Electronic structure and bonding in actinyl ions and their analogs

Differential uranyl(v) oxo-group bonding between the uranium and metal cations from groups 1, 2, 4, and 12; a high energy resolution X-ray absorption, computational, and synthetic study

Uranyl Pacman takes them all: the bonding of s- and d-block cations to uranyl is compared by experiment, spectroscopy and theory.

The uranyl(vi) ‘Pacman’ complex [(UO₂)(py)(H₂L)] A (L = polypyrrolic Schiff-base macrocycle) is reduced by Cp₂Ti(η²-Me₃SiC Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CSiMe₃) and [Cp₂TiCl]₂ to oxo-titanated uranyl(v) complexes [(py)(Cp₂Ti^IIIOUO)(py)(H₂L)] 1 and [(ClCp₂Ti^IVOUO)(py)(H₂L)] 2. Combination of Zr^II and Zr^IV synthons with A yields the first Zr^IV–uranyl(v) complex, [(ClCp₂ZrOUO)(py)(H₂L)] 3. Similarly, combinations of Ae⁰ and Ae^II synthons (Ae = alkaline earth) afford the mono-oxo metalated uranyl(v) complexes [(py)₂(ClMgOUO)(py)(H₂L)] 4, [(py)₂(thf)₂(ICaOUO)(py) (H₂L)] 5; the zinc complexes [(py)₂(XZnOUO)(py)(H₂L)] (X = Cl 6, I 7) are formed in a similar manner. In contrast, the direct reactions of Rb or Cs metal with A generate the first mono-rubidiated and mono-caesiated uranyl(v) complexes; monomeric [(py)₃(RbOUO)(py)(H₂L)] 8 and hexameric [(MOUO)(py)(H₂L)]₆ (M = Rb 8b or Cs 9). In these uranyl(v) complexes, the pyrrole N–H atoms show strengthened hydrogen-bonding interactions with the endo-oxos, classified computationally as moderate-strength hydrogen bonds. Computational DFT MO (density functional theory molecular orbital) and EDA (energy decomposition analysis), uranium M₄ edge HR-XANES (High Energy Resolution X-ray Absorption Near Edge Structure) and 3d4f RIXS (Resonant Inelastic X-ray Scattering) have been used (the latter two for the first time for uranyl(v) in 7 (ZnI)) to compare the covalent character in the U^V–O and O–M bonds and show the 5f orbitals in uranyl(vi) complex A are unexpectedly more delocalised than in the uranyl(v) 7 (ZnI) complex. The O_exo–Zn bonds have a larger covalent contribution compared to the Mg–O_exo/Ca–O_exo bonds, and more covalency is found in the U–O_exo bond in 7 (ZnI), in agreement with the calculations.

Charge densities in actinide compounds: strategies for data reduction and model building

The data quality requirements for charge density studies on actinide compounds are extreme. Important steps in data collection and reduction required to obtain such data are summarized and evaluated. The steps involved in building an augmented Hansen-Coppens multipole model for an actinide pseudo-atom are provided. The number and choice of radial functions, in particular the definition of the core, valence and pseudo-valence terms are discussed. The conclusions in this paper are based on a re-examination and improvement of a previously reported study on [PPh4][UF6]. Topological analysis of the total electron density shows remarkable agreement between experiment and theory; however, there are significant differences in the Laplacian distribution close to the uranium atoms which may be due to the effective core potential employed for the theoretical calculations.

Inverse Trans Influence in Low-Valence Actinide-Group 10 Metal Complexes of Phosphinoaryl Oxides: A Theoretical Study via Tuning Metals and Donor Ligands

