Heterometallic FeIII/K Coordination Polymer with a Wide Thermal Hysteretic Spin Transition at Room Temperature

The anionic Fe-III complex exhibiting cooperative spin transition with a wide thermal hysteresis near room temperature, K[Fe(5-Brthsa)(2)] (5-Brthsa-H-2=5-bromosalicylaldehyde thiosemicarbazone), is reported. The hysteresis (Delta=69 K in the first cycle) shows a one-step transition in heating mode and a two-step transition in cooling mode. X-ray structure analysis showed that the coexistence of hydrogen bond and cation-pi interactions, as well as alkali metal coordination bonds, to give 2D coordination polymer structure. This result is contrary to previous reports of broad thermal hysteresis induced by coordination bonds of FeII spin crossover coordination polymers (with 1D/3D structures), and by strong intermolecular interactions in the molecular packing through pi-pi stacking or hydrogen-bond networks. As a consequence, the importance, or the very good suitability of alkali metal-based coordination bonds and cation-pi interactions for communicating cooperative interactions in spin-crossover (SCO) compounds must be reconsidered.