The First Use of a ReX$_{5}$ Synthon to Modulate Fe$^{III}$ Spin Crossover via Supramolecular Halogen⋅⋅⋅Halogen Interactions

We have added the {Re$^{IV}$X$_{5}$}$^{−}$ (X=Br, Cl) synthon to a pocket‐based ligand to provide supramolecular design using halogen⋅⋅⋅halogen interactions within an Fe$^{III}$ system that has the potential to undergo spin crossover (SCO). By removing the solvent from the crystal lattice, we “switch on” halogen⋅⋅⋅halogen interactions between neighboring molecules, providing a supramolecular cooperative pathway for SCO. Furthermore, changes to the halogen‐based interaction allow us to modify the temperature and nature of the SCO event.