Room temperature hysteretic spin transition in 1D iron(II) coordination polymers

Abstract

The 1D spin transition compound [Fe(L1)3](ClO4)2 (1) with L1 = ethyl-4H-1,2,4-triazol-4-yl-acetate, a novel neutral bidentate ligand, has been synthesised. The temperature dependence of the high-spin molar fraction derived from 57Fe Mossbauer spectroscopy reveals an exceptionally abrupt single step transition between low-spin and high-spin states with a hysteresis loop of width 5 K (Tc↑ = 298 K and Tc↓ = 293 K). This spin transition operating around room temperature presents striking reversible thermochromism from white at 295 K to pink at ice temperature, behaving as an optical alert towards temperature variations. This first order phase transition was additionally followed by differential scanning calorimetry in good agreement with Mössbauer spectroscopy data. The spin transition properties of a freshly prepared sample of 1, dried under a N2(g) stream which was formulated as [Fe(L1)3](ClO4)2·MeOH (2), are shifted below room temperature (T1/2↑ = 273 K and T1/2↓ = 263 K), thus showing a remarkable influence of solvent inclusion on the spin state of these chain compounds.