ActinideLos Alamos National Research Laboratory Quarterly 1st quarter 2004


N u c l e a r M a t e r i a l s R e s e a r c h a n d T e c h n o l o g y Research Quarterly

ContentsActinide Research Quarterly

1 Heavy-element chemistry at Los Alamos

2 Researchers discover fl uorescence from a new class of actinyl transitions

7 Classic high-symmetry species

11 Exploring - interactions

15 Unusual tetraoxo coordination in heptavalent

20 The correlation between Raman and bond lengths

23 Developing a comprehensive picture of the chemical behavior of the 5f elements

28 orbitals

30 Lewis and bases

31 Electronic structure and bonding in 5f-element complexes

36 XAFS spectroscopy as a probe of actinide speciation

42 Structural characterization of compounds targeted in separation

46 Powder x-ray diffraction

47 Single-crystal x-ray diffraction

48 Unusual system shows potential to revolutionize technologies

52 Russian storage facility commissioned

Nuclear Materials Technology/Los Alamos National Laboratory Heavy-element chemistry at Los Alamos

he chemistry and physics of the actinide elements (, neptunium, plutonium, etc.) have always been of prime Timportance at Los Alamos, beginning with the early efforts to characterize the chemistry of plutonium during the . This continuing institutional investment in fundamental actinide science has provided the technical basis for process and separation chemistry and related to the national security mission of the Laboratory. As a result, Los Alamos has the expertise, capabilities, and facilities to carry out a wide range of chemical research with the actinide elements, including inorganic, organometallic, and -state synthesis; a wide variety of spectroscopic characterization techniques; and mod- ern electronic structure theory. The actinide series marks the emergence of 5f in the valence shell. In the pure elements, those to the left of plutonium have delocalized (bonding) electrons, while elements to the right of - nium have localized (nonbonding) electrons. Plutonium is trapped in the middle, and for the delta- metal, the electrons seem to be in a unique state of being neither fully delocalized nor localized, which leads to novel electronic interactions and unusual physical and chemical behavior. The concept of localized or delocalized 5f electrons also pervades the bonding descriptions of many of the actinide mol- ecules and compounds. In the normal nomenclature of chemistry, the delocalized electrons are those involved in covalent bonding, while the localized electrons give rise to ionic behavior. Understanding the nature of bonding in actinide materials remains a computational and experimental challenge. A full understanding of the nature of the in actinide systems must access the relative roles of all atomic orbitals in chemical bonding. The articles in this issue of Actinide Research Quarterly describe the Los Alamos approach to understanding covalency in actinide and materials. This strategy combines synthetic chemistry, spectroscopic characterization, and theory and modeling to understand and predict the chemical and physical properties of actinide materials. This multidisciplinary approach is the strength of the Los Alamos heavy-element chemistry discipline and provides the scientifi c means to formulate rational approaches to solve complex actinide problems in a wide variety of environments. David L. Clark Gordon D. Jarvinen

The actinide series.

NuclearNuclear Materials Technology/Technology/LosLos AlamosAlamos NationalNational LaboratoryLaboratory 1 Actinide Research Quarterly

Uncharted scientifi c territory Researchers discover fl uorescence from a new class of actinyl transitions

he fl uorescence observed following the interaction of light with actinide species has fascinated people for mil- Tlennia, in large part because of its visually striking effect. An artifact of yellow-colored, uranium-doped was found near Naples, , and determined to have been made in 79 A.D. Early 19th century European glassmakers added small amounts of uranium to their glass formulations as a coloring agent to make Canary-glass objects in which the appearance is clearly enhanced by the characteristic yellow-green fl uorescence. As recently as the late Early 19th century 1930s, uranium were added to the glazes employed to deco- European glassmakers rate the brilliant red dinnerware developed by Fiestaware. Although added small amounts the production of uranium glass in the United States was halted in of uranium to their 1942, the marvelous colors of these uranium-containing objets d’ formulations as a continue to appeal to enthusiasts today. coloring agent to make Canary-glass objects. Intrigue A uranium-glass candy The historical importance of fl uorescence is not only dish is shown before artistic, but also scientifi c as well. The Canary-glass objects popular illumination (top left) and after illumination in the 19th century attracted the interest of the Scottish scholar Sir (center left) with David Brewster, who mentioned the fi rst scientifi c observation of light. The visually appealing green fl uorescence of uranium-containing glass Canary-glass Bohemian in 1849. The term for this fundamental phenomenon, fl uorescence, perfume bottle (bottom was later introduced by Sir George Gabriel Stokes in 1852 in his left) dates from the paper titled “The Change in the Refrangibility (wavelength) of mid-19th century. As Light.” In this historic work, he described fl uorescence from recently as the 1930s, uranium oxide analytes: uranium oxides were added to the glazes “The intervals between the absorption bands of green uranite were used to create the nearly equal to the intervals between the bright bands of which brilliant red color of dinnerware developed the derived spectrum consisted in the case of yellow uranite. After by Fiestaware, such having seen both systems, I could not fail to be impressed with the as the cup and saucer conviction of a most intimate connexion [sic] between the causes shown below. of the two phenomena, unconnected as at fi rst sight they might Photos by Mick Greenbank appear. The more I examined the compounds This article was of uranium, the more this conviction was contributed by Los strengthened in my mind.” Alamos researchers Marianne Wilkerson This work contributed to the rule that emit- and Harry J. Dewey ted light is always of longer wavelength than of the Chemistry that of the exciting light, a principle that is now Division and John Berg of the called Stokes’ Law. In modern years, fl uores- Nuclear Materials cence from compounds containing uranyl Technology Division. has attracted widespread attention as a tool for understanding the complex electronic structure of actinide molecules, characterizing speciation

2 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004

of uranium materials, and detecting the presence of uranium. Although investigations of fl uorescence from other actinide species are not as extensive, reports from Russian scientists suggest that fl uorescence from neptunium analytes is quite sensitive. However, the molecular composition of their neptunium samples is unknown.

Discovery Researchers at Los Alamos have discovered the fi rst example of Laser-induced fl uorescence from an actinyl species other than uranyl. A team led by fl uorescence Marianne Wilkerson and Harry Dewey of the Chemistry Division and spectroscopy is John Berg of the Nuclear Materials Technology Division has established used to probe the from a a capability at the Integrated Spectroscopy Laboratory to detect near- sample containing the electronic structure of infrared photons emitted by actinide compounds following visible neptunyl compounds. neptunyl tetrachloride laser excitation. The spectra arise from a 5f-5f transition, opening up the John Berg collects can be induced fl uorescence data from ability to explore the electronic structure of actinide compounds in the using excitation from a sample containing near-infrared region of the electromagnetic spectrum by using sensitive a simple neon and selective spectroscopic capabilities. This research is part of a Basic laser source. Cs2U(Np)O2Cl4 while Marianne Wilkerson Energy Sciences (DOE BES) program to inves- adjusts the sample. tigate chemical behavior of heavy elements.

Actinide challenges and limitations Many innate chemical properties of mole- cules are a consequence of electronic structure, or the manner in which electrons are arranged in orbitals around bonding , and the electronic structures of molecules containing actinide elements are especially intriguing and challenging to understand. The large radii and lack of strongly directional bonding of actinide atoms contribute to a large variety of molecu- lar arrangements. Only actinide-containing molecules can house electrons in the 5f groupgroup of orbitals, and the large number of orbitals in this 5f groupgroup can interact with other avail- able molecular orbitals, generating intricate electronic structures. Optical spectroscopy is a powerful tool for probing electronic structures of molecules, but the spectral signatures of

Nuclear Materials Technology/Los Alamos National Laboratory 3 Actinide Research Quarterly

The neptunyl tetrachloride ion actinide compounds are complicated due to extensive numbers of spectral lines. As a re- The neptunium compound chosen to initiate this fl uorescence sult, correlations between the molecular struc- investigation is Cs NpO Cl . The structure of this relatively simple 2 2 4 tures and optical spectra of actinide molecules consists of a neptunium coordinated in a pseudo- octahedral fashion by two apical oxo groups and four equatorial remain diffi cult to predict or even describe. 2- such that the NpO2Cl4 anion is centrosymmetric and has approximate D4h symmetry. The charge of the dianion is balanced Opportunities by two cesium cations. This class of molecular is particularly To date, a large number of the spectro- amenable to fl uorescence studies because the preparation of scopic studies of actinide compounds have Cs2NpO2Cl4 is easily carried out in air, the simple chloride ligands lack any intraligand vibrational modes that could give rise to complicating focused on understanding the electronic vibronic structure or fl uorescence quenching, and the lack of water structure and optical spectra of uranyl, a in the crystalline lattice eliminates another potential source of ubiquitous anion consisting of a metal that fl uorescence quenching. is coordinated by two ligands in a trans fashion. The electronic spectra of uranyl Dilution, or doping, of a fl uorescent ion or molecule into a host crystal is often necessary to minimize self-quenching and to offer some compounds are characterized by a particular degree of control over the local environment. Model host material type of interaction in which an is ex- should have sites of appropriate size for accommodating the dopant, cited from an orbital that is mostly associated be chemically unreactive with the dopant, and lack any high-frequency with the oxygen atoms to an orbital that is vibrational modes that could serve as acceptors for radiationless more representative of the central metal atom. deactivation. Cs2UO2Cl4 is an ideal host for inclusion of Cs2NpO2Cl4 because it is isostructural and only slightly larger in size, and its These so-called ligand-to-metal charge- spectroscopic signature transfer (LMCT) transitions are readily offers a large optical window detected using fl uorescence, which in some throughout the near-infrared cases is visible to the unaided eye in the mes- into most of the visible merizing yellow and green colors of uranium spectrum. Furthermore, there is a fortuitous wealth oxide compounds. of supporting information Efforts toward understanding the elec-

on both Cs2UO2Cl4 and tronic structures of all other actinide species, Cs2NpO2Cl4 obtained however, are much less common. One compli- from complementary cating factor is that there is an additional class spectroscopic methods. of transitions available to these other species: transitions that correspond to promotion of an electron from one state of predominantly 5f metal orbital character to another state of pri- marily 5f metal orbital character.character. As suggested Researchers used the neptunium earlier, the number of possible excitations compound Cs2NpO2Cl4 in their involving 5f electronselectrons complicates theoreticaltheoretical fl uorescence investigation. Neptunium is treatment. Furthermore, fl uorescence transi- green, oxygen is red, is black, tions involving electrons in 5f orbitals will not and cesium is blue. be found in the easily detected visible region of the optical spectrum, but rather in the near- infrared region, where detection technology has been much less advanced until recently.

4 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004

The discovery that fl uorescence from the neptunyl ion is observable, 2+ particularly near-infrared fl uorescence, opens uncharted scientifi c ter- ritory for the study of a signifi cant class of transitions available to other stable forms of uranium, neptunium, plutonium, and . The O signatures mapped out by 5f-5f transitions areare relativelyrelatively simpler than those of LMCT transitions with regard to the numbers of electronic Understanding the M states and potential relaxation pathways. The ability to probe these electronic structures transitions with molecular fl uorescence will enrich our understand- of actinide species ing of the fundamental properties of 5f orbitals, such as their rolerole in other than uranyl O actinide bonding, transition probabilities as a function of temperature, is complicated by and the lifetimes of excited states. the fact that there is Many spectroscopic an additional class studies of actinide Applications of transitions—the compounds Researchers are attempting to develop near-infrared fl uorescence 5f-5f transition— have focused on to complement a variety of other techniques for a Laboratory Directed available to these other species. In this understanding the Research and Development-Directed Research (LDRD-DR) project electronic structure type of transition, an aimed at a fundamental scientifi c understanding of the interaction of and optical spectra electron is promoted of the uranyl ion, a actinide molecules with oxide minerals within the solid ()/ from one state cation consisting water interface. By coupling these experiments with transport models of predominantly of a metal that is and other techniques, researchers hope to develop a better understand- 5f metal orbital coordinated by two ing of interfacial reactions of in the environment. character to another oxygen ligands in Collective results suggest that neptunium fl uorescence may have state of primarily a trans fashion. practical application as a detection capability. Russian researchers 5f metal orbital This is the structure claimed more than ten years ago that a neptunium as character. This of an actinyl ion low as a few picograms could be achieved through fl uorescence detec- fi gure shows the (M = uranium, tion in the red region of the visible spectrum. New data suggests that transitions available to the neptunyl ion. neptunium, this region may not be where even the most intense signal plutonium, or should occur. americium). The electronic The Los Alamos team has discovered that the spectra of uranyl neptunyl ion fl uoresces with reasonable effi ciency compounds, which in the near-infrared not only at liquid- are characterized temperature, but more signifi cantly at room Np 6d by so-called ligand- temperature. Furthermore, fl uorescence is readily to-metal charge- achieved using excitation by red light of a simple transfer transitions, helium neon laser or diode lasers common in Np 5f are detected using consumer electronic devices. Specifi c optical transi- fl uorescence, but tions also can be identifi ed by their characteristic some of them energy and by an additional identifying tag, the are visible to the length of time that an electron resides in an excited unaided eye, as O 2p seen in the yellow state before the excitation decays in a fl uorescence and green colors of transition. Measurements by Los Alamos research- uranium glass. ers reveal that low-lying excited states of neptunyl may have reasonably long lifetimes, comparable to Np 6p those of uranyl species.

Nuclear Materials Technology/Los Alamos National Laboratory 5 Actinide Research Quarterly

How does a molecule interact with a photon of light?

Every molecule has a characteristic set of electronic states, each of which is defi ned by a particular arrangement of the molecule’s electrons into molecular orbitals. These electronic states lie at different energies. Most of the time a molecule resides in the state of the lowest energy, known as its ground state, but there are accessible higher-energy states known as excited states. One way Future that a molecule may be transformed into an excited state is by It had been presumed that fl uorescence, absorbing a photon of light whose energy matches the difference the workhorse technique for understanding in energy between the ground state and an excited state of the uranyl LMCT transitions, was unemployable molecule. When this absorption occurs, the electrons rearrange to for observing the 5f excitations that areare much form the excited state. A plot of the tendency to absorb light versus more common in all other dioxo actinide spe- the photon energy is known as an absorption spectrum and exhibits peaks that correspond to energy differences between the ground cies. The team’s discovery of 5f-5f fl uoruorescenceescence and excited states. from the neptunyl ion has revealed a new and exciting tool for future fundamental studies of Because a molecule is inherently unstable in an excited state, it electronic structure and applied development tends to return to the ground state by any of several pathways. The of detection technologies. length of time that the molecule resides in an excited state before it decays to a lower-energy state is termed the excited state lifetime. One of the decay pathways is by emission of a photon in a process known as luminescence, a word coined from the Latin word for light—lumen. Luminescence is a relatively rare decay pathway for excited states of most molecules, but if it occurs, it greatly eases the detection and study of excited states, making its discovery in a neptunium-containing molecule particularly interesting.

The characteristic vibrations of the molecule may also be excited by either absorption or luminescent processes. Much of the complex structure in an absorption spectrum is due to excitation of one or more quanta of vibrational energy in addition to the change of

electronic states. Relative Intensity

y 6000 6400 6800 Energy (cm-1)

Recent advances in detection technology gy have made it possible to observe fl uorescence

Relative Absorption Intensit transitions in the near-infrared region, as shown Energy in this fl uorescence spectrum of Cs2NpO2Cl4, the compound used by Los Alamos researchers in the Relative Ener early part of their investigation. Relative Emission Intensity Bond Length Energy

The absorption versus luminescent process is defi ned in this illustration. The red curve represents the potential energy of a lower-lying electronic state such as the ground state and the blue curve represents the potential energy of an excited electronic state. The black levels on each curve symbolize the energy of levels of a vibrational mode. When an absorption process occurs, a photon of appropriate energy is absorbed by a molecule, promoting the molecule to a vibrational level of a higher- lying electronic state. Conversely, when a luminescent process occurs, a photon is emitted by a molecule in an excited state, allowing it to relax to a vibrational level of a lower-lying state.

