Colossal Anisotropic Thermal Expansion through Coupling Spin Crossover and Rhombus Deformation in a Hexanuclear {(Fe4Fe2II)-Fe-III} Compound

Colossal and anisotropic thermal expansion is a key function for microscale or nanoscale actuators in material science. Herein, we present a hexanuclear compound of [(Tp*)Fe-III(CN)(3)](4)[FeII(Ppmp)] (2)center dot 2CH(3)OH (1, Tp*= hydrotris(3,5-dimethyl-pyrazol-1-yl)borate and Ppmp= 2-[3-(2'-pyridyl)pyrazol-1-ylmethyl]pyridine), which has a rhombic core structure abbreviated as {(Fe2FeI2)-Fe-III-I-I}. Magnetic susceptibility measurements and single-crystal X-ray diffraction analyses revealed that 1 underwent thermally-induced spin transition with the thermal hysteresis. The FeII site in 1 behaved as a spin crossover (SCO) unit, and significant deformation of its octahedron was observed during the spin transition process. Moreover, the distortion of the FeII centers actuated anisotropic deformation of the rhombic {(Fe2Fe2II)-Fe-III} core, which was spread over the whole crystal through the subsequent molecular rearrangements, leading to the colossal anisotropic thermal expansion. Our results provide a rational strategy for realizing the colossal anisotropic thermal expansion and shape memory effects by tuning the magnetic bistability.