Colossal Anisotropic Thermal Expansion through Coupling Spin Crossover and Rhombus Deformation in a Hexanuclear {FeIII4FeII2} 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*)FeIII(CN)3]4[FeII(Ppmp)]2⋅2 CH3OH (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 {FeIII2FeII2}. 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 {FeIII2FeII2} 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. A new hexanuclear spin crossover compound [(Tp*)FeIII(CN)3]4[FeII(Ppmp)]2⋅2 CH3OH (1) was synthesized by assembling the spin crossover FeII units and asymmetric Ppmp ligands into a rhombic core structure. It showed the colossal positive and negative anisotropic thermal expansion coupled with spin transition.