title: Strong cooperativeness in the mononuclear iron(II) derivative exhibiting an abrupt spin transition above 400 K.
abstract: The spin crossover system, [Fe(bzimpy)(2)](ClO(4))(2).0.25H(2)O, 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 DeltaT = 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 A). The high cooperativeness has been assigned to a perfect pi-stacking of the benzimidazole rings in the crystal lattice at a distance as short as 3.6 A. Variable-temperature IR data and the heat capacity measurements match well the magnetic data. The thermodynamic properties are DeltaH = 17 kJ mol(-)(1), DeltaS = 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.
authors.affiliation: Department of Inorganic Chemistry, Slovak Technical University, SK-812 37 Bratislava, Slovakia.
authors.forename: R
authors.initials: R
authors.lastname: Boca
authors.forename: M
authors.initials: M
authors.lastname: Boca
authors.forename: L
authors.initials: L
authors.lastname: Dlhán
authors.forename: K
authors.initials: K
authors.lastname: Falk
authors.forename: H
authors.initials: H
authors.lastname: Fuess
authors.forename: W
authors.initials: W
authors.lastname: Haase
authors.forename: R
authors.initials: R
authors.lastname: Jarosciak
authors.forename: B
authors.initials: B
authors.lastname: Papánková
authors.forename: F
authors.initials: F
authors.lastname: Renz
authors.forename: M
authors.initials: M
authors.lastname: Vrbová
authors.forename: R
authors.initials: R
authors.lastname: Werner