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 V values of 1.4 and 1.9 cm³ mol^-1, respectively. An associative activation mode (I_a or A mechanism) was obtained for the Tm³⁺ complex (V = 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.