School of Chemistry - Theses
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ItemNeutron Spectroscopy and Magnetic Properties of Lanthanoid(III)-dioxolene Compounds( 2022)This thesis presents a series of studies on the electronic structure and magnetic properties of several families of lanthanoid(III)-dioxolene compounds. Compounds of the trivalent lanthanoid (Ln) ions are the current best performing single-molecule magnets (SMMs), and efforts to improve their properties include design of the coordination geometry, as well as control of spin-phonon, magnetic exchange, and intermolecular dipolar interactions. This thesis presents an exploration of the effect of Ln(III)-radical magnetic exchange coupling, as well as other solid-state effects, on the SMM behaviour of Ln(III) dioxolene compounds. Slow magnetic relaxation in Ln-SMMs can be modulated by the introduction of magnetic exchange coupling, however, quantifying the magnitude of magnetic exchange coupling in many Ln(III) systems is difficult using conventional magnetometric techniques, due to the often large orbital angular momentum contribution to the magnetic moment of Ln(III) ions. Spectroscopic techniques are therefore required to determine the magnetic exchange coupling for non Gd(III) systems. Inelastic neutron scattering (INS) is used in this work, alongside magnetometry, EPR spectroscopy, and luminescence measurements, to experimentally determine both the crystal field (CF) splitting and magnetic exchange coupling in several families of Ln(III)-dioxolene compounds. The two families of compounds [LnTp2trop] (Tp– = tris-pyrazolylborate; tropH = tropolone) and [LnTp2dbsq] (dbsqH = 3,5-di-tert-butylsemiquinone) are investigated, and a trend in the magnitude of the antiferromagnetic magnetic exchange coupling |JLn-SQ| is found, increasing from Tb to Yb in the isostructural series of compounds. For the compound [Tb(18-c-6) Br4CatNO3] (18-c-6 = 18-crown-6; Br4CatH2 = tetrabromocatecholate), INS is used to measure the magnitude of the CF splitting of the Tb(III) in a highly axial coordination environment. The one electron oxidised compounds [Ln(18-c-6)X4SQNO3] . I3 (X4SQH = tetrahalosemiquinone; X = Cl, Br) were then synthesised, and the magnetic exchange coupling was determined for the Gd(III) analogues by magnetometry, and the Nd(III) and Tb(III) congeners by INS. The implications of the magnetic exchange coupling on the slow magnetic relaxation of both the [LnTp2dbsq] and [Ln(18-c-6)X4SQNO3] . I3 families of compounds is investigated. Slow magnetic relaxation in zero-field is engendered for the Tb(III) congeners by the magnetic exchange bias. For the case of stronger magnetic exchange coupling in the Kramers systems, the Ln(III) radical exchange coupling leads to the loss of a doubly degenerate ground state and the magnetic bistability. For the [Ln(18-c-6)X4SQNO3] . I3 compounds, the axial ligand environment and magnetic exchange coupling leads to unusual slow magnetic relaxation for the Gd(III) and Eu(III) congeners. Modulation of the slow magnetic relaxation in these systems by effects other than Ln(III)- radical exchange is also observed for several of the compounds. The effect of intermolecular dipolar interactions and crystal packing on the magnetic relaxation of the SMM analogues adds to the understanding of the contributing factors on the observed relaxation of Ln(III)-SMMs.
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ItemValence tautomerism and exchange interactions in cobalt-dioxolene complexes( 2019)This thesis presents an experimental and computational investigation of cobalt and cobalt-dioxolene complexes, focusing on valence tautomeric (VT) behavior, single-molecule magnet (SMM) properties and magnetic exchange interactions. Understanding of the electronic structure and principles governing the successful switching behavior of these molecules has led to the synthesis of new VT complexes and recommendations for future directed design. A density functional theory (DFT) method has been developed for the prediction of VT in a family of mononuclear cobalt-dioxolene complexes. The DFT method is able to accurately reproduce trends in spin-state energetics and has afforded the prediction of VT in [Co(Me3tpa)(Br4diox)]+ (Me3tpa = tris(6-methyl-2-pyridylmethyl)amine; Br4diox = tetrabromo-1,2-dioxolene). Subsequent synthesis of this complex confirmed a temperature-dependent VT equilibrium of LS-Co(III)-catecholate and HS-Co(II)-semiquinonate species in solution, with a solvent-tunable transition temperature. Solid-state magnetometry studies revealed incomplete VT interconversions on heating above room temperature. Analysis of the electrochemical data reported for literature VT compounds led to an experimentally-derived guiding parameter for the observation of valence tautomerism. The nature of the exchange interaction between the octahedral HS-Co(II) ion and the coordinated semiquinonate radical in the high-temperature VT state is a long-held question in literature, complicated by significant spin-orbit coupling of HS-Co(II). In this work, the exchange coupling has been elucidated using an anisotropic exchange Hamiltonian in conjunction with multistate Restricted Active Space Self-Consistent Field (RASSCF) ab initio modeling and wavefunction analysis, with comparison to magnetic susceptibility, magnetization and inelastic neutron scattering data. We report dominant ferromagnetic exchange coupling, which contains significant spin-orbit coupling contributions, and is of a similar magnitude to the cobalt(II) crystal field parameters. Furthermore, it is revealed that intermolecular exchange interactions must be incorporated for the accurate reproduction of magnetic and spectroscopic data. The competing effects of exchange interactions and cobalt(II) single ion anisotropy on the magnetization dynamics of HS-Co(II) and