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.