Crystal environment induced symmetry reduction (CEISR): deep analysis of para-chloroacetophenone azine and generalization

Abstract

The morphologies of sixteen crystal structures of ten symmetrical X_para–Ph–(Me)CN–NC (Me)–Ph–X_para azines: 1 (Cl), 2 (Br), 3 (I), 4 (OH), 5 (PhO), 6 (CF₃), 7 (F), 8 (Me), 9 (NO₂), and 10 (PrO) are described. All the azines crystallize forming idioteloamphiphile monolayers (IAMs) and their morphologies fall into the two main classes of “flat” and “shiplap” depending on the lateral offsets between next-neighbors in the monolayer. Ab initio studies show that the (E,E)-configured p-disubstituted acetophenone azines are C₂-symmetric in the gas phase and that the azine and phenyl twists are correlated and give rise to P- and M-chirality. The crystal structures of 1–10 are true racemates and, remarkably, the majority feature asymmetric (C₁) azines rather than dissymmetric (C₂) azines. To understand the origin of the symmetry reduction, we studied the supramolecular structures and IAM characteristics of the five azines that crystallize with a flat morphology 1-I (Cl), 2-Ia (Br), 3 (I), 5 (PhO), and 10 (PrO). The deep analysis of chloroazine 1-I shows that the symmetry reduction is the result of qualitatively different coordination environments of the two chloroarene moieties and presents a case of crystal environment induced symmetry reduction (CEISR). The crystal structure of 1-I is of the type “AABB Kick/Flat” and the advantages of the double-stripe motif are explained. Careful analysis of the crystal structure showed arene–arene T-contacts of the types edge-to-face (e|f), face-to-edge (f|e), and face-to-face (f|f), a new arene–azine interaction (e|Az), and H⋯Cl, Cl⋯π, and Cl⋯Cl contacts. Intra- and interlayer interaction inventory analyses show in a compelling fashion that the arenes A_i and A_s in each azine engage in qualitatively different intermolecular interactions. To demonstrate the usefulness of the interaction inventory analysis, we also compare and contrast the crystal environments of the C₁-symmetric azines in crystals of 2-Ia (Br) and 10 (PrO) and the C₂-symmetric azines in crystals of 3 (I) and 5 (PhO). Intermolecular interactions are quantified using Hirshfeld surface analyses, pairwise interaction energies, and electrostatic potential maps. We will also describe a fast method for the detection of CEISR based on N⋯H Hirshfeld 2D fingerprint plots.