Prototypical iron(ii) complex with 4-amino-1,2,4-triazole reinvestigated: an unexpected impact of water on spin transition

Abstract

The magnetic and thermodynamic properties of the prototypical 1D polymeric complex Fe(ATrz)₃(NO₃)₂·H₂O (ATrz = 4-amino-1,2,4-triazole) were reinvestigated to gain an insight into the impact of water molecules on the spin transition. Variations in the outerspheric water molecule content in the complex induce drastic and unpredictable changes in its spin crossover regimes. Under vacuum the complex loses water molecules and shows a wide (ca. 30 K) and reproducible hysteresis loop, T_c↑ = 337–345 K and T_c↓ = 316–313 K. In sealed ampoules the complex Fe(ATrz)₃(NO₃)₂·H₂O shows a narrow hysteresis (ca. 1–4 K), T_c↑ = 326–329 K and T_c↓ = 326–324 K. After adsorption of water the complex Fe(ATrz)₃(NO₃)₂·nH₂O (n = 1.25–1.6) demonstrates a narrow two-step spin transition. In all these cases the kinetics of the LS → HS and HS → LS transitions has decelerating non-cooperative character. For the system Fe(ATrz)₃(NO₃)₂·nH₂O (n = 3.6–16.6) wide hysteresis (ca. 5–20 K) re-appears near room temperature (T_c↑ = 319–321 K and T_c↓ = 300–315 K). Surprisingly, the kinetics of the HS → LS spin transition for the systems with high water content switches from decelerating to sigmoidal (cooperative). The activation energy of the LS → HS transition was estimated for the first time for iron(II) spin crossover complexes with 1,2,4-triazoles (ca. 1000–2000 kJ mol⁻¹). The systems Fe(ATrz)₃(NO₃)₂ and Fe(ATrz)₃(NO₃)₂·nH₂O show compensation effects (ΔH − ΔS, ln A − E_a). A correlation between the T_onset↑, the ΔH values and the water content in the complex is observed: the highest ΔH values (27–29 kJ mol⁻¹) and the lowest T_onset↑ values (317–320 K) correspond to the samples with high water content, whereas the lowest ΔH values (19–23 kJ mol⁻¹) and the highest T_onset↑ values (337–345 K) correspond to water-free samples, Fe(ATrz)₃(NO₃)₂. Our results provide the first experimental evidence that the presence of water (and even air humidity) produces dramatic changes in the spin crossover behavior of the prototypical 1D polymeric complex Fe(ATrz)₃(NO₃)₂·H₂O (ATrz = 4-amino-1,2,4-triazole).