Disorder–Order Transformation and Significant Dislocation Motion Cooperating with a Surprisingly Large Hysteretic Magnetic Transition in a Nickel–Bisdithiolene Spin System

The compound [4′-CF3bzPy]­[Ni­(mnt)2] (1) (where 4′-CF3bzPy = 1-(4′-(trifluoromethyl)­benzyl)­pyridinium and
mnt2– = maleonitriledithiolate) was synthesized
and displays a magnetic bistability with a surprisingly large thermal
hysteresis loop (∼49 K). X-ray crystallographic studies reveal
that in the high-temperature (HT) phase the anions and cations form
mixed stacks, with alternating anion dimers (AA) and cation dimers
(CC) in an ...AACCAACC... fashion along the crystallographic a + b direction, and disordered CF3 groups in the cations are aligned into a molecular layer
parallel to the crystallographic (001) plane. However, in the low-temperature
(LT) phase, the c-axis length of the unit cell is
roughly doubled, and the asymmetric unit switches from one [4′-CF3bzPy]­[Ni­(mnt)2] pair in the HT phase to two [4′-CF3bzPy]­[Ni­(mnt)2] pairs. Most interestingly, the
CF3 group in the cations becomes ordered, and the conformation
of one of two crystallographically different cations changes significantly.
A dislocation motion between the neighboring molecular layers emerges
as well. The analyses of the magnetic susceptibilities and the density
functional theory calculations suggest that the antiferromagnetic
exchange interaction within one of two types of [Ni­(mnt)2]22– dimers in the LT phase is much
stronger than that within the [Ni­(mnt)2]22– dimer in the HT phase. The lattice reorganization
during this phase transition is proposed to be responsible for the
wide thermal hysteresis loop.