Oxygen Exchange in Uranyl Hydroxide via Two “Nonclassical” Ions

Nature and coordination geometry of geologically relevant aqueous Uranium(VI) complexes up to 400 ºC: A review and new data

The structure of the uranyl aqua ion (UO₂²⁺) and a number of its inorganic complexes (specifically, UO₂Cl⁺, UO₂Cl₂⁰, UO₂SO₄⁰, [Formula: see text] , [Formula: see text] and UO₂OH₄^2-) have been characterised using X-Ray absorption spectroscopy/extended X-Ray absorption fine structure (XAS/EXAFS) at temperatures ranging from 25 to 326 ºC. Results of ab initio molecular dynamics (MD) calculations are also reported for uranyl in chloride and sulfate-bearing fluids from 25 to 400 ºC and 600 bar to 20 kilobar (kb). These results are reported alongside a comprehensive review of prior structural characterisation work with particular focus given to EXAFS works to provide a consistent and up-to-date view of the structure of these complexes under conditions relevant to U mobility in ore-forming systems and around high-grade nuclear waste repositories. Regarding reported EXAFS results, average equatorial coordination was found to decrease in uranyl and its sulfate and chloride complexes as temperature rose - the extent of this decrease differed between species and solution compositions but typically resulted in an equatorial coordination number of ∼3-4 at temperatures above 200 ºC. The [Formula: see text] complex was observed at temperatures from 25 to 247 ºC and exhibited no major structural changes over this temperature range. UO₂OH₄^2- exhibited only minor structural changes over a temperature range from 88 to 326 ºC and was suggested to manifest fivefold coordination with four hydroxyl molecules and one water molecule around its equator. Average coordination values derived from fits of the reported EXAFS data were compared to average coordination values calculated using the experimentally derived thermodynamic data for chloride complexes reported by Dargent et al. (2013) and Migdisov et al. (2018b), and for sulfate complexes reported by Alcorn et al. (2019) and Kalintsev et al. (2019). Sulfate EXAFS data were well described by available thermodynamic data, and chloride EXAFS data were described well by the thermodynamic data of Migdisov et al. (2018b), but not by the data of Dargent et al. (2013). The ab initio molecular dynamics calculations confirmed the trends in equatorial coordination observed with EXAFS and were also able to provide an insight into the effect of pressure in equatorial water coordination - for a given temperature, higher pressures appear to lead to a greater number of equatorially bound waters counteracting the temperature effect.

Hydrolysis of Small Oxo/Hydroxo Molecules Containing High Oxidation State Actinides (Th, Pa, U, Np, Pu): A Computational Study

The energetics of hydrolysis reactions for high oxidation states of oxo/hydroxo monomeric actinide species (Th^IVO₂, Pa^IVO₂, U^IVO₂, Pa^VO₂(OH), U^VO₂(OH), U^VIO₃, Np^VIO₃, Np^VIIO₃(OH), and Pu^VIIO₃(OH)) were calculated at the CCSD(T) level. The first step is the formation of a Lewis acid/base adduct with H₂O (hydration), followed by a proton transfer to form a dihydroxide molecule (hydrolysis); this process is repeated until all oxo groups are hydrolyzed. The physisorption (hydration) for each H₂O addition was predicted to be exothermic, ca. -20 kcal/mol. The hydrolysis products are preferred energetically over the hydration products for the +IV and +V oxidation states. The compounds with An^VI are a turning point in terms of favoring hydration over hydrolysis. For An^VIIO₃(OH), hydration products are preferred, and only two waters can bind; the complete hydrolysis process is now endothermic, and the oxidation state for the An in An(OH)₇ is +VI with two OH groups each having one-half an electron. The natural bond order charges and the reaction energies provide insights into the nature of the hydrolysis/hydration processes. The actinide charges and bond ionicity generally decrease across the period. The ionic character decreases as the oxidation state and coordination number increase so that covalency increases moving to the right in the actinide period.

