Plinio Di Bernardo

Complexes of technetium(IV) and (III) with tertiary phosphines

Abstract The synthesis and characterization of the complexes trans-TcX4L2 and mer-TcX3L3 (X = Cl and Br, and L = PMe2Ph, PEt2Ph and PPh3) are reported. Configurations were deduced by far-IR and 1H NMR studies. Magnetic susceptibility and 1H NMR results for mer-TcCl3(PMe2Ph)3 are compared with those for analogues d4 complexes.

Complexation of uranium(VI) with malonate at variable temperatures

Summary The complexation between uranium(VI) and malonate in 1.05 mol kg−1 NaClO4 was studied at variable temperatures (25, 35, 45, 55 and 70 °C). The formation constants of three successive complexes, UO2(OOCCH2COO), UO2(OOCCH2COO)22− and UO2(OOCCH2COO)34−, and the molar enthalpies of complexation were determined by potentiometry and calorimetry. The heat capacity of the complexation, Δ Cop,m(MLj), is calculated to be 96 ± 12, 195 ± 15 and 267 ± 22 J K−1 mol−1 for j=1, 2 and 3, respectively. Extended X-ray Absorption Fine Structure Spectroscopy helped to characterize the coordination modes in the complexes in solution. UV/Vis absorption and luminescence spectra at different temperatures provided qualitative information on the temperature effect. The effect of temperature on the complexation between uranium(VI) and malonate is discussed in terms of the electrostatic model and compared with the complexation between uranium(VI) and acetate.

Lanthanide(III) trifluoromethanesulfonate complexes in anhydrous acetonitrile

Abstract Anhydrous solutions of lanthanide(III) trifluoromethanesulfonates (triflates) in acetonitrile have been prepared and studied. Solubility measurements revealed that among the salts considered (Ln(SO 3 CF 3 ) 3 ; LnLaLu) only the heavier ones (LnDyLu) are soluble (S>150 mmol dm −3 ) in strictly anhydrous conditions. Additions of small quantities of water proved that the solubility of lighter salts depends strongly on the presence of this reagent in solution. The ability of the triflate ion to coordinate the tervalent lanthanides in anhydrous solutions has been investigated by means of conductometric and FT-IR techniques. Quantitative IR measurements allowed detection of the main species in solution to be Ln(SO 3 CF 3 ) 2 + and Ln(SO 3 CF 3 ) 3 . Their apparent equilibrium constants are reported. The competition of perchlorate and triflate for the Ln(SO 3 CF 3 ) 2 + ion has also been studied. Measurements of the formation constant of the mixed complex, Ln(SO 3 CF 3 ) 2 ClO 4 , suggested that the affinity of the perchlorate ion for the Ln(SO 3 CF 3 ) 2 + complex is about ten times lower than that of the third triflate. The effects of added water or dimethyl sulfoxide on the dissociation of the triflate complexes in AN were also obtained.

Energetics and structure of uranium(VI)-acetate complexes in dimethyl sulfoxide.

The thermodynamics of the complexation between uranium(VI) and acetate in dimethyl sulfoxide (DMSO) was studied at 298 K in an ionic medium of 0.1 mol dm(-3) tetrabutyl ammonium perchlorate. The results show that the uranyl ion forms three strong successive mononuclear complexes with acetate. The complexes, both enthalpically and entropically stabilized, are significantly more stable in DMSO than in water. This feature can be ascribed to the weak solvation of acetate in DMSO. The thermodynamic parameters for the formation of the uranium(VI) complexes with acetate in DMSO are compared with those with ethylenediamine in the same solvent. The difference between the two ligand systems reveals that, for the complexation reactions involving charge neutralization, the reorganization of the solvent gives a very important contribution to the overall complexation energetics. The coordination mode of acetate in the uranyl complexes and the changes of the solvation sphere of UO(2)(2+) upon complexation were investigated by FT-IR spectroscopy in DMSO and in acetonitrile/DMSO mixtures. In addition, DFT calculations were performed to provide an accurate description of the complexation at the molecular level. The experimental and calculated results suggest that acetate is solely bidentate to UO(2)(2+) in the 1:1 and 1:3 complexes but mono- and bidentate in the 1:2 complexes. The DFT calculations also indicate that the medium effects must always be taken into account in order to gain accurate information on the complex formation in solution. In fact, the relative stability of the reaction products changes markedly when the DFT calculations are carried out in vacuum or in DMSO solution.

