A simple solvent-free method for post-synthetic exchange of anions in an iron(ii) spin crossover material is described.

We describe the synthesis of switchable spin crossover materials via mechanochemistry for the first time. Three chemically diverse spin crossover materials have been produced using solvent-free grinding.Crucially, cooperative spin transition behavior and crystallinity is retained, presenting exciting opportunities for the discovery of new materials with switchable magnetic, optical and structural properties.Spin crossover (SCO) materials have seen a surge in interest in recent years as a result of their optical, magnetic and structural bistability in technologically accessible temperature ranges, giving rise to a number of potential applications in sensing, display and actuator technologies.1,2 The bistability in these systems stems from their propensity to switch reversibly between high-spin (HS) and low-spin (LS) states in response to physical stimuli including temperature, pressure, light irradiation, guest molecules and magnetic field. 3 There are hundreds of known SCO-active materials in the literature including molecular coordination complexes, 1D coordination polymers and 2-/3-dimensional metal organic frameworks. The cooperativity of the SCO phenomenon (abruptness of the transition, hysteretic effects etc.) is a result of electron-phonon coupling between SCO centers in the solid state through elastic interactions in the lattice. 4 The degree of cooperativity is important for application, and has been shown to be sensitive to the crystallinity of the material, particle size and the presence of solvents in the lattice. 2,5 Synthesis of bulk SCO materials has typically been carried out via traditional solution state chemistry, although more complex techniques for the production of nanoparticles and thin films have been developed in the last decade. Remarkably, all existing techniques use solvents and can be timeconsuming; often requiring inert atmospheres and multiple synthetic steps. This aspect is particularly limiting in the search for new SCO-active materials. Herein, we present for the first time the rapid and facile synthesis of three chemically different classes of SCO material, using mechanochemical techniques. We show that microcrystalline samples of SCO-active molecular materials, 1D coordination networks and 3D MOFs can be produced using this technique and crucially the cooperative behaviour of the spin transition is maintained. Finally, we highlight how this technique can be used to accelerate SCO research. Mechanochemistry refers to the reaction of materials through the application of mechanical energy, often through grinding in the solid state. While mechanochemistry has long been used in the synthesis of inorganic materials and composites, in recent years it has also been applied in the synthesis of molecular systems, coordination complexes and frameworks, co-crystals and supramolecular networks. 7,8 Mechanistic explanations for the success of the mechanochemical approach are numerous and as yet no single model can be applied to the varied range of systems that can be produced in this manner....

The effects of mechanochemical synthesis and postsynthetic grinding on the spin crossover material [Fe(atrz)3](BF4)2 was examined in detail using a combination of X-ray diffraction, magnetometry, EXAFS and TEM. Mechanochemical synthesis yielded a different polymorph (β-phase) to the solution synthesised sample (α-phase), with a lower temperature spin crossover. Milling duration did not significantly affect this temperature but did result in the production of smaller nanoparticles with a narrower size distribution. It is also possible to convert from α- to the β-phase via postsynthetic grinding.

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