The recognition and in-depth understanding of inverse trans influence (ITI) have successfully guided the synthesis of novel actinide complexes and enriched actinide chemistry. Those complexes, however, are mainly limited to the involvement of high-valence actinide and/or metal-ligand multiple bonds. Examples containing both low oxidation state actinide and metal-metal single bond remain rare. Herein, more than 20 actinide-transition metal (An-TM) complexes of phosphinoaryl oxide ligands have been designed in accordance with several experimentally known analogs, by changing the metal atoms (An = Th, Pa, U, Np, and Pu; and TM = Ni, Pd, and Pt), actinide oxidation states (IV and III) and metal-metal axial donor ligands (X = Me₃SiO, F, Cl, Br, and I). The relativistic density functional theory study of structural (trans-An-X and cis-An-O toward An-TM), bonding (topological electron/energy density), and electronic properties reveals the order of the ITI stabilizing actinide-metal bond. Computed electron affinity (EA) values, related to the electrochemical reduction, linearly correlate with experimentally measured reduction potentials. Although the same ITI order for the ligand donors was shown as in a previous study, the correlation between electrochemical reduction and the ITI was found to be weak when the actinide atoms were changed. For most complexes, the reduction is primarily of an actinide-based mechanism with minor participation of transition metal and phosphinoaryl oxide, whereas that of thorium-nickel complexes is different.

Metal Bonding with 3d and 6d Orbitals: An EPR and ENDOR Spectroscopic Investigation of Ti3+-Al and Th3+-Al Heterobimetallic Complexes

Accessing covalent bonding interactions between actinides and ligating atoms remains a central problem in the field. Our current understanding of actinide bonding is limited because of a paucity of diverse classes of compounds and the lack of established models. We recently synthesized a thorium (Th)–aluminum (Al) heterobimetallic molecule that represents a new class of low-valent Th-containing compounds. To gain further insight into this system and actinide–metal bonding more generally, it is useful to study their underlying electronic structures. Here, we report characterization by electron paramagnetic resonance (EPR) and electron–nuclear double resonance (ENDOR) spectroscopy of two heterobimetallic compounds: (i) a Cp^tt₂ThH₃AlCTMS₃ [TMS = Si(CH₃)₃; Cp^tt = 1,3-di-tert-butylcyclopentadienyl] complex with bridging hydrides and (ii) an actinide-free Cp₂TiH₃AlCTMS₃ (Cp = cyclopentadienyl) analogue. Analyses of the hyperfine interactions between the paramagnetic trivalent metal centers and the surrounding magnetic nuclei, ¹H and ²⁷Al, yield spin distributions over both complexes. These results show that while the bridging hydrides in the two complexes have similar hyperfine couplings (a_iso = −9.7 and −10.7 MHz, respectively), the spin density on the Al ion in the Th³⁺ complex is ∼5-fold larger than that in the titanium(3+) (Ti³⁺) analogue. This suggests a direct orbital overlap between Th and Al, leading to a covalent interaction between Th and Al. Our quantitative investigation by a pulse EPR technique deepens our understanding of actinide bonding to main-group elements.

The electronic structures of Ti³⁺−Al and Th³⁺−Al heterobimetallic complexes are probed by electron−nuclear double resonance spectroscopy, revealing a much larger spin density on the Al center in the latter and the presence of a covalent Th−Al bonding interaction caused by the direct orbital overlap between Th and Al.

Persistent Superprotonic Conductivity in the Order of 10−1 S·cm−1 Achieved Through Thermally Induced Structural Transformation of a Uranyl Coordination Polymer

Despite tremendous efforts having been made in the exploration of new high-performance proton-conducting materials, systems with superprotonic conductivity higher than 10−1 S·cm−1 are scarcely reported. We show here the utilization of bridging uranyl oxo atoms, traditionally termed cation–cation interaction (CCI), as the hydrogen bond acceptor to build a dense and ordered hydrogen bond network, affording a unique uranyl-based proton-conducting coordination polymer (H3O)4UO2(PO4)2 (HUP-1). This compound contains a densely connected hydronium network that is substantially stabilized by uranyl oxo atoms and exhibits high proton conductivities over a wide temperature range. At 98 °C, 98% relative humidity, a superprotonic conductivity of 1.02 × 10−1 S·cm−1 is observed for the system, one of the highest values reported for a solid-state proton-conducting material. This property originates from the thermally induced phase transformation from HUP-1 to another uranyl compound also with a CCI bond, (H3O)UO2PO4·(H2O)3 (HUP-2), accompanied by the partial generation of phosphorus acid that is further trapped in the structure of HUP-2, demonstrated by solid-state NMR analysis. The superprotonic conductivity of H3PO4@HUP-2 is persistent under the testing condition.

