Bis[(<i>E</i>)‑2,6-bis(1<i>H</i>‑pyrazol-1-yl)-4-styrylpyridine]iron(II)
Complex: Relationship between Thermal Spin Crossover and Crystal Solvent
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Abstract
The crystal structures and thermal spin-crossover (SCO)
behavior of <b>[Fe(</b><i><b>E</b></i><b>-dpsp)</b><sub><b>2</b></sub><b>](BF</b><sub><b>4</b></sub><b>)</b><sub><b>2</b></sub><b>·X</b> [<i><b>E</b></i><b>-dpsp</b> = (<i>E</i>)-2,6-bis(1<i>H</i>-pyrazol-1-yl)-4-styrylpyridine; <b>X</b> = 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 <b>[Fe(</b><i><b>E</b></i><b>-dpsp)</b><sub><b>2</b></sub><b>](BF</b><sub><b>4</b></sub><b>)</b><sub><b>2</b></sub><b>·acetone</b> and <b>[Fe(</b><i><b>E</b></i><b>-dpsp)</b><sub><b>2</b></sub><b>](BF</b><sub><b>4</b></sub><b>)</b><sub><b>2</b></sub><b>·4MeNO</b><sub><b>2</b></sub>, detailed analyses at various temperatures are
conducted. The magnetic properties of bulk microcrystalline samples
of <b>[Fe(</b><i><b>E</b></i><b>-dpsp)</b><sub><b>2</b></sub><b>](BF</b><sub><b>4</b></sub><b>)</b><sub><b>2</b></sub><b>·X</b> are assessed
using a SQUID magnetometer. Among the series, only <b>[Fe(</b><i><b>E</b></i><b>-dpsp)</b><sub><b>2</b></sub><b>](BF</b><sub><b>4</b></sub><b>)</b><sub><b>2</b></sub><b>·acetone</b> undergoes peculiar
thermal SCO, such as a precipitous and hysteretic spin-state change
(<i>T</i><sub>1/2↑</sub> = 179 K, <i>T</i><sub>1/2↓</sub> = 164 K, and Δ<i>T</i><sub>1/2</sub> = 15 K) and frozen-in effect. All single crystals of <b>[Fe(</b><i><b>E</b></i><b>-dpsp)</b><sub><b>2</b></sub><b>](BF</b><sub><b>4</b></sub><b>)</b><sub><b>2</b></sub><b>·X</b> are free from
intermolecular interaction except for <b>[Fe(</b><i><b>E</b></i><b>-dpsp)</b><sub><b>2</b></sub><b>](BF</b><sub><b>4</b></sub><b>)</b><sub><b>2</b></sub><b>·acetone</b>: One of the phenyl rings in <b>[Fe(</b><i><b>E</b></i><b>-dpsp)</b><sub><b>2</b></sub><b>](BF</b><sub><b>4</b></sub><b>)</b><sub><b>2</b></sub><b>·acetone</b> 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 <b>[Fe(</b><i><b>E</b></i><b>-dpsp)</b><sub><b>2</b></sub><b>](BF</b><sub><b>4</b></sub><b>)</b><sub><b>2</b></sub><b>·acetone</b>
