Non-classical Fe-II spin-crossover behaviour in polymeric iron(II) compounds of formula [Fe(NH(2)trz)(3)]X-2 center dot xH(2)O (NH(2)trz equals 4-amino-1,2,4-triazole; X equals derivatives of naphthalene sulfonate)

A new series of iron(II) spin-crossover materials of formula [Fe(NH(2)trz)(3)]X-2 . xH(2)O [X = 1-naphthalene sulfonate (1 ns), 2-naphthalene sulfonate (2ns), 4-hydroxy-1-naphthalene sulfonate (40H-1ns), 4-amino-1-naphthalene sulfonate (4NH(2)-1ns) and 6-hydroxy-2-naphthalene sulfonate (60H-2ns)] are reported. The structure of these compounds consists of linear chains in which the Fe(II) ions are linked by triple N-1,N-2-1,2,4-triazole bridges. All compounds show non-classical spin-crossover behaviour. Optical and magnetic measurements recorded upon heating show an abrupt low-spin to high-spin transition accompanied by a pronounced thermochromic effect occuring between 330 and 340 K depending on the anion. Thermogravimetric analyses show that the transition is induced by the removal of the two lattice water molecules, which initially stabilized the low-spin state. The dynamical character of this transition has been monitored by extended Fe-57 Mossbauer spectroscopic studies on [Fe(NH(2)trz)(3)](2ns)(2) . xH(2)O. Upon cooling, the dehydrated modifications show classical spin-crossover behaviour with hysteresis at much lower temperatures, ranging from 229 to 297 K depending on the anion. [Fe(NH(2)trz)(3)](2ns)(2) represents one of the first iron(II) spin-crossover materials showing a spin transition in the close vicinity of room temperature (290 K) accompanied by hysteresis (14 K).