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.

Effect of robust π-π stacking synthon on the formation of mercury coordination compounds; an unusual pseudo-square planar geometry.

In this study, three Hg(II) complexes, [HgCl2(L(2-naph))]n, 1, [HgBr2(L(2-naph))]n, 2 and [HgI2(L(2-naph))2], 3 where L(2-naph) is N-(naphthalene-2-yl)pyrazine-2-carboxamide ligand have been synthesized and characterized. X-ray single crystal diffraction analysis of these compounds reveals that 1 and 2 are isostructural coordination polymers and 3 is a discrete compound. In comparison to homologue complexes containing N-(naphthalene-1-yl)pyrazine-2-carboxamide ligand, interestingly, structural analysis clearly shows that displacing substituent position plays an important role in the formation of the supramolecular organization of molecular complexes. The common feature in the crystal packing of these complexes is that there is a strong tendency to form π-π stacking interaction between pyrazine and naphthalene rings. These π-π stacking interaction synthons affect the coordination geometry and structural assembly. Also, theoretical methods show the π-π stacking interaction energies within a range of -64.13 to -70.51 kJ mol(-1). It is notable that in 3, cooperation of intermolecular π-π stacking synthon and intramolecular C-H(pyz)···I-Hg hydrogen bond resulted in the formation of unusual pseudo-square planar geometry around the Hg(II) center. This study reveals an undeniable contribution of π-π stacking interaction to the organization and stabilization of some of the crystal structures reported here.

catena-Poly[[bis-(pyrazine-2-carbox-amide-κN)mercury(II)]-di-µ-bromido].

In the crystal structure of the title compound, [HgBr(2)(C(5)H(5)N(3)O)(2)](n), the Hg(II) cation is located on an inversion center and is coordinated by two N atoms from the pyrazine rings and four bridging Br(-) anions in a distorted octa-hedral geometry. The Br(-) anions bridge the Hg(II) cations with significantly different Hg-Br bond distances of 2.4775 (8) and 3.1122 (8) Å, forming polymeric chains running along the a axis. Inter-molecular N-H⋯O and N-H⋯N hydrogen bonds are effective in the stabilization of the crystal structure.

catena-Poly[[bis-(pyrazine-2-carbox-amide)mercury(II)]-di-µ-chlorido].

In the polymeric title compound, [HgCl(2)(C(5)H(5)N(3)O)(2)](n), the Hg(II) atom (site symmetry ) adopts a distorted trans-HgN(2)Cl(4) octa-hedral coordination geometry. In the crystal, adjacent mercury ions are bridged by pairs of chloride ions, generating infinite [100] chains, and N-H⋯O and N-H⋯(N,N) hydrogen bonds help to consolidate the packing.