Effect of Ligand Chirality and Hyperconjugation on the Thermodynamic Stability of a Tris(aquated) Gd III Complex: Synthesis, Characterization, and T 1 ‐Weighted Phantom MR Image Study

The hexadentate chiral alanine‐based ligand, Li 3 cbda, in a reaction with hydrated GdCl 3 at pH ~ 6.5 provided the corresponding nine‐coordinated, neutral Gd III complex ( 1 ). The hydration state ( q ) of complex 1 was 3.1 ± 0.1, as determined by luminescence decay time measurements of Tb III congener in H 2 O and D 2 O. Complex 1 exhibited thermodynamic stability, log  K GdL = 14.64 in 0.15 m NaCl solution at 25 °C. At 1.41 T, pH ≈ 7.4, and 25 °C, the complex offered longitudinal relaxivity, r 1 = 10.95 m m –1  s –1 , which was in accordance with three inner‐sphere water molecules. While, the relaxivity value remained unaffected in the pH range of 4.0 to 7.4, under basic conditions (pH ≥ 10) a reduction in the relaxivity value to 6.68 m m –1  s –1 was observed. A similar phenomenon was also realized in the presence of endogenous anions (100 equivalents of F – and HCO 3 – ), which implied the removal of coordinated‐water molecules under the conditions. Phantom MR imaging under the clinical scanner at 1.5 T showed the enhancement in the image intensity with increasing concentration of the complex. As expected, the image intensity of 1 m m complex 1 was about three times compared to 1 m m MultiHance® solution.