Stoichiometry of An(iii)–DMDOHEMA complexes formed during solvent extraction

Abstract

N,N′-Dimethyl,N,N′-dioctylhexylethoxymalonamide (DMDOHEMA) is used to separate An(III) and Ln(III) from fission products in several liquid–liquid extraction processes that aim at recycling actinides. The stoichiometry of the extracted complexes is important for a complete understanding of the processes. The presented work focuses on the complexation of Cm(III) with DMDOHEMA studied by TRLFS in mono- and biphasic (solvent extraction) systems. The formation of [Cm(DMDOHEMA)_n]³⁺ (n = 1–3) in 1-octanol containing 1.7 mol L⁻¹ of water with log β′₁ = 2.6 ± 0.3, log β′₂ = 4.0 ± 0.5, log β′₃ = 4.3 ± 0.5 was confirmed. In addition, fluorescence lifetime measurements indicated the formation of a 1 : 4 complex. Furthermore, solvent extraction experiments were performed, varying the proton and nitrate concentrations. TRLFS measurements of organic phases confirmed the existence of two species, [Cm(DMDOHEMA)₃(NO₃)(H₂O)_1–2]²⁺ (dominant at high proton and nitrate concentrations) and [Cm(DMDOHEMA)₄(H₂O)]³⁺ (dominant at low proton and nitrate concentrations). To support the proposed stoichiometries, vibronic side-band spectroscopy (VSBS) was employed, allowing the observation of vibrations of functional groups coordinated to the probed metal ion. Clear differences between the vibronic side bands of the 1 : 3 and 1 : 4 complex in the range of 900–1300 cm⁻¹ were observed. Vibrational spectra calculated by DFT complimented the experimental data and confirmed the proposed stoichiometries. They revealed a monodentate coordination mode of the nitrate and two water molecules in the 1 : 3 complex.