School of Chemistry - Theses
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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.