Interaction of Metal Oxido Compounds with B(C6 F5 )3

Lewis acid-base pair chemistry has been placed on a new level with the discovery that adduct formation between an electron donor (Lewis base) and acceptor (Lewis acid) can be inhibited by the introduction of steric demand, thus preserving the reactivity of both Lewis centers, resulting in highly unusual chemistry. Some of these highly versatile frustrated Lewis pairs (FLP) are capable of splitting a variety of small molecules, such as dihydrogen, in a heterolytic and even catalytic manner. This is in sharp contrast to classical reactions where the inert substrate must be activated by a metal-based catalyst. Very recently, research has emerged combining the two concepts, namely the formation of FLPs in which a metal compound represents the Lewis base, allowing for novel chemistry by using the heterolytic splitting power of both together with the redox reactivity of the metal. Such reactivity is not restricted to the metal center itself being a Lewis acid or base, also ancillary ligands can be used as part of the Lewis pair, still with the benefit of the redox-active metal center nearby. This Minireview is designed to highlight the novel reactions arising from the combination of metal oxido transition-metal or rare-earth-metal compounds with the Lewis acid B(C₆ F₅ )₃ . It covers a wide area of chemistry including small molecule activation, hydrogenation and hydrosilylation catalysis, and olefin metathesis, substantiating the broad influence of the novel concept. Future goals of this young and exciting area are briefly discussed.

Fabrication of blue organic light-emitting diodes from novel uranium complexes: synthesis, characterization, and electroluminescence studies of uranium anthracene-9-carboxylate complexes

In this study, three uranium(vi) complexes, [UO2(C15H9O2)2(CH3CH2OH)2]·2CH3CH2OH (1), [U2O4(C15H9O2)2(CH3O)2(CH3OH)2]·2CH3OH (2), and [U2O4(C15H9O2)4(CH3OH)2]·2H2O (3), were prepared by reacting anthracene-9-carboxylic acid with uranyl acetate dihydrate using various ligand to uranyl acetate ratios in different solvents. The infrared and UV-Vis spectra along with elemental and thermal analyses showed the formation of mono- and dinuclear anthracene-9-carboxylate complexes of uranium. A 1 to 3 molar ratio of uranyl acetate to anthracene-9-carboxylic acid in ethanol resulted in the formation of the mononuclear complex 1, whereas a 1 to 2 and 1 to 3 molar ratio of uranyl acetate to anthracene-9-carboxylic acid in methanol produced the dinuclear complexes 2 and 3, respectively. Single-crystal structure determinations of 1, 2 and 3 revealed hexagonal bipyramidal geometries for the mononuclear uranium complex of 1 and a pentagonal geometry for the dinuclear uranium complexes of 2 and 3. The single-crystal structures of complexes 2 and 3 showed π-π interactions in contrast to complex 1. The strong π-π interactions in complex 2 and 3 lead to an enhanced photoluminescence intensity in comparison with 1 without π-π interaction. The optical properties of the prepared complexes are associated with the ligand-induced resonant system. The fluorescent uranium complex 1 that showed a blue emission upon excitation at 270 nm was used for the fabrication of a blue organic light-emitting diode (BOLED), an industrially important OLED, using a simple solution-process fabrication method.

Emergence of the structure-directing role of f-orbital overlap-driven covalency

FEUDAL (f’s essentially unaffected, d’s accommodate ligands) is a longstanding bonding model in actinide chemistry, in which metal-ligand binding uses 6d-orbitals, with the 5f remaining non-bonding. The inverse-trans-influence (ITI) is a case where the model may break down, and it has been suggested that ionic and covalent effects work synergistically in the ITI. Here, we report an experimentally grounded computational study that quantitatively explores the ITI, and in particular the structure-directing role of f-orbital covalency. Strong donor ligands generate a cis-ligand-directing electrostatic potential (ESP) at the metal centre. When f-orbital participation, via overlap-driven covalency, becomes dominant via short actinide-element distances, this ionic ESP effect is overcome, favouring a trans-ligand-directed geometry. This study contradicts the accepted ITI paradigm in that here ionic and covalent effects work against each other, and suggests a clearly non-FEUDAL, structure-directing role for the f-orbitals.

