Synthesis and spin-crossover characteristics of polynuclear 4-(2´-hydroxy-ethyl)-1,2,4-triazole Fe(II) molecular materials

A new series of Fe(II) spin-crossover materials of formula [Fe(hyetrz)3](Anion)2•xH2O, where hyetrz = 4-(2´-hydroxy-ethyl)-1,2,4-triazole and Anion = Cl–, NO3–, Br–, I–, BF4–, ClO4–, PF6–, have been prepared and the spin transition characteristics studied. 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. Most of the hydrated compounds show non-classical spin-crossover behaviour associated with the removal of lattice water molecules, which initially stabilize the low-spin state. However for two of them, the perchlorate and the iodide compounds, the transition temperature is shifted to higher temperatures by dehydration. For the corresponding dehydrated compounds, the transition temperature T1/2 increases with decreasing anion radii. [Fe(hyetrz)3]I2 represents one of the few Fe(II) spin-crossover materials showing a spin transition in the close vicinity of room temperature (291 K) accompanied by a thermal hysteresis (12 K).