Structural diversity in a family of quasi-tetrahedral organic molecules: From van der Waals solids to helices and molecular complexes

The crystal structure and packing of a series of six nearly tetrahedral organic compounds 1-6, of general formula PhNH-(CArArAr3)-Ar-1-Ar-2 [1(Ph, Ph, Ph), 2(Ph, Ph, 2-Py), 3(Ph, Ph, 3-Py), 4(Ph, Ph, 4-Py), 5(Ph, 2-Py, 2-Py), and 6(2-Py, 2-Py, 2-Py)] were investigated by X-ray crystallography and supported, in some cases, by ab initio calculations, IR spectroscopy, and H-1 NMR. All molecules have in common an aniline group, but they differ in the number of phenyl groups that are replaced by pyridyl ones (1, 2, 5, 6) or the position of the nitrogen heteroatom at the pyridyl ring (2-4). Purely van der Waals solids are obtained when the molecule can form an intramolecular N-H center dot center dot center dot N hydrogen bond (2, 5, 6), and, in this case, a correlation between melting point and structure was attempted. However, when intermolecular N-H center dot center dot center dot N bonds were established, helices (4) or tetrameric (two organic molecules + two solvent molecules) molecular complexes (3) were obtained.