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.