Extraction and complexation studies with 2,6-bis(5-(tert-butyl)-1H-pyrazol-3-yl)pyridine in the presence of 2-bromohexanoic acid

Abstract

To improve the understanding of the extraction chemistry of An(III) and Ln(III) with N-donor ligands 2,6-bis(5-(tert-butyl)-1H-pyrazol-3-yl)pyridine (C4-BPP) in the presence of 2-bromohexanoic acid was investigated. Extraction studies showed an excellent separation factor of SF_{Am(III)/Eu(III)} ≈ 200 and SF_{Am(III)/Nd(III)} ≈ 60 in comparison with the structurally similar ligand 2,6-bis(5-neopentyl-1H-pyrazol-3-yl)pyridine C5-BPP (SF_{Am(III)/Eu(III)} ≈ 100), even though C5-BPP showed significantly higher stability constants. Time-resolved laser fluorescence spectroscopy (TRLFS) studies revealed the formation of the ternary 1 : 1 and 1 : 2 complexes [Eu(C4-BPP)_n(2-bromohexanoate)_m]^(3−m)+ (n = 1–2) ( and ). [Eu(C4-BPP)₂(2-bromohexanoate)_m]^(3−m)+ was the relevant complex species in solvent extraction. In contrast, Cm(III) form stable 1 : 3 complexes. The ability of 2-bromohexanoic acid to replace C4-BPP from the inner coordination sphere of Eu(III) but not from Cm(III) is due to a more favorable complexation of Cm(III) over Eu(III) with C4-BPP. This resulted in a notably more efficient separation of An(III) and Ln(III) in comparison with C5-BPP.