HS-Co(II)-radical complexes have been examined. We report two new pseudo-octahedral HS-Co(II) SMMs, [Co(Me3tpa)(Br4cat)] (Br4cat2- = tetrabromocatecholate) and [Co(Me3tpa)(trop)](PF6) (trop- = tropolonate), with fully characterized electronic structures including low temperature X-band and W-Band electron paramagnetic resonance spectroscopy, and RASSCF ab initio calculations. Comparison with the absence of slow magnetic relaxation observed for the radical analogue, [Co(Me3tpa)(3,5-tBu2sq)]+ (3,5-tBu2sq•- = 3,5-di-tert-butyl-semiquinonate), highlights the importance of Ising exchange coupling or an odd total number of electrons for the manifestation of SMM behavior. The principles governing two-step VT transitions in dinuclear cobalt complexes are presented, building on the report of the first compound to exhibit a two-step VT interconversion in both solid and solution states, which incorporates the bis(dioxolene) ligand, 3,3,3′,3′-tetramethyl-1,1′-spirobi(indan)-5,5′,6,6′-tetraol (spiroH4). Five new dinuclear cobalt complexes have been synthesized based on the bridging ligand, 3,3,3′,3′-tetramethyl-1,1′-spirobi(indan)-4,4′,7,7′-tetrabromo-5,5′,6,6′-tetraol (Br4spiroH4). Complex [{Co(Me2tpa)}2(Br4spiro)]2+ (Me2tpa = bis(6-methyl-2-pyridylmethyl)(2-pyridyl-methyl)amine) exhibits a VT transition above room temperature, while complex [{Co(Me3tpa)}2(Br4spiro)]2+ exhibits a partial VT transition centered at 175 K. Synthesis of zinc analogues has enabled characterization of the isolated Br4spiro ligand, particularly in the mixed-valence (Br4spirocat-sq)3- state. Correlation of the electrochemical and mixed-valence properties of the compounds with the step-wise VT behavior has afforded guiding principles for the design of dinuclear complexes that exhibit a two-step VT transition.
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ItemStudies of the interaction of metal complexes with ligands and biomolecules in the gas phase using mass spectrometry( 2005)Introduction of soft ionization techniques, such as electrospray ionization (ESI), has resulted in extensive use of mass spectrometry based techniques to study biomolecules in the gas phase. Despite thorough studies of the gas-phase chemistry of even-electron biomolecules, the examination of their odd-electron counterparts has to this point been much less extensive due to the inefficiency of ESI in generating such species. Among various methods that could be employed to generate and study odd-electron biomolecules in the gas phase, redox processes involving metal ions and homolytic cleavage of metallated amino acid or peptide derivatives would be attractive from a chemical perspective since, in principle, a wide range of metals and biomolecules or biomolecule derivatives could be explored. An important aspect of these approaches is that they can be carried out on a wide range of tandem mass spectrometers equipped with electrospray ionization and collision induced dissociation capabilities. (For complete abstract open document)
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ItemThe synthesis of bicyclic lactams using the concept of self-terminating radical cyclisations( 2012)This thesis explores the scope of self-terminating radical cyclisations for the synthesis of bicyclic lactams, which could be of potential pharmaceutical interest. Self-terminating radical cyclisations enable transformation of suitable alkynes to bicyclic compounds under tin-free conditions using O-centred radicals as oxidants. Firstly, β-Lactam substituted alkynes 96 were synthesised, which could undergo the NO3• induced cascade to generate bicyclic β-lactams 97. Energy and geometry calculations were performed to generate potential energy surfaces of the NO3• induced radical cascade with 96 and predicted that the reaction cascade is both thermodynamically and kinetically favourable. The success of the self-terminating radical cyclisations with the NO3• and lactam substituted alkynes relies upon the favourability of the HAT step (e.g. 98 to 99), which is the rate-determining step. Thus the presence of the ring clamp reduces the degrees of freedom of the chains to allow the vinyl radical site and hydrogen atom donor to be in favourable configurations for HAT. Thus, focus was put back on the NO3• induced cascade with other lactam substituted alkynes. In order to reduce the ring strain imposed by the β-lactam ring clamp, it was replaced with a larger ring, i.e. a γ/δ-lactam ring. Energy calculations predicted potential energy surfaces for the NO3• induced radical cascade with the lactam substituted alkynes 190 and these indeed suggested that the radical cascade became more favourable as the lactam ring increased in size. Experimental studies gave strong indications that radical reactions of NO3• with the γ/δ-lactam substituted alkynes 190a/b afforded bicyclic lactams 191a/b and 202a/b via the self-terminating radical cyclisation. Interestingly, the formation of 1,2-diketones 203a/b were also present in the reaction mixture, suggesting that the self-terminating radical cyclisation was not the most favourable pathway. The molecular formulae of the products formed in the radical reaction were confirmed with HRMS. CID analysis gave structural evidence for the bicyclic framework of the bicyclic lactam products, e.g. bicyclic δ-lactams 191b and 202b possess a [4.3] and [4.4] bicyclic framework respectively. The addition of a single O-atom to the γ/δ-lactam substituted alkynes 190a/b, observed from HRMS analysis, can be explained by the self-terminating radical cyclisation. Overall, there was great difficulty in the synthesis of the lactam substituted alkynes, and reaction of them with NO3• via the self-terminating radical cyclisation granted limited success. It can be concluded that generating bicyclic lactams via self-terminating radical cyclisations is not a suitable method of choice.