Guest-Dependent Pressure-Induced Spin Crossover in Fe [M (CN) ] (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 (2-pyrpy) ] [M (CN) ] }⋅4 MeOH⋅6 H 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 M 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 centres. The hydrated phases show a partial SCO with approximately two out-of-four Fe 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 centres undergoing high-spin to low-spin transition at 1 GPa.