Structural Investigation of Substituent Effect on Hydrogen Bonding in (S)-Phenylglycine Amide Benzaldimines

A detailed structural analysis of 23 new crystal structures of (S)-phenylglycine amide benzaldimines with various substituents (CH<inf>3</inf>, Ph, OCH<inf>3</inf>, F, Cl, Br, NO<inf>2</inf>) on the benzylidene is performed in this contribution. These compounds belong to the highly studied family of Schiff bases. Etter's nomenclature and Hirshfeld surfaces are used to describe respectively the strong hydrogen bonds and the secondary interactions existing in these compounds. Surprisingly, all 23 obtained structures can be sorted into five types according to their hydrogen bonding motifs. The potential interplay of steric and electronic effects of the substituents on the resulting bonding patterns, conformational features and packing was investigated. Our analysis revealed that neither mesomeric/inductive factors of halogens nor π-π stacking, C-H···π, and other hydrophobic interactions affect the structural outcome. The type affiliation is rather due to the interplay of three parameters: (1) the number of strong hydrogen bonds forming the motif (thermodynamic factor), (2) the ease with which the motif is formed (kinetic factor), and (3) the capacity of the motif to accommodate substituents on the different positions (steric factor). It was thus possible to suggest a stability ranking of the five structural types and to identify stable forms when polymorphism was encountered. (Figure Presented).