Complexation of Pr(Ⅲ) with Tetraalkyl Diglycolamides in Wet n-octanol, Aqueous Solution, Solid State and Solvent Extraction

To further understand the complexation of An(Ⅲ) and Ln(Ⅲ) with tetraalkyl diglycolamides (TRDGA), the complexation of Pr(Ⅲ) with water-insoluble N, N'-dimethyl-N, N'-dioctyl-diglycolamide (DMDODGA, L) and water-soluble tetramethyl-diglycolamide (TMDGA, L') was investigated in wet n-octanol, aqueous solution, solid state and solvent extraction respectively by using spectrophotometry and X-ray crystallography. Three successive Pr(Ⅲ) complex species (1:1, 1:2 and 1:3 of metal ion to ligand) for both ligands were identified in both solutions by using spectral titration method. The stability constants and standard reference absorption spectra of these complexes were acquired by spectra deconvolution. The almost identical spectra of the corresponding Pr(Ⅲ)-DMDODGA complexes in wet n-octanol and the Pr(Ⅲ)-TMDGA complexes in aqueous solution indicate each related pair shares similar coordination geometry. By using slow evaporation method, PrL3(ClO4)3 and PrL'3(ClO4)3 single crystals of sufficient quality were grown successfully from ethanol solution and aqueous solution respectively, and their structures were determined by using X-ray diffraction method. The bond lengths and stability constants of these complexes were discussed and compared with those of corresponding Am(Ⅲ)-TMDGA and Pr(Ⅲ)-TEDGA complexes. The diffuse reflectance spectra of PrL'3(ClO4)3 solid compound collected and the absorption spectra of PrL33+ and PrL'33+ in solutions have the same characteristics, which suggests that Pr(Ⅲ) in these 1:3 complexes shares a same inner coordination sphere. And the X-ray diffraction analysis results show that Pr(Ⅲ) ions in these two 1:3 crystals are both coordinated with three tridentate ligands (DMDODGA or TMDGA) to form a geometry of distorted tricapped trigonal prism (TCTP). Using varying concentrations of DMDODGA in 40% n-octonol-60% kerosene (V/V) as initial organic phases and 1.0 mol/L HNO3-0.05 mol/L Pr(NO3)3 as initial aqueous phases, a set of samples of the extracted Pr(Ⅲ) complexes were prepared and their absorption spectra were collected. The factor analyses of these spectra show that there were no less than two extracted Pr(Ⅲ) species existing in these samples. To probe into the role of nitrate played in the extraction process, using the mixture of Pr(Ⅲ) and DMDODGA in wet n-octanol as the starting solution, a nitrate involved spectral titration experiment was carried out. Unlike the way it does in aqueous solutions, the result shows that the nitrite may bind to metal center of the Pr(Ⅲ)-DMDODGA complexes directly in wet n-octanol. Combining all of the experimental results, the extracted Pr(Ⅲ) complex species were identified as PrL2(NO3)(H2O)2+ and PrL33+ for the first time. © 2022, Editorial Board of Atomic Energy Science and Technology. All right reserved.