Structural Investigations of solvated Spin Crossover Complexes and Co-crystals

Spin Crossover materials show promising properties as multifunctional materials with potential uses in electronics and sensors, for example. Ideally, the spin crossover needs to be tuned for a desired purpose. Co-crystallisation and solvation of spin crossover materials show promise in the ability to tune the thermal behaviour of these system’s spin transition. In previous work, co-crystallisation of [Fe(3-bpp)2] 2+ (where 3-bpp is 2,6-bis(pyrazol-3-yl)pyridine) with a range of dipyridyl based coformers proved to be successful in the formation of an array different supramolecular structures and this work aims to determine if this success can be repeated with three other spin crossover complexes. This work reports the mechanochemical synthesis of three complexes: [Fe(pbi)3][BF4]2, [Fe(bpbi)2][BF4]2 and [Fe(himpy)3][BF4]2. A series of solvated crystal structures of [Fe(pbi)3][BF4]2 are presented with the associated thermally induced spin crossover properties investigated through single-crystal X-ray diffraction and reflectivity experiments. All of the solvates undergo spin crossover below 320 K. Three co-crystal solvates are reported: [Fe(pbi)3][BF4]2 and 4’4,-azopyridine, [Fe(bpbi)2][BF4]2 and acridine, [Fe(bpbi)2][BF4]2 and 4’4,-trimethylenepyridine. Each crystal structure is accompanied by a structural investigation. The difficulties faced with co-crystallisation of these three complexes is discussed with respect to co-crystallisations of Fe(3-bpp)2.