Departament de Química Inorgànica i Orgànica , Universitat de Barcelona , Diagonal 645, 08028 Barcelona , Spain.
Institute of Nanoscience and Nanotechnology (IN2UB) , Universitat de Barcelona 08007 Barcelona , Spain.
Inorg Chem. 2018 Sep 4;57(17):11019-11026. doi: 10.1021/acs.inorgchem.8b01625. Epub 2018 Aug 22.
Spin-crossover (SCO) molecular solids are valued switchable materials for their common abrupt and reversible thermal transitions, large thermal hysteresis, or guest-dependent effects. These properties usually involve crystallographic transitions coupled to the SCO events. These phenomena are of great value for the understanding of solid-state transformations and also for exploiting them. We present here a lattice of the complex FeL(bbp) (1; L and bbp are tris-imine ligands) featuring an unprecedented rich succession of SCO and crystallographic phase transformations. Magnetometry measurements unveil a thermally irreversible sequence of spin conversions that delineate four different thermal pathways. All of these are single-crystal-to-single-crystal processes and can thus be monitored by single crystal X-ray diffraction using one unique specimen. Fresh crystals of 1 contain one molecule of acetone per Fe center (1·ac) that abandons the lattice upon warming at the same time that a SCO from an ordered mixed spin state (1:1 high spin/low spin; HS/LS) to a fully HS state, 1, occurs. This crystallographic phase, accessed through a template effect by the solvent, converts into another one, 1, upon cooling, as triggered by a HS to LS SCO. Warming of 1 induces a new SCO (LS to ordered HS/LS) coupled to another crystallographic phase transition, 1 → 1. The fully HS state of 1 can not be reached before decomposition of the compound. Instead, this phase cycles between the HS/LS and the LS states through superimposable pathways, different from that of the prerequired 1 → 1 phase change. Analysis of the thermal variation of the free energy, G, through density functional theory methods provides trends in agreement with the observation of these transformations and clarifies the possible metastable nature of the various phases identified. This unique behavior allows the access to four different magnetic responses depending on the thermal history of the sample, within a given range of temperatures near the ambient conditions.
螺旋翻转（SCO）分子固体因其常见的急剧和可逆的热转变、大的热滞回线或客体依赖性效应而被视为可切换的材料。这些性质通常涉及与 SCO 事件耦合的晶体学转变。这些现象对于理解固态转变以及利用它们具有重要价值。我们在这里展示了一个由配合物FeL(bbp)（1；L 和 bbp 是三齿亚胺配体）组成的晶格，其具有前所未有的丰富的 SCO 和晶体相转变序列。磁测量揭示了一个热不可逆的自旋转换序列，该序列描绘了四个不同的热途径。所有这些都是单晶到单晶的过程，因此可以通过使用一个独特的样本的单晶 X 射线衍射来监测。1 的新鲜晶体含有每个 Fe 中心一个丙酮分子（1·ac），当升温时，它会离开晶格，同时从有序混合自旋态（1:1 高自旋/低自旋；HS/LS）到完全 HS 态的 SCO 发生，1，发生。这种晶体相通过溶剂的模板效应进入，在冷却时，由于 HS 到 LS 的 SCO，转化为另一种晶体相，1。1 的升温诱导另一个 SCO（LS 到有序 HS/LS）与另一个晶体相转变，1→1，耦合。在化合物分解之前，不能达到 1 的完全 HS 态。相反，该相通过与预先需要的 1→1 相变化不同的重叠途径在 HS/LS 和 LS 态之间循环。通过密度泛函理论方法分析自由能 G 的热变，提供了与这些转变的观察结果一致的趋势，并澄清了所确定的各种相的可能亚稳性质。这种独特的行为允许根据样品的热历史在环境条件附近的给定温度范围内访问四种不同的磁响应。
