6‑(Tetrazol-5-yl)-2,2′-bipyridine: A Highly Selective Ligand for the Separation of Lanthanides(III) and Actinides(III)

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 1:2 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 1:2 and 1:3 complexes with increasing HN4bipy concentration; for europium­(III), formation of 1:1 and 1:3 complexes is observed. Although the solid-state samarium complexes were confirmed as 1:2 species the 1:2 europium­(III) solution complex in ethanol was not identified with TRLFS. The determined conditional stability constant for the 1:3 fully coordinated curium­(III) complex species is more than 2 orders of magnitude higher than that for europium­(III) (log β3[Cm­(N4bipy)3] = 13.8 and log β3[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 1:2 and 1:3 curium­(III) complexes, but no ternary complexes were observed. The stability constants for the 1:3 metal ion–N4bipy complexes equate to a theoretical separation factor, SFCmIII/EuIII ≈ 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.