Coordination of the uranyl ion in solution and ionic liquids : a combined UV-Vis absorption and EXAFS study

The uranyl ion (UO22+) has been extensively studied for decades and nowadays it is still a hot topic in a number of contemporary issues like nuclear waste treatment and the Balkan syndrome. Therefore, besides our fundamental interest in this complex system, the aim of this study was to provide a convenient and straightforward approach to identify the structure of various uranyl complexes formed in solution. To achieve this goal, spectroscopic techniques like UV-Vis absorption spectroscopy, luminescence and excitation spectroscopy as well as magnetic circular dichroism (MCD) were used, thereby focusing on typical eye-catching features like intense peaks and vibrational fine structure. This vibrational fine structure in the spectra of uranyl compounds is affected by the symmetry of the first coordination sphere of UO22+. In this work, we obtained the optical spectra of a number of symmetry groups, i.e. D4h, D3h, D2h and D3 coordination symmetry, which can be used as fingerprints of a certain symmetry group. These spectroscopic data were complemented with Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy on the LIII-edge of uranium. This modern experimental technique enables us to obtain structural information like bond distances, on solution species. Moreover, the equatorial bond distances can be related with the coordination number of the uranyl ion. For example, a U-Oeq bond distance between 2.34 Å and 2.42 Å is characteristic for a fivefold coordination of the uranyl unit. We have chosen some well-known systems like [UO2(H2O)5]2+ and [UO2Cl4]2- to get familiar with the curve fitting procedure of EXAFS data. Afterwards we applied the knowledge we had obtained from these simple compounds to more complicated systems like [UO2(NO3)3]- and UO2(NO3)2(TBP)2. These uranium LIII-edge EXAFS data have given additional evidence for the geometry of the complexes proposed by optical spectroscopic techniques. It is obvious that the unique combination of optical spectroscopic techniques, available fingerprint spectra and uranium LIII-edge EXAFS spectroscopy provides us with a valuable tool for determining the first coordination sphere of unknown uranyl complexes in solution. The most important conclusions of this work are summarized in the following paragraphs. The optical spectra of the solution species have been compared with the spectra measured on single crystals. There is a good agreement between the energy values of the different electronic transitions of the single crystals and those of the same species in solution. We have to remark, however, that the spectroscopic properties of a uranyl complex in solution differ from those of the same compound in the solid state. First, in solution the absorption and luminescence bands are much broader. Consequently, overlap between the several electronic and vibronic transitions in the uranyl spectrum is inevitable. Furthermore, the molecules are randomly oriented in solution, so any polarization can be observed, making the spectrum in solution more complicated to analyse. All electronic transitions in the spectrum of the uranyl ion (D∞h) are parity forbidden by the Laporte selection rule. Two intensity mechanisms are currently invoked: either the static ligand field or the coupling of vibrations of ungerade parity (vibronic coupling). The maximum coordination of the uranyl ion with chloride ions in non-aqueous solution is four. The UV-Vis absorption spectrum can be explained in the centrosymmetric D4h coordination symmetry. Thus, the spectrum is purely vibronic in nature. The UV-Vis absorption spectrum of the uranyl tetrachloro complex [UO2Cl4]2- is dominated by the vibronic coupling mechanism. Intensity is induced by coupling of the asymmetric stretching (νa, a2u) and the bending (νb, eu) vibrations of the uranyl ion itself and especially of one equatorial ligand vibration, i.e. ν10 (b1u). This U-Cl out-of-plane bending, transforming as the fxyz orbital, is coupled to the first electronic transition from Σg+ to Πg (from A1g to Eg) and to one component of the transition from Σg+ to Δg (from A1g to B2g). The fourfold coordination of the uranyl ion with chloride ions is also demonstrated by uranium LIII-edge EXAFS spectroscopy. The [UO2Cl4]2- polyhedron contains two axial oxygen atoms at 1.77 Å and four chloride ligands at 2.68 Å. The UV-Vis absorption spectra of complexes