Thermal hysteresis induced by external pressure in a 3D Hofmann-type SCO-MOF.

Two isostructural 3D Hofmann-type frameworks, [Fe(dbdpe)M(CN)]·4HO (M = Pt for 1 and Pd for 2), were synthesized based on a bis-monodentate ligand dbdpe (1,2-dibromo-1,2-di(pyridin-4-yl)ethane). Both compounds underwent similar one-step incomplete spin crossover (SCO) processes in the presence of lattice water molecules, i.e., T↓ = 185 K for 1 and 187 K for 2 without any hysteresis loop, while their dehydrated products exhibited paramagnetic behaviours. The application of pressure on 1 broadened the hysteresis loop to 13-25 K and shifted the transition temperature from 185 to 298 K, whereas the SCO completeness was not well improved. Variable temperature X-ray crystallographic studies clearly confirmed the incomplete SCO behaviors, and the intermolecular hydrogen bonds might promote the cooperativity in the SCO processes.