Pybox-Iron(II) Spin-Crossover Complexes with Substituent Effects from the 4-Position of the Pyridine Ring (Pybox = 2,6-Bis(oxazolin-2-yl)pyridine)

Spin-crossover (SCO) behavior of a series of [Fe(X-pybox)₂](ClO₄)₂ was investigated, where X-pybox stands for 4-X-substituted 2,6-bis(oxazolin-2-yl)pyridine with X = H, Cl, Ph, CH₃O, and CH₃S. We confirmed that the mother compound [Fe(H-pybox)₂](ClO₄)₂ underwent SCO above room temperature. After X was introduced, the SCO temperatures (T_1/2) were modulated as 310, 230, and 330 K for X = Cl, Ph, and CH₃S, respectively. The CH₃O derivative possessed the high-spin state down to 2 K. Crystallographic analysis for X = H, Cl, CH₃O, and CH₃S was successful, being consistent with the results of the magnetic study. Distorted coordination structures stabilize the HS (high-spin) state, and the highest degree of the coordination structure distortion is found in the CH₃O derivative. A plot of T_1/2 against the Hammett substituent constant σ_p showed a positive relation. Solution susceptometry was also performed to remove intermolecular interaction and rigid crystal lattice effects, and the T_1/2’s were determined as 260, 270, 240, 170, and 210 K for X = H, Cl, Ph, CH₃O, and CH₃S, respectively, in acetone. The substituent effect on T_1/2 became very distinct, and it is clarified that electron-donating groups stabilize the HS state. View Full-Text