Design and Development of Tuneable Spin-Crossover Compounds as Alternative Switchable Components in Information Storage Media

Abstract

The following work regards the synthesis and characterisation of compounds with the potential to undergo a spin crossover (SCO). A SCO describes an electronic transition between the LS and HS states in response to perturbation by light, pressure, magnetic-field or thermal stimuli. These materials consist of d4 – d7 first row transition metal ions chelated to a suitable organic ligand system. By inducing a SCO transition, a compound exhibits exploitable changes in magnetism, colour and conductivity, making them candidates for use in information storage media. For conventional systems, the SCO must occur around room temperature with a practical hysteresis. Here, the LS state and HS state can correspond to, for instance, the binary states used in computational memory. Currently, a suitable compound remains elusive. To this end, this thesis challenges the design and development of cationic Fe(II) and Co(II) complexes by intelligent ligand modification. Amide- and thioamide-functionalisation of Fe(II) 2,6-bis(pyrazol-1-yl)pyridine (bpp) complexes, of the form [Fe(ligand)2](X)2, where X = BF4- or ClO4-, has been used to modulate the SCO properties. Complexes were found to exhibit a range of magnetic, from being locked in the LS or HS state, to abrupt SCO with hysteresis up to 96 K. Crystallisation of several compounds was achieved by slow vapour diffusion. Intermolecular hydrogen bonding between NH-groups and anions were responsible for isostructural packing between salts, which led to similar SCO properties. In addition, amide-linked homo- and heterobimetallic coordination polymers were synthesised and found to exhibit gradual SCO, attributed to their amorphous nature. Finally, an attempt was made to establish a more ‘green’ and efficient synthesis of bpp and related ligands. As a result, several novel ligand species and their Fe(II) and Co(II) complexes were synthesised. Amide-functionalised 2,6-bis(benzimidazol-2-yl)pyridine (bbip) Fe(II) complexes were found to exhibit SCO behaviour, while ester functionalised versions persisted in the LS state. Compounds were analysed by TGA, DSC, Evan’s method NMR spectroscopy, SQUID magnetometry, UV/Vis, Raman spectroscopy, single crystal and powder X-ray diffraction. Surface deposits of coordination polymers were also characterised by SEM and AFM following drop casting and spin coating onto silicon wafers. Overall, this has been a highly fruitful investigation leading to a total of 33 novel organic compounds and 46 novel complexes, 26 of which displayed some form of SCO.