Effect of hydration on coordination properties of uranyl(VI) complexes. A first-principles molecular dynamics study

Results from Car-Parrinello molecular dynamics simulations are reported for [UO2(OH2)5]2+, UO2(NO3)2(OH2)2, and UO2(NO3)2(eta2-tmma) (tmma = tetramethylmalonamide) in the gas phase and in aqueous solution. The distances between uranyl and neutral ligands such as water and tmma are decreased by up to 0.2 angstroms upon hydration, whereas those between uranyl and the nitrate ion are increased by up to 0.08 angstroms. According to pointwise thermodynamic integration involving constrained molecular dynamics simulations, solvation facilitates the transition of the chelating nitrate ligand to a eta1-bonding mode: the free energy of UO2(eta2-NO3)(eta1-NO3)(OH2)2 relative to the bis-chelating minimum drops from 3.9 kcal/mol in vacuo to 1.4 kcal/mol in water. Optimizations in a polarizable continuum (specifically, the conductor-like screening model in conjunction with the zero-order regular approximation and triple-zeta Slater basis sets) can qualitatively reproduce the geometrical changes from explicit hydration.     

Coordination environment of aqueous uranyl(VI) ion

Water dissociation from [UO2(OH2)5]2+ is studied with Car-Parrinello molecular dynamics simulations (using the BLYP density functional) in the gas phase and in aqueous solution. Free energies, DeltaA, are estimated from pointwise thermodynamic integration using one U-O(H2) distance as a reaction coordinate. While an isomeric, four-coordinate complex, [UO2(OH2)4]2+.H2O, is more stable than the five-coordinate reactant in the gas phase (DeltaA = -2.2 kcal/mol), the former is strongly disfavored in water (DeltaA = +8.7 kcal/mol).     

Theoretical study of electron transfer in uranyl(VI)–uranyl(V) complexes in solution

The rates of the electron self‐exchange between uranyl(VI) and uranyl(V) complexes in solution have been investigated in detail with quantum chemical methods. The calculations have shown that the bond length of U? Oyl is elongated by 0.1 Å when the extra electron is localized on the sites. The diabatic potential surfaces are obtained. The inner reorganization energies are 212.6 and 226.8 kJ mol^?1 for hydroxide and fluoride bridge systems, respectively. The solvent reorganization energies are 28.12 and 31.60 kJ mol^?1 for hydroxide and fluoride bridge systems, respectively. The nuclear frequency factors are 3.17 × 10¹³ and 3.12 × 10¹³ s^?1 for hydroxide and fluoride bridge systems, respectively. The electronic coupling matrix elements are 1.89 and 4.06 kJ mol^?1 for hydroxide and fluoride bridge systems, respectively. The electron‐transfer rates of our calculations are 12.95 and 0.819 M^?1 s^?1 for hydroxide and fluoride bridge systems, respectively. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Mechanism of water exchange in aqueous uranyl(VI) ion. A density functional molecular dynamics study

Constrained Car-Parrinello molecular dynamics simulations and thermodynamic integration have been performed for an associative pathway of water exchange between aqueous [UO2(OH2)5]2+ and bulk water. The simulated free energy of activation for this process, 6.7 kcal mol(-1), is significantly lower than that computed for a purely dissociative mechanism, 10.8 kcal mol(-1). Because the transient hexahydrate is indicated to have no chemically significant lifetime, the exchange mechanism can be classified as associative interchange.     

Coordination of uranyl(VI) carbonate species in aqueous solutions

There are very few examples in nature for U(VI) compounds with carbonate ligands other than the well known tricarbonates. Especially examples of U(VI) dicarbonato compounds are nearly completely missing. Even in aqueous solutions, the dicarbonato complex was found as a species of minorimportance only. On the basis of structural data on the ligands H₂O and carbonate as well as the available data on U(VI) coordination compounds, steric requirements of equatorial coordination are studied for aqueous solution species. A pentagonally coordinated monocarbonato species [UO₂CO₃(H₂O)₃] is found as the most likely coordination. For the dicarbonato species, hexagonally coordinated [UO₂(CO₃)₂(H₂O)₂] with D_2h symmetry is found as most probable structure. Possible causes of the instability of U(VI) dicarbonato species are discussed.     

N-ethyl- and N,N-di-isopropylacetamide complexes of some actinoid tetranitrates and uranyl(VI) nitrate

Complexes of composition Th(NO₃)₄·3L (L = MeCONHEt [ea]), M(NO₃)₄·2.5L (M=Th, L = Mecon(Prⁱ)₂ (dipa) and M = U, L = ea and dipa), Pu(NO₃)₄·2L (L = ea and dipa) and UO₂NO₃·2L (L = dipa) have been prepared. Their IR, Raman (thorium compounds only) and electronic spectra are reported.     

