Cadmium(II) chloride, bromide and iodide complexes with 4,4'-bipyridazine: when are diazine and halide bridges (in)compatible?

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.