In actinide chemistry, a longstanding bonding model describes metal-ligand binding using 6d-orbitals, with the 5f-orbitals remaining non-bonding. Here the authors explore the inverse-trans-influence — a case where the model breaks down — finding that the f-orbitals play a crucial role in dictating a trans-ligand-directed geometry.

Ion association with tetra-n-alkylammonium cations stabilizes higher-oxidation-state neptunium dioxocations

Extended-coordination sphere interactions between dissolved metals and other ions, including electrolyte cations, are not known to perturb the electrochemical behavior of metal cations in water. Herein, we report the stabilization of higher-oxidation-state Np dioxocations in aqueous chloride solutions by hydrophobic tetra-n-alkylammonium (TAA⁺) cations—an effect not exerted by fully hydrated Li⁺ cations under similar conditions. Experimental and molecular dynamics simulation results indicate that TAA⁺ cations not only drive enhanced coordination of anionic Cl^– ligands to Np^V/VI but also associate with the resulting Np complexes via non-covalent interactions, which together decrease the electrode potential of the Np^VI/Np^V couple by up to 220 mV (ΔΔG = −22.2 kJ mol⁻¹). Understanding the solvation-dependent interplay between electrolyte cations and metal–oxo species opens an avenue for controlling the formation and redox properties of metal complexes in solution. It also provides valuable mechanistic insights into actinide separation processes that widely use quaternary ammonium cations as extractants or in room temperature ionic liquids.

The electrochemical behaviour of redox-active metal cations foremost depends on the metal centre’s inner-sphere coordination environment. Here the authors show that electrolyte cations unexpectedly stabilize higher-oxidation-state neptunium dioxocations in water through extended-coordination sphere interactions.

Rare-earth metal and actinide organoimide chemistry

Elaborate synthesis schemes pave the way to f-element and group 3 complexes with multiply bonded imido ligands displaying intriguing reactivity.

Dihydroxo-bridged diuranyl(vi) complexes with 2-((2-(6-chloropyridazin-3-yl)hydrazono)methyl)-4-R-phenols: structural insights and visible light driven photocatalytic activities

Complexes of the {(UO₂)₂(μ-OH)₂}²⁺ core with N,N,O-donor 2-((2-(6-chloropyridazin-3-yl)hydrazono)methyl)-4-R-phenolates and their visible light-induced photocatalytic organic dye degradation abilities are reported.

Assessing the accuracy of simplified coupled cluster methods for electronic excited states in f0 actinide compounds

We scrutinize the performance of different variants of equation of motion coupled cluster (EOM-CC) methods to predict electronic excitation energies and excited state potential energy surfaces in closed-shell actinide species.

Elucidating cation–cation interactions in neptunyl dications using multi-reference ab initio theory

Understanding the binding mechanism in neptunyl clusters formed due to cation–cation interactions is of crucial importance in nuclear waste reprocessing and related areas of research.

High-Pressure Synthesis, Structural, and Spectroscopic Studies of the Ni-U-O System

The first comprehensive structural study of the Ni-U-O system is reported. Single crystals of α-NiUO₄, β-NiUO₄, and NiU₃O₁₀ were synthesized, and their structures were refined-using synchrotron single-crystal X-ray diffraction data supported by X-ray absorption spectroscopic measurements. α-NiUO₄ adopts an orthorhombic structure in space group Pbcn and is isostructural to CrUO₄ containing corrugated two-dimensional (2D) layers of corner-sharing UO₆ polyhedra and edge-sharing one-dimensional (1D) zigzag α-PbO₂ rutile-like chains of NiO₆ polyhedra in the [001] direction. β-NiUO₄ is isostructural to MgUO₄ and has an orthorhombic structure in space group Ibmm, which contains alternating 1D chains of edge-sharing UO₆ and NiO₆ polyhedra in the [001] direction as in regular TiO₂ rutile. NiU₃O₁₀ forms a triclinic structure in space group P1̅ and is isostructural with CuU₃O₁₀, where it forms a three-dimensional (3D) framework structure built through a mixture of UO₆ and UO₇ polyhedra in which the NiO₆ polyhedra sit isolated within the framework. X-ray absorption near-edge structure (XANES) measurements, conducted using XANES mapping of single crystals, support the presence of hexavalent uranium in the three structures. The polymorphs of NiUO₄ were found to only form under high-pressure and high-temperature conditions (≥4 GPa and 700 °C). It is argued that this is a consequence of the relative size difference between the Ni²⁺ and U⁶⁺ cations, where the Ni²⁺ cation is effectively too small for the Ibmm structure and too large for the Pbcn structure to form under ambient pressure conditions. This does not appear to be an issue for NiU₃O₁₀, which forms under ambient pressure conditions, where NiO₆ polyhedra sit isolated within the framework of 3D connected UO₆/UO₇ polyhedra. Synthesis conditions indicate that β-NiUO₄ is the preferred higher-pressure phase and that the transformation to this occurs irreversibly at a temperature between 950 and 1000 °C, when P = 4 GPa. The routes toward the synthesis of the oxides and the associated structural and spectroscopic results are described with respect to the structural chemistry of the Ni-U-O system, the larger AUO₄ family of oxides (A = divalent or trivalent cation), and also their relation to the rutile-related family of oxides.

