PARAMAGNETIC GD-III-FE-III HETEROBIMETALLIC COMPLEXES OF DTPA-BIS-SALICYLAMIDE

The reaction between DTPA (diethylenetriaminepenta-acetic acid)-anhydr ide and p-aminosalicylic acid (PAS) affords a novel ligand, DTPA(PAS) 2!, able to form stable heterobimetallic complexes with Gd3+ and Fe3ions. The lanthanide ion occupies an internal coordination cage formed by three nitrogen atoms, two carboxylate and two carboxoamido groups of the ligand, whereas the outer salicylic moieties form stable chelat e rings with Fe(III) ions. The stoichiometry of the resulting heterobi metallic complexes, established by measurements of water proton relaxa tion enhancement, is (H2O)-Gd-DTPA(PAS)2!2-Fe(H2O)2 or (H2O)-Gd-DTPA (PAS)2!3-Fe depending on the pH of the aqueous solution. The individua l contributions to the observed relaxation enhancement from Gd3+ and F e3+ paramagnetic ions have been clearly distinguished and analysed.