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^III complexes <b>1F</b>, <b>1Cl</b>, <b>1Br</b>, and <b>1I</b> were isolated. The temperature dependence of magnetic susceptibility and crystal structure revealed that <b>1F</b>, <b>1Cl</b>, and <b>1Br</b> are all isostructural and exhibit SCO with the rotational motion of the cation and ligands, whereas <b>1I</b> shows incomplete SCO. Note that <b>1Cl</b> and <b>1Br</b> showed irreversible and reversible cooperative SCO transitions, respectively. Short intermolecular contacts between the Fe^III 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 <b>1Cl</b> and <b>1Br</b>, whereas the disorder due to the freezing of ligand rotations in <b>1Cl</b> is revealed to inhibit the SCO cooperativity.