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

Abstract

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(L^2-quin)₂]_n, 1, [HgI₂(L^2-quin)], 2, [HgCl₂(L^3-quin)]_n, 3, [Hg₃Br₆(L^3-quin)₂]_n, 4, and [Hg₃I₆(L^3-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, L^2-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 L^2-naph adducts to L^2-quin and L^3-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.