Guest‐Dependent Pressure‐Induced Spin Crossover in Fe II 4 [M IV (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, cyanido‐bridged coordination clusters {[Fe II (2‐pyrpy) 2 ] 4 [M IV (CN) 8 ] 2 } ⋅ 4 MeOH ⋅ 6 H 2 O (M=Mo ( 1 solv ), M=W ( 2 solv ) and 2‐pyrpy=2‐(1‐pyrazolyl)pyridine are presented, 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 centres. The hydrated phases show a partial SCO with approximately two out‐of‐four Fe II centres 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 centres undergoing high‐spin to low‐spin transition at 1 GPa.