Actinides in Molecules : Exotic Properties Probed by X-Ray Absorption Spectroscopy

O21-04

Actinides in molecules : exotic properties probed by X-ray Absorption Spectroscopy

C. Den Auwer1, P. Moisy1, P. Guilbaud1, D. Guillaumont1, E. Simoni2, S. D. Conradson3

1CEA Marcoule DEN/DRCP/SCPS, 30207 Bagnols-sur-Cèze Cedex, France 2 Université Paris Sud, IPN, 91405 Orsay Cedex, France 3 Los Alamos Nat. Lab., MST Division, Los Alamos, NM 87545, USA

Abstract – Dealing with actinide elements in molecular chemistry may result in particularly attractive and exotic physico-chemical properties. In solution, one of the spectroscopic tools able to selectively probe the structural or electronic properties of these molecules is the X-ray absorption process. Different aspects of absorption edge or EXAFS analysis related to actinide studies are presented, including phenomenological and semi-quantitative approaches.

INTRODUCTION ThO2 versus the linear one of U, Np and Pu elements has been the subject of numerous Due to their incomplete 5f and 6d electronic theoretical works questioning the extent of the 5f shells, the actinide elements present some and 6d participation in the bonding molecular unusual physical and chemical properties. orbitals involving the two oxygen atoms [2]. Moreover, the actinide very large atomic The use of X-ray Absorption Spectroscopy in the numbers are also responsible for very important hard X-ray regime to investigate actinide relativistic effects, among which the scalar effect molecular adducts has been of increasing is at the origin of the so-called lanthanidic and interests in the past decade. This is particularly actinidic contraction. Another consequence is true for species in solutions, when that the elements of the first half part of the complementary speciation techniques as optical actinide series exhibit a large number of stable or and vibrational spectroscopies or quantum metastable formal oxidation states ranking from chemical investigation can be employed. Thus, III to VII depending on the atomic number. the X-ray probe can be considered as a Thus, the question of delocalized vs localized fingerprint of the structure of the cation behavior of the 5f electrons still remains a polyhedron. Additionally, in the edge regime, critical issue to be addressed from both electronic properties as valence orbital experimental and theoretical points of view. population and localization can be obtained. Oxidation states III and IV are formally spherically symmetric cations while oxidation MEASURING THE X RAY ABSORPTION sates V and VI have a transdioxo skeletons with coordination sites in the equatorial plane. Although one of the major advantage of XAS is Because most of the actinide complexes (from the ability to tune the absorption edge for a given uranium to americium) with oxidation states element, most of the actinyl studies have dealt n+ higher than IV do contain the actinyl AnO2 (n with LIII and more rarely LI edges [3]. Reasons = 1, 2) moieties, there are of technological and for this limitation are mainly technical because it environmental primary importance. Furthermore, allows measurement across multiple barrier the ubiquity of the uranyl cation in the confinement of the sample. General geosphere, because of its remarkable stability, considerations related to the manipulation of makes it one of the most extensively investigated radionuclides at the synchrotron sources have cation within the actinide family. The major been summarized in the Proceeding of the 2nd characteristic of actinyl cations is the occurrence Euroconference and NEA Workshop on of quasi linear O-An-O unit with a short An-O Speciation, Techniques and Facilities for multiple bond and a bond length ranking from Radioactive Materials at Synchrotron Light 1.7 to 1.9 Å depending on the cation. This Sources [4]. characteristic is also specific of the actinide The X-ray absorption spectrum of a given family as only one analogue (the osmyl cation element is commonly divided in two different 2+ OsO2 ) is known in the d block [1]. This is a regions, the pre-edge and edge region, called good evidence of the bonding particularity that XANES, and the post-edge region, called involves significant participation of the 5f EXAFS, which corresponds to significantly n+ orbitals in the HOMO of the AnO2 unit. As a different processes. In the edge region, low matter of fact, the bent structure of molecular energy excited states are considered and

