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Uranium Thiocyanate Compounds in Multiple Oxidation States Stefano

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Uranium Thiocyanate Compounds in Multiple Oxidation States A thesis presented to the University of Dublin, Trinity College for the degree of Doctor of Philosophy in Chemistry by Stefano Nuzzo Under the supervision of Prof. Robert J. Baker School of Chemistry Trinity College Dublin 2019 Declaration I hereby declare that this thesis has not been submitted as an exercise for a degree at this or any other university and it is entirely my own work. I agree to deposit this thesis in the University’s open access institutional or allow the library to do so on my behalf, subject to Irish Copyright Legislation and Trinity College Library conditions of use and acknowledgement. … ………. Stefano Nuzzo II Summary In this thesis a series of high symmetry uranium(IV) and thorium(IV) thiocyanate n complexes of the type A4[An(NCS)8] (An = Th, U; A = Cs, Me4N, Et4N, Pr4N) have been structurally and spectroscopically characterized. It was shown that the size and shape of the counter cation determine the coordination symmetry around the actinide ion. Interestingly, this effect proved to have a significant influence on the magnetic properties 4− of the [U(NCS)8] ion, as revealed by SQUID and specific heat capacity measurements. Inelastic neutron scattering and far-infrared spectroscopic measurements have been performed on the uranium compounds to examine the corresponding low-lying energy level electronic structures, while a computational approach using the CONDON framework has been used to quantify these energy states. Overall, the experimental and computational data suggest that the energy gap between the ground and the first excited state varies considerably so that for A = Et4N a magnetic singlet ground state is predicted; however, a distortion of the coordination geometry from cubic toward square antiprismatic reduce this energy gap such that a magnetic triplet ground state is observable for A = Cs at low temperatures. This represents the first example of a triplet electronic ground state for a uranium(IV) compound; thus, this study underlines the importance of the coordination geometry to understand the magnetic properties of uranium(IV) compounds. Moreover, a spectroscopic and magnetic analysis has been carried out also on two uranium(IV) selenocyanate complexes of the type A4[U(NCS)8] n (A = Et4N, Pr4N); however, no significant differences in the electronic structure have been observed with respect to the equivalents thiocyanate compounds. 4− The reactivity of the [U(NCS)8] ion has also been examined in organic media of diverse polarity and with several different counter cations. From oxidation processes of Cs4[U(NCS)8], two mixed-valent uranium compounds have been isolated and structurally characterized. A spectroscopic, magnetic and computational analysis has been performed to unequivocally delineate the electronic configuration of these systems; a photophysical study also revealed the presence of both uranium(IV) and uranyl(VI) together in one compound. A reaction between Na4[U(NCS)8] and [Co(bipy)3][PF6]2 led to the formation of a compound of formula [Co(bipy)3][U(NCS)8] for which spectroscopic and magnetic properties suggest the presence of a pentavalent uranium(V) ion, but the mechanism of the synthesis is not clear. The synthesis of homoleptic uranium(III) thiocyanate complexes, from reductions of uranium(IV) precursors, have also been attempted, but III 4− without success; however, a spectroelectrochemistry study on [U(NCS)8] has shown that the addition of one electron to the metal ion causes the dissociation of all the π-donor [NCS]− ligands and decomposition of the sample. Theoretical calculations have also revealed an enhanced ionicity for the U(III)–N compared to the U(IV)–N bond, which can explain the observed decomposition. A series of uranyl(VI) thiocyanate [NCS]− and selenocyanate [NCSe]− complexes, of the n type [R4N]3[UO2(NCS)5 (R4 = Bu4, Me3Bz, Et3Bz), [Ph4P][UO2(NCS)3(NO3)] and n [R4N]3[UO2(NCSe)5] (R4 = Me4, Pr4, Et3Bz) have also been described. X-ray crystallography, NMR, vibrational and photoluminescence spectroscopy and theoretical methods have been used to explore the non-covalent interactions present in the structures of these compounds, namely: charge assisted C—H…O=U and C—H…S(e) hydrogen bonding, and Se…Se or S…S chalcogenide interactions. Chalcogenide interactions have been observed in some of the structures and were more prevalent in the [NCSe]− series than in the [NCS]−. Charge assisted hydrogen bonding to the –yl oxygens were also noticed, but they were weak and not able to strongly influence the spectroscopic properties of the uranyl moiety. Finally, a detailed structural, spectroscopic and theoretical study on a series of 