Are supramolecular synthons stable as discrete units? Results from a series of salts of 4-(4-hydroxyphenylazo)benzoic acid and its derivatives with aminopyridines

Analysis of a supramolecular synthon is an integral part of understanding, describing, and designing non-covalent assembly. The energies of synthons play a key role in the formation and disruption to result in the conversion to a new assembly. It also provides scopes to understand their collective effects by being a part of an assembly. We have performed a structural study and analyzed the synthons in 4-(4-hydroxyphenylazo)benzoic acid (H2AZOBEN) and its salts with 2-aminopyridine, 4-aminopyridine, and pyrimidin-2-amine derivatives to show the collective contributions to stability from synthons. The energies of identified synthons from observed crystal structures were independently optimized by DFT calculation with M062X density functional together with the QZVP and 6-311+G(d,p) basis set. MM2-surface charge-density calculation on the salt of formic acid with aminopyridine or aminopyrimidine established that the geometrical arrangements in discrete combinations are not through the complementary hydrogen-bonded heterodimers, but the anion and cation have perpendicular orientations. The crystal structure of the hydrate of H2AZOBEN showed the presence of a hydrogen-bonded water bridge, and theory has suggested that this was the key feature in the stabilization of the assembly. Similarly, the gain in energy by hydration in hydrates was compared with the energy of different synthons in anhydrous forms, to show that hydration is the key to the stability of those. The combined effects of different synthons were found as a factor in providing relative stability compared to a single one. In the assemblies of methoxy or ethoxy derived H2AZOBEN, the ethereal oxygen did not participate in the scheme of hydrogen bonds. This is attributed to the C-H…π interactions of the C-H bond of the methyl and ethyl groups provide directional property to the assembly and also provide an environment to the O-atom without a good hydrogen bond acceptor in its proximity.