ATALANTE 2004 Nîmes (France) June 21-25, 2004 1 O21-04 transitions to bound states or quasi-bound states attributed to the multiple scattering contribution are often observed. In a simple one-electron of the transdioxo unit and shoulder C (even if it molecular-orbital (MO) picture, the authorized is less intense in the simulation than in the LUMO or partially vacant valence orbitals are experimental spectrum). Although some probed by the photoelectron according to the differences appear between each calculation dipolar transition rule. Quadrupolar transition or corresponding to each snapshot, the broad phonon coupling can also contribute to the edge character of the edge spectrum precludes the as observed for several transition metals. Within selection of one preferred conformation over the one-electron multiple scattering picture, both another one by comparison with the experiment. absorption coefficient µ and l-projected density The spectrum obtained when averaging the 6 of states (DOS) ρ have smooth atomic calculations (which should represent the backgrounds modulated by the oscillating averaged conformation of the molecule in water) function χ. Transitions to bound or quasi-bound is indeed in very good agreement with the 2+ states resulting from mixing with continuum experiment. The geometry of [OsO2(H2O)5] states occur bellow or around the vacuum level. molecular skeleton exhibits a strong distortion Above the continuum threshold, in the EXAFS compared to the ones of U and Np : an axial regime, the final one-electron state belongs to the distance of 1.75 Å and very short equatorial continuum and the spectrum is dominated by distances of 2.03 Å. A clear signature of these constructive or destructive interferences structural changes is observed on Figure 1. contained in χ. 3 A ACTINIDE MOLECULAR CHEMISTRY 2.5 Uranyl B The molecular systems selected for this 2 proceeding are formally divided in four parts C average from simple to complex molecular edifices : 1.5 aquo form of the actinide and transition metal 6 snapshots 2+ 2+ cations and more specifically UO2 , NpO2 , 1 2+ 2+ experiment OsO2 ; reactivity of UO2 upon uptake onto mineral surfaces; U, Np, Pu, Am involved in 0.5 Osmyl

three-dimensional molecular building blocks; Normalized abosrbance (a.u.) metalloprotein behavior upon uptake of U, Np 0 experiment radionuclides.

-0.5

Learning from edge spectroscopy of aqueous -20 0 20 40 60 80 100 species Relative energy (eV) Fig. 1: Experimental and calculated (Feff8.2) LIII 2+ 2+ Three of the four molecular structures considered edge of [UO2(H2O)5] and [OsO2(H2O)5] . 2+ here are solvent adducts: [UO2(H2O)5] , 2+ 2- [NpO2(H2O)5] and [NpO2(OH)4] [5]. In order Figure 2 shows the LIII edges of neptunyl in both 2+ to account for the solvent part beyond the first acidic ([NpO2(H2O)5] ) and basic 2- coordination sphere, an approach using ([NpO2(OH)4] ) aqueous solutions. The molecular dynamic calculations has been structural distortion of the molecular skeleton performed for these systems in aqueous solution, from acidic to basic media corresponds to an leading to a sampling for both the first sphere elongation of 0.07 Å of the axial distances and a hydrogen atoms and the surrounding water contraction of 0.21 Å of the equatorial distances. molecules around the actinyl rod. The fourth Comparison of the corresponding experimental 2 adduct [OsO2(OH)4] , selected here as a point of LIII edges shows that this deformation leads to comparison with the actinide elements is dramatic changes in the edge spectrum: strong insoluble in aqueous solutions. Figure 1 differences in the intensity of A, shifts of B and 2+ compares the experimental LIII edge of C in the spectrum of [NpO2(H2O)5] compared 2+ 2- [UO2(H2O)5] with 6 XANES calculations based to [NpO2(OH)4] . These differences are well on clusters obtained from 6 snapshots of the reproduced by the calculation. A comparison molecular dynamical calculation. Qualitatively, with Figure 1 shows that the spectra of 2+ 2+ all the calculations reproduce the XANES [UO2(H2O)5] and [NpO2(H2O)5] are very features, i.e. the strong white line A, shoulder B comparable due to the similarity of both

