Weak hydrogen bonds in the molecular packing of N-tetraalkyl terephthalamides

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 inter-molecular 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.