Thermal Hysteresis in a Spin-Crossover
Fe<sup>III</sup> Quinolylsalicylaldimine Complex, Fe<sup>III</sup>(5-Br-qsal)<sub>2</sub>Ni(dmit)<sub>2</sub>·solv: Solvent Effects
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Abstract
The
Fe<sup>III</sup> complexes Fe(5-Br-qsal)<sub>2</sub>Ni(dmit)<sub>2</sub>·solv with solv = CH<sub>2</sub>Cl<sub>2</sub> (<b>1</b>) and (CH<sub>3</sub>)<sub>2</sub>CO (<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><sub>1/2</sub> ↓ ∼273 K and <i>T</i><sub>1/2</sub> ↑ ∼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