Visualization of Weak Interaction Effects on N₂O Schiff Base Ligands in Iron(II) Spin Crossover Complexes

Abstract

Single crystals of an iron(II) spin crossover (SCO) complex containing an N₂O Schiff base ligand, [FeII(qsal-COOH)₂], were successfully prepared. The crystal structures from a single-crystal X-ray diffraction study were obtained at 100 and 170 K and yielded information regarding both the low-spin and high-spin states, without any crystal breakage being observed after the phase transition. Structural analysis was performed to visualize the origin of the SCO phenomenon in the [FeII(qsal-COOH)₂] complex. We report that the fundamental requirements for an abrupt SCO, particularly in Fe-qsal complexes, are not only a combination of weak interaction effects, such as π–π, P4AE (parallel four-fold aryl embrace), and hydrogen bonding, but also the presence of solvents in the crystalline structure, since the solvent molecules can influence the interactions between the [FeII(qsal-X)₂] molecules.