Synthesis, Structure, and Characterization of a Novel One-Dimensional Mixed-Ligand Coordination Polymer of Copper(II)

A novel Cu(II) mixed-ligand complex, {[Cu(sq)(L)(H2O)]center dot 0.9H(2)O}(n) (1) (sq = squarate dianion, L = 5,5'-dimethyl-2,2'-bipyridine), has been synthesized and characterized by elemental analyses, infrared (IR), and X-ray single-crystal diffraction. In 1, each copper(II) ion has a five-coordinated environment with square pyramidal geometry. Aone-dimensional (1D) chain structure of the complex is constructed by squarate in mu-1,2-bis(monodentate) coordination mode and further forms a two-dimensional (2D) framework via "metal chelate-metal chelate ring" pi-pi stacking interactions between the adjacent chains. This unexpected pi-pi stacking interaction provides structural evidence for themetalloaromaticity of the Cu(II)-(aromatic alpha,alpha'-diimines) chelate rings.