The Relationship Between Spin Crossover (SCO) Behaviors, Cation and Ligand Motions, and Intermolecular Interactions in a Series of Anionic SCO Fe(III) Complexes with Halogen-Substituted Azobisphenolate Ligands.

To investigate the halogen substitution effect on the anionic spin crossover (SCO) complexes, azobisphenolate ligands with 5,5'-dihalogen substituents from fluorine to iodine were synthesized, and their anionic Fe complexes , , , and were isolated. The temperature dependence of magnetic susceptibility and crystal structure revealed that , , and are all isostructural and exhibit SCO with the rotational motion of the cation and ligands, whereas shows incomplete SCO. Note that and showed irreversible and reversible cooperative SCO transitions, respectively. Short intermolecular contacts between the Fe complex anions were found despite Coulomb repulsions for all the complexes. The topological analysis of the electron density distributions revealed the existence of X···X halogen bonds, C-H···X, C-H···N, and C-H···O hydrogen bonds, and C-H···π interactions are evident. The dimensionality of intermolecular interactions is suggested to be responsible for the cooperative SCO transitions in and , whereas the disorder due to the freezing of ligand rotations in is revealed to inhibit the SCO cooperativity.