Anion, Solvent and Time Dependence of High-Spin-Low-Spin Interactions in a 3D Coordination Polymer

A thorough investigation into the role of the counterion and occluded solvent on the structure and spin-crossover behaviour of the threefold interpenetrated network [Fe(4ditz)(3)](2+) [4ditz = 1,4-bis(tetrazol-1-yl) butane I is presented. As seen by X-ray single crystal diffraction, changing the hexafluorophosphate anion in the previously investigated trigonal phase [Fe(4ditz)(3)](PF6)(2) to tetrafluoroborate causes the butylene spacers to adopt three different conformations, whereupon the symmetry is reduced to triclinic (space group Pi). X-ray powder diffraction and NMR spectroscopy provide evidence for the incorporation of statistically distributed ethanol molecules in the cavities of the compound, whose amount depends on whether absolute ethanol or a binary system, ethanol/water (95:5, v/v), is used as solvent. The phase precipitated from EtOH/H2O features a metastable high-spin (HS) state when rapidly cooled, a sharp spin transition to a mixed HS/LS (1:1) state at around 90 K on slow cooling with an approximately 10 K hysteresis and LIESST appears, whereas the sample synthesised from absolute ethanol shows a more gradual spin transition at the same temperature without hysteresis.