Ab initio molecular dynamics studies of hydroxide coordination of alkaline earth metals and uranyl

Ab initio molecular dynamics (AIMD) simulations of the Mg2+, Ca2+, Sr2+ and UO22+ ions in either a pure aqueous environment or an environment containing two hydroxide ions have been carried out at the density functional level of theory, employing the generalised gradient approximation via the PBE exchange-correlation functional. Calculated mean M-O bond lengths in the first solvation shell of the aquo systems compared very well to existing experimental and computational literature, with bond lengths well within values measured previously and coordination numbers in line with previously calculated values. When applied to systems containing additional hydroxide ions, the methodology revealed increased bond lengths in all systems. Proton transfer events (PTEs) were recorded and were found to be most prevalent in the strontium hydroxide systems, likely due to the low charge density of the ion and the consequent lack of hydroxide coordination. For all alkaline earths, intrashell PTEs which occurred outside of the first solvation shell were most prevalent. Only three PTEs were identified in the entire simulation data of the uranium dihydroxide system, indicating the clear impact of the increased charge density of the hexavalent uranium ion on the strength of metal-oxygen bonds in aqueous solution. Broadly, systems containing more charge dense ions were found to exhibit fewer PTEs than those containing ions of lower charge density.

Structural and Computational Characterization of a Bridging Zwitterionic-Amidoxime Uranyl Complex

A bridging (μ₂) neutral zwitterionic amidoxime binding mode previously unobserved between amidoximes and uranyl is reported and compared to other uranyl amidoxime complexes. Density functional theory computations show the dinuclear complex exhibits a shallow potential energy surface allowing for facile inclusion of a nonbonding water molecule in the solid-state.

Structural Analysis of the Complexation of Uranyl, Neptunyl, Plutonyl, and Americyl with Cyclic Imide Dioximes

Knowledge-based design of extracting agents for selective binding of actinides is essential in stock-pile stewardship, environmental remediation, separations, and nuclear fuel disposal. Robust computational protocols are critical for in depth understanding of structural properties and to further advance the design of selective ligands. In particular, rapid radiochemical separations require predictive capabilities for binding in the gas phase. This study focuses on gas-phase binding preferences of cyclic imide dioximes to uranyl, neptunyl, plutonyl, and americyl. Structural properties, electron withdrawing effects, and their effects on binding preferences are studied with natural bond-order population analysis. The aromatic amidoximes are found to have a larger electron-donation effect than the aliphatic amidoximes. It is also found that plutonyl is more electron withdrawing than uranyl, neptunyl, and americyl when bound to the cyclic imide dioximes studied.

Double uranium oxo cations derived from uranyl by borane or silane reduction

A new type of double uranium oxo cation [O-U-O-U-O]4+ is prepared by selective oxygen-atom abstraction from macrocyclic uranyl complexes using either boranes or silanes. A significant degree of multiple U[double bond, length as m-dash]O bonding is evident throughout the U2O3 core, but either trans-,cis- or trans-,trans-OUOUO motifs can be isolated as boron- or silicon-capped oxo complexes. Further controlled deoxygenation of the borylated system is also possible.

Controlling uranyl oxo group interactions to group 14 elements using polypyrrolic Schiff-base macrocyclic ligands

Heterodinuclear uranyl/group 14 complexes of the aryl- and anthracenyl-linked Schiff-base macrocyclic ligands L^Me and L^A were synthesised by reaction of UO₂(H₂L) with M{N(SiMe₃)₂}₂ (M = Ge, Sn, Pb). For complexes of the anthracenyl-linked ligand (L^A) the group 14 metal sits out of the N₄-donor plane by up to 0.7 Å resulting in relatively short MOUO distances which decrease down the group; however, the solid state structures and IR spectroscopic analyses suggest little interaction occurs between the oxo and group 14 metal. In contrast, the smaller aryl-linked ligand (L^Me) enforces greater interaction between the metals; only the Pb^II complex was cleanly accessible although this complex was relatively unstable in the presence of HN(SiMe₃)₂ and some organic oxidants. In this case, the equatorial coordination of pyridine-N-oxide causes a 0.08 Å elongation of the endo UO bond and a clear interaction of the uranyl ion with the Pb(ii) cation in the second donor compartment.

Understanding the origins of Oyl–U–Oylbending in the uranyl (UO22+) ion

Although rare, O_yl–U–O_ylbending in the uranyl (UO₂²⁺) ion can be effected by either steric perturbation or electronic perturbation.

