Complexation of Np(V) with the dicarboxylates malonate and succinate: complex stoichiometry, thermodynamic data and structural information


Complexation of Np(V) with the dicarboxylates malonate and succinate: complex stoichiometry, thermodynamic data and structural information

Maiwald, M. M.; Müller, K.; Heim, K.; Rothe, J.; Dardenne, K.; Roßberg, A.; Koke, C.; Trumm, M.; Skerencak-Frech, A.; Panak, P. J.

The complexation of Np(V) with malonate and succinate is studied by different spectroscopic techniques, namely attenuated total reflection FT-IR (ATR FT-IR) and extended X-ray absorption fine structure (EXAFS) spectroscopy, as well as by quantum chemistry to determine the speciation, thermodynamic data and structural information of the formed complexes. For complex stoichiometries and thermodynamic functions (log beta0n(T), Delta rH0n, Delta rS0n) near infrared absorption spectroscopy (Vis/NIR) is applied. The complexation reactions are investigated as a function of the total ligand concentration ([Mal2-]total, [Succ2-]total), ionic strength (Im = 0.5 – 4.0 mol kg-1 Na+(Cl-/ClO4-)) and temperature (T = 20 – 85 °C). Besides the solvated NpO2+ ion, the formation of two Np(V) species with the stoichiometry NpO2(L)n1-2n (n = 1, 2, L = Mal2-, Succ2-) is observed. With increasing temperature the molar fractions of both complex species increase and the application of the law of mass action yields the temperature dependent conditional stability constants log beta`n(T) at given ionic strengths. The log beta`n(T) are extrapolated to IUPAC reference state conditions (Im = 0) according to the specific ion interaction theory (SIT) revealing thermodynamic log 0n(T) values. For all formed complexes (NpO2(Mal)-: log 01(25 °C) = 3.36 ± 0.11, NpO2(Mal)23-: log 02(25 °C) = 3.95 ± 0.19, NpO2(Succ)-: log 01(25 °C) = 2.05 ± 0.45, NpO2(Succ)23-: log 02(25 °C) = 0.75 ± 1.22) the stability constants increase with increasing temperature confirming an endothermic complexation reaction. The temperature dependence of the thermodynamic stability constants is described by the integrated Van’t Hoff equation yielding the standard reaction enthalpies and entropies for the complexation reactions. In addition, the sum of the specific binary ion-ion interaction coefficients 0n(T) for the complexation reactions are obtained from SIT modelling as a function of the temperature.
The structure of the complexes and the coordination mode of malonate and succinate are investigated using EXAFS spectroscopy, ATR-FT-IR spectroscopy and quantum chemical calculations. The results show, that in case of malonate 6-membered chelate complexes are formed, whereas the formation of 7-membered rings with succinate is energetically unfavourable in the equatorial plane of the Np(V) ion (as NpO2+ cation).

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