Lanthanum(III) and uranyl(VI) diglycolamide complexes: synthetic precursors and structural studies involving nitrate complexation

The synthesis and structural characterization of lanthanum(III) and uranyl(VI) complexes coordinated by tridentate diglycolamide (DGA) ligands O(CH2C(O)NR2)2[R=i-Pr (L1), i-Bu (L2)] are described. Reaction of L with UO2Cl2(H2O) n forms the uranyl(VI) cis-dichloride adducts UO2Cl2L [L=L1 (1a), L2 (1b)], while reaction of excess L with the corresponding metal nitrate hydrate produces [LaL3][La(NO3)6] [L=L1 (2a), L2 (2b)] for lanthanum and UO2(NO3)2L [L=L1 (3a), L2 (3b)] for uranium. Compounds 2b and 3a have been structurally characterized. The solid-state structure of the cation of 2b shows a triple-stranded helical arrangement of three tridentate DGA ligands with approximate D3 point-group symmetry, while the counteranion consists of six bidentate nitrate ligands coordinated around a second La center. The solid-state structure of 3a shows a tridentate DGA ligand coordinated along the equatorial plane perpendicular to the OUO unit as well as two nitrate ligands, one bidentate and oriented in the equatorial plane and the other monodentate and oriented parallel to the uranyl unit with the oxygen donor atom situated above the mean equatorial plane. Ambient-temperature NMR spectra for 3a and 3b indicated an averaged chemical environment of high symmetry consistent with fluxional nitrate hapticity, while spectroscopic data obtained at -30 degrees C revealed lower symmetry consistent with the slow-exchange limit for this process.     

Binding of pertechnetate to uranyl(VI) in aqueous solution. A density functional theory molecular dynamics study

According to constrained Car-Parrinello molecular dynamics simulations and thermodynamic integration, the free binding energy between uranyl hydrate and pertechnetate in aqueous solution is significantly lower than that between uranyl and nitrate, namely, by 1.7 kcal mol(-1). This is the first study of the differential binding of these two ligands to uranyl, which can have implications for the separability of uranium and technetium during the reprocessing of nuclear waste.     

Comparative study of uranyl(VI) and -(V) carbonato complexes in an aqueous solution

Electrochemical, complexation, and electronic properties of uranyl(VI) and -(V) carbonato complexes in an aqueous Na2CO3 solution have been investigated to define the appropriate conditions for preparing pure uranyl(V) samples and to understand the difference in coordination character between UO22+ and UO2+. Cyclic voltammetry using three different working electrodes of platinum, gold, and glassy carbon has suggested that the electrochemical reaction of uranyl(VI) carbonate species proceeds quasi-reversibly. Electrolysis of UO22+ has been performed in Na2CO3 solutions of more than 0.8 M with a limited pH range of 11.7 < pH < 12.0 using a platinum mesh electrode. It produces a high purity of the uranyl(V) carbonate solution, which has been confirmed to be stable for at least 2 weeks in a sealed glass cuvette. Extended X-ray absorption fine structure (EXAFS) measurements revealed the structural arrangement of uranyl(VI) and -(V) tricarbonato complexes, [UO2(CO3)3]n- [n = 4 for uranyl(VI), 5 for uranyl(V)]. The bond distances of U-Oax, U-Oeq, U-C, and U-Odist are determined to be 1.81, 2.44, 2.92, and 4.17 A for the uranyl(VI) complex and 1.91, 2.50, 2.93, and 4.23 A for the uranyl(V) complex, respectively. The validity of the structural parameters obtained from EXAFS has been supported by quantum chemical calculations for the uranyl(VI) complex. The uranium LI- and LIII-edge X-ray absorption near-edge structure spectra have been interpreted in terms of electron transitions and multiple-scattering features.     

Electron transfer in uranyl(VI)-uranyl(V) complexes in solution

The rates and mechanisms of the electron self-exchange between U(V) and U(VI) in solution have been studied with quantum chemical methods. Both outer-sphere and inner-sphere mechanisms have been investigated; the former for the aqua ions, the latter for binuclear complexes containing hydroxide, fluoride, and carbonate as bridging ligand. The calculated rate constant for the self-exchange reaction UO(2)(+)(aq) + UO(2)(2+)(aq) <=>UO(2)(2+)(aq) + UO(2)(+)(aq), at 25 degrees C, is k = 26 M(-1) s(-1). The lower limit of the rate of electron transfer in the inner-sphere complexes is estimated to be in the range 2 x 10(4) to 4 x 10(6) M(-1) s(-1), indicating that the rate for the overall exchange reaction may be determined by the rate of formation and dissociation of the binuclear complex. The activation energy for the outer-sphere model calculated from the Marcus model is nearly the same as that obtained by a direct calculation of the precursor- and transition-state energy. A simple model with one water ligand is shown to recover 60% of the reorganization energy. This finding is important because it indicates the possibility to carry out theoretical studies of electron-transfer reactions involving M(3+) and M(4+) actinide species that have eight or nine water ligands in the first coordination sphere.     