6 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004

Homoleptic nine-coordinate An(III) and Ln(III) complexescomplexes Classic high-symmetryhigh-symmetry species that advanceadvance our understandingunderstanding of f-element structure,structure, bonding, and dynamicsdynamics

nderstanding the behavior of actinide and ions in water and other solvent media remainsremains one of the grgreateat Uexperimental and theoreticaltheoretical challenges in f-element chemis- try.try. The behavior of actinide ions in aqueous solution is highly relevantrelevant This article was to issues in processing,processing, concerns about actinide-ion contributed by Los transport in groundwater,groundwater, and hydrologicalhydrological implications of spent fuel Alamos researcher storage strategies. The strongstrong Lewis acidity of the actinide ions leads to Mary P. Neu of the complex, highly pH-dependent equilibria in aqueous media. (Lewis Chemistry Division acids areare substances with a strongstrong affi nity for accepting electronelectron pairs, and Jason L. Sonnenberg and but which do not necessarily directlydirectly afaffectfect hydrogenhydrogen ion concentration). Bruce E. Bursten of Understanding the naturenature of f-element– the Department of solvent interactions is also important for fuel Chemistry, The Ohio cycle processesprocesses such as actinide/lanthanide State University. separations and requirerequire an understanding of the intimate coordinationcoordination spheresphere of solvent molecules about actinide and lanthanide ions, of the interactions of the primary coordinationcoordination complexes with the bulk solvent that surround them, and of the dynamical processes between the coordinated and bulk solvent molecules. Successful modeling of the behavior of lanthanide and actinide ions in solution begins with a detailed understanding of the direct interactions between the ions and solvent molecules as ligands. The level of understand- ing necessary for such modeling can greatly benefi t from studying the bonding, electronic structure, spectroscopy, and chemical proper- This thermal ellipsoid representation shows the ideal ties of well-defi ned coordination complexes tricapped, trigonal-prismatic structure of the Pu(III) of the early actinides. Given the importance aquo complex (the trifl ate anion is not shown). The six of isolating the important ion-solvent interac- symmetry-equivalent prismatic oxygen ligands (red) tion, the strategy of using noncoordinating defi ne the vertices of the trigonal . The three or very weakly coordinating counterions was capping oxygen ligands (blue) are positioned outside adopted; exemplary is the trifl uoromethane the rectangular faces of the prism. The plutonium atom sulfonate anion (CF SO –, commonly (green) is in the center. 3 3 called trifl ate). Trifl ate salts of Pu(III) and Am(III) aquo

complexes—[An(OH2)9][OTf]3—were prepared, inin wwhichhich tthehe ooxygenxygen aatomstoms of the nine-coordinated water molecules adopt a highly symmetric tricapped, trigonal-prismatic (TCTP) geometry about the Pu3+ and Am3+ ions. Lanthanide trifl ates having the same compositions and analogous geometries are well characterized, allowing for a direct comparison between the structure and bonding in An(III) and Ln(III) complexes.

Nuclear Materials Technology/Los Alamos National Laboratory 7 Actinide Research Quarterly

High-Symmetry Species

Because the valence electronic structure of ions requires relativistic quantum chemical actinide ions are dominated by atomic 5f techniques. and 6d orbitals, whereas that of the lantha- Once the primary coordination sphere nide ions involves the 4f and 5d orbitals, (fi rst layer of solvent molecules surround- the comparison of these two classes of com- ing the central dissolved ion) is correctly plexes might provide valuable insight into described, molecular dynamics will be how electronic structure affects geometric used to model the interaction of the fi rst- structure and reactivity. coordination-sphere complex with the The TCTP structure has six symmetry- bulk solvent. The crystalline experimental equivalent prismatic ligands at the vertices complexes, which isolate the fi rst coordina- of a trigonal prism. The remaining three tion sphere, thus provide an opportunity to Theoretical studies ligands are capping ligands positioned out- test the quantum chemical description of the have shown that the side the rectangular metal-ligand interac- vacant 6d atomic faces of the trigonal tions. In the present orbitals of the prism. The capping studies, actinide atom (in this ligands are equiva- functional theory case plutonium) are lent to one another (DFT) has been used the principal sites by symmetry, but to perform this of ligand electron are unique from assessment. Scalar acceptance. This contour plot of the six prismatic relativistic effects 3+ ligands. As such, the were included via Pu(H2O)9 shows the water ligands metal-ligand dis- the zero-order regu- donating primarily tances for prismatic lar approximation into the plutonium and capping ligands (ZORA) method. 6d orbitals. can be different as The structure of 3+ demonstrated in isolated Pu(H2O)9 the Pu(III) aquo was calculated using structure, which relativistic DFT with

contains Pu-OH2 the ion constrained bond distances of 2.574(3) Å for the capping to a rigorous TCTP geometry. These waters and 2.476(2) Å for the prismatic calculations will suffi ce for discussing the

waters. Bond distances for the Am(III) com- σ donation from the H2O ligands into ap- plex are slightly shorter, correlating well propriate acceptor orbitals on the plutonium with the decrease in across the center. The calculated Pu-O bond distances

actinide series. (Pu–Ocap = 2.619 Å and Pu–Opris = 2.540 Å) We are engaged in computational mod- areare slightly longer than those observed in elling of these actinide species in solution. the . Interactions involving

Our theoretical studies of solvated f-element the second of each H2O ligand will ions (those chemically bound to solvent involve subtle Pu–O π effects. π bonding de- molecules such as water) will ultimately use scribes a particular type of overlap between a combination of and the atomic orbitals of bonding electrons, molecular dynamics. The electronic struc- differing from the more symmetric σ elec- tural description of the direct interactions tron-density overlap. of the solvent molecules with the f-element

8 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004

It is important to note that the calcula- different spatial extents of the atomic orbitals

tions predict correctly that the Pu–Ocap between the two rows of the .

distance is longer than the Pu–Opris distance. From experimental studies, we can com- Relaxing the symmetry to allow the water pare the plutonium and americium aquo molecules greater rotational freedom, which structures with those for of will allow them to maximize their π bond- similar ionic radii, Nd(III) and Sm(III). The ing to the Pu3+ center, could lead to some capping waters have nearly identical bond shortening of the Pu-O bonds and improve the agreement with the observed crystal structure. Pu + 3AgPF6 MeCN [Pu(MeCN)9][PF6]3 + 3Ag Overall, there is clearly good agreement between experiment and theory on these complex systems under the constraints of high symmetry and without specifi c consideration of anion effects. 3+ The calculations on Pu(H2O)9 provide insights into the nature

of the Pu-OH2 bonding. As expected, the water ligands act as electron-pair donors (Lewis bases) that do- lengths, while the An(III) prismatic waters To study structure 3+ nate electron density to the Pu ion, which are approximately 0.03 Å shorter than those and bonding serves as an electron acceptor (Lewis ). of the Ln(III) with the same ionic radii. properties Our previous theoretical studies of organo- Electronic structure calculations aimed at of actinides, actinide complexes demonstrated that the explaining these differences are under way. researchers dissolve vacant 6d atomic orbitals of the actinide To further explore the structure and Pu-239 metal strips atom served as the principal sites of ligand bonding properties, we have recently (shown in the vial electron acceptance, whereas the 5f orbitals prepared nonaqueous trivalent actinide above left) or metal were used to house any metal-based elec- complexes that are nearly isostructural with powder in water trons. The same bonding paradigms the aquo complexes and contain weaker or and place them under an appear to be present in Pu(H O) 3+. The σ-donor ligands coordinated to the metal. 2 9 argon atmosphere. water ligands donate primarily into the For example, [Pu(NCMe) ][PF ] •MeCN 9 6 3 The resulting solids plutonium 6d orbitals. was prepared by treating an acetonitrile (shown in the photo Because a neutral plutonium atom has suspension of Pu-239 metal turnings under at right and in this eight valence electrons, the Pu3+ ion is argon atmosphere with three equivalents case plutonium

expected to have fi ve metal-based electrons. of either AgPF6 or TlPF6. Compared with strips dissolved Our research indicates that this is indeed the Pu–O in the aquo complex, the average in acetonitrile) the case and that those electrons reside Pu–N distance (2.572 Å) is 0.047 Å longer. are analyzed in nearly pure plutonium 5f orbitals. This The geometry of the acetonitrile complex is by a number of dichotomy in the roles of the actinide 5f distorted rather than ideal TCTP, perhaps methods, including and 6d orbitals will change somewhat for because the PF – ion is more strongly coordi- single-crystal x-ray 6 diffraction. lanthanide complexes because of the nating than the trifl ate anion.

Nuclear Materials Technology/Los Alamos National Laboratory 9 Actinide Research Quarterly

High-Symmetry Species

Diffuse refl ectance Compared to nine-coordinate acetonitrile spectra obtained on Ln(III) tricapped trigonal prismatic adducts ground crystals of (Ln = La, Sm, and Pr), the three capping Ln–N the solids and optical distances are on average slightly shorter than absorbance spectra of the six prismatic ones in the crystal structures

the solutions for the of La(∆avg = 0.016 Å) and Pr (∆avg = 0.004 Å).

aquo and acetonitrile This trendtrend reversesreverses in the Sm complex (∆avg = complexes are nearly 0.008 Å). However,However, the difdifferenceference between the superimposable, average prismatic and average capping dis- 3+ but each feature in tance in a particular Ln(NCMe)9 compound the spectrum of the is small. acetonitrile complex Published DFT calculations of the nine- is red-shifted by coordinate acetonitrile Ln3+ (Ln = Eu, Yb, and approximately La) solvates predicted that the three capping 3+ 15 nm, providing distances of Ln(NCMe)9 would be longer spectroscopic than the six prismatic distances. Qualitatively, evidence of the the uranium complex follows the trend difference in bond predicted by DFT, but the plutonium complex strengths. follows the trend of the experimentally deter- In contrast to the mined lanthanide structures. plutonium complex, We will continue to combine experimental Optical absorbance and diffuse refl ectance spectra the uranium aceto- and theoretical methods to explore the rich suggest that Pu3+ has veryvery similar coordination geometries in the solution and solid states of nitrile adduct we pre- chemistry of f-element complexes in solution. the acetonitrile and aquo complexes. This in turn pared displays nearly Such judicious combination of synthetic and suggests that the ion is nine-coordinate in solid and ideal TCTP geometry. with modern computa- solution forms and in both solvent environments. The six prismatic tional modelling provides understanding of The acetonitrile spectra are shifted by about 15 nm U–N distances are a very complex and relevant topic in current relative to the aquo spectra, refl ecting the differing 2.60 Å. The difference f-element chemistry. Lewis basicity of the ligands. The top two spectra between the aver- This research is supported by the DOE are diffuse refl ectance spectra of the solids. The age prismatic M–N Offi ce of Basic Energy Sciences. bottom two spectra are the absorbance spectra of distances of Pu and U the solutions. complexes is 0.026 Å, which is close to the 0.025 Å difference in the atomic radii of U and Pu. The three 2.65 Å capping U–N interactions in the U complex are longer than the prismatic interactions by 0.05 Å, and 0.087 Å longer than the average Pu–N capping interactions in the plutonium acetonitrile complex. This U–N capping distance is 0.062 Å longer than that predicted by atomic radii differences between U(III) and Pu(III).

10 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004 Capability complements experimental probes Exploring metal-ligand interactions in actinide complexes using density functional calculations

odern techniques can provide insights This article was into electronic properties of actinide complexes such as the contributed by Los nature of the chemical bonding, the structures and relative Alamos researchers M P. Jeffrey Hay and energies of related compounds, and the nature of electronic excited states. The results are obtained from quantum mechanical calculations Richard L. Martin of the Theoretical that treat the interactions of the electrons with the nuclei in the mol- Division. ecule. From the solution of the molecular Schröedinger equation, wave functions can be obtained that describe the electrons, and, in turn, the Calculated structures overall electronic density in the molecule. 2+ for [UO2(H2O)5] (left) The ability to perform reliable calculations on actinide species is a + and [(NpO2)(crown)])(crown)] result of several developments over many years, including theo- (right) complexes from retical methods and increasing computational capabilities. In density functional theory particular, recent advances in density functional theory (DFT) calculations are shown have made possible calculations at right. The fi gure on large chemical on the left is a typical systems compared example of an actinyl species comprised to conventional of a linear bonding quantum chemistry linkage (the central approaches. O–U–O molecule) that Another devel- is coordinated with fi ve opment involves molecules of water techniques to treat bound to the actinide just the valence atom in the central electrons (those in the plane (U=blue, O=red, outermost “shells” of and H=white). The fi gure atoms that are most [UO (H O) ]2+ + on the right shows [UO2(H2 O)5 ]2+ [NpO[NpO2(cr(crown-6)]own-6)] + likely to participate 2 2 5 2 a recently identifi ed organic ligand known in the formation as a —the of chemical bonds) in molecules containing heavy elements such as structure encircling the actinides. In addition, these approaches simultaneously incorporate the neptunium molecule effects of relativity on the valence electrons, since relativistic effects are (Np=blue, O=red, crucial in this region of the periodic table. C=grey, and H=white). Oxo complexes as testbeds Among the most prevalent motifs in the chemistry of higher-valent actinide species are actinyl species comprised of a linear bonding 2+ 2+ linkage [O=U=O] as in the uranyl (UO2 ) species that generally coordinates to other molecules (ligands) by forming bonds around a 2+ central plane about the UO2 unit. Typical examples are the aquo com- 2+ plex [UO2(H2O)5] and its Np and Pu counterparts that represent the dominant species found in aqueous solution at low pH for the +5 and +6 oxidation states of the actinides. Recent calculations have examined how the molecular structures and calculated vibrational frequencies of the O=An=O2+ unit systematically vary across the actinides from U to Np to Pu.

Nuclear Materials Technology/Los Alamos National Laboratory 11 Actinide Research Quarterly

Metal-Ligand Interactions

These two organoactinide species illustrate the longer than observed by XAFS role density functional theory plays in (2.42 Å). understanding In addition, solvent effects, bonding in the which are crucial for predicting actinides. The top reaction energies in solution, left fi gure is the can be included by an effective calculated structure medium model that includes a of the U(VI) complex polarizable dielectric medium Cp* U(NPh) , which 2 2 surrounding the molecule (for has no unpaired example, water or an aqueous electrons. The top solution of chloride). right fi gure is the One of the challenges in calculated structure of the U(IV) complex of Cp* U(CH -N-N- actinides has been to character- 2 3 ize and selectively remove such CR2)2, which has two unpaired 5 f species from waste streams, electrons. The lower in chemical processing, or left fi gure is a contour in geochemical formations. plot of one of the Our synthetic chemistry col- valence orbitals in leagues at Los Alamos recently Cp* U(NPh) . The 2 2 identifi ed an organic ligand lower right fi gure known as a crown ether that is a contour plot binds to the Np(V) oxidation of a similar orbital in Cp* U(CH -N- state of neptunyl to form a 2 3 + [(NpO2)(crown)] complex. N-CR2)2. The blue and red indicate (See ARQ 3rd quarter 1998.) positive and The calculated structure negative contours, from density functional respectively. The calculations is in good agree- larger amount of ment with the crystal structure participation by the obtained experimentally. The U atom in the These predictions can be compared with calculations give a good prediction of the bonding for the Raman frequency for the O=Np=O vibration complex on the data from experimental probes in solution (776 cm-1 calculated, 780 cm-1 experimental), left is indicated such as XAFS and . Both by the relatively theory and experiment give the same values the predicted Np–O bond length (1.81 Å cal- larger proportion for the axial An=O bond length in this series culated, 1.80 Å experimental), as well as other of contours on the in going from U (1.76 Å) to Np (1.75 Å) to Pu structural features. metal relative to the (1.74 Å). The calculated Raman frequencies The only other analogous compound 2+ 2+ N-containing ligands. of the O=An=O unit are decreasing from synthesized to date contains the UO2 . By contrast, for the 908–805 cm-1, which may be compared with Calculations have been performed for the complex on the right, the observed experimental range of 872– entire series of complexes for both +5 and a larger proportion of 835 cm-1 across the same series. The calculated +6 oxidations states of U, Np, and Pu for the contours is found q+ bond length between the actinide and the the [AnO2(crown)] species. For most cases on the N-containing the calculations represent predictions where ligands. equatorial oxygen atoms (2.52 Å) is somewhat

12 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004

experimental species have not been isolated to examine the nature of the bonding, geometrical date. This illustrates how theory can comple- structure, and of these com- ment experimental probes to help identify plexes. It is often useful to examine the spatial and characterize actinide species of potential nature and relative energies of the molecular environmental interest that may be diffi cult to orbitals to understand chemical bonding as isolate or study experimentally. well as to explain electronic spectroscopy. In

the Cp*2U(NPh)2 complex, the energy levels Understanding bonding and reactivity of the ligands themselves are stabilized by Among the novel features of actinide chem- interactions with both 6d and 5f orbitals from istry is the ability of these complexes to involve the U. The energy levels correspond physically both 5f and 6d orbitals in forming chemical to the binding energy needed to remove or bonds. In the actinide series, proceeding across excite an electron at a particular energy, since the periodic table entails formally fi lling up in quantum theory, the molecular energy levels the 5f shell. The neighboring 6d shell is also have discrete values. energetically accessible, and this shell This schematic even lies lower in energy than the diagram of Schematic levels -- Cp* U (NPh) 5f shell in the early actinides. The princi- 2 2 Cp*2U(NPh)2 shows pal means by which theory can provide the interaction of ligand orbitals (on an understanding of the bonding in this Visible, uv the left) with 6d and actinide series comes from examination excitation 6d 5 f orbitals of U (on of the molecular orbitals. These are the right) to form the the one-electron wave functions for molecular orbitals in 5f individual electrons in the molecule in the U complex (in the the DFT calculations, from which the middle). The vertical total electron density for the molecule scale corresponds to ultimately is obtained. the energy needed to Two representative organoactinide remove an electron species illustrate the role of theory: the from a particular fi lled Increasing binding energy orbital (electrons bis(imido) U(VI) complex Cp*2U(NPh)2 and the bis(hydrazonato) U(IV) complex are indicated by arrows) and excite it Cp* U(CH -N-N-CR ) . In the calcula- 6p 2 3 2 2 to an empty orbital, tions, a Cp group (C H ) was used to 5 5 as occurs when the Cp* and NPh +6 model the permethyl cyclopentiadienyl Cp*2U(NPh)2 U levels molecule absorbs levels levels group, C5Me5 (Cp*) as shown in the energy in the visible fi gure. For more information on the syn- or UV region of the thesis of these compounds, see the article on In visible and ultraviolet spectroscopy, the spectrum. page 23. The U(VI) complex has no unpaired incident light excites an electron from a fi lled electrons, while the U(IV) complex has two to an unfi lled level as indicated by the arrow. unpaired 5f electrons.electrons. The molecular orbitals In photoelectron spectroscopy, higher energy describing the other valence electrons can be x-ray sources are used to ionize the molecule viewed as arising primarily from the ligands by completely ejecting an electron from the bound to the uranium with some participation different energy levels. by the uranium orbitals. DFT calculations have been performed on both complexes to

