Bis[(E)‑2,6-bis(1H‑pyrazol-1-yl)-4-styrylpyridine]iron(II) Complex: Relationship between Thermal Spin Crossover and Crystal Solvent

The crystal structures and thermal spin-crossover (SCO) behavior of [Fe­(E-dpsp)2]­(BF4)2·X [E-dpsp = (E)-2,6-bis­(1H-pyrazol-1-yl)-4-styrylpyridine; X = crystal solvent] are investigated. The titled iron­(II) complex features polymorphology induced by crystal solvents, which is identified by means of single-crystal X-ray diffraction analysis: For [Fe­(E-dpsp)2]­(BF4)2·acetone and [Fe­(E-dpsp)2]­(BF4)2·4MeNO2, detailed analyses at various temperatures are conducted. The magnetic properties of bulk microcrystalline samples of [Fe­(E-dpsp)2]­(BF4)2·X are assessed using a SQUID magnetometer. Among the series, only [Fe­(E-dpsp)2]­(BF4)2·acetone undergoes peculiar thermal SCO, such as a precipitous and hysteretic spin-state change (T1/2↑ = 179 K, T1/2↓ = 164 K, and ΔT1/2 = 15 K) and frozen-in effect. All single crystals of [Fe­(E-dpsp)2]­(BF4)2·X are free from intermolecular interaction except for [Fe­(E-dpsp)2]­(BF4)2·acetone: One of the phenyl rings in [Fe­(E-dpsp)2]­(BF4)2·acetone is twisted appreciably and features an intermolecular H–H short contact with one of the neighboring complexes to form a one-dimensional network. The twisted phenyl group also participates in π–π stacking with one of the pyrazolyl rings of another neighboring molecule, constructing a dimeric couple. These intermolecular interactions would induce cooperative effects, which leads to the good thermal SCO phenomenon of [Fe­(E-dpsp)2]­(BF4)2·acetone.