One of the most
interesting phenomena, the thermal induced spin crossover (SCO), arises from an
ability of the transition metal octahedral complexes with 3d4-3d7 configuration
to adopt two different electronic states. In iron(II) complexes, the high
spin (HS) to low spin (LS) HS(S = 2)
LS(S = 0) transition triggered by
a change of temperature, an application of pressure, magnetic field, or by
light irradiation involves severe alterations of magnetic, optical, and
dielectric properties which are the basis of potential applications. For
practical applications, SCO materials have to exhibit an abrupt spin transition
with a wide loop of hysteresis. According to the model of elastic interactions
developed by Spiering, a perturbation produced by shortening of the Fe–N bond
length involves a compression of a crystal lattice contributing additionally to
a stabilization of the LS form of iron(II).
LS(S = 0) transition triggered by
a change of temperature, an application of pressure, magnetic field, or by
light irradiation involves severe alterations of magnetic, optical, and
dielectric properties which are the basis of potential applications. For
practical applications, SCO materials have to exhibit an abrupt spin transition
with a wide loop of hysteresis. According to the model of elastic interactions
developed by Spiering, a perturbation produced by shortening of the Fe–N bond
length involves a compression of a crystal lattice contributing additionally to
a stabilization of the LS form of iron(II).
[Fe(ebtz)2(C2H5CN)2](ClO4)2 was
prepared in the reaction of 1,2-di(tetrazol-2-yl)ethane (ebtz) with Fe(ClO4)2·6H2O
in propionitrile. The compound crystallizes as a one-dimensional (1D) network,
where bridging of neighboring iron(II) ions by two ebtz ligand molecules
results in formation of a [Fe(ebtz)2]∞ polymeric
skeleton. The 1D chains are assembled into supramolecular layers with axially
coordinated nitrile molecules directed outward. The complex in the high spin
(HS) form reveals a very rare feature, namely, a bent geometry of the
Fe–N–C(propionitrile) fragment (149.1(3)° at 250 K). The HS to low spin (LS)
HS→LS transition triggers reorientation of the propionitrile molecule resulting
in accommodation of a typical linear geometry of the Fe–N–C(nitrile) fragment.
The switching of the propionitrile molecule orientation in relation to the
coordination octahedron is associated with increase of the distance between the
supramolecular layers. When the crystal is in the LS phase, raising the
temperature does not cause reduction of the distance between supramolecular
layers, which contributes to further stabilization of the more linear geometry
of Fe–N–C(C2H5) and the LS form of the complex. Thus, a
combination of Fe–N–C(C2H5) geometry lability and lattice
effects contributes to the appearance of hysteretic behavior (T1/2↓ ≈
112 K, T1/2↑ ≈ 141 K).
Agata
Białońska, Robert Bronisz. (2012). Role of Fe–N–C Geometry Flip-Flop in
Bistability in Fe(tetrazol-2-yl)4(C2H5CN)2-Type Core Based Coordination Network
Journal Of The
American Chemical Society, DOI: 10.1021/ic300880w
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