln(AAZTAPh–NO2)(H2O)q]− (Ln = Gd3+, Dy3+, or Tm3+) were determined by 1H nuclear magnetic resonance (NMR) measurements. The number of inner-sphere water molecules was found to change from two to one when going from Dy3+ to Tm3+. The calculated water exchange rate constants obtained by variable-temperature proton transverse relaxation rates are 3.9 × 106, 0.46 × 106, and 0.014 × 106 s–1 at 298 K for Gd3+, Dy3+, and Tm3+, respectively. Variable-pressure measurements were used to assess the water exchange mechanism. The results indicate an associative and dissociative interchange mechanism for Gd3+ and Dy3+ complexes with ΔV⧧ values of −1.4 and 1.9 cm3 mol–1, respectively. An associative activation mode (Ia or A mechanism) was obtained for the Tm3+ complex (ΔV⧧ = −5.6 cm3 mol–1). Moreover, [Dy(AAZTAPh–NO2)(H2O)2]− with a very high transverse relaxivity value was found as a potential candidate for negative contrast agents for high-field imaging applications.