Pressure-Induced Multistep Hysteretic Spin-Crossover Behavior of a Hofmann Coordination Polymer Containing FeN6–Pd(CN)4–FeN4O2 Backbone

Spin-state switchable materials exhibit multistep spin transition (ST) driven by strong through-bond and through-space cooperativity resulting in competing elastic interactions that stimulate collective lattice distortions. While the origin of the multistep ST has been elucidated through single-crystal X-ray diffraction, the influence of external hydrostatic pressure remains unexplored. Hofmann-type coordination polymers (HCPs) being porous materials offer easy modification of guest molecules and modulation of elastic interactions required for multistep ST and also to investigate the effect of hydrostatic pressure. Herein, a unique cyanide-bridged Fe^II-Pd^II two-dimensional (2D) HCP has been synthesized using 5-bromoisonicotinamide (5-bna) ligand in ethanol-water mixture, having a polymeric backbone of Fe^IIN₆-Pd(CN)₄-Fe^IIN₄O₂, formulated as {Fe(5-bna)(H₂O)[Pd(CN)₄]}·C₂H₅OH (1Pd), displaying thermal and pressure-induced multistep hysteretic ST. 1Pd exhibits a hysteretic and abrupt two-step ST with an 85 K plateau corresponding to the HS, HS_0.66LS_0.34, and HS_0.50LS_0.50 spin states, further verified by variable temperature SC-XRD and powder X-ray diffraction. Host-host and host-guest hydrogen-bonding and halogen-π interactions influence the type of elastic interactions and stabilize the intermediate spin state. The impact of hydrostatic pressure on the noncovalent interactions influenced the multistep ST, transforming it into a gradual, single-step ST, with the increase in hysteresis at higher pressures.