An unusual gadolinium ten-coordinated dimeric complex in the series of MRI contrast agents: Na[Gd(H2O)AAZTA] · 3H2O

The crystal structures of the ligand AAZTA (6-amino-6-methylperhydro-1,4-diazepine tetraacetic acid) and of its Gd complex Na[Gd(H₂O)AAZTA] · 3H₂O have been determined by single crystal X-ray diffraction. The AAZTA ligand crystallizes in a zwitterion form with two deprotonated carboxylic groups and two protonated tertiary nitrogen atoms. Two independent molecules of about a similar conformation and three water molecules as asymmetric units, are present in the crystal. In the solid state the gadolinium complex is a centrosymmetric dimer with a bicapped square antiprismatic coordination geometry around each metal ion. Two symmetric bridging carboxylate groups determine the dimer formation with Gd-O(2) and Gd-O(2)′ bond distances rather comparable of 2.526(4) and 2.548(4) Å, respectively, while the Gd-O(1) (inner sphere water) bond distance is 2.443(5) Å. A network of hydrogen bonds between the water of inner and outer spheres and the AAZTA carboxylic groups is present in the crystal structure.