Uranyl Carbonate Complexes in Aqueous Solution and Their Ligand NMR Chemical Shifts and 17O Quadrupolar Relaxation Studied by ab Initio Molecular Dynamics

Dynamic structural effects, NMR ligand chemical shifts, and ¹⁷O NMR quadrupolar relaxation rates are investigated in the series of complexes UO₂²⁺, UO₂(CO₃)₃^4-, and (UO₂)₃(CO₃)₆^6-. Car-Parrinello molecular dynamics (CPMD) is used to simulate the dynamics of the complexes in water. NMR properties are computed on clusters extracted from the CPMD trajectories. In the UO₂²⁺ complex, coordination at the uranium center by water molecules causes a decrease of around 300 ppm for the uranyl ¹⁷O chemical shift. The final value of this chemical shift is within 40 ppm of the experimental range. The UO₂(CO₃)₃^4- and (UO₂)₃(CO₃)₆^6- complexes show a solvent dependence of the terminal carbonate ¹⁷O and ¹³C chemical shifts that is less pronounced than that for the uranyl oxygen atom. Corrections to the chemical shift from hybrid functionals and spin-orbit coupling improve the accuracy of chemical shifts if the sensitivity of the uranyl chemical shift to the uranyl bond length (estimated at 140 ppm per 0.1 Å from trajectory data) is taken into consideration. The experimentally reported trend in the two unique ¹³C chemical shifts is correctly reproduced for (UO₂)₃(CO₃)₆^6-. NMR relaxation rate data support large ¹⁷O peak widths, but remain below those noted in the experimental literature. Comparison of relaxation data for solvent-including versus solvent-free models suggest that carbonate ligand motion overshadows explicit solvent effects.

Theoretical investigation on the mechanism and dynamics of oxo exchange of neptunyl(VI) hydroxide in aqueous solution

Four types of reaction mechanisms for the oxo ligand exchange of monomeric and dimeric neptunyl(VI) hydroxide in aqueous solution were explored computationally using density functional theory (DFT) and ab initio classical molecular dynamics. The obtained results were compared with previous studies on the oxo exchange of uranyl hydroxide, as well as with experiments. It is found that the stable T-shaped [NpO3(OH)3](3-) intermediate is a key species for oxo exchange in the proton transfer in mononuclear Path I and binuclear Path IV, similar to the case of uranyl(VI) hydroxide. Path I is thought to be the preferred oxo exchange mechanism for neptunyl(VI) hydroxide in our calculations, due to the lower activation energy (22.7 and 13.1 kcal mol(-1) for ΔG(‡) and ΔH(‡), respectively) of the overall reaction. Path II via a cis-neptunyl structure assisted by a water molecule might be a competitive channel against Path I with a mononuclear mechanism, owing to a rapid dynamical process occurring in Path II. In Path IV with the binuclear mechanism, oxo exchange is accomplished via the interaction between [NpO2(OH)4](2-) and T-shaped [NpO3(OH)3](3-) with a low activation energy for the rate-determining step, however, the overall energy required to fulfill the reaction is slightly higher than that in mononuclear Path I, suggesting a possible binuclear process in the higher energy region. The chemical bonding evolution along the reaction pathways was discussed by using topological methodologies of the electron localization function (ELF).

Combining luminescence spectroscopy, parallel factor analysis and quantum chemistry to reveal metal speciation - a case study of uranyl(vi) hydrolysis

This study of aqueous metal speciation is an advanced combination of theoretical and experimental methods. Continuous wave (CW) and time-resolved laser-induced fluorescence spectroscopy (TRLFS) data of uranyl(vi) hydrolysis were analyzed using parallel factor analysis (PARAFAC). Distribution patterns of five major species were thereby derived under a fixed uranyl concentration (10-5 M) over a wide pH range from 2 to 11. UV (180 nm to 370 nm) excitation spectra were extracted for individual species. Time-dependent density functional theory (TD-DFT) calculations revealed ligand excitation (water, hydroxo, oxo) in this region and ligand-to-metal charge transfer (LMCT) responsible for luminescence. Thus excitation in the UV region is extreme ligand sensitive and specific. Combining findings from PARAFAC and DFT the [UO2(H2O)5]2+ cation (aquo complex 1 : 0) and four hydroxo complexes (1 : 1, 3 : 5, 3 : 7 and 1 : 3) were identified. The methodological concept used here is applicable to luminescent metals in general and thus enables acquisition of refined structural and thermodynamical data of lanthanide and actinide complexation.

Uranyl extraction by N,N-dialkylamide ligands studied using static and dynamic DFT simulations

DFT/MM-MD simulations highlight the structure and dynamics of mixed uranyl/nitrato/monoamides (L) complexes at an “oil”/water interface.

