Strong Cooperativeness in the Mononuclear Iron(II) Derivative Exhibiting an Abrupt Spin Transition above 400 K

The spin crossover system, [Fe(bzimpy)2](ClO4)2·0.25H2O, was reinvestigated above room temperature (bzimpy = 2,6-bis(benzimidazol-2-yl)pyridine). The system exhibits an abrupt low-spin to high-spin transition at T c = 403 K. Liberation of a fractional amount of water does not affect the spin crossover: the system is perfectly reversible with a hysteresis width of ΔT = 12 K. The existence of the hysteresis at such high temperature determines that the lowest limit of the solid-state cooperativity parameter is J/k > 403 K despite long iron(II) separations (10 Å). The high cooperativeness has been assigned to a perfect π-stacking of the benzimidazole rings in the crystal lattice at a distance as short as 3.6 Å. Variable-temperature IR data and the heat capacity measurements match well the magnetic data. The thermodynamic properties are ΔH = 17 kJ mol-1, ΔS = 43 J K-1 mol-1, so that the entropy of the spin transition shows a considerable contribution from the molecular vibrations. A theoretical model has been applied in fitting the magnetic data along the whole hysteresis path. A statistical distribution of the cooperativity parameter led to the feature that angled walls of the hysteresis loop are well reproduced.