A density functional theory study of uranium(VI) nitrate monoamide complexes

Density functional theory calculations were performed on uranyl complexed with nitrate and monoamide ligands (L) [UO(2)(NO(3))(2)·2L]. The obtained results show that the complex stability is mainly governed by two factors: (i) the maximization of the polarizability of the coordinating ligand and (ii) the minimization of the steric hindrance effects. Furthermore, the electrostatic interaction between ligands and uranium(vi) was found to be a crucial parameter for the complex stability. These results pave the way to the definition of (quantitative) property/structure relationships for the in silico screening of monoamide ligands with improved extraction efficiency of uranium(vi) in nitrate acidic solution.     

Infrared and thermal studies of uranyl(VI) complexes of antipyrine

Recently, the isolation and characterisation of a large number of uranyl(VI) complexes with neutral unidentate oxygen donor ligands having XO bonds (XC, N, P, S or As) have been reported [1–5]. Antipyrine (1-phenyl-2,3-dimethyl-5-pyrazolone), containing a polar carbonyl group, is found to form complexes with metal ions [6-13]. This communication describes the isolation and physico-chemical properties of the complexes formed by uranyl(VI) ions.     

Uranium(VI) sulfilimine complexes: a new class of nitrogen analogues of the uranyl ion

The compound tetraphenylphosphonium tetrachlorooxo-S,S-diphenylsulfiliminatouranium, [Ph4P][UOCl4(NSPh2)], has been prepared in high yield from [Ph4P][UOCl5] and [Ph2S=NSiMe3]. An X-ray structure of this compound shows that the uranium atom has a pseudooctahedral geometry with oxygen and nitrogen atoms in trans positions. The structure of the analogous phosphoriminato complex [Ph4P][UOCl4(NPPh3)] has been determined for comparison. Derivatization of the sulfide group shows that only a limited range of functionalization confers stability toward reduction. The emission spectrum of the first electronic excited state reveals a greatly reduced energy compared with that of the uranyl ion. This red shift in the transition is consistent with the weakening of the U-N bond relative to the U-O bond.     

Solvation of Co(III)-cysteinato complexes in water: a DFT-based molecular dynamics study

Structural, dynamical, and vibrational properties of complexes made of metal cobalt(III) coordinated to different amounts of cysteine molecules were investigated with DFT-based Car-Parrinello molecular dynamics (CPMD) simulations in liquid water solution. The systems are composed of Co(III):3Cys and Co(III):2Cys immersed in liquid water which are modeled by about 110 explicit water molecules, thus one of the biggest molecular systems studied with ab initio molecular simulations so far. In such a way, we were able to investigate structural and dynamical properties of a model of a typical metal binding site used by several proteins. Cobalt, mainly a toxicological agent, can replace the natural binding metal and thus modify the biochemical activity. The structure of the surrounding solvent around the metal-ligands complexes is reported in detail, as well as the metal-ligands coordination bonds, using radial distribution functions and electronic analyses with Mayer bond orders. Structures of the Cocysteine complexes are found in very good agreement with EXAFS experimental data, stressing the importance of considering the surrounding solvent in the modeling. A vibrational analysis is also conducted and compared to experiment, which strengthens the reliability of the solvent interactions with the Cocysteine complexes from our molecular dynamics simulations, as well as the dynamics of the systems. From this preliminary analysis, we could suggest a vibrational fingerprint able to distinguish Co(III):2Cys from Co(III):3Cys. Our simulations also show the importance of considering a quantum explicit solvent, as solute-to-solvent proton transfer events have been observed.     

Reactions of uranyl(VI) complexes of acid dihydrazides with β-diketones

Summary Uranyl(VI) complexes of malonic acid dihydrazide (MDH₂) and phthalic acid dihydrazide (PDH₂) and the products of their reactions with four -diketones have been characterised by elemental analysis and by electrical conductance, and spectral (i.r. and electronic) measurements. The MDH₂ and PDH₂ complexes UO₂(L)₂(H₂O)₂ are eight coordinate whereas the macrocyclic UO₂(L)(H₂O)₂ complexes are six coordinate. In each complex MDH₂ and PDH₂ act as bidentate liglands having the coordination sites at secondary amide-nitrogen atoms.     

