Anisotropic Thermal Expansion and Asymmetric Spin-Crossover Hysteresis in [Fe ^III ₄ Fe ^II ₂ ] Clusters Driven by Elastic Anisotropy

Anisotropic thermal expansion (ATE) provides an effective means to translate subtle molecular motions into directional lattice strain and cooperative functional responses in solid-state materials. Herein, we report two cyano-bridged hexanuclear [FeIII4FeII2] clusters, [FeIII(Tp*)(CN)3]4[FeII(5pemp)]2 (1) and [FeIII(Tp*)(CN)3]4[FeII(4pemp)]2 (2). By subtly shifting the position of the methyl substituent on the auxiliary ligand, we achieve rational control over supramolecular interactions between neighboring clusters, thereby modulating the elastic anisotropy of the lattice. Compound 2 exhibits pronounced anisotropic elastic responses that give rise to anisotropic thermal expansion, including negative thermal expansion (NTE) along one principal axis X2. This anisotropic lattice response is further coupled to the SCO behavior, resulting in a rate-dependent and asymmetric thermal hysteresis in the magnetic susceptibility. In contrast, compound 1, which features a more isotropic supramolecular interaction network, displays conventional, hysteresis-free SCO behavior. DFT calculations show that the directional supramolecular effects modulate the natural populations at the spin-active FeII centers, providing a theoretical basis for the distinct transition temperatures T1/2 observed in 1 and 2. These results demonstrate that the site-specific modulation of intermolecular interactions provides an effective strategy to construct anisotropic elastic frameworks in polynuclear SCO clusters, enabling the rational design of materials that integrate anisotropic thermal expansion with tunable spin-transition pathways and kinetics.