exhibiting a trigonal D3h coordination symmetry like [UO2(NO3)3]- and [UO2(CH3COO)3]-, both formed in organic solvents, are characterized by very sharp, intense peaks at the low energy side of the spectrum, i.e. between 21000 cm-1 and 24000 cm-1. The corresponding MCD signals are intense negative A-terms. The spectra of [UO2(NO3)3]- and [UO2(CH3COO)3]- are affected by the static ligand field. In D3h symmetry, the transition from Σg+ to Δg (from A’1 to E’) is electronically allowed along the x- and y-axis, thereby inducing intensity in the first region of the spectrum (21000 cm-1 - 24000 cm-1), which results in the typical sharp and intense peaks. Uranium LIII-edge EXAFS data have given evidence for the trigonal symmetry of [UO2(NO3)3]- and [UO2(CH3COO)3]- species in non-aqueous solution. The uranyl ion is surrounded by three bidentate nitrate groups at 2.48 Å in the equatorial plane in the uranyl trinitrato complex. Special features at larger distances, indicating the presence of distal oxygen atoms at 4.16 Å, are observed in the Fourier transform due to the linear arrangement within the nitrate groups. The uranium LIII-edge EXAFS spectrum of [UO2(CH3COO)3]- exhibits the same structural features as that of [UO2(NO3)3]- and can be explained in a similar way. The first coordination sphere of the uranyl ion in [UO2(CH3COO)3]- consists of three acetate ligands with the U-Oeq distance equal to 2.48 Å. The focusing effect is also important within the acetate groups, thereby identifying the distal carbon atoms at a distance of 4.37 Å. Whereas the UV-Vis absorption spectra of [UO2Cl4]2- and [UO2(NO3)3]- are dominated by the σu+δu configuration, the σu+φu configuration plays a dominant role in the spectrum of the inclusion complex [UO2(18-crown-6)]2+. At higher energies the transition from Σg+ to Γg (σu+φu) appears, which is typical for a D3 coordination symmetry. The identification of six equatorial oxygen atoms at 2.46 Å and twelve equatorial carbon atoms at 3.51 Å in the EXAFS spectrum and the corresponding Fourier transform indicates the inclusion of UO22+ in the crown ether cavity. The U-Oeq and U-Ceq bond distances cover a large range, which is reflected in the high Debye-Waller factors σ². The uranyl complexes mentioned above are obtained in non-aqueous solvents (acetonitrile, acetone). The stability constants of the different complexes in these solvents are not known, unlike those in aqueous solution. The ligand affinity towards the uranyl ion changes from non-aqueous solvents to aqueous solutions. Whereas [UO2Cl4]2- and [UO2(NO3)3]- are formed in acetonitrile and acetone, no significant complex formation occurs between the uranyl ion and chloride or nitrate ions in aqueous solution. This weak complex formation is confirmed by the absence of spectral changes in the UV-Vis absorption spectra with respect to the spectrum of the “free” uranyl ion, the observation of the typical features of [UO2(H2O)5]2+ in the EXAFS spectra and the species distribution calculations using the known stability constants. Ionic liquids are salts, composed of an organic cation and an organic/inorganic anion, with a melting point below 100 °C. Currently, studies are performed to replace the classical organic solvents by these ionic liquids in numerous fields of chemistry like separation processes, catalysis and organic synthesis. Due to the growing interest in ionic liquids, we have investigated the speciation of uranyl complexes in imidazolium-based and pyrrolidinium-based ionic liquids by means of spectroscopic techniques (UV-Vis absorption spectroscopy, luminescence and excitation spectroscopy, magnetic circular dichroism, uranium LIII-edge EXAFS spectroscopy). Thereby, the anionic component of the ionic liquids was varied. The comparison of the spectroscopic properties in ionic liquids with the fingerprint spectra in non-aqueous solvents unambiguously points to the formation of [UO2Cl4]2-, [UO2(NO3)3]- and [UO2(CH3COO)3]- species as well as to the presence of the inclusion complex [UO2(18-crown-6)]2+ in the ionic liquids [C4mim][Tf2N] and [bmpyr][Tf2N]. The cations and the anions of the ionic liquids seem to have no influence on the positions of the electronic transitions. Indeed, there is a good agreement with the corresponding UV-Vis data in non-aqueous solution. The presence of a [UO2(NO3)3]- species in [C4mim][Tf2N] is demonstrated by uranium LIII-edge EXAFS spectroscopy. The structural parameters of the [UO2(NO3)3]- coordination