10-[(4- halo-2,6-diisopropylphenyl)imino]phenanthren-9-ones and derivatives of the phenanthrene-9,10-dione ligand has been discussed. Particular attention has been reserved on the analysis of the supramolecular non-covalent interactions exhibited by the structures of these molecules, such as “π–π stacking”, C–H…π, C–X lone pair…π, C– X…H–C and C–X…X–C. DFT and AIM analysis have also been used to compare the strength of these interactions. It was found that the lone pair···π interactions were the dominant halogen type interactions and a computational study showed that these were stabilizing and of the same order of magnitude as interactions from lone pairs deriving from oxygen or nitrogen donor atoms. However, among all the interactions, the most important one appeared to be a weak hydrogen bond between the C–H of a phenanthrene and the carbonyl oxygen. These ligands were also reduced by reaction with potassium metal to form iminoalkoxy semiquinone radical anions. A spectroscopic analysis confirmed the formation of radical species, but these were isolated as powders only and it was not possible to grow single crystals. Reactions between UCl4, as source of U(IV), and these radicals have been also attempted. However, the desired compounds, that could potentially show magnetic coupling interactions between the radical and the uranium metal, have not been isolated. IV Acknowledgement Firstly, I would like to express my sincere gratitude to my supervisor Prof. Robert James Baker for his continuous support during my PhD study and related research, for his enormous patience with my mistakes. His guidance helped me in all the time of research and writing of this thesis. Thank you very, very much. Besides my supervisor, I would like to thank Dr. Brendan Twamley for his enormous contribute throughout my project; thank you for all the crystal structures that you refined for me, for your help in the lab and your encouragements. I also would like to thank Dr. John O’ Brien for being the “NMR god” and, of course, Dr. Manuel Ruether for “simply” being the king of the spectroscopic instruments; thank you both for being always present. Obviously, I also want to thank all the members of the stuff in the School of Chemistry of Trinity College. My sincere thanks go also to Professor Michael Probert from University of New Castle for welcoming me in his group even if for a short period, for diffracting two of my single crystals at 4 K and for explaining me how a single crystal can be diffracted at different pressures. Moreover, I would like to express my gratitude to Dr. Marco Evangelisti and Dr. Giulia Lorusso (from University of Saragoza), for measuring the magnetic properties of my samples, to Dr. Duc Lee and Dr. Helen C. Walker (from the ISIS facility) for the INS experiments, to Silvia Capelli (from ISIS Neutron and Muon Facility) for the single- crystal neutron diffraction analysis, to Dr. Milan Orlita (from Grenoble) for the FIR measurements, to Dr. James a. Platts (from Cardiff University) and Dr. A. Kerridge (from Lancaster university) and Dr. P. Kӧgerel (from Aachen University) for the computational calculations. I thank my fellow labmates, in particular Dr. Harrison Omorodin and Samuel Edwards, for their help in the lab and for all the fun we have had in the last four years. A big thank goes in particular to Dr. Saptarshi Biswas for simply being Saptarshi and for his suggestions and helps during difficult experiments. I wish to thank all my friends in Dublin, in particular Luca and Imma, and the members of the Colavita group: Khairul, Adam, Federico, Johana, James, Ronan, Guido, Alessandro, Carlota and Letizia for being a very nice office-mates. Last but not the least, I want to thank my family: my parents, my brothers and my sister for their support throughout my project and during the writing of this thesis. V Table of Contents List of Appendices List of Figures List of Tables List of Schemes List of Abbreviations Chapter 1: Introduction 1.1 Overview 2 1.2 Actinides and Nuclear Energy 3 1.3 Chemical Characteristics of Actinides 8 1.3.1 Spin-Orbit Coupling 15 1.4 Photophysical Properties of Actinides 16 1.4.1 Structures and Photophysical Properties of Uranyl(VI) Ion 18 1.4.2 Photophysical Properties of Uranyl(V) Ion 22 1.4.3 Photophysical Properties of Actinide(IV) Ions 23 1.4.4 Photophysical Properties of Actinide(III) Ions 27 1.5 Magnetic Properties of Actinide Ions 28 1.5.1 Magnetic Susceptibility of Uranium Ions 28 1.6 The thiocyanate Ion 32 1.7 Aims of the Project 33 1.7 References 34 Chapter 2: Structural and spectroscopic investigation of A4[U(NCE)8] (A = Cs, n Me4N, Et4N, Pr4N; E = S, Se) 2.1 Introduction 45 2.2 Systematic Structural and Spectroscopic Study of A4[An(NCS)8] (An = Th, U; A n = Cs, Me4N, Et4N, Pr4N) 50 2.2.1 Synthesis and Structural Characterization 50 2.2.2 Structural Distortion
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