A fluorescence study on the complexation of Sm(III), Eu(III) and Tb(III) with tetraalkyldiglycolamides (TRDGA) in aqueous solution, in solid state, and in solvent extraction
Tetraalkyldiglycolamide (TRDGA) ligands have potential utilization for the separation of actinide and lanthanide ions in the nuclear industry as chelates in aqueous solution if water-soluble or as extractants in organic solvents if water-insoluble. Here, a spectral titration method is extensively applied to investigating the complexation of fluorescent Sm(III), Eu(III), and Tb(III) with TRDGA ligands in aqueous solutions and a solvent extraction system. In aqueous solutions using N,N,N′,N′-tetramethyldiglycolamide (TMDGA, LI) as chelate, three successive complex species of Ln(III), including [LnLI]3+, [LnLI2]3+, and [LnLI3]3+, are identified for each Ln(III) (Ln = Sm, Eu, and Tb), and their stability constants are determined with fluorescence spectral titration method at 25 °C in 1 M NaNO3. The coordination mode in [LnLI3]3+ is illustrated by single-crystal structures of the solid compounds LnLI3(ClO4)3 (Ln = Sm, Eu, Tb, and LI = TMDGA) grown from aqueous solutions by slow evaporation. The crystal structures show that in the complexes Ln(III) ions are coordinated by nine oxygen atoms from three tridentate LI ligands in a distorted tricapped trigonal prism geometry. To provide parallels to solvent extraction chemistry, the extracted Ln(III) complexes with N,N′-dimethyl-N,N′-dioctyldiglycolamide (DMDODGA, LII, a lipophilic analogue of TMDGA) are prepared, and the fluorescence spectra are collected as well for comparison. The fluorescence spectra of the extracted Ln(III) complexes with LII in an organic phase of 40–60% (v/v) n-octanol–kerosene are nearly identical to the corresponding deconvoluted spectra of [LnLI3]3+ in aqueous solution. The similarity in fluorescence spectra suggests that Ln(III) ion in the extracted complexes is also coordinated by three tridentate LII ligands and that the nitrate anions acting just as counterions do not directly bond to Ln(III) in the organic phase of solvent extraction.