To develop a selective ligand for the separation of lanthanides(III) and actinides(III) the coordination chemistry of the chelating N-donor ligand 2,6-bis(1-(p-tolyl)-1H-1,2,3-triazol-4-yl)pyridine (BTTP) was investigated. The two isostructural lanthanide compounds [Ln(BTTP)3(OTf)3] (Ln = Eu (1), Sm (2); OTf = trifluoromethanesulfonate) were synthesized and fully characterized. The solid-state structures of both compounds were established by single-crystal X-ray diffraction. The complexation of Cm(III) and Eu(III) with BTTP in acetonitrile was studied using time-resolved laser fluorescence spectroscopy. With increasing BTTP concentration Cm(III) 1:2 and 1:3 complexes and Eu(III) 1:1 and 1:3 complexes are identified. The conditional stability constants of the 1:3 complex species with BTTP are log β3 = 14.0 for Cm(III) and log β3 = 10.3 for Eu(III). Both M(III) 1:3 complexes are prone to decomplexation with increasing acidity.

The complexation of Cm(III) and Eu(III) with 2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine (nPr–BTP) in methanol–water mixtures with varying water content is investigated by TRLFS. For both metal ions, the exclusive formation of the 1:3 complex is observed in each solvent mixture. Stability constants are determined by analysis of the fluorescence spectra. As a result, an immense influence of the water content is found. The stability constant of the [Cm(nPr–BTP)3]3+ complex increases by three orders of magnitude when the water content in methanol–water mixtures is reduced (logβ3 = 14.3 ± 0.1 at 50 vol% water content; logβ3 = 17.4 ± 0.4 at 5 vol% water content). Due to the preferential solvation of Cm(III) by water in methanol–water mixtures, the increase is moderate at water contents between 50 vol% and 20 vol% and becomes steeper at lower water contents. For the [Eu(nPr–BTP)3]3+ complex, an analogous evolution of the stability constant is observed (logβ3 = 12.0 ± 0.1 at 50 vol% water content; logβ3 = 14.7 ± 0.4 at 5 vol% water content). Therefore, the difference between the stability constants of the Cm(III) and Eu(III) 1:3 complexes is constant at all solvent mixtures investigated (Δlogβ3 = 2.3 ± 0.3) which means that the selectivity of the N-donor ligand is not influenced by the solvent.

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.