Spin crossover in iron(II) and cobalt(II) complexes including multifunctional materials

At the outset, the primary focus of this Ph.D. project has been directed towards the synthesis of new 2,2’-dipyridylamino(dpa)- and 1,2,4-triazole(trz)- based ligands to be incorporated into spin crossover (SCO) iron(II) complexes. Initial aims sought the synthesis of a wide and diverse range of SCO compounds containing novel dpa- and trz-based ligands in order to determine what sort of control could be exercised over magnetic properties. New ligands synthesised for the project incorporated a range of nondpa- or trz-substituents with, for example, different capacities to participate in hydrogen bonding or aryl interactions (such as !-stacking), with different degrees of bulkiness, flexibility, or additional binding sites, for metals or other guests, that are distinct and separate from the dpa or trz coordination sites. This fundamental study also probed the influence of second coordination sphere entities, including the effects of solvation and counter ion species, upon the spin transition. Structure-function correlations for this work were vital to further the understanding of the factors influential in determining the magnetic properties of crystalline SCO compounds, by varying the nature of the ligand, solvent or counter-ion. As there is currently great interest in the study of multifunctional (‘hybrid’) molecular materials, a further aspect of this Ph.D. project was aimed towards the synthesis of ligands incorporating a moiety capable of a secondary function, in addition to SCO. This led to two main areas of study, the first of which consisted of the synthesis and coordination of a number of ferrocene-appended ligands and attempts have been made to analyse how one-electron changes at the ferrocene group (located external to the coordination site) influences the spin transition. This includes a system in which a new ferrocene-trz ligand and its 1-D polynuclear complexes were studied using magnetic, electrochemical and Mössbauer measurements (Chapter 6), as well as synthesis of discrete FeII and CoII complexes incorporating ferrocene-based ligands (Chapter 7). Our second approach at obtaining multifunctional SCO materials has led us to pursue the synthesis and coordination of crown-appended dpa-based ligands which have the potential to show, in addition to SCO, encapsulation of alkali metal ions in the aza-crown ether, which ideally would result in guest dependent magnetic properties. This work appears in Chapter 4. Throughout this study, where possible, all complexes have been comprehensively characterised, primarily using single crystal X-ray structural determination and variable temperature magnetic susceptibility measurements, although the vast array of complexes appearing in this thesis have been characterised using a number of other techniques including PXRD, Mössbauer spectroscopy, electrochemical studies, magnetic- and photo-magnetic measurements and to a lesser extent heat-capacity measurements.