Reactions of Cadmium(II) Halides and Di-2-Pyridyl Ketone Oxime: One-Dimensional Coordination Polymers

The coordination chemistry of 2-pyridyl ketoximes continues to attract the interest of many inorganic chemistry groups around the world for a variety of reasons. Cadmium(II) complexes of such ligands have provided models of solvent extraction of this toxic metal ion from aqueous environments using 2-pyridyl ketoxime extractants. Di-2-pyridyl ketone oxime (dpkoxH) is a unique member of this family of ligands because its substituent on the oxime carbon bears another potential donor site, i.e., a second 2-pyridyl group. The goal of this study was to investigate the reactions of cadmium(II) halides and dpkoxH in order to assess the structural role (if any) of the halogeno ligand and compare the products with their zinc(II) analogs. The synthetic studies provided access to complexes {[CdCl₂(dpkoxH)∙2H₂O]}_n (1∙2H₂O), {[CdBr₂(dpkoxH)]}_n (2) and {[CdI₂(dpkoxH)]}_n (3) in 50-60% yields. The structures of the complexes were determined by single-crystal X-ray crystallography. The compounds consist of structurally similar 1D zigzag chains, but only 2 and 3 are strictly isomorphous. Neighboring Cd^II atoms are alternately doubly bridged by halogeno and dpkoxH ligands, the latter adopting the η¹:η¹:η¹:μ (or 2.0111 using Harris notation) coordination mode. A terminal halogeno group completes distorted octahedral coordination at each metal ion, and the coordination sphere of the Cd^II atoms is {Cd^II(η¹ - X)(μ - X)₂(N_pyridyl)₂(N_oxime)} (X = Cl, Br, I). The trans-donor-atom pairs in 1∙2H₂O are Cl_terminal/N_oxime and two Cl_bridging/N_pyridyl; on the contrary, these donor-atom pairs are X_terminal/N_pyridyl, X_bridging/N_oxime, and X_bridging/N_pyridyl (X = Br, I). There are intrachain H-bonding interactions in the structures. The packing of the chains in 1∙2H₂O is achieved via π-π stacking interactions, while the 3D architecture of the isomorphous 2 and 3 is built via C-H∙∙∙C_g (C_g is the centroid of one pyridyl ring) and π-π overlaps. The molecular structures of 1∙2H₂O and 2 are different compared with their [ZnX₂(dpkoxH)] (X = Cl, Br) analogs. The polymeric compounds were characterized by IR and Raman spectroscopies in the solid state, and the data were interpreted in terms of the known molecular structures. The solid-state structures of the complexes are not retained in DMSO, as proven via NMR (¹H, ¹³C, and ¹¹³Cd NMR) spectroscopy and molar conductivity data. The complexes completely release the coordinated dpkoxH molecule, and the dominant species in solution seem to be [Cd(DMSO)₆]²⁺ in the case of the chloro and bromo complexes and [CdI₂(DMSO)₄].