Spektroskopische und thermodynamische Untersuchung der Komplexierung von Cm(III) und Eu(III) mit hydrophilen Bis-Triazinylpyridinen

Abstract

In the present work the complexation of Cm(III) and Eu(III) with a hydrophilic 2,6-bis-(1,2,4-triazinyl)-pyridine (aq-BTP) is studied. Aq-BTP complexes actinides(III) selectively over lanthanides(III) in nitric acid solution. The object of this work is the identification and the spectroscopic and thermodynamic characterization of the Cm(III) and Eu(III) complex species present in solution. The results should contribute to a better fundamental understanding of the driving force behind BTPs selectivity towards trivalent actinides on a molecular level. Time-resolved laser fluorescence spectroscopy (TRLFS), luminescence and UV/Vis spectroscopy are applied. Information on the structure of M(III)-aq-BTP complex species is obtained from density functional theory. Three different M(III) complex species containing one, two or three aq-BTP ligands are identified in H2O at pH 3.0. Relative fluorescence intensity factors are determined for each of the [M(aq-BTP)n] complexes (M = Cm(III)/Eu(III), n = 1 – 3). These factors are required to quantify the complexes. The stability constant logβ3 of the [Cm(aq-BTP)3] complex (which is the one relevant to extraction processes) is two orders of magnitude higher than that of the corresponding Eu(III) complex. This difference is in agreement with the separation factor (SF Am(III)/Eu(III) = 150) determined experimentally by liquid-liquid extraction. The difference in the stability constants originates from the different reaction enthalpies for the formation of the [M(aq-BTP)3] complexes. These results represent the thermodynamic driving force for the aq-BTPs selectivity towards trivalent actinides over lanthanides. Comparing the stability constants of the [M(aq-BTP)n] species (M = Cm(III)/Eu(III), n = 1 – 3) shows an increasing selectivity with increasing number of coordinated aq-BTP ligands. Hence, high selectivity is achieved if the f-element ions are fully coordinated by nine N-donor atoms (three aq-BTP ligands). A less polar solvent (i-PrOH:H2O 1:1) results in a destabilization of the highly charged [M(aq-BTP)3] complexes (z = −9), while a more polar solvent (0.1 M NaClO4, pH 3.0) stabilizes these species. However, the solvent does not change the selectivity (∆logβ3 = 2.1, 0.1 M NaClO4, pH 3.0). In the separation process the trivalent lanthanides are extracted from the trivalent actinides from aqueous solution with 0.5 M nitric acid. Hence, the complexation of Cm(III) and Eu(III) with aq-BTP was studied in HNO3. In contrast to the complexation in H2O at pH 3.0, the [M(aq-BTP)3] complex is directly formed under acidic conditions; the M(III) complexes with one and two aq-BTP ligands are suppressed. The selectivity (∆logβ3 = 2.1) is almost identical to the values found in H2O (pH 3.0), in 0.1 M NaClO4 (pH 3.0) and in extraction studies. As already shown in H2O at pH 3.0, this difference is attributed to a difference in the reaction enthalpy of the complex formation. Comparing stability constants determined in 0.5 M HClO4 and in 0.5 M HNO3 shows no influence of the counter ion (ClO−4 , NO−3 ) on aq-BTPs selectivity. However, a significant influence on the spectroscopic properties of the [M(aq-BTP)3] complexes is found; e.g., the fluorescence intensity of the [Eu(aq-BTP)3] complex is by a factor of four higher in 0.5 M HClO4 as compared to 0.5 M HNO3. This originates from different interactions between the anions in the outer coordination sphere and the coordinated aq-BTP ligands. Thereby, it is shown that the difference in the fluorescence intensity correlates with the experimentally determined quantum yield of [Eu(aq-BTP)3] . Another important aim of the work is the investigation of the [M(aq-BTP)n ] (M = Cm(III)/Eu(III), n = 1 – 3) complex structure and bonding. The [Eu(aq-BTP)n] complexes 5D0→7F0 emission band position and the corresponding structural composition of the first Eu(III) coordination sphere are correlated. According to this correlation, the solution coordination structure of Eu(III) complexes coordinated with varying N-heteroaromatic ligands can easily be predicted. The nephelauxetic parameters which were determined in this context indicate a higher covalent degree in the Eu(III)-N(triazine)-bonds in comparison to the Eu(III)-N(pyridine)-bond. Furthermore, the bond lengths between f-element ion and donor nitrogen atoms were calculated by density functional theory and compared to the literature values of hydrophobic [M(n-Pr-BTP)3]3+and [M(H-BTP)3]3+ complexes. The calculated bond lengths between the f-ion and the pyridine nitrogens are in agreement with the literature. However, the distance between the f-ion and the triazine nitrogen is 5 pm longer than the M(III)-N(triazine) distance of [M(H-BTP)3]3+. This is a consequence of the sterically demanding sulfophenyl substituent of aq-BTP and/or the electrostatic repulsion of the negatively charged -SO−3 group. The comprehensive thermodynamic and spectroscopic investigations of the present work contribute to a better molecular understanding of the coordination chemistry of trivalent 4f- and 5f-element ions and hydrophilic bis-triazinylpyridine ligands under conditions relevant to a process. The determined thermodynamic and spectroscopic data are of major importance for the future intelligent design of improved extraction systems and for process optimization.