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

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 ßn°(Θ), ΔrHn°, ΔrSn°), near infrared absorption spectroscopy (vis/NIR) is applied. The complexation reactions are investigated as a function of the total concentration of malonate ([Mal2-]total) and succinate ([Succ2-]total), ionic strength [Im = 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)n1-2n (n = 1, 2, L = Mal2-, Succ2-) is observed. With increasing temperature, the molar fractions of both complex species increase and the temperature-dependent conditional stability constants log ßn'(Θ) at given ionic strengths are determined by the law of mass action. The log ßn'(Θ) are extrapolated to IUPAC reference-state conditions (Im = 0) according to the specific ion interaction theory (SIT), revealing thermodynamic log ßn°(Θ) values. For all formed complexes, [NpO2(Mal)-: log ß1°(25 °C) = 3.36 ± 0.11, NpO2(Mal)23-: log ß2°(25 °C) = 3.95 ± 0.19, NpO2(Succ)-: log ß1°(25 °C) = 2.05 ± 0.45, NpO2(Succ)23-: log ß2°(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 ßn°(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).

Spectroscopic characterisation and thermodynamics of the complexation of Np(V) with sulfate up to 200 °C.

In the present work the complexation of Np(V) with sulfate in aqueous solution is studied in a temperature range up to 200 °C by absorption spectroscopy. For this purpose, a new spectroscopic setup is implemented and tested for its suitability for Vis/NIR absorption spectroscopy at elevated temperatures. The complexation of Np(V) with sulfate is studied as a function of the total ligand concentration at various temperatures (T = 25-200 °C) and ionic strengths (Im(NaClO4) = 1.0-4.0 mol kg-1 NaClO4). The exclusive formation of NpO2(SO4)- up to 200 °C is confirmed by peak deconvolution and slope analyses. The thermodynamic stability constants log ß01(T) are obtained from linear regressions according to the specific ion interaction theory (SIT). A systematic increase of the log ß01(T) is observed with increasing temperature, resulting in a linear correlation of log ß01(T) with T-1. The magnitude of the increase is 1.9 logarithmic units at 200 °C in comparison to log ß01(25 °C) = 1.05 ± 0.16. Thus, the standard reaction enthalpy and entropy (ΔrH0m, ΔrS0m) are determined with the integrated Van't Hoff equation revealing ΔrH0m = 31.0 ± 1.0 kJ mol-1 and ΔrS0m = 123 ± 9 J mol-1 K-1. In addition, the stoichiometric sum of the specific binary ion-ion interaction coefficient (Δε01(T)) is determined up to 200 °C showing an insignificant temperature dependence. Thus, a temperature-independent ε(Na+, NpO2(SO4)-) = 0.07 ± 0.11 is calculated for the temperature range up to 200 °C. Comparison of the present results with literature data confirms the excellent applicability of the new high-temperature absorption spectroscopic setup for complexation studies up to 200 °C.

Thermodynamics and Structure of Neptunium(V) Complexes with Formate. Spectroscopic and Theoretical Study.

The temperature and ionic strength dependences of the complex formation of NpO2+ with formate in aqueous solution are studied by absorption spectroscopy (Im = 0.5-4.0 mol kg-1, T = 20-85 °C, [Form-]total = 0-0.65 mol kg-1), extended X-ray absorption fine structure spectroscopy (EXAFS) and quantum chemical methods. The complex stoichiometry and the thermodynamic functions of the complexation reactions are determined by peak deconvolution of the absorption spectra and slope analyses. Besides the solvated NpO2+ ion, two NpO2+ formate species (NpO2(Form)n1-n; n = 1, 2) are identified. Application of the law of mass action yields the temperature dependent conditional stability constants log ß'n(T) at a given ionic strength. These data are extrapolated to IUPAC reference state conditions (Im = 0) using the specific ion interaction theory (SIT). The results show, that log ß01(20 °C) = 0.67 ± 0.04 decreases by approximately 0.1 logarithmic units with increasing temperature, log ß02(20 °C) = 0.11 ± 0.11 increases by about 0.2 logarithmic units. The temperature dependence of the log ß0n(T) values is modeled with the integrated Van't Hoff equation yielding the standard reaction enthalpy ΔrH0 and entropy ΔrS0 of the complexation reactions. The results show that the formation of NpO2(Form) is exothermic (ΔrH01 = -2.8 ± 0.9 kJ mol-1) whereas the formation of NpO2(Form)2- is endothermic (ΔrH02 = 6.7 ± 4.1 kJ mol-1). Furthermore, the binary ion-ion interaction coefficients εT(i,k) of the formed complexes are determined in NaClO4 and NaCl media as a function of the temperature. The coordination mode of formate toward the metal ion is investigated by EXAFS spectroscopy and quantum chemical calculations. A coordination of the ligand via only one O atom of formate to the metal ion is identified.

A spectroscopic study of the complexation reaction of trivalent lanthanides with a synthetic acrylate based PCE-superplasticizer.

