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 = Gd(3+), Dy(3+), or Tm(3+)) were determined by (1)H nuclear magnetic resonance (NMR) measurements. The number of inner-sphere water molecules was found to change from two to one when going from Dy(3+) to Tm(3+). The calculated water exchange rate constants obtained by variable-temperature proton transverse relaxation rates are 3.9 × 10(6), 0.46 × 10(6), and 0.014 × 10(6) s(-1) at 298 K for Gd(3+), Dy(3+), and Tm(3+), respectively. Variable-pressure measurements were used to assess the water exchange mechanism. The results indicate an associative and dissociative interchange mechanism for Gd(3+) and Dy(3+) complexes with ΔV(⧧) values of -1.4 and 1.9 cm(3) mol(-1), respectively. An associative activation mode (Ia or A mechanism) was obtained for the Tm(3+) complex (ΔV(⧧) = -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.