Syntheses, crystal structures, and luminescence properties of three cadmium coordination polymers assembled from a semi-rigid tetracarboxylate ligand

Crystal engineering is considerably useful in the preparation of functional coordination polymers. In the present study, a new semi-rigid tetracarboxylate, 4,5-bis(4-carboxyphenoxy)phthalic acid (H4L), was designed and applied for assembling metal coordination polymers. Self-assembling process involving Cd(II) metal ions, H4L and 4,4′-bipyridine (4,4′-bpy), (E)-1,2-bis(pyridin-4-yl)ethane (bpe) or 1,4-bis(1-imidazolyl)benzene (bib) resulted into the formation of coordination polymers, including [Cd2(L)(4,4′-bpy)1.5(H2O)]n·(H2O)n (1), [Cd2(L)(bpe)1.5(H2O)]n·(H2O)3.5n (2), and [Cd2(L)(bib)1.5]n·(H2O)n (3). Through X-ray single crystal diffraction analysis, the three compounds 1–3 were found to exhibit isostructural three-dimensional polymeric structures composed of two-dimensional secondary layers pillared by different rod-like N-donor ligands. Compared with the uncoordinated H4L exhibiting a symmetric crystal structure, the molecular configurations of L4− in 1–3 were tuned due to the complexation of cadmium ion and the introduction of different N-donors. Additionally, the coordination geometries of cadmium ions and luminescence properties in compounds 1–3 were finely tuned due to the introduction of different lengths of rod-like ligands. The thermostability and luminescence properties of compounds 1–3 were determined. The present results provide further understanding of the crystal engineering of semi-rigid ligands to a certain extent.