Cd(II) coordination polymers based on expanded N,N′-heteroaromatic donor ligands

The isomeric N donor linkers bis(pyridin-4-ylmethylene)naphthalene-1,5-diamine (L1) and bis(pyridin-3-ylmethylene)naphthalene-1,5-diamine (L2) have been used with acetate and iodide as co-ligands to generate four new cadmium(II) coordination polymers. [CdL1(OAc)₂]_n (1), [CdL1(I)₂]_n (2), [CdL2(OAc)₂]_n (3) and [CdL2(I)₂]_n (4) were characterized by elemental analysis, X-ray powder diffraction, IR spectroscopy, thermal analysis and single crystal X-ray diffraction. In line with the concept of crystal engineering, their structures share common motifs: the heteroaromatic linkers generate chains, subtending Cd⋯Cd distances consistent with their N donor arrangement. The co-ligands control the coordination number of the Cd(II) cations and interchain connectivity: The acetate moieties either adopt bridging and chelating or exclusively bridging modes and crosslink essentially linear chains based on octahedral metal centers; in the former case, the ladder-like one-dimensional ribbon 1 is generated, whereas the latter connectivity results in the two-dimensional layer structure 3. In contrast, iodide acts as terminal ligand towards tetrahedral Cd(II), and hence 2 and 4 are single-stranded 1D zig-zag polymers. In comparison to the uncoordinated heteroaromatic L1 and L2, the Cd coordination polymers show slightly red-shifted luminescence spectra of significantly higher intensity due to the more rigid arrangement of the π systems.