Ab Initio Study of Covalency in the Ground versus Core-Excited States and X-ray Absorption Spectra of Actinide Complexes

Relativistic multireference ab initio wave function calculations within the restricted active space (RAS) framework were performed to calculate metal and ligand X-ray absorption (XAS) near-edge spectroscopy (XANES) intensities for the metal M_4,5 edges of [PuO₂(H₂O)₅]²⁺, [An^VIO₂]²⁺ (An = U, Np, Pu), and [AmCl₆]^3- and the Cl K edge of the Am complex. The extent of An(5f)-ligand bonding was determined via natural localized molecular orbital analyses of the relevant spin-orbit coupled multireference states. The calculated spectra are in good agreement with experiments and allow a detailed assignment of the observed spectral features. The XANES M_4,5-edge spectra are representative of the actinide orbital covalency in the probed core-excited states, which may be different from the ground-state covalency. An assignment of ground-state An orbital covalency based on XAS spectra should therefore be made with caution.

Elucidation of the inverse trans influence in uranyl and its imido and carbene analogues via quantum chemical simulation

The inverse trans influence (ITI) is investigated in uranyl, UO22+, and its isoelectronic imido (U(NH)22+) and carbene (U(CH2)22+) analogues at the density functional and complete active space self consistent field levels of theory. The quantum theory of atoms in molecules is employed to quantify, for the first time, the effect of the ITI on covalent bond character and its relationship to bond lengths and complex stability.

Sensing Uranyl(VI) Ions by Coordination and Energy Transfer to a Luminescent Europium(III) Complex

The release of uranyl(VI) is a hazardous environmental issue, with limited ways to monitor accumulation in situ. Here, we present a method for the detection of uranyl(VI) ions through the utilization of a unique fluorescence energy transfer process to europium(III). Our system displays the first example of a "turn-on" europium(III) emission process with a small, water-soluble lanthanide complex triggered by uranyl(VI) ions.

Iron Vacancies Accommodate Uranyl Incorporation into Hematite

Radiotoxic uranium contamination in natural systems and nuclear waste containment can be sequestered by incorporation into naturally abundant iron (oxyhydr)oxides such as hematite (α-Fe₂O₃) during mineral growth. The stability and properties of the resulting uranium-doped material are impacted by the local coordination environment of incorporated uranium. While measurements of uranium coordination in hematite have been attempted using extended X-ray absorption fine structure (EXAFS) analysis, traditional shell-by-shell EXAFS fitting yields ambiguous results. We used hybrid functional ab initio molecular dynamics (AIMD) simulations for various defect configurations to generate synthetic EXAFS spectra which were combined with adsorbed uranyl spectra to fit experimental U L₃-edge EXAFS for U⁶⁺-doped hematite. We discovered that the hematite crystal structure accommodates a trans-dioxo uranyl-like configuration for U⁶⁺ that substitutes for structural Fe³⁺, which requires two partially protonated Fe vacancies situated at opposing corner-sharing sites. Surprisingly, the best match to experiment included significant proportions of vacancy configurations other than the minimum-energy configuration, pointing to the importance of incorporation mechanisms and kinetics in determining the state of an impurity incorporated into a host phase under low temperature hydrothermal conditions.

