Thermodynamic and spectroscopic studies of lanthanides(III) complexation with polyamines in dimethyl sulfoxide

The thermodynamic parameters of complexation of Ln(III) cations with tris(2-aminoethyl)amine (tren) and tetraethylenepentamine (tetren) were determined in dimethyl sulfoxide (DMSO) by potentiometry and calorimetry. The excitation and emission spectra and luminescence decay constants of Eu3+ and Tb3+ complexed by tren and tetren, as well as those of the same lanthanides(III) complexed with diethylenetriamine (dien) and triethylenetetramine (trien), were also obtained in the same solvent. The combination of thermodynamic and spectroscopic data showed that, in the 1:1 complexes, all nitrogens of the ligands are bound to the lanthanides except in the case of tren, in which the pendant N is bound. For the larger ligands (trien, tren, tetren) in the higher complexes (ML2), there was less complete binding by available donors, presumably due to steric crowding. FT-IR studies were carried out in an acetonitrile/DMSO mixture, suitably chosen to follow the changes in the primary solvation sphere of lanthanide(III) due to complexation of amine groups. Results show that the mean number of molecules of DMSO removed from the inner coordination sphere of lanthanides(III) is lower than ligand denticity and that the coordination number of the metal ions increases with amine complexation from approximately 8 to approximately 10. Independently of the number and structure of the amines, linear trends, similar for all lanthanides, were obtained by plotting the values of DeltaGj degrees, DeltaHj degrees, and TDeltaSj degrees for the complexation of ethylenediamine (en), dien, trien, tren, and tetren as a function of the number of amine metal-coordinated nitrogen atoms. The main factors on which the thermodynamic functions of lanthanide(III) complexation reactions in DMSO depend are discussed.     

Chemical equilibria in the binary and ternary uranyl(VI)-hydroxide-peroxide systems

The composition and equilibrium constants of the complexes formed in the binary U(VI)-hydroxide and the ternary U(VI)-hydroxide-peroxide systems have been studied using potentiometric and spectrophotometric data at 25 °C in a 0.100 M tetramethylammonium nitrate medium. The data for the binary U(VI) hydroxide complexes were in good agreement with previous studies. In the ternary system two complexes were identified, [UO(2)(OH)(O(2))](-) and [(UO(2))(2)(OH)(O(2))(2)](-). Under our experimental conditions the former is predominant over a broad p[H(+)] region from 9.5 to 11.5, while the second is found in significant amounts at p[H(+)] < 10.5. The formation of the ternary peroxide complexes results in a strong increase in the molar absorptivity of the test solutions. The absorption spectrum for [(UO(2))(2)(OH)(O(2))(2)](-) was resolved into two components with peaks at 353 and 308 nm with molar absorptivity of 16200 and 20300 M(-1) cm(-1), respectively, suggesting that the electronic transitions are dipole allowed. The molar absorptivity of [(UO(2))(OH)(O(2))](-) at the same wave lengths are significantly lower, but still about one to two orders of magnitude larger than the values for UO(2)(2+)(aq) and the binary uranyl(VI) hydroxide complexes. It is of interest to note that [(UO(2))(OH)(O(2))](-) might be the building block in cluster compounds such as [UO(2)(OH)(O(2))](60)(60-) studied by Burns et al. (P. C. Burns, K. A. Kubatko, G. Sigmon, B. J. Fryer, J. E. Gagnon, M. R. Antonio and L. Soderholm, Angew. Chem. 2005, 117, 2173-2177). Speciation calculations using the known equilibrium constants for the U(vi) hydroxide and peroxide complexes show that the latter are important in alkaline solutions even at very low total concentrations of peroxide, suggesting that they may be involved when the uranium minerals Studtite and meta-Studtite are formed by α-radiolysis of water. Radiolysis will be much larger in repositories for spent nuclear fuel where hydrogen peroxide might contribute both to the corrosion of the fuel and to transport of uranium in a ground water system.     

Thermodynamic Studies of the Complexation between Neodymium and Acetate at Elevated Temperatures

Complexation of neodymium (III) with acetate in 2.2 mol-kg^-1 NaClO₄ solution was studied at elevated temperatures (45 and 70°C) by potentiometry, calorimetry, and optical spectroscopy. The formation constants of the consecutive complexes, Nd(OOCCH₃),²⁺ Nd(OOCCH₃) ₂ ⁺ , and Nd(OOCCH₃)₃, and the molar enthalpies of complexation at these temperatures were determined. The stability of the three complexes increases with increased temperatures, because of increased positive entropy change at higher temperatures, which exceeds the increased values of the positive (endothermic) enthalpy. The molar heat capacity changes of complexation C_p,m(MLj) (J-K^-1-mol^-1) for Nd(OOCCH₃) _j ^(3-j)+ in the temperature range from 25 to 70°C were calculated to be: 102 ± 13 (j = 1); 122 ± 19 (j = 2); and 239 ± 27 (j = 3). The effect of temperature on the complexation is discussed in terms of the electrostatic model.     

Interaction of thorium(IV) with nitrate in aqueous solution: medium effect or weak complexation?