polyhedron in the ionic liquid are comparable with those in acetonitrile. Uranium LIII-edge EXAFS measurements on the other complexes studied in ionic liquids, will be performed in future. The presence of small inorganic ligands inhibits the formation of the inclusion complex [UO2(18-crown-6)]2+ in ionic liquids. This is consistent with the observations in acetonitrile and propylene carbonate. Once a trace of chloride or bromide ions is added to the ionic liquid, the crown ether is removed from the first coordination sphere, as established by UV-Vis absorption spectroscopy and crystal structure determinations. During the synthesis of imidazolium-based ionic liquids, one has to take care that all 1-methylimidazole has reacted. Otherwise, hydrolysis products of the uranyl ion are formed in the ionic liquids, giving a broad, structureless band in the UV-Vis absorption spectra. Furthermore, kinetic effects are involved in the sample preparation of uranyl-containing ionic liquids. It would be very interesting to study these kinetic effects in more detail in future, for example by means of UV-Vis absorption spectroscopy. Tri-n-butylphosphate (TBP) is a commonly used neutral extracting agent in liquid-liquid extraction processes for nuclear waste treatment. Knowledge of the geometry of the species involved in these extraction procedures can provide insight in the development of new, more selective extracting agents. Therefore, we have investigated the first coordination sphere of the uranyl ion in a tri-n-butylphosphate solution containing UO2(NO3)2∙6H2O. Both 31P NMR spectroscopy and UV-Vis absorption spectroscopy indicate the formation of the UO2(NO3)2(TBP)2 complex in solution. The vibrational fine structure in the spectrum can be attributed to a D2h coordination symmetry. Hence, the spectrum is purely vibronic in nature. In the MCD spectrum, only B-terms are observed, which is consistent with the removal of all degeneracy in D2h symmetry. Furthermore, we were able to calculate the equilibrium constant Keq and the thermodynamic parameters (DH, DS) of the complex formation reaction between UO2(NO3)2·6H2O and tri-n-butylphosphate, based on the 31P NMR data. Uranium LIII-edge EXAFS spectroscopy also reveals the coordination of two bidentate coordinated nitrate groups (U-Oeq = 2.52 Å) and two monodentate phosphate groups (U-Oeq = 2.37 Å). Complex multiple scattering features have to be included in the analysis of the spectra due to the linear arrangement within the nitrate groups and the tri-n-butylphosphate ligands. Investigation of the structure of UO2(NO3)2(TBP)2 in ionic liquids might contribute to the studies of the potential replacement of organic solvents in liquid-liquid extraction processes. The photochemistry of the actinide compounds is almost exclusively dominated by the uranyl ion. However, the mechanisms of most of these photochemical reactions are not known yet. An interesting topic is the structure of uranyl oxalato complexes involved in these photochemical reactions. We have presented a different perspective on the complex formation of the uranyl ion with oxalate ions. Based on spectroscopic measurements in acetone solution, we have proposed a dimeric species with a bridging oxalate group, where each uranyl unit is a pentagonal bipyramid. In addition, the UV-Vis absorption spectra, which exhibit an increase in intensity in the low energy part, are consistent with a dimeric structure with D2 coordination symmetry. Furthermore, we believe that the out-of-plane bending ν10 (a in D2) can induce an intramolecular twisting mechanism, thereby destroying the oxalate ligands. Hopefully our hypothesis will be confirmed in future by uranium LIII-edge EXAFS spectroscopy and theoretical calculations on dimeric structures. It has been emphasized in our studies that for a metal-to-ligand ratio of 1:2 or 1:3 a di- or tricomplex is not necessarily formed, which is often overlooked in the literature. Many mistakes are also made by assuming that the dissolution of a solid uranyl compound will give the same structure as the solid in solution. For example, the salt UO2(NO3)2∙6H2O dissolved in aqueous solution is fully dissociated, thereby forming the hydrated “free” uranyl ion. It is obvious that uranium LIII-edge EXAFS spectroscopy offers good prospects for future work concerning the coordination environment of the uranyl ion in solution as well as in ionic liquids. However, an important disadvantage is encountered using