Impacts of Oxo Interactions on Np(V) Crown Ether Complexes

Intermolecular interactions between the oxo group of an actinyl cation and other metal cations (i.e., cation-cation interactions) are dependent on the strength of the actinyl bond. These cation-cation interactions are prominently observed for the neptunyl cation [Np(V)O₂]⁺ and are sufficiently stable enough to explore using a variety of chemical techniques. Herein, we investigate these intermolecular interactions in the neptunyl 18-crown-6 system, because this macrocyclic ligand provides both stable coordination and the proper sterics to engage the oxo group in bonding with both low-valent metal cations and neighboring neptunyl units. We report the structural and spectroscopic characterization of five neptunyl, [Np(V,VI)O₂]^+,2+, compounds: Np1a ([NpO₂(18-crown-6)]ClO₄), Np1b ([NpO₂(18-crown-6)]AuCl₄), Na-Np ([Np(V)O₂(18-crown-6)(Na(H₂O)(18-crown-6)][Np(VI)O₂Cl₄], Np-Np ([NpO₂(18-crown-6)](NpO₂Cl₂NO₃)], and Np-Cl (NpO₂Cl(H₂O)_1.75). Each of these compounds were prepared from the ambient reactions of Np(V) in HX (where X = Cl, NO₃) with the 18-crown-6 ether molecule. Structural information obtained from single-crystal X-ray diffraction data was paired with solid-state and solution Raman spectroscopy to provide information on the interaction of the neptunyl oxo atom with neighboring cations. Neptunyl (Np═O) bond lengths are not perturbed upon interaction with the Na⁺ cation (Na-Np), but elongation is observed upon formation of a neptunyl-neptunyl interaction (Np-Np). This is also the first structurally characterized isolated, molecular complex that contains a simple T-shaped neptunyl-neptunyl interaction. Raman spectroscopy indicates little perturbation to the neptunyl bond until the formation of the neptunyl-neptunyl motif, which also results in activation of the ν₃ asymmetric stretch. Additional spectroscopic studies indicated that the neptunyl 18-crown-6 inclusion complexes form in solution and persist in the presence of other low-valence cations.

Silyl-Phosphino-Carbene Complexes of Uranium(IV)

Unprecedented silyl-phosphino-carbene complexes of uranium(IV) are presented, where before all covalent actinide-carbon double bonds were stabilised by phosphorus(V) substituents or restricted to matrix isolation experiments. Conversion of [U(BIPMTMS )(Cl)(μ-Cl)2 Li(THF)2 ] (1, BIPMTMS =C(PPh2 NSiMe3 )2 ) into [U(BIPMTMS )(Cl){CH(Ph)(SiMe3 )}] (2), and addition of [Li{CH(SiMe3 )(PPh2 )}(THF)]/Me2 NCH2 CH2 NMe2 (TMEDA) gave [U{C(SiMe3 )(PPh2 )}(BIPMTMS )(μ-Cl)Li(TMEDA)(μ-TMEDA)0.5 ]2 (3) by α-hydrogen abstraction. Addition of 2,2,2-cryptand or two equivalents of 4-N,N-dimethylaminopyridine (DMAP) to 3 gave [U{C(SiMe3 )(PPh2 )}(BIPMTMS )(Cl)][Li(2,2,2-cryptand)] (4) or [U{C(SiMe3 )(PPh2 )}(BIPMTMS )(DMAP)2 ] (5). The characterisation data for 3-5 suggest that whilst there is evidence for 3-centre P-C-U π-bonding character, the U=C double bond component is dominant in each case. These U=C bonds are the closest to a true uranium alkylidene yet outside of matrix isolation experiments.

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.

Quantification of f-element covalency through analysis of the electron density: insights from simulation

The electronic structure of f-element compounds is complex due to a combination of relativistic effects, strong electron correlation and weak crystal field environments. However, a quantitative understanding of bonding in these compounds is becoming increasingly technologically relevant. Recently, bonding interpretations based on analyses of the physically observable electronic density have gained popularity and, in this Feature Article, the utility of such density-based approaches is demonstrated. Application of Bader's Quantum Theory of Atoms in Molecules (QTAIM) is shown to elucidate many properties including bonding trends, orbital overlap and energy degeneracy-driven covalency, oxidation state identification and bond stability, demonstrating the increasingly important role that simulation and analysis play in the area of f-element bond characterisation.