Nuclear Materials Technology/Los Alamos National Laboratory 13 Actinide Research Quarterly

Metal-Ligand Interactions

If the ligands had their formal charge (such as –1 for Cp*, –1 for hydrazonato and –2 for imido) the U atom in turn would have +6 and +4 charges associated with its formal valence in the two molecules. The calculations show this ionic picture to be too extreme, since the DFT results give signifi cantly reduced charges on the uranium of only +1.0 and +1.3 for the U(VI) and U(IV) complexes, respectively. When all of the bonding electrons are included, this analysis also shows a net “population” of 2.2 6d electrons and 2.5 5f electronselectrons for the U(VI) complex compared to 1.6 6d electrons and 0.8 5f electronselectrons for the U(IV) complex, to which must be added the two nonbonding 5f elec- trons for a total 2.8 5f-electron-electron population. A large portion of the metal’s participation in the bonding can be traced to a few key orbitals in each system. Two representative orbitals that are qualitatively similar looking are shown in the panels in the fi gure on page 12. The orbital on the left in the bis(imido) complex has substantial U 5f involvement (35% 5f of the overall orbital contain- ing a pair of electrons) compared to only 11% 5f involvement in the bis(hydrazonato) complex. This translates into greater covalent bonding interactions (in fact multiple-bonding character) in the bis(imido) complex. While these aspects are beyond the scope of this discussion, it is informative to consider the electronic spectroscopy of these molecules in terms of excitations from these bonding orbitals to unfi lled orbitals. If the unfi lled orbitals correspond to 5f or 6d metal orbitals, they would be denoted as charge-transfer excitations compared to ligand-based excitations if both pairs of orbitals reside on the ligand. In addition, as in the case of the U(IV) complex with a 5f2 confi guration, there are also low-lying excitations arising from fi lled to unfi lled 5f excitations that give rise to the absorption spectra in the visible and near-IR region. Finally, the energies and orbital makeup of the occupied levels can be used to compare with photoelectron spectra that probe the energies. These examples serve to show how DFT electronic structure calcula- tions can be used to calculate molecular structure and spectroscopic properties of actinide complexes. The close agreement between theory and experiment for known compounds gives us reasonable confi dence in our ability to use DFT theory to predict properties of compounds that have not yet been prepared and provides valuable insight and guidance for interpreting the nature of covalency and chemical bonding and interpretation of spectroscopic properties. The research on aqueous complexes was supported by a Laboratory Directed Research and Development (LDRD) project administered by the Glenn T. Seaborg Institute at Los Alamos National Laboratory. The organoactinide research is supported by the DOE Offi ce of Basic Energy Sciences.

14 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004

A unique role for 5f orbitals in bonding Unusual tetraoxo coordination in heptavalent neptunium

he light actinide elements—uranium, neptunium, plutonium, americium, and —in their highest oxidation states This article was T(V–VII) have an unusually high affi nity for formation of contributed by Los metal-oxygen bonds and demonstrate a high degree of covalency and Alamos researchers metal-ligand multiple bonding. This is particularly true for oxo com- David L. Clark of the 2+ plexes, where, for example, the U=O multiple bond in the UO2 ion is Nuclear Materials remarkably short, and the mean bond strength is similar to that of the Technology Division; Phillip D. Palmer, C=O bond in CO2. These unusually strong bonds can be traced to the ability of the C. Drew Tai, and uranium atom to use a combination of 5f, 6d, and 6p atomic orbitals D. Webster Keogh in chemical bonding. Understanding the relative roles of the actinide of the Chemistry 5f, 6d, and 6p atomic orbitals in forming such strong bonds has been a Division; Steven D. Conradson, of the fundamental challenge in since the recognition of the presence of the 5f series, and a good deal of the fundamental chemistry and Technology and physics of light actinides has been devoted to understanding the Division; and Robert nature of these bonding interactions. J. Donohoe of the Historically, the relative roles of 5f/6d/6p atomic Biosciences Division.. orbitals in forming chemical bonds within the linear 2+ The remarkable strength dioxo ion (AnO2 ) of hexavalent actinides generated of actinide-oxygen bonds: a healthy scientifi c debate. Much of this debate origi- a comparison of gas-phase nated at Los Alamos, as exemplifi ed by the seminal

bond enthalpies arguments on why ThO2 is bent in the gas phase and 2+ UO2 is always linear. It took the clever application UO 710 kJ/mole of two-photon spectroscopy and oxygen k-edge x-ray 2 absorption spectroscopy by Robert Denning of Oxford 2+ UO2 701 kJ/mole University to sort it all out. What we learned from those studies has changed MoO2 587 kJ/mole CO 802 kJ/mole our fundamental thinking about valence orbitals in 2 heavy-element chemistry. The radial distributions of the early actinide 6s and 6p atomic orbitals are not buried within the core of the atom but lie in the valence region and must be considered to be active in chemical bonding. The ability of the 6p orbitals to hybridize with the 5f can lead to unusually strongstrong σ bonding, and this type of 6p-5f hybridization was shown to be a pre-pre- 2+ dominant contributor to the strong covalent bonds in linear UO2 ions. More recently, we have turned our attention to light actinide ions in even higher oxidation states to look for evidence of increased covalency and 5f orbital participation in bonding. Our early studies have focused on developing the basic understanding of the molecular structure of actinide ions in the heptavalent state ( VII) based on x-ray diffraction and absorption, and assessing the relative bond strengths as determined by nuclear magnetic and vibrational spectroscopy.

Nuclear Materials Technology/Los Alamos National Laboratory 15 Actinide Research Quarterly

The heptavalent oxidation state of actinide ions is rare, but has been known since the late 1960s due largely to the pioneering efforts of Russian scientists V.I. Spitsyn and N.N. Krot. X-ray diffraction data show that heptavalent neptunium can exist in the familiar trans dioxo

form, as illustrated by the structure of CsNpO4 shown in I, or in an unusual tetraoxo form with a square planar arrangement of O atoms, as

indicated by the structure of Li5NpO6 shown in II, though we note that the identity of this structure has been recently questioned by L.R. Morss and coworkers at Argonne National Laboratory.


Solid-state structure of CsNpO4. Solid-state structure of Li5NpO6. This This structure has a puckered layer structure has an isolated unit, with four

of NpO4 sheets,sheets, with axial Np=O equatorial Np=O bonds above and Molecular structure determination bonds above and below the layer. below the plane of metal atoms, and two of the NpO (OH) 3- ion 4 2 Np atoms are green, O atoms are short Np–O bonds within the plane. Np The molecular structure of the NpO (OH) 3- ion 4 2 red, and Cs atoms are purple. atoms are green, O atoms are red, and (top) in the solid state was determined by x-ray Li atoms are purple. . This thermal ellipsoid drawing emphasizes the pseudo-octahedral about the central neptunium ion. To understand the nature of chemical bonding in the unusual square This ion has four short Np–O bonds (1.88 Å) perpendicular to the page and two long Np–OH planar tetraoxo ion, we have been studying the physical and spectro- bonds (2.33 Å) in the plane of the page. The scopic properties of discrete molecular forms of heptavalent neptunium structural parameters for the NpO (OH) 3- ion and plutonium. For example, we have determined the molecular struc- 4 2 3- (bottom) were also determined by XAFS ture of the discrete NpO4(OH)2 ion by x-ray diffraction in the solid state spectroscopy. This fi gure shows the Fourier and by XAFS spectroscopy in both the solid state and solution. Using transform of the XAFS spectrum (solid black a technique pioneered by Krot and coworkers at Moscow’s Institute of line) and the theoretical fi t (dashed red line). 3- , we isolated the NpO4(OH)2 ion from alkaline solu- The components of the fi t, shown beneath the 3+ tion, using a rather large Co(NH3)6 cation, to give a solid compound of spectrum with negative amplitudes, correspond formula [Co(NH3)6][NpO4(OH)2]·2H2O. to individual shells of atoms. Note that there are The solid-state structure determined by x-ray diffraction is shown no atoms at 2.72 and 3.76 Å. The peaks labeled in the fi gure at upper left. The tetraoxo ion of formula NpO (OH) 3- dis- “ms,” which are routinely observed in XAFS data 4 2 of linear ions, are due to multiple scattering of a plays a highly unusual geometry with four oxo ligands in an equatorial - photoelectron off the oxygen atoms in the linear plane and two trans OH ligands, one above and one below this plane. O=Np=O unit. The radii of the coordination The four planar O ligands have an average Np–O distance of 1.878(5) shells observed in the XAFS (1.88 and 2.30 Å, and the two Np–OH bonds areare 2.238(4) Å. XAFS spectroscopyspectroscopy was Å) match the solid-state structure, giving high 3- confi dence that the NpO4(OH)2 anion is present in solution.

16 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004

used to determine structural details of the To better understand the relative strengths 3- Np(VII) ion both in highly alkaline solution of the Np=O bond in NpO4(OH)2 we turn to and in the solid state. The fi gure at lower left other forms of spectroscopic characterization. shows a representative Fourier transform Raman spectroscopy can provide sensitive (without phase corrections) of the k3-weighted insights into the strength of the M=O bonds XAFS spectra, the theoretical fi t to the data, and has found wide utility in the study of ac- and illustrates the single-shell contributions to tinide oxo complexes. The idealized molecular 3- the fi t. These bonding parameters are nearly structure of NpO4(OH)2 possesses D4h sym- identical to those found in the crystal structure metry, and this tetraoxo fragment III should determination described above. display two separate vibrations in the Raman

It is very unusual for a metal ion to have spectrum—a totally symmetric mode of A1g four coplanar oxo ligands as indicated in III. symmetry and an asymmetric mode of B1g For transition in their highest oxidation symmetry, as shown in V. states, the tetraoxo ions of general formula - 2- MO4 (Mn, Tc, Re) or MO4 (Cr, Mo, W) are always tetrahedral as shown in IV. In the tet- OH OH rahedral geometry, all fi ve metal d orbitals can O O O O be used to maximize the M–O bonding. The Np Np unusual planar arrangement of O atoms in O O O O 3- NpO4(OH)2 can be traced to the presence of 5f OH OH orbitals and the use of a combination of 5f, 6d, A1g B and 6p orbitals in chemical bonding. 1g V

OH 3- O - Raman spectra recorded for Np(VII) solu- O O tions at varying concentrations and Np M O in the solid state always showed a single invar- O O O O iant Raman peak. Since it was possible that OH the two vibrational modes were coincidentally III IV overlapping one another, depolarization ratio studies were used to assess whether the mode we were observing was the totally symmetric

The 1.88 Å Np=O bonds in the planar (A1g) vibration, the asymmetric (B1g) vibration, 3- NpO4(OH)2 ion are rather long when com- or an overlap of the two. TotallyTotally symmetric pared with other Np=O bonds in dioxo spe- vibrations give rise to polarized Raman lines, 2+ cies, which average aroundaround 1.75 Å for NpO2 whereas nontotally symmetric vibrations give ions. tetraoxo ions are even Raman lines that are depolarized. shorter, as evidenced by the comparison with - the ReO4 ion, which shows four Re=O bonds at 1.69(2) Å. Thus, the long Np=O bonds ob- 3- served in NpO4(OH)2 suggest that the Np=O bond is weakened relative to the Np=O bond 2+ in NpO2 ions.

Nuclear Materials Technology/Los Alamos National Laboratory 17 Actinide Research Quarterly

In the fi gure at upper left, a marked decrease is seen in the intensity of the Raman band when observed in a perpendicular as opposed to

parallel orientation, thus identifying a symmetric A1g vibration. The de-

crease in intensity of the A1g band also serves to demonstrate that therethere

was not a B1g band masked underneath this mode, since the B1g band should still be presentpresent at full intensity in the fi guregure at lower left. Raman

spectroscopy on single crystals also revealed a single A1g vibrational mode identical to that observed in the solution. The reasonreason that we don’t observe the asymmetric vibration in our studies is under evalu- ation, but it is possible that we are observing resonance enhancement of the totally symmetric mode. Additional studies where we vary the frequency of the exciting laser are in progress. For single-crystal samples, the symmetrical Np=O vibrational mode is observed at the extremely low frequency of 716 cm-1, while in 2.2 M Np(VII) LiOH solution this value increases slightly to 735 cm-1. These low vi- brational frequencies confi rm that the Np=O bond is weakened relative 200 I to the more common dioxo ions, which show typical Np=O stretching modes in the higher-frequency (stronger bonds) range 850–780 cm-1. The 150 longer, weaker bonds observed in the tetraoxo ion are an indication that there is much less π-electron donation from the oxo ligands to the metal 100 center in tetraoxo versus dioxo ions. This assessment is supported by 17 Relative Intensity I the observation of an O NMR chemical shift at δ 1470 ppm, which can be compared to the value of 1125 parts per million observed for UO 2+ 50 2 under the same solution conditions. The theory of NMR chemical shifts is quite complicated, but in 600 650 700 750 800 850 900 Wavelength (cm-1) general, a lower electron density on an oxygen atom leads to a greater downfi eld shift. Our data are therefore consistent with the notion that 3- the NpO4(OH)2 ion has more electron density on the oxygen atom 3- Polarized Raman spectra of the NpO4(OH)2 relative to the transdioxo ions. The combination of this data—a long ion identify the vibrational symmetry Np=O bond, a low (Np=O) vibrational frequency, and a high-fi eld 17O A totally symmetric vibrational mode gives rise resonance—all point to weaker bonding in the Np=O bonds of tetraoxo to a polarized Raman line, and a vibration with ions relative to dioxo ions. Our current efforts are focused on quantify- lower symmetry is depolarized. When polarized laser is employed, we can monitor ing the relative contributions of σ and π bonding and the relative roles the polarization of the scattered Raman line to of the 5f, 6d, and 6p orbitals in forming these bonds in the square planar 3- determine the symmetry of the vibration. This NpO4 unit in NpO4(OH)2 . 3- This research is supported by the DOE Offi ce of Basic Energy Sciences. fi gure shows a solution spectrum of NpO4(OH)2 recorded with perpendicular and parallel polarization of the exciting laser. The observed frequency is clearly polarized, and can be

assigned to the totally symmetric A1g vibration.vibration.

