A water-tuned reversible spin transition with the largest hysteresis loop in 3D Hofmann frameworks pillared by flexible ligands

Using rigid linear ligands as pillars for constructing 3D Hofmann-type frameworks is an important strategy to synthesize spin-crossover (SCO) materials with a large hysteresis loop. However, application of flexible pillars for building 3D Hofmann SCO networks remains a big challenge. Herein, by judiciously choosing a known ligand, N,N′-bis(4-picolinoyl)hydrazine (bph) as a flexible pillar, two novel isostructural 3D Hofmann SCO frameworks [Fe(bph)M(CN)₄]·4.5H₂O (1·4.5H₂O-c, M = Pt²⁺; 2·4.5H₂O-c, M = Pd²⁺; c = crystal) have been successfully synthesized. Both 1·4.5H₂O-c and 2·4.5H₂O-c exhibited an abrupt spin transition with a hysteresis loop of 60 and 30 K, respectively, representing the largest hysteresis width found for the 3D Hofmann SCO frameworks built from flexible pillars until now. In contrast, the dehydrated complexes 1 and 2 obtained from the dehydration of 1·4.5H₂O-c and 2·4.5H₂O-c, respectively, only showed a gradual spin transition without any hysteresis loop. Notably, both 1·and 2 can be transformed again into powdery 1·4.5H₂O-p and 2·4.5H₂O-p (p = powder) on exposure to water. However, the rehydrated 1·4.5H₂O-p and 2·4.5H₂O-p merely indicated incomplete two-step SCO behaviors with small hysteresis loops but higher transition temperatures compared with 1·4.5H₂O-c and 2·4.5H₂O-c. X-ray crystallographic studies at 296 and 100 K revealed that the rare large hysteresis loops observed in 1·4.5H₂O-c and 2·4.5H₂O-c may be ascribed to the strong cooperativity in the SCO process owing to the rich hydrogen bond interactions between the flexible bph ligands and lattice water molecules.