Do An(iii) and Ln(iii) ions form heteroleptic complexes with diglycolamide and hydrophilic BT(B)P ligands in solvent extraction systems? A spectroscopic and DFT study

Abstract

Time-Resolved Laser Fluorescence Spectroscopy (TRLFS) was used to study the complexation of Cm(III) and Eu(III) in solvent extraction systems consisting of an organic phase containing TODGA (tetra-n-octyl diglycolamide) as an extracting agent and an aqueous phase containing hydrophilic complexing agents, tetrasodium salts of 2,6-bis(5,6-di(3-sulphophenyl)-1,2,4-triazin-3-yl)pyridine, SO₃-Ph-BTP⁴⁻, or 6,6′-bis(5,6-di(3-sulphophenyl)-1,2,4-triazin-3-yl)-2,2′-bipyridine, SO₃-Ph-BTBP⁴⁻. Only one complex, homoleptic [Cm(TODGA)₃](NO₃)₃, was found in the organic phase. No heteroleptic complexes of Cm(III) with TODGA and SO₃-Ph-BTP⁴⁻ have been detected. In contrast, in dilute aqueous HClO₄ solutions containing TEDGA (a hydrophilic homologue of TODGA) and SO₃-Ph-BTP⁴⁻ ligands, seven different Cm(III) species were identified by their distinct emission spectra. Apart from the emission bands corresponding to the hydrated Cm³⁺ cation and to the complexes [Cm(SO₃-Ph-BTP)]⁻ and [Cm(TEDGA)_n]³⁺ (n = 1–3), two further emission bands were ascribed to heteroleptic complexes [Cm(TEDGA)(SO₃-Ph-BTP)]⁻ and [Cm(TEDGA)₂(SO₃-Ph-BTP)]⁻. In a similar aqueous solution containing TEDGA and SO₃-Ph-BTBP⁴⁻, only one heteroleptic complex was identified, [Cm(TEDGA)(SO₃-Ph-BTBP)]⁻. In addition, quantum chemical optimisation of the structures of the postulated heteroleptic complexes was performed.