Impact of ligand spacer and counter-anion in selected 1D iron(II) spin crossover coordination polymers

A new 1D coordination polymer [Fe(βAlatrz)₃](ClO₄)₂ ? H₂O (1) with a neutral bidentate ligand, βAlatrz = 4H-1,2,4-triazol-4-yl-propionate, was prepared and its magnetic behavior was investigated by temperature dependent magnetic susceptibility measurements and ⁵⁷Fe M?ssbauer spectroscopy. The temperature dependence of the high-spin molar fraction derived from ⁵⁷Fe M?ssbauer spectroscopy recorded on cooling below room temperature reveals a gradual single step transition with T_1/2 = 173?K between high-spin and low-spin states in agreement with magnetic susceptibility measurements. 1 presents striking reversible thermochromism from white, at room temperature, to pink on quench cooling to liquid nitrogen. The phase transition is of first order as deduced from differential scanning calorimetry, with T_1/2 matching the one determined by both SQUID and ⁵⁷Fe M?ssbauer spectroscopy. A brief assessment has been made among closely related 1D coordination polymers to perceive the effect of ligand spacer length and anion effect on the spin crossover behavior of these new materials.