Guest-dependent pressure-induced spin crossover in FeII4[MIV(CN)8]2 (M = Mo, W) cluster-based material showing persistent solvent-driven structural transformations.

Discrete molecular species that can perform certain functions in response to multiple external stimuli constitute a special class of multifunctional molecular materials called smart molecules. Herein, we present cyanido-bridged coordination clusters {[Fe II (2pyrpy) 2 ] 4 [M IV (CN) 8 ] 2 }·4MeOH∙6H 2 O (M = Mo ( 1solv ), M = W ( 2solv ) and 2pyrpy = 2-(1-pyrazolyl)pyridine) which show persistent solvent driven single-crystal-to-single-crystal transformations upon sorption/desorption of water and methanol molecules. Three full desolvation-resolvation cycles with the concomitant change of the host molecules do not damage the single crystals. More importantly, the Fe 4 M 2 molecules constitute a unique example where the presence of the guests directly affects the pressure-induced thermal spin crossover (SCO) phenomenon occurring at the Fe II centers. The hydrated phases show a partial SCO with approximately two out-of-four Fe II centers undergoing a gradual thermal SCO at 1 GPa, while in the anhydrous form the pressure-induced SCO effect is almost quenched with only 15% of the Fe II centers undergoing high-spin to low-spin transition at 1 GPa.