Complexation of Ho(III) with tetraalkyl-diglycolamide in aqueous solutions and a solid state compared in organic solutions of solvent extraction

• Three complex species of Ho(III) with TMDGA are identified in aqueous solutions.
• Single crystals of Ho(TMDGA)3·3ClO4 are grown and its structure is determined.
• The extracted complex of Ho(III) with DMDODGA is confirmed to be Ho(TMDGA)3·3ClO4.

The complexation of Ho(III) with tetramethyl-diglycolamide (TMDGA) and N,N′-dimethyl-N,N′-dioctyl-diglycolamide (DMDODGA) were investigated with spectrophotometry and X-ray crystallography. Single crystals of a solid compound HoL3(ClO4)3 (L = TMDGA) were grown from aqueous solutions by slow evaporation. The crystal structure of HoL3(ClO4)3 shows that in the solid compound Ho(III) is coordinated by nine oxygen atoms from three tridentate TMDGA molecules in a distorted tricapped trigonal prism (TCTP) geometry. In aqueous solution, three successive complex species, HoL3 +, HoL23 +, and HoL33 + (L = TMDGA) were identified and their stability constants were determined to be 2.20 ± 0.09, 4.48 ± 0.18, 5.88 ± 0.18, respectively, with spectral titration method at 25 °C and 1 M ionic strength (1 M NaNO3). The UV–Vis absorption/reflection spectra of the 1:3 species HoL33 + (L = TMDGA) in aqueous solution/solid state HoL3(ClO4)3 compound were very well comparable to the absorption spectra of the extracted samples of Ho(III) with DMDODGA in various organic solvents in solvent extraction. The similarity in the spectra suggest that Ho(III) in the extracted samples is also coordinated by three tridentate DMDODGA with similar coordination geometry as that in HoL33 + (L = TMDGA) in aqueous solution/solid HoL3(ClO4)3 compound. In the organic phase of solvent extraction with DMDODGA as extractant, the nitrate anions do not directly bond to Ho(III) in the extracted complex but just act as far away counter-ion to neutralize the positive charge of HoL33 + (L = DMDODGA), and the diluents do not have much influence on the formation of the extracted Ho(III)-DMDODGA complex.

Similar complexes of Ho(III) with tetraalkyl-diglycolamide ligands formed in aqueous solution/solid state compound/solvent extraction systems.Figure optionsDownload as PowerPoint slide