Designing Solid Solutions of Enantiomers: Lack of Enantioselectivity of Chiral Naphthalimide Derivatives in the Solid State

The enantiomers of a previously reported naphthalimide derivative are shown in this study to form a solid solution; furthermore, on the basis of the knowledge of solid solution structural aspects other naphthalimide derivatives have been synthesized and shown to lack the enantioselectivity in the solid state. The structural origin of solid solution formation is the same as observed in most of the cases in the literaturequasi-centrosymmetric structures form at nonracemic compositions where the most abundant enantiomer adjusts its conformation to mimic the absent one. Such solid solutions belong to the type showing some enantioselectivity. An extended single crystal X-ray diffraction study of the crystals of different enantiomeric compositions reveals the nature of the disorder in studied solid solutions. Intermolecular interactions are analyzed in terms of Hirshfeld surfaces and by means of density functional theory calculations to explore the differences of isostructural quasi-centrosymmetric (enantiopure) and genuine centrosymmetric (racemic) packings to shed light on the energetic aspects of solid solution formation as well as to explain the origin of partial enantioselectivity. Furthermore, lattice energy calculations explain why two structurally distinct solid solutions (around the racemic and near the pure enantiomer regions) form as found for one of the studied compounds.