COMPLEXATION OF INDIUM(III), GALLIUM(III), IRON(III), GADOLINIUM(III), AND NEODYMIUM(III) IONS WITH AMINO DIPHOSPHONIC ACIDS IN AQUEOUS-SOLUTION

The compounds (Me2N)CH(PO3H2)2(MAMDP) and MeC(NH2)(PO3H2)2(AEDP) have been synthesized. These compounds have been titrated with base and the protonation constants determined. Titration data of pH against added acid or base show that AEDP acts as a ligand toward In3+ and that MAMDP binds as a ligand to In3+, Ga3+, Fe3+, Gd3+, and Nd3+. From a least-squares fit of the pH titration curves, the stability and protonation constants have been obtained for solutions containing these ligands and these trivalent metal ions. The ligands bind to the trivalent metal ions via the phosphonate oxygens, although it is likely that hydrogen bonding occurs between water and the phosphonate ligand. For the ions In3+, Ga3+, and Fe3+, the logarithms of the stability constants log K101 and log K102 (where K101 = [ML2(5-)]/[M3+][L4-] and K102 = [ML2(5-)]/[M3+][L4-12) have the respective values of 30.0, 25.8, 28.8 and 35.8, 33.7, 34.3. For the lanthanide ions Nd3+ and Gd3+ the values are lower at 16.0, 17.6 and 20.4, 20.7, respectively. This pattern of stability constants follows the trend for the complexation of EDTA with these trivalent metal ions. The compound MAMDP remains coordinated to the metal ions in solutions of low acidity because protonation occurs at the free rather than the complexed phosphonate oxygens.