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 H-1 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 x 10(6), 0.46 x 10(6), and 0.014 x 10(6) 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 Delta V double dagger values of -1.4 and 1.9 cm(3) mol(-1), respectively. An associative activation mode (Ia or A mechanism) was obtained for the Tm3+ complex (Delta V double dagger = -5.6 cm(3) 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.