Solvent Effects on the Structural Packing and Spin-Crossover Properties of a Mononuclear Iron(II) Complex

Solvent molecules are known to have a remarkable effect on the crystal structures and magnetic properties of spin-crossover (SCO) compounds. On the basis of our previous works on this topic, we have synthesized a series of SCO Fe(II) compounds, [Fe(tpa)(NCSe)₂]·Solv (tpa = tris(2-pyridylmethyl)amine; Solv = 0.5CH₃OH for 1·CH₃OH, 0.5C₂H₅OH for 1·C₂H₅OH, 0.25H₂O for 1·H₂O, 0.5CH₃CN for 1·CH₃CN-A, CH₃CN for 1·CH₃CN-B, and CH₂Cl₂ for 1·CH₂Cl₂), by crystallization of the molecular complex [Fe(tpa)(NCSe)₂] from the respective solvents. Single-crystal X-ray crystallographic studies show that the molecular packing structures and intermolecular interactions of these compounds are subtly changed by the lattice solvent molecules; thus, their SCO properties can be differentiated from each other. All of the solvated compounds undergo one-step SCO behavior with the order of critical temperatures being T_c(1·CH₃CN-B) < T_c(1·CH₂Cl₂) < T_c(1·CH₃CN-A) < T_c(1·C₂H₅OH) ≈ T_c(1·CH₃OH) < T_c(1·H₂O), of which thermal hysteresis loops of 3 K width (T_c→ = 255 K and T_c↔ = 252 K) and 10 K width (T_c→ = 256 K and T_c↔= 246 K) are observed for 1·CH₃CN-B and 1·CH₂Cl₂, respectively.