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

Abstract

Phenyl-substituted derivatives of 2,2′∶6′,2′′-terpyridine and a corresponding bipyridine–pyrazine derivative have been shown to have metal extraction properties and separation factors for americium(III) over europium(III) which are comparable to those previously obtained for 2,2′∶6′,2′′-terpyridine (L¹). The extracting agents in either tert-butylbenzene (TBB) or hydrogenated 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 solution in synergistic combination with 2-bromodecanoic acid. In contrast to L¹, the new hydrophobic ligands have little or no solubility in the aqueous phase. In an effort to better understand the nature of the species which may be involved in the extraction process, a series of metal–L¹ complexes which cover the lanthanides have been prepared. Five different structural types have been established for the lanthanide coordination complexes. In type 1, [M(NO₃)₃(L¹)(H₂O)] (M = Nd), the metal is 10-coordinate being bonded to one terdentate L¹ ligand, three bidentate nitrates and a water molecule. In type 2, [M(NO₃)₂(L¹)₂]⁺[M(NO₃)₄(L¹)]⁻ (M = Nd, Sm, Tb, Dy and Ho), the metal atom in the cation is 10-coordinate, being bonded to two terdentate L¹ ligands and two bidentate nitrates; in the anion the metal is also 10-coordinate, being bonded to one terdentate L¹ ligand and four nitrates, of which three are bidentate and one unidentate. In type 3, [M(NO₃)₃(L¹)(H₂O)]·L¹ (M = Ho, Er, Tm and Yb), the metal is 10-coordinate, being bonded to three bidentate nitrates, one terdentate L¹ and a water molecule. In addition, an L¹ ligand is found in the asymmetric unit which is hydrogen-bonded to the coordinated water molecule. In type 4, [M(NO₃)₃(L¹)(H₂O)] (M = Tm), the metal is 9-coordinate, being bonded to two bidentate nitrates, one unidentate nitrate, one terdentate L¹ ligand and a water molecule. In type 5, [M(NO₃)₃(L¹)] (M = Yb), the metal is 9-coordinate, being bonded to three bidentate nitrates and one terdentate L¹ ligand. A sixth structural type was observed for M = La in the crystal structure [(H₂L¹)(NO₃)]⁺[(H₂L¹)]²⁺[La(NO₃)₆]³⁻. The metal is not bound to L¹ 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 concentrations in the aqueous phase, where protonation of L¹ can occur.