EXAFS spectroscopy. In case of a mixture of species, the EXAFS data will only give average coordination numbers and bond distances, which hampers the clarification of the structure of solution species. Therefore, we would suggest that the combination of UV-Vis absorption spectroscopy, luminescence spectroscopy, where possible magnetic circular dichroism, group theoretical analysis as well as uranium LIII-edge EXAFS spectroscopy, NMR spectroscopy, theoretical calculations and principal component analysis is an excellent tool for elucidating the geometry and the composition of the first coordination sphere of several unknown uranyl complexes. The determination of the structure of the intermediately formed chloro and nitrato complexes in non-aqueous solvents by a combination of the techniques mentioned above is a real challenge! But, at first, it would be very helpful to get insight in the structure of the solvated uranyl ion in anhydrous acetonitrile.INTRODUCTION 1 THEORETICAL SECTION CHAPTER 1: THE URANYL ION: A UNIQUE CHEMICAL SPECIES 9 1 Scientific history of uranium and discovery of radioactivity 9 2 Occurrence of uranium and radiation hazards 11 3 The uranyl ion UO22+ 14 4 Spectral properties of the uranyl ion UO22+ 15 References 20 CHAPTER 2: THEORY 23 1 Molecular orbitals 23 2 - or - coupling scheme in the uranyl ion? 29 3 Equatorial ligand field perturbation 32 4 Zeeman perturbation and magnetic circular dichroism (MCD) 39 5 Selection rules and intensity mechanisms 45 5.1 Magnetic dipole transitions 46 5.2 Static ligand field – induced electric dipole transitions 47 5.3 Dynamic ligand field or vibronic coupling 49 6 Summary 52 References 56 CHAPTER 3: EXTENDED X-RAY ABSORPTION FINE STRUCTURE (EXAFS) SPECTROSCOPY 59 1 X-rays and synchrotron radiation 59 2 Physical principles of EXAFS and EXAFS equation 62 3 Data analysis 69 3.1 Data reduction 69 3.2 Curve fitting 72 3.3 Scattering paths 74 4 Advantages and limitations of EXAFS spectroscopy 76 5 Sample preparation, experimental setup and data analysis used 77 References 80 EXPERIMENTAL SECTION CHAPTER 4: EXPERIMENTAL PROCEDURES 81 1 Reagents 81 2 Synthesis of uranyl salts 81 3 Spectrophotometric measurements 83 4 31P NMR spectroscopy 85 References 86 CHAPTER 5: COORDINATION EFFECTS ON THE SPECTRA OF THE URANYL ION UO22+ 88 1 Introduction 88 2 Experimental details 90 3 Data analysis of uranium LIII-edge EXAFS spectra 92 3.1 [UO2(H2O)5]2+ and [UO2Cl4]2- 93 3.2 [UO2(NO3)3]- 93 3.3 [UO2(CH3COO)3]- 95 3.4 [UO2(18-crown-6)]2+ 96 4 Results and discussion 98 4.1 “Free” uranyl ion (hydrated uranyl ion) 98 4.2 Coordination with chloride ions 106 4.3 Coordination with nitrate ions 119 4.4 Coordination with 18-crown-6 140 5 Conclusions 150 References 154 CHAPTER 6: SPECIATION OF URANYL COMPLEXES IN IONIC LIQUIDS 163 1 Introduction 163 2 Experimental details 167 3 Results and discussion 171 3.1 “Free” uranyl ion (hydrated uranyl ion) 171 3.2 [UO2Cl4]2- 174 3.3 [UO2(NO3)3]- and [UO2(CH3COO)3]- 178 3.4 [UO2(18-crown-6)]2+ 190 4 Conclusions and outlook 199 References 202 CHAPTER 7: SPECTROSCOPIC PROPERTIES OF UO2(NO3)2(TBP)2 207 1 Introduction 207 2 Experimental details 209 3 Results and discussion 212 4 Conclusions 227 References 229 CHAPTER 8: THE PHOTOCHEMICAL BEHAVIOUR OF THE URANYL ION: A DIFFERENT PERSPECTIVE AND OUTLOOK 233 1 Photochemistry of the uranyl ion 233 2 Experimental details 236 3 Uranyl oxalato complexes in acetone 237 4 Discussion and outlook 241 References 247 GENERAL CONCLUSIONS AND SUMMARY 249 SAMENVATTING 257 PUBLICATIONS AND ATTENDED CONFERENCES 265 A APPENDICES APPENDIX 1: SAFETY ISSUES ON HANDLING URANYL SAMPLES 267 APPENDIX 2: MOLECULAR ORBITALS 269 1 Altmann-Herzig definition and relationship to 269 2 Group orbitals of the two axial oxygen atoms of UO22+ 271 3 Molecular orbitals of the uranyl ion 273 3.1 Bonding molecular orbitals 273 3.2 Non-bonding molecular orbitals 273 3.3 Anti-bonding molecular orbitals 274 APPENDIX 3: SUPPORTING MATERIAL: SPECTRA AND NUMERICAL DATA 275 1 [UO2Cl4]2- 276 1.1 Acetonitrile 276 1.2 Ionic liquids [C4mim][Tf2N] and [bmpyr][Tf2N] 283 2 [UO2(NO3)3]- 288 2.1 Acetonitrile 288 2.2 Ionic liquids [C4mim][Tf2N] and [bmpyr][Tf2N] 292 3 [UO2(18-crown-6)]2+ 297 3.1 Acetonitrile 297 3.2 Ionic liquids [C4mim][Tf2N] and [bmpyr][Tf2N] 302 APPENDIX 4: CRYSTALLOGRAPHIC DATA 305 1 Crystal structure of [(UO2)2(2-OH)2(H2O)6] [UO2Br4] (18-crown-6)4 305 2 Crystal structure of [C6mim]2[UO2Br4] 309 3 Experimental details 312status: publishe