Rigid and Compact Binuclear Bis‐hydrated Gd‐complexes as High Relaxivity MRI Agents

The first binuclear Gd‐complex of the 12‐membered pyridine‐based polyaminocarboxylate macrocyclic ligand PCTA was synthesized by C−C connection of the pyridine units through two different synthetic procedures. A dimeric AAZTA‐ligand was also synthesized with the aim to compare the relaxometric results or the two ditopic Gd‐complexes. Thus, the (1)H relaxometric study on [Gd(2)PCTA(2)(H(2)O)(4)] and on [Gd(2)AAZTA(2)(H(2)O)(4)](2−) highlighted the remarkable rigidity and compactness of the two binuclear complexes, which results in molar relaxivities (per Gd), at 1.5 T and 298 K of ca. 12–12.6 mM(−1) s(−1) with an increase of ca. 80 % at 1.5 T and 298 K (+70 % at 310 K) with respect to the corresponding mononuclear complexes.