Non-classical FeII spin-crossover behaviour in polymeric iron(II) compounds of formula [Fe(NH2trz)3]X2xH2O (NH2trz=4-amino-1,2,4-triazole; X=derivatives of naphthalene sulfonate)
A new series of iron(ii) spin-crossover materials of formula [Fe(NH2trz)3 ]X2xH2O [X=1-naphthalene sulfonate (1ns), 2-naphthalene sulfonate (2ns), 4-hydroxy-1-naphthalene sulfonate (4OH-1ns), 4-amino-1-naphthalene sulfonate (4NH2-1ns) and 6-hydroxy-2-naphthalene sulfonate (6OH-2ns)] are reported. The structure of these compounds consists of linear chains in which the Fe(ii) ions are linked by triple N1 ,N2-1,2,4-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 57Fe Mössbauer spectroscopic studies on [Fe(NH2trz)3 ](2ns)2xH2O. 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(NH2trz)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).