Substituent Effect in Spin-Crossover Behavior of Iron(II)-Ar-pybox Complexes (Ar-pybox= 4-Aryl-2,6-bis(oxazolin-2-yl)pyridine)

摘要

Spin-crossover (SCO) is a reversible transition between low-spin (LS) and high-spin (HS) states by external stimuli like heat. The SCO behavior of [Fe(Ar-pybox)_2](C1O_4)2 was investigated, where Ar-pybox stands for 4-aryl-2,6-bis(oxazolin-2-yl)pyridine with Ar= 4-pyridyl (4Py), 3-thienyl (3Th), and phenyl (Ph). They were characterized by means of single-crystal X-ray diffraction study, being consistent with the results of the magnetic measurements. The SCO temperatures (T_(1/2)) in the polycrystalline state were determined to be 360 and 230 K for Ar= 4Py and Ph, respectively. The 3Th derivative possessed a HS state in all the temperature range. The solution susceptometry was also performed to purge intermolecular interaction and rigid crystal lattice effects, affording T_(1/2)= 310, 240, and 240 K for Ar= 4Py, 3Th, and Ph, respectively, in acetone. The substituent effect analysis using the Hammett substituent constant (σ_p) clarified that electron-withdrawing groups raise T_(1/2). A plausible model describing the substituent effect on T_(1/2) is proposed based on d-π interaction. The present result is regarded as a successful example of crystal field engineering