18 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004

The relative roles of 5f, 6d, and 6p atomic orbitals 2+ Why UO2 is linear and isoelectronic ThO2 is bent

2+ also based on theoretical calculations, The UO2 ion is well known in actinide chemistry and displays a linear geometry in every known was that U could use a combination of 5f example of its complexes. In contrast, the and 6p (normally(normally considered to be in the core) orbitals to form an unusually strong isoelectronic ThO2 molecule is bent in the gas phase with an O–Th–O angle of 122°. The σ bond in the linear geometry. theoretical understanding of these differences has taken nearly 20 years to understand fully Twenty years later and after the and hinges on a detailed understanding of the application of very sophisticated relative roles of the atomic 5f, 6d, and 6p orbitals spectroscopy (two-photon laser and in forming the metal oxygen bond in these two oxygen k-edge x-ray absorption) and more systems. modern theoretical calculations based on the hybrid density functional approach, In the early 1980s, there were two points of view: we have found a resolution of these two one based on U–Oπ bonding (Bill Wadt at Los points of view. In the fi nal analysis, both σ Alamos) and another based on U–Oσ bonding and π bonding are found to be important. (Roald Hoffmann at Cornell University). The Both fπ and dπ bonding occur, but the Los Alamos view was that for U, the 5f levelslevels dπ bonding is signifi cantly stronger. are signifi cantly lower in energy than the 6d, Moreover, in a strange and unexpected while for Th, the 6d levellevel is lowerlower than the 5f. way, we also learned that the fσ hybridizes Theoretical calculations showed that with 6pσ, and the resulting f-p U–Oσ 6dπ bonding was important for both Th and U, bond is much stronger than the U –Oσ but that U could also use the 5f orbitals forfor π bond formed by the d orbitals. Therefore,Therefore, bonding. A bent geometry was preferred for Th the dominant covalent interactions in due to the dominance of the 6dπ bonding, and linear OUO2+ are due to the strong 5f-6pσ a linear geometry was preferred for U because bonding and 6dπ bonding. a linear geometry could use both 6d and 5f orbitals for bonding. The Cornell point of view,

Nuclear Materials Technology/Los Alamos National Laboratory 19 Actinide Research Quarterly The correlation between Raman spectroscopy and bond lengths in high-valent actinide complexes

he correlation of vibrational energies to bond strength and molecular structure has a long tradition in inorganic chemis- try. For example, Badger’s rules allow the metal-metal bond This article was T length for second- and third-row transition dimetallic complexes to be contributed by predicted based on the bond vibrational energy. In the course of our Los Alamos studies of structure and bonding in actinide complexes in high-oxida- researchers tion states, we have examined a variety of axial dioxo systems in which C. Drew Tait, D. Webster Keogh, the linear O=An=O moiety is both highly sensitive to the electronic Mary P. Neu, Sean confi guration of the complex and also yields a characteristic Raman R. Reilly, Wolfgang band, identifi ed as the symmetric stretch ν1. Runde, and Brian With only a handful of crystallographic studies of actinyl com- L. Scott of the plexes available, until recently there has been a relative paucity of the Chemistry Division; structural data needed for the development of structure/vibrational Robert J. Donohoe band correlations. Now, however, x-ray beam-line studies, especially of the Biosciences XAFS spectroscopy, are used Division; David to characterize the structural L. Clark of the properties of solution and poly- Nuclear Materials crystalline actinide samples, and Technology Division; Steven D. creating a much more expansive Conradson of the structural data for actinide Materials Science complexes. As a result, we can and Technology now correlate the energy of the Division; and Scott ν1 Raman band, either from the A. Ekberg, formerly literature or our own data, with of the Chemistry structural data derived from Division. both crystallographic and solu- tion XAFS studies to seek the common relations between the vibrational energies and An–O bond lengths. Plots comparing the struc- The correlation of tural and vibrational data for representative U, Np, and Pu dioxo spe- actinyl bond distance cies are shown in the fi gure above, where the An–O bond length (y-axis) to vibrational energy is maintained between the different actinides to facilitate comparisons. for a variety of actinide The units on the x-axis refl ect the fact that the mathematics of relations metals and oxidation like Badger’s rule are such that the bond distance is a linear function states. of the vibrational energy to the –2/3 power. Clear trends can be seen within a particular oxidation state of an actinide, for example U(VI) and Np(V); however, unlike dimetallic transition metal complexes, no universal trend between the actinides or even between oxidation states of the same actinide is observed.

20 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004

The sensitivity of Raman band energy to Across the actinide series, for a given oxida- the bond distance (slope) is much greater for tion state, the metal decreases as Np(V) than for U(VI), which in turn is more the increases. This actinide con- sensitive than that observed for Np(VI). Pu(VI) traction, similar to the well-studied lanthanide -1 manifests shifts in ν1 > 40 cm even though the contraction, results in the prediction that the changes in the Pu=O bond length are very lim- metal–oxygen bond length should get shorter ited. Thus, for Np(V), bond distance is a more as one proceeds along the actinide row of the sensitive indicator of changes in the equatorial periodic table. bonding structure than are the vibrational Such a result is indeed observed in the data, while for Pu(VI) the opposite is true. structural data but yields a surprising correla- Given the successful application of tion with the vibrational energies. Generally, Badger’s rule for entire rows of transition shorter bond lengths yield higher energies for metal stretches, the lack of a consistent the corresponding stretching mode; however, trend for the symmetric “-yl” stretches, which the opposite relation is found here. A com- is also a two-atom motion, is perhaps surpris- parison of the Np(V) → Pu(V) and Np(VI) → ing. Thus, the identity of the actinide itself, Pu(VI) aquo complexes, as well as the its oxidation state, and the equatorial ligand U(VI) → Np(VI) → Pu(VI) tris carbonato content must all be considered to be of poten- complexes and Np(VI) → Pu(VI) tetrachloro tial infl uence in determining the combined complexes all show decreasing stretching distance and bond-energy properties of the frequencies accompanying the shorter bonds. actinyl structural element. This result can be interpreted as follows. For example, encompassing dioxo Np(V) The bonds to the “-yl” include con- into a rather large crown ether minimizes the tributions from the 5f orbitals. The primary equatorial (ether) interactions and causes the physical cause of the actinide (and lanthanide) axial “-yl” bond to be considerably shorter contraction is decreased shielding of the core than that for the tris carbonato species, where positive charge across the series: as electrons interacts very strongly in the equato- are added to the outermost frontier orbitals rial position and engages bonding electrons they do a poorer job of screening the charge that might otherwise interact with the axial from the additional crowded at the oxygens. Another means of comparison can atomic core. be made in terms of f-orbital occupancy. Thus, Decreased shielding, in turn, causes the we can fi nd consistency in the structural and 5f orbitals to shrink towardtoward the corcore,e, and vibrational properties of the f 1 confi guration their directionality decreases as they become + 2+ complexes of [UO2 ] and [NpO2 ]. However, more diffuse. Therefore, although the axial such comparisons must be carefully made. oxygens “chase” these orbitals inward across As an example, the structural and vibrational the actinide series (the vibrational potential properties of f 0 dioxo U(VI) complexes cannot energy curve minimum moves to a smaller be compared with Np(VII) complexes because equilibrium distance), their overlap with the the latter adopt a tetraoxo core structure. increasingly buried 5f orbitals still decreases,decreases, Comparing the structural and vibrational and the bond order is reduced (the potential data from similar complexes of different energy surface becomes less curved). actinides provides an interesting perspective.

Nuclear Materials Technology/Los Alamos National Laboratory 21 Actinide Research Quarterly

Badger’s rule

Fundamental manifestations of chemical bonding include the strength of the bonds that are formed as well as their length. Intuitively, there should be a link between the bond strength and the subsequent length of that bond, and this link is provided by Badger-type rules. However, with the vibration viewed as a potential energy curve, the position of the potential energy minimum (bond length) does not necessarily have a unique link to the width/steepness of the potential energy curve (bond strength).

In fact, an example is mentioned in the main article for the tris carbonato species where weaker dioxo bonds are accompanied by shorter bond lengths. Therefore, care must be taken to tune the Badger-type relationships and to test how wide their application is. Many of the Badger-type relationships involve stretches that are described as being due to the vibration of two atoms, as in the case of metal-metal bonding. The linear dioxo moiety involving the early actinides in high-valence states is an uncommon structural motif in chemistry and requires its own Badger-type relationships.

Badger’s rule is named for the late Richard McLean Badger, whose career as a student, teacher, and researcher at the California Institute of Technology spanned more than 50 years. His “rule,” which relates a change in stretching frequency with a change in bond lengths, was the result of his extensive study the force constant and internuclear distance in a large number of diatomic molecules.

Badger-type relations provide a link between bond strength and bond length, which in turn, provides important constraints to check the theoretical calculations (with their associated approximation methods) performed to inform us of the orbitals used in constructing the mol- ecule. Alternatively, bond lengths of unknown or new molecules can be estimated from vibrational information on the bond strengths. Because growing single crystals for x-ray diffraction studies is diffi cult and sometimes not possible, this is a valuable tool to have. This research was supported by a Laboratory Directed Research and Development (LDRD) project administered by the Glenn T. Seaborg Institute of Transactinium Science at Los Alamos National Laboratory. EXAFS experiments were performed at the Stanford Synchrotron Radiation Laboratory, which is supported by the DOE Offi ce of Basic Energy Sciences.

22 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004 Organometallic actinide chemistry Developing a comprehensive picture of the chemical behavior of the 5f elements

here has been a recent resurgence of interest in understanding the electronic structure of complexes of the actinide elements. TThis is driven principally by the desire to predict the behavior of actinide species in a variety of applications within the DOE complex, from waste repositories to the design of selective chemical separation This article was methodologies. contributed by Los Generally, present models describing the electronic structure of the Alamos researchers actinides are most relevant to chemistry performed under aerobic condi- Jaqueline L. tions and do not provide a comprehensive set of chemical environments Kiplinger of the in which to probe characteristics of metal-ligand bonding. To assess the Chemistry Division validity of theoretical treatments already developed and ensure that and Carol J. Burns chemical behavior is predictable under a wider array of process condi- of the Offi ce of the Deputy Director for tions, it is important to study a broader range of ligands and chemical National Security. environments than those commonly examined and their infl uence on observable chemical behavior. Nonaqueous provides access to the broad- est possible set of ligands with which to examine the infl uence of coor- dination environment on the electronic structure and chemical reactivity of the actinide elements. We have chosen to focus particular attention on the generation and investigation of actinide-ligand multiple bonds to probe the potential for involvement of metal 5f electronselectrons in chemical bonding, examine the stability of functional groups isoelectronic with the oxo ligand, and demonstrate reaction patterns unique to f elements.

Nonaqueous analogues to the uranyl ion The uranyl ion (1) is the dominant form of uranium in aerobic media and features a linear O≡U≡O frag- ment with uranium in its highest formal oxidation state, U(VI). More than a decade ago, we prepared the fi rst example of an organometallic U(VI) complex,

(C5Me5)2U(=N-Ph)2 (2). This complex was remarkable in that it also represented the fi rst example of a bis(imido) 2+ 2+ U(NPh)2 electronic analogue to the uranyl ion, UO2 . However, unlike the uranyl ion, this uranium com- plex displays an unusual nonlinear or cis arrangement of the two imido functional groups (U=N). The fact that complex 2 contained a bent N=U=N core challenged accepted motifs for bonding in actinide complexes. The uranyl ion Complex 2 is a 20-electron species that has no transition metal analogue, (1) and the U(VI) bis(imido) complex, raising signifi cant questions about the electronic structure of this com- (C5Me5)2U(=N-Ph)2 pound and in particular the participation of valence metal orbitals (5f, (2). 6d, and 7p) in ligand-to-actinide π bonding.

Nuclear Materials Technology/Los Alamos National Laboratory 23 Actinide Research Quarterly

Organo- metallic Actinide Chemistry

We examined the extent of f-orbital participa- is the four-electron reductive cleavage of the tion in this hexavalent uranium complex double bond in azobenzene (Ph-N=N-Ph) by through collaborative interactions with (C5Me5)2U(Cl)(NaCl) generated by reduction both experimental and theoretical scientists. of complex 6 by Na/Hg to give the bis(imido) Bruce Bursten of The Ohio State University complex 2, a process not often observed for and P. Jeffrey Hay of Los Alamos conducted transition elements. These reactions, shown theoretical investigations on simplifi ed model in Scheme 1, underscore some differences be- complexes and demonstrated that 5f-orbital tween the transition metals and the actinides involvement can contribute to the stabilization and suggest the possible involvement of the 5f of nitrogen donors in a manner that is not pos- orbitals in chemical reactions of uranium. sible for the transition metals. Jennifer Green of Oxford University confi rmed these results, In search of reactive using photoelectron spectroscopy, which actinide-ligand multiple bonds revealed a small but signifi cant f-orbital-based One of the most exciting discoveries in bonding interaction. organometallic chemistry over the past 15 Complex 2 can be prepared by a variety years was the observation that d0 transi- of methods as shown in Scheme 1. The most tion-metal imido complexes readily react general route is two-electron oxidative atom with - bonds. Analogous f0 transfer using organic (RN ) (e.g., 3→2, 3 actinide imido complexes might be expected 5→2). Another method that has proven useful to display similar reactivity patterns to their in the synthesis of bis(imido) transition metal early transition-metal counterparts; however, complexes is the reductive cleavage of 1,2-di- as in the case of uranyl compounds, the most substituted , in which the cleavage characteristic chemical property of uranium of hydrazo compounds (R-NHNH-R) allows imido bonds is their decided lack of for the introduction of a single organoimido chemical reactivity. . Recently, we However, the discovered a method reaction of complex to access reactive 4 with 1,2-diphenyl- uranium imido com- to produce plexes. We found the bis(imido) com- that diazo- plex 2 provides the (R2CN2) can be used fi rst example of the to prepare U(VI) reductive cleavage of bis(imido) complex- a hydrazo compound es that are reactive yielding two ogano- toward alkanes, as imido functional illustrated in Scheme groups at a single 2. Reaction of the metal center. The most U(IV) monoimido unusual chemistry exhibited by these bis(pentamethyl- Scheme 1. Synthetic scheme for the cyclopentadienyl) preparation of the uranium complexes U(VI) bis(imido) complex 2.

24 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004

complex 7 with diphenyldiazomethane gener- U=N-2,4,6-t-Bu3C6H2 fragment in complex 8 ates the U(VI) bis(imido) complex 8. Complex has been activated to give complex 9.

8 does not lose N2 to give a U(VI) alkylidene This reaction type has no equivalent in complex (an alkylidene is a complex possess- transition metal chemistry and represents a ing a metal-carbon, or M=C, functional group). new pattern of reactivity available for high- This is in marked contrast to the chemistry valent actinide imido complexes. The uranium observed for isoelectronic N2O and organo- center is bound to three nitrogen atoms. Two azides (RN3), which have been exploited for exhibit short U–N distances associated with the preparation of U(VI) oxo (U=O) and imido uranium-amide linkages. The remaining U–N complexes. The oxidation of the uranium metal center from U(IV) to (VI) is clearly dem- onstrated by NMR spectroscopy and electronic absorption spectra of complex 8. The electronic absorption spectrum shows no f→f transi-transi- tions in the near-IR region but does show a length is consistent with a uranium-nitrogen Scheme 2. Synthetic broad, intense and featureless charge-transfer dative interaction. Notably, the U–N(H)−C scheme showing band in the visible region, which is consistent fragment is bent, which differs signifi cantly the reaction of the 0 U(IV) monoimido with the assignment of an f U(VI) metal center. from the nearly linear U−N−Cipso unit present The NMR spectra of complex 8 is in the parentparent bis(imido) complex 8. complex 2 with temperature invariant between –75 and 60 °C Ph2CN2 to generate but displays anomalous chemical shifts, sug- f-electron delocalization in the bis(imido)U(VI) complex 8, gesting that complex 8 is a temperature-inde- organometallic actinide complexes and its thermal pendent paramagnet, a characteristic property We are also interested in understanding the transformation into of organometallic complexes of U(VI). An degree to which valence metal orbitals con- a cyclometallated x-ray crystal structure of complex 8 confi rmed tribute to bonding and reactivity. Covalency complex 9. the presence of two organoimido groups ter- (the mixing of orbitals) in metal-ligand bond- minally bound to the uranium center and the ing is quite prevalent in transition metal com- overall structure of complex 8. plexes but is suggested to play a reduced role Proton NMR spectroscopy signaled the in the chemistry of the actinides. We recently quantitative formation of the novel cyclometal- discovered several classes of organo-uranium lated U(IV) bis(amide) complex 9 upon ther- complexes in which the oxidation state of the molysis of a solution of complex 8; the NMR metal, and thus the location of the valence spectrum of the product is paramagnetically electrons, is not easily determined. The shifted, which indicates that the product is a spectroscopy, reactivity, and structural chem- reduced f2 U(IV) species. X-ray crystallography istry of these compounds suggest that the 5f unambiguously ascertained that the C–H electrons are far more involved in chemical bond of one of the t-butyl groups from the bonding than previously thought.

Nuclear Materials Technology/Los Alamos National Laboratory 25 Actinide Research Quarterly

Organo- metallic Actinide Chemistry

The nearly linear U−N−C fragments are consistent with the formulation of the ligands as ketimido groups with the metal center accepting additional electron density from the nitrogen lone pairs. Spectroscopic data suggest donation of Scheme 3. Synthetic We have investigated the reaction electronic density from the metal center onto scheme for the chemistry of low-valent organouranium the ketimido ligands in this system. preparation of complexes with diazoalkanes, which are uranium diazoalkane Complex 11 is reminiscent of high-valent known to generate transition metal alkylidene complex 10 and U(VI) bis(imido) complexes that have signifi - complexes under similar conditions. its transformation cant U-N multiple bond character as typifi ed Reaction of the U(IV) complex 6 with into the U(IV) by complex 2. The metal center in complex diphenyldiazomethane under reducing bis(ketimido) 11 is not U(VI), which is evident upon conditions generates complex 10 as indicated complex 11. comparison of the U N distances in the two in Scheme 3. This species is unlike existing − systems as well as the NMR and UV-visible- transition-metal diazoalkane complexes in near-IR spectra. The complex is formally an that it possesses two diazoalkane molecules f 2 system, yet it does not behave like known bound to a single uranium metal center. One U(IV) amido complexes. The uranium diazoalkane is coordinated to the uranium ketimido complex is surprisingly unreactive, metal center in an η1 fashion and forms a displays unusual electronic properties, and uranium imido bond. The second diazoalkane is able to support the uranium metal center is coordinated to the uranium in an η2 manner, in a variety of oxidation states ranging from possibly through the N=N π system. (III) to (V). The exceedingly long N−N bond distance The physical properties and chemical indicates that substantial π backbonding stability of this U(IV) complex indicate occurs between the (C Me ) U fragment and 5 5 2 higher U–N bond order due to signifi cant an empty orbital on the diazoalkane unit, ligand-to-metal bonding in the uranium which has antibonding character, resulting in π ketimido interactions. The combined data a lengthening of the N-N bond. Interestingly, suggest electronic delocalization throughout theoretical calculations reveal an f 1π* electronic the N U N core and indicate that the 5f confi guration and suggest delocalization of a − − electrons in midvalent organouranium 5f electronelectron throughoutthroughout the N=N=C framework complexes might be far more involved in of the η2-coordinated diazoalkane. chemical bonding and reactivity than Upon standing in solution, the bis(diazo- previously thought. ) complex 10 transforms into the novel uranium bis(ketimido) complex 11, via loss of N from two diazoalkane fragments. The The role of f orbitals 2 in reactions of actinides U−N bond distances are signifi cantly shorter The role of the 5f orbitals in bonding and than other structurally characterized U(IV) reactivity is still one of the most intriguing bis(amido) complexes such as compound 9.

