Spin-Crossover Tuned Rotation of Pyrazolyl Rings in a 2D Iron(II) Complex towards Synergetic Magnetic and Dielectric Transitions

Materials showing synergy of magnetic and dielectric transitions are promising candidates for future molecular devices. The challenge is how to realize synergy between spin and dielectric transitions with responses to external stimuli. Herein, we design a 2D spin crossover (SCO) complex, [Fe-II(dpa)][(pzTp)Fe-III(CN)(3)](2) (1) (dpa=1,2-bis(4-pyridyl)ethyne and pzTp=tetrakis(pyrazolyl)borate). The local structural changes about the Fe-II ion were propagated to the whole crystal through the rigid bridging ligands (dpa), leading to elastic interactions to realize the abrupt SCO and rotational movements of polar apical pyrazolyl rings in the [(pzTp)Fe-III(CN)(3)](-) units. Dielectric measurements confirmed a substantial dielectric change (Delta epsilon '=2.3) upon the spin transition. This work provides a rational strategy to couple the spin transition and rotation of polar components, which is crucial for the synergetic switch of magnetism and dielectricity.