Thermal Hysteresis in a Spin-Crossover Fe^III Quinolylsalicylaldimine Complex, Fe^III(5-Br-qsal)₂Ni(dmit)₂·solv: Solvent Effects

The Fe^III complexes Fe­(5-Br-qsal)₂Ni­(dmit)₂·solv with solv = CH₂Cl₂ (<b>1</b>) and (CH₃)₂CO (<b>2</b>) were synthesized, and their structural and magnetic properties were studied. While magnetization and Mössbauer spectroscopy data of <b>1</b> showed a gradual spin transition, compound <b>2</b> evidenced an abrupt transition with a thermal hysteresis of 13 K close to room temperature (<i>T</i>_1/2 ↓ ∼273 K and <i>T</i>_1/2 ↑ ∼286 K). A similar packing arrangement of segregated layers of cations and anions was found for <b>1</b> and <b>2</b>. In both low-spin, LS, structures there are a large number of short intra- and interchain contacts. This number is lower in the high-spin, HS, phases, particularly in the case of <b>1</b>. The significant loss of strong π–π interactions in the cationic chains and short contacts in the anionic chains in the HS structure of <b>1</b> leads to alternating strong and weak bonds between cations along the cationic chains and the formation of unconnected dimers along the anionic chains. This is consistent with a significant weakening of the extended interactions in <b>1</b>. On the other hand, in the HS phase of <b>2</b> the 3D dimensionality of the short contacts observed in the LS phases is preserved. The effect of distinct solvent molecules on the intermolecular spacings explains the different spin crossover behaviors of the title compounds