Microcalorimetric titrations were performed to study the Th(IV)/nitrate interaction in aqueous solution. The results show the formation of a weak mononuclear complex of Th(IV) with nitrate and allow the determination of the complexation thermodynamic parameters at 298 K and ionic strength 1.0 mol dm(-3). The reaction is endothermic and entropy driven. Data and comparison with similar actinide(IV) complexes allow to confirm the inner-sphere nature of the Th(NO(3))(3+) complex.     

Crystal and molecular structures of two europium(III) nitrate complexes with triphenylphosphine oxide

The X-ray structures of the complexes Eu(NO₃)(Ph₃PO)₃(acetone)₂ (A) (Ph₃PO = triphenylphosphine oxide) and Eu(NO₃)₃(Ph₃PO)₂(ethanol) (B) have been solved by the heavy-atom method, by using the three-dimensional Patterson-Fourier synthesis. The crystals are both monoclinic and belong to the space group P2₁/n, with Z = 4. The cell dimensions are: a = 27.825(4) Å, b = 19.422(4) Å, c = 11.238(2) Å, β = 94.9(3)° for A; and a = 22.193(4) Å, b = 10.866(2) Å, c = 17.101(3) Å, β = 105.6(3)° for B. In both complexes the europium(III) ion is ennea-coordinated to three chelate nitrate groups and three oxygens of the Ph₃PO ligands for A and two of the Ph₃PO and one of the ethanol for B. The acetone molecules of A are outside the coordination sphere of the metal and disordered.     

Complexation of UVI with 1-hydroxyethane-1,1-diphosphonic acid in acidic to basic solutions

Complexation of UVI with 1-hydroxyethane-1,1-diphosphonic acid (HEDPA) in acidic to basic solutions has been studied with multiple techniques. A number of 1:1 (UO2H3L), 1:2 (UO2HjL2 where j = +4, +3, +2, +1, 0, and -1), and 2:2 [(UO2)2HjL2 where j = +1, 0, and -1] complexes form, but the 1:2 complexes are the major species in a wide pH range. Thermodynamic parameters (formation constants and enthalpy and entropy of complexation) were determined by potentiometry and calorimetry. Data indicate that the complexation of UVI with HEDPA is exothermic, favored by the enthalpy of complexation. This is in contrast to the complexation of UVI with dicarboxylic acids in which the enthalpy term usually is unfavorable. Results from electrospray ionization mass spectrometry and 31P NMR have confirmed the presence of 1:1, 1:2, and 2:2 UVIHEDPA complexes.     

Thermal treatments and characterizations of pellets for SPES direct target

In this work targets of graphite-rich lanthanum dicarbide have been produced through carburization and sintering of suitable precursor pellets obtained by mixing and pressing lanthanum oxide and graphite powders. The results about the control of the production process and the investigation of the physical properties of the pellets, performed respectively through thermal analyses (TG/DTA) and X-ray diffraction (XRD), are reported and discussed.     

The SPES direct UCx target

A possible solution for a target system aimed at the production of exotic nuclei as a result of high energy fissions in ²³⁸U compounds has been analyzed. The proposed configuration is constituted by a primary proton beam (40 MeV, 0.2 mA) directly impinging on uranium carbide disks inserted within a cylindrical carbon box. This system has been conceived to obtain both a high number of neutron rich atoms (originated from about 10¹³ fissions/s) and a low power deposition in the target. In order to extract the fission fragments, the box has to be hold at 2000^○C. The thermal analysis shows the capability of the thermal radiation to cool the disks with a reasonable margin below the material melting point. Moreover, the analyses of the thermo-mechanical behaviour and of the effusion times confirm the promising features of this target configuration.     

Hydrolysis of plutonium(VI) at variable temperatures (283-343 K)

The hydrolysis of Pu^VI was studied at variable temperatures (283–343 K) by potentiometry, microcalorimetry, and spectrophotometry. Three hydrolysis reactions, mPuO₂²⁺+nH₂O=(PuO₂)_m(OH)_n+nH⁺), in which (n,m)=(1,1), (2,2), and (5,3), were invoked to describe the potentiometric and calorimetric data. The equilibrium constants (*β_n,m) were determined by potentiometry at 283, 298, 313, 328, and 343 K. As the temperature was increased from 283 to 343 K, *β_1,1, *β_2,2, and *β_5,3, increased by 1, 1.5, and 4 orders of magnitude, respectively. The enhancement of hydrolysis at elevated temperatures is mainly due to the significant increase of the degree of ionization of water as the temperature increases. Measurements by microcalorimetry indicate that the three hydrolysis reactions are all endothermic at 298.15 K, with enthalpies of (35.0±3.4) kJ mol^?1, (65.4±1.0) kJ mol^?1, and (127.7±1.7) kJ mol^?1 for ΔH_1,1, ΔH_2,2, and ΔH_5,3, respectively. The hydrolysis constants at infinite dilution have been obtained with the Specific Ion Interaction approach. The applicability of three approaches for estimating the equilibrium constants at different temperatures, including the constant enthalpy approach, the DQUANT equation, and the Ryzhenko–Bryzgalin model, were evaluated with the data from this work.