6-(Tetrazol-5-yl)-2,2'-bipyridine: a highly selective ligand for the separation of lanthanides(III) and actinides(III).

ABSTRACT

The coordination structure in the solid state and solution complexation behavior of 6-(tetrazol-5-yl)-2,2'-bipyridine (HN4bipy) with samarium(III) was investigated as a model system for actinide(III)/lanthanide(III) separations. Two different solid 12 complexes, [Sm(N4bipy)2(OH)(H2O)2] (1) and [Sm(N4bipy)2(HCOO)(H2O)2] (2), were obtained from the reaction of samarium(III) nitrate with HN4bipy in isopropyl alcohol, resuspension in N,N-dimethylformamide (DMF), and slow crystallization. The formate anion coordinated to samarium in 2 is formed by decomposition of DMF to formic acid and dimethylamine. Time-resolved laser fluorescence spectroscopy (TRLFS) studies were performed with curium(III) and europium(III) by using HN4bipy as the ligand. Curium(III) is observed to form 12 and 13 complexes with increasing HN4bipy concentration; for europium(III), formation of 11 and 13 complexes is observed. Although the solid-state samarium complexes were confirmed as 12 species the 12 europium(III) solution complex in ethanol was not identified with TRLFS. The determined conditional stability constant for the 13 fully coordinated curium(III) complex species is more than 2 orders of magnitude higher than that for europium(III) (log ß3[Cm(N4bipy)3] = 13.8 and log ß3[Eu(N4bipy)3] = 11.1). The presence of added 2-bromodecanoic acid as a lipophilic anion source reduces the stability constant for formation of the 12 and 13 curium(III) complexes, but no ternary complexes were observed. The stability constants for the 13 metal ion-N4bipy complexes equate to a theoretical separation factor, SF(Cm(III)/Eu(III)) ≈ 500. However, the low solubility of the HN4bipy ligand in nonpolar solvents typically used in actinide-lanthanide liquid-liquid extractions prevents its use as a partitioning extractant until a more lipophilic HN4bipy-type ligand is developed.