Structural trends in one and two dimensional coordination polymers of cadmium(ii) with halide bridges and pyridine-type ligands

Abstract

Cadmium dihalides were reacted with pyridine derivatives as Lewis bases, and the solid state structures of 15 reaction products were studied by X-ray diffraction; all but one of these structures are reported for the first time. Among the 15 complexes there are 13 coordination polymers containing six-coordinated cadmium with four bridging halides in the equatorial and two pyridine ligands in the axial positions of a pseudo octahedron. For unidentate ligands such as pyridine and 3-chloro-, bromo- or methylpyridine chain polymers of composition [CdX₂(L)₂]¹_∞ were formed. The six combinations of Cl, Br, or I as bridging halides with 3-halopyridine and the two of chloride and bromide bridges with 3-methylpyridine lead to eight isomorphous compounds in the space group P2₁/c. In a second structure type 4,4′-bipyridyl was used to link neighbouring chains to two-dimensional nets with [CdX₂(bpy)]²_∞ stoichiometry. The chloro and bromo derivatives are isomorphous and crystallise in a space group Pban whereas the iodide bridged complex was obtained in the supergroup Cmmm. The halogen substituted pyridines represent the most versatile ligands for the formation of coordination polymers—they can be accommodated in the whole range of halide bridged [M(μ-X)₂]¹_∞ chains (M = Zn, Cd) covering metal⋯metal distances between 3.654 and 4.143 Å. In contrast to the above mentioned halide bridged topologies terminal rather than bridging iodide ligands occur in [CdI₂(py)₂] and [CdI₂(3-Mepy)₂]: these complexes are mononuclear with the metal center in tetrahedral coordination.