Weak Hydrogen Bonds in the Molecular Packing of N-Tetraalkyl Terephthalamides

Abstract

We present the structures of eight terephthaldiamides, seven of which (1 - 7) are fully substituted at both nitrogens and so cannot form classical hydrogen bonds. The structure of N,N´-dimethyl- N,N´-diphenylterephthalamide (7) represents a new polymorph. Where possible, the molecules tend to exhibit inversion symmetry. The amide groups are rotated significantly out of the central aromatic plane, by 33° to 84° (average 54°). The carbonyl carbon of the amide group may lie significantly (ca. 0.1 - 0.2 Å ) out of the aromatic ring plane. The packing patterns exploit those types of contact still available, namely C-H· · ·O (especially) and C-H· · ·π. The smaller substituents in general lead to simpler packing patterns such as layer structures; more complex substituents can lead to three-dimensional patterns of great complexity. The central ring tends to play less of a role as the substituents become larger. Phenyl substituents often use their para hydrogen atoms to form intermolecular contacts, presumably because they are sterically more exposed. One terephthaldiamide (8) with two NHR groups forms a mixed “classical and weak” bifurcated (N-H, o-C-H)· · ·O hydrogen bond system.