Separation of americium(III) from europium(III) with tridentate heterocyclic nitrogen ligands and crystallographic studies of complexes formed by 2,2': 6 ',2-terpyridine with the lanthanides

Phenyl-substituted derivatives of 2,2' : 6',2 "-terpyridine and a correspon ding bipyridine-pyrazine derivative have been shown to have metal extractio n properties and separation factors for americium(III) over europium(III) w hich are comparable to those previously obtained for 2,2' : 6',2 "-terpyrid ine (L-1). The extracting agents in either tert-butylbenzene (TBB) or hydro genated tetrapropene (TPH) gave D-Am/D-Eu separation factors (SFs) between 7 and 9 when used to extract the metal ions from 0.01-0.1 M nitric acid sol ution in synergistic combination with 2-bromodecanoic acid. In contrast to L-1, the new hydrophobic ligands have little or no solubility in the aqueou s phase. In an effort to better understand the nature of the species which may be involved in the extraction process, a series of metal-L-1 complexes which cover the lanthanides have been prepared. Five different structural t ypes have been established for the lanthanide coordination complexes. In ty pe 1, [M(NO3)(3)(L-1)(H2O)] (M = Nd), the metal is 10-coordinate being bond ed to one terdentate L-1 ligand, three bidentate nitrates and a water molec ule. In type 2, [M(NO3)(2)(L-1)(2)](+)[M(NO3)(4)(L-1)](-) (M = Nd, Sm, Tb, Dy and Ho), the metal atom in the cation is 10-coordinate, being bonded to two terdentate L-1 ligands and two bidentate nitrates; in the anion the met al is also 10-coordinate, being bonded to one terdentate L-1 ligand and fou r nitrates, of which three are bidentate and one unidentate. In type 3, [M( NO3)(3)(L-1)(H2O)]. L-1 (M = Ho, Er, Tm and Yb), the metal is 10-coordinate , being bonded to three bidentate nitrates, one terdentate L-1 and a water molecule. In addition, an L-1 ligand is found in the asymmetric unit which is hydrogen-bonded to the coordinated water molecule. In type 4, [M(NO3)(3) (L-1)(H2O)] (M = Tm), the metal is 9-coordinate, being bonded to two bident ate nitrates, one unidentate nitrate, one terdentate L-1 ligand and a water molecule. In type 5, [M(NO3)(3)(L-1)] (M = Yb), the metal is 9-coordinate, being bonded to three bidentate nitrates and one terdentate L-1 ligand. A sixth structural type was observed for M = La in the crystal structure [(H2 L1)(NO3)](+)[(H2L1)](2+)[La(NO3)(6)](3-). The metal is not bound to L-1 but instead forms the well-known hexanitrate anion. This complex may give some indication of the type of species which could be formed at higher acid con centrations in the aqueous phase, where protonation of L-1 can occur.