Role of Protonation and Isomerism in the Supramolecular Architectures of Heteroaryl-2-imidazole Compounds: Crystal Packing Patterns and Energetics

The influence of isomerism and protonation on crystal packing patterns of three isomeric heteroaryl-2-imidazoles and one related compound was investigated by using single-crystal X-ray diffraction and density functional theory. The comparison of their crystal structures helped unveil the details of the energetic balance controlling supramolecular packing and an unprecedented supramolecular synthon. The neutral species were featured by a whole crystal close packing like a herringbone pattern, which was mainly favored by C-H center dot center dot center dot pi interactions. On the other hand, the crystal packing of the protonated species was characterized by pi-stacking layers being supported by pi center dot center dot center dot pi and anion-pi interactions. Energy framework analysis revealed that both Coulombic interactions between hydrogen bond chains and dispersion energies mainly contributed to the stabilization of the herringbone pattern assembly, whereas a predominant contribution from Coulombic energy frameworks to total energy in protonated forms was observed. The different patterns displayed pieces of evidence of not only the important participation of the heteroaryl moieties in the supramolecular assembly but also the significant contribution of protonation hampering the N-H center dot center dot center dot N hydrogen bonding interactions in the imidazole group. A thorough characterization of the compounds by means of thermogravimetric analysis, differential scanning calorimetry, Fourier-transform infrared spectroscopy, NMR, and electrospray ionization-(+)-MS is also presented. (AU)