An Fe II spin-crossover complex becomes increasingly cooperative with ageing

The polypyrazolyl ligand H4L, a functionalised derivative of 3-bpp [bpp = bis(pyrazol-3-yl)pyridine], has been used to synthesise an FeII spin-crossover (SCO) system [Fe(H4L)2](ClO4)2·2THF·H2O (1). A subsequent magnetic and structural study revealed that the compound undergoes an incomplete transition with a small hysteresis loop of 8 K and a large residual high spin (HS) fraction. This is consistent with single crystal diffraction results at 250 and 90 K, which show a slight modification of the crystal packing and Fe?N bond lengths. Ageing of the samples leads to more cooperative SCO behaviour, as evidenced by a concomitant increase in the width of the hysteresis loops (reaching up to 20 K) and a decrease in the residual HS fraction at low temperatures. These increases in the bistable domain of the sample, corroborated by measuring the changes in heat capacity through differential scanning calorimetry, are accompanied by a loss of crystallinity, which is attributed to solvent loss and the absorption of atmospheric water. Grinding an aged sample yields a narrower hysteresis loop at higher temperatures. Compound 1 was compared with two previously observed systems of the same family that exhibit different SCO behaviour, [Fe(H4L)2](ClO4)2·H2O·2(CH3)2CO (2) and [Fe(H4L)2](ClO4)2·2C3H7OH (3), by using Hirshfeld surface analysis to have a global view of the intermolecular interactions as enhanced by the functional groups of H4L.