Stimuli-responsive spin crossover behavior in 3D Fe(ii) porous coordination polymers for guest molecules†

Structurally characterized porous spin crossover compounds are attractive types of materials due to their properties that can be regulated under several stimuli, resulting in drastic changes in their optical, electrical, and magnetic responses, leading to potential applications in chemical sensing, memory devices, actuators, etc. In this work, a new 3D Fe^II spin crossover porous coordination polymer, [Fe(tpe)₂dca]ClO₄·5CHCl₃·3CH₃OH (1, tpe = trans-1,2-bis(4-pyridyl)ethene; dca = N(CN)₂⁻), which accommodates guest molecules in its cavities to modulate its magnetic and optical properties, was prepared. 1 was characterized by X-ray diffraction in its fully solvated form by flash cooling single crystals at 100 K, thermogravimetric analysis, elemental analysis and its spin crossover tracked by magnetic susceptibility, and studied by differential scanning calorimetry on single crystals. Compound 1 displays gradual and incomplete spin crossover behaviour with a transition temperature of T_1/2 ∼ 155 K. An optical microscopy study carried out on one single crystal shows an abrupt transition around 180 K with a darkening of the crystal in the low-spin phase, although no clear evidence of an apparent size change was observed. When compound 1 loses its guest molecules partially, [Fe(tpe)₂dca]ClO₄·CHCl₃·2H₂O (2) is obtained in air atmosphere, which is paramagnetic. In addition, the complex [Fe(bpa)₂(NCS)₂]·solvent (bpa = 9,10-bis(4-pyridyl)anthracene, 3) remains paramagnetic down to 100 K, as confirmed by single crystal X-ray diffraction, due to the strong distortion of its octahedral coordination sphere as well as its rigid structure.