ATALANTE 2004 Nîmes (France) June 21-25, 2004 2 O21-04 skeletons. The strong differences observed in the relate the expected surface complexes to the 2- edge spectrum of [NpO2(OH)4] confirm that the structure of the surface coordination complexes. geometry of the first coordination sphere plays a Therefore, the stoichiometry of the chemical major role in the shape of the edge. As described species involved in such processes should be for the uranium edge, the position of feature B deduced from molecular-scale information given with respect to A is indicative of the axial Np-O by spectroscopic investigations. Among the 2- distance. In [NpO2(OH)4] , B is shifted to lower questions that XAS has addressed are inner 2+ energies compared to [NpO2(H2O)5] , in versus outer-sphere complexation, formation of agreement with a larger axial distance for the polynuclear precipitates, and mono versus hydroxide with respect to the hydrate. The polydentate surface binding modes. Although identity of the product ∆ER2 is indeed verified one of the drawbacks of the technique is to within the measurement error: average the signal over all the possible species, ∆ER2=12.5(eV)•1.75(Å)2=38 for tentative site by site data analysis has been 2+ 2 2 [NpO2(H2O)5] ; ∆ER =10.0(eV)•1.82(Å) =33 successfully undertaken. In Figure 3, examples 2- 2+ for [NpO2(OH)4] . Conversely, a careful of EXAFS spectra of UO2 /H2O/substrate analysis of the edge shape and feature positions (substrate = LaPO4, ZrP2O7 and TiO2) surface may lead to reliable structural information about complexes are presented [6]. the near structure around the actinide cation. 1.6 U(VI)/aq A 1.4 2 U(VI)/LaPO (p) 4 [ NpO (H O) ]2+ B 2 2 5 1.2 U(VI)/ZrP O (p) C 2 7 1 1.5 U(VI)/TiO (p) 2 0.8 [ NpO (OH) ]2- 2 4 U(VI)/TiO (110) 2 1 0.6 U(VI)/TiO (001) 2

EXAFS amplitude (k*CHI(k)) (a.u.) 0.4 Normalized absorbance (a.u.) 0.5 0.2

2 4 6 8 10 12 14 -1 k (Å ) 0 -20 0 20 40 60 80 100 120 Fig. 3 : Experimental EXAFS LIII edge of uranyl Relative energy (eV) sorbed onto mineral powders (LaPO4, ZrPO4, Fig. 2: Experimental (….) and calculated (-) TiO2) and mineral single crystal surfaces (TiO2 2+ (Feff8.2) LIII edge of [NpO2(H2O)5] and plans (110) and (001)). 2- [NpO2(OH)4] . EXAFS data analysis of the uranyl sorbed onto Interaction with mineral surfaces the (110) plane shows a splitting of the equatorial U-O bond distances between U- In order to model the interactions between heavy O(surface) and U-O(out). Two possible sorption metal ions and mineral surfaces in aqueous sites are available for bidentate complexation on medium, a detailed knowledge of the physico- the (110) surface : shared-edge distorted TiO6 chemical reactions at the solid-solution interface polyhedra and shared-summit polyhedra. Both is required. These interfacial reactions depend sites are present in the surface complexes in mainly on several geochemical parameters such proportions that are unknown from data analysis. as pH, redox potential and ionic strength of the Figure 4 shows a schematic representation of the aqueous medium, the speciation of the metal ion surface complex in the case of the shared-edge and the surface properties of the substrates sorption site of the (110) surface. The structural (acidity and surface sites density), which makes parameters are comparable to the one obtained a quantitative description rather complicated. from uranyl sorption on polycrystalline rutile. Nevertheless, the macroscopic thermodynamics The shortening of the U-O equatorial bond from do not give any accurate information about the 2.42 Å for aqueous uranyl to 2.32 Å for U- elementary chemical reactions used to determine O(surface) suggests an increase in covalency of the equilibrium constants. Thus, it is necessary to the U-O(surface) bond. This would not be

ATALANTE 2004 Nîmes (France) June 21-25, 2004 3 O21-04 surprising, taking into account the lack of ability to bind to proteic domains with great coordination for the rutile surface oxygens and affinity and in particular transferrin. This protein the possibility of charge delocalization along the is a glycoprotein made of 670 amino acids and is surface. used as a regulator of Fe(III) carriage. Its tertiary structure is made of two equivalent lobes (C and 3 Oout @ 2.49 Å N) with one possible complexation site each. The Fe coordination is in a distorted octahedral conformation with two coordinating tyrosines, 2 Oyl @ 1.79 Å one aspartate and one histidine (imidazole 2 Osurf @ 2.33 Å function). An additional synergetic anion (in U vivo, carbonate) is positioned between the protein and the cation. Figure 5 shows the ovotransferrin tertiary structure with Fe(III) cation (PDB ref. 1NFT). U...Ti @ 3.05 Å

Bulk TiO2

Fig. 4: Schematic representation of the uranyl complex upon sorption onto TiO2(110) crystal surface.

