RESUMO
In poly[di-µ-chlorido-µ-(4,4'-bipyridazine)-κ(2)N(1):N(1')-cadmium(II)], [CdCl(2)(C(8)H(6)N(4))](n), (I), and its isomorphous bromide analogue, [CdBr(2)(C(8)H(6)N(4))](n), (II), the halide atom lies on a mirror plane and the Cd(II) ion resides at the intersection of two perpendicular mirror planes with m2m site symmetry. The pyridazine rings of the ligand lie in a mirror plane and are related to each other by a second mirror plane perpendicular to the first. The compounds adopt the characteristic structure of the [M(II)X(2)(bipy)] type (bipy is bipyridine) based on crosslinking of [Cd(µ-X)(2)](n) chains [Cd-Cl = 2.5955 (9) and 2.6688 (9) Å; Cd-Br = 2.7089 (4) and 2.8041 (3) Å] by bitopic rod-like organic ligands [Cd-N = 2.368 (3)-2.380 (3) Å]. This feature is discussed in terms of supramolecular stabilization, implying that the periodicity of the inorganic chain [Cd···Cd = 3.7802 (4) Å in (I) and 3.9432 (3) Å in (II)] is favourable for extensive parallel π-π stacking of monodentate pyridazine rings, with centroid-centroid distances of 3.7751 (4) Å in (I) and 3.9359 (4) Å in (II). This is not the case for the longer iodide bridges, which cannot stabilize such a pattern. In poly[tetra-µ-iodido-µ(4)-(4,4'-bipyridazine)-κ(4)N(1):N(2):N(1'):N(2')-dicadmium(II)], [Cd(2)I(4)(C(8)H(6)N(4))](n), (III), the ligands are situated across a centre of inversion; they are tetradentate [Cd-N = 2.488 (2) and 2.516 (2) Å] and link successive [Cd(µ-I)(2)](n) chains [Cd-I = 2.8816 (3)-3.0069 (4) Å] into corrugated layers.
RESUMO
Coordination polymers offer a significant potential for applications in adsorption, guest and anion recognition and sensing. Their structure commonly provides binding sites for such specific interactions as pi-pi stacking and XH...pi hydrogen bonding. The latter reflects the ability of the pi-cloud to interact with positively polarized atoms. An electrostatic interaction between anionic species and electron deficient heterocycles, which parallels the above binding scheme, is also possible and very recently the existence of anion-pi interactions was proved in the solid state and in solution. This effect may be significant also for biomolecule/solution interfaces, as it occurs in protein structures. In fact, such interactions could be especially relevant for host-guest chemistry of coordination polymers, particularly for functionalization of hydrophobic crystal cavities and for the design of geometrically rigid anion receptors. However, typical electron deficient heterocycles such as 1,3,5-triazines and 1,2,4,5-tetrazines are very weak donors and they are hardly suitable for bridging metal ions and the generation of coordination frameworks. As a system that combines efficient donor properties towards transition metal ions and a pronounced ability for anion-pi interactions we have developed unsubstituted pyridazino[4,5-d]pyridazine, which was readily accessible by a novel one-pot synthesis involving inverse electron demand Diels-Alder cycloaddition (Scheme 1). Unusual anion binding properties of the ligand may be clearly related to its electron-deficiency (LUMO energy -1.591 vs. -0.288 eV for the parent pyridazine), influenced also by N-coordination to such Lewis acids as metal ions.
