Chiral Behavior and Racemization of Salen-Type Ligands and Their Iron Complexes: An X-Ray Crystal Structure Study

Chiral salen-type metal complexes are promising for diverse applications, yet their crystals often exhibit complicated chiral behavior. We investigated the crystal structures of chlorine-substituted salen-type ligands incorporating a 1,2-diphenylethylenediamine moiety and their iron complexes, which revealed varied chiral phenomena. Synthesis of the ligands using (1S,2S)-, (1R,2R)-, and racemic starting amines exclusively produced racemic ligands. A similar racemization of the ligand was observed during the formation of a dinuclear oxidized Fe(III)-O-Fe(III) complex from FeSO4·6H2O (1). Conversely, complexation with FeCl3 maintained the optically active (1S,2S)-configuration even in the iron(III) complex (2). To elucidate these structural features, intermolecular interactions were quantified via Hirshfeld surface analysis. Additionally, generative AI (Gemini, Google) was employed for data interpretation to discuss the correlation between the structural characteristics of chirality and Hirshfeld surface analysis.