Towards more complex systems: the metalloprotein case.

Among the 18000 protein structures contained in the Protein Data Bank today, one-third are metalloproteins, i.e. chemical combinations of protein atoms with metallic cations such as iron, calcium, copper, zinc and magnesium for the most biological ones. When tightly bound to protein ligands, metal ions are critical to the Fig. 5: tertiary structure of the Fe(IIII) function, structure, and stability of proteins, by ovotransferrin N-lobe complex. only allowing specific interactions to take place and/or selective chemistry to occur. First experimental results from Np(IV) Metallobiomolecules are thus considered as complexation by human transferrin [7] in the elaborated inorganic complexes with well- presence of nitrosotriacetic acid (NTA) have designed metal active site structures. A protein- been obtained at the Np L edge. A preliminary bound metal site consists of one or more metallic III model assuming that the Np cation fills the Fe cations and all protein side chain and exogenous site of the protein has been attempted. It suggests bridging and terminal ligands that define the first that the cation is surrounded by one NTA coordination sphere of each metal ion. Such chelate, two tyrosines from the protein and two metallic sites participate in the accomplishment water molecules. The average distance Np-ligand of the most essential biological and chemical distance is 2.55 Å compared to 2.07 Å for the Fe functions supported by proteins. Although the case [8]. various interaction processes between the metallic cation and the protein have been widely CONCLUSION studied in all the fields of biochemistry, focus on the specific actinide family is more seldom. In From speciation to simulation, XAS has proven particular, the knowledge of transportation, to be one of the useful probe for actinide fixation and interaction mechanisms of these chemistry. Clearly the XAS probe by itself will cations in the biologically active sites are only not fully describe the electronic valence states of partially understood. the molecule and a palette of complementary Stable actinide cations at the formal oxidation techniques and calculations must be employed. state IV (Th, Np, Pu) but also oxocations are of Also, limitation of XAS data analysis to first concern in case of contamination, for their phenomenological aspects is often restrictive and

ATALANTE 2004 Nîmes (France) June 21-25, 2004 4 O21-04 coupling simulation codes to experimental results is essential. In that sense, the selection of various transition channels from core spectroscopy to higher primary quantum numbers is a unique tool based on the tunability of the synchrotron radiation.

REFERENCES

1. N. Kaltsoyannis, P. Scott, The f elements, Oxford Science Publisher, Zeneca (1999). 2. P. J. Hay, R. L. Martin, G. Schreckenbach, J. Phys. Chem. A, 104, 6259 (2000). 3. C. Den Auwer, E. Simoni, S. D. Conradson, C. Madic, Eur. J. Inorg. Chem., 21, 3843 (2003). 4. Proc. of the Workshop on Speciation, Techniques and facilities for Radioactive Materials at Sunchrotron Light Sources, OECD – NEA, 4 – 6 October, Grenoble, France (1998) and Proc. 2nd Euroconference and NEA Workshop on Speciation, Techniques and Facilities for Radioactive Materials at Synchrotron Light Sources, OECD – NEA, September, Grenoble, France (2001). 5. C. Den Auwer, D. Guillaumont, P. Guilbaud, S. D. Conradson, J. J. Rehr, A. Ankudinov, E. Simoni, New J. Chem. Accepted for publication. 6. C. Den Auwer, R. Drot, E. Simoni, S. D. Conradson, M. Gailhanou, J. Mustre de Leon, New J. Chem., 27, 648 (2003). 7. R. Racine, Ph. Moisy, F. Paquet, H. Métivier, C. Madic, Radiochim. Acta, 91, 115 (2003). 7. K. Mitzutani, H. Yamashita, H. Kurokawa, B. Mikami, M. Hirose, J. Biol. Chem., 274, 10190 (1999).

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