Influence of N-heteroaromatic π-π stacking on supramolecular assembly and coordination geometry; effect of a single-atom change in the ligand.

In order to understand how the polarization of aromatic systems, through the introduction of a nitrogen heteroatom, affects the π-π interactions and crystal packing of mercury coordination compounds, in this study, N-(quinolin-2-yl)pyrazine-2-carboxamide and N-(quinolin-3-yl)pyrazine-2-carboxamide ligands were employed in the synthesis of five Hg(II) complexes, [HgBr2(L2=quin)2]n, 1, [HgI2(L2=quin)], 2, [HgCl2(L3=quin)]n, 3, [Hg3Br6(L3=quin)2]n, 4, and [Hg3I6(L3=quin)2]n, 5. X-ray single crystal diffraction analysis of these compounds revealed that all the complexes have polymeric structures except complex 2, which is a discrete compound. Complexes 1 and 3 have 1D and 2D polymeric structures, respectively, while complexes 4 and 5 are 3D coordination polymers. In comparison to homologous complexes containing the N-(naphthalene-2-yl)pyrazine-2-carboxamide ligand, L2=naph, interestingly, the structural analysis clearly shows that the replacement of the naphthyl CH group with a nitrogen atom changes the spatial extent of the π-electron cloud and the polarity of the aromatic ring, from L2=naph adducts to L2=quin and L3=quin adducts, and the propensity to form π-π interactions increases. These π-π stacking interaction synthons affect the coordination geometry and structural assembly. This study reveals the undeniable contribution of π-π stacking interactions to the organization and stabilization of some of the crystal structures reported here.