A new heptadentate picolinate-based ligand and its corresponding Gd(iii) complex: the effect of pendant picolinate versus acetate on complex properties

Abstract

To attain high relaxivity as well as stability, a new water-soluble, water-coordinated Gd(iii) complex, which was synthesised by reacting equimolar amounts of picolinate-based ligand H4peada and GdCl₃·xH₂O at pH ∼ 6.5, was examined. The number of inner sphere water molecules (q) in the complex was found to be 1.7 ± 0.1 from luminescence lifetime measurements of its Tb(iii) congener, complex 2. At 1.41 T, 25 °C, and pH = 7.4, the longitudinal relaxivity (r₁) value of the complex was found to be 6.08 mM^-1 s^-1, which remained almost constant in the pH range 4-10. The r₁ relaxivity value has not been affected in the presence of a 100 fold excess of bicarbonate and phosphate anions, whereas in the case of fluoride ions, the value dropped to 4.6 mM^-1 s^-1 due to a binding interaction of fluoride ions by replacing inner sphere water molecules of the complex. From the potentiometric titration method, the stability constant of the complex was found to be log K_GdL = 17.0 ± 0.08 (in 0.15 M KCl and 25 °C). At pH = 7.4, the pGd value of ligand H4peada was found to be 14.01 which was comparable to the commercially available MRI contrast agent Omniscan®. Phantom MR images of the complex under a clinical MR scanner at 1.5 T also demonstrated the usefulness of complex 1 as a potential MRI contrast agent.