Americium(iii) and europium(iii) complex formation with lactate at elevated temperatures studied by spectroscopy and quantum chemical calculations

Abstract

Thermodynamic parameters for the complex formation of Am(iii) and Eu(iii) with lactate were determined with UV-vis and time-resolved laser-induced fluorescence spectroscopy (TRLFS) in a temperature range between 25 and 70 °C. The reaction enthalpy decreased with increasing ionic strength. ATR FT-IR and NMR spectroscopy in combination with density functional theory (DFT) calculations revealed structural details of the Eu(iii) lactate 1 : 1 complex: a chelating coordination mode of the lactate with a monodentate binding carboxylate group and the hydroxyl group being deprotonated.

Role of the Hydroxo Group in the Coordination of Citric Acid to Trivalent Americium

“…For all the three systems, the free Am(III) species shows the luminescence lifetime in the range from 23.8 to 25.8 ns. This corresponds to 8–9 water molecules, which is in agreement with the reported studies , . The lifetime and luminescence decay increased significantly when the ligand titration proceeded, suggesting a decrease in the number of water molecules in the primary coordination sphere of Am(III).…”

“…These calculated distances are consistent with Am‐O distances reported in the Cambridge Structural Data base (CSD); Am‐COO – (monodentate) distances range from 2.38 to 2.55 Å, , and Am‐OH 2 distance is 2.513 Å . These values are also consistent with the one determined for Eu‐lactate or Am‐formate systems from DFT. According to DFT calculations, the Am‐O 2.1–2.2 Å short bond length found when fitting the EXAFS spectra for the Am‐citric‐pH‐1 system can only be attributed to the deprotonated hydroxyl function bonding to the americium ion.…”

“…Therefore, despite p K a s above the pH value of 1, Am(III) may be coordinated by two deprotonated carboxylic acids (central and one terminal according to p K a s) and the deprotonated hydroxo group, HCit 3– . A previous study has already described a Eu‐lactate 1:1 complex with a deprotonated hydroxo group within the coordination sphere . For this ligand, the authors suggest that the deprotonation of the hydroxo group occurs at pH 6 but not at pH 3.…”

“…The lifetime and luminescence decay increased significantly when the ligand titration proceeded, suggesting a decrease in the number of water molecules in the primary coordination sphere of Am(III). The luminescence decay of each sample, which is caused by a faster exchange of the ligands surrounding Am(III) than the decay rate of the excited state,, was fitted with a mono‐exponential curve, regardless of the number of expected species in the system. The obtained lifetime (τ) reflects the average of all the species in the sample mixture.…”

The interaction of Eu(iii) with organoborates – a further approach to understand the complexation in the An/Ln(iii)–borate system

“…Assuming that the lower limits of the Eu(III)-lactate complex constants are valid (lg β EuLac = 2.51 and lg β EuLac 2 = 4.45) 65 we obtained the following range of the complexation constant for the Eu-BLac complex: lg β Eu(BLac) = 1.94 ± 0.22 to 2.18 ± 0.26. If, in contrast, the upper limits of the Eu(III)lactate complex constants are taken to be (lg β DyLac = 3.09 and lg β DyLac 2 = 5.38; Dy(III) as an Eu(III) analog) 66 lg β Eu(BLac) ranges from 2.37 ± 0.26 to 2.61 ± 0.15.…”