Bonding trends across the series of tricarbonato-actinyl anions [(AnO2)(CO3)3]4- (An = U-Cm): the plutonium turn

Actinyl-tricarbonato anions [(AnO₂)(CO₃)₃]^4- (An = U-Cm) in various environments were investigated using theoretical approaches of quantum-mechanics, molecular-mechanics and cluster-models. Cations and solvent molecules in the 2^nd coordination sphere affect the equatorial An←O_eq bonds more than the axial An[triple bond, length as m-dash]O_ax bonds. Common actinide contraction is found for calculated and experimental axial bond lengths of ₉₂U to ₉₄Pu, though no longer for ₉₄Pu to ₉₆Cm. The tendency of U to Pu forming actinyl(vi) species dwindles away toward Cm, which already features the preferred An^III/Ln^III oxidation state of the later actinides and all lanthanides. The well known change from d-type to typical U-Pu-Cm type and then to f-type behavior is labeled as the plutonium turn, a phenomenon that is caused by f-orbital energy-decrease and f-orbital localization with increase of both nuclear charge and oxidation state, and a non-linear variation of effective f-electron population across the actinide series. Both orbital and configuration mixing and occupation of antibonding 5f type orbitals increase, weakening the AnO_ax bonds and reducing the highest possible oxidation states of the later actinides.

Highly Sensitive Detection of UV Radiation Using a Uranium Coordination Polymer

The accurate detection of UV radiation is required in a wide range of chemical industries and environmental or biological related applications. Conventional methods taking advantage of semiconductor photodetectors suffer from several drawbacks such as sophisticated synthesis and manufacturing procedure, not being able to measure the accumulated UV dosage as well as high defect density in the material. Searching for new strategies or materials serving as precise UV dosage sensor with extremely low detection limit is still highly desirable. In this work, a radiation resistant uranium coordination polymer [UO₂(L)(DMF)] (L = 5-nitroisophthalic acid, DMF = N,N-dimethylformamide, denoted as compound 1) was successfully synthesized through mild solvothermal method and investigated as a unique UV probe with the detection limit of 2.4 × 10^-7 J. On the basis of the UV dosage dependent luminescence spectra, EPR analysis, single crystal structure investigation, and the DFT calculation, the UV-induced radical quenching mechanism was confirmed. Importantly, the generated radicals are of significant stability which offers the opportunity for measuring the accumulated UV radiation dosage. Furthermore, the powder material of compound 1 was further upgraded into membrane material without loss in luminescence intensity to investigate the real application potentials. To the best of our knowledge, compound 1 represents the most sensitive coordination polymer based UV dosage probe reported to date.

On the multi-reference nature of plutonium oxides: PuO22+, PuO2, PuO3 and PuO2(OH)2

Actinide-containing complexes present formidable challenges for electronic structure methods due to the large number of degenerate or quasi-degenerate electronic states arising from partially occupied 5f and 6d shells. Conventional multi-reference methods can treat active spaces that are often at the upper limit of what is required for a proper treatment of species with complex electronic structures, leaving no room for verifying their suitability. In this work we address the issue of properly defining the active spaces in such calculations, and introduce a protocol to determine optimal active spaces based on the use of the Density Matrix Renormalization Group algorithm and concepts of quantum information theory. We apply the protocol to elucidate the electronic structure and bonding mechanism of volatile plutonium oxides (PuO₃ and PuO₂(OH)₂), species associated with nuclear safety issues for which little is known about the electronic structure and energetics. We show how, within a scalar relativistic framework, orbital-pair correlations can be used to guide the definition of optimal active spaces which provide an accurate description of static/non-dynamic electron correlation, as well as to analyse the chemical bonding beyond a simple orbital model. From this bonding analysis we are able to show that the addition of oxo- or hydroxo-groups to the plutonium dioxide species considerably changes the π-bonding mechanism with respect to the bare triatomics, resulting in bent structures with a considerable multi-reference character.