Magnetic-dielectric switching with wide hysteresis in an Fe(II) spin-crossover compound

Materials featuring hysteretic magnetic and dielectric properties assume great significance in data storage and switching devices. Nevertheless, such materials with a wide hysteresis loop remain scarce. Here, by introducing a large anion with ordered-disordered potential, a mononuclear Fe(II) compound [Fe(3-bpp)₂](NTf₂)₂·3-bpp·H₂O (1, 3-bpp = 2,6-bis(pyrazol-3-yl)pyridine and NTf₂^- = bis(trifluoromethanesulphonyl)imide) was synthesized. 1 exhibits a scan rate-dependent hysteretic spin crossover (SCO) behavior. The spin transition temperatures (T_1/2) at 2 K min^-1 are 239 and 278 K in the cooling and heating modes, respectively, indicating a hysteresis width of 39 K. Most importantly, a hysteretic dielectric property is synergistic with the SCO process. The entropy variations evaluated by differential scanning calorimetry (DSC) reach as high as 107.6/96.3 J mol^-1 K^-1 in the cooling/heating modes, which are higher than those typically observed for Fe(II) SCO compounds. Structural analysis reveals that the order-disorder transition of NTf₂^- anions triggers a significant rearrangement of hydrogen bonds, which subsequently leads to a remarkable alteration in the geometry of the [Fe(3-bpp)₂]²⁺ cation. The notable variations in the Φ and θ parameters are responsible for the hysteretic SCO behavior. The SCO process incorporating the order-disorder transition of the NTf₂^- anions gives rise to a synergistic effect with dielectric bistability. Thus, this study presents novel perspectives on the magnetic-dielectric coupling mechanism and offers a promising path for the development of multi-functional SCO materials.