26 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004

questions in actinide chemistry and continues their involvement in stabilizing metal-ligand to be a topic of considerable debate among multiple bonds. In particular, the observation theoretical and experimental . The of “valence-ambiguous” behavior in organo- high nodality of the 5f orbitals and the ex- uranium complexes challenges the once com- panded electron count of many organoactinide mon belief that 5f orbitalsorbitals aarere energeticallyenergetically complexes provide the opportunity for the inaccessible and serve merely to house actinide metals to engage in bonding schemes unpaired electrons. With our knowledge of that are not available to the transition metals, the role of both oxidation state and ancillary leading to new patterns of reactivity. While ligands in stabilizing metal-ligand multiple there is spectroscopic and physical evidence to bonds, it will be possible to extend this work to suggest that f orbitals areare key to the stabiliza- other as yet unrealized molecular targets, such tion of many high-valent (V, VI) organo- as species with metal-carbon multiple bonds. actinide complexes, fewer examples of 5f As in the case of actinyl compounds, spe- orbitals directing reactivity are known in tetra- cies containing multiply bound functional valent actinide chemistry. groups have most often been found to be In our studies, we have found that diazo- unreactive. Recent advances, however, shed alkanes readily insert into both M-C bonds light on the means to enhance the reactivity of bis(alkyl) complexes (C5Me5)2MR2 to yield of actinide-ligand multiple bonds, revealing bis(hydrazonato) complexes (see compound metal-mediated transformations uniquely 13 as shown in Scheme 4). These 22-electron enabled by 5f orbitals. Further studies will complexes have no transition metal analogues; explore methods to rationalize and exploit the reaction of diphenyldiazomethane with this behavior. derivatives affords only mono in- The development of the chemistry of sertion products. Because transition metals do transition metals with multiply bound ligands not have valence f orbitals, these uranium and altered chemists’ views of the scope of reactiv- bis(hydrazonato) complexes provide ity possible for these metals. Development of a unique opportunity to investigate f-orbital the metal-ligand multiple bond chemistry of participation in the reaction chemistry of the actinides will similarly expand the reach of organometallic actinide compounds. modern f-element chemistry,chemistry, while also setting Theoretical calculations on related model the organometallic chemistry of the actinides Scheme 4. Synthetic complexes reveal that the 5f orbitals interact apart from that of the transition metals. route to the Th(IV) to a small extent with the N– N π orbitals and This research is supported by the DOE bis(hydrazonato) the N–N σ orbitals. These results suggest that Offi ce of Basic Energy Sciences. complex 13. 5f orbitals can contribute to the stabilization of side-bound N–N units for which no appropriate symmetry match exists for transition metals.

Future directions The use of novel ancillary ligand frame- works has provided the opportunity to access complexes in a variety of oxidation states that feature examples of actinide-ligand multiple bonding, yielding a much deeper understanding of 5f-orbital-orbital eenergeticsnergetics aandnd

Nuclear Materials Technology/Los Alamos National Laboratory 27 Actinide Research Quarterly

What are valence orbitals?

An atom consists of two basic parts: the nucleus and the electrons. The nucleus is the central core of an atom and is made up of protons and . Electrons are very light, negatively charged particles that surround the positively charged nucleus. Early models of the atom depicted the electrons circling the nucleus in fi xed orbits, much like revolving around the sun.

Current theory suggests that electrons are housed in orbitals. An orbital is a region of space where there is a high probability of fi nding an electron. There are four basic types of orbitals: s, p, d, and f. An s orbital has a spherical shape and can hold two electrons. There are three p orbitals, eacheach of whichwhich has the same basic dumbbell shape but differ in its orientation in space. The p orbitals can hold up to six electrons.

There are fi ve d orbitals, whichwhich havehave more complicated shapes than s and p orbitals. The shape and orientation of the d orbitals, which together can hold up to 10 electrons, are shown to the right.

28 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004

The chemical and physical behavior of the elements results from the confi guration of the outermost electrons. These electrons, called the valence electrons, are the most loosely held and interact with those in other atoms to form chemical bonds. The type of orbital (s, p, d, or f) that the vvalencealence electronselectrons reside in is a function of the elements’ position in the periodic table. For example, elements having a partially fi lled set of d orbitals are called transition, or d-,-block, elements. These elements use electrons in the d orbitals forfor bonding and chemical reactivity.

Orbitals are often preceded by numerical In contrast to the transition elements, designations, i.e. 4f, 5d, 3p, etc. This the seven f orbitals, whichwhich are foundfound in number is an indication of the size and lanthanides and actinides, are less well energy of the orbital. A larger number understood. The 14 electrons that can reside indicates a larger and higher energy in these orbitals are highly contracted orbital. Thus, electrons in the 3s orbital (i.e., held close to the nucleus) and are of sodium (Na) are higher in energy not thought to overlap to any great degree and farther away from the nucleus than with the valence orbitals of neighboring electrons found in the 2s orbital of atoms. Thus, bonding in the lanthanides and (Li). actinides is thought to rely more heavily on the p and d orbitals. As such,such, the rolerole of the f orbitals in bonding and reactivity has been a subject of considerable debate.


Nuclear Materials Technology/Los Alamos National Laboratory 29 Actinide Research Quarterly

In 1923, J.N. Bronsted and T.M. Lowry independently suggested that acids be defi ned as proton donors and bases as proton acceptors. Thus, an acid-base reaction is simply the transfer of a proton (H+) from an acid to a base. The usefulness of this defi nition lies in its ability to handle

any solvent containing protons such as (NH3) or sulfuric acid (H2SO4). � � ����� ��� �����

������� ���� ���� Water is a unique species that can act as either an acid or a base. � � ��� � �� �������� ���� ����

� � ��� � ��� ��� ������ ���� ���� A more general defi nition of acids and bases was set forth by G.N. Lewis, who defi ned an acid as an electron- pair acceptor and a base as an electron-pair donor. While encompassing the examples shown above, the Lewis defi nition includes reactions in which no are involved. Neutralization in the Lewis theory consists of the formation of a new bond and includes the generation of

acid-base adducts such as R3N:BF3.

���������� ������� ���� ���� The of metal ions and the formation of coordination compounds are also examples of acid-base reactions in which the metal ion acts as an acid and the solvent (or ligand) acts as a base.

�� �� ��� ��������� ���������� ���� ����

�� �� ��� ���������� ���������� ���� ���� Lewis’ defi nition of acids and bases is widely used because of its simplicity and wide applicability. —G.G.

30 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004

New perspectives from organometallic chemistry Electronic structure and bonding in 5f-element complexescomplexes

he electronic structure in discrete molecular systems This article was containing actinide metals is dominated by states made contributed by Los Tup from the seven 5f orbitals on the actinide ion. These 5f Alamos researchers orbitals possess a host of interesting properties. From a chemical and David E. Morris electronic structure perspective, one of the more interesting aspects of and Jacqueline the 5f orbitals is their radial extension from the nucleus. This is a very L. Kiplinger of the important consideration in the formation of chemical bonds. Only those Chemistry Division. orbitals that extend to the periphery of the atom are actively engaged in direct chemical bonding. Because of this increased spatial extension of the 5f orbitals, electrons in these orbitals are much more strongly infl uenced by the presence of neighboring atoms in the actinide-containing molecules than, for example, the 4f electronselectrons in the molecules of the lanthanide series metals. Thus, the measurable properties of the electrons in these 5f orbitals will provideprovide directdirect information on the other atoms, ions, and/or molecules in the immediate coordination environment of the actinide metal. In fact, the spatial extension of the 5f orbitals providesprovides To help us achieve our objectives in understanding an opportunity for them to overlap electronic structure and bonding in the actinides, directly with the orbitals on these we are creating and investigating an entirely neighboring atoms. This type of orbital new class of actinide molecules derived from overlap, referred to as a covalent organometallic chemistry. Here we show four bonding interaction, has been the different types of structures that we have developed and are investigating. All of these are based on the subject of debate in actinide chemistry for decades. (C5Me5)2U core (represented by the two pentagons connected to the U metal center). The simplest Much of our understanding of molecules are typifi ed by (4), which contains twotwo the relationship between electronic

methyl (CH3) groupsgroups bound to the metal. The more structure and chemical bonding interesting systems contain ligands that can bind to in actinide molecules comes from the metal in unusual ways—like the ketimide ligands investigations of classical coordination (7) that possess a metal-nitrogen bond of ~1.5, the compounds such as the salts hydrazonato ligands (8) that each havehave twotwo nitrogen (e.g., UCl4). These studies have atoms bound to the metal, and the imido ligands (9) been invaluable and have laid the that have a bona fi de double bond between the metal foundation for interpretation of and the nitrogen atom on the ligand. spectral data. However, to further our understanding of the more subtle

Nuclear Materials Technology/Los Alamos National Laboratory 31 Actinide Research Quarterly

Structure and Bonding

aspects of the electronic/molecular structure link and to explore in greater detail the existence of direct 5f-orbital participation in bonding, we need to access chemical systems that allow greater control over and variability in the molecular structure. Organometallic actinide chemistry provides an opportunity to exercise precise control over the structure of the coordination environment around the actinide ion, while also allowing the introduction of a broader range of ligands than possible in classical coordination chemistry. Thus, in addition to classical ligands like the Cl- ion, unusual ligands can be introduced that are capable of participating in both σ and π bonding with the metal center. We recently investigated the electronic structure and bonding in a series of tetravalent organouranium complexes of the general formula

(C5Me5)2U(R)(R´) where R and R´ are ancillary ligands including halide, trifl ate, alkyl, imido, hydrazonato, and ketimido. Two of the most useful experimental probes of electronic structure in discrete molecular systems are electronic absorption spectroscopy and , in particular cyclic voltammetry. The former Cyclic voltammetry provides information about technique, also commonly referred to as UV-visible-near-infrared or the energy required to add or remove electrons optical spectroscopy, provides information about the energies and from molecules by scanning the potential in a intensities of transitions between the electronic states in a molecule. cyclic fashion and measuring the current that For the organoactinide complexes, we will be interested in transitions results. Peaks in the current response (positive for addition of electrons, negative for removal of between the electronic states derived from the 5f orbitals and between electrons) signal the onset of electron transfer, states derived from electron excitations from these 5f orbitals to other and the potential at which these peaks occur molecular orbitals, both metal- and ligand-based. provides a relative measure of the energy of Voltammetry is based on the addition or removal of electrons the process. Here we show voltammetry for from molecules at a solution/solid electrode interface. Because these three molecules. Complex 1 (red) only exhibits electron transfer processes occur from the highest occupied orbitals a single wave (the terminology for a combined (oxidation) or to the lowest unoccupied molecular orbitals (reduction), positive and negative peak that are slightly this technique gives a direct measure of the stability of the oxidation offset from each other in potential) at ~-1.8 V state of the parent complex and the energy to change that oxidation assigned to the U(IV) / U(III) transformation state by ± one or more electrons. Ideally, the trends in the data provided in this complex. Complex 4 (blue)(blue) has similar by both experimental probes can be related back to trends in structural features but at more negative potential values. (Note that voltammograms are plotted with variations to obtain a deeper understanding of the infl uence of more negative potentials to the right.) molecular structure on electronic structure. Complex 7 (green)(green) exhibitsexhibits a wavewave at ~-2.5 V for U(IV) / U(III) and a wave at ~-0.5 V. This properties from additional wave is assigned to a U(IV) / U(V) cyclic voltammetric studies oxidation process. This new wave signals the To investigate the infl uence of the ancillary ligands (R, R´) on the onset of new bonding features associated with energetics of the metal-based redox couples, we focused on the redox the nitrogen-containing ligands. waves attributable to the uranium metal-based processes. All of the complexes can be classifi ed into only two categories: Category 1 includes complexes where R and R´ are chloride, alkyl, or trifl ate groups; Category 2 includes complexes where at least one of the R or R´

32 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004

groups is a hydrazonato, imido, or ketimido nitrogen donor ligand. The ancillary ligands in the Category 1 complexes are all principally σ donors, and the voltammetry in this category is dominated by a single reduction process from U(IV) to Electronic absorption spectra U(III) that occurs between ~ –1.8 and –2.7 V provide us with important +/0 versus [(C5H5)2Fe] . A useful comparison is information about the the variability in the U(IV)/U(III) redox po- energies required to promote tential for differing ligand sets. For example, it electrons between various has been shown that the uranium metal center states within the molecules. becomes more diffi cult to reduce in the series For the actinide molecules studied here, there are two (C5H5)3UCl < (C5H5)4U < (C5Me5)2UCl2. This corresponds to the stabilization of the tetra- important yet distinct types of transitions that occur in valent oxidation state due to the electron- different energy regions. donating ability of the ligands, with donor The fi rst, shown in green, - - strength decreasing from C5Me5 to C5H5 to are associated with changes - Cl . For the Category 1 members, the reduction in electronic states derived potential refl ects the more strongly electron- from the actinide 5f orbitals. donating nature of the alkyl groups relative The second, shown in blue, to the chloride ion. No complexes in the fi rst derive from transition in which category show any evidence for a metal-based the electron is promoted oxidation process (i.e., U(IV) to U(V)). from an orbital localized on All complexes in Category 2 contain one one portion of the molecule or two nitrogen-containing ligands that have (for example, the metal) to another. We are currently the ability to interact with the metal center in attempting to develop both σ and π modes via the nitrogen lone pair theoretical justifi cations for orbitals and/or the π orbital system on the the observed differences in ligand (for the hydrazonato complexes). All molar absorptivity for these these Category 2 complexes exhibit a revers- transitions in the structurally ible reduction wave at potentials between ~–2 different complexes (4, 8, and and –2.6 V that is attributable to a metal-based 7). WeWe believebelieve these intensity U(IV)/U(III) process. The distinguishing fea- differences are a direct ture of these Category 2 complexes is the ap- indicator of the extent of pearance of an oxidative process in the region covalent interaction between from ~+0.1 to –0.6 V. The U(V) species appears the uranium metal center and the ligands. to be stable on a voltammetric time-scale, with no evidence for chemical reactivity such as . The nitrogen ligands in the Category 2 complexes provide the same measure of stabilization of tetravalent uranium as do the alkyl ligands in the bis and tris alkyl complexes in Category 1. Thus, the potential

Nuclear Materials Technology/Los Alamos National Laboratory 33 Actinide Research Quarterly

