The crystallographic phase transition for a ferric thiosemicarbazone spin crossover complex studied by X-ray powder diffraction

The crystal structure of a spin-transition compound, namely the thiosemicarbazone ferric complex Li[Fe(5BrThsa)2]·H2O, was solved from powder X-ray diffraction data at temperatures where the high-spin (373 K) and low-spin (150 K) phases prevail. The methodology is based on traditional approaches (direct methods) combined with direct space strategy. Both phases crystallise in the monoclinic system P21/c. At 373 K, the characteristics of the [FeN2O2S2] coordination core are consistent with those reported for high-spin iron(III) thiosemicarbazone complexes: a distorted coordination polyhedron and non-equivalent metal–ligand bond lengths. When the temperature is reduced to 150 K, a decrease of the β angle from ca. 101° (373 K) to ca. 90° (150 K) is the only major modification of the cell parameters. The low-spin molecular structure reveals significant differences in bond lengths and bond angles compared to the high-spin structure. Finally, an extended hydrogen-bond network is implicated in the cooperative phase transition, as supported by strong intermolecular contacts between the ferric complexes and the water molecules and the crystallographic phase transition is associated with pronounced lattice reorganization.