Complexation of Np(V) with the Dicarboxylates, Malonate, and Succinate: Complex Stoichiometry, Thermodynamic Data, and Structural Information

Martin M Maiwald; Katharina Müller; Karsten Heim; Jörg Rothe; Kathy Dardenne; André Rossberg; Carsten Koke; Michael Trumm; Andrej Skerencak-Frech; Petra J Panak

doi:10.1021/acs.inorgchem.1c01966

Complexation of Np(V) with the Dicarboxylates, Malonate, and Succinate: Complex Stoichiometry, Thermodynamic Data, and Structural Information

Inorg Chem. 2021 Dec 20;60(24):18674-18686. doi: 10.1021/acs.inorgchem.1c01966. Epub 2021 Nov 25.

Authors

Affiliations

¹ Ruprecht Karls Universität Heidelberg, Physikalisch-Chemisches Institut, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany.
² Karlsruher Institut für Technologie (KIT), Institut für Nukleare Entsorgung (INE), 76344 Eggenstein-Leopoldshafen, Germany.
³ Helmholtz-Zentrum Dresden-Rossendorf, Institut für Ressourcenökologie, Bautzner Landstraße 400, 01328 Dresden, Germany.

PMID: 34821502
DOI: 10.1021/acs.inorgchem.1c01966

Abstract

The complexation of Np(V) with malonate and succinate is studied by different spectroscopic techniques, namely, attenuated total reflection Fourier transform infrared (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 the thermodynamic functions (log β_n^°(Θ), Δ_rH_n^°, Δ_rS_n^°), near infrared absorption spectroscopy (vis/NIR) is applied. The complexation reactions are investigated as a function of the total concentration of malonate ([Mal^2-]_total) and succinate ([Succ^2-]_total), ionic strength [I_m = 0.5-4.0 mol kg^-1 Na⁺(Cl^-/ClO₄^-)], and temperature (Θ = 20-85 °C). Besides the solvated NpO₂⁺ ion, the formation of two Np(V) species with the stoichiometry NpO₂(L)_n^1-2n (n = 1, 2, L = Mal^2-, Succ^2-) is observed. With increasing temperature, the molar fractions of both complex species increase and the temperature-dependent conditional stability constants log β_n^'(Θ) at given ionic strengths are determined by the law of mass action. The log β_n^'(Θ) are extrapolated to IUPAC reference-state conditions (I_m = 0) according to the specific ion interaction theory (SIT), revealing thermodynamic log β_n^°(Θ) values. For all formed complexes, [NpO₂(Mal)^-: log β₁^°(25 °C) = 3.36 ± 0.11, NpO₂(Mal)₂^3-: log β₂^°(25 °C) = 3.95 ± 0.19, NpO₂(Succ)^-: log β₁^°(25 °C) = 2.05 ± 0.45, NpO₂(Succ)₂^3-: log β₂^°(25 °C) = 0.75 ± 1.22], an increase of the stability constants with increasing temperature was observed. This confirmed an endothermic complexation reaction. The temperature dependence of the log β_n^°(T) values is described by the integrated Van't Hoff equation, and the standard reaction enthalpies and entropies for the complexation reactions are determined. Furthermore, the sum of the specific binary ion-ion interaction coefficients Δεn^°(Θ) for the complexation reactions are obtained as a function of the t from the respective SIT modeling as a function of the temperature. In addition to the thermodynamic data, the structures of the complexes and the coordination modes of malonate and succinate are investigated using EXAFS spectroscopy, ATR-FT-IR spectroscopy, and quantum chemical calculations. The results show that in the case of malonate, six-membered chelate complexes are formed, whereas for succinate, seven-membered rings form. The latter ones are energetically unfavorable due to the limited space in the equatorial plane of the Np(V) ion (as NpO₂⁺ cation).