Structure and Bonding

of the U(IV)/U(III) couple is shifted to quite (f-f bands). The second regionregion lies at energiesenergies negative values compared, for example, to the above ~15,000 cm-1. In general, this region will bis halide. However, in the case of the nitrogen contain transitions derived from promotion of ligand complexes, the metal center becomes a 5f electronelectron into higher energyenergy metal-based 6d so electron rich that the one-electron oxidation orbitals as well as transitions to ligand-based process is also readily accessible and yields nonbonding and antibonding orbitals. These a stable pentavalent complex. The difference latter transitions are referred to as metal-to- between these Category 2 complexes and ligand charge transfer (MLCT) because the those in Category 1 is the ability of the nitro- electron is transferred from a metal-based gen ligands to engage in π bonding with the orbital onto an orbital localized on a ligand. metal center. Undoubtedly, in the complexes These electronic spectral data also fall into with imido, ketimido, or hydrazonato ligands, distinct classes related to the nature of the

the stability of the pentavalent oxidation ancillary ligands on the (C5Me5)2U core. The state derives from the additional π interaction fi rst class of complexes includes those that between these nitrogen ligands and the metal have ancillary ligands of essentially σ-donor center, stabilizing the high valent oxidation character. All of the complexes in this class state in a way that simple σ-donor ligands have a large number of narrow bands in the f-f alone cannot. region at similar energies. The most important One of the more interesting trends in the distinguishing characteristics of the spectra for redox energetics is that in the Category 2 this class of complex are the relative weakness complexes the potential separation between of both the f-f bands and the less-well-resolved,less-well-resolved, the U(V)/U(IV) couple and the U(IV)/U(III) higher-energy bands associated with f-d and couple remains nearly constant over the MLCT transitions. entire series of complexes (~2.1 V). A similar The second class of complexes contains potential separation has been found in most a hydrazonato ligand. For complexes in this of the other published voltammetric studies class, there is also a high degree of similarity of U(IV) complexes having both in the number and energy of the bands in the U(V)/U(IV) and U(IV)/U(III) couples. Thus, f-f regionregion of the spectra, as well as in the broad,broad, it appears that this potential separation is poorly resolved bands in the high-energy consistent across most U(IV) cyclopentadienyl region. For spectra in this class, however, the complexes having high electron density at the intensity of the bands in the f-f regionregion is on metal center and is not a strong function of the average about 50% greater than for those in the nature of the ancillary ligands. class discussed above, and for the broad band in the higher energy region, the intensity has Electronic structure increased dramatically. and properties The fi nal class of complexes includes the from optical imido and ketimido complexes. For spectra The UV-visible-near-infrared electronic in this last class there is an apparent decrease

absorption spectra for these (C5Me5)2U(R)(R´) in the number of f-f bands in the low-energylow-energy complexes all have two different energy re- region. The higher-energy region becomes gions of interest. The fi rst region, which lies in signifi cantly more structured, although the the energy region below ~15,000 cm-1, contains bands remain unresolved. The f-f bands areare the electronic transitions between the states even more intense in this class, and the bands derived from the 5f 2 electronic confi guration in the higher-energy region are comparable in

34 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004

intensity to those in the second class. Notably, transitions to facilitate coupling. The ligands the higher-energy bands extend to much also provide a mechanism for enhanced cova- lower energy in the spectra for this class than lent interaction with the metal center because in those of the previous two classes. of the presence of π orbitals of proper symme- The intensity variation across these three try to mix with the metal f orbitals. Thus, they classes of spectral data sets is clearly the provide a direct experimental observable (the most interesting and signifi cant result. The f-f band intensities) for the degreedegree of covalent intensities of the f-f bands for the fi rst class of interaction between the metal f orbitals and complexes are nominal for actinide complexes the ancillary ligands. The increase in intensity having a 5f 2 electron confi guration, and in the f-f bands in the thirdthird class of complexes refl ect the Laporte spin-forbidden character refl ects the fact that the MLCT transitions for of these transitions. The substantial increase these complexes extend to much lower energy in intensity observed in the f-f bands for the and can therefore interact more strongly with second and third classes suggest the presence the f-f states. of a new intensity-generating mechanism. The existence, intensity, and energetic proximity of Conclusions the broad, unstructured, higher-energy bands Important trends have been identifi ed in these spectra appear to hold the key to this in both the electrochemical and the spectral unusual trend in intensity behavior in the data for these organouranium complexes. f-f bands. Furthermore, these trends are very much We propose that the increase in intensity related to each other. The voltammetric data in the f-f bands for the second and thirdthird class illustrate that the nitrogen-bearing ligands of complexes is a direct result of an inten- result in greater electron density at the metal sity-stealing mechanism in the f-f transition center, destabilizing the U(III) oxidation state moment integral due to coupling of these elec- and signifi cantly stabilizing the U(V) oxidation tronic states to the higher-energy MLCT states state. These same ligand systems also induce in these nitrogen-containing complexes. This increased intensity in the metal-based f-f type of intensity-stealing mechanism refl ects a transitions by providing an intensity-stealing signifi cant degree of covalency in the bonding mechanism that refl ects stronger coupling to between the metal and ligand responsible for the charge-transfer excited states and indirectly the charge-transfer transition. The extent to demonstrates a greater degree of covalency which the mechanism adds intensity to the in the metal-ligand bonding than is observed f-f transitions is also relatedrelated to the energeticenergetic in the σ-donor ligand systems. Ultimately, separation between the metal-based states and these studies demonstrate that these important the charge-transfer states. The closer in energy trends can be most readily identifi ed when the states, the stronger the coupling and the synthetic methodologies are available to intensity-stealing process. provide a broad range of coordination The key feature of the present series of ura- environments to investigate. nium metallocene complexes that has made This research is supported by the DOE it possible to readily identify this interesting Offi ce of Basic Energy Sciences. electronic phenomenon is the incorporation of nitrogen-bearing ligands. These ligands in- troduce low-energy, ligand-based orbitals that engender the lower energy charge-transfer

Nuclear Materials Technology/Los Alamos National Laboratory 35 Actinide Research Quarterly

Provides a powerful examination of complicated mixtures of elements XAFS spectroscopy as a probe of actinide speciation with oxygen ligation

ne of the more fascinating characteristics of the coordination chemistry of the light actinide elements is the wide range Oof oxidation states (or valence) that they exhibit, coupled to a correspondingly broad span of structure, bonding, and reactivity. In its typical location at the extreme of behavior, plutonium in aqueous This article was solution can be found in oxidation states from III to as high as VII, and contributed by under the appropriate conditions at low pH, oxidation states III, IV, V, Los Alamos and VI can coexist. researchers Steven One of the most incisive methods that has been used for probing the D. Conradson of the structures associated with this chemistry is x-ray absorption fi ne struc- Materials Science ture (XAFS) spectroscopy. X-ray absorption techniques have a variety and Technology of advantages for the study of plutonium in diverse matrices. Every Division and David element absorbs x-rays at a different, characteristic energy that increases L. Clark of the with (Z), giving XAFS a high degree of element se- Nuclear Materials lectivity. In some samples x-ray fl uorescence from other elements can Technology Division. interfere with the signal of interest. In that case, one can move from examination of the L edge to another edge (or characteristic absorp- The photograph shows III the aquo ions of Pu tion energy) in the L shell, such as L , or L . This fl exibility makes XAFS II I in 1M very powerful for examination of complicated mixtures of elements. solution. Pu(V) is in With third-generation sources, XAFS can be measured to the parts per million concentrations of plutonium typical of many residues. For solution at pH=7 and standards, only a few milligrams of plutonium yield optimum spectra. Pu(VII) is in 2.5M Such small sample sizes are benefi cial when dealing with radioactive materials both to limit radiation exposure to the research scientist and to solution. limit the hazard to the environment and the public. Furthermore, the x-rays can be focused, allowing spot sizes of several microns in diameter instead of the typical 1.5 by 12 mm size of the native beam. This focusing, in addition to allowing microquanti- ties of plutonium to be probed, can be used to do spectromicroscopy that allows pinpointing or rastering selected ar- eas and imaging versus averaging of the chemi- cal environment across a sample.

36 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004

Plutonium has a high affi nity for oxygen- these two primary features, can be used to containing molecules and it is therefore determine the oxidation state of the target important to understanding the fundamental (x-ray absorbing) element in solution or the coordination chemistry and properties of plu- solid state. The exact energy at which these tonium-oxygen bonds. From this perspective, features appear depends on the ionization the plutonium aquo ions are the “baseline” energy of the ion. plutonium species in aqueous solution This increases with the because they only have water molecules as ion’s actual charge, so in general the XANES ligands to the central plutonium ion. Other shifts toward higher energy with increasing species form as different ligands substitute valence. We have observed this shift in the for one or more of the water molecules in the plutonium aquo ions. The fi gure below shows coordination sphere of the plutonium ion. a detailed view of the XANES spectra for Knowing the characteristics of the aquo a plutonium , Pu(0); the aquo ions of ions in detail can provide a starting point for Pu(III), (IV), (V), and (VI) all in perchloric acid understanding other plutonium complexes. solution; and Pu(VII) in a lithium hydroxide The aquo ions of plutonium in the (III) and solution. The shift in energy between succes- (IV) oxidation states have the general formula sive oxidation states is clearly visible, with 3+ 4+ Pu(H2O)n and Pu(H2O)n , respectively, which the exception of the spectra of the Pu(IV)–(V) are often designated simply as Pu3+(aq) and complexes where there is actually a small Pu4+(aq). But plutonium in the (V) or (VI) oxi- negative shift. dation state has such a large positive charge that, in aqueous solution, it extracts the O In this XANES from water to form the transdioxo () Pu(0)/Ga-stabilized cations PuO + and PuO 2+. The corresponding spectra of Pu(0) and 2 2 Pu(III)/HCIO the aqueous species aquo ions therefore have the general formulas 4 of Pu(III)–Pu(VII), + 2+ Pu(IV)/HCIO4 PuO2(H2O)n and PuO2(H2O)n , which are the absorption edge conveniently designated as PuO +(aq) and 2 Pu(V)/HCIO4 shifts toward higher PuO 2+(aq). energy and increases 2 Pu(VI)/HCIO Because the chemical environment sur- 4 as a function of rounding the plutonium ion in each solution Pu(VII)/LiOH oxidation state. The is very consistent (the only ligands are water change in energy molecules), we could look for behavioral or between Pu4+(aq) and PuO +(aq) is structural trends among the four oxidation 2 quite small or even states. Plutonium in the (VII) oxidation state is Pu Absorbtion a powerful oxidant and can only be stabilized negative, refl ecting a decrease in the for study in alkaline solutions, where its actual charge on reaction with water is thought to produce a the plutonium in the 3- tetraoxo anion of formula PuO4(OH)2 . plutonyl(V) species, but the two oxidation Identifying oxidation states states can easily There is structure to the x-ray absorption be distinguished by spectra and this x-ray absorption near-edge other features, such structure (XANES), consisting of the absorp- as the “-yl” shoulder tion edge, the absorption peak, and fi ne struc- 18045 18055 18065 18075 18085 in the XANES eV ture on (or before for some transition metals) spectrum.

Nuclear Materials Technology/Los Alamos National Laboratory 37 Actinide Research Quarterly

Actinide Speciation

Other parts of the XANES, however, can be used to help correlate a spectrum with an oxidation state. For example, the shoulder that ap- pears just after the main absorption peak in the spectra of Pu(V) or (VI) complexes in solution—the “actinyl” (or “-yl”) shoulder—can be used to distinguish those oxidation states from Pu(IV). We have observed nearly identical plots for other plutonium species with oxygen ligand

) environments—including plutonium , , carboxylates, 3 k

and oxides—in solution and the solid state. (In fact, other than the ~450 eV shift in the ionization energy as Z changes, we observe nearly (R, 3+

χ Pu (aq)

identical spectra for uranium and neptunium compounds as well.) Because the edge energies are independent of the chemical form of the plutonium, they can be used to identify the oxidation state of pluto- nium complexes in unknown chemical matrices. There is still specula- tion about the underlying reason behind the small shift between the Pu4+(aq) (IV) and (V) oxidation states. The shift toward higher energies depends on the actual charge of the ion, rather than its “formal” valence, and evidently the actual charge does not vary much between the two states. One possible explanation is linked to the formation of the plutonyl cations. The plutonyl cations have a linear structure, O=Pu=O. Recent PuO +(aq) 2 calculations have shown that bonding between the plutonium and oxy-

Radial Structure Function, gen atoms in the plutonyl has a substantial covalent character, reducing the actual charge of the central plutonium ion. The actual charge of the + plutonium ion in the Pu(V) complex PuO2 (aq) may therefore be very 2+ 4+ PuO2 (aq) similar to the actual charge of the plutonium ion in Pu (aq).

The structure of plutonium aquo ions 0 1 2 3 4 5 While changes in the local chemical environment surrounding plu- R-φ(Å) tonium have little effect on the XANES, they do have a signifi cant effect on the oscillations in the absorbance that occur at energies beyond the edge. In fact, the local 3+ In the image above, the Fourier transforms of the EXAFS of Pu (aq) molecular structure can be determined from and Pu4+(aq)—the top two blue lines—show only one major peak, which these oscillations, known as extended x-ray indicates that all the oxygen atoms of the water ligands lie in a single coordination shell. The amplitude of the peak indicates between eight absorption fi ne structure (EXAFS). Data from and nine oxygen atoms per shell. The bond length between (III) and (IV) the EXAFS region provide information about complexes contracts by 4% with the increase in charge, whereas the the local atomic-scale environment. number of ligands stays roughly the same. The molecule in the upper For simple structures with well-separated right shows a possible structure for n = 9, the tricapped trigonal prism. shells of neighbor atoms, a Fourier transform of + 2+ The Fourier transforms of the EXAFS of PuO2 (aq) and PuO2 (aq)—the the data results in a (phase-shifted and inverse bottom two blue lines—show two large peaks, which indicate two well- distance-squared weighted) radial distribution defi ned coordination shells. The fi rst peak corresponds to the two function that can be interpreted as shells of oxygen atoms of the plutonyl moiety, which are located at 1.74 Å from near-neighbor atoms surrounding the central the central plutonium ion. The second peak corresponds to the oxygen metal ion. The position and intensity of the atoms of the water ligands, which are located about 2.4 Å from the central peaks in the Fourier transform are related to plutonium ion. The equatorial coordination of the Pu(V) complex compared with the Pu(VI) complex shows a signifi cantly smaller number of water the absorber-scatterer distance and the number ligands, which are located at a longer distance. The molecule in the of atoms in each shell. lower right shows a possible structure for n = 5 water ligands, the pentagonal bipyramid.

38 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004

The fi gure on page 38 shows the Fourier to the two oxygen atoms in the plutonyl ion. transforms of EXAFS data for plutonium Analysis of the data indicates a Pu=O distance aquo ions in oxidation states (III)–(VI). For of 1.74 Å. The second peak in the Fourier Pu3+(aq) and Pu4+(aq), the data show only a transform corresponds to the oxygen atoms single large peak, which indicates that all the of the water ligands. It is known from other nearest-neighbor atoms (the oxygen atoms studies that all these oxygen atoms bond in the of the water ligands) lie evenly dispersed at equatorial plane of the plutonyl moiety, and the same distance from the plutonium ion. we deduce Pu–O distances that range from 2.4 All of these results were confi rmed by curve- to 2.5 Å. fi tting analysis, which also showed a Pu–O A signifi cant result of our research is the + distance of approximately 2.49 Å for the (III) fi nding that the PuO2 (aq) has a lower number state and 2.39 Å for the (IV) state. The shorter of water ligands (n = 4 to 5), all at a longer 2+ bond lengths for the (IV) state are associated bond distance, than PuO2 (aq), where n = 5 to with the higher charge of the central ion. The 6. This fi nding confi rms a trend that was seen number of water molecules bound to the plu- in studies of the actinyl ions of uranium and tonium is similar for both species, with neptunium, namely, that An(V) species ap- n = 8 or 9. (The Pu3+(aq) ion with n = 9 has peared to coordinate fewer ligands than An(VI) been isolated and characterized in the solid species. Because plutonium exhibits aquo ions state, see the article on page 7.) in four oxidation states, our experiments are + 2+ The transform of PuO2 (aq) and PuO2 (aq) the fi rst to observe this trend directly. data shows two peaks. The fi rst corresponds

To overcome the problem of samples having to be prepared at Los Alamos but studied at a synchrotron located a thousand miles away, researchers developed an electrochemical 0 minutes cell (left) where 25 minutes they could prepare samples in situ at Np Absorption 50 minutes the beam line. The 75 minutes series of XANES 100 minutes measurements (right) show the successful conversion of Np(VI)–(VII).

17605 17615 17625 17635 17645 eV

Nuclear Materials Technology/Los Alamos National Laboratory 39 Actinide Research Quarterly

Actinide Speciation

When other information is taken into account, the XAFS data are consistent with a + 2+ bipyramidal coordination geometry for PuO2 (aq) and PuO2 (aq). The plutonyl moiety forms the axis of the bipyramid, and, depending on conditions and the ligand used, the geometry may be a tetragonal bipyramid (four ligands in the equatorial plane), a pentagonal bipyramid (fi ve ligands), or a hexagonal biypramid (six ligands with bidentate species such 2– as CO3 ). We have used the plutonium aquo ions to establish the baseline data and oxidation state trends necessary to determine the oxidation states of plutonium complexes in matrices of unknown composition. This background data and understanding have allowed us to apply XAFS spectroscopy to characterize plutonium and other actinide species in a wide variety of chemical environments ranging from contaminated soils (see ARQ 1st Quarter, 2002) to process solutions used in our weapons production mission.

Unusual oxidation states—spectroelectrochemistry It is diffi cult to study the unusual oxidation state (VII) at a synchrotron facility due to the length of time between sample preparation at Los Alamos and the actual measurement of its spectrum at a synchrotron facility a thousand miles away. To overcome this obstacle, we developed an electrochemical cell wherein we could prepare Np(VII) and Pu(VII) in situ at the beam line. (For a discussion of related structural studies of Np(VII), see the article on page 15.) In alkaline solution, Np(VI) exists as a dioxo ion, while Np(VII) displays a tetraoxo unit. These changes in alter the spectral shape of the XANES, which shows two distinct peaks of comparable amplitude for the tetraoxo species instead of the single primary peak and shoulder characteristic of the dioxo structure. A 0.8 eV shift is observed in the edge between Np(VI) and (VII) complexes, consistent with the higher charge of the latter. The research on aqueous complexes was supported by a Laboratory Directed Research and Development (LDRD) project administered by the Glenn T. Seaborg Institute at Los Alamos National Laboratory. The alkaline chemistry of Np(VII) is supported by the DOE Offi ce of Basic Energy Sciences. XAFS experiments were performed at the Stanford Synchrotron Radiation Laboratory, which is supported by the DOE Offi ce of Basic Energy Sciences.

