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

The
FeIII complexes Fe­(5-Br-qsal)2Ni­(dmit)2·solv with solv = CH2Cl2 (1) and (CH3)2CO (2) were synthesized,
and their structural and magnetic properties were studied. While magnetization
and Mössbauer spectroscopy data of 1 showed a
gradual spin transition, compound 2 evidenced an abrupt
transition with a thermal hysteresis of 13 K close to room temperature
(T1/2 ↓ ∼273 K and T1/2 ↑ ∼286 K). A similar packing
arrangement of segregated layers of cations and anions was found for 1 and 2. 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 1. The significant loss of strong π–π interactions
in the cationic chains and short contacts in the anionic chains in
the HS structure of 1 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 1. On the other hand, in the HS phase of 2 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.

本研究成功合成了FeIII络合物Fe-(5-Br-qsal)2Ni-(dmit)2·solv，其中solv分别为CH2Cl2（1）和(CH3)2CO（2），并对它们的结构及磁性特性进行了深入研究。对于化合物1，其磁化和莫塞巴光谱数据揭示了逐渐的磁旋转变，而化合物2则表现出急剧的转变，其热滞后现象接近室温，约为13 K（T1/2↓约273 K，T1/2↑约286 K）。1和2均显示出阳离子和阴离子分离层的一致堆积排列。在两种低自旋LS结构中，存在大量的短程链内和链间接触。而在高自旋HS相中，尤其是化合物1，这一数目有所降低。在1的HS结构中，阳离子链中显著损失的强π-π相互作用以及阴离子链中的短程接触，导致阳离子链上强弱键交替出现，并在阴离子链上形成不连接的二聚体。这与1中扩展相互作用的显著减弱相一致。另一方面，在2的HS相中，LS相中观察到的短程接触的3D维度得到了保留。不同溶剂分子对分子间间距的影响，解释了标题化合物不同的磁旋转变现象。