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).
[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