Preparation, spectroscopic and high field NMR relaxometry studies of gadolinium(III) complexes with the asymmetric tetraamine 1,4,7,11-tetraazaundecane

The reaction of Gd(III) with asymmetric tetramine 1,4,7,11-tetraazaundecane (2,2,3-tet, L1) ligand has been studied via NMR spectroscopy. The ligand proton longitudinal relaxation rates (R1) have been used to estimate the distances of these protons from the Gd(III) center, in Gd(III)–L1 reaction solutions, in H2O/D2O 5/1 mixtures.Two Gd(III) complexes [Gd(III)(L1)(NH3)(H2O)4](CH3COO)3·2H2O (1) and [Gd(III)(L1)(NH3)(H2O)2]Cl3·EtOH (2) have been isolated and characterized by elemental analyses, TGA, IR, NMR and relaxometry measurements. The NMR relaxation measurements of 2 in aqueous solutions have been performed, under various temperature or concentration conditions, and compared with those of the commercial contrast agents Gd(III)–DTPA and Gd(III)–DTPA-BMA. It has also been studied the influence of (i) the Gd(III) inner-sphere water molecule number (q) alteration and (ii) the steric constraint enhancement on the metal site, over the relaxation rate values of the parent aqueous solution of Gd(III)–2,2,3-tet, and of the aqueous solutions of 2.