Intermolecular interactions in the crystal chemistry of N,N'-diphenylisophthalamide, pyridine-2,6-dicarboxylic acid bisphenylamide, and related compounds

The synthesis, characterization, and crystal chemistry of N,N'-diphenylisophthalamide (1) and pyridine-2,6-dicarboxylic acid bisphenylamide (2) are described. Through a combination of single-crystal X-ray diffraction and molecular orbital and crystal packing calculations, the important intermolecular interactions have been determined. The structures have been compared with the closely related structures, N,N'-bis(3-hydroxyphenyl)isophthalamide (3) and N,N'-dimethyl-N,N'-diphenylisophthalam (4). Crystalline 1 and 2 are isostructural, but there are subtle differences in the conformations and packing as a consequence of intramolecular hydrogen bonding in 2. This reduces the deviation from planarity in the molecular conformation of 2 and consequently lengthens the intermolecular hydrogen bonding distances. This is reflected in the lattice energies of 1 and 2 (-40.9 and -38.7 kcal/mol, respectively) and in the stacking energies of these compounds. For the compounds that do not contain an N atom in the central ring the progression 3, 1, 4 represents a reduction in the hydrogen-bonding options, reflected in the respective lattice energies: -51.3, -40.9, and -33.3 kcal/mol. The difference between 3 and 1 is in excellent agreement with predictions based on group contributions in structure 3. In 4 there are no hydrogen bonding options and so C-H ... O interactions play a much more important role.