Ligand-Engineered Spin Crossover in Fe(II)-Based Molecular and Metal–Organic Framework Systems

Some transition metals have valence electrons that can
form either
high- or low-spin complexes depending on their ligand field. The interconversion
between high-to-low or low-to-high spin can also be achieved by changing
the temperature, promoting a spin crossover (SCO) event. Such transitions
are potentially useful for quantum data storage, catalysis, and beyond.
Here we examine the spin-crossover properties of Fe­(ta)2 (iron triazolate), a Fe2+-containing metal–organic
framework (MOF) that is known to undergo spin crossover. We compare
the electronic and geometric properties of the MOF to a related molecular
system, monitoring electronic properties before, during, and after
the spin transitions. Our data reveals that long-range cooperativity
affects the energetics of spin crossover in the MOF, but not the molecule.
We attribute these differences to electronic dissimilarities in the
ligands and structural differences in the crystal connectivity, and
offer design strategies to control SCO in framework materials.

部分过渡金属（transition metals）的价电子（valence electrons）可依据所处配体场（ligand field）的差异，形成高自旋配合物（high-spin complexes）与低自旋配合物（low-spin complexes）。高自旋与低自旋之间的相互转化，也可通过改变温度实现，该过程被称为自旋交叉（spin crossover, SCO）事件。这类自旋转变在量子数据存储、催化等领域具备潜在应用价值。

本研究考察了二(三唑)合铁（Fe(ta)₂，iron triazolate）的自旋交叉特性，该物质为含Fe²+的金属有机框架（metal–organic framework, MOF），已知其可发生自旋交叉现象。我们将该金属有机框架的电子与几何结构特性与相关分子体系进行对比，并监测自旋转变过程前后及过程中的电子特性变化。本研究数据显示，长程协同性会影响该金属有机框架中自旋交叉过程的能量特性，但对分子体系并无此类影响。我们将这些差异归因于配体的电子特性差异与晶体连接方式的结构差异，并提出了调控框架材料中自旋交叉（SCO）过程的设计策略。