Discrete metallosupramolecular complexes, including metallomacrocycles, helicates and cages, have emerged as the product of vast incremental improvements in coordination chemistry. These intricate chemical systems have potential applications in quantum data storage, catalysis, separation and trapping, sensing devices as well as drug delivery. The ability to design and synthesise spin-crossover systems, with controlled structures and properties, continues to evade the chemical and physical community. Currently, there is a lot of guess-and-check, lead-optimisation and post addendum methodology in the design of compounds with useful physical properties. This project aims to design and synthesise a range of supramolecular systems and investigate how their magnetic properties can be manipulated to explore the relationship between design, structure and function, or as it applies specifically to spin-crossover (SCO), the magneto-structural correlations. A deeper understanding of the factors affecting the design of more complex molecular systems with predictable structures and functions will enable chemists to reliably produce materials with intentional properties for use in future molecular devices. Further deep studies relating structure to function in a wide variety of chemical systems, are necessary to provide a more extensive collection of magneto-structural correlations, from which the design of future SCO systems can be better informed, and additionally, the development of new theories and models can be based. Firstly, a series of three Ln(III)-based complexes were synthesised via the Schiff-base condensation of N,N-diethylsalicylaldehyde (SAL) and tris(2-aminoethyl)amine (TREN) and characterised in order to assess the efficiency of the 4-position N-diethylamino electron-donating substituent as a modulator of the fluorescence and magnetic susceptibility of this commonly employed TRENSAL N4O3-donor cavity ligand. The Eu(III) compound exhibited efficient sensitisation of the metal centre and metal centred fluorescence, while the Dy(III) complex demonstrated signs of single molecule magnet behaviour in DC susceptibility measurements. Next, a mononuclear hexadentate complex, of the form [FeL](BF4)2, based on the 4-thioimidazole donor moiety, was designed to exhibit high-temperature SCO and was investigated using variable temperature X-ray photoelectron spectroscopy (VT-XPS). The aim of this study was to identify a method by which the high-spin (HS) fraction in the surface layers of a SCO material could be quantified using XPS, in order to allow the characterisation of future thin-film SCO materials and devices. Magnetic susceptibility measurements were used to calibrate XPS spectral fractions, and a HS-fraction curve was obtained that closely resembled that of the χmT Vs T results. Finally, three series of five Fe(II) dinuclear triple helicate compounds, of the form [Fe2L3], were synthesised, characterised and explored via magnetic susceptibility and single-crystal X-ray diffraction experiments. The general helicate architecture of each series differed by the steric nature of the central connecting atom of the ditopic bisbidentate imidazoleimine ligand donor (C, S or O). For each of the three helicate architectures, five counter ions were investigated; BF4-, ClO4-, I-/I3-, Br- and Cl-. In this way, fifteen analogous helicate materials, each with subtle changes in crystallographic structure, were analysed to determine the impact of various structural parameters on the magnetic susceptibility of these compounds""that is to identify magneto-structural correlations present in this chemical system""and compare these to previously reported results, with a particular focus on dinuclear helicate compounds. A range of relationships were found between selected magnetic and structural parameters. The most extensive of which were the observed dependence of the T1/2 on the strength of the anion-to-imidazole hydrogen-bonding at the external 4-position-imidazole H-N, and the relationship between the completion of SCO and the degree of intermolecular steric crowding of the Fe(II) coordination environment. As such, in this dinuclear triple helicate chemical system, the T1/2 can be systematically tuned by substitution of the hydrogen bond acceptor, and the extent of SCO""that is the HS Fe(II) fraction remaining at low temperatures""can be systematically tuned by manipulation of the packing of adjacent helicates throughout the crystal lattice. Encouraged by these results, co-crystallisation of a helicate compound with selected organic compounds (1,4-diiodo-2,3,5,6-tettrafluorobenzene (DITFB) and 1,3,5-benzenetricarboxylic acid respectively (BTC)) was performed in order to further investigate structure-function relationships in these dinuclear triple helicate compounds. This investigation demonstrated the various influences of steric congestion of the SCO centres on the completeness of the spin-transition in these compounds.
| Date of Award | 2018 |
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| Original language | English |
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