40 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004

Transuranics away from home

When Los Alamos programs call for during the Tiger Team era, the fi rst experiments performing experiments with transuranics at a at the Stanford Synchrotron Light Source synchrotron x-ray source, how do you do it? were performed in July 1993 after an approval To safely study process that took almost two years. transuranics at the There are only three light sources in the Stanford Synchrotron United States: Stanford Synchrotron Light Since that fi rst 10-day run, scientists, Light Source, Source, Argonne National Laboratory, and technicians, and students from Los Alamos Brookhaven National Laboratory. Less than and many other institutions—including researchers had to 1 ppm of plutonium in a 200-mg Rocky Argonne, Pacifi c Northwest and Oak Ridge outfi t the experimental Flats concrete sample may be close to an National Laboratories; Great Britain’s Atomic area with controlled, exempt quantity, but Pu-238-spiked alloys or Weapons Establishment (AWE) ’s Centre fi ltered ventilation Hanford tank sludges with contact doses of de Valduc; the Australian Nuclear Science and continuous air 20 mrem/hr per hour begin to approach the and Technology Organization; the Institute monitors. limits for these nonnuclear facilities. And of Transuranium when the sample is a salt in a beryllia holder Elements (ITU), that is supposed to be melted at 800 °C or an Karlsrühe, Germany; electrochemical cell, then handling even a few the Paul Scherrer milligrams of plutonium becomes a problem. Institute; Rocky Flats; and a number of The answer to this question is to temporarily both foreign and U.S. construct a piece of equipment like that used universities—have in Los Alamos’ Plutonium Facility (PF-4) at the collaborated in XAFS experimental site. and x-ray-scattering measurements on By working with personnel from Los Alamos’ transuranic-containing Health Physics Operations Group—integrated samples that require safety management long before the phrase the unique properties of was in common use—sample holders are synchrotron radiation. designed, fabricated, tested, loaded, and These measurements shipped to the synchrotron, where both the provide results for transfer (samples can never be enclosed programs concerned in less than three layers of containment with weapons at any time) and experimental areas have materials, nuclear been outfi tted with controlled, HEPA-fi ltered fuels, environmental ventilation, continuous air monitors, and restoration, waste other equipment far less common at the light storage, separations sources than it is here. chemistry, and other aspects of both While Los Alamos radiological control basic and applied technicians place samples in tertiary actinide chemistry and containers and mount them in the materials science. diffractometer or on the XAFS sample positioner and monitor the operations, Since this pioneering the scientists on the team heat, cool, and step by Los Alamos, electrolyze the samples and perform the many other groups actual measurements according to the 24/7 have duplicated synchrotron schedule. this feat, and measurements Although many measurements on on transuranic transuranics had already been performed samples have become so common that most An aerial view of the at synchrotrons—including many by Los light sources are now being equipped with Stanford Synchrotron Alamos—prior to the initiation of this program, dedicated facilities for radioactive samples. Light Source. this was nevertheless the fi rst time that a long-term program of this type was proposed And when we’re done, we break it down, put involving many different types of materials it away until the next run, and return home to and conditions on a continuous basis, and catch up on our sleep and analyze the data. just as the awareness of safety concerns began to be heightened. Conceived in 1991,

Nuclear Materials Technology/Los Alamos National Laboratory 41 Unraveling the coordination chemistry will improve plutonium separation by precipitation processes Structural characterization of compounds targeted in plutonium separation for the past 50 years

ince the early days of its discovery, the chemical separation and purifi cation of plutonium has been directly correlated This article was Swith its applications in weapons and technolo- contributed by Los gies. However, large-scale processing schemes were often dictated by Alamos researchers historically required chemical conditions that were found to be the most Wolfgang Runde convenient for precipitation, , or solvent extraction. The and Brian L. Scott pragmatic focus on developing a separation scheme outweighed the of the Chemistry need for a more fundamental understanding of the technical basis for Division; and plutonium separation. Amanda C. Bean, In more modern times, changing regulatory requirements for lower Kent Abney, and levels of contaminants in waste waters requires process optimization Paul H. Smith of the that, without a fundamental understanding of how the process works, Nuclear Materials Technology Division. will likely not be achieved. Examples are the precipitation of plutonium 2– (C2O4 ) and iodate – (IO3 ) compounds, which were recog- nized early on as excellent candidates for plutonium separa- tion owing to their remarkably low solu- bility in slightly acidic solutions. Oxalate forms strong complexes with the lower oxida- tion states, Pu(III) and (IV), whereas iodate can effectively precipi- tate Pu(III), (IV), and (VI). In fact, the use of oxalate to separate plutonium from solution has long been implemented into plutonium Blue Pu(III) oxalate purifi cation and waste-treatment processes, while plutonium iodate crystals. precipitation has been used less abundantly for analytical purposes. Despite the fact that oxalate and iodate compounds have been applied for about 50 years in large-scale separations, the nature and composi- tion of the actual precipitates are still under debate. Unraveling the un- certainties in the chemistry of these compounds is necessary to improve and optimize the precipitation processes currently used.

42 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004

Plutonium Separation

The solid plutonium oxalate system is that the binary Pu incorporate cations dominated by two hydrates (crystals incorpo- (Ca2+, Na+, or K+ ions) in exchange for lattice rating water molecules into their structures), waters. This exchange would increase the • • Pu2(C2O4)3 10H2O and Pu(C2O4)2 6H2O, for distance between the plutonium oxalate layers which only powder x-ray diffraction data have as has been observed in many-layered mineral been reported. Several different hydrates are phases. In the presence of K+ cations and under suggested to exist at different temperatures, without strong experimental evidence regard- ing how the water molecules are actually bound within the respective crystals. To gain better insight into the oxalates, we obtained single crystals of blue Pu(III) oxalate by slowly reducing Pu(IV) oxalate in 1 M HCl acid at ambient temperature. The x-ray diffraction analysis revealed this compound to be the hydrated Pu(III) oxalate of formula • Pu2(C2O4)3(H2O)6 3H2O. (We write the formula this way to indicate that three H2O molecules are directly bound to each Pu atom, while Ball and stick illustration of the Crystal packing in three H2O molecules are incorporated in the 3- • crystalline lattice.) Pu(C2O4)3(H2O)3 unit in the solid Pu(III) Pu2(C2O4)3(H2O)6 3H2O viewed down • The two-dimensional compound consists oxalate Pu2(C2O4)3(H2O)6 3H2O. The dark the crystallographic b axis. The two- lines represent the trigonal prism with dimensional structure contains layers of [PuO9] polyhedra that are linked by [C2O4] groups and a network of interstitial water mol- the Pu atom (green) in the center, which that orient themselves in the ac plane. is coordinated to nine O atoms (red). The PuO units are represented as ecules. The plutonium atom is coordinated to 9 The prism is capped by three O atoms green polyhedra; the planar oxalate six oxygen atoms from three bidentate oxalate 2- contributed from three oxalate (C2O4 ) groups are black. ligands and three oxygen atoms from coordi- groups. C atom (black). nated waters in a distorted tricapped prismatic geometry. In this geometry,geometry, six oxygen atoms form the vertices of a trigonal antiprism and each of the three faces is capped by water. The Pu–O distances range between 2.48 Å and 2.57 Å. In the complex mixtures of actual plu- tonium process streams and the presence of high concentrations Arrangement of the oxalate ligands View of the three-dimensional structure of of pyrochemical salt in the Pu(C O ) (H O)5- units 2 4 4 2 the Pu(IV) oxalate KPu(C O ) (H O)•2H O waste such as CaCl or 2 4 2 2 2 2 that build up the Pu(IV) oxalate, along the ac plane. Water molecules (red) Na/KCl it is expected KPu(C O ) (H O)•2H O. The nine- 2 4 2 2 2 and K atoms (purple) are aligned down the coordinate Pu atom (green) is b axis. The PuO9 units are represented as surrounded by eight O atoms (red) green polyhedra; the planar oxalate groups from four oxalate ligands and one are black. water molecule. C atom (black).

Nuclear Materials Technology/Los Alamos National Laboratory 43 Actinide Research Quarterly

chelating oxalate (Pu–O = 2.48–2.54 Å) ligands are arranged around the plutonium atom while one hydroxide ligand (Pu–OH = 2.48 Å) lies above the plutonium atom. This coordination geometry is similar to Illustration of the PuO (IO ) 3- Crystal packing in PuO (IO ) •H O. The two- 2 3 5 2 3 2 2 that found for the limiting complex dimensional solid (viewed down the a axis) building block in the Pu(VI) iodate of trivalent in carbonate PuO (IO ) •H O. The Pu atom (green) contains infi nite staggered Pu(VI) iodate layers 2 3 2 2 solution, Nd(CO ) (H O)5-. It is very that are separated by water molecules. The 3 4 2 is coordinated in a pentagonal 5- likely that Pu(C2O4)4(OH) is the bipyramidal geometry to fi ve PuO7 units are represented as greengreen polyhedra;polyhedra; iodate anions, IO - (I = purple,purple, the trigonal iodate groups are purple. O atoms limiting solution complex at high 3 oxalate concentrations and assumes O = red) and to two O atoms within from lattice H2O molecules are red. the characteristic linear plutonyl, very similar coordination geometry [O=Pu=O] 2 +, moiety. as found in the extended structure of • KPu(C2O4)2OH 2H2O. hydrothermal conditions at 180 ˚C, green crys- Iodate precipitation was used for oxidation talline needles of the ternary Pu(IV) oxalate, state determination of plutonium in October • 1942 when Pu(IV) was precipitated from HNO KPu(C2O4)2OH 2H2O, were obtained. 3 Surprisingly, this compound consists of a solutions upon addition of HIO3 or KIO3. The three-dimensional framework built up from calculated molecular weight did not match the suggested formula, Pu(IO ) , indicating oxalate-linked [PuO9] polyhedra. Although 3 4 plutonium again exhibits a coordination num- the possible presence of KIO3 or HIO3 in the ber of nine, the local coordination geometry is solid or even the presence of other plutonium quite different. Eight oxygen atoms from four oxidation states. Due to the oxidizing nature of iodate, synthesizing a crystalline Pu(III) or (IV) iodate has proved an intractable task, raising some doubt as to the validity of a Pu(IV) io-

date precipitate. The use of KIO4, I2O5, or H5IO6 - - (E˚(IO4 /IO3 ) = 1.65 V) led to the complete oxi- dation of lower plutonium oxidation states to 4+ 2+ Pu(VI) (i.e., E˚(Pu /PuO2 ) = 0.98 V) and the crystallization of a Pu(VI) iodate of formula • PuO2(IO3)2 H2O . The binary plutonyl(VI) iodate, • PuO2(IO3)2 H2O , is unique in its structure and displays previously unknown actinyl-iodate coordination. This plutonyl compound is • Coordination environment of Am(III) isostructural with that of NpO2(IO3)2 H2O (green) in the pseudo-tricapped and consists of infi nite staggered layers of trigonal prismatic coordination in Crystal packing in K Am (IO ) •HIO . PuO2(IO3)2 with water molecules arranged K Am (IO ) •HIO . All oxygen atoms (red) 3 3 3 12 3 3 3 3 12 3 Views of the K (blue)-lined channels between the layers. The layers are made up of originate from eight iodate (IO -) anions,anions, 3 formed along the c axis, which are pentagonal bipyramidal [PuO ] polyhedra that which link the Am polyhedra in a three- 7 built up from alternating [AmO ] are connected by bridging pyramidal IO units. dimensional framework. The longer bond 8 3 polyhedra (green) and iodate (purple) between the Am center and the O atom ligands with the neutral HIO (purple)(purple) (dashed line) from the neutral HIO molecule 3 3 molecules staggered in the center of completes the ninefold coordination of the the channel. Am atom.

44 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004

Axial Pu=O bond lengths are 1.75Å, Although this is the fi rst architecture of its 2+ characteristic for PuO2 , and equatorial kind among the f-element iodates, this struc-struc- Pu–O distances range between 2.33 and ture suggests the possibility of the synthesis 2.42 Å. Two crystallographically unique of new microporous lanthanide iodates with - iodate anions, IO3 , connect the plutonium at- unique selectivity properties for ion-exchange, oms. The [I(1)O3] groups join two plutonium , and photochemical processes. It atoms through bridging oxygen atoms, leav- still must be shown if other molecules of ing one oxygen atom terminal; in contrast, varying size and shape can replace the trapped the other iodate group, [I(2)O3], coordinates neutral molecules and if the replacement of three plutonyl units. This coordination has cations along the channel exterior only been observed previously in the trivalent will affect the retention strength of the lanthanide iodate compounds. trapped molecules. To investigate the formation and coordina- From an academic perspective, struc- tion chemistry of Pu(III) iodates, the chemi- tural characterization of transuranium cally analogous Am(III) was used because Am complexes remains rare, and only about a remains stable in its +3 oxidation state even in dozen single crystal structures of americium the presence of any of the iodate sources men- compounds are known. As demonstrated in 3+ 2+ • tioned (E˚(Am /AmO2 ) = 1.69 V). Pink crys- K3Am3(IO3)12 HIO3, the coordination tals were successfully synthesized by reacting chemistry of americium may vary from its 243 Am(III) in 3 M HCl acid at 180 ˚C with KIO4 chemically analogous lanthanides and offers solutions. X-ray diffraction analysis revealed new insight into differences in bonding of 4f • the compound to be K3Am3(IO3)12 HIO3 , a and 5f elements. three-dimensional framework of [AmO8] units After more than 50 years of technological bridged by corner-sharing [IO3] pyramids. application of oxalate and iodate compounds, Eight oxygen atoms from eight iodate groups highly crystalline plutonium compounds with Am–O distances between 2.42(3) and have now been prepared using hydrothermal 2.60(3) Å form a distorted bicapped trigonal synthesis techniques, which has allowed for prismatic coordination polyhedron. determination of their structural nature by The most interesting aspect of this com- x-ray diffraction. under pound lies within the microporous channel hydrothermal conditions has proven to be a framework. Three [AmO8] and three [IO3] valuable method for targeting actinide com- groups form irregular hexagonal channels pounds previously only known as amorphous that are approximately 4.6 Å in diameter. or microcrystalline materials. Chemical condi-

Neutral HIO 3 molecules are staggered in the tions that mimic those used for large-scale center of these channels and potassium cat- plutonium separation and purifi cation are ions line the cavity with close contacts to the used to trigger crystallization under process

HIO3 molecule (K–O = 2.40(4) Å). The neutral conditions, and the resulting solid precipitates

HIO3 molecules are anchored in the cavity can subsequently be structurally characterized. center through a between This work was supported by the Glenn T. the oxygen atoms of the HIO3 molecules and Seaborg Institute Summer Student fellowship the americium atoms with a longer Am–O program and the Pu Stabilization and Scrap distance of 2.93(4) Å. Recovery Program, under the direction of project leader Paul H. Smith.

Nuclear Materials Technology/Los Alamos National Laboratory 45 Actinide Research Quarterly

Powder x-ray diffraction

The powder diffraction capability in Los Alamos’ Chemistry Division has two diffractometers, which are capable of running a wide variety of samples containing radioactive . This experimental technique can be used for phase identifi cation of polycrystalline samples. In addition, a new capability is being developed to determine full crystal structures from high-quality polycrystalline samples. Thorium- and uranium-containing samples can be run with a polymeric containment procedure on a 12- position autosampling attachment. Transuranic samples can be run in an airtight environmental chamber, which protects workers and the environment from exposure.

A 12-position autosampler is attached to this x-ray powder diffractometer. The round green disk in the center of the photo is a sample in position for diffraction. The diffractometer can also be fi tted with an airtight environmental chamber (inset). X-rays enter and exit through a window at the top of the chamber. The environmental chamber can be removed and loaded in a glovebox or hood.

The plot of an x-ray diffraction pattern of polycrystalline 20x103 Cs2UO2Cl4 is shown in red. The molecular structure 15 corresponding to the

diffraction pattern 10 is depicted in the Intensity background. Marianne 5 Wilkerson of Los Alamos’ Chemistry 0 Division synthesized 10 20 30 40 50 60 the sample. two-theta (°)

Photos by Josh Smith, C-OPS

46 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004

Single-crystal x-ray diffraction

The single-crystal x-ray diffraction capability in Los Alamos’ Chemistry Division is a world leader in crystal structure determination of small molecules containing actinide elements. A crystal structure is determined when a crystal is placed in an x-ray beam and the diffraction pattern is measured and analyzed, resulting in a three-dimensional picture of the molecules comprising the crystal. Researchers have determined 41 transuranic-containing structures in the past 9 years, including plutonium, neptunium, americium, and curium. In addition, more than 300 thorium- and uranium-containing crystal structures have been determined. These structures build on the results of Los Alamos scientist William Zachariasen, who in the 1950s and 1960s determined more than 180 actinide structures of extended solids and alloys. These molecular structures are the foundation for understanding materials and molecular properties and have appeared in scores of papers, including a recent issue of Nature (Nov.(Nov. 21, 2002) and the cover of the International Edition of Angewandte Chemie (1997).

