Spectrophotometric studies on solubility of lanthanoid(III) complexes with fluorinated β-diketonates in supercritical carbon dioxide

• Solubility of lanthanoid(III) complexes with 6,6,7,7,8,8,8-heptafluoro- 2,2-dimethyl-3,5-octanedionate(fod) and 1,1,5,5,5-hexafluoro-2,4-pentanedionate(hfa) complexes in supercritical carbon dioxide was measured.
• The solubility of Ln(fod)3 2H2O complexes is higher than that of the corresponding Ln(hfa)3 2H2O complexes.
• The heavy Ln(III) complexes are more soluble than the light Ln(III) ones.
• The solubility of Ln(fod or hfa)3 2H2O complexes are fairly higher compared with that of other β-deketonato complexes.

As a part of fundamental studies for examining solubility controlling factors of metal complexes and applying supercritical extraction (SCE) method to the decontamination of solid radioactive wastes, solubility of Ln(fod)3·2H2O (Ln = Pr, Nd, Eu, and Er, fod: 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate) and Ln(hfa)3·2H2O complexes (Ln = Pr, Nd, Er, and hfa: 1,1,5,5,5-hexafluoro-2,4-pentanedionate) complexes in supercritical carbon dioxide (scCO2) was measured under conditions (Temp. = 40 °C, Pressure = 10–25 MPa) by using UV–vis spectrophotometer equipped with high-pressure cell. The solubility of Pr(fod)3·2H2O, Nd(fod)3·2H2O, Eu(fod)3·2H2O, Er(fod)3·2H2O, Pr(hfa)3·2H2O, Nd(hfa)3·2H2O, and Er(hfa)3·2H2O complexes was evaluated as (1.32 ± 0.03)×10−1, (1.53 ± 0.01)×10−1, (1.59 ± 0.03)×10−1, (1.97 ± 0.01)×10−1, (6.36 ± 0.06)×10−2, (8.52 ± 0.01)×10−2, and (1.73 ± 0.01)×10−1 M, respectively, at 40 °C and 25 MPa. It was found that the solubility of Ln(fod)3·2H2O complexes is higher than that of the corresponding Ln(hfa)3·2H2O complexes and that the heavy Ln(III) complexes are more soluble than the light Ln(III) ones. These are consistent with the general tendencies that the solubility of metal complexes increases with an increase in the number of fluorine atom in their ligands and that the metal complexes largely shielded by the ligands are more soluble. Furthermore, the solubility of Ln(fod or hfa)3·2H2O complexes was found to be fairly higher compared with that of other β-deketonato complexes. This was proposed to be due to the van der Waals interaction between scCO2 and the fluorine groups in Ln(fod or hfa)3·2H2O rather than the solvation of CO2. From the present study, the Hfod compound was proposed to be one of promising extractant in the application of SCE method to the decontamination of radioactive wastes.

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