Mechanochemical sythesis of spin crossover materials for barocaloric applications

Spin crossover (SCO) materials have recently emerged as promising candidates for use in solid-state barocaloric cooling due to their intrinsic ability to undergo reversible spin-state transitions. These transitions are typically accompanied by significant entropy changes, which can be translated into sizable cooling effects. Current vapour-compression refrigeration systems rely on environmentally harmful chemicals and contribute substantially to global energy consumption, necessitating an ever growing need for a greener alternative, for which, SCO materials offer a potentially viable solution. The development of environmentally sound technologies requires equally sustainable synthetic methods. In this work, mechanochemistry, a technique that combines the use of minimal solvent, reduced reaction times, and facilitates scalability, is used. This study investigates the mechanochemical synthesis of four Fe(II) Schiff-base complexes with N4O2 coordination spheres. Included among these complexes are [Fe(HPMB)2] (HPMB = 4-hydroxy-N′-((pyridin-2-yl)methylene)benzohydrazide), and [Fe(HQNAL)2], (HQNAL = N-(8′-quinolyl)-2-hydroxy-1-naphthaldimine), two SCO compounds known to exhibit abrupt, near room temperature spin-transitions - properties that are crucial for the barocaloric process. Both compounds were successfully synthesised mechanochemically, with each synthetic pathway being further optimised to a single, one-pot reaction. The products showed good retention of their SCO behaviour, and were thoroughly characterised using single crystal X-ray diffraction, powder X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis, and nuclear magnetic resonance. Vibrating sample magnetometry was also carried out on the former compound. Two additional complexes were also investigated, for which SCO properties had not been previously reported. [Fe(NIPH)2] (NIPH = (N'-(pyridin-2-ylmethylene)isonicotinohydrazide)) was successfully synthesised, and it's SCO behaviour subsequently investigated. The syntheses of the final compound, [Fe(O-HPMB)2] (O-HPMB = 2-hydroxy-n'-(2-pyridinylmethylene)benzohydrazide), remained incomplete, providing scope for future investigation. The results of this study demonstrate that mechanochemistry provides a viable and environmentally friendly route for the synthesis of SCO materials, for potential barocaloric applications. In particular, [Fe(HPMB)2] and [Fe(HQNAL)2] retain their abrupt, near room temperature spin transitions when mechanochemically synthesised, making them promising candidates for further investigation. Pressure-dependent studies of these compounds would be an important step in assessing their suitability for barocaloric cooling and determining the magnitude of their pressure-induced thermal responses. More broadly, the one-pot mechanochemical approach developed here provides a useful platform for the discovery of new SCO materials. The rapid and highly successful technique could enable the exploration of previously unobtainable complexes, incorporating different metal centres and ligand environments, and allowing their spin transition temperatures to be fine-tuned. Overall, this work provides a solid foundation for further research into sustainable solid-state refrigeration technologies.