Aqueous solutions: state of the art in ab initio molecular dynamics

The simulation of liquids by ab initio molecular dynamics (AIMD) has been a subject of intense activity over the last two decades. The significant increase in computational resources as well as the development of new and efficient algorithms has elevated this method to the status of a standard quantum mechanical tool that is used by both experimentalists and theoreticians. As AIMD computes the electronic structure from first principles, it is free of ad hoc parametrizations and has thus been applied to a large variety of physical and chemical problems. In particular, AIMD has provided microscopic insight into the structural and dynamical properties of aqueous solutions which are often challenging to probe experimentally. In this review, after a brief theoretical description of the Born-Oppenheimer and Car-Parrinello molecular dynamics formalisms, we show how AIMD has enhanced our understanding of the properties of liquid water and its constituent ions: the proton and the hydroxide ion. Thereafter, a broad overview of the application of AIMD to other aqueous systems, such as solvated organic molecules and inorganic ions, is presented. We also briefly describe the latest theoretical developments made in AIMD, such as methods for enhanced sampling and the inclusion of nuclear quantum effects.

Promising density functional theory methods for predicting the structures of uranyl complexes

By examining the overall accuracy of different theoretical methods in predicting the U–X bond distances (of a series uranyl complexes), we found that both the global-hybrid meta-GGA functional of BB1K and the range-seperated LC-BLYP functional are fairly good (even better than the popular B3LYP method).

Controlled deprotection and reorganization of uranyl oxo groups in a binuclear macrocyclic environment

Switching on uranium(V) reactivity: The silylated uranium(V) dioxo complex [(Me(3)SiOUO)(2)(L)(2)] (A) is inert to oxidation, but after two-electron reduction to [(Me(3)SiOUO)(2)(L)](2-) (1), it can be desilylated to form [OU(μ-O)(2)UO(L)(2)](2-) (2) with reinstated uranyl character. Removal of the silyl group uncovers new redox and oxo rearrangement chemistry for uranium, thus reforming the uranyl motif and involving the U(VI/V) couple in dioxygen reduction.

Strongly coupled binuclear uranium-oxo complexes from uranyl oxo rearrangement and reductive silylation

The most common motif in uranium chemistry is the d(0)f(0) uranyl ion [UO(2)](2+) in which the oxo groups are rigorously linear and inert. Alternative geometries, such as the cis-uranyl, have been identified theoretically and implicated in oxo-atom transfer reactions that are relevant to environmental speciation and nuclear waste remediation. Single electron reduction is now known to impart greater oxo-group reactivity, but with retention of the linear OUO motif, and reactions of the oxo groups to form new covalent bonds remain rare. Here, we describe the synthesis, structure, reactivity and magnetic properties of a binuclear uranium-oxo complex. Formed through a combination of reduction and oxo-silylation and migration from a trans to a cis position, the new butterfly-shaped Si-OUO(2)UO-Si molecule shows remarkably strong U(V)-U(V) coupling and chemical inertness, suggesting that this rearranged uranium oxo motif might exist for other actinide species in the environment, and have relevance to the aggregation of actinide oxide clusters.

Comparison of free energy surfaces calculations from ab initio molecular dynamic simulations at the example of two transition metal catalyzed reactions

We carried out ab initio molecular dynamic simulations in order to determine the free energy surfaces of two selected reactions including solvents, namely a rearrangement of a ruthenium oxoester in water and a carbon dioxide addition to a palladium complex in carbon dioxide. For the latter reaction we also investigated the gas phase reaction in order to take solvent effects into account. We used two techniques to reconstruct the free energy surfaces: thermodynamic integration and metadynamics. Furthermore, we gave a reasonable error estimation of the computed free energy surface. We calculated a reaction barrier of ΔF = 59.5 ± 8.5 kJ mol(-1) for the rearrangement of a ruthenium oxoester in water from thermodynamic integration. For the carbon dioxide addition to the palladium complex in carbon dioxide we found a ΔF = 44.9 ± 3.3 kJ mol(-1) from metadynamics simulations with one collective variable. The investigation of the same reactions in the gas phase resulted in ΔF = 24.9 ± 6.7 kJ mol(-1) from thermodynamic integration, in ΔF = 26.7 ± 2.3 kJ mol(-1) from metadynamics simulations with one collective variable, and in ΔF = 27.1 ± 5.9 kJ mol(-1) from metadynamics simulations with two collective variables.