The behavior of U(IV) octahedral complexes cation2UCl6, where the cation+ is BuMeIm+ and MeBu3N+, is studied using UV/visible spectroscopy, cyclic staircase voltammetry, and rotating disk electrode ...voltammetry in hydrophobic room-temperature ionic liquids (RTILs) BuMeImTf2N and MeBu3NTf2N, where BuMeIm+ and MeBu3N+ are 1-butyl-3-methylimidazolium and tri-n-butylmethylammonium cations, respectively, and Tf2N- is the bis(trifluoromethylsulfonyl)imide anion. The absorption spectra of cation2UCl6 complexes in the RTIL solutions are similar to the diffuse solid-state reflectance spectra of the corresponding solid species, indicating that the octahedral complex UCl6 2- is the predominant chemical form of U(IV) in Tf2N--based hydrophobic ionic liquids. Hexachloro complexes of U(IV) are stable to hydrolysis in the studied RTILs. Voltammograms of UCl6 2- at the glassy carbon electrode in both RTILs and at the potential range of −2.5 to +1.0 V versus Ag/Ag(I) reveal the following electrochemical couples: UCl6 -/UCl6 2- (quasi-reversible system), UCl6 2-/UCl6 3- (quasi-reversible system), and UCl6 2-/UCl6(Tf2N) x -(3+ x ) (irreversible reduction). The voltammetric half-wave potential, E p/2, of the U(V)/U(IV) couple in BuMeImTf2N is positively shifted by 80 mV compared with that in MeBu3NTf2N. The positive shift in the E p/2 value for the quasi-reversible U(IV)/U(III) couple is much greater (250 mV) in BuMeImTf2N. Presumably, the potential shift is due to the specific interaction of BuMeIm+ with the uranium−hexachloro complex in ionic liquid. Scanning the negative potential to −3.5 V in MeBu3NTf2N solutions of UCl6 2- reveals the presence of an irreversible cathodic process at the peak potential equal to −3.12 V (at 100 mV/s and 60 °C), which could be attributed to the reduction of U(III) to U(0).
The present work describes the first structural studies of protactinium(V) in sulfuric and hydrofluoric acid media using X-ray absorption spectroscopy. The results show unambiguously the absence of ...the trans-dioxo bond that characterizes the other early actinide elements such as U and Np. In concentrated sulfuric acid (13 M), Pa(V) is proved to exhibit a single oxo bond as postulated in the literature for species in more dilute media. In a 0.5 M HF medium, XANES and EXAFS spectra indicate the absence of any oxo bond: Pa(V) exists in the form of a pure fluoro complex.
Fully relativistic molecular density–functional calculations of the electronic structures of hydrated, hydrolyzed and fluoride/chloride complexes have been performed for group–4 elements Zr, Hf, and ...element 104, Rf. Using the electronic density distribution data, relative values of the free energy change for hydrolysis and complex formation reactions were defined. The results show the following trend for the first hydrolysis step of the cationic species: Zr > Hf > Rf in agreement with experiments. For the complex formation in HF solutions, the trend to a decrease from Zr to Hf is continued with Rf, provided no hydrolysis takes place. At pH > 0, further fluorination of hydrolyzed species or fluoro–complexes has an inversed trend in the group Rf ≥ Zr > Hf, with the difference between the elements being very small. For the complex formation in HCl solutions, the trend is continued with Rf, so that Zr > Hf > Rf independently of pH. A decisive energetic factor in hydrolysis or complex formation processes proved to be a predominant electrostatic metal–ligand interaction. Trends in the
(distribution coefficient) values for the group–4 elements are expected to follow those of the complex formation.
The hydrolysis of protactinium(V) was studied at tracer scale (ca. 10(-12) M) with the solvent extraction method involving the aqueous system: Pa(V)/H2O/H+/Na+/ClO4- at 25.0degreesC for three values ...of ionic strength. Extraction experiments were conducted using the chelating agent thenoyltrifluoroacetone (TTA) in toluene. Hydrolysis constants are reported for each ionic strength investigated. An SIT modeling is presented and extrapolated constants to zero ionic strength are derived, as well as interaction coefficients of the two hydrolyzed species involved.
Hydrolysis constants of protactinium(V) at tracer scale were deduced from the variations of partition coefficient of Pa(V) in the system: TTA/toluene/Pa(V)/H2O/H+/Na+/ClO4-, as a function of TTA and ...proton concentrations, ionic strength (0.1 less than or equal to 1 mu less than or equal to 3M), and temperature (10 less than or equal to theta less than or equal to 60degreesC). Extrapolations of theses constants to zero ionic strength were performed using the SIT model. Standard thermodynamic data (under atmospheric pressure) related to the two hydrolysis equilibria involved, were derived from the temperature dependence of the hydrolysis constants at infinite dilution.
Sorption properties of Zr, Hf, Nb, Ta and Pa at trace level towards a macroporous anion exchanger in HF–HCl media were investigated with two ion exchange methods: static (batch technique) and dynamic ...(column chromatography using the elution mode). Variations of the distribution coefficients as a function of HCl and HF concentration in the range 0.02–0.8
M for both acids are presented in the form of contour plots. Some of the most promising separation possibilities are pointed out.
Single-crystal XRD data of the complex BuMeIm
2UCl
6 reveal the existence of a hydrogen bond between the most acidic H atom of the imidazolium ring (C2) and the chloride anion of the
UCl
6
2
-
...octahedron, with the shortest Cl⋯H(C2) distance equal to 2.57
Å.Visible absorption spectroscopy and
1H NMR spectroscopy confirm the existence of hydrogen bonding in RTIL BuMeImTf
2N solution, similar to that in the solid state.
The complex BuMeIm
2UCl
6 was characterized in the solid state and in a solution of BuMeImTf
2N room temperature ionic liquid using single-crystal XRD, EXAFS, visible absorption spectroscopy and NMR techniques.The structure of solid BuMeIm
2UCl
6 contains two crystallographically independent formula units (U(1) and U(2)).The U–Cl distances in
UCl
6
2
-
anions range from 2.576(6) to 2.638(4)
Å, the average values being 2.621(6) and 2.601(8)
Å for U(1) and U(2), respectively.The BuMeIm
+ cations have four different conformations of C
4H
9 chains.The shortest distances between the centres of the five-member rings of the BuMeIm
+ cations and U atoms of UCl
6
2− anions are in the range 5.53–4.90
Å.Single-crystal XRD data reveal the existence of a hydrogen bond between the most acidic H atom of the imidazolium ring (C2) and the chloride anion of the
UCl
6
2
-
octahedron, with the shortest distance Cl⋯H(C2) equal to 2.57
Å.Quantitative EXAFS measurements indicate that the octahedral complex
UCl
6
2
-
(R
U–Cl
=
2.632(2)
Å) is the predominant chemical form of U(IV) in BuMeImTf
2N solution.Visible absorption spectroscopy and
1H NMR spectroscopy confirm the existence of hydrogen bonding between the
UCl
6
2
-
anion and the acidic proton of the BuMeIm
+ cation in RTIL solution, similar to that in the solid state.
