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)center dot 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)center dot acetone and [Fe(E-dpsp)(2)] (BF4)(2)center dot 4MeNO(2), detailed analyses at various temperatures are conducted. The magnetic properties of bulk microcrystalline samples of [Fe(E-dpsp)(2)](BF4)(2)center dot X are assessed using a SQUID magnetometer. Among the series, only [Fe(E-dpsp)(2)](BF4)(2)center dot acetone undergoes peculiar thermal SCO, such as a precipitous and hysteretic spin-state change (T-1/2 up arrow = 179 K, T-1/2 down arrow = 164 K, and Delta T-1/2 = 15 K) and frozen-in effect. All single crystals of [Fe(E-dpsp)(2)](BF4)(2)center dot X are free from intermolecular interaction except for [Fe(E-dpsp)(2)](BF4)(2)center dot acetone: One of the phenyl rings in [Fe(E-dpsp)(2)](BF4)(2)center dot 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 pi-pi 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)center dot acetone.