To mitigate the associated health hazards of This close-up of working with transuranic elements, researchers a broken capillary have developed a triple-containment technique containing a crystal for the crystals used in these experiments. The shows how the triple- crystal is fi rst coated in epoxy and placed on a containment technique glass fi ber (left), the crystal and fi ber are then maintains the integrity placed in a capillary and sealed (middle), of the sample and and the capillary is coated with acrylic nail polish prevents sample (right). The acrylic coating keeps the capillary loss in the event of and contents in place in the event of breakage. breakage.

Crystallographer Brian Scott of the Chemistry Division adjusts a crystal on the single-crystal diffractometer.

The molecular structure of + NpO2(18-crown-6) .

Photos by Josh Smith, C-OPS

Nuclear Materials Technology/Los Alamos National Laboratory 47 Actinide Research Quarterly Room-temperature ionic liquids Unusual solvent system shows potential to revolutionize separation and purifi cation technologies

This article was oom-temperature ionic liquids (RTILs) represent an intriguing contributed by Los and highly unusual solvent system in which to carry out basic Alamos researchers research and develop advanced separation and purifi cation Warren J. Oldham R technologies for the actinide elements. The ionic liquids themselves of the Chemistry are the subject of intense worldwide interest, as much for their curious Division and Michael properties as for their potential in revolutionizing chemical synthesis, E. Stoll and David A. Costa of the Nuclear catalysis, separations, and electrochemistry. Materials Technology Part of the optimism surrounding ionic liquids rests on their im- Division. proved safety characteristics as compared to volatile organic and their ability to solubilize both nonpolar organic solutes as well as highly charged inorganic ions. In many cases ionic liquids combine the solvating properties of polar solvents such as water with those of non- polar organic solvents such as . The ionic liquids are character- ized as having negligible , thereby eliminating health effects through airborne exposure, and signifi cantly reducing fl am- mability hazards compared to traditional organic solvents. Additionally, these solvent systems offer a novel chemical environment that may uniquely infl uence the course of chemical reactions as compared to traditional molecular solvents. By way of defi nition, the ionic liquids used in our work are low- melting organic salts, distinct from other classes of solvents in that they are composed of separate and discrete organic cations and weakly coordinating anions. These solvents are nonproton donating and are not related to the more familiar, self-ionizing + Classical inorganic salts consist of infi nite three- solvents such as water (2 H2O = H3O + – –14 dimensional arrays of close packed spherical ions and are OH ; ionic product = 10 ), anhydrous characteristically brittle solids with high melting points (for hydrofl uoric acid, or the various example, the of NaCl is 801 °C). Low-melting “super acids.” ionic liquids are obtained through two molecular design In many respects, RTILs are most principles. First, coulombic interactions are minimized by closely related to classical molten salts. diffusing charge over several atoms in a molecule. In the In theory, some 10 trillion different case of cations, the 1,3-dialkylimidazolium cation/anion combinations can be used unit (1) is a common example that spreads the positive charge over the fi ve atoms of the heterocyclic ring. Weakly coordinating anions delocalize charge through induction to highly electronegative fl uorine atoms or by resonance as – shown explicitly for the N(SO2CF3) 2 anion. More-complexMore-complex and higher molecular weight cations and anions have also been explored, but the greater number of intermolecular contacts serve to raise melting points and increase the viscosity in such systems. For convenience in routine use as solvents, lower molecular weight constituents are preferred. The second design principle in ionic liquid synthesis is molecular asymmetry. All things being equal, low-symmetry cations and anions reduce packing effi ciency in the crystalline state and lower melting points. A dramatic – example is provided by the comparison of the PF6 and – N(SO2CF3) 2 salts of cation 1, which melt at 58 °C and –3 °C, respectively.

48 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004

to produce binary ionic liquid systems. This Salts composed of stronger Lewis acids/ fl exibility affords the potential to tune or “de- bases are more likely to exist as solids at room sign” the ionic liquid solvent for specifi c ap- temperature. These studies highlight the dif- plications. In practice, the requirements for low ferences between ionic liquids and classical viscosity and ease (low cost) of synthesis have high-temperature molten salts. The solvating limited the choice of cation to relatively low properties of the ionic liquids more closely molecular weight organic salts complemented resemble weakly coordinating organic sol- by weakly coordinating inorganic anions. vents, with the distinct difference that they Recent work has employed imidazolium or are highly conductive and the ionic con- quaternary ammonium salts of the bis(trifl uo stituents can move independently to stabilize romethanesulfonyl) anion (N(SO2CF3)2 charged solutes. – abbreviated as NTf2) because they are easily A comparison in the behavior of two prepared, chemically and thermally robust, simple U(IV) chloride complexes illustrates the and yield low-viscosity/high-conductivity an- unique chemistry observed in ionic liquids as hydrous fl uids. Because of their good solubili- compared with traditional molecular solvents. zation characteristics and wide electrochemical Tetrabutylammonium salts of U(IV) hexachlo- window, these solvents are an ideal choice for ride, [NBu4]2UCl6, readily dissolve in the ionic studying the basic electrochemical behavior of liquid solvent to give pale blue solutions with soluble actinide complexes with an eye toward a limit of about 0.1 M. Probing the developing redox-based separations technol- solutions via cyclic voltammetry experiments ogy. Our research has shown that high-quality indicates that reduction of the U(IV) complex electrochemical data can be obtained in these to U(III) or oxidation to U(V) is chemically ionic liquids on par with what is possible in reversible and that the six chloride ligands conventional electrochemical solution media. remain tightly bound to the metal center We have also focused on understanding the throughout these electron-transfer reactions. solvent (i.e., coordinating) properties of these In a similar fashion, the coordinatively ionic liquids. Using a combination of Lewis- unsaturated complex, acid- and Lewis-base-specifi c solvatochromic UCl4, also dissolves with gentle warming to probe molecules that change color as a func- give deep-green solutions with a solubility tion of chemical environment, we fi nd that limit of about 0.4 M at room temperature. It the Lewis-acid properties of the ionic liquids is known that UCl4 dissolves in coordinating depend on the organic cation and are generally organic solvents such as acetonitrile or THF comparable to aliphatic alcohols such as etha- to give eight coordinate adducts of the form, nol or . The Lewis basicity, determined UCl4(L)4 (L = donor solvent). In contrast, we – by the NTf2 anion, lies between very weakly have discovered that when dissolved in the coordinating solvents such as nitromethane ionic liquid, UCl4 undergoes a facile chloride and more-coordinating solvents such as THF. redistribution reaction to give mixtures of –2 The discovery that ionic liquids are simul- [UCl6] and additional species stabilized by taneously weakly Lewis acidic and weakly fi ve or fewer chloride ions. Well-formed x-ray-

Lewis basic is consistent with the fact that quality single crystals of [cation]2UCl6 readily these salts are liquids at room temperature. separate from these solutions, where the cation This observation indicates that they are charac- identifi ed in each case is derived from the par- terized by very low lattice energies determined ticular ionic liquid used in the experiment. by weak coulombic interactions between the cations and anions.

Nuclear Materials Technology/Los Alamos National Laboratory 49 Actinide Research Quarterly

Ionic Liquids

Ligand redistribu- Single-crystal x-ray diffraction experiments tion of this type is for a number of different metal complexes surprisingly common revealed that four different binding modes – in actinide coordina- were possible for the NTf2 anion. The anion tion chemistry, driven can bind in a monodentate fashion either by the stabilization through the nitrogen atom or through one of afforded by maximiz- the sulfonyl oxygen atoms. For less sterically ing steric saturation. constrained metal species, two different biden- For example, while tate binding modes were also identifi ed, either

UCl4(DMSO)3 appears via two oxygen atoms from each end of the to be a reasonable bis(sulfonyl)imide moiety or via a more acute complex, in fact this nitrogen-oxygen interaction. Electronically the – system exists as a com- NTf2 anion behaves as a weakly coordinating plex salt of the form anion that donates limited electron density to

[UCl2(DMSO)6][UCl6] the metal center. – (DMSO = dimethyl- The redox consequences of NTf2 ligation – sulfoxide). In NTf2- are clearly illustrated by comparison of the + + based ionic liquid 4 /3 redox couples for (C5H5)2TiCl2 and

systems, coordinately (C5H5)2Ti(NTf2)2. While the dichloride The –NTf anion 2 unsaturated uranium system is reversibly reduced at about –1.43 V provides a range of species are no doubt stabilized by interactions versus Ag/Ag+ reference electrode, the coordination modes with the anion. We have shown that these in- (C H ) Ti(NTf ) complex is reduced at –0.49 V, to accommodate 5 5 2 2 2 teractions are weak by adding two equivalents a stabilization of 0.94 V. These are very large ef- the particular electronic and steric of chloride ion (as the tetrabutylammonium fects that suggest the potential to signifi cantly –2 preferences of a salt) to yield pure [UCl6] solutions. stabilize low-valence metal species in the ionic metal ion. Softer Given the unexpected complexity of the liquid medium. The possibility of directly elec- metals, represented chloride system and the implication that the troplating very active actinide metals is also – by low-valent NTf2 anion is playing an active role in the suggested by the stabilization of low-valence transition metal coordination chemistry of unsaturated metal complexes. We recognized these observations complexes, display species, we have set out to document the as an opportunity to directly study the sim- coordination to the fundamental binding properties of this anion plest of metal systems by introducing “bare” nitrogen atom as and to help understand how these interactions metal ions into the ionic liquid solvent in the 1 2 η -N or η -N,O. High- determine resultant redox properties. absence of any competing ligands. valent transition A series of coordinatively unsaturated In the case of uranium, we found that reac- metal and actinide complexes prefer metal species was generated either through the tion of UCl4 with four equivalents of AgNTf2 oxygen coordination reaction of metal alkyl complexes with HNTf2 or anodic oxidation of a uranium electrode as η1-O or η2-O,O, or through reaction of metal halide complexes immersed in the ionic liquid solvent produced

depending on with AgNTf2, as illustrated for the (C5H4Me)3U deep green U(IV) solutions that were identical the availability of fragment shown in the following equation. when characterized by UV/vis spectroscopy. one or two open These results suggest that the U(IV) ion is coordination sites. – likely stabilized by ligation to four NTf2 an- ions, each coordinated in a bidentate fashion through the oxygen atoms. Such a binding

50 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004

mode is reminiscent of well-characterized characterization. These solvents do, however, actinide ß-diketonate complexes. allow facile spectroscopic and electrochemical characterization opportunities for the study of novel actinide coordination chemistry. The study of the actinides in Similar to the titanium complex, the effect ionic liquids is therefore complementary to of removing the chloride ligands from the parallel investigations in aqueous and U(IV) metal center is profound. In the case organic solvents, as well as high-temperature –2 + + of [UCl6] the 4 /3 reduction potential is molten salts. measured at –1.98 V versus Ag/Ag+, while for Ultimately, the prospect of preparing new

“U(NTf2)4” this reduction occurs at –0.24V, a classes of compounds and discovering new stabilization of 1.74 V. Additionally, despite reactivity patterns that might be exploited in the extreme electrophilicity (strongly electron- advanced separations technology guides this attracting) indicated by this shift in reduction work. Given the complementary roles of potential, no evidence was found for decom- aqueous/organic solvent extraction, ion position of the ionic liquid in the presence of exchange , and high-tempera- such a potentially reactive U(IV) ion. The ture molten salt electrorefi ning in current behavior of the electrochemically produced actinide processing and purifi cation practice, U(III) ions at more negative electrode poten- we believe that ionic liquids might be used to tials is under active investigation. develop new technologies that combine these The study of actinide elements in ionic liq- disparate tasks into one integrated unit opera- uid solvents presents a number of challenges tion with possible advantages in production in the areas of product isolation and structural effi ciency, safety, and space requirements.

U(IV) cations can be introduced into the ionic liquid solvent without the complication of additional halide anions by electrochemically dissolving a sacrifi cial uranium anode in a two-compartment cell. An electrochemically etched uranium anode produced by this process is shown in the photo. Spectroscopic characterization of the resulting green solutions indicate that U(IV) is the dominant oxidation state. The uranium ions are likely to be – stabilized by the NTf2 anions that compose the

ionic liquid and the proposed structure of U(NTf2)4 is indicated in the top right. A comparison of the 2 – electrochemical behavior of [UCl6] and U(NTf2) 4 is shown in the lower right. The 4+/3+ reduction potential is increased by 1.74 V upon elimination of the six chloride anions from the metal’s coordination sphere.

Nuclear Materials Technology/Los Alamos National Laboratory 51 Actinide Research Quarterly

Los Alamos played key role in bilateral effort Russian secure storage facility commissioned

This article was fter an extensive twelve-year The Laboratory’s achievements over the contributed by Los bilateral effort, the Russian Fissile life of the project were numerous, despite Alamos researcher Material Storage Facility (RFMSF) many obstacles. Within this challenging F. Jeffrey Martin, A located at Mayak was commissioned by environment Los Alamos provided the of the Nuclear Minatom Dec. 10, 2003. The RFMSF is a United Russians extensive access to Western methods Nonproliferation Division. States-sponsored secure storage facility built to and technology to perform their own nuclear hold fi ssile material derived from dismantled safety analysis that would stand up to inter- Russian nuclear weapons. The complex is national scrutiny. Lab researchers reviewed about the size of the Pentagon’s footprint and the thermal design of the RFMSF and thermal will be the largest, most secure such facility environment of the plutonium and led a large in Russia. Russia plans to load 24 metric tons bilateral thermal program of analysis and full- of weapons useable plutonium, but it has a scale experiments that confi rmed a substantial capacity of 100 metric tons, should additional thermal margin in the design. This effort excess material be declared. determined the plutonium capacity could be This $412M project is one of the most suc- increased from 33 to 100 metric tons, suffi cient cessful Nunn–Lugar programs. It is expected to hold all of Russia’s excess plutonium for the to become fully operational and begin receiv- foreseeable future. ing plutonium sometime in 2004, after all inte- Los Alamos also completed a technically grated systems testing and personnel training comprehensive safety and ecological analysis have been completed, and security measures and evaluation that confi rmed the safety ade- have been activated. Completion of this facility quacy of the facility per U.S. and International is considered to be the largest single step to- Atomic Energy Agency (IAEA) safety stan- ward safeguarding the international stockpile dards; evaluated the adequacy of the facility of weapons useable material and contribution measures for fi ssile material control, physical to international nonproliferation efforts. protection and accountability, and facility se- Los Alamos played a key and continuous curity; and provided the technical backbone of role from the project’s inception to its comple- the U.S. transparency program by conducting tion with more than 190 staff participating. an extensive radiation measurement campaign Over the history of the RFMSF project, Los for a database for nondestructive fi ssile ma- Alamos had three broad roles: to provide ad- terial mass measurements and providing vanced nuclear material facility technology to expert support to the U.S. transparency modernize the Russians’ approach to nuclear negotiating team. safety, and fi ssile material control, and ac- These many efforts culminated in the countability; to technically evaluate the facility successful completion of the facility and design, construction, and pre-operational state confi rmation that its is capable of making to confi rm the facility will meet its mission and the world a safer place. provide safe, secure, and ecologically sound storage of fi ssile material; and to provide and evaluate state-of-the-art technology for the Defense Threat Reduction Agency to support facility transparency measures for future verifi - cation activities during operations.

52 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2004

Actinide Research Quarterly is produced by Los Alamos National Laboratory

About the Quarterly ARQ Staff

Actinide Research Quarterly high- NMT Division Director lights recent achievements and ongoing pro- Steve Yarbro grams of the Nuclear Materials Technology (NMT) Division. We welcome your sugges- Scientifi c Advisors, tions and contributions. ARQ can be read on Seaborg Institute the World Wide Web at: David L. Clark Gordon D. Jarvinen AQarchive/AQhome/AQhome.html Editor Address correspondence to Meredith S. Coonley, IM-1 Actinide Research Quarterly c/o Meredith Coonley Designer Mail Stop E500 Susan L. Carlson, IM-1 Los Alamos National Laboratory Contributing Writers/Editors Los Alamos, NM 87545 Vin LoPresti, IM-1 Garth Giesbrecht, NMT-DO If you have questions, comments, suggestions, Ed Lorusso, NMT-3 or contributions, please contact the ARQ staff at [email protected]. Printing Coordination Lupe Archuleta, IM-4 Phone (505) 667-0392 Fax (505) 665-7895

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Nuclear Materials Technology/Los Alamos National Laboratory