Room-temperature switching of magnetic hysteresis by reversible single-crystal-to-single-crystal solvent exchange in imidazole-inspired Fe(II) complexes

The recent upsurge in molecular magnetism reflects its application in the areas of sensors and molecular switches. Thermal hysteresis is crucial to the molecular bistability and information storage, a wide hysteresis near room temperature is expected to be of practical sense for the molecular compound. In this work, spin crossover iron(ii) complexes [Fe(Liq)₂](BF₄)₂·(CH₃CH₂)₂O (1-Et₂O) and [Fe(Liq)₂](BF₄)₂·3H₂O (1-3H₂O) were prepared and structurally and magnetically analysed. The single-crystal-to-single-crystal (SCSC) solvation transformation and the influence on the crystal structures and magnetic hysteresis were investigated in an etherification-hydration cycle. At room temperature, X-ray diffraction experiments indicated a transformation from one crystal (1-Et₂O, P2₁2₁2) to another crystal (1-3H₂O, P2₁2₁2₁) upon humidity exposure and reversible recovery of its crystallinity upon exposure to ether vapor. The etherified phase 1-Et₂O exhibits room temperature spin crossover (T_1/2 = 305 K) but negligible thermal hysteresis, however the hydrated phase 1-3H₂O exhibits the apparent hysteresis loop (T_1/2↑ = 346 K, T_1/2↓ = 326 K) which expands to room temperature. This effect is associated with the change of intermolecular cooperativity in the etherification-hydration recyclability.