Synthesis and Physicochemical Characterization of New C‐Functionalized Derivatives of the Gadolinium(III) Complex with 3,6,10‐Tris(carboxymethyl)‐3,6,10‐triazadodecanedioic Acid (H5ttda) Exhibiting Fast Water Exchange – Potential Paramagnetic Reporters for Molecular Imaging

To confirm the observation that [Gd(ttda)] derivatives have a significantly shorter residence time τ_M of the coordinated H₂O molecule than [Gd(dtpa)], four new C‐functionalized [Gd(ttda)] complexes, [Gd(4‐Me‐ttda)] ( 1 ), [Gd(4‐Ph‐ttda)] ( 2 ), [Gd(9‐Me‐ttda)] ( 3 ), and [Gd(9‐Ph‐ttda)] ( 4 ), were prepared and characterized (H₅ttda=3,6,10‐tris(carboxymethyl)‐3,6,10‐triazadodecanedioic acid; H₅dtpa=3,6,9‐tris(carboxymethyl)‐3,6,9‐triazaundecanedioic acid). The temperature dependence of the proton relaxivity for these complexes at 0.47 T and of the ¹⁷O transverse relaxation rate of H₂¹⁷O at 7.05 T confirm that the proton relaxivity is not limited by the H₂O‐exchange rate. The residence time of the H₂O molecules in the first coordination sphere of the gadolinium complexes at 310 K, as calculated from ¹⁷O‐NMR data, is 13, 43, 2.9, and 56 ns for 1, 2, 3 , and 4 , respectively. At 310 K, the longitudinal relaxivity of 2 is higher than for the parent compound [Gd(ttda)] and the other complexes of the series. The stability of the new compounds was studied by transmetallation with Zn²⁺ ions. All the new complexes are more stable than the parent compound [Gd(ttda)].