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 ΔCp,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.

Complexation of uranium(VI) with acetate at variable temperatures

The complexation between uranium(VI) and acetate in 1.05 mol kg−1 NaClO4 was studied at variable temperatures (25, 35, 45, 55 and 70 °C). The formation constants of three successive complexes, UO2(OOCCH3)+, UO2(OOCCH3)2 and UO2(OOCCH3)3−, and the molar enthalpies of complexation were determined by potentiometry and calorimetry. Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) provided structural information to identify the coordination modes of the acetate in the complexes in solution, which helped to interpret the trends in the enthalpy and entropy of the complexation. The effect of temperature on the stability of the complexes is discussed in terms of the electrostatic model.

Lanthanide(III) complex formation with diethylenetriamine in anhydrous N,N-dimethylformamide

Abstract The formation of diethylenetriamine (dien) complexes of the lanthanide(III) ions has been studied in N,N-dimethylformamide (dmf) containing 0.1 mol dm−3 tetraethylammoniumtrifluoromethanesulfonate as constant ionic medium at 298 K. The stability constants have been obtained by potentiometry using the method of competitive reactions, silver(I) being the auxiliary central ion. The thermodynamic investigations have therefore been extended also to this metal ion. The data concerning lanthanide(III) complexation are well explained in terms of formation of two successive mononuclear complexes which are enthalpy stabilized, entropy changes being unfavourable. Dien coordinates with all N atoms both in ML and ML2 lanthanide(III) complexes; with silver(I), tridentation is achieved only when the first mononuclear complex is formed. The trends in enthalpy and entropy values of the lanthanide(III) complexes are discussed taking into account the presence of multiple solvation equilibria which involve the bare ions and the complexes in solution. Comparison with similar data previously obtained in dimethylsulfoxide (dmso) both for the lanthanide(III)- and silver(I)-dien systems, shows the remarkable influence of the solvent on the thermodynamics of complex formation.

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.

Thermodynamics of the Complexation of Uranium(VI) by oxalate in aqueous solution at 10-70oC

The protonation reactions of oxalate (ox) and the complex formation of uranium(vi) with oxalate in 1.05 mol kg(-1) NaClO(4) were studied at variable temperatures (10-70 degrees C). Three U(vi)/ox complexes (UO(2)ox(j)((2-2j)+) with j = 1, 2, 3) were identified in this temperature range. The formation constants and the molar enthalpies of complexation were determined by spectrophotometry and calorimetry. The complexation of uranium(vi) with oxalate ion is exothermic at lower temperatures (10-40 degrees C) and becomes endothermic at higher temperatures (55-70 degrees C). In spite of this, the free energy of complexation becomes more negative at higher temperatures due to increasingly more positive entropy of complexation that exceeds the increase of the enthalpy of complexation. The thermodynamic parameters at different temperatures, in conjunction with the literature data for other dicarboxylic acids, provide insight into the relative strength of U(vi) complexes with a series of dicarboxylic acids (oxalic, malonic and oxydiacetic) and rationalization for the highest stability of U(vi)/oxalate complexes in the series. The data reported in this study are of importance in predicting the migration of uranium(vi) in geological environments in the case of failure of the engineering barriers, which protect waste repositories.

Calorimetric and spectroscopic studies of Eu(III) complexation with tetramethylmalonamide and tetramethylsuccinamide in acetonitrile and dimethylsulfoxide

Abstract The complexation of Eu(III) with two N , N , N′ , N′ -alkyl-substituted diamide ligands, tetramethylmalonamide (TMMA) and tetramethylsuccinamide (TMSA), was studied in organic solvents using titration calorimetry, FT IR and luminescence spectroscopy. The formation constants and enthalpy changes of the complexation were determined in pure acetonitrile and in acetonitrile containing small amounts of dimethyl sulfoxide (DMSO). It was observed that TMMA forms stronger complexes with Eu(III) than TMSA in the latter systems, which is attributed to the structural difference between TMMA and TMSA and an entropy effect. The effect of Eu(III) solvation by DMSO on the formation of the Eu(III)–diamide complexes is also discussed.