Speciation and coordination chemistry of uranyl(VI)-citrate complexes in aqueous solution

The pH dependence of uranyl(VI) complexation by citric acid was investigated using Raman and attenuated total reflection FTIR spectroscopies and electrospray ionization mass spectrometry. pH-dependent changes in the nu(s)(UO(2)) envelope indicate that three major UO(2)(2+)-citrate complexes with progressively increasing U=O bond lengths are present over a range of pH from 2.0 to 9.5. The first species, which is the predominant form of uranyl(VI) from pH 3.0 to 5.0, contains two UO(2)(2+) groups in spectroscopically equivalent coordination environments and corresponds to the [(UO(2))(2)Cit(2)](2)(-) complex known to exist in this pH range. At pH values >6.5, [(UO(2))(2)Cit(2)](2)(-) undergoes an interconversion to form [(UO(2))(3)Cit(3)](3)(-) and (UO(2))(3)Cit(2). ESI-MS studies on solutions of varying uranyl(VI)/citrate ratios, pH, and solution counteranion were successfully used to confirm complex stoichiometries. Uranyl and citrate concentrations investigated ranged from 0.50 to 50 mM.     

On the "yl" bond weakening in uranyl(VI) coordination complexes

The U-O(yl) triple bonds in the UO(2)(2+) aquo ion are known to be weakened by replacing the first shell water with organic or inorganic ligands. Weakening of the U-O(yl) bond may enhance the reactivity of "yl" oxygens and uranyl(VI) cation-cation interactions. Density functional theory calculations as well as previously published vibrational spectroscopic data have been used to study the origin of the U-O(yl) bond weakening in uranyl(VI) coordination complexes. Natural population analyses (NPA) revealed that the electron localization on the O(yl) 2p orbital is a direct measure of the U-O(yl) bond weakening, indicating that the bond weakening is correlated to the weakening of the U-O(yl) covalent bond and not that of the ionic bond. The Mulliken analysis gives poor results for uranium to ligand electron partitioning and is thus unreliable. Further analyses of molecular orbitals near the highest occupied molecular orbital (HOMO) show that both the σ and π donating abilities of the ligands may account for the U-O(yl) bond weakening. The mechanism of the bond weakening varies with coordinating ligand so that each case needs to be examined independently.     

Polybisaminophosphazene-silver nitrate complexes: Coordination and properties

Poly[bis(propylamino)phosphazene]-silver nitrate complexes and poly[bis-(butylamino)phosphazene]-silver nitrate complexes with various salt contents, which are new polyphosphazene-salt complexes, have been prepared. The complexes were characterized by FTIR, ³¹P-NMR, and ¹³C-NMR spectroscopies, and the thermal properties were studied by DSC, TGA, and TGA/FTIR measurements. It was found that the poly[(bisamino)phosphazene]-silver coordination may occur both at the backbone nitrogen and the side chain nitrogen and incorporation of silver nitrate into the polymer lowered the decomposition temperature. The complexes can be cast as freestanding film with good dimensional stability. Compared to their parent polymers, the highest conductivities of the films are increased three to four orders in magnitude at room temperature. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 1023–1031, 1997     

Effect of nitrate, perchlorate, and water on uranyl(VI) speciation in a room-temperature ionic liquid: a spectroscopic investigation

Room-temperature ionic liquids form potentially important solvents in novel nuclear waste reprocessing methods, and the solvation, speciation, and complexation behaviors of lanthanides and actinides in these solvents are of great current interest. In the study reported here, the coordination environment of uranyl(VI) in solutions of the room-temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][Tf(2)N]) containing perchlorate, tetrabutylammonium nitrate, and water was investigated using Raman, ATR-FTIR, and NMR spectroscopies in order to better understand the role played in uranyl(VI) solution chemistry in room-temperature ionic liquids by water and other small, weakly complexing ligands. The (2)H NMR chemical shift for water in a solution of uranyl perchlorate hexahydrate in [EMIM][Tf(2)N] appears at 6.52 ppm, indicating that water is coordinated to uranyl(VI). A broad ν(OH) stretching mode at 3370 cm(-1) in the ATR-FTIR spectrum shows that this coordinated water is engaged in hydrogen bonding with water molecules in a second coordination sphere. A significant upfield shift in the (2)H NMR signal for water and the appearance of distinct ν(as)(HOH) (at 3630 cm(-1)) and ν(s)(HOH) (at 3560 cm(-1)) vibrational bands in the ATR-FTIR spectra show that coordinated water is displaced by nitrate upon formation of the UO(2)(NO(3))(2) and UO(2)(NO(3))(3)(-) complexes. The Raman spectra indicate that perchlorate complexed to uranyl(VI) is also displaced by nitrate. Our results indicate that perchlorate and water, though weakly complexing ligands, do have a role in uranyl(VI) speciation in room-temperature ionic liquids and that Raman, infrared, and NMR spectroscopies are valuable additions to the suite of tools currently used to study the chemical behavior of uranyl(VI)-ligand complexes in these solvents.