Cyanido-bridged Fe(II)–W(IV) frameworks with pyridine derivatives provide a powerful platform for tuning spin-crossover (SCO) properties through modulation of ligand field strength and cooperativity. Here, we report two isostructural three-dimensional frameworks, FeII(4-chloropyridine)42WIV(CN)8 (1) and FeII(4-bromopyridine)42WIV(CN)8 (2), both crystallizing in the orthorhombic Fddd space group. Despite their structural similarity, variations in the halogen substituent lead to markedly distinct SCO behaviors. Compound 1 shows a sharp and cooperative SCO phenomenon with a thermal hysteresis loop, involving 49% conversion of the FeII centers. By contrast, compound 2 exhibits a gradual and quenched SCO phenomenon with only 11% conversion and without thermal hysteresis. Structural analysis revealed that compound 1 possesses shorter Fe–NCN bonds and weaker halogen bonding, whereas compound 2 exhibits longer Fe–NCN bonds and stronger halogen bonding. These crystallographic differences influence the stabilization of the low-spin state and the cooperativity. These findings demonstrate that halogen substitution can affect Fe–N bonding and lattice elasticity, providing molecular-level insight and a rational design strategy for controlling bistability and cooperativity in cyanido-bridged frameworks.
Nagashima et al. (Mon,) studied this question.