The interaction between different trivalent lanthanides and a synthetic acrylate based PCE-superplasticizer (52IPEG4.5) is investigated by using a combination of laser- and synchrotron based spectroscopic techniques. Time-resolved laser fluorescence spectroscopy (TRLFS) is used to obtain thermodynamic data (stability constants (log ß'(T)), reaction enthalpy (ΔrH) and entropy(ΔrS)) of the complexation reaction of Eu(III) and 52IPEG4.5 as a function of the temperature (20-80 °C) and ligand concentration (<2 g/kg) in 0.1 mol/kg NaCl solution. Under the chosen experimental conditions, the increase in temperature mainly affects the complexation properties (loading capacity) of the macromolecule itself rather than the stability constant of the formed complex (log ß'(T) ranging between 6.5 and 5.9). The thermodynamic results are complemented by extended X-ray absorption fine structure (EXAFS) spectroscopic measurements to resolve the molecular structure of 52IPEG4.5 complexes with Eu(III), Gd(III), and Tb(III). The results show, that each metal ion is coordinated by three carboxylic groups within the 52IPEG4.5 complexes. Furthermore, the determined interatomic distances exhibit that the functional groups are attached in a bidentate end-on fashion.

4,4'-Di-tert-butyl-6-(1H-tetrazol-5-yl)-2,2'-bipyridine: modification of a highly selective N-donor ligand for the separation of trivalent actinides from lanthanides.

In the present work, the complexation and extraction behaviour of 4,4'di-tert-butyl-6-(1H-tetrazol-5-yl)-2,2'-bipyridine (HN4tbubipy) towards trivalent actinides (An(iii)) and lanthanides (Ln(iii)) is studied by spectroscopic methods, liquid-liquid extraction, and quantum chemical calculations. The ligand synthesis of HN4tbubipy as well as its application in coordination chemistry of the 4f elements is described. Reaction of HN4tbubipy with [Ln(NO3)3·6H2O] (Ln = Sm, Eu) results in [H2N4tbubipy]+[Ln(N4tbubipy)(NO3)3(H2O)]-. Both compounds have been characterized by single crystal X-ray diffraction. The solubility of the ligand in different organic solvents is determined, showing a high solubility in MeOH which decreases with the lipophilicity of the solvent. The pKa = 2.4 ± 0.2 of HN4tbubipy in EtOH (4.4 vol% H2O) is determined by absorption spectrophotometry. The complexation of Cm(iii) and Eu(iii) with HN4tbubipy is studied by time resolved laser fluorescence spectroscopy (TRLFS). For both metal ions the formation of the complexes [M(N4tbubipy)n]3-n with n = 2, 3 (M = Cm(iii), Eu(iii)) is observed. Slightly higher conditional stability constants for Eu(iii) (log ß'2(Eu(N4tbubipy)2+) = 8.9 ± 0.3, log ß'3(Eu(N4tbubipy)3) = 12.7 ± 0.5), compared to Cm(iii) (log ß'2(Cm(N4tbubipy)2+) = 8.5 ± 0.4 and log ß'3(Cm(N4tbubipy)3) = 12.4 ± 0.6) are determined. Thus, the ligand has no preference for the complexation of An(iii) over Ln(iii). Additionally, no significant extraction of Am(iii) and Eu(iii) is observed in liquid-liquid extraction experiments due to protonation of the ligand at the experimental conditions. The experimental studies are supported by quantum chemical calculations of the free ligand and the [M(N4tbubipy)3] complexes (M = Cm(iii), Gd(iii)). The results are in excellent agreement with the experimental data and provide a deeper understanding of the complexation properties of HN4tbubipy.

A thermodynamical and structural study on the complexation of trivalent lanthanides with a polycarboxylate based concrete superplasticizer.

The complexation of trivalent lanthanides with a commercial polycarboxylate based concrete superplasticizer (Glenium® 51) is investigated using different spectroscopic techniques. Time-resolved laser fluorescence spectroscopy (TRLFS) in combination with a charge neutralization model is used to determine temperature dependent conditional stability constants (log ß'(T)) for the complexation of Eu(iii) with Glenium® 51 in 0.1 mol kg-1 NaCl solution in the temperature range of 20-90 °C. Only one complex species is observed, and log ß'(T) (given in kg per mol eq) shows a very slight increase with temperature from 7.5 to 7.9. The related conditional molar reaction enthalpy (ΔrH'm) and entropy (ΔrS'm) obtained using the Van't Hoff equation show that the complexation reaction is slightly endothermic and entropy driven. The thermodynamic investigations are complemented by structural data for complexes formed with Gd(iii) or Tb(iii) and Glenium® 51 using extended X-ray absorption fine structure (EXAFS) spectroscopy. The results imply a non-chelate coordination of the trivalent metals through approximately three carboxylic functions of the polycarboxylate comb polymer which are attached predominantly in a bidentate fashion to the lanthanide under the given experimental conditions.