Room temperature spin crossover properties in a series of mixed-anion Fe(NH2trz)3(BF4)2−x(SiF6)x/2 complexes

Abstract

A series of mixed-anion Fe(NH₂trz)₃(BF₄)_2−x(SiF₆)_x/2 spin crossover complexes is obtained modifying the reaction time but also using an increase amount of tetraethyl orthosilicate as the source for the production and the incorporation of SiF₆²⁻ competing with the BF₄²⁻ anions present in the mother solution. The increase of the SiF₆²⁻ anion inclusion to the detriment of the BF₄⁻ counterpart induces a shift of the temperature transition toward high temperatures leading to interesting bistability properties around room temperature with T_1/2 spanning from 300 K to 325 K. Moreover, the implementation of a solid–liquid post synthetic modification approach from the Fe(NH₂trz)₃(BF₄)₂ parent complex with identical TEOS proportions and under certain experimental conditions lead systematically to the same Fe(NH₂trz)₃(BF₄)_1.2(SiF₆)_0.4 composition. This compound presents an abrupt spin crossover behaviour with a narrow hysteresis loop just above room temperature (320 K), which is stable under thermal cycling and along time with no specific storage conditions. Such crystalline powder sample incorporates homogeneous rod-shaped particles whose formation and physical properties can be followed simultaneously using infra-red spectroscopy, dynamic light scattering (DLS), transmission electronic microscopy (TEM) and optical reflectance. The observation of a stabilized single ca. 800 nm population of mixed-anion particles starting from insoluble various sizes (from nano- to microscale) Fe(NH₂trz)₃(BF₄)₂ particles supports the key role of the solvent (water molecules) on the separation, the reactivity and the reorganization of the 1D iron-triazole chains forming the packing of the structure.