“Reversible I ₂ Uptake Induces Record‐High Spin‐Crossover Hysteresis in a Fe(II) Hofmann‐Type MOF”

Controlling bistable spin states in Fe(II) spin-crossover (SCO) frameworks remains challenging. Here, we demonstrate that guest-induced chemical pressure provides a reversible and general strategy to amplify thermal hysteresis. In the flexible Hofmann-type MOF [Fe(pz)Pd(CN)₄], vapor-phase iodine uptake produces a record-wide thermal hysteresis of 120 K, far exceeding previously reported values. Structural and magnetic analyses confirm that the framework remains intact, and lattice compression by iodine enhances cooperative spin transitions, stabilizing both high-spin and low-spin (LS) states. Partial iodine loading induces multistep SCO with asymmetric hysteresis, whereas full saturation generates extreme bistability with symmetric hysteresis. This approach establishes guest capture as a direct, reversible tool to control SCO bistability, offering a broadly applicable design principle for multifunctional porous materials.