Solid-State Ligand-Driven Light-Induced Spin Change at Ambient Temperatures in Bis(dipyrazolylstyrylpyridine)iron(II) Complexes

We previously reported that an FeII complex
ligated
by two (Z)-2,6-di­(1H-pyrazol-1-yl)-4-styrylpyridine
ligands (Z-H) presented
a solid state ligand-driven light-induced spin change (LD-LISC) upon
one-way Z-to-E photoisomerization,
although modulation of the magnetism was trivial at ambient temperatures
(Chem. Commun. 2011, 47, 6846). Here, we report the synthesis of new derivatives of Z-H, Z-CN and Z-NO2, in which electron-withdrawing cyano and
nitro substituents are introduced at the 4-position of the styryl
group to attain a more profound photomagnetism at ambient temperatures. Z-CN and Z-NO2 undergo quantitative
one-way Z-to-E photochromism upon
excitation of the charge transfer band both in acetonitrile and in
the solid state, similar to the behavior observed for Z-H. In solution, these substituents
stabilized the low-spin (LS) states of Z-CN and Z-NO2, and the behavior was quantitatively analyzed
according to the Evans equation. The photomagnetic properties in the
solid state, on the other hand, cannot be explained in terms of the
substituent effect alone. Z-CN displayed photomagnetic properties almost identical to those of Z-H. Z-CN preferred the high-spin (HS) state at all temperatures
tested, whereas photoirradiated Z-CN yielded a lower χMT at
ambient temperatures. The behavior of Z-NO2 was counterintuitive, and
the material displayed surprising photomagnetic properties in the
solid state. Z-NO2 occupied the LS state at low temperatures and underwent
thermal spin crossover (SCO) with a T1/2 of about 270 K. The photoirradiated Z-NO2 displayed a higher value
of χMT and the modulation of χMT exceeded that of Z-H or Z-CN. Z-NO2·acetone, in which acetone molecules
were incorporated into the crystal lattice, further stabilized the
LS state (T1/2 > 300 K), thereby promoting
large modulations of the χMT values
(87% at 273 K and 64% at 300 K) upon Z-to-E photoisomerization. Single crystal X-ray structure analysis
revealed that structural factors played a vital role in the photomagnetic
properties in the solid state. Z-H and Z-CN favored
intermolecular π–π stacking among the ligand molecules.
The coordination sphere around the FeII nucleus was distorted,
which stabilized the HS state. In contrast, Z-NO2·acetone was liberated from such intermolecular π–π stacking
and coordination distortion, resulting in the stabilization of the
LS state.