4,4′-Di-tert-butyl-6-(1H-tetrazol-5-yl)-2,2′-bipyridine: modification of a highly selective N-donor ligand for the separation of trivalent actinides from lanthanides

Abstract

In the present work, the complexation and extraction behaviour of 4,4'di-tert-butyl-6-(1H-tetrazol-5-yl)-2,2'-bipyridine (HN₄^tbubipy) towards trivalent actinides (An(iii)) and lanthanides (Ln(iii)) is studied by spectroscopic methods, liquid-liquid extraction, and quantum chemical calculations. The ligand synthesis of HN₄^tbubipy as well as its application in coordination chemistry of the 4f elements is described. Reaction of HN₄^tbubipy with [Ln(NO₃)₃·6H₂O] (Ln = Sm, Eu) results in [H₂N₄^tbubipy]⁺[Ln(N₄^tbubipy)(NO₃)₃(H₂O)]^-. Both compounds have been characterized by single crystal X-ray diffraction. The solubility of the ligand in different organic solvents is determined, showing a high solubility in MeOH which decreases with the lipophilicity of the solvent. The pK_a = 2.4 ± 0.2 of HN₄^tbubipy in EtOH (4.4 vol% H₂O) is determined by absorption spectrophotometry. The complexation of Cm(iii) and Eu(iii) with HN₄^tbubipy is studied by time resolved laser fluorescence spectroscopy (TRLFS). For both metal ions the formation of the complexes [M(N₄^tbubipy)_n]^3-n with n = 2, 3 (M = Cm(iii), Eu(iii)) is observed. Slightly higher conditional stability constants for Eu(iii) (log β'₂(Eu(N₄^tbubipy)₂⁺) = 8.9 ± 0.3, log β'₃(Eu(N₄^tbubipy)₃) = 12.7 ± 0.5), compared to Cm(iii) (log β'₂(Cm(N₄^tbubipy)₂⁺) = 8.5 ± 0.4 and log β'₃(Cm(N₄^tbubipy)₃) = 12.4 ± 0.6) are determined. Thus, the ligand has no preference for the complexation of An(iii) over Ln(iii). Additionally, no significant extraction of Am(iii) and Eu(iii) is observed in liquid-liquid extraction experiments due to protonation of the ligand at the experimental conditions. The experimental studies are supported by quantum chemical calculations of the free ligand and the [M(N₄^tbubipy)₃] complexes (M = Cm(iii), Gd(iii)). The results are in excellent agreement with the experimental data and provide a deeper understanding of the complexation properties of HN₄^tbubipy.