An investigation of P,P'-di(2-ethylhexyl) methanediphosphonic acid and some of its metal complexes

Copper(II), iron(III), europium(III), thorium(IV) and uranium(VI) complexes of P,P'-di(2-ethylhexyl) methanediphosphonic acid, H(2)DEH[MDP], were prepared along with calcium and sodium salts. The compounds were characterized by elemental analysis, infrared spectroscopy, and vapor phase osmometry (VPO). Infrared spectra reveal that the asymmetric, nu(asym)(POO-), and symmetric, nu(sym)(POO-), phosphorus-oxygen stretching modes of the ligand are sensitive to the nature of the metal ion present in the compound. The dramatic variation in nu(asym)(POO-) with metal ion indicates a wide range in strength of the metal-diphosphonate interaction. The shift of both nu(asym)(POO-) and nu(sym)(POO-) to lower energies relative to their values in the sodium salt indicates symmetrical coordination of the phosphonate groups through chelate and/or bridging interactions. The difference Delta nu between the POO- stretching frequencies, [nu(asym)(POO-)-nu(sym)(POO-)], becomes smaller as the ionic potential of the metal ion increases. H(2)DEH[MDP] was found by VPO to be dimeric in toluene at 25 degrees C in the 0.2-0.005 m concentration range. Dimerization is achieved through strong intermolecular hydrogen bonding between the phosphoryl and P-OH groups of two H(2)DEH[MDP] molecules. Pre-equilibration with water or 1.0 M nitric acid does not effect the aggregation of the ligand in toluene. The apparent aggregation numbers measured for H(2)DEH[MDP] solutions contacted with aqueous metal ions indicate that the ligand has a tendency to form polynuclear complexes. Aggregation increases with increasing metal ion concentration, deprotonation of the P-OH groups, and the ionic potential of the metal ion. The absence of nitrate ion in these solutions shows that, under high metal loading conditions, cations are extracted from aqueous nitric acid by H(2)DEH[MDP] with displacement of hydrogen ions. Ligand band shifts indicate that when excess un-ionized H(2)DEH[MDP] is present in solution, the phosphoryl oxygen atoms of the acid participate in the complexation process. (C) 1997 Elsevier Science Ltd.