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).0.25H(2)O, was reinvest igated above room temperature (bzimpy = 2,6-bis(benzimidazol-2-yl)pyridine) . The system exhibits an abrupt low-spin to high-spin transition at T, = 40 3 K. Liberation of a fractional amount of water does not affect the spin cr ossover: the system is perfectly reversible with a hysteresis width of Delt aT= 12 K. The existence of the hysteresis at such high temperature determin es that the lowest limit of the solid-state cooperativity parameter is J/k > 403 K despite long iron(II) separations (10 Angstrom). The high cooperati veness has been assigned to a perfect pi -stacking of the benzimidazole rin gs in the crystal lattice at a distance as short as 3.6 Angstrom. Variable- temperature IR data and the heat capacity measurements match well the magne tic 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 consi derable 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 featu re that angled walls of the hysteresis loop are well; reproduced.