Formation of homo- and heteronuclear complexes of 1-hydroxyethylidene-1,1-diphosphonic acid with MnII and FeIII in aqueous solutions

Systems based on 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), iron( iii )—HEDP and manganese( ii )—iron( iii )—HEDP, were studied by pH-potentiometry combined with mathematical modeling and NMR. Diverse and highly stable heteronuclear iron manganese complexes were found to exist in the heteronuclear system. The formation of the MnH 3 L 2 3– , FeMnH 3 L 2 0 , Fe 2 MnHL 3 3– , and MnL 2– complexes was established. The relaxation efficiency coefficient REC 2 (relaxivity) of these complexes was evaluated at ~2000 mol –1 s –1 L. Therefore, these systems hold promise as MRI contrast agents.