Crystal Engineering of Spin-Crossover Materials

Spin-crossover (SCO) materials are transition metal complexes that can switch their spin state in response to external stimuli such as changes in temperature and pressure. They have potential applications in sensing, actuation and cooling technologies. Crystal engineering concepts have been applied to develop a huge range of new SCO materials based on the [Fe(3-bpp)2] 2+ SCO complex (where 3-bpp = 2,6-di(pyrazol-3-yl)pyridine). Chapter 2 focuses on the initial development of this approach by synthesising a series of eleven spin-crossover cocrystals using ditopic coformers, where the supramolecular architectures are dictated by the structure of the coformers. Eight of these new SCO materials were found to be SCO-active, displaying interesting switching properties including hysteresis, stepped SCO and irreversible transitions, and structure-property relationships were developed based on the findings. Chapter 3 investigates the effects of solvates and anions on SCO materials, showing that the choice of anion affects the resulting supramolecular structure of SCO cocrystals. A series of four SCO-active isostructural cocrystal solvates were synthesised and analysed by variable temperature single-crystal X ray diffraction, showing that stronger hydrogen bond acceptors stabilise the low-spin state. Chapter 4 builds on the work from Chapter 2 and Chapter 3 by developing the cocrystallisation strategy further and looking into the effects of using more complex coformers. This resulted in supramolecular structures that deviate from those seen in Chapter 2. Cocrystals were also synthesised using monotopic coformers, which do not form extended supramolecular architectures but were found to be SCO-active and showed interesting domain formation. Mixed cocrystallisation was used to synthesise new materials, demonstrating that it is another strategy that can be implemented to modify SCO V materials. Chapter 5 demonstrates that mechanochemistry using a pestle and mortar or a ball mill can be used to synthesise SCO materials and SCO cocrystals. The SO4 2- salt of the [Fe(3-bpp)2] 2+ SCO complex was synthesised using mechanochemistry and a crystal structure of the material has been reported for the first time. Liquid-assisted grinding in the ball mill was investigated, showing that specific SCO cocrystal polymorphs can be obtained selectively by modifying the milling conditions