A novel trinuclear zinc metal–organic network: Synthesis, X-ray diffraction structures, spectroscopic and biocompatibility studies

Abstract

Crystal structure and vibrational spectra of a new zinc metal–organic network, {[Zn3(tp)4]·4H2O}n, 1, have been studied. The compound was constructed from Zn(NO3)2·6H2O and H2tp ligand (H2tp=1,4-benzenedicarboxylic acid) by solvothermal synthesis in DMF. Its framework is built from the trinuclear SBUs as eight-connected nodes and tp ligands as spacers. Each node consists of three ZnII ions that are in a linear arrangement with Zn⋯Zn distance of 3.271Å. The distorted octahedral middle Zn atom is connected to six oxygen atoms (Zn–O=2.036–2.252Å) belonging to six different tp carboxylate group. Each of the two terminal Zn atoms is coordinated by four oxygen atoms (Zn–O=1.952–1.986Å) belonging to four carboxylate groups of tp ligands. Normal-coordinated analysis of the vibrational spectra together with force field calculations for Zn(O2CC)n (n=4 or 6) entities which is the most affected by coordination effect have been performed. The results show consistency of the lower values of force constants for ZnO6 entity compare to ZnO4 that could be related to the stronger inter-ligand repulsion in octahedron compare to tetrahedron. In-situ PXRD patterns show two gradual structural transformations from ambient to 400°C and a new ZnO phase is formed between 400 and 500°C that are in a good agreement with thermal gravimetric analysis. Photoluminescence measurement in the solid state shows an emission at 420nm which is in accordance with other zinc carboxylate MOFs. The potential cytotoxicity of 1 was evaluated on human amniotic epithelial cells (hAECs) by XTT assay. The results clearly showed that this compound exerted little in vitro cytotoxic effect in a wide range of concentration and time intervals.